Physical Electronics. Handbook of Vacuum Physics

184 Pages·1965·4.604 MB·English

by  A. H. Beck (Eds.)

HANDBOOK OF
VACUUM PHYSICS
VOLUME 2
PHYSICAL ELECTRONICS
EDITED BY
A. H. BECK
Engineering Laboratory, Cambridge
Part 1
C. GREY MORGAN—Fundamentals of Electric Discharges in Gases
PERGAMON PRESS
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Copyright © 1965
Pergamon Press Ltd
First edition 1965
Library of Congress Catalog Card No. 63-21443
Set in Monotype Times 10 on 12 pt.
and printed in Great Britain by
Bell and Bain Ltd., Glasgow
ACKNOWLEDGEMENTS
I am much indebted to my many colleagues in the Department of
Physics, University College of Swansea, who, under the stimulating
guidance of Professor F. Llewellyn-Jones, are enthusiastically en­
gaged in research on various aspects of electric discharges in gases.
My colleagues have given me helpful advice and have constructively
criticised the manuscript. In particular I wish to thank Professor
P. M. Davidson for invaluable discussions concerning sections 4.17
et seq on non-steady state ionization growth, and also Dr. A. J.
Davies and Mr. C. J. Evans for advice concerning section 4.14.
My thanks are also due to numerous authors for permission to
reproduce illustrations from their publications; to the American
Institute of Physics for permission to use Figs. 4-15 and 24, 25, 30,
37, 41, 42, 45; to the Institute of Physics and The Physical Society
for Figs. 18-22, 26-29, 33, 34, 35, 38, 39, 40, 44, 49 and 50; to the
John Wiley Publishing Co. Ltd. for Fig. 17; to the North Holland
Publishing Co. for Figs. 46-48 and to the Editors of Nature for
Figs. 23, 31, 32 and 36.
C. GREY MORGAN
vu
1. INTRODUCTION
1.1 Dark, glow and arc discharges
The term Electrical Discharges in Gases is used generically to
denote the passage of electricity through a gas and implicitly em­
braces the wide variety of physical phenomena which accompany
such a discharge of electricity. The discharge currents may be as
small as 10~16A in certain ionization growth studies, or be as large
as megamperes in thermonuclear and plasma physics studies. The
electric fields encountered in discharge phenomena also cover a
remarkably wide scale.
The range of technological application of electrical discharges is
enormous and ever widening: for many years several discharge
phenomena have been used to reduce and measure residual gas
pressures in vacuum equipment. An interesting example is the
pumping action of an arc discharge used in the experimental thermo­
nuclear device known as DCX-1, in which an effective pumping
speed of 104 1. sec-1 at approximately 10~6 mm Hg is obtained with
a 300A arc. Recently, a low power electrical discharge in a helium-
neon mixture has been used as an optical laser in order to produce
an infrared beam with a very small spread and a spectral line width
some 105 times narrower than that obtainable by any other means.
The possibilities offered by gas discharges for the propulsion of
space vehicles (plasma propulsion), and the direct conversion of
thermal to electrical energy are now the subject of serious and
intensive research.
More familiar applications of electrical discharges include
fluorescent lighting, mercury-arc rectifiers, hydrogen thyratrons,
trigatron switches and similar gaseous electronic devices, all of which
depend for their action upon the changes that may be brought about
in the electrical conductivity of a gas.
The electrical conductivity of a gas in its normal state is extremely
low and controlled largely by the rate of electron and positive ion
production in the gas by the incidence of cosmic rays and the presence
of local radioactivity. However, when a sufficiently large electric
field is established in the gas, the conductivity can increase by many
orders of magnitude in an extremely short time : the gas, initially an
insulator, becomes an almost perfect conductor. This transition is
3
4 HANDBOOK OF VACUUM PHYSICS
known as the breakdown of the electrical insulating properties of the
gas, or, briefly, electrical breakdown.
During the transition several distinct phases or types of electrical
discharges may be observed, each having its own particular set of
basic electrical and optical characteristics. The visible characteristics
of these phases have given rise to a number of names, for example,
dark discharge, glow discharge, and arc discharge, which typify their
physical appearance. These phases form the three fundamental
types of continuous, self-regenerating electrical discharge. Once
established they require only a suitable source of electromotive force
for their maintenance. They differ from each other in respect of their
current-voltage characteristics and intensity and distribution of
emitted radiation. A broad classification of the three types may be
made on this basis.
Thus in dark discharges the ionization currents and current
densities are generally very small, < 10~7A and the inter-electrode
voltages may be as high as 105 to 106V at high gas pressures( > several
atmospheres) for electrode separations of some centimetres. There
is negligible visible radiation.
In glow discharges, on the other hand, the currents may lie
between 10~6 and 10-1A and the maintaining voltage may be either
considerably less than that existing in the dark discharge, or greater
if the product of the gas pressure and electrode separation is suffi­
ciently small. As the name glow implies the discharge is clearly
visible.
The arc discharge is characterized by large currents, generally
greater than 10_1A with current densities > 102A cm-2 and low
inter-electrode voltages of the order of some tens of volts. It is
accompanied by intense visible radiation and high gas temperatures
(103 to 105 °K).
An experimentally measured current-voltage characteristic of dark
and glow discharges is illustrated in Fig. 1 for low pressure helium
between plane parallel silver electrodes in series with a resistance and
a source of steady (static) voltage. (Davies, Llewellyn-Jones and
Morgan, 1962.)
The curve AB represents the occurrence of transient dark discharges
intitiated by electrons liberated in the gas by the fortuitous passage of
cosmic rays and the presence of radioactive material or by very weak
illumination of the cathode by ultraviolet light. Illumination of the
cathode by stronger ultraviolet light yields enhanced photo-electric
300 / \
280 m
2601 o
240 H
Ξ
220
o
200
180
160
Gas:Pure helium, p=38-71orr -.y o
140 E
Electrodes : Silver, d = 0-8cm
120 >
-o-o-o : Measured points
100
: Schematic o
80
60 vO__ w
40
20
ιοΗΖ ισ" ιοΗΟ io-9 io-8 io7 io-6 io-5 i i ininoi—-4i i i ιnσini 3 τ ι ι nini ι ι ι nini ' ' ' "■"'IO » ' 4, ""■I'O >
1(A) w
FIG. 1. Experimentally measured current-voltage characteristic of dark and glow discharges.
L*
6 HANDBOOK OF VACUUM PHYSICS
emission and results in a steady, larger, current represented by the
curve A'B'.
The magnitude of these transient and steady currents increases with
inter-electrode voltage V. For values of V less than a critical value,
denoted by V in Fig. 1, these currents are not self-sustaining, i.e.
s
they require for their maintenance a specific source of initiatory
electrons, e.g. irradiation by ultraviolet light or a radioactive source.
In the absence of such sources only random quenched transient
currents are observable.
However, when V is made equal to V a discharge current once
s
initiated, albeit fortuitously, can continue to flow in the absence of
any further supply of initiatory electrons. This then constitutes a
self-maintained dark discharge. The voltage V at which self-
s
maintenance occurs is known as the static sparking potential or
static breakdown voltage.
By suitably controlling the circuit it is found that the current can
be allowed to increase to values of about 10~6A without any signi­
ficant changes in the value of the inter-electrode voltage, i.e. it remains
constant at V = V. For larger values of current the voltage drops
s
considerably. Thus the critical voltage at which self-maintenance
first occurs is found to be independent of the value of the current,
provided that the current is not too large. The breakdown voltage
or sparking potential is thus defined as the minimum voltage which
is able to maintain a very small current in the gas.
This non-dependence of V upon current is illustrated by the region
s
B'C and it represents the onset of breakdown. The physical signi­
ficance of (and the fundamental mechanisms responsible for) these
measurable characteristics were first examined quantitatively by
Townsend (1903) and the self-maintained dark discharge is often
called a " Townsend discharge ".
If the current is allowed to increase further to values > 10~4A
the discharge becomes faintly visible and the maintaining voltage
continues to fall until a further region DE of constant voltage
(denoted by F ) is reached. This is the normal glow maintenance
glow
potential and is independent of the magnitude of the discharge current
up to values ~ IO"1 A. In this region the current density remains
practically constant; the discharge spreads over the cathode surface
as the current increases until finally the whole surface is covered.
Further increase of current can be obtained only by increasing the
inter-electrode voltage and this regime is known as the abnormal
ELECTRIC DISCHARGES IN GASES 7
glow. A maximum voltage is reached when the current may be
~ 1A. Larger currents are accompanied by a fall of voltage to a
very low value and a considerable increase in the intensity of visible
radiation. This is the arc regime and the current may then be limited
only by the ability of the source of electromotive force to supply it.
In any combination of gas and electrode configuration the value of
the static sparking potential V is found to be a function of the
s
product of the gas pressure p and electrode separation d. This
0-8cm
Gas: Pure helium
Electrodes: Silver, d*0-8cm
_L
Pd (mmHgxcm)
FIG. 2. Paschen curve.
dependence of V upon pd was first established by de la Rue and
s
Müller, and later confirmed by Paschen (1889). Paschen concluded
from studies made with H , C0 and air over a wide range of
2 2
values of pd, that V was a function of the product/?*/ only. This is
s
known as Paschen's Law and the characteristic curve V = φ(ρά)
s
for a given electrode arrangement and gas is called a Paschen Curve.
For very small and very large values of pd, deviations occur from
Paschen's Law. These are due to the occurrence of field dependent
ionization processes. At low values of pd the presence of a relatively
weak magnetic field can produce considerable deviations. Figure 2
shows the form of a Paschen curve.
8 HANDBOOK OF VACUUM PHYSICS
For any particular system V is found to depend upon the nature
s
and state of the electrode surfaces and upon the nature and purity of
the gas.
Figures 1 and 2 refer to the steady state, i.e. time as a parameter
does not enter into the considerations. However, there are many
cases when impulsive voltages in excess of V are applied between
s
electrodes. The current is then observed to increase in time and it is
common knowledge that breakdown (sometimes called sparking or
a spark discharge) accompanied by the collapse of the applied
voltage may occur in a very short time. This is illustrated in Fig. 3a
which shows an oscillogram of a step function voltage in excess of
-*J 100 m sec h*—
—*J 3/i.sec U—
, Y
*w Vglow
Overshoot
*■%-» »glow 1 (circuit effect)
Key Key
KEY TO FIG. 3a. KEY TO FIG. 3b.
V and its subsequent collapse to the glow maintenance potential
si
K as breakdown develops. The horizontal portion between the
glow
instant of application and the collapse to K is known as the total
glow
time lag of spark breakdown. It comprises an initiatory or statistical
time lag, which is spent awaiting the arrival of an initiatory electron,
and a formative time lag, which is the subsequent time taken for the
current to grow sufficiently to cause the collapse of the applied
voltage. Figure 3a refers to breakdown in hydrogen at a low pressure
between copper electrodes. The applied voltage impulse exceeded
V by only 2 per cent and the time lag of 3/isec consisted principally
s
of the formative lag, since the statistical component was eliminated
by X-irradiation of the cathode. (Morgan, 1955, 1956.)
Figure 3b, in contrast, shows a long formative time lag (100 msec)
obtained in the study of breakdown in helium between silver elec­
trodes, referred to above.
FIG. 3a, Formative time lags in FIG. 3b. Formative time lags in FIG. 3C. Initiatory time lags in
low pressure hydrogen (constant). low pressure helium (constant). hydrogen (statistically distributed).