JPH047532B2 - - Google Patents

Description

Claim 1: An electron gun for a traveling wave tube focused by a periodic permanent magnet, which has a high output mode in which the electron beam has a first space charge density, and a high power mode in which the electron beam has a second space charge density. and a cathode and grid assembly comprising: a cathode having an emitting surface for emitting electrons to form a longitudinal electron beam; and a cathode as described above; a control grid mounted axially spaced relative to the radiation surface; and a shadow grid mounted longitudinally spaced relative to the radiation surface; Mechanically insulated: means for applying a potential difference across said cathode, shadow grid and control grid, said means being such that said low power mode electron beam and high power mode electron beam have equal diameters; 1. An electron gun that selectively applies a potential difference to both a low-output mode electron beam and a high-output mode electron beam so that the electron beam becomes 2. The electron gun according to claim 1, wherein the shadow grid and the control grid have the same grid pattern. BACKGROUND OF THE INVENTION 1. Technical Field of the Invention The present invention relates to electron guns for traveling wave tubes, and more particularly to the grid structure of electron guns whose beam is focused by periodic permanent magnets. 2. Prior Art A traveling wave tube (TWT) carries an electron beam and a radio frequency (RF) energy inside the tube, interacting with each other in a favorable manner. This rf energy is transmitted from an external rf source through the input port to this
Supplied within TWT. Further, the electron beam is supplied by an electron gun. This traveling wave tube (TWT)
Generally, it consists of a generally cylindrical body having an axially extending cavity and a cylindrical electron beam tube. A series of axially extending cavities are disposed radially around the electron beam tube, and a device is provided within each cavity for connecting or coupling the cavities to the beam tube. These series of cavities generally form the circuit of this TWT. Further, a magnetic beam focusing means is provided surrounding the TWT circuit, a magnetic field is formed by the magnetic beam focusing means, and the central axis of the magnetic beam focusing means is aligned with the central axis of the beam tube. We are doing so. This magnetic focusing means is generally of the solenoid type or periodic permanent magnet type (PPM). Due to this magnetic field, the electron stream is subjected to a force in a compressive direction, is focused, becomes a layered uniform beam, and is supplied into the electron beam tube. This TWT is often required to be capable of operating in both high-power mode and low-power mode.
The high output mode and low output mode correspond to the magnitude of the output. For example, the electron beam current in high power mode is required to be five times the electron beam current in low power mode. This beam current is determined by the amount of electrons emitted from the cathode of the electron gun. The amount of electrons emitted from the surface of this cathode is controlled by the grid. Illustrated is the electron gun grid of the device disclosed in Scott US Pat. No. 3,812,395. This electron gun was developed in 1954, for example.
Written by JRPierce, published by D. Van Nostrand.
This is a Pierce-type electron gun disclosed in “Theory and Design of Electron Beams.” The device disclosed in the Scott patent changes the voltage on the electron gun's control grid to switch between a high beam current at high power and a low beam current at low power. However, if the voltage of the control grid is reduced at low power, the number of electrons emitted from the cathode will be correspondingly reduced. This reduction in the number of electrons therefore reduces the space charge density of the beam. The reduction in the space charge density of this beam reduces the force with which it is focused radially inward by the magnetic field, thus defocusing it from its laminar focus position at high power. Theoretically, the beam current emitted from the cathode has a factor of 5.
If the beam current at low power is reduced by , the beam current at low power becomes 1/5 of the beam current at high power. However, this
In TWT, this actual beam current is lower due to beam collapse. The beam collapse defocuses the beam so that it is blocked by the electron beam tube wall. for example,
In a PPM focused TWT, only 50% of the theoretical value of the electron beam at low power is actually transmitted through the beam tube. (Focused by solenoid
(TWT is approximately 90%) In contrast, the transmission efficiency at high output is approximately 92% for the PPM type and approximately 98% for the solenoid type. The solution to this beam collapse problem is to keep the space charge density of the electron beam constant at low power, i.e., to keep the space charge density at low power the same as the space charge density at high power. The grid must be designed to do so. This is achieved by biasing the voltage applied to a portion of the control grid to suppress electron emission from the electron emitting surface of the cathode corresponding to this portion. A beam with this narrow diameter has the same space charge density as at high power, and no collapse occurs. Such conventional technology is
As disclosed in Berwick et al. US Pat. No. 4,023,061.
However, something like this poses another problem. The device disclosed in the above-mentioned Berwick patent controls the voltage applied to the control grid such that during low power operation only a small diameter beam of the beam emitted from the cathode passes. This makes the space charge density at low output equal to the space charge density at high output. However, narrowing the electron beam at these low powers reduces interaction with radio frequency signals within the TWT's circuit cavity. Radio frequency signals amplified within the cavity generate an axially extended cylindrical electric field concentric with the hollow cylindrical beam tube. For the radio frequency signal to be amplified, electrons within the beam tube must interact with the concentric electric field. However, due to its inherent characteristics, this electric field is concentrated on the inner peripheral surface of the cavity of the electron beam tube. Therefore, if the diameter of the beam is reduced, it will no longer interact with this radio frequency signal since it is much smaller than the diameter of the beam tube. above
The grid structure disclosed in the Berwick patent suffers from these interactions, typically reducing electrical efficiency by as much as 50% at low power. Additionally, the structure disclosed in Berwick's US patent had a flaw in the physical configuration of the grid structure. This control grid consists of two concentric grids. This inner grid has a smaller diameter and provides control at low power. Also,
These two grids cover a large area and
Provides control during high output. The two grids are physically and electrically isolated, with the support members of the inner grid positioned across the flow of electrons emitted at high power. Therefore, this support member obstructs the flow of electrons at high outputs and disturbs the electron radiation pattern at high outputs. In these conventional devices, the beam diameter at low output is smaller than the beam diameter at high output, resulting in lower electronic efficiency at low output. In addition, the grid structure of this electron gun has a problem in that the first control grid is supported concentrically with respect to the second control grid, which disturbs the electron radiation pattern when the outer control grid is activated. It was hot. SUMMARY OF THE INVENTION An object of the present invention is to ensure that the diameter of the electron beam at low power is approximately equal to the diameter of the electron beam at high power, and the interaction between the beam at low power and RF energy is different from that of the electron beam at high power. The object of the present invention is to provide an electron gun having an efficiency comparable to that of interaction with RF energy. Another object of the present invention is to provide an electron gun in which the electron emission pattern is not disturbed by a support member in the electron beam stream at high output. For these and other purposes, features and advantages are achieved by a novel grid structure for an electron gun. This new grid structure has a shadow grid that is different from conventional shadow grids and is electrically and physically insulated from the cathode of the electron gun. The shadow grid of the present invention is
The electron gun is configured to have a potential higher or lower than the potential of the cathode depending on whether the electron gun is in a high output state or a low output state. This control grid is connected to a voltage adjustable power supply to switch between high and low power modes. Further, in the present invention, the voltage biases of the two grids can be adjusted separately and independently. Since this voltage is regulated and the control grid has an equal effective area in both high and low power modes, the diameter of the electron beam in both high and low power modes is approximately be equal. Also, unlike the two control grids disclosed in the Berwick patent mentioned above, only one control grid is provided, so that support members etc. in the flow of the beam are provided. The flow of electrons is not disturbed.

[Brief explanation of drawings]

Figure 1 is a schematic diagram showing the structure of a conventional electron gun and grid, with particular attention to the beams at high and low powers; Figure 2 is a perspective view of a typical cathode; A diagram showing a conventional control grid that controls the TWT during output; Figure 4 is a diagram corresponding to Figure 3 showing a control grid that controls the TWT during high output; Figure 5 shows the electron gun of the present invention. and a partial cross-sectional view of the grid structure; FIG. 6 is an end view of the shadow grid and corresponding control grid of the present invention. DETAILED DESCRIPTION OF THE INVENTION In order to better understand the features of the present invention, the structure and operation of a conventional multi-mode TWT and associated electron gun, schematically shown in FIGS. 1-4, will first be described. This typical multimode electron gun includes an electrically heated cathode element 100 configured to emit electrons from its concave emitting surface 102, which may be spherical, paraboloid of revolution, etc. or formed from a hyperboloid of revolution. The cathode element 100 is made of tungsten material impregnated with barium, for example. Also, the grid structure is
A shadow grid 110 connected to the cathode 102 through a connecting member 112, a radially outer control grid 120 and a radially inner control grid 130.
These are composed of the radiation surface 10
Controls the emission of electrons from 2. Electrons are emitted from this emitting surface and, after passing through the grid structure described above, are focused into a beam by a focusing electrode 140 and accelerated by an accelerating anode 150. After this, this beam enters the TWT circuit 160 shown in FIG. The physical characteristics of the electron beam incident on the TWT circuit are determined approximately by the geometry of the grid structure and the bias voltages applied to the grid elements, shadow grid 110 and control grids 120, 130. As shown in FIG. 1, the diameter D _L of the electron beam at low power is much smaller than the diameter D _L of the electron beam at high power.
This difference in diameter is due to the difference in shape and size of control grids 120 and 130 shown in FIGS. 3 and 4, respectively. The control grid 130 includes an outer annular mounting member 132 and a central circular grid member 134, which is connected to the mounting member 132.
and a radial member 136 supporting and connecting to the radial member 136. The control grid 120 also includes an outer annular mounting member 122 and a central circular opening 124. An annular grid member 12 is provided inside the mounting member 122.
6 is provided to form the opening 124 described above. This opening 124 is connected to the grid member 134 described above.
This grid member 126 is slightly larger than
The grid member 134 occupies an area approximately equal to the entire area of the radiation surface 102 of the negative 100.
The area of this grid member 134 is the area of the cathode 100.
The area of the annular grid member 126 is approximately equal to the area of the annular portion 106 of the cathode 100. When the electron gun shown in FIG. 1 is operated in a low output mode, the cathode 100 is first heated to emit electrons. The control grid 130 is also biased to a positive voltage with respect to the cathode 100.
This attracts electrons emitted from the central portion 104 of the cathode 100. Also, the control grid 120 described above is biased to a negative voltage with respect to the cathode 100. Therefore, the annular part 1 of this cathode
Electrons emitted from 06 are inhibited from flowing through this grid structure. Therefore, in the low power mode, only the electrons emitted from the central portion 104 of the cathode 100 form an electron beam. Therefore, the diameter D _L of this electron beam is relatively small as shown in FIG. Therefore, the diameter D _L of this beam is much smaller than the diameter of the beam tube formed by the walls of the TWT circuit 160, and the interaction of this electron beam with the rf energy (mainly concentrated near the walls) is The interaction efficiency decreases to about 50% of that in high power mode. Also, when operating in high power mode, both control grids 120 and 130 apply a positive bias voltage to the cathode 100. Thus, grid member 134 attracts electrons emitted from central portion 104 of cathode 100, and grid member 126 attracts electrons emitted from central portion 104 of cathode 100.
Attracts electrons emitted from 6. Therefore, in high output mode, these grid members 1
Electrons emitted from the entire emission surface 102 of the cathode 100 are attracted by both 34 and 126.
The diameter D _L of the electron beam in this high power mode is sufficiently large as shown in FIG. This larger diameter electron beam interacts more efficiently with the rf energy concentrated near the walls of the TWT circuit 160. However, at this high output, the cathode 100
The electrons emitted from the annular portion 106 of the control grid 120 are attracted to and must pass through the annular grid 134 of the control grid 120 before passing through the support member 136 of the control grid 130. Since the support member 136 is in the path of the electrons attracted to the control grid 120, it interferes with and disrupts the flow of electrons, thereby disrupting the electron beam pattern at high power. This conventional shadow grid 110 also protects the control grids 120, 130 from overheating and melting during high power operation. The shadow grid 110 is shaped to match the shape of the control grids 120, 130 described above. Accordingly, the shadow grid 110 includes an externally disposed annular mounting member and a grid shaped and dimensioned to match the grids 134 and 126 described above. The radially extending grid members of shadow grid 110 are configured to mate with radial grid members 137 of control grid 130 and radial grid members 127 of control grid 120, respectively. Similarly, the circumferentially extending members of shadow grid 110 are configured to mate with circumferentially extending members 138 and 128 of control grids 130 and 120, respectively. This shadow grid 110 is connected to the cathode 100 so that it is at the same potential. When operating in high power mode, this electron beam current is approximately 4 Amps. The area of the circumferential and radial grid members of the control grids 120, 130 is approximately 10% of the electron emitting surface 102. Therefore, 10% of this beam current i.e.
0.4 amps are intercepted by these two control grids. In this high power mode, the control grid is powered 300 volts higher than the cathode, so the power dissipated by the control grid is approximately
It is 120 watts. This power is not immediately dissipated in the control grid, causing it to overheat and melt. However, this electron emitting surface 102 and control grids 120, 13
Since a shadow grid 110 is disposed between the electron emission surface 102 and the electron emitting surface 102, the electrons collide with the circumferential grid members and radial grid members of this shadow grid and are annihilated by the shadow grid 110, which has a low potential. be done. A corresponding minimum number of electrons in the circumferential and radial grid members of this control grid is therefore blocked. This shadow grid 1
10 is 10% of the electron beam (i.e. 0.4 amps)
and shield control grits 120 and 130. Since this shadow grid 110 is at zero potential with respect to the electron emitting surface 102, no power is consumed. The operation and characteristics of the conventional electron gun and its grid structure have been described above.The features and advantages of the grid structure of the present invention will now be described with reference to FIGS. 5 and 6. The electron gun shown in FIG. 5 has the same structure as the electron gun shown in FIG. 1 above, except that the cathode and grid structures are different. The electron gun shown in FIG. 5 is equipped with a heated cathode having an electron emitting surface 102. The electron gun shown in FIG. The emitted electrons are then controlled by the control grid, focusing electrode 140 and accelerating electrode 1.
Move to 50. And this electron beam
Inject into the TWT circuit. The grid structure of the present invention differs from grid structures used in conventional multimode TWTs in a number of ways. A shadow grid 210 of the present invention is shown in FIG. The grid consists of an annular mounting member 212 and an electrically conductive inner grid structure 213. This grid structure 213 includes a plurality of radial members 214 and circumferential members 21 that intersect with each other.
It consists of 6. These members 214 and 216 are formed of molybdenum material and intersect each other to form a plurality of 0.100 inch by 0.060 inch openings 218. This shadow grid 2
10 is provided electrically and mechanically insulated from the cathode 100. This shadow grid 210 can therefore be electrically biased and set to a potential different from that of the cathode 100 by means of a variable voltage power supply and corresponding to the mode of TWT. The structure is such that a high or low voltage can be applied. This shadow grid 210 acts as a seed for a control grid (220 in FIG. 5) similar to shadow grid 110 in FIG. 1. The feature of this shadow grid 210 is that it is provided electrically and mechanically insulated from the cathode 100 and operates at a different potential from the cathode. This shadow grid 110 generally includes an electron emitting surface 102.
approximately 0.003 inch (0.0762 mm) axially away from the Control grid 220 is of similar construction to shadow grid 210 except that its mounting member is wider than that of shadow grid 210. The grid area of control grid 220 is equal to the grid area of shadow grid 210.
The radial and circumferential grid members of the control grid 220 are also aligned to match the radial and circumferential grid members 214 and 216 of the shadow grid 210. Therefore, just as the shadow grid 110 shields the control grids 120 and 130, the shadow grid 210 also serves to shield and protect the control grid 220. When we use the term "aligned" to refer to the radial and circumferential grid members of shadow grid 210 and control grid 220, we mean that these members are aligned in a direction parallel to the direction of flow of electrons in the beam; This means that the grid 220 is arranged so that it is not exposed to electrons. It is known that if the grid matches its position and potential to the equipotential surface of the electric beam, the trajectory of the electrons will not be disturbed by the grid (except for the electrons that collide with the grid). This grid becomes “transparent” to the electron beam (“An Ultra-Laminar Terode Gun for High
Duty Cycle Applications” by Richard True
IEEE IEDM, pp. 286-289, 1979). During operation, the shadow grid 210 is maintained at a slightly positive potential and positioned according to the principles described above,
A high quality beam can be obtained. Both the shadow grid 210 and the control grid 220 are transparent to the beam. This electron gun operates like a gridless electron gun. Therefore, a well-focused layered beam is obtained during this high power operation. Also, because the shadow grid operates at a lower voltage, there is less heating due to blocking the beam current. Also, in the low power mode, the shadow grid 210 is maintained at a slightly negative potential with respect to the cathode 100. The position of the shadow grid relative to this cathode 100 does not change. This shadow grid is no longer in a "transparent" position. Since a slightly negative bias voltage is applied to this shadow grid, the emitted electron current is reduced. Thus, the space charge density of the beam can be reduced without reducing the diameter of the grid. However, this slightly negatively biased shadow grid 210 disturbs the trajectory of electrons passing through the grid. Instead of a subsequent highly laminar trajectory, this negatively biased shadow grid 210 imparts a large transverse velocity component to the emitted electrons. This increase in transverse kinetic energy of the beam increases the "beam transverse temperature." This increase in cross-beam temperature reduces the space charge density, which in turn creates a magnetic field that compresses the beam. Additionally, this increase in cross-beam temperature at low power counteracts this decrease in space charge density, making it possible to maintain the beam diameter at low power approximately equal to the beam diameter at high power. becomes. This phenomenon is explained by “Optical Theory of Thermal
“Velocity Effects in Cylindrical Beams”, G.
Herrmann, 1958, Journal of Appliud
Physics” Vol. 29, page 127.
It is known as Hermann's optical theory. The balance radius R (Hermann's radius) of a thermal beam in a magnetic field is the Brillouin radius R _BR , the beam temperature T, the focusing field B, and this focusing field at the position of the pole.
If B _C : =R _BR [1/2 + 1/2 (B ² / _C r ⁴
/ _C / B ² R ⁴ / _BR +2kmT ^r2 / _C / e ² B ² R ⁴ / _BR ) ^1/2 ] ^1/2 ...
(1) becomes. Here, K, e and m are respectively
Boltzman constant, electron charge, and mass. Also, the radius of the cathode of this electron gun is r _C. Moreover, the above R _BR is related to the space charge effect, and therefore: B ² R ² _BR =√2PΦ/πε _O (e/m) ^3/2 ...(2). where P is the space charge limited current and ε _O is the absolute permittivity of free space (8.855×10 farads/m)
and Φ is the beam voltage. Equations (1) and (2) above are “Verfication and use of Harmann′s
Optical Theory of Thermal Velocity Effects
in Electron Beams in the Low Perveance
Rqgin”, K.Amboss, IEEE Trans.ED,
Vol. 11, page 479 (1964). In a typical multimode X-band TWT, the space charge limited current varies from P=1.0 μ _P in high power mode to P=0.2 μ _P in low power mode.
Other variables in the high output mode and in the low output mode are shown in Table 1.

【table】

[Table] Table 2 shows the results of solving equations (1) and (2) above using the values in Table 1. In Table 2, when T is zero, the limit radius is R, and when _BC is zero, the temperature radius is R.

[Table] As is clear from this second table, P = 1.0, T
= 1400°K high output mode and P = 0.2, T =
The beam radius is approximately the same in both low power modes at 18000°K. This analysis result is #162
A CGH-P type standard electron gun was modified, and the shadow grid 210 had a slight (±)
A positive or negative voltage (50V) can be applied. Also, this modified electron gun
It was installed on the 8725 standard TWT made by Hughes Aircraft. In high power mode, this TWT has a positive voltage of 10 volts applied to the -25000 volt cathode. Control grid 220 is also set to -24699 volts (i.e. 301 V positive voltage with respect to the cathode) and 4
A normal operating current of amperes flows. The best beam transmission is achieved at 82.5%. This transfer efficiency is comparable to that of an unmodified electron gun in which the shadow grid was electrically connected to the cathode. Also, from 9.7
RF power of 15KW is achieved in the 9.9GHZ band. Also, in low power mode, the beam current is reduced to 1 amp and the shadow grid 210 is
potential of -25040 volts (i.e. with respect to the cathode)
115V positive voltage). Beam transmission is 90
%, RF transmission is achieved by 88%. On the other hand, in the conventional PPM type TWT shown in the figure, beam transmission is 92% at high output, and 50% at low output. A multimode electron gun is thus obtained with two grids, a shadow grid and a control grid. The shadow grid is connected to a variable voltage power supply and maintained at a slightly positive or negative voltage with respect to the cathode. In low power mode, this shadow grid is maintained at a slightly negative voltage (40 volts negative) with respect to the cathode, thereby increasing the beam cross temperature and compensating for the reduction in space charge density. Therefore, this electron beam can maintain its diameter against the focusing force of the magnetic field. Another feature of the present invention is that an existing electron gun (ie, a single-mode electron gun) can be modified into a multi-mode electron gun. first,
The shadow grid is decoupled from the cathode so that it can be maintained at a different voltage than the cathode, ie, at a voltage supplied by a variable voltage power supply. next,
The voltages on the shadow grid and the control grid are controlled such that the cross-beam temperature increases. Also, by providing a plurality of spaced apart "shadow grids" with adjustable applied voltages, a multi-output mode (ie three output mode) type electron gun can be achieved. Although the present invention has been described above with reference to FIGS. 1 to 6, these figures and embodiments are only for illustrating the present invention. Those skilled in the art can change the structure, materials, etc. without departing from the gist of the present invention. The features of the invention are limited by the scope of the following claims.