US2925528A - Electronic valves - Google Patents

Description

Feb. 16, 1960 M. c. HARTNELL-BEAVIS 2,925,523

ELECTRONIC VALVES Filed Dec. 14, 1956 FIG. 3 WE n e S t P t fl ELECTRONIC VALVES Michael Carl Hartnell-Beavis, Newbury, England Application December 14, 1956, Serial No. 628,382

Claims priority, application Great Britain December 15, 1955 6 Claims. (Cl. 315-39) This invention relates to electronic valves having an electrode structure producing a current of stably bunched electrons by the multipactor effect. The multipactor effect is a phenomenon associated with surfaces whose secondary electron emission coefficient can, within a certain primary electron energy range, be greater than one,

and is defined as the buildup of a density modulated electron current due to secondary electron emission in a structure supporting an alternating electric field.

In the specification of Mulletts copending patent application Serial No. 628,381,, filed December 14,1956, an

electronic valve is described which has a cathode structure comprising parallel plates adapted to support a radio.- frequency electric field normal to theirsurfaces and to generate electrons from both plates by the multipactor effect, with one of said plates perforated such that an anode placed beyond the perforated plate collects those bunches of electrons which pass through the perforations.

An electronic valve in accordance withthe present invention comprises a perforated cylindrical sheet cathode one surface of which has an effective secondary emission coefficient greater than unity, an electrode coaxial with and facingsaid one surface, an anode coaxial with and facing the other surface and means for setting up in the electrode/cathode, space (hereinafter referred'to asthe multipacto r space) both an axial magnetic field and a radial R.F. alternating electric field, the arrangement and applied fields being such that electrons omitted from the emissive surface return towards and either pass through or strike said surface, those passing through being col-.

lected bylthe anode. n n

The interaction of the electrons with the electric and magnetic fields tends to bunch the electrons for certain values of the magnetic field towards a stable phase with respect to the phase of the alternating-electric field and a density modulated electron current builds up in the cyclic repetition of this process. The maximum obtainable circulating electron current is limited by space charge because the field attracting the secondary electrons away A frequency multiplying amplifier valve according to the present invention may be constructed similarly to said amplifier valve and having the interaction space so dimensioned that the alternating field which builds up in the interaction space is a multiple of that in the multipactor space.

A triggered oscillator valve according to the present invention may be constructed similarly to the amplifier valve but including means for feeding back a fraction of the power generated in the interaction space into the multipactor space, an associated tuned cavity coupled to the interaction spacegoverning the frequency of oscillation and means for feeding sufiicient power into the multi-.

pactor space to initiate a multipactor build-up of electron invention embodiments thereof will now be described byv way of example with reference to the accompanying drawing, of which.

Fig. l is a diagrammatic axial section through the multipactor and interaction spaces of one embodiment of the invention.

Fig. 2 is a cross section on line 11-11 of Fig. 1.

Fig. 3 is a diagram showing the phase notation, used in describing the mode of operation and Fig.4.is a section similar to Fig. 1 through one embodiment of a triggered oscillator valve according to the invention.

In Figures 1 and 2 the multipactor space M is the an nulus between a central electrode 1 and a coaxial perforated cylindrical cathode 2; the interaction space I is the I annulus between the cathode 2 and a coaxial anode 3.

Solenoidal windings, (not shown) around the anode 3 may be themeans of providing the ,necessarylaxial magnetic field which is substantially uniform overthe multifrom the emissive surface is depressed by the charge of the electron bunch approaching this surface, and in order to reduce this limitation a D.C. voltage is applied across the multipactor space tending to accelerate the electrons away from the emissive surface. The cathode being perforated, some of the bunches of electrons pass through the perforations into the anode/cathode space, hereinafter termed the interaction space, across which is applied a D.C. voltage tending to accelerate the electrons away extracting power from the alternating field in the interaction space.

pactorspace -M. The iemissive, cathode surface-,provid-. ing the secondary-electrons is, in this embodiment, the inner surface of the cathode 2. R.F. power enters the multipactor space along input coaxial line formed by an extension 1a of the electrode 1 and cylindrical extensions 2a of the cathode 2 and is reflected at an end disk 4 which terminates the coaxial line and which is so spaced that a maximum of the R.F. field occurs in the centre of the perforated cathode 2. A negative DC. voltage is applied to the cathode 2 to provide both a D.C. field across the multipactor space M and an accelcrating field across the interaction space I. It is to be understood that these fields may equally well be pro vided by applying appropriate positive potentials to the centre electrode 1 and outer anode 3. The valve is enclosed by a cylindrical outer wall 3a, of which anode 3 forms a part, and this outer wall 3a is extended in a direction opposite to the extension 1a of the anode 1 to provide, in conjunction with an inner conductor 3b, a coaxial output line fromthe interaction space I. D.C.

open circuits and R.F. short circuits are provided at 6 and 7 and at a convenient position along the cylindrical extension 2a such that the extension 2a, supported on insulating supports, is isolated for the application of the D.C. negative potential. The interaction space I is so dimensioned that the end wall 8 thereof is electrically where n=0, 1, 2 wavelengths of the alternating field axially distant from thecentre of. the perforated cath j Patented Feb. 16, 196i).

odei. :The whole of the space inside the outer wall 3:: is evacuated to a low pressure and may be continuously pumped through ports situated at convenient points along the input and/or output lines.

In Figure 3 is shown the phase notation-'for' the-electrons inlthe multipactor space M. An electron leaving the inner surface of the cathode 2 with an initial phase 45 with respect to the time when the field is zero, is accelerated away from this surface and its trajectory is bent by the magnetic field as shown diagrammatically at 9 in Figure 2 so that it arrives back at the cathode surface with a phase with respect to the R.F. zero field. The initial phase which is stable for cyclic repetition is defined as and is such that =2n+ where n=1, 2, 3, and the essential conditions for a bunched buildup are that the impact energy is such that the effective secondary emission coefficient of the cathode surface is greater than one and that electrons emitted earlier and later than the stable phase are bunched towards this stable phase. The effective secondary emission co: efiicient is to be understood as the ratio of secondary electrons to primary electrons taking into account any losses which occur (e.g. through the perforations). These conditions are satisfied for certain values of the magnetic field strength. In those regions beyond the cathode where it is not desired, and where conditions are favourable for a multipactor build-up of electron current, it must be inhibited by making the surfaces of a material, or of such a geometry, that they have an effective secondary emission coetficient less than one. In addition the spacing of the annular gap can be so reduced that the multipactor build-up cannot occur.

Since the electrons continue to turn under the influence of the magnetic field when passing through the. perforations in. the cathode 2 and in the interaction space I, the cathode must be made as thin as is mechanically practicable and the spacing between cathode 2 and the anode 3 of thevinteraction cavity must be small. The gain in tangential velocity by the electrons in traversing the interaction space is dissipated as a heatloss on impact with the anode 3. Reduction of the interaction space gap reduces this loss and increases the efficiency of the DC. to R.F. conversion, but it also increases the R.F. losses so that a compromise spacing isrequired.

"Itis to be understood that in an'a1ternative embodiment the multipactor space .may be situated outside'the interaction space with the emissive cathode surface on the outside, the electrons passing radially'inward through perforations in the cathode into the interaction space.

Since the space charge limited circulatingelectron current in the multipactor space M .is approximately proportional to the DC. voltage biasacross the multipactor space, and the output power for a given current is approximately proportional to the DC. anode voltage applied across the interaction space I, modulation or pulsing may be applied by'varying either or both of these voltages. Pulsing may also be obtained by pulsing the R.F. input. For high power operation forced cooling may be employed. One example of a triggered oscillator according to the invention is shown in Figure 4. The multipactor space M is between the centre electrode 1 and the cathode 2. The interaction space I around the cathode 2 is situated between said cathode 2 and the coaxial outer anode 3. A negative D.C. voltage is-applied to the end wall 4 of the cathode extension 2a through rod 10 which passes through a hole in the outer wall 3:: and is supported, together with the cathode structure 2, 2a, by the insulating and sealing cap 11. The multipactor space M is so dimensioned that at the operating wavelength the cathode 2 is at a voltage maximum and the end 12 of the cathode extension 2a is electrically one half wavelength distant from the centre of the cathode. The, interaction space I is dimensioned such that at the operating wavelength its centre iaaxially, at or near the voltagemaximum with respect cavity 14 is coupled through an iris 15 (between end wall 13 and cathode extension 2a) to the interaction space and the exact frequency of operation is looked through this coupling to that of the cavity 14. The cavity 14 may be made externally tunable by known means. The axial magnetic field may be provided by solenoidal windings :round theoutside of the valve.

The extension 1a of the electrode 1 is, in the embodiment shown, the outer conductor of .a'coaxial line having an inner conductor 16. RF, power is initially fed through this line into the multipactor space M by any convenient coupling means; a hole'coupling 17 is shown schematically in Figure 4. "R.F. power is required to be fed through coaxial line 16 to initiate the current build-up since an impact energy in the region of 300 electron volts must be imparted to the electrons in the multipactor region to obtain a secondaryemission coefficient greater than one. When the electron current in the multipactor space M builds up, R.F. power is developed in the interaction space as described with reference to Figs. 1-3. This power is fed out along a coaxial output line of which the outer wall 3a constitutes the outer conductor and the extension In of the electrode 1 the inner conductor and a fraction of the power, as initially determined by the spacing of the feed gap at 12, is fed back into the multipactor space M. The power absorbed is determined finally by the space charge limitation of the circulating current in the multipactor space. The oscillations may be pulsed and/or modulated by varying the voltage appliedto the multipactor cathode 2 and the valve mayJbe evacuated through ports conveniently situated along the output line.

It is to be understood that feed back of power from sheet cathode, one surface of said cathode having an.

effective secondary emission coefiicient, greater than unity, an electrode coaxial with 'and facing said one surfaceto form a multipactor space therebetween, an anode coaxial with and, facing the other surface of. said cathode to form an interaction space therebetween, coaxial extensions of said cathodeand said electrodefor feeding R.F. power into said multipactor space, coaxial extensions of said cathode and said anode for extracting power from said interaction space, said multipactor and interaction spaces each having an end wall spaced from the center of said perforated cathode whereby said cathode and electrode 'form with said first mentioned extensions a first shorted coaxial line having a voltage antinode substantially at said center at the operating 'frequency, said cathode and said anode forming with said second-mentioned extensions a second shorted coaxial line having a voltage antinode substantially at said center at the operating frequency, and means for applying an axial magnetic field within theinteraction space.

2. An R.F. valve as claimed in claim 1 wherein said electrode is located coaxially within said cathode.

3. An R.F. valve as claimed in claim .2 wherein means are provided for feeding back a fraction of the power generated in the interaction space into the multipac tcnspace, a cavity resonator is coupled to the interactlon action space, and means are provided for feeding sufiicient R.F. power into the multipactor space from an external source to'initiate multipactor build-up.

5. An R.F. valve as claimed in claim 4 wherein said feeding means for initiating multipactor build-up comprises a conductor arranged coaxially within said electrode to form therewith a third shorted coaxial line and a coupling hole in said electrode through which R.F. power from said third line may be fed into the multipactor space.

6. An R.F. amplifying valve comprising av perforated cylindrical sheet cathode, one surface of said cathode having an eifective secondary emission coeflicient greater than unity, an electrode coaxial with and facing said one surface to form a multipactor space therebetween, an anode coaxial with and facing the other surface of said cathode to form an interaction space therebetween, means for applying an axial magnetic field within the multipactor space, means whereby R.F. power may be fed into the multipactor space, means whereby R.Fa

References Cited in the file of this patent UNITED STATES PATENTS 2,140,285 Farnsworth Dec. 13, 1938 2,172,152 Farnsworth Sept. 5, 1939 2,216,169 George et a1. Oct. 1, 1940 2,226,077 Snyder Dec. 24, 1940 2,295,396 George Sept. 8, 1942 2,574,562 Hansell Nov. 13, 1951 2,674,694 Baker Apr. 6, 1954

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