US3489944A - High power field emission microwave tube having a cathode delivering nanosecond relativistic electron beams - Google Patents

Description

3,489,944 AVING A GATHODE z WD/SEIHEIdINV I I l I l O Q WD/S3100? INVENTOR ALf-:C s. DENHOLM SAM v. NA BV f Filed May 27, 1966 S. DENHOLM ETAI- HIGH POWER FIELD EMISSION MICROWAVE TUBE H DELIVERING NANOSECOND RELATIVISTIC ELECTRON BEAMS v @v om Jan. 13, 970

M ATTORNEY United States Patent Olice 3,489,944 Patented Jan. 13, 1970 3,489,944 HIGH POWER FIELD EMISSION MICROWAVE TUBE HAVING A CATHODE DELIVERING NANOSECOND RELATIVISTIC ELECTRON BEAD/IS Alec S. Denholm, Lexington, and Samuel V. Nablo, Stoneham, Mass., assignors to Ion Physics Corporation, Burlington, Mass., a corporation of Delaware Filed May 27, 1966, Ser. No. 553,501 Int. Cl. H01j 25/10, 7/16, 19/10 U.S. Cl. 315--5.39 9 Claims ABSTRACT OF THE DISCLOSURE This invention relates generally to microwave tubes and more particularly to microwave tubes, such as klystrons, which employ microwave cavities excited by electron beams.

For many years one of the limitations in the design of high power microwave devices of the velocity modulation type, such as the klystron, has been the lack of cathode structures capable of high power operation.

In such devices, as they are now known to the art, maximum performance is limited by the electron gun and by voltage breakdown problems.

The present invention provides a means whereby power output of such devices can be raised several orders of magnitude to at least 1011 Watts.

The present invention not only increases the voltage capability of such devices but also boosts by at least two orders of magnitude the maximum current obtainable from such tubes.

The present invention achieves these and other results by the use of voltage gradients which hitherto were believed unavailable in such tube structures.

The present invention thus provides a means whereby high power pulses hitherto believed unattainable may be achieved in microwave tube structures, such as klystrons.

Broadly speaking, these and other advantages can be Obtained in such tube structures by substituting therein for the usual cathode a field emission electron accelerator device capable of providing high voltage, high power electron pulses whose duration is in the order of a few nanoseconds.

The invention thus possesses other features and advantages, some of which, with the foregoing, will be set forth in the following description of the invention taken in conjunction with the drawings, wherein:

FIGURE 1 is a simplified plan view in cross section of a klystron incorporating the present invention;

FIGURE 2 is a plan view in cross section of a radial multi beam klystron incorporating the present invention;

FIGURE 3 shows some characteristics of the pulsed electron beam; and

FIGURE 4 is an isometric presentation of a single pulse.

Referring now to FIGURE l, there is shown a klystron incorporating the basic elements of the present invention. The klystron is used herein only for purposes of illustration and it should not be construed from such a use that the invention is limited to klystrons. The klystrons used in this example comprises an electron accelerator structure 12 forming one end of an elongated envelope and a collector electrode structure 14 forming the other end of the envelope. The intermediate portion of the envelope is formed by the radio frequency interaction means or body 15 of the tube which comprises a plurality of drift tube sections 16, 17, 18, 19 and 20 spaced from cach other to form interaction gaps 22, 24, 26 and 28 and a plurality of cavity resonators 30, 32, 314, 36 bridging the interaction gaps 22, 24, 26 and 28.

The cavity resonator portions each comprise two metal end walls 38 and a ceramic cylinder 40 sealed between the end walls 38. The ceramic cylinders 40 provide vacuum type envelope walls and radio frequency windows through which radio frequency oscillations may pass into the exterior nonevacuated portions of the cavity resonators represented by dotted lines 42. Such exterior portions of the resonant cavities are conductive thus electrically connecting the drift tube sections and completing the RF interaction means or the body 15 of the tube.

A ceramic cylinder 44 is sealed between each end wall of the resonant cavity portion 36 adjacent the collector electrode 14 and a metallic flange 46 on the collector electrode 14 thus insulating the collector electrode from the remainder of the klystron.

The electron accelerator or injector structure 12 consists of three parts; a gas filled coaxial capacitor, a gas discharge switch and a field emission cathode. The capacitor is typically about 3 meters long (depending on pulse duration desired), has a capacitance of about 280 pf. and consists of an outer electrode 50 and an inner electrode 53. The outer electrode 50 is a metallic tube, capable of being pressurized (eg. to 400 p.s.i.), which is sealed at one end with an insulating disc 52 and secured at the other end to a metal ground plane 51 carried by tube body 15. This ground plane 51 has the anode 59 centrally located therein. The inner electrode 53 is insulatively maintained in spaced relationship with the outer electrode 51 by suitable devices (not shown) and electrically connected at one end through a switch 49 and a resistor 54 to a power supply 55. In the preferred embodiment this central electrode would be a cylinder resonantly charged by an insulating core power supply through an isolating inductance. The other end of electrode 53 is coupled through an electrical discharge switch 61 to a field emission cathode 57.

This field emission cathode 57 is mounted within an evacuated housing which is secured to ground plane 51, around anode 59, directly in line with the switch 61 and the central electrode 53. This housing comprises a conducting cap plate 56, carrying the cathode 57, sealed to an insulating envelope 58 which is in turn sealed to the ground plane 51. Preferably the envelope 58 is capacitively graded and formed of dielectric rings (not shown) and alternating metal ring electrodes (not shown). The length and geometry of this envelope is such as to prevent electrical iiashover. The tip of cathode 57 is maintained typically about 3 cm. from the anode 59 and the geometry at the tip is such as to provide suitable field emission characteristics. The anode 59 may be a screen, an orifice or a thin foil. Anode 59 is preferably a titanium foil approximately 0.005 cm. thick when it is desirable to atmospherically isolate the interior of housing 58 from the interior of the tube body 15.

As mentioned previously the capacitor is gas filled. The filling so used may be for example a mixture of nitrogen and carbon dioxide or sulphur hexafluoride at high pressure such that electrical breakdown is inhibited between electrode 50 which is maintained at ground potential and central electrode 53 which is maintained negative with respect to ground.

It should at this time be noted that the capacitor could be replaced with either a lumped or distributed transmission line, or the coaxial line could be insulated by a liquid or solid dielectric.

When switch 49 is closed a voltage is permitted to build up between the housing wall 50 and the center line 53. When the voltage reaches a predetermined level switch 61 is closed and a gas discharge is initiated between line 53 and plate 56. This breakdown can be initiated by the use of an auxiliary triggering electrode, by a laser beam or simply by overvolting.

This discharge transfers the energy stored in line 53 to plate 56 and electrode 57 in a controlled fashion determined by the transmission line properties of 53 and Ythe field emissionrcharacteristics ofthe field Yemission gapY existing between electrode 57 and ground plane 59. Upon transfer of this energy to electrode 57 a very high potential appears between electrode 57 and ground plane 59 causing electrons to be extracted from electrode 57 by means of the field emission process. These electrons are attracted toward anode 59 and because of their momentum the electrons pass therethrough into the tube section where they may be acted upon in a normal klystron manner.

If desired a conventional magnetic system (not shown) may be provided around the drift tube sections to prevent the electron beam from spreading excessively. Normally, however, this magnetic control can be dispensed with in the present invention for it has been discovered that by permitting a small amount of residual ionizable gas to remain in the tube body 15 that a continuously self constricting electron beam, having a sinusoidal lmotion can be created. This is possible in the present invention because of the brief, intense beam currents provided by the present invention.

Such self constriction provides many benefits whereby the overall complexity of the tube body 15 and associated equipment may be reduced. This feature is highly desirablc.

The following brief treatise discusses this effect and the theory behind it.

This effect of self-constiction of an electron stream was predicted to occur only when the streams azimuthal magnetic field achieves sufficient intensity. The inventors have discovered however that when the stream is relativistic, drifting in the absence of a longitudinal field that a partial neutralization of the stream by positive ions will be such as to aid an insufficiently intense magnetic field whereby a self constriction of the beam occurs.

The relativistic aspect of the beam is obtained through the use of the described electron injector 12 while partial neutralization of the electron stream is achieved by creating positive ions in the tube body 15. These ions are created by the beam from a gas maintained, at a partial pressure of 10v4 to 103 torr in tube body =15.

This is already distinguishable from normal tube practice where great lengths are taken to eliminate all possible gas atoms in order to prevent ionization and thus collector burnout.

The problem of collector burnout is not encountered in the present invention for when magnetic self constriction of the beam is used the collector is situated in the beam at a point of lowest possible electron density.

The generalized perveance of such a partially neutralized electron beam may be written:

2 2z K sa (l f) 1) where v=Ne2/mc2, 'y=(12)% and f is the charge density ratio, Ni/Ne in the beam. For space-charge neutralized conditions of a self-constricted beam, K is negative and analysis of the particle equations of motion in the radial electric and azimuthal magnetic fields results 4 in sinusoidal electron motion with a characteristic wavelength.

x=21rb/(-K) f2 where b is the initial beam radius. For the conditions typically encountered in conventional electron guns, the effect of the self-focusing magnetic forces on the beam trajectories are so small as to be undetectable. In fact, the electrostatic pinching of a partially neutral stream due to the effects of the beam-generated plasma, a phenomenon commonly referred to as gas-focusing, dominates beam behavior at voltages up to kev. and normally precludes well-defined observations of magnetic selffocusing effects. Y, V t ,i

This effect is readily distinguishable from the self pinching effect noted in current carrying plasmas for in such plasmas the longitudinal electric field is not zero.

The described invention generated an electron stream which met the above described characteristics and was periodically self constricted.

The characteristics of the pulsed electron beam transmitted into the drift region by the window have been determined by a number of techniques: primarily a magnetic probe was used to measure and hence Itotal; movable calorimetric probes were used to measure and map integrated pulse power; an external fast scintillatorphotodiode monitor to measure the radiation (current) profile; and direct current probes for the determination of current density. In addition, a technique for simultaneous longitudinal mapping of the radial profile of the beam current density was employed using thin-film dosimetry techniques applicable to this region that is, (0.1-5 megarads per pulse). It is with this diagnostic technique and the calorimetric probe that the self-focusing effect found in this apparatus has been studied in some detail. A pulsed electron beam current of 17,000 amperes with a FWHM value of 20 nanoseconds at a beam energy of 2.5 mev. was determined for the normal capacitor charging potential of 3.6 mv. with an entrance beam radius b=l.5 cm. Energy decrement in the window was found to be 50 kev.

If the relevant ionization cross section for air at these energies s estimated then the drift tube pressure required to yield complete stream neutrality (f=l) in a time t after the beginning of the pulse can be calculated. For the entrance conditions used, the mean electron density at the window, Ne, was found to be on effective ionization cross section of 10-18 cm.2 was assumed. Since Ne=je/q,8c and N1=jeaNa/ q, where NEL is the neutral density and je the electron current density, then f: (UNaBc). For a charge density ratio of unity, Nal: (ac)1. Since t can be no more than the 2X 10-8 sec. duration of the beam, Na must be greater than 1.7 1015 cm.-3 or 0.04 torr when unimpeded radial flow of the secondary electrons from ionization in the radial field of the beam is assumed.

The results of a typical run taken with calorirnetric probes located at z=20 cm. are shown in FIG. 3. The probes used consisted of stainless steel discs l6 mm. in diameter which were located with respect to the symmetry axis at r=0 cm. and r=0.8 cm. The probes response at each pressure point was related to the flux of both energy and charge as shown on the ordinates. Pulse reproducibility from the accelerator is sufficiently good (-3%) that no data normalization was required. As shown, maximum compression of the beam was realized at a pressure of 0.2 torr and, for this case, with less than 20% beam energy loss. The time dependent behavior on the beam current density observed with axial current probes in independent experiments revealed its expected degradation during the early portion of the pulse when aNact L No correction was made for the electron backscatter losses expected in the probe (about at 2.5 mev.) in reducing the data. The general features of this curve are determined by the motion of the secondary electrons from the beam-plasma.

An isometric presentation of the data taken during a single pulse at a beam energy of 2.5l mev. is shown in FIG. 4. The peak values of je shown in the figure are of reduced accuracy due to the failure of the film monitoring technique at current densities above 1800 amperes/cm.2 (5 megarads delivered dose). Independent calorimetric measurements have shown current densities of over 5000 amperes/crn.2 at the first pinch (5.5 cm.). The integrated profiles near the entrance plane indicate l0=17,000 arnperes which falls off to about 13,000 amperes beyond the first pinch, the main loss occurring from the divergent component of the stream at the beam periphery near the entrance window. The 17,000 a. figure has been confirmed by bremsstrahlung measurements.

Using the relations for the generalized perveance as given in Eq. 1, a value of K=0.33 is derived for a cylindrical beam equivalent to that used to obtain the data of FIG. 3 under fully compensated (f=1) conditions. Using the predictions of the simplified model from Eq. 2, the characteristic wavelength A=16 cm. for the entrance radius b=l.5 cm. The expected periodic motion of the drifting 2.5 mev. stream has been observed over 3 nodes (at 5.5, 13 and 19 cm.) with a gradual degradation of the sinusoidal shape of the envelope into cylindrical geometry and is not subject to the usual instabilities. It has been propagated over 3 meters with a loss of but one-half in current density and total current at a drift tube pressure of 0.3 torr. The wave length of this self constriction in the electron stream was found to be about 13 cm. and its frequency determined at approximately 2.3 10+g c.p.s.

As is well known in the klystron art radio frequency oscillations may be introduced into the resonant cavities. Such oscillations will produce varying electrostatic fields across the interaction gaps associated with such resonant cavities to modulate the velocity of the electrons in the beam as they cross the interaction gaps. Such modulation of the velocity of the electrons in the beam results in bunching of the electrons as they proceed along the drift tube. The bunches of the electrons thus obtained cause varying electrostatic fields at each of the succeeding interaction gaps as they pass thereacross thus introducing oscillations in the cavities associated with such gaps. Such oscillations will reinforce the electrostatic field at each of the gaps which will further increase the bunching of the electrons as they proceed along the drift tube. The oscillations produced in the cavity adjacent the collector electrode will be of the greatest intensity and may be coupled out of the cavity. The electrons proceed onto the collector electrode 14 where the residual energy is dissipated in the form of heat.

Because in the described apparatus the center line 53 may be for example the terminal electrode of an electrostatic belt generator capable of having a potential of at least three million volts and currents of in excess of kiloamperes it is desirable that the target 14 be of low Z material and locally shielded in order to reduce and to prevent leakage of X-radiation generated by the electrons striking the target. Since the line 53, as described, substantially acts as a pulse generator a controlled repetitive discharge occurs. This line, which is shown in its simplest form, could also be fashioned in the Blumlein form and switching could be accomplished at ground potential. Each time line 53 discharges to plate 56 electrons are extracted from electrode 57 and enter the tube section 15 in short bursts or quanta of high energy electrons.

For this reason the varying electrostatic fields appearing at each of the interaction gaps 22, 24, 26 and 28 as they pass thereacross will introduce significantly greater oscillations in the cavities associated with the gaps. These oscillations of course reinforce the electrostatic field at each of the gaps which further increases the bunching of the electrons as they proceed along the drift tubes. In this instance because the bunching has already occurred and the subsequent interaction gaps further increase the bunching, the oscillations produced will be of significantly greater intensity than any previously obtained in the prior art.

With reference now to FIGURE 2 there is shown a slight modification of the present invention. This figure shows a sectional view of a radial klystron where there is provided a plurality of klystron bodies radially arranged around an evacuated chamber 160. Each body 115 is identical to body 15 of FIGURE 1 and the Same numbers refer to the same component parts. Disposed in the chamber is a multipoint field emission cathode 157 each point of which is each maintained in close proximity to a respective anode 159. The supporting cylinders 151 in this view must be of ceramic or glass or other suitable dielectric material to insulate the pulse voltage and permit the desired electric field between the foil windows 159 and the electrodes 157. In this instance for the sake of simplicity that portion of the accelerator 12 equivalent to line 53 and its associated apparatus has been replaced by black box labeled pulse generator.

This device operates in exactly the same manner as the previously described apparatus.

By means of the apparatus described in this FIGURE 2 waveguide problems, etc. are decreased or eliminated. The described apparatus may be used in radar applications and the like in which the output power per channel must be reduced. For example if 10 klystron bodies 115 are radially arranged around chamber 160 the power per body is reduced by one order of magnitude. Furthermore, since each electrode 157 is simultaneously emitting electrons each klystron body will receive a precisely phased quanta of electrons which are identical in power and energy thus reducing much of the necessary switching apparatus and associated equipment needed to achieve the Same results in present day radar applications.

It should of course be understood that regenerative coupling means can be added to the described apparatus. Since such means are old and well-known to the art they are shown here, in FIGURE 1, as a black box 191 coupled to each cavity 30, 32, 34 and 36. It is, of course, obvious that such regenerative coupling means could also be added to the apparatus of FIGURE 2.

Having now described several embodiments of the present invention, it is respectfully requested that the invention described herein be limited only by the following claims and not by the specific embodiments described.

What is claimed is:

1. A high power microwave amplifying device for amplifying micro-wave signals to power levels at least millions of watts comprising means for producing a kilo ampere charge of electrons, a c-ollector of low Z rnaterial to reduce X-radiation generated `by said electrons striking the collector, and a radio frequency interaction structure coupling said producing means and said collector, said producing means comprising at least one field emission cathode, a source of energy coupled to said cathode, means for delivering the energy from said source to said cathode and a ground plane between said cathode and said radio frequency interaction structure to extract said energy from said cathode, after said energy has been delivered to said cathode in the form of a nonasecond relativistic electron 4beam through said structure to said collector, said structure comprising a plurality of alternating drift spaces and resonant cavities with electric `fields established in said cavities to bunch the electrons in said beam by alternatively increasing and decreasing their velocities.

2. The device of claim 1 wherein there is a multiplicity of said collectors radially disposed about said producing means and a multiplicity of said interaction structures coupling each of said collectors to said producing means and said means comprises a plurality of field emission cahtodes radially disposed in line with said interaction structures and said collectors whereby each of said collectors receives a proportionate share of said stream.

3. The device of claim 1 wherein said means comprises an electrostatic belt generator whose terminal forms the central conductor of a coaxial capacitor coupled by a gas discharged switch to a field emission cathode structure housed in an evacuated structure and isolated from said interaction structure by a vacuum gap and a grounded anode.

4. The device of claim 1 wherein said means comprises a pulse forming system consisting of a power supply electrically connected to a fluid insulated transmission line coupled by a gas discharge switch to a field emission cathode contained in an evacuated envelope, said cathode being electrically isolated from said interaction structure -by a vacuum gap and a grounded anode.

5. The device of claim 1 wherein the -pressure in said interaction structure is maintained between 10*3 and 104 tOrr.

6. The device of claim 1 wherein said interaction structure contains means for aiding the azimuthal magnetic field associated with said electron beam to cause self constriction of said beam to assure a periodic pattern while said beam is passing through said interaction structure to said collector, said pattern having a frequency in the order of 109 c.p.s. being characteristic of self focusing drifting electron beams connotating the motion characteristic of relativistic electrons moving under the influence of self-generated magnetic fields.

7. A microwave electron discharge device comprising a housing containing means for generating a kilo ampere pulse of megavolt electrons, said generating means comprising a pulse generator, a field emission cathode coupled to said pulse generator, and means for delivering the energy from said pulse generator to said cathode, means for acting on said pulse to amplify a microwave signal and means separating said acting means from said generating means, said separating means comprising a ground plane between said cathode and said acting means to extract said energy from said cathode after said energy has been delivered to said cathode, into said acting means, said acting means comprising a collector of low Z material to reduce X-radiation generated by said electron striking said collector, a plurality of alternating electrodes and cavity resonators, means for establishing electric elds in said cavities to velocity -modulate electrons passing therethrough, and regenerative coupling means between said cavities to create sustained oscillations in said device.

8. The device of claim 7 wherein said pulse generator comprises an electrostatic belt generator whose terminal forms the central conductor of a gas insulated capacitor coupled through a gas discharge switch to the field emission cathode.

9. The device of claim 7 wherein said -pulse generator comprises a high voltage generator connected to the central conductor of a gas insulated capacitor coupled through a gas discharge switch to the field emission cathode.

References Cited UNITED STATES PATENTS 2,958,804 11/1960 Badger et al. 315-539 3,054,962 9/1962 Opitz 328-227 3,131,300 4/'1964 Jeter et al. 328-227X 3,270,243 8/1966 Kerst 328-228X 3,344,298 9/1967 Martin 313-57 OTHER REFERENCES Techniques For The Study of Self-Focusing Electron Streams, by Graybill et al., April 1966, Proceedings of the 8th Annual Electron & Laser Beam Symposium, on pp. 465-470 relied upon.

HERMAN KARL SAALBACH, Primary Examiner SAXFIELD CHATMON, JR., Assistant Examiner U.S. Cl. X.R.

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