U.S. patent number 3,610,986 [Application Number 05/033,578] was granted by the patent office on 1971-10-05 for electron beam source including a pilot nonthermionic, electron source.
This patent grant is currently assigned to Union Carbide Corporation. Invention is credited to James R. King.
United States Patent |
3,610,986 |
King |
October 5, 1971 |
ELECTRON BEAM SOURCE INCLUDING A PILOT NONTHERMIONIC, ELECTRON
SOURCE
Abstract
A device for generating an electron beam including a pilot gun
which generates a pilot electron beam by nonthermionic means. The
pilot electron beam is used to bombard a main cathode from which a
main electron beam is generated.
Inventors: |
King; James R. (Tustin,
CA) |
Assignee: |
Union Carbide Corporation (New
York, NY)
|
Family
ID: |
21871217 |
Appl.
No.: |
05/033,578 |
Filed: |
May 1, 1970 |
Current U.S.
Class: |
313/305;
313/231.01; 313/318.01 |
Current CPC
Class: |
H01J
37/06 (20130101) |
Current International
Class: |
H01J
37/06 (20060101); H01j 005/50 (); H01j
017/26 () |
Field of
Search: |
;313/63,74,230,231,318
;250/41.9SB |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lake; Roy
Assistant Examiner: Demeo; Palmer C.
Claims
What is claimed is:
1. An electron beam gun comprising:
(A) a pilot gun having
(i) An enclosure provided with a relatively low-pressure ionizable
gaseous medium providing the source of electrons from said pilot
gun;
(ii) A pilot cathode positioned in one wall of said enclosure, said
pilot cathode having a concave surface to give a convergent beam of
electrons having a focal point beyond the Faraday dark space;
(iii) A pilot anode positioned in said wall around and spaced from
said pilot cathode the potential difference between said pilot
cathode and pilot anode providing the electron accelerating voltage
for the electrons generated from said gaseous medium.
(B) A main cathode at the same potential as said pilot anode
positioned in the wall opposite said pilot cathode and at a
distance from said pilot cathode beyond the Faraday dark space such
that said main cathode is bombarded by electrons from said pilot
gun and thereby itself emits electrons.
(C) A main anode spaced from said main cathode and having a high
electron accelerating voltage therebetween to accelerate the
electrons emitted from said main cathode toward a workpiece.
2. An electron beam gun according to claim 1 wherein said pilot
cathode has a concave surface whose radius of curvature is
approximately twice the cathode radius.
3. An electron beam gun according to claim 1 including means for
flowing gas from a source thereof into said enclosure to provide
and maintain said low-pressure ionizable gaseous medium.
4. An electron beam gun according to claim 1 including a
self-contained source of gas in said enclosure.
5. An electron beam gun according to claim 3 and including a
control ring positioned in said wall around said means for flowing
gas into said enclosure and in contact with the Faraday dark space
when the gun is in operation to thereby carry a high negative
potential which repels electrons.
6. An electron beam gun according to claim 4 having a control cup
fitted with an aperture at the bottom, in proximity to and in line
with the cathode emitter for the purpose of aiding in the
stabilization of the focal region of the beam, is positioned in
said enclosure.
7. An electron beam gun according to claim 1 wherein power is
supplied to said pilot gun through power cables having an outside
diameter of about 0.250 and a conductor of as small as 400 circular
mils cross section.
Description
This invention relates to an improved electron beam gun and more
particularly to such a gun which eliminates electromagnetic fields
around the main cathode of the gun and vastly increases the
operating life of the heated emitter of the main gun.
Apparatus for generating electron beams by utilizing a heat source
which effects emission of electrons from a cathode by thermionic
emission are well known. Also well known are the gaseous type
electron generating guns frequently referred to as the plasma or
cold cathode-type gun. It is also generally acknowledged that the
power density of a hot cathode gun is greater than that achieved by
a cold cathode gun. Up until now, thermionic emission was achieved
in several ways, the most common being resistance heating of the
filament. This, of course, depends upon body resistance of the
filament material to the passage of high electrical currents.
Resistance heating has several accumulative drawbacks, not the
least of which is related to the fact that emission is an area
dependent function, and therefore, the greater the emission
required from the gun, the larger the emitting area must be. The
larger the area, the lower the resistance of the filament as
resistance is inversely proportional to the cross sectional area of
the conductor. As a consequence of H=I.sup.2 R the heating current
must be greater to obtain greater emission which in turn creates a
greater magnetic field around the filament, and of course, requires
cables of larger cross section to carry the required current.
The first of the problems is to maintain a uniform heating current
through the cathode which is changing resistance with temperature.
Further, there is a decided change in contact resistance at the
point where the cathode is clamped to the electrical heating
supply. These clamps must function both mechanically and
electrically to hold the cathode mechanically firm while it is
being heated from ambient temperature to approximately 2300.degree.
K and to hold the cathode with unvarying electrical contact to
conduct the required high current throughout the temperature range
and while held for prolonged periods at high temperature.
A resistance-heated cathode has a very sensitive geometry, or
distribution of resistance, such that it is difficult to restrict
emission to the desired area. The cathode must be designed so that
the area of highest resistance, and, therefore, the greatest heat,
is concentrated at the center of the filament. Under ideal
conditions this area, and only this area, is brought to emission
temperature. In addition to the geometry of the cathode itself, the
attachments to the filament must be designed to function as heat
sinks which will prevent extraneous heating.
The magnetic field surrounding the filament, as a result of the
heating current through the filament, enters the interelectrode
space between the anode and cathode of the electron beam gun and
causes an unwanted diversion of electrons and thereby, a bending of
the electron beam. It can be shown that the initial angle of
deflection exerted by the filament field depends upon the heating
current which is varied for many reasons to satisfy the emission
requirements. Further, the initial angle of deflection depends upon
the velocity of the electrons as they enter the interelectrode
space as determined by the square root of the accelerating voltage,
thus creating a complex interdependency.
A second method of heating the emitter for the welding gun is less
common due to technical difficulties involved in the solution, but
nevertheless, exists in practice. This system is often called an
indirectly heated or bombarded cathode, and it consists of a small
electron beam gun behind the main gun. The sole purpose of the
first gun is to produce an electron beam that is accelerated at low
DC voltage to bombard the emitter of the main gun. In this case,
advantage is taken of the kinetic energy conversion to heat energy
at the point of impact of the accelerated electrons, e=1/2mv.sup.2.
The bombarding gun consists of a resistance-heated filament which
is placed at some negative potential, say 1000 volts negative. The
emitter of the welding gun is the target for the bombarding gun and
is made positive with respect to the filament of the bombarding
gun. Basically, the emitter is serving as the target anode of a
crude work focus gun. It should be noted that approximately the
same power is required to heat the given mass of the emitter,
regardless of the method of heating used. In this case, however,
lower currents are used in the milliampere range in conjunction
with a higher voltage, than is used for resistance heating.
This technique is extremely effective and produces a very narrow
and straight beam due to the absence of a magnetic field around the
emitter of the main gun. The other relative disadvantages of the
magnetically induced mechanical forces are also known eliminated.
The system, however, has the inherent weakness that the bombarding
gun is making use of a resistance-heated filament which will also
eventually fail due to the geometry, thinness, and magnetically
induced mechanical stresses.
Accordingly, it is the main object of this invention to provide a
novel electron beam generating gun which eliminates unwanted
electromagnetic deflection of the beam, decreases thermal
aberration, and has vastly improved emitter life.
Another object is to provide a gun that can be used with electrical
supply cables which are small in size and are sufficiently flexible
to permit convenient coiling thereof in the gun chamber.
A further object is to provide a gun wherein the main cathode
temperature is varied in response to changes in the main beam
current in order to maintain constant current or to vary current
according to a predetermined program such as a slope or taper
program.
Still another object is to provide a gun provided with means for
controlling backfiring through the gas due to the negative
potential of the main cathode with respect to the chamber
walls.
These and other objects will either be pointed out or become
apparent from the following description and drawings wherein;
FIG. 1 is a front elevation view partially in cross section of an
embodiment of the invention.
FIG. 2 is a graphic comparison of voltage distribution and light
distribution in a normal glow discharge.
FIG. 3 is a sketch showing the important regions of the glow
discharge in relation to the cold cathode gun.
FIG. 4 is a front elevation view partially in cross section of a
second embodiment of the invention.
FIG. 5 is a front elevation view partially in cross section of a
third embodiment of the invention.
FIG. 6 is a graph of a typical curve showing variation of beam
current and control ring voltage as a function of pressure.
Briefly stated, the objects of the invention are accomplished by an
electron beam gun which includes a pilot gun comprising an
enclosure provided with a relatively low-pressure ionizable gaseous
medium which serves as the source of electrons for the gun.
Positioned in one wall of the enclosure is the pilot cathode. The
pilot cathode has a concave surface with a radius of curvature
approximately twice the cathode radius to give a convergent beam of
electrons having a focal point beyond the Faraday dark space.
Positioned in the same wall as the pilot cathode concentric with
and spaced from the pilot cathode is a ring-shaped pilot anode. An
accelerating voltage for the electrons is applied between the pilot
cathode and pilot anode.
A main cathode emitter is positioned in the wall of the enclosure
opposite the pilot cathode and is maintained at the same potential
as the pilot anode. The distance of the main cathode emitter from
the pilot cathode is always beyond the Faraday dark space when
operated within the desired pressure range. The main cathode
emitter is bombarded on its upper surface by electrons from the
pilot gun and thereby itself emits electrons to form a main
electron beam from its lower surface. The cathode emitter is
retained in a structure which surrounds the emitter 360.degree. but
exposes its top surface to the bombarding beam and exposes a
circular disc of a diameter appropriate to the desired emission
area on its lower surface. The structure surrounding the emitter on
the top side is a part of its electrical circuit of the cold
cathode pilot gun. The structure in the vicinity of the emitter may
be flat, concave, or convex. The convex curvature is preferred as
it may aid in forming the equipotential surfaces between the pilot
cathode and the emitter.
The lower side of the structure is electrically a part of the main
gun and its shape primarily determines the shape of the
equipotential surfaces between the main cathode electrode and the
anode electrode. It is taught by the science of electron optics
that the geometry of the equipotential surfaces between the cathode
electrode and the anode electrode determines the electrostatic lens
effect on the passage of electrons.
It can be seen that the upper and lower parts of the structure
supporting the cathode emitter function simultaneously in two
different circuits. The upper portion functions as a part of the
anode electrode of the pilot gun and the lower portion functions as
the cathode electrode of the main gun.
Spaced from the main cathode electrode is a main anode. Impressed
between the main cathode electrode and main anode is an
accelerating voltage to accelerate the electrons emitted from the
main cathode emitter toward a workpiece. The accelerating voltage
between the main cathode and anode is usually high relative to the
pilot accelerating voltage but is not dependent upon any particular
ratio or relationship.
The pilot gun of this invention is a plasma or cold cathode gun
containing a low-pressure ionizable gas usually maintained at a
pressure of from about 5 to 30 microns, although the pilot gun can
be operated at higher or lower pressures depending on the gas,
voltage and electrode spacing. The gases can be ambient gases or
inert gases, although inert gases are preferred for protection of
the main cathode emitter. The pilot gun can be operated at various
voltages and power levels; however, in this invention the pilot gun
is preferably operated at a voltage up to about 10 kilovolts (kv.)
and a maximum current of 50 ma.
The main gun is usually operated at a pressure of approximately 0.1
micron or less, at a voltage of about 60 kilovolts (kv.) and a
current of 500 ma. which is wholly dependent upon the area and
temperature of the emitter to produce an electron beam of 30
kilowatts. The work is usually placed in a chamber maintained at a
pressure in the area of about 5 microns, although the pressure
range might be at any value from infinitely low to 1 or above 1
atmosphere when the gun is used for high vacuum, intermediate
vacuum, or in atmosphere. However, utilizing the concept of the
invention, electron beams of different powers can be obtained by
changing pressures, voltages, electrode spacing and kinds of
electrodes and gases within the pilot gun such that emitters of
greater or lesser area are heated to the appropriate emitting
temperature.
Referring now to the drawing in FIG. 1, the electron beam gun is
shown generally at G. The pilot gun portion of gun G is shown at P.
Such pilot gun has a pilot cathode member 1 positioned in one wall
3 of an insulator enclosure 5 of pilot gun P. Outside of the pilot
cathode 1 and insulated therefrom by insulator material 2 is a
pilot anode 7 having a flange plate 9 at its top portion. Outside
the pilot anode 7 is a control ring 11. Positioned in the wall 13
opposite the pilot cathode 1 is the main cathode emitter 15. Main
cathode 16 is electrically connected to pilot anode 7 by conductor
17. The main function of conductor 17 is to carry electrons back
from cathode emitter 15 through pilot anode 7 to pilot cathode 1.
The main cathode emitter 15 is preferably a tungsten disc, although
any good electron emitting material may be used such as tantalum or
coatings having a low work function. Supporting main cathode
emitter 15 is main cathode electrode 16. Spaced from said main
cathode 16 and downstream therefrom is the main anode 19.
Downstream from the main anode 19 is a main beam focusing coil
21.
The pilot cathode may be made of any suitable pure, alloyed or
coated material such as molybdenum, copper, steel, and etc.,
although aluminum is preferred. It is well known that plasma
cathode guns, having a flat cathode orifice plate produce a
divergent electron beam. The power density of such beams is usually
low so that some focusing means must be employed to form a
convergent beam having higher power density which can be used as a
heat source. In this invention the pilot cathode has a shallow
concavity which serves as the plasma cavity in which the plasma
that is the source of electrons is generated. This cavity has the
characteristics of an electron lens. With this type of lens, when
electrons approach the lens from the side of lower field strength,
in this case the plasma cavity in the pilot cathode, the lens
action is convergent. This convergence is obtained by the concave
curvature of the cavity which results in equipotential surfaces
which are convex and, therefore, exert a convergent action on the
electrons.
It was discovered that a successful curvature of the pilot cathode
approximates a shallow curve having a true radius, although Pierce
shapes or parabolic shapes will function to a lesser degree. It was
found that the radius of the curve is related to the O.D. of the
cathode such that the radius is approximately equal to twice the
cathode radius. The choice of radius determines the depth of the
curve; however, curves of smaller radii will function, provided
that the depth does not exceed 0.130 to 0.180 inches. However, the
most stable operation is achieved with larger radii.
The vertical position of the focal point was inversely related, to
the depth of curvature; i.e., the deeper curvature, the nearer the
focal point to the face of the cathode. On one test unit the focal
point of a curvature of 0.750 radius and a depth of 0.250" was
actually up inside the cavity, whereas a depth of 0.080" projected
the focal point several inches from the cathode, and a depth of
0.130 projected the focal point approximately 2.5 inches from the
cathode. As the focal point is projected farther away, the diameter
of the focal region is also increased, thereby reducing the power
density.
The plasma cathode gun involves the phenomenon of normal glow
discharge. A normal glow discharge is divided up into areas of
light and dark spaces of varying intensity. The bright areas are
either areas of high ionization or areas of recombination of
electrons and positive ions. The dark areas are conversely those of
low ionization and recombination. It is well known that the space
nearest the cathode is called the Aston dark space. The cathode
glow space is the area in which the electrons have reached the
critical minimum velocity for the given gas pressure. Next is the
cathode dark space. In the cathode dark space, the electrons are
above critical velocity and leave behind a large quantity of
positive ions that were originated in the cathode glow space. The
negative glow region is one of maximum potential drop and is
extremely bright (the brightest of the entire column) and is an
area of maximum ionization and recombination due to the presence of
large numbers of ions, electrons, and gas molecules. The dark space
beyond the negative glow region is the Faraday dark space and is
one of potential minimum, with respect to cathode potential, due to
the large number of electrons leaving the negative glow region. The
electrons are slowed below critical ionizing velocity in this
region and negative charges predominate. The so-called positive
column is a plasma of equal charges of negative and positive ions.
Following these are the anode glow and anode dark spaces which are
of little consequence.
A graphic comparison of voltage distribution and light distribution
between parallel electrodes is shown in FIG. 2. The main regions of
discharge in relationship to the pilot gun elements are as shown in
FIG. 3, wherein smaller parts bear similar reference
characters.
It was found that a work surface (ground or positive potential) may
be positioned at any point along the vertical height of the
positive column with little change in beam current as long as it
does not come into physical contact with the Faraday dark space 35
(see FIG. 3). Upon contacting the dark space with the work surface,
the beam is suddenly reduced in intensity from its maximum value to
some extremely low value which may be only one-tenth of the
maximum. If the work is further raised until it contacts the now
faint negative glow region 34, the beam current will decrease even
more, or will extinguish entirely. Due to the drastic change as
these areas are approached, we say that the discharge "quenches"
when contact with the Faraday dark space is made.
Accordingly, while it is desirable from a beam power intensity
point of view to have a focal point as close as possible to the
cathode to form a small focal circle having a high-power density,
it is not possible to utilize such a short focal distance due to
the position of the Faraday dark space.
The distance of the Faraday dark space from the face of the cathode
is determined primarily by the operating pressure, and secondarily,
by the gun design and the applied voltage. Therefore, for a given
set of design parameters the dark space must be tolerated at
whatever distance it is positioned. It is known, as discussed
hereinabove, that any surface that penetrates the dark space will
diminish (quench) the beam current, and thus it is imperative that
the focal point distance from the pilot cathode be chosen to be
somewhat greater than the pilot cathode to the lower edge of the
Faraday dark space distance.
In order to accomplish a focal distance greater than the dark space
distance, some power density must be sacrificed due to a larger
beam cross section at the longer distance.
In operation, in the embodiment shown in FIG. 1, the pressure in
the pilot gun P is maintained between 10 and 20 microns and is
controlled by flowing gas from a source through an inlet 23 into a
"bubbler unit" 25 filled with high dielectric fluid compatible with
a vacuum environment. The purpose of the "bubbler" will be
described hereinafter. From the bubbler unit 25 the gas flows into
chamber C formed by enclosure 5. The top of the pilot gun P has a
hole or holes 27 which communicates directly with the vacuum
chamber VC so that the pilot gun chamber C is continually pumped by
the vacuum system connected to the chamber VC at port 29. The gas
might also be ducted out of the gun by a tube and independent
pumping system. The pressure in the chamber C is maintained by the
flowing gas e.g. 100 cc./min. and such flow is small enough not to
affect the actual pressure of the vacuum chamber VC. The number and
size of the gas bleed holes is related to the desired pressure
within the enclosure and the desired flow rate. The source of the
gas might also be a self-contained source such as a self-enclosed
gas bottle or an appropriate quantity of a porous material such as
activated alumina, activated charcoal, or zeolite which will serve
as a virtual leak in the enclosure. A material having a vapor
pressure in the region of interest could also be contained within
the cavity as a source of gas or vapor.
The pilot cathode 1 requires only about 0.025 amperes to provide
enough power (about 250 watts) at 10 kilovolts to heat the main
cathode 15 to emission temperature. Up until now, the emitters in
electron beam guns were customarily resistance-heated by passing an
electric current therethrough. Since large beam currents require a
large heating current on the order of 65 amperes, a strong
electromagnetic field is created which surrounds the emitter. The
field electromagnetically bends the electron beam away from the
axis of the gun and therefore, is a source of beam distortion. The
present invention utilizes a power electron beam e.g. 250 watts or
less to heat the emitter or main cathode for the high-powered
30,000 -watt electron gun. This system has the advantage of
eliminating the electromagnetic field surrounding the emitter. The
pilot cathode itself is virtually indestructible and will not
require frequent replacement. The main emitter may now fail only
due to erosion or sputtering which is an uncontrollable process of
vaporization of extremely hot metals under vacuum conditions.
The present electron gun utilizes a temperature controlled emitter
or main cathode for purposes of varying beam current. In practice,
the magnitude of the main beam current is monitored by an
appropriate circuit which sends a signal to a comparator circuit. A
reference signal is also sent to the comparator circuit and the
difference in signals is then sent to the power supply of the pilot
gun to raise or lower the power delivered thereto to increase or
decrease the pilot electron beam power. This, of course, raises or
lowers the temperature of the emitter material, thus producing only
the required quantity of electrons, thereby increasing the average
life of the emitter or main cathode.
Another advantage of the gun of this invention is that because the
cold cathode technique requires only about 0.025 amperes, compared
to the 65 amperes used in prior guns, cables 31 and 33, which
supply the pilot gun P, can be small and flexible so as to permit
such cables to be conveniently coiled in and out of the chamber VC.
For example, it is possible for a 0.250 -inch diameter cable to
replace a 2.5 -inch diameter cable used with prior art guns,
because that part of the cable bulk due to the cross section
required to carry high current and the heating effect of the high
current is eliminated.
The gun G includes a pilot gun P operating at 10 kv. and a main gun
M operating at about 0.1 -micron pressure at 60 kv. Electrically,
the two guns are independent, but they are connected through
conductor 17, described above, to a common cable 31. Accordingly,
cable 31 carries an overall maximum potential of 70 kv. The main
cathode emitter 15 carries 60 kv. and contacts the gas chamber C of
pilot gun P and therefore, there is a 60 kv. potential that might
be applied through the gas to the walls of the vacuum chamber VC.
Thus, the gun could fire in the reverse direction by the 60 kv. if
the 60 kv. can "see" a positive surface through the gas outlet or
inlet holes.
The gun G is provided with several features for preventing the
above-described backfiring.
The first feature to eliminate backfiring through the gas input
line 23 is the aforementioned "bubbler unit" 25. This unit
partially filled with dielectric fluid provides physical
discontinuity between electrical ground and the piping attached to
the gun. This combination retards any backfiring tendency through
the inlet passage.
A second method of eliminating backfiring through the inlet line is
to feed the gas into the chamber through an insulated tube
connected to a gas bottle which is insulated from ground. Of course
the two techniques may be employed simultaneously.
Another feature incorporated in the gun to minimize backfiring
involves the use of the flange 9 on pilot anode 7. The flange 9 is
at the same potential as the pilot anode 7 and 60 kv. main cathode
electrode 16. However, flange 9 is on the outside of the insulator
material 2, putting it closer to the chamber walls. This
effectively moves the field stress outside the gas region.
A third means of minimizing the tendency to backfire is to
interpose a staggered hole baffle between the gas bleed holes and
the gas plasma in the enclosure such that there is no electrical
"line of sight" for the voltage to "see" positive ground through
the bleed holes. For example, three bleed holes might be provided
in the top of the gun spaced 120.degree. apart such that the bleed
holes appear at 0.degree., 120.degree., and 240.degree.. A ring
baffle mounted below the area carrying the bleed holes would carry
appropriately sized holes but spaced 60.degree. , 180.degree. , and
300.degree. or any other position just so that the gas can find a
path between the staggered holes. Due to the fact that they never
are in exact alignment electrical lines of stress cannot "see"
through them.
The lower baffle ring may either be an insulator or a conductor.
When it is made of a conductive material a fourth means of
backfiring suppression is provided.
A fourth feature used to minimize backfiring involves incorporating
a control ring outside the pilot anode through which the gas inlet
and outlet passages penetrate. The outer control ring contacts the
Faraday dark space, and thus carries a high negative potential,
which is related to pressure and applied voltage as shown in FIG.
6. As electrons approach the high negative potential in the control
ring, they are repelled while positive ions are neutralized. The
control ring may or may not be connected to the pilot anode through
a high value of resistance.
The negative potential on the control ring is generated by taking
advantage of the various regions of electrical potential created in
a cold cathode discharge. It was found that the thickness of these
regions and the outside diameter of the Faraday dark space is
inversely influenced by ambient pressure. Thus, as pressure
increases, the diameter of the region shrinks and thereby will move
across a control ring positioned as shown. The ring acts like a
slider of a potentiometer, and picks up a varying voltage across
the dark space. The varying voltage picked up by the control ring
is a function of voltage, pressure, current, cathode to ground
surface distance, and position or geometry of the dark space.
Because of these relationships it can be seen that variety of
control functions are possible both directly related to the
invention of the pilot gun as well as other devices.
As shown in FIG. 6, there is a definite relationship between
control ring voltage and beam current where pressure, electrode
spacing and accelerating voltage are held constant.
By applying a supplementary voltage to this ring the dark space
dimensions may also be varied to cause the beam current to vary.
When the negative voltage picked up by this ring is decreased by
applying a positive potential or by grounding the ring's
self-potential through a potentiometer of approximately 1 megohm,
the current will be reduced as the resistance is reduced. A greater
reduction in current may be obtained by choosing the parameters of
pressure and spacing such that a reduction in the negative
potential on the control ring will cause the dark space to increase
in depth until its lower edge makes contact with a grounded ring or
surface. In this manner the control ring may be used to modulate
current flowing in the beam.
As the control ring potential also varies with changing cathode to
ground surface distance when voltage and pressure are maintained
constant, the ring may be used as a transducer which produces an
output voltage variation that is the analog of changing cathode to
surface distance as might be required as a control signal for a
surface contour follower or a frictionless electronic cam
follower.
Both the beam current and control ring voltage vary inversely as
pressure varies when accelerating voltage and cathode to surface
distance are held constant. Either or both parameters may be used
to measure pressure over a range of at least 3 to more than 50
microns with a very high rate of response.
In the subject invention the negative potential picked up by the
control ring with the discharge region is used as a self-biasing
potential to help reduce backfiring through the gas. This
self-biasing could also be aided by the application of other
voltage sources to the ring.
A further use of the self-biasing biasing potential picked up by
the control ring which again be supplemented by other voltage
sources is shown in FIG. 5 which is another embodiment of the
invention.
FIG. 5 is similar to FIG. 4 in most respects but has another
element 226 added which functions to stabilize the focus of the
cold cathode gun. Parts in FIG. 4 similar to the parts in FIG. 1
bear the same reference number, with the addition of one hundred
thereto. Parts in FIG. 5 which are similar to those in FIG. 4 bear
the same reference number, increased by one hundred.
A second embodiment of the gun is pictured in FIG. 4 generally
shown at G'. The pilot portion of gun G' is shown at P'. Such pilot
gun has a pilot cathode member 101, positioned in an insulator ring
102 which insulates the pilot cathode from the pilot anode 107.
Attached to the pilot anode 107 is a metal enclosure 104 which is
at the same potential as the pilot anode 107. Between the pilot
anode 107 and the enclosure 104 is a control ring 111 supported by
and spaced from the anode by insulator ring 106 which also contains
gas inlet hole 124 connected to gas inlet tube 123. Positioned in
the wall of the metal enclosure 104 opposite and in line with pilot
cathode 101 is the main cathode emitter 115. The main cathode
emitter is preferably a tungsten disc although any good electron
emitting material such as tantalum or other metals coated with
materials having a low work function could be used. Supporting the
main cathode emitter 115 is main cathode electrode 116. Opposite
and in line with main cathode electrode 116 is main anode 119
followed in line by main beam focusing coil 121. The entire
assembly of the pilot gun is supported by a main gun insulator 114
which in turn is housed and supported by shielding cylinder 120
which is at ground or positive potential.
The two embodiments differ in that: FIG. 1 the pilot gun is
assembled into and electrically insulative housing with wire 17
acting to close the electrical circuit between main cathode emitter
15 and pilot anode 7. In FIG. 4 the pilot gun is assembled into an
electrically conductive housing in which the metal enclosure 104 is
common to the main cathode emitter 115 and pilot anode electrode
107.
Both systems function similarly although the electrical field
distribution and shape of equipotential surfaces differ somewhat.
An insulating sleeve may be inserted into the enclosure between the
walls of the metal enclosure and the gas space which would produce
a field distribution in the FIG. 4 gun which would be more similar
to the field distribution in the FIG. 1 gun if desired.
It is possible to utilize a variable negative bias control in place
of or in conjunction with temperature variation of the main cathode
emitter for current control for the main gun. In such case, the
main cathode emitter 15 is insulated from the main cathode
electrode and electrically connected to a variable bias supply
which makes the emitter negative with respect to the main cathode
electrode.
A third embodiment FIG. 5 utilized a wire mesh cup or solid cup 226
attached to the control ring 211, spaced with a gap or by an
insulating cylinder or insulating cone to prevent contact with
metal enclosure 204. The cup at negative potential tends to remove
positive ions from the beam region while at the same time creating
a pinching or focusing action on the beam of electrons.
It is known that the focal length of the beam and its power density
at the focal point is related to the lens effect of the virtual
anode represented by the equipotential surface at the lower
extremity of the Faraday dark space. The area of the pilot cathode
involved in the creation of the beam determines the amount of
current delivered by the gun. A low-current beam would make use of
only the center portion of the pilot cathode, say an included angle
of 20.degree. as more current is drawn from the gun more of the
cathode area glows and a greater included angle of, say 30.degree.
is involved. If boundary rays (straight lines) are drawn from the
edges of the glowing area to intersect the axis of the gun the
point of intersection represents the focal point along the axis. It
can, therefore, be seen that the focal point moves up as more
current is drawn and down as less current is drawn. When the angle
is small, the focal region is collimated to some degree and the
beam is in approximate focus over some distance, say 0.250" or
greater. At larger angles, the focal region approximates a
point.
The shape of the lower edge of the dark space and the dimensions of
the dark space are also a function of the changing involvement of
the area of the pilot cathode.
Without further refinements it can be said that the shape of
cathode determines the range of focus for the range of current and
pressure anticipated. Focus may further be accomplished by the use
of permanent magnet fields or by appropriately shaped
electromagnetic fields as taught by the science of electron
optics.
It can be seen, through an experimental glass enclosure that the
dark space grows thicker and comes closer to the cathode emitter 15
when the pressure approaches 5 microns as the reduced beam current
tends to collimate the beam or to move the focal point beyond the
emitter surface. Under these conditions the beam current tends to
its minimum and the voltage on the control electrode tends toward
its maximum which could approximate 250 to 2,000 volts under
certain conditions. On the other hand, when the pressure rises to
approach 20 microns, the dark space thins and moves closer to the
pilot cathode surface and a greater portion of the pilot cathode is
involved in the glow, thus, increasing the beam current.
Under these conditions the voltage on the control ring may drop to
25 or 50 volts as the pressure approaches 20 microns and beam
current approaches maximum. The focal point moves higher and the
beam at the emitter surface is divergent.
By connecting the cup 226 to control ring 211 the negative
potential from the control ring is applied to the sides and bottom
of the cup. The bottom of the cup is fitted with an aperture, the
lips of which may either be the thickness of the cup material or
increased by inserting a tube in the aperture to increase its
electrically effective length. This aperture is large enough, say
0.250" or larger, to pass the main body of the beam when the focal
point is at the midpoint of the aperture but small enough to
intercept a small portion of the fringe electrons from the beam.
The charge on the cup will be the sum of two electrostatic charges.
If the focal point of the beam then moves higher the portion of the
charge obtained from the control ring will decrease but at the same
time, the portion of the charge coming from the interception of the
fringe electrons of a divergent beam will increase. The net
increase in the voltage on the cup and control ring will force the
dark space to thicken (or move downward), thus forcing the focal
point downward until the original equilibrium of the focal point in
the aperture is reinstated.
This explanation could be made in terms of the fact that an
increased negative potential on the control cup would attract and
neutralize more positive ions, reduce the bombarding effect and
secondary emission from the pilot cathode, and change the angle of
area involvement on the face of the pilot cathode but the sense of
explanation would remain the same.
The above explanation describes the stabilization of the focus of
the beam to compensate for increases in pressure within the
enclosure. A similar action takes place that (although less
critical due to collimating effects at lower pressures) stabilizes
the focus when pressure decreases.
As the pressure in the enclosure decreases the control ring
potential will rise which would tend to decrease beam current and
to decrease the angle of the pilot cathode area involved also the
focal point would tend to move downward. Under these conditions the
beam is of smaller diameter and does not tend to impinge fringe
electrons upon the aperture edges to the same degree that
high-pressure defocalization does. However, the relatively great
increase of voltage on the control cup will tend to remove more
positive ions from the beam which will neutralize a portion of the
negative potential on the control ring which will allow the dark
space to rise, increase the area of cathode involvement, increase
beam current and to move the focal point upward to reinstate focal
equilibrium in the aperture.
At all times the electrical servosystem will drive the accelerating
potential in the proper direction to keep the emission from the
cathode emitter at the preset value. The above stabilization is
only an aid and the working of the system is not wholly dependent
upon it.
Having described the invention with reference to certain preferred
embodiments, it should be understood that certain modifications may
be made to parts of the invention with respect to the arrangement
thereof without departing from the spirit and scope of the
invention.
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