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Патент USA US3087066

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April 23, 1963
Filed July 14, 1961
2 Shéets-Sheet 1
April 23, 1963
Filed July 14, 1961
2 Sheets-Sheet 2
FZm Do m DOm
Unitd States Patent O??ce
Joseph J. Murray, Oakiand, Cali?, assignor to the United
Patented Apr. 23, 1963
an incipient property common to all, such as electron emis
sion from the cathode, is controlled, sparking is effectively
To meet the above requirement the cathode electrode
material may have a speci?c resistivity in the range from
1 to 1012 ohm-centimeters.
To utilize the resistive effect described above, certain
additional physical conditions must be ful?lled which
The present invention relates to high voltage electrode
limit the selection of suitable cathode materials. One
structure and more particularly to means for suppressing 10 consideration is the time constant for approach to equilib
sparking between spaced apart electrodes whereby high
rium charge distribution on the cathode surface. For an
intensity electric ?elds may be maintained therebetween.
isolated smooth surface this time constant is essentially the
The present invention was originally developed to pro
product of volume resistivity times capacitivity. To ob
duce a high intensity electric ?eld in a velocity spectrom
tain su?iciently conductor-like behavior for the cathode
eter. This form of spectrometer is used in conjunction 15 material for static conditions, time-constants of the order
with momentum analyzing systems to produce angular
of one millisecond (10"3 sec.) are acceptable.
and spatial displacement of particles of different mass
A second consideration in selecting the cathode mate
which occur in secondary charged particle beams pro
rial is the effective quenching of cold electron emission
duced by accelerators such as proton synchrotrons of the
from the surface of the material by inducing a backward
class providing energies in the order of several billion
electric ?eld through the material which must ‘be com
electron-volts. In the velocity spectrometer, a magnetic
parable to the ?eld strength causing the cold emission.
?eld is crossed with the electric ?eld to effect the sepa
Such backward electric ?eld may be of the order offer
rating function, the magnetic ?eld impelling particles in
greater than 10 million volts per centimeter.
one direction and the electric ?eld impelling the particles
A third consideration is the stability of the cathode
in the opposite. For particles with a certain velocity the 2 Ul material including thermal stability wherein the cathode
two forces just balance and only such particles appear
does not vaporize, dielectric stability wherein there is no
at the terminal end of the spectrometer. The intensity of
breakdown with the internal electric ?elds encountered,
the electric ?eld which can be maintained in the spectrom
and mechanical stability wherein the material Will not be
eter is a factor which limits the maximum particle energy
ruptured by the physical stresses imposed by the high po
at which the separating function will be effective. The 3 tentials present. A still further desirable consideration
present invention allows the electric ?eld intensity to be
is that the above properties should be realized in a dynami
increased by two to three times with a corresponding
cally stable steady state. As Will hereniafter be discussed
(though not linearly related) increase in beam particle
in greater detail, the most commonly available materials
energy or, in appropriate cases, a reduction in the size
which meet the foregoing requirements are various types.
35 of glass, for example soda-lime glass. Most glasses, how
of the ‘apparatus for a given particle energy.
While the invention has been found highly useful in
ever, must be heated to a temperature as high as a few
the above described spectrometer application it is by no
hundred degrees centigrade in order to lower the resistivity
means limited to such usage and is equally applicable in
into the desired range.
diverse other forms of equipment in which high electrical
Accordingly the invention, in its preferred form com
gradients are required. In general the invention allows 40 prises a pair of spaced apart electrodes having facing
much stronger electric ?elds to be maintained between
parallel ?at inner surfaces and a high voltage source con
electrodes of given spacing than has heretofore been
nected to establish a high potential difference between the
electrodes. Disposed against the inner surface of the more
The electric ?elds to be discussed herein are of the
negative electrode is a layer of glass, or other material
type produced between two spaced apart electrodes sepa
having the characteritics herein discussed, which layer
States of America as represented by the United States
Atomic Energy Commission
Filed .luiy 14, 1961, Ser. No. 124,242
7 Claims. (Cl. 250—41.9)
rated by a vacuum or near vacuum with pressures usually
less than 10-3 millimeters of mercury. A direct current
voltage is connected between the electrodes to provide
effectively constitutes the negative electrode.
It is therefore an object of the present invention to
provide a means for creating more intense steady-state
an electric ?eld within the vacuum therebetween. With
electrical ?elds than have heretofore been generally
this arrangement, it has generally been possible to hold 50 obtainable.
more intense ?elds between closely spaced electrodes than
It is another object of the present invention to provide
across more widely spaced electrodes owing to the greater
an electrode structure for establishing a very high gradi
likelihood of a sustained are occurring with greater spac~
ent electric ?eld, said structure acting to suppress incipient
55 arcing across the ?eld.
Considering now the structural characteristics of the
It is yet another object of the present invention to
present invention, the limiting class of vacuum sparks
provide an electrode structure which inhibits the emission
in the above described electrode arrangement (but not
of electrons from the electrode in vacuum thereby allow
necessarily all sparks) appears to involve in the incipient
ing an increase in the intensity of the electric field asso
stage the emission of electrons from very small areas of
ciated with the electrode.
the surface of the cathode electrode at the eventual spark
It is still another object of the invention to provide
sites. On this basis it has now been found that if the
more intense electrical ?elds to facilitate the control and
volume (speci?c) resistivity of the cathode is high enough,
analysis of charged particle beams of energies higher than
and certain other conditions are met, the degenerative
that heretofore conviently handled.
It is yet another object of the invention to provide a
high voltage electrode material having a combination of
electrical characteristics which suppress incipient sparks
whereby an extremely high voltage may be held on said
The invention will be better understood by reference to
effect of the local voltage drop at the electron emitting
sites will prevent instabilities which would otherwise cause
sparks. Concurrently, the potential at other points on
the surface of the cathode is essentially unchanged by
the process so that the major portion of the electric ?eld
is undisturbed by the incipient spark and extinguishing
thereof. Even though the ultimate nature of sparks may
differ widely depending on a diversity of circumstances, if
the accompanying drawings of which:
FIGURE 1 is a perspective view of a pair of high
sense an upper limit for 'r of the order of a millisecond
voltage electrodes with energizing means for providing
an extremely intense electrical ?eld, a portion of the elec
is preferred. Thus a ?rst condition is
trodes havingbeen broken away to show a cross section
thereof and to permit an indication of the resistive current
paths associated with are suppression,
If there is to be any effective quenching of cold emis
sion, the backward :?eld, E, induced on the vacuum side
FIGURE 2 is a graph of the volume resistivity as a
function of temperature for’ materials usable for the nega
of the cathode surface at the emitting site must be com
parable to the ?eld strength required to cause appreciable
tive-electrode in the. structure of FIGURE 1,
cold emission, or in order to magnitude 10" volts/cm.
FIGURE 3 is an equivalent circuit diagram of the
A second preferred condition is, then,
apparatus of FIGURE 1, and
FIGURE 4 is a simpli?ed perspective view showing
components ofv a velocity spectrometer illustrating a
typical usage of the invention.
where i is the emission current density and so is the
.Referring now to FIGURE 1, there is shown a ?at
capacitivity of vacuum
generally rectangular positive electrode 11 formed of a 15
8.85 ><10-14 sec.
highly conductive material such as stainless steel. A ?at
negative electrode 12 comprised of glass, or a comparable
material,..having a fairly ‘high speci?c resistivity which
Finally, it is required as a third condition
may be of the order. of from lto 10GB ohm-crn., is dis
posed in, a spaced apart parallel relationship with the 20 (3) that stability of the following types be provided:
(a) thermal—no appreciable cathode vaporization,
’ positive electrode, 11. A backing plate 13 of stainless
(b) dielectric-no breakdown with internal electric
steel or other highly conductive material, contacts the
?elds of the order
backsurface of the negative glass electrode12on the side
opposite thepositive electrode 11, the backing plate 13
beingparallel to electrode 12 and disposed thereagainst.
The edges of the electrode members 11, 12 and 13 are
(c)- mechanical—no rupture for stresses of the order
preferably, roundedto suppress sparking and all exposed
surfaces are polished to reduce irregularities from which
eleptrpn emission would tend to occur. To provide for
maximum voltage holdingrability, the space between elec
trodesgll andrilz is evacuated, and‘ accordingly a suitable
(d) dynamic stability,
vacuum envelope, indicated schematically at 14, surrounds
the electrodes 11 and 12. A high voltage power supply
1_6.5h_as a positive terminal connected to the positive elec
trode 11 and has a negative terminal connected to the
where “dynamic stability” implies stability in temporal
sense, as determined by the dynamical relationship among
the various parameters of the system in the absence of
more or less violent and discontinuous,instabilities of
the former types. The system involved here is so complex
thatonecan do little more analytically than recognize the
need for dynamic stability, and condition (3d) therefore
cannot .very well in?uence the choice, of ,material.
Furthermore, although its ful?llment is important, condi
backing plate 13 of._the negative electrode 12.
I When a high voltage from source 16 is applied, there
isavery intenseelectric ?eld between the negative elec
trode 12'and the positive electrode 11 as indicated by
tion (3c).woulyd not be expected to impose severe restric
electric ?eld lines 15 in FIGURE 1. Since the backing 40 tions on,.the choice of material, since the stress associated
with .a ?eld of 107 ,volts/cm. is only about 6-00 psi.
plate 13 contacts nearly all of one surface of the nega
Condition (3b), however, suggests the choice of a mate
tive electrode 12, there is provided a short uniform path
for the positive charge carriers from the surface of the
negative electrode 12 facing the‘ positive electrode 11,
although such uniformity is not essential to the function
rialwithghigh dielectric strength and high capacitivity.
ing of the invention in all instances. If an incipient spark
site should for any reasonexist at a point 17 on the nega~
tive' electrode 12, the initial effect will be electron emis
sion as indicated schematically at 18. The necessary
supply of electrons 18 for initiatingand sustaining the
spark must ilow through the fairly high resistance of the
In, addition, condition/(3a) together with condition (2)
can be used to approximate ,a lower limit for p asfollows:
Given a critical temperature, Tc (certainly less than
the melting temperature of vthe cathode material), and
assuminglthat ohmiclosses are dissipated solely by thermal
negative electrode 12 along paths indicated schematically
conductivity, it can be demonstrated with the aid of
simplifying-models or dimensional analysis that the rela
tionship between E and the other parameters of the system
is dominated by the term:
in FIGURE 1 by resistances 19. The voltage drop caused
by this electron current through the high resistance of the
negative electrode 12 to point 17 causes the voltage at 55 where ATj.,—_T,‘_I_T,,m5,,n,is the temperature rise at the
emitting site on the cathode surface, r is the “radius” of
point 17 to approach that of positive electrode 11, thus
the site, and k is thermal conductivity. C is a numerical
reducing the voltage at the spark site 17 and quenching
factor of order unitywhich depends on the geometrical
the electron emission.
Since the threshold for signi?cant sparking between
electrodes 11 and 12 is raised by the foregoing effect, a
much higher potential di?erence may be maintained be
details of the site. Setting C equal to unity and using
condition ‘(2), E1107 _v./_cm., one gets an order-of
magnitude estimate of a lower limit p,
tween the electrodes relative to conventional metal elec
trodes. Increases of the potential ditference by factors of
from two to three have been realized and electrical ?elds
having intensities up to 2.5 million volts per centimeter
have been maintained.
With respect to the selection of a suitable material for
the cathode electrode 12, one consideration is the time
constant for approach to equilibrium charge distribution 70
on the cathode surface. For an isolated smooth surface
this time constant, 7- is given by the product of volume
resistivity, p, and capacitivity, e,: when inductive effects
and more complex relaxation phenomena are ignored.
in ohm-cm” for r in cm. ‘and kAT in watt cmrl.
. There is experimental evidence indicating that the ef
fective 11 for tungsten is of the order 10"5 to 10-5 cm., and
forpresent purposes this may be assumed to be generally
true for other materials under consideration. For metals
semiconductors with I z melting, kAT varies be
tween 1012 and 103 watts/cm, so that using 10 as a rep
resent‘a-tive value for 6/60 gives the condition for metals
and semiconductors: pill)-3 to 10 ohm-cm. This is an
unattainable range of resistivity for metals and ‘is possible
In order to realize conductor-like behavior in a static 75 but marginal for semiconductors; that is, these materials
would probably melt before an appreciable back ?eld, E,
For glasses, on the other hand, with TczTmealmg,
the value of kAT is about 2 watts/cm., so that with 6/60
again set equal to 10; pZl to 102 ohm-cm. This is a
lower limit of resistivity always exceeded by a wide
margin in glasses but at the same time much less than
the upper limit, PZlOTB/eIIQ‘IO ohm-cm. set by condi
tion (1), thus leaving a wide acceptable range of values
velop, spreading out along a line directed toward the
widest part of the gap.
The luminous patches are una?ected by magnetic ?elds
of several hundred gauss transverse to the electric ?eld,
which would be expected to cause noticeable distortion or
motion of the patches if electrons where involved as sig
ni?cant agents in effecting the exchange (electrons are
evidently involved at least in a passive role, as evidenced
by high X-ray yields).
of p which can be attained in heated glass. In FIGURE 2 10
After prolonged operation (several hours) with a stable
nominal values of volume resistivity as a function of
ion exchange in progress, inspection has revealed appreci
temperature for soda-lime glass, Pyrex, and fused quartz
are indicated. Condition (3b) is also satis?ed by these
able erosion of the cathode elect-rode 12 in the vicinity of
the luminous patches, sometimes without any other dam
glasses even though their capacitivities are not especially
age at all. Occasionally currents in excess of 1 ma. can
high, ranging from 3.5 for fused quartz to about 10 for 15 flow before an ion-exchange discharge becomes unstable.
soda~lime glass, since the intrinsic dielectric strength for
At currents greater than about 200 ,ua., however, cathode
each is so very high about 5-106 v./ cm. These considera
damage in addition to erosion usually occurs, apparently
tions make heated glass a preferred material for the
as a result of local heating which causes the surface of the
negative electrode 12.
glass to chip.‘
Tests have shown that the electric ?eld starts at the 20
Such an ion-exchange process occurring at an essen
surface of the glass electrode 12 and not at the backing
tially ?xed threshold voltage should, if allowed to persist,
plate 13, indicating that the glass is functioning in a
limit the advantages of the invention where large elec
manner resembling that of a plate of a capacitor and
trode spacings are required. It has been found however
does not act as a dielectric between the backing plate and
that the effect may be suppressed effectively by the intro
the positive electrode 11. In fact, the backing plate 13 25 duction of any one of several different gases into the sys
may be replaced by a point contact and in the absence or"
tem at pressures from a few tenths of a micron to a few
appreciable current ?ow the electric ?eld is essentially
it of Hg. Air, H2, and A at similar pressures all produce
essentially the same effect~the discharge disappears and
Referring now to FIGURE 3, the operation of the
the voltage applied to the system may be raised immedi
invention will be further clari?ed by reference to an
ately and permanently to a higher value at which some
equivalent circuit analog of the electrode structure which
factor other than gap discharge become a limitation.
is shown for the condition where no sparking is occurring.
At close electrode spacings, -i.e. one millimeter or less,
A capacitor 51 is equivalent to the vacuum gap between
the introduction of an inert gas at pressures up to several
the positive and negative electrodes 11 and 12 of FIG
URE 1 while the negative electrode 12 may be shown by 35 p. of Hg has no effect except to cause a substantial in
crease in the normal quiescent gap current. Essentially
a capacitor 52 and resistor 53 in parallel, the parallel
the same maximum ?eld can be reached with or without
combination being connected in series with the capacitor
gas pressure.
51. The capacitor 52 and resistor 53 combination will
Thus if the apparatus of FIGURE 1 is to be operated at
typically have a time constant of the order of 1-l0—3
exceeding 300! kv., and with electrode spacings
seconds. A potential from the source 54- applied to the
exceeding one millimeter, gas pressure within the vacuum
R-C combination appears entirely across the vacuum gap
enclosure 14 should be adjusted to be within the above
capacitor 51. However, when a spark occurs across the
speci?ed range of values.
vacuum gap, the potential is almost entirely across the
Referring now to FIGURE 4, a typical usage of the in
parallel resistor 53 and capacitor 52. A time in tie order
of milliseconds is required for the redistribution of vari 45 vention is shown, the apparatus being a parallel plate
velocity spectrometer 21 of a type used in the study of
ous charge carriers in the glass electrode, therefore the
nuclear phenomena. This apparatus is used to sort out
invention responds to DC voltage variations up to the
nuclear particles of a selected velocity from a beam 20 of
kilocycle range.
high energy particles having various differing velocities
In the operation of the invention at voltages in excess
which beam is generally obtained from a particle acceler
of 300 kv. a new phenomenon occurred, one not observed
ator 22 which might, for instance, be a synchrotron.
at lower voltages, namely an ion~exchange process. With
The spectrometer comprises a pair of spaced apart
a sudden onset, identically shaped luminous patches ap
electrodes essentially similar to those hereinbefore de
peared on the electrodes Ill and 12, stable at threshold
scribed with reference to FIGURE 1 and thus includes a
but rapidly becoming intense and developing into unstable
long rectangular negative glass electrode 12’ having a
discharges as the potential was raised only a few percent
conductive backing plate 13' disposed in a parallel spaced
above threshold. The threshold voltage for onset of the
apart relationship with a ?at rectangular positive electrode
ion exchange increased at ?rst when the system was
11', the electrodes having the specialized characteristics
forced to discharge repeatedly, but was observed to
stabilize eventually at about 400‘ kv. almost independently
of the spacing of the electrodes for spacing between 1.5
and 7 cm.
previously described. A pair of spaced parallel rectangu
lar magnetic ?eld coils 27 and 28 are disposed along the
opposite sides of the electrodes 11' and 12’ with the plane
of the coils perpendicular to the electrodes and parallel to
Identi?cation of the discharge as an ion-exchange proc
‘beam 26. A magnet power supply 29‘ is connected to the
ess is based on the following observations. The luminous
coils 27 and 28 so that current through both is in the same
patches, sometimes covering an area of 1 cm.2 or more,
were most intense and, in spite of a certain amount of 65 direction and a magnetic ?eld is created ‘between the two
electrodes 11' and 12' at right angles to the electric ?eld
movement, tended to occur mainly at depressed areas of
therebetween, and to beam 20‘, as shown by magnetic ?eld
the anode electrode 11 which was not a particularly ?at
surface, ‘but with relative deviations from ?atness of as
much as 2 mm.
lines 31. A high voltage power supply 32 has a negative
voltage terminal connected to the backing plate 13' and
This is a natural consequence in an ion
has a positive voltage terminal connected to the electrode
exchange if the secondary ions are emitted with essentially 70 11’ establishing the electric ?eld 33. A vacuum envelope
zero velocity. A striking demonstration of the same
effect occurs when the electrodes 11 and 12 are not paral
lel. In this case a single luminous spot may appear on
each electrode at the onset of a discharge, but as the volt
34 encloses the spectrometer and avacuum pump 36 is
coupled thereto. To control the temperature of the glass
cathode 12’ for adjusting the resistivity thereof as herein
before described, an electrical radiant heating element 39
age is r-aised a number of regularly spaced patches de 75 is disposed beneath the negative electrode and the back
I 3,087,056
ing plate 13',- and is connected to a suitable source of
current '41 through a variable resistor 42.
In operation, the beam 20 of accelerated charged par
ticles is directed between the two electrodes 11’ and 12’
The electric ?eld 33 tends tov de?ect the beam particles in
one direction and the magnetic ?eld 31 tends to de?ect
the beam particles in the opposite. For particles of a
particular velocity the two de?ection forces will equalize
less than l()—3 second, a second electrode having a ?at
surface parallel to said ?rst electrode and spaced there
from to ‘form a ?eld ‘gap, means providing a vacuum in
said ?eld gap, a highly conductive backing means con
tacting the surf-ace of said ?rst electrode which is remote
from said second electrode, and a high voltage power
supply having a positive terminal connected to said second
electrode and a negative terminal connected to said back
ing means.
and such particles pass along the linear path 20 Without
In apparatus for producing an intense electric ?eld,
de?ection while other particles are de?ected vand removed 10 the3.combination
comp-rising a ?rst electrode and a second
from the beam by striking the electrodes 11’ or 12'.
apart therefrom to form- a ?eld gap, said
Thus, from the input beam 20, there remains an output
?rst electrode being formed of glass, a high voltage power
beam 20" comprised only of particles having the desired
supply having a negative terminal connected to said ?rst
energy which energy is determined by the electrical and
electrode and having a positive terminal connected to said
magnetic ?eld intensities.‘ The output beam 20" is di
netic ?elds are usually made adjustable for obtaining the
second electrode, and a vacuum enclosure surrounding
said ?eld gap.
4. In apparatus for producing an intense electric ?eld
particular magnetic and electric ?eld strengths for sepa
rating any particles of a speci?c desired velocity.
The inclusion of the present invention in this type of
the speci?c resistivity of said glass.
rected toward a detector 38 which may be a bubble cham
her for example. The intensities of the electric and mag
apparatus allows a much more intense electric ?eld to be
obtained before sparking occurs. Consequently a sub
stantial reduction in the physical size of the spectrometer
substantially as described in claim 3, the further combina
tion of means ?or heating said ?rst electrode to regulate
5. Apparatus for producing an intense electric ?eld as
described in claim 3 wherein said ?rst electrode is com
prised of soda-lime glass, and comprising the further
of a heating element disposal proximal said
may be obtained or the range of the spectrometer may be 25 combination
for maintaining the speci?c resistivity
extended in comparison
prior instruments of this
thereof within the range 1 to 101° ohm-centimeters.
embodiment of the spectrometer having ten foot
long electrodes, a gap spacing of ?ve centimeters, a soft
glass negative electrode operating at 450 kv. at a tempera
ture of 100° C., and having an argon pressure of about
one micron, has been operated in an 800ilm.e.v./c. K—
beam generated by a proton synchrotron. Under normal
6. In a separator for selecting charged particles having
a particular characteristic from a heterogeneous beam of
particles, the combination comprising a pair of electrodes
spaced apart to de?ne a ?eld gap along which said hetero
geneous beam of particles may be directed, a ?rst of said
electrodes being comprised of glass having a speci?c resis
rtivity exceeding one ohm-centimeter and having a bulk
conditions the spark rate varies between a few per day
time constant less than 10*3 second, a high voltage power
and a ‘few per hour, much of which is attributable to 35 supply having a negative potential terminal connected to
sparks i'on the supporting insulators. After two months
said ?rst electrode and having a positive potential ter
of operation, no damage to the electrodes had occurred.
rninal connected to the second of said electrodes whereby
Operation of the spectrometer in a beamycomposed of
particles of known charge mass and momentum provided
an additional means for verifying that the expected elec
tric ?eld actually is present between the electrodes.
It will be apparent to those skilled in the art that nu
an electrical ?eld is established between said electrodes,
means providing a magnetic ?eld between said electrodes
which magnetic ?eld is normal to said electric ?eld and
normal to said beam of particles, a magnet power supply
connected to said magnetic ?eld means, a vacuum tank
inerous variations and modi?cations may be made within
surrounding ‘said ?eld gap, a vacuum pump connected
the spirit and scope of the invention and thus it is not in 45 with said tank, and means for heating said ?rst electrode
tended to limit the. invention except as de?ned in the fol
to bring the electrical characteristics thereo? within said
lowing claims.
What is claimed is:
1. In apparatus for providing an electrical ?eld, the
combination comprising a ?rst and a second electrode dis
posed in spaced apart relationship, said ?rst electrode
being comprised of a material characterized by a high
dielectric strength and by having a volume resistivity
greater than one ohm-centimeter and a time constant of
less than 10-3 second, a conductor means contacting sub
stantially the entire surface of said ?rst electrode on the
speci?ed ranges.
7. In apparatus for producing an intense electrical
?eld, the combination comprising a ?rst electrode formed
of a material having ‘a volume resistivity and time con
stant substantially equivalent to that of heated glass, 2.
second electrode spaced apart ‘from said‘?nst electrode to
form a ?eld gap, a high voltage power supply having a
negative terminal connected to said ?rst electrode and a
positive terminal connected to said second electrode, and
side thereof opposite from said second electrode, and
means ?or connecting said second electrode to a positive
a vacuum enclosure surrounding‘said electrodes and said
?eld gap.
potential and said conductor means to a relatively nega
References Cited in the ?le of this patent
tive potential.
2. In apparatus for maintaining an intense electric ?eld
without excessive sparking, the combination comprising a
?rst ?at electrode ?ormed of a material characterized by
having a high dielectric strength and a volume resistivity
greater than one ohm-centimeter and a time constant of
Bowie _______________ __ Aug. 13, 1940
De Forest ____________ __ Feb. 22, 1949
Sto'ckcr _______________ __ Jan. 10, 1961
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