Low volume, lightweight, high voltage electron gun

A low volume, lightweight, high voltage electron gun having an overall size and weight of approximately 50% the size and weight of a conventional electron gun of the same output. This is accomplished by interposing between the cathode and the walls of the housing of the electron gun a thin piece of dielectric material of preselected breakdown strength. The dielectric material is in direct contact with both the cathode and the housing of the electron gun, with the voltage stress between the walls of the housing and the cathode being such that it does not exceed the breakdown strength of the dielectric material.

BACKGROUND OF THE INVENTION 
This invention relates generally to electron guns, and, more particularly 
to an electron gun which is capable of producing a large output with 
reduced volume and being extremely lightweight so as to be easily 
incorporated within a laser system. 
An electron gun is a device capable of producing a continuous of pulsed 
stream of electrons. Since the advent of the electric discharge or 
electro-dynamic laser, the electron gun has found increased utility in the 
laster field. Conventional high voltage electron guns or vacuum electron 
guns are generally large in volume and consequently quite heavy due to the 
large vacuum separation which is required between the cathode and vacuum 
enclosure in order to stand off the high voltage on the cathode. 
Unfortunately size and weight are essential limitations in the construction 
of lasers. Since conventional high voltage vacuum electron guns are both 
large in size and extremely heavy in weight, substantial usage of, for 
example, electro-dynamic lasers has been subsequently limited. It is 
therefore essential to provide, not only in the laser field, but in any 
other field in which the emission of electrons is necessary, to produce an 
electron gun which is also of low volume and light weight construction. 
SUMMARY OF THE INVENTION 
The high voltage electron gun of this invention is capable of producing 
either a continuous or pulsed stream of electrons, and yet having a 
considerably smaller volume and less weight than the electron guns of the 
past and thereby overcoming the shortcomings of past electron guns as 
pointed out hereinabove. 
The electron gun of the instant invention substantially eliminates the 
problems of the past by placing the vacuum enclosure or housing almost 
directly adjacent the cathode, being separated therefrom by only a thin 
dielectric spacer. Consequently, the reduction of volume of the electron 
gun of this invention also eliminates the requirement of a large vacuum 
pump such as required with electron guns of the past. 
This electron gun is made up of a housing or enclosure in which the cathode 
is mounted. The cathode is mounted within the housing, separated from the 
walls of the housing by a thin piece of dielectric material sandwiched 
therebetween. Since a high vacuum is required in the enclosure of the 
electron gun, it is essential that the dielectric material have a low 
outgassing property. In addition, the thickness of the dielectric material 
must be sufficient to prevent breakdown between the cathode and the walls 
of the enclosure. The dielectric material not only separates the cathode 
from the adjacent housing wall, but also continues along the inside of the 
housing even in the area in which the cathode is not located terminating 
adjacent the bottom wall of the housing. The surface of the dielectric 
material located between the bottom of the cathode and the bottom wall of 
the housing is preferably corrugated to provide a longer path for 
breakdown. Located in the bottom wall of the housing and positioned 
opposite the cathode is an output window through which the electron beam 
can pass. 
As a result of the configuration of the high voltage electron gun of this 
invention, it is possible to have a resultant volume savings of over 50% 
and a corresponding weight savings compared to the conventional high 
voltage electron guns of the past. Furthermore, since the volume of the 
electron gun of this invention is substantially half the volume of past 
electron guns a much smaller vacuum pump can be utilized therewith even 
further reducing the overall weight of the electron gun. 
It is therefore an object of this invention to provide a high voltage 
electron gun of less volume and less weight than electron guns of the 
past. 
It is another object of this invention to provide a high voltage electron 
gun which has substantially reduced the vacuum pump requirements. 
It is still another object of this invention to provide a high voltage 
electron gun which is easily adaptable for use within a laser system. 
It is still a further object of this invention to provide a high volume 
electron gun which is economical to produce and which utilizes 
conventional, currently available components. 
For a better understanding of the present invention, together with other 
and further objects thereof, reference is made to the following 
description taken in conjunction with the accompanying drawing and its 
scope will be pointed out in the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Reference is now made to FIG. 1 of the drawing which shows a 
cross-sectional view of a conventional electron gun 10. As can be seen in 
FIG. 1 of the drawing, the volume of electron gun 10 is relatively large. 
In addition, for proper operation of the electron gun 10, cathode 12, 
which is mounted within a vacuum enclosure or housing 14 by a high voltage 
bushing 16, is separated by a great distance from all the walls 18 of the 
enclosure or housing 14 of electron gun 10. This relationship is 
maintained in order to stand off the high voltage on cathode 12. 
The electron gun 20 of this invention, shown clearly in FIGS. 2 and 3 of 
the drawing, and drawn in substantially the same scale as electron gun 10 
of FIG. 1 is approximately half the volume of the conventional electron 
gun 10. Such a reduced volume is accomplished by interposing a thin piece 
of dielectric material 22 between cathode 24 and the walls 26 of the 
grounded enclosure or housing 28 of electron gun 20. Cathode 24 is secured 
within housing 28, by any suitable securing means such a high strength, 
low outgassing adhesive such as epoxy. Enclosure or housing 28 is made of 
any good quality metal such as stainless steel, aluminum or titanium or a 
composite material such as boron epoxy. The dielectric material 22 may be 
made, for example, of any of a series of readily available polyethylenes. 
In defining the relationship between the dielectric material 22 and the 
cathode 24, dielectric material 22 is not only interposed directly between 
cathode 24 and walls 26 of housing 28, but also adjacent that portion of 
wall 26 of housing 28 which is not directly adjacent cathode 24. In fact, 
in order to provide a longer path for breakdown, that portion 30 of 
dielectric material 22 not in contact with cathode 24 is corrugated. It is 
also critical that the voltage stress between walls 26 of housing 28 (the 
anode) and cathode 24 be such that it does not exceed the breakdown 
strength of the dielectric material 22 utilized. 
The thickness of the dielectric material required can be determined from 
the following two formulas: 
A. In the portion of electron gun 20 where cathode 24 and the adjacent wall 
26 can be viewed as two parallel plates the electric field (E) is given by 
EQU E=V/d (1) 
where V is the voltage on cathode 24 and d is the separation between 
cathode 24 and wall 26. If V is in volts and d is in meters the dimensions 
of E are volts/meter. 
B. In the portion of electron gun 20 where cathode 24 and wall 26 can be 
considered as two concentric cylinders the electric field is given by 
EQU E=V/[R.sub.1 1n(R.sub.2 /R.sub.1)] (2) 
where R.sub.1 is the radius of the inner cylinder, R.sub.2 is the radius of 
the outer cylinder and ln is the natural logarithm. 
As best shown in FIG. 3 of the drawing any suitable high voltage source 32 
of, for example, 100,000-200,000 volts is operably connected to cathode 24 
through a wall 26 of housing 28. Any suitable conventional vacuum pump 34 
capable of producing a vacuum of approximately 10.sup.-5 to 10.sup.-7 torr 
within housing 28 is operably connected by a vacuum line 36 to the 
interior of housing 28. As with the conventional electron gun 10 shown in 
FIG. 1 of the drawing, any suitable output window 38 which may form an 
integral part of bottom wall 26 of housing 28 can be utilized to allow for 
the exiting of the resultant electron beam 40. 
As an example of the desired reduction in overall volume of electron gun 
20, assume 200,000 volts on cathode 24. Typical dielectric strengths for 
polystyrene range from 1.85 to 2.76.times.10.sup.5 v/cm. This is the value 
at which breakdown could occur. If we choose an average value of 
2.3.times.10.sup.5 v/cm for the dielectric strength then the minimum 
thickness of the dielectric 22 using formula (1) is 
EQU d=(V/E)=(200,000/230,000(v/v/cm)=0.87 cm. 
To be safe the dielectric should be at least three times as thick or about 
2.61 cm. This dimension should be compared to the ten to fifteen cm 
dimension for vacuum electron guns of the past. 
The design of the electron gun 20 of this invention results in a volume 
reduction of over 50% compared to the conventional electron gun 10 as 
shown in FIG. 1 of the drawing. This volume reduction allows for a 
corresponding reduction in weight in electron gun 20 not only due to the 
decrease in material utilized in the construction thereof, but also as a 
result of a smaller vacuum pump being required with the instant invention. 
Although this invention has been described with reference to a particular 
embodiment, it will be understood to those skilled in the art that this 
invention is also capable of further and other embodiments within the 
spirit and scope of the appended claims.