Grid structure for certain plural mode electron guns

A cathode/grid assembly for effecting a dual power mode electron gun for use with a traveling wave tube. The conventional "shadow grid" is both electrically and mechanically isolated from the cathode. A variable voltage source is connected to the shadow grid to bias it slightly above or slightly below the cathode potential, depending on the power mode. The electron emitting surface area of the cathode is the same in both modes. By changing the bias on the shadow grid, the transverse beam temperature may be increased to compensate for the reduced space charge density of the low power mode. Thus, the diameter of the low power beam is substantially the same as the diameter of the high power beam. This ensures good beam transmission and high electron beam rf interaction in the low power mode.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to electron guns for use with traveling wave tubes 
and, more particularly, to the grid structure of such electron guns as are 
focused by means of periodic-permanent-magnets. 
2. Description of the Prior Art 
A traveling wave tube (TWT) is a device for bringing together within the 
same space an electron beam and radio-frequency (rf) energy for 
interaction with one another to produce a desired effect. The rf energy is 
provided from an external source to the rf input port of the TWT. The 
electron beam is provided from an electron gun. A traveling wave tube 
(TWT) typically comprises a generally cylindrical body having a hollow 
axially-extending, cylindrical electron beam tube. A series of 
axially-extending cavities are radially positioned around the beam tube, 
each of the cavities having an aperture which connects or couples that 
cavity to the beam tube. These series of cavities are generally referred 
to as the circuit of the TWT. A magnetic focusing means, which is also 
radially positioned around the TWT circuit, produces a magnetic field, the 
axis of which coincides with the axis of the beam tube. The magnetic 
focusing means is generally either a solenoid type or a 
periodic-permanent-magnet (PPM) type. The magnetic field functions to 
exert a compressive force on the electron beam to focus it and confine it 
to flow within the electron beam tube in a laminar and uniform manner. 
TWTs are frequently used in applications which require the TWT to be 
capable of alternately operating at a low power level and a high power 
level. The high and low power levels refer to the power level of the 
output power. Typically, the applications call for the high power level to 
require an electron beam current five times the electron beam current of 
the low power mode. The beam current is determined by the amount of 
electrons emitted from the cathode of the electron gun. To control the 
amount of electrons emitted from the cathode surface, electron gun grids 
are used. An illustration of the use of electron gun grids may be found in 
the device described in U.S. Pat. No. 3,812,395 issued to Scott. The 
electron gun is typically a Piercegun as described in "Theory and Design 
of Electron Beams" by J. R. Pierce, D. Van Nostrand Company, Inc., 1954. 
In the device disclosed by Scott, the voltage on an electron gun control 
grid is varied to alternately provide a high beam current during the high 
power mode and a low beam current during the low power mode. However, the 
decreased voltage on the control grid during the low power mode causes a 
corresponding decrease in the amount of electrons emitted from the 
cathode. The decrease in the number of electrons reduces the beam space 
charge density. The reduced beam space charge density is thus less 
effective at counter-balancing the radially-inward compressive force of 
the magnetic field and, as a result, the electron beam collapses or 
defocuses from the focused, laminar state of the high power mode. 
Theoretically, the available low beam current should be one fifth of the 
amount of the available high beam current if the beam current emitted at 
the cathode is decreased by a factor of five. However, the low beam 
current that is actually available in the TWT is less because of the 
collapse of the beam. The collapse of the beam causes the beam to become 
defocused, with the result that a part of the beam intercepts the wall of 
the electron beam tube. For example, in a PPM-focused TWT, only 
approximately 50% of the theoretical amount of the low power beam current 
is actually transmitted through the beam tube (as compared to 
approximately 90% for a solenoid-focused TWT). In contrast, the 
transmission percentage during the high power mode is approximately 92% of 
the theoretical for the PPM type and 98% for the solenoid-focused type. 
To alleviate the collapsing beam problem, control grids are designed and 
used to maintain the space charge density constant in the electron beam 
during the low power mode, that is, to maintain the low power mode space 
charge density at the level of the high power mode space charge density. 
This is generally accomplished by biasing the voltage on a portion of a 
control grid to cause a corresponding portion of the cathode electron 
emitting surface to be non-emitting. This produces a smaller diameter 
electron beam during the low power mode. The smaller diameter beam, which 
has a space charge density equal to that of the beam of the high power 
mode, does not collapse. Such a prior art control grid operation is 
disclosed in U.S. Pat. No. 4,023,061, issued to Berwick et al. However, 
this solution gives rise to problems of its own. 
The control grid, of a device as disclosed in Berwick, has been biased to 
permit only a small diameter beam to be emitted from the cathode during 
the low power mode of operation. This effects a space charge density in 
the low power mode equal to the space charge density of the high power 
beam. However, the now narrow low power mode electron beam interacts 
poorly with the radio-frequency signal that is present at the cavity 
apertures of the TWT circuit. The radio-frequency signal, propagating 
through the cavities, creates an axially-extending cylindrical electric 
field that is also concentric with the hollow cylindrical beam tube. In 
order to amplify the radio-frequency signal, electrons in the beam tube 
must interact with the axial electric field. However, the electric field, 
due to its inherent properties, generally concentrates at the inner 
peripheral surface of the electron beam tube adjacent the cavity 
apertures. Thus, the narrow beam, which has a diameter that is much 
smaller than the diameter of the beam tube, interacts inefficiently with 
the radio-frequency signal. The efficiency of interaction, generally 
referred to as the electronic efficiency, is generally decreased by as 
much as 50% during the low power mode for the grid structures similar to 
that of the device shown in Berwick. 
A second disadvantage of devices similar to that of Berwick results from 
the physical structure of the grid assembly. The control grid is comprised 
of two concentric grids. The radially inner grid is smaller and controls 
the low power mode. The radially outer grid is annular and circumscribes 
the radially inner grid. Together, the two grids cover a larger area and 
control the high power mode. Because the two grids are physically and 
electrically insulated from one another, the support structure of the 
radially inner grid must traverse the flow of emitted electrons during the 
high power operating mode. The support structure will thus intercept 
electrons in the high power mode and cause distortions in the electron 
optics of the high power mode. 
Any prior art devices which have a low power beam of a diameter 
significantly less than the diameter of the high power beam will suffer 
from reduced electronic efficiency in the low power mode. In addition, any 
electron gun grid structure which has a first control grid supported 
concentrically with respect to a second control grid will suffer 
distortions in the electron optics when the beam operates in the mode 
wherein the radially outer control grid is activated. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide an electron 
gun having a low power electron beam with a diameter substantially equal 
to the diameter of the high power electron beam whereby the efficiency of 
interaction of the low power beam with the rf energy is comparable to the 
efficiency of interaction of the high power beam with the rf energy. 
It is a further object of the invention to provide an electron gun having 
electron optics which, while operating in the high power mode, are not 
distorted by the presence of support members of a control grid within the 
flow of the electron beam. 
These and other objects, features and advantages are effected by providing 
a novel grid structure for the electron gun. This novel grid structure 
comprises a shadow grid which, unlike shadow grids of the prior art, is 
electrically and physically insulated from the cathode of the electron 
gun. Because it is electrically insulated, the shadow grid can be, and is, 
operated at an adjustable voltage bias with respect to the cathode. The 
shadow grid of the invention may be biased above or below the potential of 
the cathode, depending on whether the electron gun is to be operated in 
the high power or low power mode. The control grid is also connected to an 
adjustable voltage source to effect a high power mode and a low power 
mode. The voltage biases of the two grids are separately adjusted 
according to the invention. Proper adjustment of the voltages, together 
with the fact that the control grid has the same effective area for both 
high and low power modes, produces an electron beam which is substantially 
the same diameter in both the high and low power modes. Because only a 
single control grid is used, instead of two concentric control grids as 
shown in Berwick, the electron flow is not distorted by the presence of a 
grid support structure within the beam flow path.

DETAILED DESCRIPTION OF THE INVENTION 
To more fully appreciate the advantages of the present invention, it is 
helpful to first discuss the characteristics of the structure and 
operation of prior art dual-mode TWTs and the associated electron guns as 
typified by the device shown diagrammatically in FIGS. 1-4. 
The typical dual-mode electron gun is provided with an electrically heated 
cathode element 100 which emits electrons from a concave emission surface 
102 which may be a spherical, parabolic, or hyperbolic surface of 
revolution. The cathode element 100 may, for example, comprise a 
barium-impregnated tungsten material. A grid structure, comprising a 
shadow grid 110 electrically connected to cathode 100 such as by 
connection 112, a radially outer control grid 120 and a radially inner 
control grid 130, is provided to control the emission of electrons from 
the emission surface 102. After the electrons have been emitted from the 
surface 102 and have passed through the grid structure, the electrons are 
focused into a beam configuration by a focus electrode 140 and accelerated 
by accelerating anode 150. Thereafter, the beam enters the TWT circuit 
shown as 160 in FIG. 1. 
The physical characteristics of the electron beam as it enters the TWT 
circuit are substantially determined by the geometric configuration of the 
grid structure and the bias voltages applied to the grid elements, i.e., 
the shadow grid 110, and control grids 120 and 130. As shown in FIG. 1, 
the diameter D.sub.L of the low power electron beam is significantly less 
than the diameter D.sub.H of the high power electron beam. This difference 
is a direct result of the geometry of the control grids 130 and 120 shown 
in FIGS. 3 and 4, respectively. 
Control grid 130 has a radially outer and annular mounting structure 132 
and a central circular grid structure 134 supportively connected to the 
mounting structure 132 by radial elements 136. Control grid 120 has a 
radially outer and annular mounting structure 122 and a central circular 
aperture 124. An annular grid structure 126 is secured to the radially 
inner bound of mounting structure 122 and defines the aperture 124. 
Aperture 124 is slightly larger than grid structure 134 and grid structure 
126 together with grid structure 134 occupy an area substantially equal to 
the entire surface area 102 of cathode 100. The area of grid structure 134 
substantially corresponds in area and size to central area 104 of cathode 
100 and annular grid structure 126 substantially corresponds in area and 
size to annular area 106 of cathode 100. 
To operate the electron gun of FIG. 1 in its low power mode the cathode 100 
is first heated in order to enable it to emit electrons. Control grid 130 
is electrically biased positively with respect to cathode 100. This will 
tend to attract electrons from the central circular area 104 of cathode 
100. Control grid 120 is electrically biased negatively with respect to 
cathode 100. This tends to inhibit the flow of electrons through the grid 
assembly from annular cathode surface 106. Hence, in the low power mode, 
the electron beam is comprised only of electrons emitted from the central 
circular area 104 of the cathode 100. The electron beam will thus have a 
relatively small diameter D.sub.L as shown in FIG. 1. Because the beam 
diameter D.sub.L is significantly less than the beam tube diameter defined 
by the walls of the TWT circuit 160, the efficiency of interaction of the 
electron beam with the rf energy (which is concentrated near the walls) is 
greatly reduced and in the vicinity of only 50% of the efficiency of 
interaction for the high power mode. 
To operate the electron gun in its high power mode, both control grid 120 
and control grid 130 are electrically biased positively with respect to 
cathode 100. Grid 134 thus tends to attract electrons from area 104 of the 
cathode 100 and grid 126 tends to attract electrons emitted from annular 
area 106 of cathode 100. Thus, in this high power mode, electrons are 
attracted by the combined grids 134 and 126 from substantially the entire 
emission surface 102 of cathode 100. The electron beam produced in the 
high power mode will have a significantly larger diameter D.sub.H as 
indicated in FIG. 1. This larger diameter electron beam is able to 
interact more efficiently with the rf energy which is concentrated near 
the walls of the TWT circuit 160. However, it should be noted that in the 
high power mode, the electrons emitted from the annular area 106 of the 
cathode 100, and attracted by the annular grid 126 of control grid 120, 
after passing through control grid 120 must pass by the support members 
136 of control grid 130. The presence of support members 136 within the 
path of electrons attracted by control grid 120 interferes with, and 
creates disturbances within, the electron optical characteristics of the 
high power electron beam. 
The shadow grid 110 of the prior art grid structure serves to protect the 
control grids 120 and 130 from overheating and melting, especially in the 
high power operating mode. Shadow grid 110 has a structure substantially 
identical to the combined structure of control grids 130 and 120. Thus, 
shadow grid 110 would have an annular radially outer mounting structure 
and would have a grid structure of a size and shape equal to the size and 
shape of the superimposed grids 134 and 126. The radially extending grid 
elements of shadow grid 110 would substantially align with the radially 
extending elements 137 of control grid 130 and with the radially extending 
elements 127 of control grid 120. Similarly, the circularly extending 
elements of shadow grid 110 would substantially align with circularly 
extending elements 138 and 128 of control grids 130 and 120, respectively. 
The shadow grid 110 is electrically connected to, and at the same 
electrical potential as, the cathode 100. 
When operating in the high power mode, the electron beam current is on the 
order of four amperes. The combined area of circular and radial grid 
elements of control grids 120 and 130 is about ten percent of the electron 
emission surface 102. Thus, approximately ten percent of the beam current, 
or 0.4 amperes, would be intercepted by the two control grids. Because the 
control grids, in the high power mode, operate at about 300 volts above 
the potential of the cathode, the power that would have to be dissipated 
by the two control grids is on the order of 120 watts. This amount of 
power is not readily dissipated by the control grids and they would 
quickly overheat and melt. By interposing shadow grid 110 between the 
electron emission surface 102 and the control grids 120 and 130, the 
emission of electrons from the cathode surface 102, directly opposite the 
radial elements and circular elements of the shadow grid, is suppressed by 
the low potential of the shadow grid 110. Hence, only minimal electrons 
are intercepted by the radial and circular elements of the control grids. 
The shadow grid 110 will suppress the ten percent of the electron beam 
current (i.e., 0.4 amperes) and thus shield the control grids 120 and 130. 
Since the shadow grid 110 is at zero volts potential with respect to the 
electron emission surface 102 it is not required to dissipate any power. 
Having described the operation and characteristics of prior art electron 
guns and their grid structure, the characteristics, features and 
advantages of the grid structure of the present invention may now readily 
be perceived as described below with reference to FIGS. 5 and 6. 
The electron gun of FIG. 5 is substantially identical to the electron gun 
of FIG. 1 with the exception of the novel structure of the cathode and 
grid elements. The electron gun of FIG. 5 has a heated cathode 100 having 
an electron emission surface 102. A grid assembly controls the flow of 
emitted electrons toward a focus electrode 140 and toward an accelerating 
anode 150. The electron beam then enters the TWT circuit. The novel grid 
structrure of the invention is distinguished from the grid structure used 
in dual mode TWTs of the prior art in several respects. 
The shadow grid 210 of the present invention is shown in FIG. 6. It 
comprises an annular mounting member 212 and an electrically conductive 
interior grid structure 213. The grid structure 213 is formed by a 
plurality of intersecting radial elements 214 and circular elements 216. 
The elements 214 and 216 may typically be a molybdenum material and 
intersect to form a plurality of apertures 218 which are roughly 0.100 
inches by 0.060 inches. The shadow grid 210 is mounted to be in both 
electrical and mechanical isolation from the cathode 100. Thus, the shadow 
grid 210 can be electrically biased, such as by a variable voltage source 
240, to operate at an electrical potential different from that of the 
cathode 100 and either above or below the potential of the cathode 100 
depending upon the mode of operation of the TWT. The shadow grid 210 
performs the same function of shielding the control grid (220 in FIG. 5) 
as does shadow grid 110 of FIG. 1. The distinguishing features of shadow 
grid 210 are that it is electrically and mechanically insulated from 
cathode 100 and it is operated at a voltage other than that of the 
cathode. The shadow grid 110 may typically be axially spaced from electron 
emission surface 102 by about 0.003 inches (0.0762 mm). 
The control grid 220 is substantially identical to shadow grid 210 except 
its mounting member may be broader than that of shadow grid 210. The grid 
area of control grid 220 is substantially the same size as the grid area 
213 of shadow grid 210. Further, the radial grid elements and circular 
grid elements of control grid 220 are aligned with the radial grid 
elements 214 and circular grid elements 216, respectively, of shadow grid 
210. Thus, shadow grid 210 can perform its protective shadow function and 
shield control grid 220 just as shadow grid 110 shields control grids 120 
and 130. The word "aligned" when used with respect to the radial and 
circular grid elements of shadow grid 210 and control grid 220 means the 
elements are aligned parallel to the localized flow of electrons within 
the beam, such that control grid 220 does not intercept electrons. 
It is well known that when a grid coincides in position and voltage with an 
equipotential surface of an electron beam, the electron trajectories are 
not disturbed by the presence of the grid (except for those electrons 
which are intercepted by the grid). The grid is effectively "invisible" to 
the electron beam. (See "An Ultra-Laminar Tetrode Gun for High Duty Cycle 
Applications" by Richard True, IEEE IEDM, pgs. 286-289, 1979). By running 
the shadow grid 210 slightly positive with respect to the cathode 100 and 
positioning the shadow grid according to the above principle, a high 
quality beam can be obtained. Both the shadow grid 210 and the control 
grid 220 will appear to be invisible to the beam. The electron gun will 
behave essentially like an ungridded gun. This high power mode of 
operation should therefore produce an excellently focused laminar beam. 
Since the shadow grid is operated at a relatively low voltage, heating by 
the interception of beam current is not significant. 
In the low power mode the shadow grid 210 is operated slightly negatively 
with respect to the cathode 100. The position of the shadow grid with 
respect to the cathode 100 is not changed. The shadow grid 210 is thus no 
longer located so as to be "invisible". The slightly negative bias of the 
shadow grid reduces the emitted electron current. Since the diameters of 
the grids have not changed, the space charge density of the beam has been 
reduced. However, the slightly negative bias of the shadow grid 210 has 
also disrupted the path of electrons passing through the grid. Instead of 
following a highly laminar path, the negative bias of the shadow grid 210 
imparts a large transverse velocity component to the emitted electrons. 
This increase in the beam's transverse kinetic energy effectively raises 
the "transverse beam temperature". This increase in transverse beam 
temperature offsets, to some extent, the decrease in space charge density, 
thereby countering the tendency of the magnetic field to compress the 
electron beam. By raising the transverse beam temperature of the low power 
beam sufficiently to counteract the decrease in space charge density, it 
is possible to maintain the diameter of the low power beam substantially 
equal to the diameter of the high power beam. This is in accord with 
Herrmann's optical theory of thermal velocity effects as stated in 
"Optical Theory of Thermal Velocity Effects in Cylindrical Beams", by G. 
Herrmann in Journal of Applied Physics, Vol. 29, p. 127, 1958. 
The thermal beam equilibrium radius R (the "Herrmann radius") in the 
magnetic field is related to the Brillouin radius R.sub.BR, the beam 
temperature T, the magnetic focusing field B and its value at the cathode 
B.sub.c by the equation: 
##EQU1## 
where k, e and m are respectively the Boltzman constant and the charge and 
mass of an electron. The radius of the cathode of the electron gun is 
r.sub.c. 
The term R.sub.BR accounts for the space charge effect since: 
##EQU2## 
where P is the beam perveance, .epsilon..sub.o is the permittivity of free 
space (8.855.times.10.sup.-12 farads/meter) and .PHI. is the beam voltage. 
Equations (1) and (2) are from "Verification and Use of Herrmann's Optical 
Theory of Thermal Velocity Effects in Electron Beams in the Low Perveance 
Regime" by K. Amboss, IEEE Trans. ED, Vol. 11, p. 479 (1964). 
For a typical dual mode X-band TWT the perveance will change from 
P=1.0.mu.P in the high power mode to P=0.2.mu.P in the low power mode. 
Other typical tube parameters for high an low power modes are set out in 
Table 1. 
TABLE 1 
______________________________________ 
Power Mode 
Parameter High Low 
______________________________________ 
Radius of Curvature 
0.684(1.73736 cm) 
0.684 
of Cathode (in.) 
Radius of Cathode (in.) 
0.311(.7899 cm) 
0.311 
Semiangle of 27 27 
Convergence (degrees) 
Radius of Pole 0.1(.254 cm) 0.1 
Piece (in.) 
Voltage (volts) 24,500 24,500 
Maximum Field (Gauss) 
2250 2250 
Cathode Field (Gauss) 
3.937 3.937 
##STR1## 1.0 0.2 
Temperature of 1,400 18,000* 
Cathode (degrees K) 
First Anode 24,500 24,500 
Volts (volts) 
Magnetic Field 0.985(2.5019 cm) 
0.985 
Period (in.) 
______________________________________ 
*effective transverse temperature 
Using the values of Table 1 to solve equations (1) and (2) leads to the 
results of Table 2. In Table 2 Confined Radius is R with T set to zero and 
Temperature Radius is R with B.sub.c set to zero. 
TABLE 2 
______________________________________ 
Parameter High Low 
______________________________________ 
Brillouin Radius 
(in.) 0.0227 0.0102 
(cm) 0.057658 0.02591 
Confined Radius 
(in.) 0.0243 0.0164 
(cm) 0.061722 0.041656 
Temperature Radius 
(in.) 0.0235 0.0238 
(cm) 0.05969 0.060452 
Total Radius (in.) 0.0249 0.0247 
(cm) 0.063246 0.062728 
______________________________________ 
Table 2 shows that substantially the same total beam radius is obtained for 
the high power mode with P=1.0 and T=1,400 degrees K (the actual operating 
temperature of the cathode) and for the low power mode with P=0.2 and 
T=18,000 degrees K. This analytical approach was verified by modifying a 
standard electron gun design #162 CGH-P to allow the shadow grid 210 to be 
operated at small positive and negative voltages (.+-.50v) with respect to 
the cathode 100. The modified gun was mounted on a standard 8725 TWT body 
manufactured by Hughes Aircraft Company. 
For the high power mode the TWT was focused with the shadow grid 210 at 10 
volts positive with respect to the cathode which was at -25,000 V. The 
control grid 220 was set to -24,699 volts (i.e., 301 V positive with 
respect to the cathode) and produced a normal operating current of four 
amps. The best beam transmission obtained was 82.5%. This value is 
comparable to the transmission obtained in production tubes using 
unmodified guns which have the shadow grid electrically connected to the 
cathode. RF power of 15 KW was obtained across the band from 9.7 to 9.9 
GHz. 
For the low power mode the beam current was reduced to one ampere by 
reducing the voltage on shadow grid 210 to -25,040 V, i.e., 40 volts 
negative with respect to the cathode. The control grid 220 was set to 
-24,885 (i.e., 115 V positive with respect to the cathode). The beam 
transmission obtained was 90%, and the rf transmission was 88%. These 
figures compare favorably to the figures of prior art PPM type TWTs which 
show a high power beam transmission of 92% and a low power mode beam 
transmission of only 50%. 
There has thus been provided a dual mode electron gun having only two 
grids, a shadow grid and a control grid. By connecting the shadow grid to 
a variable voltage source the shadow grid may be operated at a voltage 
level slightly above or below the voltage level of the cathode. In the low 
power mode, the shadow grid is operated slightly negative (40 volts 
negative) with respect to the cathode, thereby increasing the transverse 
beam temperature and compensating for the decreased space charge density. 
The beam thus is able to maintain its diameter against the compressive 
force of the magnetic fields. By virtue of the teachings of the invention, 
relatively minor changes can be made to a standard electron gun (i.e., a 
single mode gun) to permit dual mode operation of the gun. First, the 
shadow grid must be isolated from the cathode to permit it to operate at a 
voltage other than that of the cathode, i.e., at the voltage provided by a 
variable voltage source. Second, the voltages on the shadow grid and 
control grid are adjusted to achieve the required increase in transverse 
beam temperature. It is believed that with appropriate adjustment of 
voltage bias and spacing, multiple "shadow grids" could be used to effect 
a multiple mode (i.e., three or more power mode) electron gun. 
While the invention has been described with particular reference to FIGS. 
1-6, the FIGURES and the specification, as well as the specific examples 
employed, are for purposes of illustration and discussion only. Many 
changes in structure and material could be made by one of ordinary skill 
in the art without departing from the spirit and scope of the invention. 
The scope of the invention is intended to be limited only as set forth in 
the appended claims.