Table Of ContentCOPYRIGHTED
30SEPH ABRAHAM SALOOM, JR.
1951
ON THE CAUSES OF "SPONTANEOUS" MICROWAVE
OSCILLATIONS IN GASEOUS DISCHARGES
BY
JOSEPH ABRAHAM SALOOM
B.S., University of Illinois, 1948
M.S., University of Illinois, 1949
THESIS
SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEQREE OF DOCTOR OF PHILOSOPHY IN ELECTRICAL ENGINEERING
IN THE GRADUATE COLLEGE OF THE
UNIVERSITY OF ILLINOIS, 1951
URBANA, ILLINOIS
UNIVERSITY OF ILLINOIS
THE GRADUATE COLLEGE
May 18, 1951
I HEREBY RECOMMEND THAT THE THESIS PREPARED UNDER MV
SUPERVISION BY JOSEPH..
EMTTTTFT) "ON THE! CAUSES OF SPONTANEOUS MTCRQWAW. nfiflTT.T.A TTONK Tl
_GASEQUS_JDISCHA£GES!L
BE ACCEPTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR
THE DEGREE OF DOCTOR OF PHILOSOPHY IN ELEffi_!IRICAL-ENGINEKRING_.
Charge of Thesis
/ Headof ba{>aarr lmcnt
Recommendation concurred inf
Committee
- ^ ^ - f e f e ^^
on
Final Examination!
^ A/<r^g^fc
t Required for doctor's degree but not for master's.
M440
TABLE OF CONTENTS
ACKNOWLEDGMENTS . *,.
y
I. INTRODUCTION . . . . .d
II. DEGREE OF GENERALITY OF WEHNER OSCILLATOR ..,10
A. CHARACTERISTICS OF WEHNER OSCILLATOR . 10
B. USE OF UNPROTECTED OXIDE COATED CATHODE IN WEHNER OSCILLATOR . ll
#
C. USE OF PROTECTED OXIDE COATED CATHODE IN WEHNER OSCILLATOR »16
D. THE GLASS REPELLER 18
E. LOW VOLTAGE OSCILLATIONS ..•..„ 18
F. DISCUSSION... . ......••20
III* NEW EFFECTS AND PROPERTIES OBSERVED IN THE GAS DISCHARGE OSCILLATOR.....23
A. SPECTRUM OF SIGNAL OBTAINED FROM OSCILLATOR. 23
B. POSITIVE GRID OSCILLATIONS., 29
C. DARK SPACES . . 31
#
IV. ON SOME PROBLEMS CONSIDERED IMPORTANT..... . , . .. . .. .33
V» DESIGN CHARACTERISTICS CF NEW EXPERIMENTAL TUBE .. 35
#
VI. RADIO FREQUENCY PROBE MEASUREMENTS IN THE DISCHARGE , k6
A. E.F. POWER DISTRIBUTION IN THE DISCHARGE...... ...«1;6
B. THE BIASED PROBE . J47
0t
C. THE EXTERNALLY DRIVEN OSCILLATOR. £l
D. DISCUSSION OF RESULTS #...... £3
VH. PHASE MEASUREMENTS ..56
A. MEASUREMENT OF THE PHASE DIFFERENCE OF TWO MICROWAVE SIGNALS .56
B. PHASE MEASUREMENTS IN THE DISCHARGE ...58
C. PHASE MEASUREMENTS IN THE DARK SPACES • .5?
D. DISCUSSION 6*2
iv
TABLE OF CONTENTS, continued
VIII. CONCLUSIONS....* 65
BIBLIOGRAPHY 66
VITA .6?
.
ACKNOWLEDGMENT
The author wishes to express his appreciation and gratitude to Professor
R. P. Wellinger for the invaluable advice and assistance so generously af
forded. Special thanks also go to Professor H. von Foerster for his interest
and suggestions, to Mr. James Etter and Mr. Edward Ernst for their aid in ex
perimental work, and to Mr. B. Marshall, Mr. J. Lowe, Mr. C. Boley, Mr. R.
Parrill and Mr. A. Wilson for their superb cooperation in machining and assem
bling of the intricate structures used in this work. The work described in
this thesis was sponsored by the United States Army Air Force under Contract
W33-038-ac-lli7U2.
1
CHAPTER I
INTRODUCTION
The purpose of this thesis is to continue the investigations on the causes
of microwave oscillations which originate in gaseous discharges at low pressures.
These oscillations occur in the decimeter and sometimes in the centimeter range
and have the advantages that they may be tuned electrically over a wide range and
that they will generate appreciable amounts of power. Since no tuned circuit
external to the gas discharge is required to determine the frequency of these
oscillations, they are often designated as spontaneous oscillations.
In 1925, Irving Langmuir showed, by an analysis of the current received by
a collector placed opposite a hot cathode within a low pressure mercury vapor
discharge, that there are three groups of electrons present? (1) primary
electrons, which retain practically all the momentum they acquired in passing
through the positive ion sheath around the cathode, (2) secondary electrons, mov
ing in random directions with a Maxwellian velocity distribution corresponding to
a temperature roughly proportional to the primaries, (3) ultimate electrons, hav
ing a Maxwellian distribution of velocities corresponding to a much lower
temperature than that of the secondaries. I. Langmuir discovered during these
measurements that, under a wide range of conditions, the primary electrons experi
enced abnormal increases in energy, i.e., there was an appreciable fraction of
electrons acquiring velocities greater than those corresponding to the voltage
drop across the tube, and a larger number of electrons which had less than the
expected energy, so that the total energy stayed constant. A natural supposition
was that electrical oscillations in the discharge caused the scattering both by
subjecting the electrons to rapidly changing fields and by causing potential
changes on the electrodes, but I. Langmuir did not detect any oscillations in the
2
experiments he made on the scattering of electrons.
17
In 1926, F. M. Penning ' studied the scattering of electrons in mercury
vapour discharges at low pressures, as suggested by Langmuir, and concluded that
it was possible that the scattering was accompanied by and caused by oscillations.
Penning worked with a three element tube, slightly different from the ones used
by Langmuir, and also with a two element tube with the anode surrounding the
cathode, and found oscillations with wavelengths ranging from 50 to 100 cm. It
was during these experiments that Penning noticed the influence of the tube
envelope on the character of the oscillations in the diode.
In 1929, L. Tonks and I. Langmuir " developed a theory of electronic and
ionic oscillations in an ionized gas. While the theory offered no explanation for
the cause of the observed oscillations, the frequency range predicted for the most
rapid oscillations, namely, ultimate electron oscillations, agreed with experiment,
According to this theory two types of oscillations are possible: electronic
oscillations occurring in the 1000 megacycle region and ionic oscillations occur
ring in the kilocycle range, both types of oscillations being independent of the
external circuit. According to this theory, when the electrons oscillate, the J
positive ions behave like a jelly with a uniform density of positive charge. Im
bedded in this jelly and relatively free to move, is an initially uniform electron
density of the same absolute value. They are able to show that if the electrons
are displaced, they will execute harmonic motion about their equilibrium position.
The formula they derive gives the frequency of oscillations as a function of the
electron density. The whole plasma* is assumed to oscillate as a unit, but no
indication is given of how the energy is transferred from the primary electrons
to the ultimate electrons. Merrill and
* Plasma is the name commonly used to designate that portion of a discharge in
which the concentrations of positive ions and electrons are approximately equal
and are relatively high.
3
Webb proposed the theory that primary electrons are scattered after their passage
through a thin layer of oscillating plasma.
Work in this field of research appears to have experienced a lull until 1939
when Merrill and Webb ^ performed an elegant investigation of the scattering of
electrons in mercury vapour discharges at low pressures. Working with a simple
diode and using a very accurately positioned probe, they found regions of intense
scattering and oscillations in planes parallel to the anode and cathode. The
frequency of these oscillations checked with the formula published by Tonks and
Langmuir within ten per cent. Dimensional analysis shows that this formula
7
uC =^ 4-TTOe "
Where
U3 is the angular frequency in radians per sec.
Q
t"*"^ is the mass of the electron
€. is the charge on the electron
*l is the density of the electrons per cur.
is the only manner in which these quantities can be arranged in an expression of
the dimensions of U-J . Throughout the field of gaseous discharges this ex
0
pression is used repeatedly. It is shown in this thesis that Langmuir»s formula
does not predict the exact frequency for the Wehner oscillator but certainly
determines the range in which they can be expected. The electron density appear
ing in the Langmuir formula is for a uniform plasma which is very difficult to
obtain in practice. For plasmas in which decimeter waves can be expected, the
electron density is of the order of 1010 electrons per cm?. Merrill and Webb
were unable to find any mechanism by which the energy is transferred from the
h
primary beam to the ultimate electrons, but they concluded that the primary beam
is an essential part of the oscillation. Although they could not find any intense
oscillations near the cathode, they concluded that a region of intense oscil
lations must exist very close to the cathode surface, causing the observed
scattering.
In 19U2, G. Wehner, investigating a tube with three electrodes, discovered a
quantitative relation between the wavelength of the oscillation and the square
root of the anode voltage. He could show the proportionality between these
quantities, thus establishing a direct relation between the transit time of the
fast electrons from one point of the tube to another and the period of the oscil
lations. He advanced a theory based on the model of the klystron. He observed
that the layers of intense scattering reported by Merrill and Webb appear with
the oscillations and concluded that they are similar to the gap of a klystron
oscillator, the intermediate plasma corresponding to the drift space. In the
Wehner structure, the discharge is maintained between two parallel electrodes kept
approximately at the same potential as is shown in Fig. 1. The cathode consists
of a graphite grid, drawing the electrons from an ignitron, used as a convenient
source of electrons. The anode surrounds the plasma and a typical value of the
anode voltage is 200 volts. According to the theory proposed by G. Wehner, the
primary beam of electrons is accelerated in the dark space and velocity modulated
in an oscillating layer located at the edge of the dark space. The beam finally
delivers its energy into a second 'oscillating layer, if there is one present, and,
if not, returns to the first oscillating layer to give up its energy, in the same
manner as in a reflex klystron. The nature of the feedback between the layers is
not explained, nor is the nature of the layer, nor the mechanism of coupling the
power out*of the oscillator.
