US3226595A - Low noise electron gun - Google Patents

Description

1965 J. BERGHAMMER ETAL 3,226,595

LOW NOISE ELECTRON GUN Filed March 31, 1960 INVENTORS Ja /{Away it'id/MMMEA 74/VZE) 5100M United States Patent 3,226,595 LOW NOISE ELECTRON GUN Johannes Berghammer, Princeton, and Stanley Bloom, Plainfield, N.J., assignors to Radio Corporation of America, a corporation of Delaware Filed Mar. 31, 1960, Ser. No. 19,079 1 Claim. (Cl. 315-16) The present invention relates to an apparatus and method for producing a low noise electron beam from a thermionic cathode. The invention is useful in any electron beam tube requiring a low noise beam, such as traveling wave tubes and beam klystrons.

The electron stream or beam emitted by a thermionic cathode emerges with random noise in the form of current and velocity fluctuations due to the random emission of electrons from the cathode and to their thermal velocity spread, and other causes. This noise consists of an infinite number of waves at different frequencies. When the beam is used in an amplifier tube for amplifying signals in a given operating frequency range, the noise waves having frequencies in that range are amplified along with the signal and result in an undesirably low signal-tonoise ratio, or noise factor. Thus, it is desirable to reduce or minimize the initial noise in the beam in the operating frequency range as much as possble.

Most so-called low noise tubes now in use merely transform the initial noise in the beam in the gun region so that the beam enters the helix or other R.F. interaction region in a favorable condition, without actually reducing the total noise content of the beam appreciably.

The object of the present invention is to produce a low noise electron gun that will reduce or attenuate the total noise in the beam from a thermionic cathode to a very low value.

A further object is to combine such a noise reducing gun region with a subsequent exponential transformer region ahead of the RIF. interaction region of a traveling wave tube, for example, to obtain a minimum overall noise factor for the tube.

These and other objects .are attained, in accordance with the present invention, in an electron gun comprising a thermionic cathode and at least two annular electrodes spaced substantially along the beam path from the cathode by applying suitable low positive voltages to the two electrodes relative to the cathode to produce a low velocity beam having a virtual cathode in the region between the two electrodes and having a plasma frequency in the virtual cathode region substantially greater than the desired operating frequency.

In the accompanying drawing:

FIG. '1 is an axial sectional view of the electron gun portion of an electron beam tube embodying the present invention;

FIG. 2 is an enlarged axial, sectional view of a modified cathode that may be used in FIG. 1;

FIG. 3 is a graph used in explaining the invention, and

FIG. 4 is an enlarged axial sectional view of an alternative cathode and associated accelerating electrode that may be used instead of the cathodes of FIGS. 1 and 2.

In FIG. 1 is shown a beam tube having an envelope 1 containing an electron gun structure 3 comprising a thermionic cathode 5 having an electron emissive face 7 on one end and a recess 9 at the other end for receiving a conventional heater (not shown). Preferably, the cathode 5 is cylindrical with a circular end face 7 coaxial and normal to the longitudinal axis of the envelope 1. The cathode 5 may be formed with a relatively narrow peripheral portion of the end face 7 sloped away from the central portion of the end face 7 and provided with an annular emissive coating 1'1, as shown in FIG. 2. The narrow coated sloped edge reduces erratic edge emission of electrons that contributes toward the noise in a beam from a cathode having a square edge as shown in FIG. 1, limits the emission to the outer portion of the cathode to produce a uniform hollow beam 13 along the axis of the tube, and increases the lifetime of the cathode. Moreover, the sloped surface permits the equipotential surfaces to form more smoothly around the emitting surface.

The beam 13 emitted by the cathode 5 is accelerated by relatively small positive voltages applied to two aligned afinular accelerating electrodes 15 and 17 spaced substantially along the beam path from the cathode and spaced apart by a distance d, as shown in FIG. 1. An example of a suitable potential distribution along the beam edge between the cathode 5 and electrode 17 is shown by the curve 19 at the top of FIG, 1. Preferably, the positive voltage applied to electrode 15 is not greater than about 10 volts and that applied to electrode 17 is from about 3 volts to about 30 volts, in order to keep the average beam velocity v to a relatively small value.

To produce the desired virtual cathode in the space between electrodes 15 and 17, and also to make the plasma frequency, w in the virtual cathode substantially greater than the operating frequency, two thoustand megacycles, for example, requires a relatively large beam current density, of the order of ma./cm. In order to assist the accelerating electrode 15 in drawing out the required beam current, an additional annular accelerating electrode 21 is coaxially positioned around the cathode 5. This electrode is preferably of truncated-cone shape with its inner edge positioned behind the end face 7, as shown in FIG. 1, so that it can be operated at a small positive voltage of several volts without intercepting electrons of the beam. The field of electrode 21 fringes around in front of the end face 7 and thereby increases the beam accelerating electric field gradient.

The electrons in the beam 13 are confined or constrained to traverse paths parallel to the axis of the tube by an axial magnetic field, such as that produced by a coil 23 coaxially surrounding the envelope 1.

Preferably, separate adjustable voltage sources 25, 27 and 29 are used to apply the desired positive D.C. voltages to electrodes 15, 17 and 21 respectively, relative to the cathode 5, as shown schematically in the drawing.

In the operation of the tube, the cathode is heated by conventional means to thermionic emitting temperature and the voltage sources 25, 27 and 29 are adjusted, for the particular geometry used, to cause a virtual cathode to be formed in the beam path between electrodes 15 and 17. The virtual cathode is indicated by the dip of the voltage distribution curve 19 slightly below the base line, which is the Zero or cathode potential. It will be shown how the electron gas or plasma constituting such a virtual cathode can be caused to support only evanescent waves, i.e., perturbations that decay in the direction away from the source, and hence, reduce the total noise in the electron beam.

First, consider a stationary (non-drifting) electron gas without thermal or other motions. The oscillation frequency, w, of the electron gas is then just the plasma frequency, w Thus we write Next, suppose that a velocity spread exists in the electron gas in a confining magnetic field in the z direction. Let 6 represent the velocity spread; +6/2 is the highest velocity, and -6/2 is the lowest velocity. Then, the dispersion Equation 1 becomes Solving this equation for [3, we obtain two solutions, as follows:

(10/110 (.0 /U (.0 a1.2= ,1 /1a (1 where the subscripts l and 2 for #2 represent the two roots (or waves) for the plus and minus signs, respec tively, and

is the relative velocity spread. In terms of beam temperature a =3kT /mv where k is Boltzmans constant and T is the beam temperature in degrees Kelvin. Equation 4 may be written fiLz fio flp where and The term [3,, is the plasma propagation constant. Examination of Equation 5 shows that B is imaginary when Fig 6) in which case [3 (Equation 4) is complex. It is known that if the propagation constant of a medium is complex, waves propagated by that medium must be either amplified or reactively attenuated by the medium. By using criteria given in a paper entitled Kinamatics of Growing Waves, by P. A. Sturrock, Physical Review, vol. 112, No. 5, pp. 1488-1503, December 1, 1958, it can be shown from dispersion Equation 3 that for these waves there is no growth with time possible, hence, these waves are evanescent.

Thus it can be seen that the total noise fluctuations in an electron beam can be reduced by establishing conditions in the beam path satisfying condition 6.

FIG. 3 shows the curve 31 for the equation The shaded area above this curve represents condition 6. It can be seen that condition 6 includes the following which means that some electrons must have negative velocities. This, in turn, means that a virtual cathode must exist in the electron stream to satisfy conditions 6 and 8. Condition 9 says that the plasma frequency cu (Bi /7716 11 .where e/ m is the charge to mass ratio of the electron,

i is the beam current density, and e is the dielectric constant of a vacuum, this requires a relatively high D.C. current density i,,. If, for example, the virtual cathode is produced under conditions in which a is not much greater than unity, it is necessary that u be substantially greater than 0:, in order to satisfy condition 6. Conversely, if u is not much greater than w, a must be substantially greater than unity.

The electron gun shown in FIG. 1 is designed to produce both a virtual cathode and a large D.C. current density. The gun is operated, in accordance with the present invention, with applied voltages such that (l) a virtual cathode is formed in the beam path between electrodes 15 and 17, and (2) the plasma frequency in the virtual cathode is substantially greater than the operating frequency of the tube.

In order to produce a virtual cathode in a beam be-- tween two spaced electrodes, the beam current density must exceed a certain critical value, given, for example, on page of an article entitled Effects of Space Charge in the Grid-Anode Region of Vacuum Tubes, by Salzberg and Haeif, RCA Review, January 1938, as follows:

ara' m in c.g.s. units. Converted to practical units and present nomenclature, Equation 12 becomes where V and V are the positive voltages applied to electrodes 15 and 17, respectively, and d is the distance between these two electrodes. Thus, for a particular set of values for V V and d, it is necessary to inject a beam current density I which is somewhat greater than I from Equation 13. The combined accelerating field produced by electrodes 15 and 21 is sufiicient to inject the current density required to produce the desired virtual cathode between electrodes 15 and 17 and make w substantially greater than w.

The tube shown in FIG. 1 further includes three additional annular accelerating electrodes 33, and 37 spaced along the beam path beyond electrode 17. The last accelerating electrode 37 includes a drift tube portion 39 to which a helix 41 is connected by an RF. coupling portion 43 adapted to be coupled to an external transmission line. Preferably electrodes 33 and 35 are positioned so that the distance between the virtual cathode (between electrodes 15 and 17) and electrode 33 is less than the spacing between electrodes 33 and 35, as shown, and suitable voltages are applied to electrodes 33, 35 and 37 by a voltage source 45 to produce an exponential rise in space potential from electrode 17 to electrode 37. As an example, the voltages applied to the various electrodes may be as follows:

Electrode: Volts 5 0 21 5 15 3 17 3 33 5O 35 37 250 Preliminary experiments have been made with a low noise traveling Wave tube having gun electrodes similar to those shown in FIG. 1 but without the sloped edge on the cathode, normally operated as an exponential transformer type gun to give an overall noise factor of 56 db. When this tube was operated, in accordance with the present invention, with suitable voltages on electrodes 15, 5, 17 and 21 to produce a virtual cathode in the beam between electrodes 15 and 17 an overall noise factor of about 3 db was obtained at a frequency of 3000 megacycles. It is believed that noise factors as low as 1 db can be obtained by optimum electrode design and operating voltages.

FIG. 4 shows an alternative cathode 47 in the form of a hollow cylinder having an annular fiat end face 49 coated with emissive material. The cathode 47 is coaxially surrounded by an annular electrode 51, like electrode 21 of FIG. 1. An additional accelerating electrode 53 in the form of a cylindrical rod is coaxially mounted within the hollow cathode 47 with a conical end portion protruding somewhat beyond the end face 49. In operation, low positive voltages are applied to electrodes 51 and 53, relative to the cathode 47, to augment the accelerating field of electrode 15. If desired the inner and/or outer edges of the cathode end face 49 may be sloped like the cathode face 7 in FIG. 1, for the same purpose.

What is claimed is:

An electron gun for producing a low noise electron beam along a predetermined axis, for use in a given operating frequency range, comprising a cylindrical thermionic cathode having a circular electron emissive end surface coaxial with and normal to said axis, the peripheral portion of said end surface being sloped backward and outward from the central portion thereof and coated with emissive material, two annular accelerating electrodes coaxially disposed along said axis in spaced relation to each other and spaced substantially from said cathode, and means including at least one voltage source connected between said cathode and said accelerating electrodes for applying such low positive voltages to said electrodes relative to said cathode as to produce a low-velocity confined beam having a virtual cathode in the region between said accelerating electrodes and having a relative velocity spread in said virtual cathode, where w is any operating frequency in said range, to is the plasma frequency and w is greater than (0, said means including a third annular accelerating electrode coaxially surrounding said thermionic cathode, with the cathode protruding into the space between said first and third accelerating electrodes.

References Cited by the Examiner RCA Review; June, 1960; page 244 cited.

GEORGE N. WESTBY, Primary Examiner.

RALPH G. NILSON, ROBERT SEGAL, Examiners.

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