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Zero-Bias Gridded Gun

Field of the invention
The invention relates to electron guns widely used in linear-beam microwave tubes such as klystrons and
travelling-wave tubes . Such guns typically have a concave emitting cathode surface from which a converg ing stream of electrons is drawn by an accelerating anode in front of the cathode . The converged beam passes through a hole in the anode to enter the tube ' s interaction region .
Such guns are often made with a control grid covering
the emissive surface and spaced slightly from it . The control grid is usually driven by a rectangular-wave
pulser to produce a pulsed electron beam. The grid is pulsed negative wi th respect to the cathode to turn the beam off and intermittently pulsed somewhat positive
to turn the beam on for a short time .

Prior Art
Convergent electron guns for linear-beam tubes
typically have a focussing electrode surrounding the
emitting cathode to shape the electric fields for proper convergence of the beam. It has been known to insulate this focussing electrode from the cathode and use it as a control electrode to modulate the beam. The cut-off amplification factor for this control electrode is only
1.5 to 2.0 maximum. Hence, the modulating voltage to
pulse the beam completely off must be at least half the value of the beam accelerating voltage, making the cost, size and power consumption of the modulator unreasonable.
U.S. patent No. 3,183,402 issued May 11, 1965 to L. T. Zitelli illustrates an improvement to this control electrode. FIG. 1 is a schematic diagram of Zitelli 's gun. Concave cathode 14 is surrounded by the hollow focus electrode 15. In addition, a hole 19 through the center of cathode 14

OMPI contains an insulated central probe electrode 16 whose face projects beyond the surface of cathode 14. The electron beam drawn from cathode 14 by accelerating anode 18 is- thus slightly hollow because probe 16 is non emitting. Probe 16 and focus electrode 15 are tied to gether by a conductor 8. A pulsed modulating voltage may be applied to them to turn the beam on and off.
Alternatively, as shown in FIG. 1 which corresponds to FIG. 3 of the Zitelli patent, the control electrodes may be connected to a small positive bias voltage as shown and the cathode 14 is then pulsed negative via conductor 17 to turn the beam on. The addition of the center post electrode raised the cut-off amplification factor to about 3.0, thus making a modest improvement in the demands on the modulator. The control electrodes of this prior art cannot truly be classed as grids because they do not cover the surface of the cathode to produce a high amplification factor. Rather, they are removed from the electron beam and must exert their influence on the electric field from a distance, thus the low amplifi cation factor.
Actual grids covering the cathode surface and situa in the electron stream have been used in linear-beam tub where the duty cycle, that is the ratio of beam on-time beam off-time, is small. U.S. patent 3,843,902 issued

October 22, 1974 to George V. Miram and Gerhard B. Kuehn illustrates an advanced prior-art grid. FIG. 2, copied from the above patent, illustrates the general range of geometries used in the prior art. Here the generally co cave cathode 20 is substantially covered by a grid 22 spaced a distance d_ from its emissive surface 24. In th particular invention emissive surface 24 is composed of

OMPI large number of small concave depressions with non-emissive grid elements 26 covering the spaces between them. The conductive web elements 28 of control grid 6 are aligned with the non-emissive "shadow" grid elements 26 so that the small bea lets of electrons are focused through the apertures 29 of grid 6 and miss the conductive web
elements 28. Since grid 6 is run positive with respect to cathode 20 when beam current is being drawn, any interception of electrons by web elements 28 causes undesirable secondary emission as well' as heating of the grid and consequently thermionic emission from it. Such a grid can provide quite a high amplification factor, of the order of 100 or more depending on the ratio of grid element spacing a_ to grid-to-cathode spacing cL FIG. 2 illustrates a typical state-of-the-art geometry where _a is 1.5 to 2.0 times d_. It was known from the prior art of receiving-type grid-controlled radio tubes that the grid element spacing could be made large so that when the grid operated at zero bias useful currents could be drawn to the anode.
However, it was universally believed in the linear beam tube art that unbearable distortions of the trajectories of the electrons in the beam would result unless the grid were operated at a potential equal to the potential that would occur in space at the position of the grid if the grid were not there. FIG. 3, taken also from U.S. patent No. 3,843,902 illustrates the electron trajectories and the equipotential surfaces calculated for a section of the gun of FIG. 2. The uniformity of the equipotentials in the grid aperture inside element 6 shows that the grid potential was indeed very close to the space
potential .

Summary of the Invention
An object of the invention is to provide a convergent linear-beam gun with a control grid of fairly high ampli-fication factor which can be operated at a potential no more positive than the cathode. A further object is to provide a gun whose current can be switched on and off with a low pulse voltage. A further object is to provid a gun with a grid which requires no voltage bias with respect to the cathode, thereby simplifying the modulato A further object is to provide a gun which can be operat at very high duty cycles without excess heating of the grid.
. These objects have been achieved by a novel configuration of the grid elements. The inventors found the surprising results that, when the spacing between grid web elements is many times the spacing between the grid elements and the cathode surface, the grid can then be run at cathode potential while drawing current from the cathode without distorting the electron trajectories so seriously that the tube would be unusable. This result was not derivable from the widely spaced grids of the receiving-tube art, because in that art the shape of the electron trajectories after passing through the grid was not highly critical as it is in the linear-beam tube art

Brief Discussion of the Drawings
FIG. 1 is a sketch of the prior arrangement of non-intercepting beam control electrodes.
FIG. 2 is a sketch of a prior-art gun in which the grid was operated at a potential positive with respect to the cathode.
FIG. 3 shows the electron trajectories and equipote tial lines of the gun of FIG. 2.
FIG. 4 is a schematic partial section of a gun according to the present invention.
FIG. 5 shows the range of geometries of typical prior-art guns compared to the geometries of successful guns embodying the invention.

Description of the Preferred Embodiments
FIG. 4 is a partly perspective, partly sectional sketch of a gun embodying the invention. Thermionic cathode 30 has a spherically concave emissive surface 31 such as an oxide coated surface. Cathode 30 is supporte

— by a thin metallic cylinder 32 of low thermal conductivity, from a rigid support member 34 which latter is eventually mounted on a vacuum envelope and cathode voltage insulator (not shown). A heater 36, shown schematically, raises cathode 30 to a thermionic emitting temperature. A grid structure 40 is supported on a grid insulator 42 from
mounting member 34. A conductive grid lead 44 traverses through a hole in insulator 42 to connect with external grid lead 46 insulated from' supporting member 34 by in-sulating member 48. Grid 40 comprises radial and azimuthal web members 50, 52 which are disposed a small distance ά in front of emissive surface 31. Apertures 53 in grid 40 between web members 50, 52 have transverse dimensions <k which are much larger than the grid-to-cathode spacing d .
A hollow anode 54 may be included as part of the electron gun or alternatively may be built and regarded as a
separate element. Anode 54, when operated at a high
positive voltage with respect to cathode 30, draws a converging stream of electrons 56 which pass through an
aperture 58 in anode 54 to form the required linear-beam outline 60. The novelty of the gun lies in the combination of the method of operation and the novel geometric arrangement which makes this operation possible.
FIG. 5 illustrates the range of geometries involved compared to the prior art. The left-hand side of FIG. 5 is taken from the well-known book "Vacuum Tubes" by K. R.
Spangenberg, McGraw-Hill, New York, 1948. It illustrates the range of geometries covered by various approximate formulas used in the prior art for calculating the ampli-fication factor. The variables are the screening fraction S, which is just the fraction of the cathode shaded by the diameters of the assumed round parallel wires, and the cathode-grid spacing factor, which is the ratio of the spacing between grid wires to the spacing from grid wires to cathode. The different cross-hatchings repre--sent the regions for which various approximate formulas apply. Note that cathode-grid spacing factors below

- EXtr
ι> IPO ^
2 ftNAT\θ about 2.5 are the range considered by this comprehensive prior-art review. At the right-hand side of FIG. 5, the cross-hatched region 5 illustrates the range of geometri which have been found workable in the present invention with zero grid bias. It is likely that a more extensive range of cathode-grid spacing factors may be useful, including ratios around 10 and perhaps as low as 5. For ratios above 15 the amplification factor becomes quite low. For a gun with microperveance 1.0 the amplifica-tion factor was a useful value of 9.
Another factor which affects the perfection of focus of the beam is the ratio of the cathode-grid spacing d_ to the overall diameter D of the cathode. The inventors have found that good beam optics can be main-tained when d/D lies between 0.01 and 0.04.
It will be obvious to those skilled in the art that if the control grid is constrained to remain at cathode potential during the time beam current is drawn, that increasing the amplification factor by increasing the screening fraction or decreasing the cathode-grid spacing factor will inevitably decrease the perveance of the gun. Thus, a compromise between amplification factor and perveance must always be made. Other limitations are that the cathode-grid spacing factor a/d must be quite large and the ratio d/D must be small to avoid undue distortion of the electron trajectories. When these conditions are fulfilled, one can use the desirabl method of modulation in which the grid is at zero bias during the current pulse, thus eliminating electron bombardment of the grid with consequent overheating and secondary- emission, and also simplifying pulse modulator. The invention thus comprises this desirable method of modulation.
It will be obvious to those skilled in the art that many embodiments may be made within the 'scope of the invention, which is intended to be limited only by the following claims and their legal equivalents.