Title: Ion beam-plasma interactions
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Title: Ion beam-plasma interactions
Physical Description: vii, 106 leaves : illus. ; 28 cm.
Language: English
Creator: Dunnill, William Arthur, 1937-
Publication Date: 1965
Copyright Date: 1965
 Subjects
Subject: Plasma (Ionized gases)   ( lcsh )
Magnetic fields   ( lcsh )
Physics thesis Ph. D
Dissertations, Academic -- Physics -- UF
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Thesis: Thesis--University of Florida, 1965.
Bibliography: Bibliography: leaves 104-105.
Additional Physical Form: Also available on World Wide Web
General Note: Manuscript copy.
General Note: Vita.
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Bibliographic ID: UF00097898
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: alephbibnum - 000565956
oclc - 13609435
notis - ACZ2380

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ION BEAM-PLASMA INTERACTIONS























By

WILLIAM ARTHUR DUNNILL


A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL OF
THE UNIVERSITY OF FLORIDA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE
DEGREE OF DOCTOR OF PHILOSOPHY












UNIVERSITY OF FLORIDA


June, 1965



















ACKNOWLEDGMENTS


The author wishes to thank the members of his committee:

Professors J. W. Flowers, J. Kronsbein, T. L. Bailey, F. E.

Dunnam, and R. W. Cowan for their assistance throughout his

graduate program. Especially he wishes to express his thanks

to Dr. Flowers for suggesting the topic of this dissertation

and for general assistance and suggestions. He also wishes to

show his appreciation to Dr. Kronsbein for the many discussions

covering theoretical work in the field.

The author is also very grateful to his wife Alice for her

patience and understanding during the final stages of his work.

Finally, the author wishes to gratefully acknowledge the

partial support given him by the Atomic Energy Commission,

under Contract AT-(40-1) 2783.


















TABLE OF CONTENTS


Page

ACKNOWLEDGMENTS ................... .. ii

LIST OF ILLUSTRATIONS . . . . . . . . iv

CHAPTER

I. INTRODUCTION . . . . . . . . . 1

II. THEORY . . . . . . . . ... .. 4

III. APPARATUS AND PROCEDURE . . . . .. 28

IV. RESULTS . . . . . . . .... . 56

V. CONCLUSIONS . . . . . . . ... 94

LIST OF REFERENCES . . . . . . . . . 104

BIOGRAPHICAL SKETCH . . . . . . . . ... 106










LIST OF ILLUSTRATIONS


Figure Page

1. Voltage drop measured across two probes
0.345 m apart as a function of the
magnetic field . . . . . . . . 10

2. Escape flux of ions transversally to the
magnetic field from a cold-cathode P. I. G.
discharge working with hydrogen ...... 12

3. Escape flux of ions transversally to the
magnetic field for B > 1000 gauss . . .. 13

4. The critical magnetic field is plotted as a
function of the pressure . . . . .. 15

5. Left-handed screw instability for m = 1 . . 18

6. A cross sectional view of the upper part of
Fig. 5 . . . . . . . . . .19

7. A cross section of the Penning discharge with
the charge distribution and resultant drifts 23

8. High energy dual beam experimental apparatus 30

9. Magnetic field strength along the axis of
the cone is plotted as a function of the
distance from the cone tip . . . ... 32

10. High current ion source . . . . ... 37

11. Detail of the cathode of the high current
source . . . . . . . .... . 39

12. The magnetic field along the arc axis is
plotted as a function of the distance
from the anode . . . . . . .. 40

13. Side and front view of the water-cooled
magnetic anode . . . . . . ... 42










Figure


14. Radial dependence of the magnetic field
on the surface of the anode is plotted
for three different magnet currents .... 43

15. Diagram of the pulse circuit and the arc . .. 45

16. Disk pulsing arrangement . . . . ... 46

17. Diagram of the current pulse circuit and
the arc . . . . . . . . ... .47

18. Reflex arc discharge apparatus . . . ... 51

19. Detail of the anode-button structure of
the reflex arc . . . . . . ... .53

20. Detail of the moveable cathode of the
reflex are discharge . . . . . ... 54

21. Collector potential in an air plasma is
plotted as a function of the magnetic
field . . . . . . . .... . 57

22. Collector potential is plotted as a function
of the hydrogen ion beam energy ...... 59

23. Collector potential in an argon plasma is
plotted as a function of the magnetic field 60

24. Collector potential in a helium plasma is
plotted as a function of the magnetic field 61

25. Collector potential is plotted as a function
of the magnetic field . . . . ... 63

26. Collector current Ic is plotted as a function
of the source pressure for a 2.5-amp arc
with a 1-inch-diameter shield aperture . . 65

27. Collector current Ic is plotted as a function
of the collector voltage V for a 2.5-amp
arc . . . . . . . . . . 67

28. Collector current Ic is plotted as a function
of the collector voltage Vc for a 1.5-amp
arc . . . . . . . . ... 68


Page








Figure


29. The beam current from a 1.5-amp arc is plotted
as a function of the radially increasing
anode magnetic field . . . . . ... 70

30. The beam current from a 1.5-amp arc is plotted
as a function of the radially decreasing
anode magnetic field . . . . . ... 71

31. Pulse voltage measured across 47-ohm resistor
between the collector and ground . . .. 73

32a. The pulsed beam voltage resulting from a
60-sec.- 200-volt arc pulse is shown as
a function of time . . . . . .. 74

32b. The pulsed beam voltage resulting from a
60-sec 400-volt arc pulse is shown as
a function of time . . . . . .. 74

33. The arc current is plotted as a function of
the maximum sustainable pulse voltage at a
pressure of .22 microns . . . . .. 75

34. The pulsed beam voltage resulting from a 6-amp
arc pulse is plotted as a function of time . 77

35. The oscillogram represents the variable current
fluctuation in time to the probe and arc
electrodes . . . . . . . ... .79

36. The frequency is plotted as a function of
the magnetic field for different cathode-
button separation distances . . . ... 80

37. The frequency is plotted as a function of
the cathode-button separation distance
for different magnetic fields . . ... 81

38. The frequency is plotted as a function of the
pressure for given cathode-button separation
distances . . . . . . . ... .83

39. The oscillogram represents the constant current
fluctuation in time to the probe and arc
electrodes . . . . . . . ... .84


Page









Figure


40. The frequency is plotted as a function of
the button radius . . . . . ... .85

41. The frequency is plotted as function of
the arc current . . . . . . ... .86

42. The floating potential of the variable
probe is plotted as a function of the
radial position of the probe . . . ... 88

43. The arc voltage is plotted as a function
of the magnetic field for different
cathode-button separation distances ... . 90

44. The arc voltage is plotted as a function of
the cathode-button separation distance for
different button diameters . . . ... 91

45. The arc voltage is plotted as a function of
the button diameter for three different
pressures . . . . . . . . .. 92


Page














CHAPTER I

INTRODUCTION



The interaction of a beam of charged particles with a plasma

has been studied in great detail since Bohm and Gross have shown

that a constant velocity homogeneous beam moving through a homo-
1
generous plasma is unstable. Since the plasma density n even
12 14
for a dense plasma, is generally small (no 10 10 parti-

cles/cm3), the energy lost per unit length by a charged particle

is insignificant, being of the order of 10-3 10-5 eV per cm.2

However, in the majority of cases, the beam becomes self-modulated,

leading to a coherent interaction between the particles in the

beam and plasma. The energy lost by the beam particles in exciting

oscillations can be considerable, being of the order of 103 10
7 8 2
eV per cm for particle bunches of N = 10 10 The fact that

this interaction energy is larger for beams and charged particle

bunches, than it is for individual particles, may be either an

advantage or disadvantage. For example, advantage may be taken of

this phenomena when devising means of injecting plasma into magnetic

traps and in increasing the energy of the plasma particles.








2



For instabilities to arise, the number of particles in the

beam giving up energy to the electromagnetic field must exceed

the number of particles in the plasma absorbing energy from the

field. This requirement has been shown to impose certain re-

strictions on the particle velocity distributions in the beam
3
and in the plasma. If the ions alone have a peaked velocity,

the electron species must have an average thermal energy cover-

ing the ion velocity range for instabilities to occur. This

indicates a restriction on ion beam instabilities unless the

electron distribution is of a special nature. This leads one

to think that it is really the electrons that govern instabili-

ties, even in the ion beam case.

Thus far in all devices designed to study means of con-

trolling fusion, instabilities of various natures have arisen

with the result being somewhat the same in each case. As the

density or energy of the charged particles has been increased

toward thermonuclear values, large particle losses from the

containing region have suddenly appeared, effectively destroying

the plasma. These instabilities are normally accompanied by

radio frequency noise over a large portion of the spectrum.

Clearly, the understanding of such oscillations is of fundamen-

tal importance in understanding beam-plasma systems.












In this dissertation the work with possible oscillatory

systems has been divided into three phases. The first phase

has been an investigation of high energy beam-plasma combina-

tions in which the beam was a hydrogen ion beam in the energy

range from 10 to 20 keV with currents of 5 to 10 ma. The

second phase concerned a low energy ion beam in the range of

several hundreds of volts. Finally, a discharge of a reflex

type has been studied as a beam-plasma interacting system.

For this system both an ion beam or stream and an electron

beam are present at low energies with relatively large current

densities.













CHAPTER II

THEORY



In the investigation of plasma-ion beam interactions, re-

sults of other experiments and theories have been useful. Since

a great deal of work has been done on studying plasma-electron

beam interactions, it has been advantageous to compare this work

with analogous plasma-ion beam experiments.


Dual Beam-Plasma Interaction Theory

The first phase of this work, in a sense, concerned a dual

ion beam-plasma interaction, so the analogous two electron beam-

plasma interaction has been investigated and studied. Standing

waves of longitudinal plasma electron oscillations have been

produced between boundary electrodes by oppositely directed,

independent, interpenetrating electron beams passing through a
4
plasma. The theory describing this two stream instability

determined by Bohm and Gross may be obtained by examining the

dispersion relation found by considering two oppositely directed

beams moving through a background plasma.5











In order to solve for the organized motion of particles in

a plasma, consider a system whose velocity distribution is given

by

dN = n f(V )dV = n f(U + V )dU (1)


where Uo is the velocity in the wave system and V1 is the

velocity of the wave in the laboratory system.

The average potential D(x), assumed to vary trigonometrically in

space and time, causes each particle to undergo a periodic change

of velocity and a corresponding change of its contribution to the

density.

Solving for the total charge density p by integrating over

all U and inserting this into Poisson's equation gives




+1 + 2c(1X2
0
o 24p 4(xl)( V 1/2 (2)

mU


Assuming O(x) is static in the wave system restricts us to tra-

veling wave solutions for all quantities of the form (x V1t).
2
By assuming small values for 2eO(x)/mU we can expand the

square root, obtaining the linear approximation


2 4n f((V )dV
-o o o (3)
m J(V V1)
f, 0












The dispersion relation found by writing V1 = u/k and V\7 2

-k 2 yields the non-trivial solution of


41n 0 f(V )dV (4)
1 o o (4)

m (w kVo0 2


For the two beam-plasma interaction system, we let f(Vo) be 8/2,

when IVo lies between a and a 8, and f(Vo) be 0 for all

other values of V This means each beam has a velocity spread

of width 8 while the total system has a zero mean velocity.

Neglecting collisions and restricting k to small values

yields the two roots


k a + k a+ (a )2 + a(a 6) (5)


and w2 -k2a(a 6) (6)


Unless 8 = a, the second root corresponds to an imaginary fre-

quency, representing the two beam-plasma instability.

To experimentally investigate the dual beam-plasma interac-

tion, it is necessary to produce either two beams or a reflected

component of a single beam available from a single source.

Since two sources were unavailable, it was decided to use a

reflected component of an ion beam to initiate the dual beam

studies.













When an electron beam impinges on a target in the presence

of a plasma, the negative potential build-up becomes related to

the beam velocity and its distribution. Electrons can then be

reflected providing essentially the situation of two opposing

beams with intermodulation, and thus feedback of any growing

disturbance on either beam. For the ion beam case a similar

behaviour has been found difficult to achieve since the ion beam

will not support positive charge and potential required in the

region of reflection. This is primarily due to the high mobi-

lity of electrons to the target, allowing charge, and hence,

potential neutralization to take place.

It has been found that intense hydrogen ion beams are

automatically space-charge neutralized when focused by a mag-

netic solenoid lens unless an electron drain is provided.

If electrons are drawn out of the drift space by a positive bias

on the target, for example, a spreading may be visually observed

in the beam. This is consistent with the expected behaviour of

the beam with no neutralization. If a transverse magnetic field

is applied near the target, space-charge neutralization takes

place whether the target is biased or not. When the target is

positively biased, the transverse magnetic field from the tar-

get bias creates an E x B electron trap in front of the target.

This electron build-up effectively cancels the positive bias on












the target, so the electrons remain in the drift space and

charge neutralize the ion beam. With this thought in mind, a

transverse magnetic field in the target region of the ion

beam could allow positive potential build-up sufficient to

produce the required reflected ion beam component.


High Current Source and Reflex Discharge Theory

The theory discussing the processes involved in the se-

cond and third phases of this tract will now be considered.

This grouping is done since both experiments are concerned

with the fact that particle diffusion rates may be coupled to

the low frequency oscillations found in the discharge. Much

work has been done on this problem since Bohm found that

single particle ambipolar diffusion theory does not predict

the particle diffusion rates across magnetic field lines in

various discharges. Bohm postulated that fluctuations of the

charge density may produce electric fields which give rise to

particle drifts across magnetic fields. Simon has shown

that this discrepancy between theory and experiment is due to

the fact that diffusion across the magnetic field is not am-

bipolar in character in all experiments. This absence of

ambipolar diffusion is due to the highly anisotropic conduc-

tivity in the medium, resulting in the observed higher diffu-











sion rate. Electrons moving parallel to the magnetic field

neutralize the space charge set up by the ions moving perpen-

dicular to the magnetic field. This electron flow parallel

to the magnetic field is completed through the conducting

electrodes at the ends of the discharge and is called the

short-circuit effect. With this in mind, Hoh and Lehnert

devised an experiment eliminating the short-circuit effect by

using a very long discharge tube with a diverging magnetic

field at the tube ends. Their experiments showed that the

classical theory for ambipolar diffusion held below a criti-

cal magnetic field B Above this field value, an instability

set in resulting in a diffusion rate much greater than that

given by the classical theory. The increased diffusion was

inferred from noting that the potential difference in Fig. 1

along the column increased when the magnetic field was above
9
B Measurements of the ion current to a negatively charged

probe at the tube wall supported this view. The characteris-

tics remained approximately the same for hydrogen, helium,

argon and krypton plasmas. The fact that the short-circuit

effect was eliminated by the geometry of the system and that

the noise in the discharge current increased when abnormal

diffusion set in indicated oscillations, turbulence, or other

asymmetric large scale motion in the plasma. Bonnal et al.



















pO (mm Hg)


O 0
>0-
N's


Theoretical curve for He~
Theoretical curve for He+
Theoretical curve for le+
O G O
2.13 1.56 0.80


D


B 0
c A

A A 0


0
0

B
c
N
N\


.1 .2 .3
Magnetic Field


.4 .5
(volt-sec/m2 )


Fig. 1. Voltage drop measured across two
probes 0.345 m apart as a function of the
magnetic field. Full curves are calculated
from the theory for molecular ions and the
dashed curves are for atomic ions.


280




240


200




160




120











measured the particle escape flux as a function of the magnetic

field and pressure.0 Figures 2 and 3 show that above a critical

magnetic field Bc, a new diffusion mode appears accompanied by

noise emission in which the escape flux increases with the mag-

netic field. Above B the escape flux decreases again along

with the noise level. As the pressure is decreased, both B
c
and B shift toward lower magnetic field values.
m
Other instability mechanisms accounting for enhanced dif-

fusion could be due to unstable wall sheaths or sound wave

resonance effects. Bohm's criterion for a stable wall sheath

formation is

1/2
v+ (kT/m+)/2 (7)


where v+ is the ion velocity perpendicular to the wall on en-
7 -2
tering the sheath. Since v+ a B the velocity will decrease

as the magnetic field is increased. For a sufficient magnetic

field strength, Eq. (7) will not hold and the wall sheath will

become unstable.1

Further theoretical investigations of the instability in

the positive column in a longitudinal magnetic field were car-
12
ried out by Kadomtsev and Nedospasov.2 They showed that a

screw-shaped disturbance in the plasma column was responsible

for the observed phenomenon. Experimental results summarized












Pressure (mm Hg)

1.6 x 10-2
2.4 x 10-2
3.0 x 10-2


B (gauss)

230
320
360


250 500 750
Magnetic Field (gauss)

Fig. 2. Escape flux of ions transversally to the
magnetic field from a cold-cathode P.I.C. discharge
working with hydrogen, The discharge length is
110 mm, diameter 30 mm, voltage drop 400 volts and
current 200 ma.













Pressure (mm Hg)

4 2.1 x 10-2
-2
5 2.5 x 102
6 3.0 x 10'2


B (gauss)

1800
2100
2150


2000 3000 4000
Magnetic Field (gauss)

Fig. 3. Escape flux of ions transversally to
the magnetic field for B > 1000 gauss.


1000












in Fig. 4 have shown that it is indeed the screw-shaped dis-

turbance rather than the wall sheath instability that is re-
13
sponsible for enhanced diffusion.3 Due to the importance of

the screw instability, an outline of the theory and a physi-

cal description of the mechanisms involved will be given below.14, 15

The plasma density of a cylindrical positive column in a

magnetic field is given by


n (r) = 0Jo(aor/R) a 4 2.405 (8)


where r denotes the radial distance from the cylinder axis, N
o
is the plasma density on the axis, J is the Bessel function
o
16
of zero order, and R is the tube radius. The radial ambi-

polar electric field is


E (r) = ( o/n )(dn /dr) (9)


where $ = (D. D )/(b + b )




22 2
D = D /(1 + 'r ) b = b /(1 + ^T2)


D = D /(1 + L+) b b /(l + W +) (10)


D_ = bkT_/e D+ b kT /e


where D is the diffusion coefficient, b is the mobility, w

the gyrofrequency, T the collision time with neutrals, and










R = .90 cm I = 200 ma
R = 1.27 cm I = 400 ma
where R = tube radius


0
0
O ,R = .90 cm


R = 1.27 cm


O A



A 0
^/y/


R = .90 cm


\R = 1.27 cm


1 2 3 4 5
Pressure (mm Hg)

Kadomtsev's theory

- - Wall sheath instability theory

Fig. 4. The critical magnetic field is plotted
as a function of the pressure.











T the electron temperature.14' 17 The same notation with sub-

script + is used for the ions, where k is the Boltzmann constant

and e is the charge. It is assumed that the collision frequency

of the ions is much larger than their gyrofrequency in the posi-

tive column. The production rate of electrons and ions in the

positive column is16


Z D (a /R) (11)


where D = (D b + D b )/(b + b)
a -b+j +" -I +1


The left-handed screw-shaped density perturbation n'

which is originally quasi-neutral, is superimposed on the

steady-state density distribution from Eq. (8), where


n' = n1(r)cos(me + kz) (12)


In this notation, nl(r) denotes the radial variation of the

perturbed density, e the azimuthal angle, z the distance along

the axis, and k the wave number. Since the temperature and mo-

bility of the ions are considerably less than those of the elec-

trons, it is assumed that the rate of growth and dissipation of

the ion part of the screw are small in comparison with those of

the electron screw.











A perturbed space charge p', given by Eq. (13), will arise

when the electron screw is given a small angular displacement 6

relative to the ion screw in the positive e direction.


p' = enl(r) cos(mO + kz) cos(mO + kz E) (13)


S-e6nl(r)sin(me + kz)


It can be seen in Fig. 5 that p' produces an azimuthal electric

field E-. The subsequent Es/Bz drift feeds electrons from the

main density distribution n (r) into the screw perturbation.

When the net gain of electrons averaged over the screw

(n/2 > e > -At/2 in Fig. 6) is larger than the losses, the elec-

tron screw will grow, i. e. when


E' n n D
Se 1D 1 -L n'
B r [z 3z r 6e r


-1 n' 1
(rD -) - rn'b E ) -zn' (14)
r 6r ar r 6r -_ or


The first three terms on the right side of Eq. (14) represent

electron losses in the screw due to diffusion along and per-

pendicular to the magnetic field. The fourth term represents

conduction losses in the unperturbed ambipolar field while the

last term gives the rate of electron production.

From the field quantities given in Fig. 5, it can be seen

that E tends to "lift up" the electron screw relative to the
oz







18
z


Er xB

Ex Bz Eor
E




V =V
-z o




v \ ,



oz




















0

Fig. 5. Left-handed screw instability for m = 1. The
perturbed density distribution of ions is given by the
body confined by the full lines. The corresponding
electron distribution is indicated by the dashed lines.










B V
Z 0

0S x/2
E

S++
+++ +
+++++++
+++++

E' x B










A/2
Fig. 6. A cross sectional view of the upper part
of Fig. 5. This shows a more realistic charge
distribution and the outward E' x B drift which is
responsible for instability.













ion screw. This is equivalent to a rotation in the positive e

direction of the electron screw. The charge separation in a

given plane produces the azimuthal electric field Ee. Note

that the initial destabilizing mechanism is induced by the axial

electric field. The ambipolar electric field, however, tends

to cause the electron screw to rotate in the negative e direc-

tion relative to the ion screw. When this negative rotation

plus the stabilizing diffusion and conduction effects balance

the rotation in the positive e direction, the particle distri-

bution will become steady with electrons moving in screw-shaped

paths inside the ion screw. This balance takes place at the

critical magnetic field B

Other work has been done in which rotational instabilities

were observed. Perkins and Post found that the plasma rota-

ted eccentrically as one body while escaping radially across

the magnetic field lines when instabilities occurred in their

hot electron and hot ion plasmas.l8

Simon and Guest investigated the screw-type instability
19
in perhaps a more realistic manner.9 Instead of requiring an

external longitudinal electric field to initiate the spiral

behaviour, they assumed the direct streaming of ions and elec-

trons out of the plasma to the end walls provided the mechanism.

Although ambipolar fields tend to balance the ion and electron












currents, exact cancellation does not occur. This seems rea-

sonable since ions diffuse more rapidly in the directions

perpendicular to the magnetic field. Thus, there is net current

flow along the axis and a helical perturbation may be unstable.

Another important feature of their work is the differentiation

between the phenomena taking place in the positive column and

that which takes place in the low pressure region outside of the

main discharge. Kadomtsev and Nedospasov considered only the

positive column region where the ion collision frequency was

greater than the ion gyrofrequency. Thus w,+ < 1 indicating

that the magnetic field acts only on the electrons, where ( is

the ion cyclotron frequency and T+ the ion collision time. In
-3
the low pressure region (po 10 mm), ~ +T+ and Cr_ are both

greater than 1, so that the magnetic field influences both types

of particles. The particle conservation equations are also

different for both regions. In the low pressure case ions are

produced only along the axis of the discharge while in the

positive column, continuous ionization of the neutral gas is

always going on. For low pressure conditions, Guest and Simon

found that the frequency of rotation wk, to the first approxi-

mation, is independent of B and is given by


(k = (iV+/4d)(l + T_/T+)


(15)












where d is the discharge length, and V is the average particle

velocity to the end walls. From experimental considerations,

this model indicated a rotation frequency of 25 kc, in good

agreement with the measured values. The rotation frequency

dependence on the discharge length for constant pressure and

magnetic field was measured.20 The low pressure conditions were

satisfied by injecting a beam from a duoplasmatron ion source

into background gases of hydrogen, helium, and nitrogen at pres-

sures from 0.05 to 2 p. The discharge length d was varied con-

tinuously from 10 to 110 cm. Frequency measurements showed the

inverse wall separation dependence from d = 12 to d = 18 cm.

For larger d, however, irregularities or other types of oscilla-

tions occurred.

A "three fluid" model has been used to predict diffusion

processes and turbulence in weakly ionized plasmas in a Penning

discharge.1 As shown in Fig. 7, the plasma column has a radial

electric field E due to the Penning discharge geometry and is

immersed in an axial magnetic field B. An azimuthal perturbation

applied to the axisymmetric plasma allows the electrons to drift

in the 0 direction with a velocity ve = Er/B. The neutrals,

acting as a third fluid, exert a larger frictional drag on the

ions than they do on the electrons. Thus the ion perturbation

lags behind the electrons setting up an Ee field. The. E x B
































/ V /r Ee xB


r+ 4-










Fig. 7. A cross section of the Penning discharge
with the charge distribution and resultant drifts.











drift drives the plasma outward and amplifies the perturbation.

This differs from the screw instability in that a radial electric

field, rather than an axial electric field, drives the pertur-

bation. The dissipative effects also allow a smooth transition

to the unstable mode in contrast to the sharp onset of the screw

instability. This model is in good agreement with critical mag-

netic fields found experimentally in a reflex discharge by other

workers.

Chen and Cooper found that the longitudinal wavelength of

the measured oscillations in a reflex discharge were much too
22
long to be associated with ion waves.22 From measured wavelengths,

they calculated a wave velocity of the order of 2 x 10 cm/sec

which is much smaller than the Alfv6n velocity (-10 cm/sec) and

much larger than the acoustic velocity (-106 cm/sec). The

dispersion relation obtained from the "three fluid" model does,

however, give results for the critical magnetic field which

agree with the measured Chen and Cooper values. Crawford

attempted to explain this phenomenon by a radial electrostatic

sound wave mechanism but he noted that an azimuthally varying

mode was strongly excited and easily detected.23

Alexeff and Neidigh have carried out extensive experi-

mental studies on the ionic sound waves in plasmas predicted by

Tonks and Langmuir.24' 25 The sound waves were found in both a











magnetically supported plasma column formed by a reflex dis-

charge, and in the plasma of a spherical chamber with no B

field. Resonant modes restricted the ionic sound waves to

steady oscillations of discrete frequencies.

The basic equation leading to the ionic sound waves

derived by Tonks and Langmuir is


v = ne2/(lm+ + ne2m+X2/ykT_) 1/2 (16)


where v is the frequency in cycles/sec, n is the electron or
3
ion density in particles/cm e is the electron charge in esu,

m+ is the mass of the ion in grams, X is the ionic wavelength in

cm, 7 is the adiabatic compression coefficient of the electron

gas, k is Boltzmann's constant in ergs/OK, and T is the elec-

tron temperature in K. The plasma is assumed to be charge

neutral, the ions singly ionized, and the electron temperature

much higher than the ion temperature.

If only wavelengths X much larger than the Debye cutoff

length L are considered, where


L = (kT /4ne2) /2, (17)


then v = X- (ykT/m+) /2 (18)


where Eq. (18) is the dispersion relation for ionic sound waves.












With the assumptions that the ions and neutral gas atoms

have the same mass and the coherent velocity of the ions due to

the ionic sound wave motion is small compared to the thermal

velocities of the ions, it can be shown that the ionic sound

waves can be strongly damped out by neutral background gas. It

is then possible for the lower frequency standing waves to be

damped out while allowing excitation of the higher frequency

waves to take place. So, at higher pressures, only higher over-

tones of the fundamental frequency should be observable.

Experimental verification of the ion sound wave model was

attempted in a reflex discharge arc. With proper adjustments of

the source parameters, pure sinusoidal wave forms were measured.

The frequency was found to be independent of the magnetic field,

the gas density, and the plasma supply voltage and current. How-

ever, if the filament temperature of the source was changed

enough, the frequency would change discontinuously from the fun-

damental to the first overtone. Occasionally, overtones up to the

fourth order were observed. The different plasma gases used indi-
-1/2
cated that the frequency was proportional to m It was also

noted that by varying the length of the column, it was found that

the frequency was approximately inversely proportional to the

column length.







27



Since it appears that no one theory has been developed

which accurately explains the phenomena observed in the dif-

ferent experiments, it will be the object here to compare our

results presented in the fourth chapter, with the instability

models discussed above.












CHAPTER III

APPARATUS AND PROCEDURE


The apparatus used in these experiments will be divided

into three parts. Initially a high energy, low current hydro-

gen ion beam extracted from a duoplasmatron ion source was used

to investigate ion beam-plasma interactions. Secondly, a hollow

cathode, low energy, higher current ion source was developed to

further the interaction studies. Finally, a reflex discharge of

special design was developed and used to terminate the studies

covered in this tract.


Dual Beam-Plasma System

Experimental work previous to this effort had investigated

the passage of a single high energy ion beam through a plasma.26

No strong interaction of the single beam was found within the

limitations of the beam magnitudes and path lengths available.

For two beams which counterstream, the interactions, if existent,

should be greatly amplified by the associated mechanisms for

modulation and feedback processes. Lacking two complete duo-

plasmatron sources, an investigation was made in the attempt to

reflect a single beam from a region of positive potential. If

this were found to be possible to a sufficient extent, a dual

ion beam system would be achieved in a manner in which an













electron beam is often found to behave. In addition, the beha-

viour of an ion beam under reflection was thought to be strongly

indicative of its ability to excite plasma oscillations by col-

lective coupling. To investigate this reflection possibility,

the ion beam was arranged to fall upon a suitable and versatile

target after passing through a plasma in the target vicinity.

The duoplasmatron source, including the extraction and fo-

cusing electrodes was purchased as a single unit and then en-

closed in a vacuum tight, cylindrical brass container. The con-

tainer was insulated from the source by a 3/4-inch lucite ring as

shown in Fig. 8. It was then mounted horizontally on a steel and

aluminum frame. A sylphon bellows was attached to the orifice of

the lens system to allow for mechanical adjustment. A valve was

inserted between the bellows and the rest of the system. With
-6
the valve closed, pressure was maintained at 106 mm Hg in the

source by a N. R. C. type HS6 oil diffusion pump with a pumping

speed of 1500 liters/sec. This pump was mounted in a brass con-

tainer directly below the Einzel lens. Thus work could be done

on the rest of the system without disturbing the high vacuum in

the source. Two 4-inch-diameter Pyrex crosses, connected by a

1/4-inch-diameter, water-cooled, brass, beam defining aperture,

were mounted in series next to the valve. A C. V. C. type MCF

700 diffusion pump with a pumping speed of 500 liters/sec was














c
0



p- i
o0
SCa

0 0
3 1-1
p












mounted on the bottom of each cross and a conducting plate with

a Hoke leak valve was mounted on the top of each cross. A 45*

conical pole piece, silver-soldered into the brass end plate,

was used as the beam target.

A thin mica disk insulated the conical pole piece target

from the wire-wound iron core magnet, allowing this end plate

structure to float in potential. The magnetic field strength of

the target was measured as a function of the distance along the

conical axis with a Radio Frequency Hall Probe Gaussmeter. The

field characteristics are shown in Fig. 9. The strongly diver-

gent magnetic field obtained with this target geometry gave rise

to the required transverse component of the magnetic field. A

copper disk with a 1/8-inch-diameter hole in its center and

electrically insulated from all other components was placed in

front of the cone tip. A vacuum tube voltmeter was used to mea-

sure the potential build-up between the pole piece and ground.

The power supplies required to run the source, extract and

focus the beam, and to reflect the beam are summarized in Table 1.

Before the source was started, the Einzel lens was "condi-

tioned" by slowly raising the extraction voltage to 45 keV.

Threaded nylon bolts were used to hold the brass container and

lucite insulator ring to the source. Even with the threaded lu-

cite, breakdown occurred through some of the bolts. After



















-O,;X





















O L10








I 0
1000


0 I = 2.0 amp
S I = 5.0 amp
X I = 6.0 amp


X

2 ] ^


2000


3000


4000


Magnetic Field (gauss)

Fig. 9. Magnetic field strength along the axis of the
cone is plotted as a function of the distance from the
cone tip. Three different values of magnet current are
used.












TABLE I

Power Supplies for Source Operation

Arc (current regulated) 450 volt 5 amp

Source magnet 150 volt 10 amp

Filament 6 volt 100 amp

Palladium leak 7.5 volt 30 amp

Extraction focus

Extraction 50 kV 50 ma

Focus 10 kV 25 ma

Reflection magnet 150 volt 10 amp



trying various cleaning methods, it was found that the breakdown

could be stopped by dipping the bolts in corona "dope" and then

bolting the container and lucite ring to the source. A 250-

watt string of resistors allowed 1 ma to drain to ground at 10

kV, eliminating most of the corona losses and stabilizing the

high voltage source.

The standard procedure for operating the duoplasmatron ion

source is as follows. The palladium leak was first turned on,

bringing the source pressure up to a desired pressure. The cur-

rent to the source magnet was then set at 2 amps. The filament

current was adjusted to approximately 22 amps, and the arc was

then ignited. Before extraction, the source magnet current was












generally set back to approximately 1 amp, since this gave the

best results. To be certain the pressure would remain essen-

tially constant, the arc was allowed to run for five minutes.

The pressure could always be adjusted by varying the current

in the palladium leak or by varying the filament current.

Increasing the filament current lowered the pressure and in-

creasing the palladium leak current increased the pressure.

The focus potential was then set at 3 kV. Finally, the beam

was extracted by raising the accelerating voltage to 10 to 20

kV. The beam was easily seen in a darkened room and optical

focusing was readily attained by adjustment of the focus po-

tential and the source magnet current.

During a typical run, the source, accelerating and focu-

sing characteristics were kept constant. Normally, the source

pressure remained constant without adjustment; for some runs,

however, the palladium leak current and/or the filament current

had to be changed to keep this pressure constant.

When steady beam conditions were realized, gas was emitted

through the leak valve into the target region at a rate set to

establish a given background pressure. The potential build-up

on the pole piece and the interaction frequencies were then

measured as a function of the magnetic field in the collector

region. These measurements were repeated for different












background pressures, beam energies, and plasma gases and are

discussed in the next chapter.


High Current Source System

One limitation of the collective behaviour of the high

energy beam is associated with the limited beam current.

Higher beam currents of appreciable amounts have not been

possible from any known sources from which ions are acceler-

ated. The major factor in this limitation is associated with

space charge arising from charge separation. However, there

appeared the possibility of using, as a source, ions from a

long cylindrical arc operating in the magnetic field. From

such a source both the ions and electrons diffuse radially

from the main column. The ions are accelerated toward the

cathode by the arc operating voltage. They then have the pos-

sibility of forming an intense beam if they do not strike the

cathode or a support of the cathode. At the same time limita-

tions found on the arc potentials, for the arc geometry reason-

ably available, have restricted the beam to low potentials;

but this is of little interest toward interacting possibili-

ties. In principle, large beams and energetic beams could

likely be achieved. The limitations of such a source would

depend upon the diffusion rate and not space charge. Moreover,













there appeared the possibility of pulsing this arc to obtain

a high energy beam, again without a space charge limitation.

At this stage, however, it was appreciated that the ion

stream alone does not seem to be capable of collective inter-

action in the presence of low temperature thermal electrons.

The dual beam interacting aspects for two such simple, intense

ion sources were not therefore carried to completion. Instead,

aspects of the diffusion process appear extremely important

and the arc source has been studied to uncover some of these

processes which are also beam interaction processes. These

possibilities have been investigated and the apparatus devel-

oped is presented in this section.

The hollow cathode, low energy, high current ion source,

hereafter called the high current source, is shown in Fig. 10.

The high current source, focusing magnets, and collector were

attached to a 4-inch-diameter Pyrex cross. The entire assembly

was mounted on a steel and aluminum frame.

The anode structure initially consisted of an insulated

water-cooled brass plate mounted on one arm of the Pyrex cross.

On the opposite side of the cross another brass plate, floating

in potential, collected the ion beam. After several trial ca-

thode arrangements, a satisfactory water-cooled cathode struc-

ture was developed and mounted on top of the cross.










Hoke Leak Valve


Focusing Magnet Coil # 2

x


Brass
Collector


6" MCF Diffusion Pump


Fig. 10. High current ion source.












Details of the cathode are shown in Fig. 11. The hollow

1/8-inch-diameter tantalum tube served both as a gas feed

and as the self-heated cathode. The gas flow was controlled

by having a Hoke leak valve and needle valve in series with

the argon tank. A section of Tygon tubing inserted between

the needle valve and Hoke valve insulated the cathode from

ground. A 1/8-inch-diameter tungsten post shielded the

tantalum cathode at the bend and prevented cutting due to

ion impact at this vulnerable point. A 3 1/2-inch-diameter

copper shield disk with a 1/2 to 1-inch-diameter beam aper-

ture was placed on the collector side of the cathode tip.

This shield, which could be grounded or allowed to float,

had several purposes. When grounded, the shield and grounded

collector essentially defined a field free region for the

beam to move in. Potentials applied to the shield gave it

the properties of an accelerating electrode. The shield also

reduced the tantalum sputtering to the Pyrex walls in the beam

and collector regions.

The 450-volt current-regulated arc supply was connected

between the cathode and anode in series with a 71-ohm surge-

limiting resistor. A 30-volt 50-amp D.C. supply powered

the focusing magnets, producing a mirror-shaped field along

the axis of the discharge. The spacial dependence of the field

along the axis is shown in Fig. 12.














1/8"-diameter Copper


as Feed
Water Inlet

SWater Outlet


Copper Jacket


1/8"-diameter
Tungsten Post


1/8"-diameter Tantalum Tube


Fig. 11. Detail of the cathode of the high current
source.

















450





400


Power 5



350

0 Power 3

-o
-4

300
-,4
P Power 1




250






200






150 I I -!\
0 1 2 3 4 5 6
Axial Distance (inches)

Fig. 12. The magnetic field along the arc axis is
plotted as a function of the distance from the anode.
Three settings on the power supply are shown.












The water-cooled magnetic anode designed to allow a more

flexible magnetic field variation in the active arc column is

shown in Fig. 13. A 1 1/4-inch-diameter water-cooled copper

disk was hard soldered to a truncated soft iron core. The

iron core, 1/8-inch-diameter water lines, and probe fixtures

were soldered to the brass anode plate. A thin mica disk

insulated the anode assembly from the external iron core

electromagnet and rubber tubes insulated the water lines.

Fig. 14 gives the radial dependence of the magnetic field on

the surface of the anode.

The 9 pin header allowed potential measurements at five

different radial positions along the surface of the anode. A

variable probe allowed potential measurements from the center

of the arc to the Pyrex walls and from the anode surface to

the cathode. The variable probe consisted of a 40-mil tung-

sten wire epoxied into a 5/16-inch-diameter capillary tube.

The tube and wire were bent at 90 so that the external rota-

tion of the probe allowed a complete radial sweep of the arc

region. An 0-ring sleeve fitting maintained the high vacuum

when the probe was adjusted.

The equipment used in the three methods of pulsing the

arc is described below. First a two-section artificial

transmission line matched roughly with the characteristic arc
































































0, C:

o<
0 <


S0.

3 u
















I = 2 amp
I = 5 amp
I = 10 amp


0 OAx

00-_


Radial Distance from Axis (inches)


Fig. 14. Radial dependence of the magnetic field
on the surface of the anode is plotted for three
different magnet currents.


2000


1000













impedance was used to apply an over-voltage to the anode of

the arc for 60 microseconds. Each section of the line consisted

of a .46-millihenry inductor and a .5-microfarad capacitor. The

capacitors in the circuit in Fig. 15 were charged with a Model

30-5-1 Del Electronics power supply. The magnitude of the

pulse was governed by setting the spark gap at different values.

The effects of the pulse on the beam current were obtained by

monitoring the voltage drop across a variable resistor between

the collector and ground with a Tektronix Model 517 CRO.

A variation of pulsing the arc potential is shown in Fig.

16. Two insulated copper disks with 1-inch-diameter holes

were mounted next to the anode. The pulse circuit was then

connected to the outer disk with the inner disk shielding the

anode. The pulse circuit, arc circuit, and beam monitoring

were the same as above.

The circuit in Fig. 17 was used to provide a current pulse

to the arc. The pulse circuit used the same power supply and

spark gap arrangement as above. A 1-microfarad capacitor, 1000-

ohm resistor, and the spark gap were connected in series with

the arc. A 12-ohm resistor attached at the cathode was also

tied in series with the pulse circuit. The 517 CRO was used

to measure the potential drop across this resistor and across

the variable resistor between the collector and ground.























.46 mh .46 mh p
Spark Gap



.5 pf .5 pf

tage





Arc Anode
Ar



Copper
Shield


7 1 9





Arc Supply -


c Cathode


Collector


Fig. 15. Diagram of the pulse circuit and the arc.


High Vol
























To Pulse Copper Disks
Circuit

















Water-cooled Anode


Tantalum Cathode









ICopper Shield


Fig. 16. Disk pulsing arrangement.






















12 n



1 Pf

6 MQ Spark Gap
--\^----\\V-Q 0-
1 KO
Copper
Shields

Arc
Anode '


Arc Supply


Tantalum
, Cathode


Collector


Fig. 17. Diagram of the current pulse circuit
and the arc.












A 3 1/2-inch-diameter copper disk was placed behind the

original shield surrounding the cathode. This second disk

was grounded while the original shield was allowed to float.

A Faraday Cup arrangement was periodically used in place of

the collector to check for secondary emission.

The standard procedure for operating the high current

source is as follows. A base pressure of 10-6 mm Hg or less

generally indicated no leak problems. The water for cooling

the electrodes was turned on and a check against electrical

shorts was made. The argon gas pressure was increased to

approximately 1 atmosphere and the magnetic field was turned

on. The current-regulated arc supply was then adjusted until

a glow discharge ran between the anode and tantalum cathode.

When the tantalum turned red hot, the power was increased

until the arc was struck. The arc current was generally set at

1.0 amp and the pressure reduced by adjusting the leak valves

to the .1 micron-range.

During a typical run, the above procedure was followed

with minor adjustments made depending on the electrode structure.

For the initial set-up shown in Fig. 10, the collected steady

ion beam current was measured as a function of the collector

potential by using a Lambda Electronics Corporation Model 71

current-regulated supply. These measurements were repeated for

different pressures, arc currents, and disk-hole diameters.












No changes were necessary in the arc striking procedure

when the magnetic anode was inserted in place of the flat

plate anode. For constant arc conditions, the potential on the

variable probe and on the anode probes was measured as a func-

tion of the magnetic field. The collector current, variable

probe current, and probe oscillations were also measured as a

function of the magnetic field. The field was adjusted by

varying the current to the anode magnet and by changing the

polarity of the focusing magnets as well as the current.

The oscillations were observed with either a Tektronix Model

517 or with a Tektronix Model 545 CRO. The measurements were

then repeated for different arc currents and pressures.

The arc striking procedures were again the same as de-

scribed above when the pulsing circuits were incorporated into

the experiment. For the circuit shown in Fig. 15, a voltage

pulse was applied directly to the arc. The pulse voltage in

Fig. 16 was applied to the region immediately surrounding the

discharge. Fig. 17 shows the circuit used to provide a current

pulse to the arc. The pulse effects on the collector current

were observed by noting the potential drop across a variable

resistor between the collector and ground. These measurements

were repeated for various pulse voltages, arc currents, and

pressures; and oscillograms were recorded with a Wollensak 75 mm

scope camera.













Reflex Arc Discharge Assembly

A number of observations on the beam collected from the high

current ion source indicated that oscillatory behaviour of the

arc was very likely associated with the diffusion rate. The

source was therefore re-designed to provide the highly oscil-

latory reflex mode of the arc. It was also designed to allow

mechanical facility for adjustment in all dimensions in order to

determine if such adjustment was related to resonant modes

of oscillation excited by beam-plasma interaction within the

arc column.

The reflex discharge set up by changing the anode and ca-

thode structures of the high current source, is shown in Fig.

18. The vacuum chamber, magnetic focusing coils, and power

supplies were unchanged. The water-cooled anode-button arrange-

ment was mounted on one arm of the Pyrex cross while the

adjustable cathode was placed on the opposite arm of the cross.

The vacuum chamber was again evacuated from the bottom by a

500 liter/sec diffusion pump, while the top of the cross was

covered with an aluminum plate. A 4-inch section of 1/8-inch-

diameter tantalum tubing was sealed to a hollow 5/16-inch-dia-

meter brass rod. The cathode assembly could be varied longi-

tudinally through the 0-ring vacuum seal in the cathode plate.

Water-cooled copper reflex buttons, 1/8-inch thick and of















Aluminum Plate


Variable


5/16"-diameter
Hollow Brass Rod


MCF 6" Diffusion
Pump


Fig. 18. Reflex arc discharge apparatus.


1r '"
Water
Lines


Water-
cooled
Brass
Anode


Gas Feed


7 Magnet
Coils


W


F7













varying diameters, were positioned 3/8 inch in front of the

brass anode plate. Figures 19 and 20 show the details of the

reflex discharge electrodes.

Operating procedures for the reflex discharge are identical

to those of the high current source except for one point. It

was found that the arc struck easier when the anode and button

were electrically tied together. Once the arc struck, the

button and cathode were generally grounded while the anode car-

ried the high voltage. It should be noted that the discharge

would not operate at low pressure whenever a hole was burned

into the wall of the tantalum cathode.

During a typical run with a fixed button assembly, the

pressure was set by carefully adjusting the tank valve and the

two needle valves. For a given arc current and anode-cathode

separation, the arc potential and oscillations measured with

a Tektronix 545 CRO could be picked up on the variable probe

or on any of the ungrounded electrodes. With the pressure and

electrode separation remaining the same, the arc current was

changed and the magnetic field dependence of the arc potential

and oscillations were measured again. The electrode separation

was then changed and the above procedure repeated. Finally, the

pressure was changed by adjusting the needle valves and the

whole experiment was repeated. The button diameter was then







































































































90) )
4 Wr

0o -r4
3 1-1
























5/16"-diameter



Hollow Brass Rod


"0" Ring


-- \Zf_ _\ r \ \_


Threaded
Brass Force Cap
Sleeve


Brass Plate


Threaded Force Cap
1/e"-diameter -5/16"-diameter
Ta rt lum Tube \ /16"-di\ete
Tantalu -\ Z \_ \\\\ \ -k

Flare / ___ Hollow Brass Rod
Fitting


Fig. 20. Detail of the moveable cathode of the
reflex arc discharge.









55



changed by replacing the old button with a new one. Four

different button diameters were used in compiling the data

which are presented and discussed in the next chapter.











CHAPTER IV

RESULTS



The results and graphs presented below are divided into

three groups, consistent with the general trend of this paper.


Dual Beam-Plasma Results

In order to have an idea of the energy range of the re-

flected ion beam, the potential build-up on the target collec-

tor was measured as a function of the impinging beam energy

and the magnetic field. Fig. 21 shows the collector potential

versus the magnetic field for a beam energy of 15 keV. The

beam consisted mainly of H3 since the source was operated in
27
the high pressure (~1000 microns) region.2 Air, at three

different pressures was used in these measurements to create

the background plasma. From the figure it can be seen that for

low magnetic field strengths, the collector potential increased

rapidly as the magnetic field was increased. For magnetic

fields on the cone tip above a range of 600 to 900 gauss, the

collector potential decreased as the magnetic field was

increased. It was also noted that the collector potential

increased as the background pressure was decreased.














Background Air Pressure (mm Hg)

0 5 x 10-6
-4
A 4x 10
X 5 x 104


x
- X


A



A



A



A
A


x


A


\x A



X/ A


0


0









OO
0/


O i i


S80 90 100 110
Collector Potential (volts)

Fig. 21. Collector potential in an air plasma is plotted
as a function of the magnetic field. Three different air
pressures are used in conjunction with a 15-keV hydrogen
ion beam.


2 1-


1 -













In Fig. 22 the collector potential is plotted as a func-

tion of the beam energy for a given magnetic field strength at

two different background air pressures. It can be seen that

the potential increased as the beam energy was increased. It

should be noted, however, that the beam current increased

slightly as the accelerating potential was increased.

In Fig. 23 the potential on the collector is plotted as a

function of the magnetic field for a 15.5-keV, 4-ma beam and

for given background pressures in argon. The source parameters
+ +
were adjusted so that the beam consisted mainly of H2 and H1.

From the figure it can be seen that in the low pressure region

the potential decreased slightly as the magnetic field was

increased to around 600 gauss. For magnetic fields above this

value, the collector potential increased. It can also be seen

that for a given B, the potential decreased as the pressure was

increased for pressures below .3 microns. For a given argon

pressure above .3 microns, the collector potential was essen-

tially unaffected by the magnetic field. In fact, in this

higher pressure range, the potential increased as the pressure

was increased for a given magnetic field strength.

The collector potential dependence on the magnetic field

strength using helium as the background gas is shown in Fig.

24. The source parameters were again adjusted so that the


















Background

0
L


Air Pressure (mm Hg)
-4
4 x 10-
5 x 10-5


) 100

Collector Potential


(volts)


Fig. 22. Collector potential is plotted as a function
of the hydrogen ion beam energy. Two different air
plasma pressures are used in a constant magnetic field
powered at 4 amps.


20 -


15 -


I _













Background Argon

O 5.0 x 10-6
1.5 x 10-5


'x A







1 //
-x 40


100


Pressure (mm Hg)

\ 3.0 x 10'4
0 3.0 x 10-2


SO/




0



0


110


120


Collector Potential (volts)

Fig. 23. Collector potential in an argon plasma is
plotted as a function of the magnetic field. Four
different argon pressures are used in conjunction
with a 15.5-keV, 4-ma hydrogen ion beam.









Background


Helium Pressure (mm Hg)
0 5.0 x 10'6
A 1.3 x 10-5
X 7.0 x 10-5
O 2.0 x 10-1


/
A 0
/



0
A 0


30
Collector


Potential (volts)


Fig. 24. Collector potential in a helium plasma is
plotted as a function of the magnetic field. Four
different helium pressures are used in conjunction
with a 15-keV, 3-ma hydrogen ion beam.


O


O
0












/7


A O


I O



S/O

0


x


x





x


x


x
X


X


X


X


X


X


S X
I X


A












+ +
15 keV, 3 ma beam consisted mainly of H2 and H1. As in the

case for argon, the potential decreased slightly as the mag-

netic field was increased up to 600 gauss. Above this field

value, the collector potential increased and for a given

magnetic field strength, the potential decreased as the pres-

sure was increased.

Using hydrogen as the background gas, Fig. 25 shows the

collector potential versus the magnetic field for a 16-keV,
+ +
4-ma beam consisting mostly of H- and H+ ions. It can be
2 1
seen that the slopes of the curves for the various pressures

are very similar to those found using argon and helium as

the background gases. The potential decreased slightly at

first and then increased for values of B above 600 gauss.

Again for a given magnetic field strength, the collector poten-

tial decreased as the pressure in the collector region was

increased.

The slopes of the potential versus magnetic field curves

were the same for the different gases with air being the excep-

tion as noted above. This exception, however, may be explained

by the fact that two system conditions were different when

air was used. First, the pressure changes with air were made

by simply capping off one or more pumps whereas the other gas

pressure changes were made by adjusting the gas flow with leak












Background Hydrogen Pressure (mm Hg)

0 3.0 x 10-6
A 4.0 x 10-5
X 1.0 x 10-4


6- X A/



5 X A 0



S4 A

-4

3- X I 0



2 X






X A 0
/
0 X A 0 iA
80 90 100 110 120

Collector Potential (volts)

Fig. 25. Collector potential is plotted as a function of
the magnetic field. Three different hydrogen pressures
are used in conjunction with a 16-keV, 4-ma hydrogen beam.













valves while leaving all the pumps open. Second, the beam used
+
in the air experiments consisted mainly of H3 ions, while the

beam used to interact in argon, helium and hydrogen consisted
+ +
mainly of H2 and H1.

All the potentials measured on the collector were relatively

low compared to the beam accelerating potential, but may have

given rise to a reflected beam component in any or all of the

background plasmas. However, in each case, no coherent or

decipherable oscillations could be found in the collector region

or on the collector itself. This effect was found to occur in

all of the gases used to create the background plasmas. The

fact that the potential changes were the same for all gases

indicates that the plasma ions were relatively unimportant in

determining the final target potentials.


High Current Source Results

A relatively high current argon ion beam was extracted

from the source shown in Fig. 10 when the cathode was grounded.

A typical ion current-pressure dependence curve is shown in Fig.

26 for an arc current of 2.5 amp and a 1-inch-diameter shield

aperture. From the figure it can be seen that no appreciable

current was collected until the pressure dropped to the 1-micron

range. The current then rapidly increased to 200 ma for a


























o 0
/ \










O O

0
0

O0


I I


I I I I II


1 x 10-4
1 x10


1 x 10-3


Pressure (mm Hg)

Fig. 26. Collector current I is plotted as a function
of the source pressure for a 5.5-amp arc with a 1-inch-
diameter shield aperture.


250 [-


200


150 -


0
/
0


I I I I I I I


2 x 10-5


. . . I I I . .













pressure of .2 microns. As the pressure was lowered even

further, the ion current decreased. Fig. 27 describes the

voltage-current characteristic curve for the collector at a

pressure of .1 microns. The ion current remained essentially

constant as the collector was biased from -500 volts to +30

volts. A further voltage increase caused a sharp current re-

versal indicating the onset of electron streaming to the

positively biased collector.

The maximum ion current obtained from the source with a

3/4-inch-diameter shield aperture was 190 ma. The current

dependence on the pressure followed the curve in Fig. 26

with a slight downward shift of the current for each pressure

point. When the arc current was reduced, the beam current

dropped. The voltage-current characteristic curve for the

collector when the arc current was 1.5 amp and the pressure

was .3 microns is shown in Fig. 28. Again the beam current

remained essentially the same when the collector was nega-

tively biased. The total collector current dropped to zero

quite abruptly when the bias potential was increased to 100

volts indicating that most of the beam particles had energies

in the 100-volt range. This was slightly less than the voltage

drop across the arc. It should be noted that although the

magnetic field has to be turned on for the beam to appear, the



















100


- C


0


50 1


-100 -50


-50 +


-100 4-


Fig. 27. Collector current I is plotted
of the collector voltage V for a 2.5-amp
inch-diameter shield aperture is used and
pressure is .1 microns.


as function
arc. A 1-
the source


50


k.Y I
50


















100+


50


-50 t


-100


O


100






0






0


Fig. 28. Collector current I is plotted as a function
of the collector voltage V for a 1.5-amp arc. A 3/4-
inch-diameter shield aperture is used and the source
pressure is .3 microns.














beam current appears to be unaffected by a limited change in

the magnetic field. This point, however, was not studied in

detail.

The anode arrangement shown in Fig. 13 was used to show

that the additional magnetic field could be used to enhance

the beam current. Fig. 29 shows how the beam current varies

as a function of the magnetic field for a 1.5-amp arc. It

can be seen that the beam current decreased for low anode

magnetic fields, increased above the initial beam current, and

finally decreased to an almost null current. This was observed

with the coils polarized to give a total magnetic field, which

increased in the radial direction. With the magnet coils

polarized to give a total field decreasing in the radial

direction, the beam current had the anode magnetic field de-

pendence shown in Fig. 30. Here it can be noted that as the

anode field was increased, the beam current increased until a

saturation point was reached. It was also noted that a small

beam current became evident as the anode field was increased

when both coils were off. The potentials measured on the

probes, although dependent on the polarity and strength of the

fields, could not be used to interpret the observed phenomena.

The noise picked up on the probes varied from 220 cycles/sec to

107 cycles/sec. Reproducable frequency measurements were hard
















0


0


-


- C
O\


Coils 1 and 2 on


Coil 1 on only




0
\o


15 20


25 30


Beam Current (ma)

Fig. 29. The beam current from a 1.5-amp arc is
plotted as a function of the radially increasing
anode magnetic field.


O


0


O


0




0


"-














0
I

I
Coil i on only/ 0


0/
/
/
/


o/


dI


I


O




OCoils 1 and 2
on



0


0



/
0
/O
O


I I I


10 20 30 40
Beam Current (ma)
Fig. 30. The beam current from a 1.5-amp arc is
plotted as a function of the radially decreasing
anode magnetic field.


S01












to obtain so no correlation could be made between the arc

parameters, field strengths and radio frequency noise.

The pulse circuit shown in Fig. 15 was used to apply a

variable pulse to the arc while the effect on the beam

current was measured by noting the potential drop across

a 2.4-ohm resistor between the collector and ground. A

typical oscillogram of the pulsed beam current is shown in

Fig. 31. For given source conditions, the pulsed beam current

increased somewhat proportionally to the applied arc pulse as

shown in Fig. 32. In Fig. 32a, a 200-volt pulse on the arc

producing the .24-volt drop measured across the 2.4-ohm

resistor indicated a pulse current of 100 ma above the steady

beam current. In Fig. 32b, a 400-volt arc pulse producing the

12-volt drop measured across the 47-ohm resistor indicated a

250-ma current pulse above the steady 60-ma beam. The pressure

in the system for this run was .7 microns and a further increase

in the amplitude of arc pulse resulted in breakdown. In

general, the maximum pulse voltage the arc could sustain for a

given arc current, was proportional to the pressure. Fig. 33

shows that at constant pressure, arcs of higher currents could

hold higher amplitude voltage pulses before breakdown occurred.

The anode arrangement shown in Fig. 16 was used to apply

a variable pulse to the copper ring concentric with the arc










73












0
CO




0






20 psec/division




Fig. 31. Pulse voltage measured across 47-ohm resistor
between the collector and ground. The 1.5-amp arc run-
ning at a pressure of .2 microns had a 60-psec 340-
volt square pulse applied to it.


























20 psec/division


Fig. 32a.
60-psec -
of time.


The pulsed beam voltage resulting from a
200-volt arc pulse is shown as a function


20 psec/division


Fig. 32b.
60-psec -
of time.


The pulsed beam voltage resulting from a
400-volt arc pulse is shown as a function






















1.8 -





1.6



i 0

1.4
oO








1.2 0




I -
1.0
200 300 400
Pulse Voltage (volts)

Fig. 33. The arc current is plotted as a function
of the maximum sustainable pulse voltage at a pressure
of .22 microns.













axis. The effects on the beam current were again determined

by measuring the potential drop across a resistor between the

collector and ground. Since the beam ions were not directly

extracted from the main discharge, pulsing the outer disk

seemed to have some possibility of pulsing the beam current

without the loading effects on the main discharge. In operation,

however, it was not possible to hold any appreciable potential

difference on the pulsed disk relative to the anode. Therefore,

the pulsed beam current using this geometry had the same

characteristics as those found when the anode was pulsed.

The circuit shown in Fig. 17 was used to provide a cons-

tant current pulse to the arc. The control of current with

this circuit provided a smoother potential to the arc with

over-voltages of several hundred volts for approximately .5

milliseconds duration. Fig. 34 is a typical oscillogram of

the pulsed beam current found by again measuring the potential

drop across a resistor between the collector and ground. From

the figure it can be seen that the pulsed beam current was 20

ma above the steady 30-ma beam current. The pulsed currents

obtained with this circuit were always considerably lower than

those found using the two previous pulse circuits. It was

also found that the amplitude of the beam current pulse did not

depend on the amplitude of the applied pulse. The pulsed

























0


-v4



0






20 psec/division

Fig. 34. The pulsed beam voltage resulting from a 6-amp
arc pulse is plotted as a function of time. The arc
current was 1.5 amps and the pulse voltage was measured
across a 100-ohm resistor between the collector and
ground.













current did, however, increase when the steady beam current was

increased.


Reflex Discharge Results

Fig. 35 represents a typical oscillogram of the frequen-

cies observed on the variable probe or on any of the ungrounded

electrodes when using the discharge arrangement shown in Fig.

18. For the source parameters given in Fig. 35, a frequency

of 4.2 x 10 cycles/sec was observed. This low frequency

range, which was investigated by changing the magnetic field,

pressure, cathode-button separation, and button radius, was

found only when the magnetic field was on while the button was

insulated from the anode. Fig. 36 shows the frequency depend-

ence on the magnetic field while using the separation distance

as a constant parameter. The graph shows, as in all cases,

that the observed frequency increases as the magnetic field

B is increased. The amplitudes of the increased frequencies,

however, appeared to be independent of B. The minimum B

necessary for the low frequency oscillations to become apparent

was not investigated because the magnet power supply was not

continuously variable.

The frequency dependence on the cathode-button separation

is shown in Fig. 37. By adjusting the cathode, the separation

was varied from 1/8 inch to 7 inches. For values above 1 inch,









79


















0





0








10 psec/division

Arc current = 1 amp

Electrode separation = 2 inches

Button radius = 3/4 inch

Pressure = .2 microns

Magnet power on 1


Fig. 35. The oscillogram represents the variable current
fluctuation in time to the probe and arc electrodes.












Cathode-button


Separation

0 1/8

A 3/8
X 1

o 2
9 3
A 4

* 6


Distance (inches)


A O D s p X









A OL O X









* A CO X


I I I


Frequency x


104


cycles/sec


Fig. 36. The frequency is plotted as a function of the
magnetic field for different cathode-button separation
distances. The arc was operated at 1.0 amp with a 3/4-
inch-diameter button at a pressure of .2 microns.


.... I I
r


L














- X



0 A
\ 0


O


Magnetic Field (power positions)

0 1

A 3

X 5


X


A 'X


\A
0 AX



0 A X






0 A X ,
6 S/. /


Frequency x


10 cycles/sec


Fig. 37. The frequency is plotted as a function of the
cathode-button separation distance for different magnetic
fields. The arc was operated at 1.0 amp with a 3/4-inch-
diameter button at a pressure of .2 microns.


C


I I


1 ^


I I | I


I












the frequency decreased as the separation was increased. For

separation values below 1 inch, the frequency increased as

the separation was increased. The frequency dependence on

the pressure is shown in Fig. 38. In general, the frequency

increased for given source conditions as the pressure was de-

creased. It should be noted that these low frequency oscil-

lations were never observed until the pressure dropped below

the 1.0 microns range. This was the same pressure range at

which the ion beam was extracted from the high current source

shown in Fig. 10. Although the frequency dependence on the

pressure was not too critical in the low pressure region, it

was found that the amplitude decreased as the pressure was

increased. Above the 1.0 micron-range,,megacycle noise,,as

shown in Fig. 39, independent of the magnetic field, was ob-

served on the probe.

By taking points from curves plotted using different

buttons, a button-radius versus frequency curve was obtained.

Fig. 40 shows that the frequency decreased as the button radius

was increased. This dependence was found to exist for all

magnetic field strengths, electrode separations, and pressures.

A series of measurements were made plotting frequency as

a function of the magnetic field, pressure, and electrode

separation for different arc currents. Fig. 41 shows that the









83








Cathode-button Separation Distance (inches)

0 2

11 X 0 A 3
X 4

0 5





9



S 8 OX
o





6 X & O
Mi 7



6 X 0



2 4 6 8 10

Frequency x 10 cycles/sec

Fig. 38. The frequency is plotted as a function of the
pressure for given cathode-button separation distances.
A 1 1/4-inch-diameter button was used while the arc was
operated at 1.0 amp with the magnet power on 1.



























En










1 1sec/division


Arc current = 1.0 amp

Magnetic field = 0

Electrode separation = 2 inches

Button radius = 1 inch

Pressure = 50 microns


Fig. 39. The oscillogram represents the constant current
fluctuation in time to the probe and arc electrodes.












Magnet Power
0 1

A A 3

0 5


Cathode-button Separation Distance
2 inches

4 inches


* A E OA[



0 A \ O n




\ \ \

\ \ \
t n \ -A"-* ~::::r: ~-


2 3 4 5 6 7 8

Frequency x 10 cycles/sec

Fig. 40. The frequency is plotted as a function of the
button radius. The arc is operated at 1.0 amp at a pressure
of .2 microns for given magnetic fields and cathode-button
separation distances.


1 1/2













Magnet Power

0 1

A A 3

0 5


0










I
I
I




I


Cathode-button Separation Distance

2 inches

4 inches


SA


A 0


Frequency x 104


cycles/sec


Fig. 41. The frequency is plotted as a function of the arc
current. The cathode-button separation distance and magne-
tic field is varied using a 1 1/4-inch-diameter button at a
pressure of .2 microns.


1.5


1.0













low frequency oscillations were relatively independent of the

arc current.

The probe position dependence of the collected signals

was carefully examined. It was found that the observed sig-

nals were independent of the longitudinal probe position, but

the radial positioning of the probe was found to be important.

In almost all cases observed, the cleanest and highest ampli-

tude signal was found when the probe was located at the edge

of the button. The floating potential on the Langmuir probe

at the button edge was always positive as indicated in Fig.

42. As the probe was rotated toward the center, the amplitude

of the signal decreased slightly. For the negative potential

range, generally found within 1/8 inch from the arc axis, the

coherent signals were washed out by background noise. This

discontinuity in the ability to measure coherent signals

indicates that the rotational modes of oscillations were no

longer coupled to the longitudinal modes in the center of the

arc. Therefore, most of the investigations were carried out

with the probe set radially at the button edge.

Although the diffusion current from the arc was not measured,

the arc voltage V was measured as a function of the magnetic
field, electrode separation, and button radius. By assuming the
field, electrode separation, and button radius. By assuming the

















40 -


/ 0


OW
1-1-I

0 > 20
SE




ton Edge \


-20'

Y


+*


-40


,E

O
/
/
/


/


'F
IB


Button Edge


ladial Probe


2
Position (inches)


Magnet Power
1

5 ----


Fig. 42. The floating potential of the variable probe
is plotted as a function of the radial position of the
probe. Two different magnetic fields are used for an
arc current of 1.0 amp at a pressure of .2 microns.


But


' ' ' ~ '


I














arc voltage to be proportional to the longitudinal electric

field a comparison of Fig. 1 with Fig. 43 shows that the pres-

sure and magnetic field values were in the range allowing

enhanced diffusion to take place. As the magnetic field was

increased, the arc potential increased in direct contrast to

what is expected for collisional diffusion, but agreeing with

enhanced diffusion experiments.

In Fig. 44, the arc voltage is plotted as a function of

the electrode separation. From this figure it can be seen that

the arc voltage decreased as the separation was increased infer-

ring a decrease of enhanced diffusion. This arc voltage de-

crease, however, is the normal arc behaviour for increased

discharge lengths so the above statement concerning diffusion

dependence should not be taken too seriously.

Fig. 45 shows a plot of the arc voltage versus the button

diameter. The voltage increased as the diameter was increased

up to 2 inches. For the 2 1/2-inch-diameter button, V decreased
a

indicating either a decrease in the diffusion current or a change

in the arc operating mode.

The Langmuir probe was used to measure the ion density n+

and the electron temperature T of the reflex discharge. Al-

though there is no theoretical basis for the Langmuir theory to
























240


230


220 H


Cathode-button Separation Distance (inches)
0 1/4

A 1/2

X 1

C 2

4 1/2

A 5 1/2



00



O x


O E
F1-I


AAL

AA


Magnet Power

Fig. 43. The arc voltage is plotted as a function of the
magnetic field for different cathode-button separation
distances. A 2-inch-diameter button was used while the
arc was operated at 1.0 amp at a pressure of .15 microns.


B


210


I I I > B















Button Diameter (inches)

0 3/4

A 2

X 2 1/2

220 -A---
A A




210



0

o0
W) 00-
CU 200 0

0
> 0

I-'I

190 0






180
0 2 4 6
Separation Distance (inches)

Fig. 44. The arc voltage is plotted as a function of the
cathode-button separation distance for different button
diameters. The magnet power was on 1 while the arc was
operated at 1.0 amp at a pressure of .2 microns.









92





Pressure (microns)
O
O .11

230 / .15

/ .20

X
220 -

OA


210 -




0 200 0



X/
0 190




180 -

A

170
0 1 2 3

Button Diameter (inches)

Fig. 45. The arc voltage is plotted as a function of the
button diameter for three different pressures. The arc
was operated at 1.0 amp with the magnet power on 1 and a
separation distance of 2 inches.




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