U.S. patent number 3,671,798 [Application Number 05/097,343] was granted by the patent office on 1972-06-20 for method and apparatus for limiting field-emission current.
This patent grant is currently assigned to The United States of America as represented by the. Invention is credited to Wayne L. Lees.
United States Patent |
3,671,798 |
|
June 20, 1972 |
METHOD AND APPARATUS FOR LIMITING FIELD-EMISSION CURRENT
Abstract
Self-protected electrodes which inherently limit field-emitted
currents to a safe value and also stabilize such currents are
disclosed. The electrodes are characterized by a plurality of
columnar conductors connected at one end to a common potential
source. The electrodes are insulated from one another along their
lengths whereby the effective or exposed surfaces thereof are
subdivided into a mosaic of conducting patches which are insulated
from one another.
Inventors: |
Wayne L. Lees (Lexington,
MA) |
Assignee: |
The United States of America as
represented by the (N/A)
|
Family
ID: |
22262887 |
Appl.
No.: |
05/097,343 |
Filed: |
December 11, 1970 |
Current U.S.
Class: |
313/336; 313/351;
313/309; 315/36 |
Current CPC
Class: |
H01J
1/3042 (20130101); H01J 19/40 (20130101) |
Current International
Class: |
H01J
19/00 (20060101); H01J 1/30 (20060101); H01J
19/40 (20060101); H01J 1/304 (20060101); H01j
001/16 (); H01j 019/10 () |
Field of
Search: |
;313/306,307,308,309,325,336,351 ;315/35,36 ;328/8 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
A Thin-Film Field Emission Cathode, by C. A. Spindt; Journal of
Applied .
Physics, Vol. 39 No. 7 pp. 3504-3505 June 1968..
|
Primary Examiner: John W. Huckert
Assistant Examiner: Andrew J. James
Attorney, Agent or Firm: Monte F. Mott Wilfred Grifka John
R. Manning Paul F. McCaul
Claims
1. A self-protecting electrode for use in rarified gaseous
environments comprising: a plurality of spaced elongated columnar
conductors, each of the columnar conductors having first and second
ends and having sufficient resistance between the ends to provide a
predetermined potential shift in the potential of one end with
respect to the other in response to the flow of a predetermined
field-emission current through the respective columnar conductors,
the distance between any pair of said conductors in a direction
perpendicular to their lengths being greater than the length of
either conductor in said direction; insulating material for
insulating said conductors one from another along their lengths
with at least a portion of each conductor extending from its first
end being surrounded by said insulating means and inhibited from
exposure to the other conductors; and means for electrically
connecting the first ends of said conductors to a
2. The electrode of claim 1 wherein said conductors are of
substantially equal length, are very long when compared to their
cross-sectional area and the second ends of said conductors being
of substantially the same
3. The apparatus of claim 2 wherein said insulating material
extends the length of said conductors, with only the second ends of
said conductors being exposed and each conductor along its entire
length being covered by said insulating means to inhibit its
exposure along its length to adjacent
4. A self-protecting electrode for use in rarified gaseous
environments comprising: a plurality of columnar conductors, each
of the columnar conductors having first and second ends and having
sufficient resistance between the ends to provide a predetermined
potential shift in the potential of one end with respect to the
other in response to the flow of a predetermined field-emission
current through the respective columnar conductors; insulating
material for insulating said conductors, one from another along
said lengths, said insulating material extending along a portion of
said conductors from their first ends, the second ends of the
conductors and portions thereof adjacent said second ends extending
above the insulating material; and means for electrically
connecting the first ends of said conductors to a
5. The apparatus of claim 4 further comprising: a layer of
conductive material disposed on the surface of the insulating
material through which the conductors extend, said layer of
conductive
6. An electrode comprising: a substantially flat electrically
conductive base member; a plurality of electrically conductive
columnar members, each extending vertically from said base member
with a first end in electrical contact with said base member and an
opposite second end remote from said base member; and insulating
material extending from said base member and surrounding at least a
portion of each columnar member from its first end toward its
second end, whereby at least a portion of each columnar member is
inhibited from exposure to the other columnar members, the distance
between any pair of said columnar members in a direction parallel
to said base member being greater than the width of either columnar
member in said
7. The electrode of claim 6 wherein said columnar members are of
substantially equal length, are very long when compared to their
cross-sectional area and the second ends of said columnar members
being of
8. The apparatus of claim 7 wherein said insulating material
extends the entire length of said columnar members, with only the
second ends of said columnar members being exposed and each
columnar member along its entire length being covered by said
insulating material to inhibit its exposure
9. The apparatus of claim 7 wherein said insulating material
extends along a portion of said columnar members from their first
ends, the second ends of the columnar members and portions thereof
adjacent said second ends
10. The apparatus of claim 9 further comprising: a layer of
conductive material disposed on the surface of the insulating
material through which the columnar members extend, said layer of
conductive material being electrically isolated from said columnar
members.
Description
ORIGIN OF THE INVENTION
The invention described herein was made by an employee of the
United States Government and may be manufactured and used by or for
the Government for governmental purposes without the payment of any
royalties thereon or therefor.
The present invention is directed to self-protecting electrodes.
More particularly, the present invention relates to the limiting to
a safe value and/or the stabilization of field-emission currents
such as those which may be associated with inherently present but
undesired protrusions on the surfaces of electrodes of high vacuum
devices. Accordingly, the general objects of the present invention
are to provide novel and improved methods and apparatus of such
character.
While not limited thereto in its utility, a principal object of the
present invention is to limit the current of the concentrated arc
formed at the cathode of evacuated electronic devices when vacuum
electric breakdown occurs. In the prior art, it has been common
practice to limit the average current in the pulse occurring upon
breakdown by including in the cathode circuit a series connected
resistance or inductance having an impedance so large that the
concomitant electrode-potential pulse is comparable with the
potential before breakdown, This practice, however, allows at least
the energy stored in the electrode charge to be dissipated in the
arc with consequent electrode damage.
The present invention can be described best with reference to the
mechanism of vacuum electric breakdown. Vacuum electric breakdown
is the uncontrolled passage of a surge of electric current between
two conductors separated by an evacuated gap. When a potential
difference is established between two such conductors, the electric
fields at the conductors' surfaces are calculable. Such fields,
calculated in terms of gross surface features, the so-called
macroscopic electric fields, attain magnitudes typically on the
order of 0.1 to 1 megavolt per centimeter at the cathode before
breakdown occurs. The actual vacuum breakdown or arcing between
electrodes, since it occurs when the gas density in the vacuum is
too low to support a glow discharge, must be attributed to the
pulling of electrons from the cathode by the electric field. This
phenomenon is known as filed emission. The fields required at a
cathode in order for appreciable filed-emission current to flow,
however, are several hundred times as strong as the fields that may
predicted on macroscopic surfaces. Recent studies have supported
the assumption that, in order for these high field strengths to be
present, vacuum sparking must accordingly be initiated at very
small projections, not otherwise visible, occurring on gross
electrode surfaces whose macroscopic fields were of comparatively
low strength.
Thus, it is now well accepted that sparks in a high vacuum arise
from sharp, filed-enhancing projections on the conductive cathode
surfaces involved. Typically, vacuum electric breakdown is
preceded, as the filed strength between a pair of electrodes is
increased, by field-emission current. This phenomenon may be more
clearly understood by reference to FIGS. 1A and 1B of the
accompanying drawing. FIG. 1A is a representation of an ideal
uniform field between a pair of electrodes separated by an
evacuated gap. FIG. 1B represents field variations which, in actual
practice, occur. The lower electrode or cathode of FIG. 1B has
protrusions on its surface. It is to be noted that no electrode
surface is free of these field-enhancing projections. For example,
tests have been made on single-crystal tungsten which has been
electropolished to remove mechanically strained metal and trapped
abrasive. When employed as a cathode, such a tungsten electrode was
found to possess field-emitting projections which were not apparent
until the field was applied. As may be seen from FIG. 1B, the
protrusions on the electrode surface will concentrate electric flux
at their tips, to some extent shadowing the adjoining electrode
surface, so that the field at the microscopic tip is greatly
enhanced. As a result of these high local fields, field emission
from these protrusions will occur.
As is well known, field emission is the escape of electrons through
the surface of a conductor into a sufficiently high attractive
field. This field must be at least 10 million volts per centimeter
to extract measurable current from most metals. As the field at the
electrode is increased, the field-emitted current increases very
sharply until, at a tip field on the order of 10.sup.8 Volts per
centimeter with a corresponding field-emitted current density on
the order of 10.sup.8 Amperes per centimeter squared, the field
emission becomes unstable and the current increases quickly by
several decades. The resultant vacuum electric breakdown,
characterized by a spark or arc, is limited typically by the
destruction of the emitting protrusion and of a portion of the
electrode surface. If the electrodes are supplied from a source of
low impedance, as is necessary in many technical applications, the
breakdown can do substantial damage to the electrodes before the
current is interrupted by protective devices.
It is also to be noted that the destruction of a portion of the
electrode surface which occurs upon vacuum electric breakdown
results in the production of vapor. As will be obvious,
accumulation of a sufficient concentration of vapor between the
electrodes will result in a glow discharge with corresponding
destructive breakdown currents. Also, while sparks destroy their
own initiating points, they tend to splash metal and thus to
establish new surface protrusions. This effect leads to the
reduction of the field strength at which a subsequent vacuum
electric breakdown will occur.
The present invention overcomes the above-discussed and other
disadvantages of the prior art and in so doing provides a novel,
self-protective electrode which may also be used as a stabilized
field-emission source. The electrodes of the present invention are
characterized by a plurality of columnar conductors which, to a
spacially displaced anode, will appear as a mosaic of conducting
patches insulated from one another. The exposed surfaces of the
conductors which comprise the present electrodes are microscopic
but remain large when compared to typical field-enhancing
protrusions. Also, each of the conductors is very long when
compared to its effective surface area, the opposite ends of each
conductor being connected to a common potential source.
When employed as a stabilized field-emission electrode, the
electrodes of the present invention may also be characterized by a
control film or electrode which surrounds but is insulated from the
base of each of the individual conductors. The control film may
serve to provide an equipotential against which tip-potential
fluctuations are stabilized and may also be employed to control the
level of field emission from each conductor to a common value.
It is therefore an object of the present invention to limit the
current of the concentrated arc formed at a cathode in vacuum
electric breakdown.
It is also an object of the present invention to destroy
field-emitting protrusions on an electrode surface in such a manner
that insufficient vapor to maintain an arc is produced.
It is another object of the present invention to provide electrodes
which are comprised of a plurality of conducting columns, the
individual columns having sufficient resistance to provide a
limiting potential shift with the maximum allowable field-emission
current.
It is a further object of the present invention to stabilize
field-emitted currents.
The present invention may be better understood and its numerous
advantages will become apparent to those skilled in the art by
reference to the accompanying drawing wherein like reference
numerals refer to like elements in the various figures and in
which:
FIG. 1, comprising the FIGS. 1A, 1B, 1C and 1D, is a schematic
presentation of the problem solved by the present invention and its
manner of solution;
FIG. 1A representing the theoretical field distribution between a
pair of electrodes in a vacuum;
FIG. 1B representing the actual field distribution in the prior
art, and
FIGS. 1C and 1D representing the modifications in field strength
resulting from use of the present invention;
FIG. 2 is an isometric view of a first embodiment of the present
invention;
FIG. 3 is an isometric view, partly in section, of a second
embodiment of the present invention employed as a stabilized
field-emission electrode; and
FIGS. 4A and 4B are partial, cross-sectional views of the
embodiment of FIG. 3 depicting field distribution for the
embodiment of FIG. 3 under each of two operating conditions.
DESCRIPTION OF THE PREFERRED EMBODIMENT
As discussed above, FIG. 1A depicts the ideal field distribution
between a pair of electrodes separated by an evacuated gap whereas
FIG. 1B depicts the actual field distribution, prior to vacuum
electric breakdown, which arises due to field-enhancing projections
on the electrode surface. As also previously noted, it is an object
of the present invention to prevent vacuum electric breakdown and,
in so doing, to destroy or erode the field-enhancing projections on
the electrode surface slowly and smoothly, thereby obviating the
danger of establishing new points (through explosive destruction of
a projection) and insuring against the production of sufficient
vapor to maintain a glow discharge. Thus, the electrode
construction of the present invention is characterized by the
self-limitation of field-emitted current.
The manner in which the foregoing is accomplished may be understood
from a consideration of FIGS. 1C and 1D. As alluded to above,
field-emission current is very sensitive to the magnitude of the
field acting at the electrode surface. Because of this, minor
fractional changes in field strength can alter the field-emitted
current by several decades. Also, it must be remembered that the
field-enhancing projections which characterize all electrode
surfaces are of microscopic size. Accordingly, since each
protrusion is very small, the distance over which the macroscopic
field must be varied in order to control field-emission from each
protrusion is correspondingly small. As is well known, high fields
can be developed over short distances with small potential
differences. Thus, in accordance with the present invention and as
may be seen from FIG. 2, the effective surface of an electrode is
subdivided into a mosaic of conducting patches which are insulated
from one another and which have dimensions that are microscopic but
remain large compared with the size of the field-enhancing
protrusions. Each of these conducting patches is connected to a
base or common-potential plane through separate resistive columns,
such columns being indicated in FIG. 1C by the resistors R. A
field-enhancing projection on a conducting patch which emits
current i will make that patch more positive than its neighbors by
iR, where R is the column resistance. This potential difference
will reduce the applied field at the emitter by Ri (b/a), where a
is the inscribed radius of the patch, and b is a geometric
factor.
To restate the foregoing, when a potential is applied, the field at
first is distributed uniformly over the conducting surface-patches
formed by the ends of the conducting columns, as indicated at FIG.
1A, except for local enhancement at a protrusion, as shown in FIG.
1B. When the field has been increased until a sharp protrusion on
one patch begins to emit electron current i, supplied through its
column resistance R, that surface patch becomes more positive than
its neighbors by iR, thereby creating an opposing field at its
surface as shown by the broken lines in FIG. 1C. The resultant
gross field is reduced at the emitting patch, as shown in FIG. 1D,
thereby limiting the field-emitted current. It is to be noted that
the filed-emitted current will not be eliminated but rather will be
self-limited to a safe value whereby field emission from the
surface protrusion will continue smoothly until the current-heated
protrusion has been eroded away or rounded, the phenomenon of
thermal evaporation being known in the art.
With reference now to FIG. 2, a first embodiment of the present
invention is shown. The embodiment of FIG. 2 comprises a plurality
of square shaped conductors 10 which are insulated from one another
by means of insulation 12. Conductors 10 are electrically connected
to one another at first ends by being in contact with conductive
metal plate 13. A centrally located one of electrodes 10 is shown
as having a field-enhancing projection 14 on its second or upper
end, the second ends of the conductors forming the conducting
patches of the resulting mosaic electrode. It should be noted that,
for proper operation of the invention, it is not necessary that the
projection 14 be centered in the mosaic patch as indicated in FIG.
2. Rather, the reverse field developed by the increment iR and
therefore the current-limiting effect increases as the projection
is placed nearer the edge of the conducting path. It may be shown
that the reverse field at the center of a conducting patch is on
the order of Ri divided by a, a being the inscribed radius of the
patch. For flat-top columns with negligible insulator thickness,
the field-emission current i reduces the applied field by: where:
b/c is the column shape factor, h is column height, p is column
resistivity, ca.sup. 2 is column cross-sectional area. 10.sup.- If
a is 1 micron, then Ri should become on the order of 10 volts when
i approaches the intended field-emission current limit. If the
conducting column under the patch having the field-enhancing
protrusion has a cross-sectional area of 10-116 .sup.8 cm.sup.2 and
a length of 1 centimeter, and is formed of a material having a
resistivity of 10 microhm-centimeters, then its resistance R
becomes 1,000 ohms and a 10 volt excursion Ri requires a
field-emission current of 10 milliamperes. Such a current would
approach typical breakdown density if emitted from an area of
10.sup.-.sup.10 cm.sup.2. The uncontrolled current rise at
breakdown, however, would be prohibited by the attendant increase
in the potential difference Ri and the resultant greatly increased
opposing field. Accordingly, the effect of a high but limited
field-emission current would not be a spark or other catastrophic
breakdown but rather would be a simple ablation and rounding of the
emitting protrusion so that its field-enhancement factor would be
reduced. It should be noted that the resistivity of the insulation
material 12 must be high enough so that the leakage current to
adjoining columns is small compared with i .
While the individual conducting columns 10 have high resistances,
the macroscopic conductivity of the columnar electrode in a
direction perpendicular to its surface is approximately that of the
solid conductor material. The ratio of this conductivity to that of
the solid is the ratio of the conductor cross-section area to the
combined conductor and insulator cross section. Thus, for example
in a vacuum capacitor, such a columnar electrode can carry a large
radio frequency current density, on the order of 100 amperes per
centimeter squared, without undue heating while any breakdown
pulses superimposed on this current are unidirectional and will be
limited to a few milliamperes.
As will be obvious from the foregoing discussion, the conducting
columns of the embodiment of FIG. 2 must be of small size since the
reverse field produced by a potential increment Ri varies inversely
with the radius of the patch while the resistance R in turn varies
inversely with the square of this radius. The embodiment of FIG. 2
may be produced, for example, by commercial techniques for drawing
metallic wires in a glass matrix. These techniques are known and
are similar to those employed, for example, in forming fiber-optic
bundles. Another technique which may be employed in the production
of the embodiment of FIG. 2 and which preferably would be employed
in manufacturing the embodiment of FIG. 3 would be the
unidriectional solidification of a eutectic melt. It is known that
metallic "whiskers," in either a conductive or nonconductive
supporting matrix, may be obtained by freezing certain alloys from
a melt in one direction. For further information on the production
of such single crystal "whisker" composites, reference may be had
to an article entitled "Metals with Grown-in Whiskers" by M.
Salkind and F. Lemkey which appeared at pages 52-64 of
"International Science and Technology," March 1967 or to an article
entitled "Whisker Composites by Unidirectional Solidification" by
M. Salkind et al. which appeared at pages 52-60 of "Chemical
Engineering Progress," Volume 62, No. 3, March 1966.
Considering now FIG. 3, an electrode array which has been formed by
one of the above-noted techniques is shown. As depicted in FIG. 3,
the array is to be employed as a stabilized, field emission source.
The array comprises a plurality of rod-like conductors 16 supported
in an insulating matrix 18. It is, of course, to be understood that
the supporting matrix 18 has been chemically etched away to expose
the rod-like conductors 16. It is also to be understood that the
composite has been suitably sliced so that the lower ends of all of
the conducting rods 16 are exposed thereby enabling electrical
contact to be made between each of rods 16 and a planar electrode
20. The embodiment of FIG. 3 is also shown as comprising a
reference or control film 22 of conducting material which is
isolated from each of conductors 16. It is, however, to be observed
that control film 22 would not be employed where the electrode is
utilized merely because of its superior breakdown limiting
characteristics as, for example, in a vacuum capacitor. For such
uses, in order to maximize the ratio of effective conductor area to
total electrode surface area, it would be desirable to increase the
effective surface area of conductive rods 16 by building these
elements up through electroplating, or by melting exposed portions
of the elements to globular form, with care in either case to
insure that the rods would remain electrically isolated from one
another at the exposed upper surface of the supporting matrix
18.
It can be shown that a small fractional change of surface field
produces a greater fractional change in field-emitted current at
low fields than at high. Accordingly, a reverse-field increment
that reduces current significantly at an applied field high enough
to initiate breakdown is still more effective at lower fields, and
therefore the same structure than can limit vacuum breakdown at
high field-emission current densities may become, after some
modification, a stabilized electron source for field emission at
lower current densities. With regard to modification, the major
difference is the addition of the control film 22 as shown in FIG.
3. In addition, the step of increasing the effective surface area
of the conductors 16 would not, of course, be performed if the
device were to be used as a stabilized field-emitting array rather
than merely as a self-limiting electrode. The protruding tips of
the conductors 16 will, of course, concentrate the field in the
same way as do the random protuberances, such as projection 14 of
FIG. 2, on which breakdown occurs. In the preferred embodiment the
conducting rods 16 are made small enough so as to produce the
required field enhancement even when rounded at their ends. This
rounded form, in which the current density in the "resistor"
columns is nearly as large as that in the filed-emitting tips, will
develop the greatest stabilizing voltage in response to a current
increment.
The control film 22 serves both to provide an equipotential against
which tip-potential fluctuations are stabilized and also to control
the common level of field emission. Since the broad surface offered
by control film 22 provides a near termination for electric flux
lines originating on the tip of a conductor 16, a potential rise
due to increased current flow in the conductor leads to a reverse
field larger and more predictable than would be developed if the
neighboring surfaces were only the other conductors and an exposed
insulating surface. In particular, an exposed insulating surface is
undesirable since it would be the source of poorly controlled
fields from the slowly migrating charges with resulting drift of
the field-emission currents. The control film 22 also serves to
adjust the field applied to all conductor tips simultaneously
through the vehicle of regulating tip potential with respect to
conducting base 20. It must be observed, of course, that if the
conductors 16 protrude so far that most of the lines of flux
terminate on adjoining conductors, the control film will have
little influence.
The effect on the fields at the rounded ends of conductors 16, as
the control film is made more positive than the conductor tips, is
shown schematically in FIGS. 4A and 4B. In FIG. 4A, the Ri voltage
at the tips of conductors 16 is zero. As the control film potential
is increased in a positive direction, the concentration of flux
lines at the tips of conductors 16 is increased by the addition of
the lines from the film that terminate on the protruding conductors
16. Presuming that the total flux from the remotely located anode
is not changed appreciably, the resulting increased tip field is
accompanied by a decrease of anode flux to the control film 22 as
shown in FIG. 4B. Obviously, if the control film is made more
negative, the anode flux to the control film increases while the
tip field decreases. Thus, it may be seen that the combined action
of the control film and the self-limiting effect of the columnar
conductors results in a controllable field-emission electrode which
is stabilized and which will not be subject to catastrophic
breakdown.
While preferred embodiments have been shown and described, various
modifications and substitutions may be made thereto without
departing from the spirit and scope of the present invention.
Accordingly, it is to be understood that the present invention has
been described by way of illustration and not limitation.
* * * * *