The design and operation of a field emission array (FEA) cathode and the subsequent demonstration of the first FEA gyrotron are presented. Up to 10 mA from 30 000 tips was achieved reproducibly from each of ten chips in a gyrotron environment, namely, a vacuum 1 x 10(-8) mbar, -50 kV potential with multiple chip operation, The design parameters of the FEA gun were similar to those of a magnetron injection gun with an achievable electron beam current of 50-100 mA and measured power 720 W cw. Coherent microwave radiation was detected in both TE(02) at 30.1 GHz and TE(03) at 43.6 GHz, with a starting current of 1 mA

Cooke, S J

Cross, A W

Garven, M

Phelps, A D R

Spark, S N

English

University of Strathclyde Institutional Repository

Strathprints Institutional RepositoryGarven, M and Spark, S N and Cross, A W and Cooke, S J and Phelps, A D R (1996) Gyrotronexperiments employing a field emission array cathode. Physical Review Letters, 77 (11). pp. 2320-2323. ISSN 0031-9007Strathprints is designed to allow users to access the research output of the University of Strathclyde.Copyright c© and Moral Rights for the papers on this site are retained by the individual authorsand/or other copyright owners. You may not engage in further distribution of the material for anyprofitmaking activities or any commercial gain. You may freely distribute both the url (http://strathprints.strath.ac.uk/) and the content of this paper for research or study, educational, ornot-for-profit purposes without prior permission or charge.Any correspondence concerning this service should be sent to Strathprints administrator:mailto:strathprints@strath.ac.ukhttp://strathprints.strath.ac.uk/VOLUME 77, NUMBER 11 P HY S I CA L REV I EW LE T T ER S 9 SEPTEMBER 1996Gyrotron Experiments Employing a Field Emission Array CathodeM. Garven,* S. N. Spark,† A.W. Cross, S. J. Cooke, and A. D. R. PhelpsDepartment of Physics and Applied Physics, University of Strathclyde, Glasgow, G4 0NG, United Kingdom(Received 2 April 1996)The design and operation of a field emission array (FEA) cathode and the subsequent demonstrationof the first FEA gyrotron are presented. Up to 10 mA from 30000 tips was achieved reproducibly fromeach of ten chips in a gyrotron environment, namely, a vacuum 1 3 1028 mbar, 250 kV potential withmultiple chip operation. The design parameters of the FEA gun were similar to those of a magnetroninjection gun with an achievable electron beam current of 50–100 mA and measured power 720 Wcw.Coherent microwave radiation was detected in both TE02 at 30.1 GHz and TE03 at 43.6 GHz, with astarting current of 1 mA. [S0031-9007(96)00889-7]PACS numbers: 84.40.Ik, 52.75.VaField emission arrays (FEAs), which are central to thenew technology of vacuum microelectronics, are emerg-ing as novel sources of electrons. Development of FEAsat research centers around the world is proceeding withapplications in microwave devices and display technol-ogy [1–6] in which they provide a compact, controllableelectron source. FEAs have the potential to revolutionizefree-electron microwave devices, since electron emissioncan in principle be modulated by small voltages (10 V) tovery high frequencies (GHz) to effectively prebunch theelectron beam for coherent microwave generation. Addi-tionally, FEAs are “cold” emitters with a fast turn-on ca-pability, require no heater current, and permit high currentdensities with spatial and temporal control. Their uniquecombination of properties makes possible smaller, lighter,and more efficient microwave devices. This possibility isrealized in this work with the demonstration of an FEAcathode used in a millimeter wave device.Vacuum microelectronics involves electron transport invacuum via the use of modern semiconductor microfab-rication technology [1]. First proposed over 25 yearsago, thin-film field-emission technology has experienceda resurgence in interest as microfabrication technologyhas matured [7]. The first functioning micron scale fieldemission devices were made by Spindt [8] and continuousimprovements to this process have been made with corre-sponding improvements in performance [2]. The use ofFEAs in microwave power tubes was first discussed byBrodie and Spindt [4].In a gyrotron, mm-wave generation is due to thecyclotron resonance maser (CRM) instability. A beamof relativistic electrons undergoing cyclotron motion ina uniform axial magnetic field interacts collectively witha propagating electromagnetic wave, such that energytransfer occurs. The cyclotron frequency ( fc ­ vcy2p)of the beam in a magnetic induction B is given viavc ­eBgm0, (1)where g ­ 1 1 jeV jym0c2 for a beam accelerating po-tential difference V and eym0 is the electron chargeymassratio. The interaction frequency is determined by the cy-clotron resonance condition v ­ svc 1 kzyz , for a waveof angular frequency v and axial wave-vector componentkz , where s ­ 1 for the fundamental interaction. In thecase of the gyrotron, kzyz ¿ v so that the effect of aspread in axial velocity is minimal and the emitted radi-ation is only mildly Doppler up-shifted from the cyclo-tron frequency.For the TEmn or TMmn mm-wave mode of a cylindricalwaveguide, the wave dispersion relation v2 ­ c2k2' 1c2k2z must also be satisfied, in which the cutoff frequencyvco ­ ck' is mode dependent. Therefore, the gyrotroninteraction frequency occurs slightly above both the modecutoff and cyclotron frequencies.Gyrotrons developed previously at Strathclyde Uni-versity have been based on both field-immersed, fieldemission plasma flare cathodes [9–11] and thermionicmagnetron injection gun (MIG) type cathodes. Recentgyrotrons have employed a thermionic cathode with mea-sured long pulse operation up to 100 ms at 1 MW beampower and a conversion efficiency of 10% at 42 GHz [12].A third type of gyrotron based on a microelectronicFEA cathode is presented here. This was designed tobe a low power, ,1 kW electron beam system operatedboth cw and long pulse, À100 ms. Previously, short-pulsed plasma flare cold cathodes have been employed ingyrotrons, which produced up to 6 MW mm-wave outputat a pulse repetition frequency of 330 Hz. This repetitionrate was limited by the power supply energy deliveryand switching capabilities of the high voltage pulsedsystem. Utilizing FEAs promises ultimate repetitionfrequencies during pulsed operation well beyond this, asmodulation does not require the switching of the highvoltage power supply.The FEAs employed in this work were fabricated byGEC-Marconi Ltd. and consisted of silicon tips approxi-mately 1 2 mm high with 10 nm tip radii, aligned be-neath holes in an extraction grid electrode. The gridsconsist of a 0.5 mm layer of niobium supported by2320 0031-9007y96y77(11)y2320(4)$10.00 © 1996 The American Physical SocietyVOLUME 77, NUMBER 11 P HY S I CA L REV I EW LE T T ER S 9 SEPTEMBER 1996an SiO2 insulating layer, in which the silicon emittersprotrude through holes typically 2 mm in diameter. Pre-vious experiments demonstrated [13] that for arrays withup to 10 000 tips, average emission currents of 1 mA pertip could be obtained under ideal UHV conditions.The key parameters for the FEA cathode, which werebased on a thermionic MIG [12], were the annular cathoderadius, 12.5 mm, and the slope angle, 4–. These parame-ters were not changed throughout the course of the experi-ments. Previous studies [13] investigated various chipdesigns and the most suitable for a gyrotron applicationwas determined in terms of optimum emission current.The final emitter chip design used was 2.25 3 2.25 mm2in area with five 6000 Si tip arrays, mounted on a 6.6 mmsquare flat package (“flatpack”). The chip and flatpackdimensions along with the MIG parameters dictated that amaximum of ten chips would fit into the FEA gun geom-etry, Fig. 1. A current of 10 mA per chip was measuredindicating that the FEA cathode could achieve total emis-sion currents of up to 100 mA from the ten chips.One of the main advantages of this new type of cathodeover its traditional counterparts is in the ability to controlthe spatial distribution of individual components of thebeam. This was achieved by individually addressing eachpackage in the FEA cathode, accomplished by supplyinga separate control voltage to the Nb grid connectionson each chip, with a common Si tip cathode potential.Such control was also found necessary during multiplechip operation to permit optimum operation of each chip,due to the slight variation in the grid voltage required byeach chip to operate at its maximum emission current.An electrically isolated, fiber optically controlled powersupply unit with ten channels, all variable from 0 to150 V, was designed for this specialized operation.FIG. 1. Cross section through the head of the FEA cathode,with two of ten chips shown. For each chip, the silicon emittersare electrically connected to the common cathode ring via theflatpack base. Individual grid voltage control was achievedvia microscopic bond wires to the package leads, which wereelectrically contacted to the grid connectors.When mounted in the final electrode configuration theFEA cathode potential could be up to 250 kV. Withthe vulnerable structure of the FEA chip, flatpack mount-ing and the 0.25 mm bond wires exposed, the result-ing electric field enhancement would cause catastrophicbreakdown between the cathode components and otherelectrodes. To prevent this the chip mounting structurewas surrounded by a field-relieving cathode shield, Fig. 1.To allow emitted electrons to propagate from the FEAcathode, ten holes, aligned with the FEAs, were made inthe smooth contoured shield.The FEA cathode emission currents are consistentwith gyrotron starting currents, provided the gyrotron isoperated with a high cavity Q (.10 000). Cavity insertswere employed to increase the cavity quality factor, Q,thereby reducing the starting current. For these experi-ments a dc central intracavity magnetic field, variablefrom 0 to 9 T, was generated using a superconductingmagnet which provided a uniform homogeneous magneticfield, varying by ,0.1% over a length of 10 cm. Theexpected gyrotron operating characteristics were beamcurrent ,100 mA, gating voltage ,100 V, acceleratingpotential ,50 kV, emission area ,1.5 cm2, annular beamdistribution, operating vacuum ,1028 mbar with a cath-ode lifetime of .100 h, and both cw and pulsed operation.The FEA cathode assembled in the gyrotron with thecathode and cavity magnetic field coils and cavity insertsis shown in Fig. 2. Electron trajectory modeling was usedto verify the electron beam propagation from the FEAcathode to the interaction cavity. Thermal diagnosticswere employed to detect the electron beam transportthrough the interaction region.A standard UHV ceramic insulator was inserted inthe electron beam transport tube, between the FEAanode and the gyrotron interaction cavity, to electricallyisolate the cavity from the diode. The anode and cavitysections were separately connected to ground throughtwo ammeters, allowing measurement of the electronreturn current from each section. Current reaching thecavity wall after passing through the interaction regionwas therefore measured directly, independently of thecurrent flowing to the anode. In the experiment theanode current was minimized by optimizing the guidingFIG. 2. Overview of the FEA gyrotron, showing the elec-trodes, magnetic field coils, ceramic insulator, and cav-ity components.2321VOLUME 77, NUMBER 11 P HY S I CA L REV I EW LE T T ER S 9 SEPTEMBER 1996magnetic fields. This provided a useful diagnostic byallowing direct detection of the current participating inthe CRM interaction.Figure 3 summarizes FEA cathode operation in termsof IyV space, where I in mA is the electron beamcurrent measured to the gyrotron drift tube and V inkV is the accelerating potential. The lower left handcorner sectioned off by the curved line represents the areawhere the FEA chips operated with minimum signs ofstress, i.e., grid currents ,100 mA and emission currents,1 mA. Chips were operated cw in this area for lifetimesof tens of hours with minimal degradation in performance.Typically a chip would operate only 30 min outsidethis area before showing signs of operational stress andincreased grid currents.There are two points which must be emphasized withrespect to the future of FEAs in high power mm-wave devices, (1) optimization of chip lifetime and (2)increase of beam power. The region of operation withoptimized lifetime may be extended well beyond theupper half of Fig. 3 simply by operating multiple chips.This work suggests that chips of the design employed inthis work will operate successfully with emission currentsup to ,6 mA. Ten such chips would therefore supply,60 mA, at 40 kV to produce a 2.4 kW electron beam.This is a modest extrapolation of these results since bothmultiple chip operation and cw 10 mA operation from asingle chip have been demonstrated.In our experiments the maximum generated power waspower supply limited since operating the FEAs outside thesafe operation line resulted in increased grid currents. Atany one time only three chips were operated which for themaximum power of 720 W represents an average of 6 mAper chip at 40 kV. The maximum measured beam powerfrom a single FEA chip was ,375 W, emitting 9.37 mAwith an accelerating potential of 40 kV, corresponding toa current density of ,0.5 Aycm2.To verify that the electron beam passed through thecavity region, thermistor heat sensors were attached to theFIG. 3. Operating conditions, IyV , showing oscillating (j )and nonoscillating s–d operation. Data points refer to singlechip operation unless otherwise indicated.beam tube to determine the positions at which electronswere incident upon the cavity wall. With this diagnostic,the electric and magnetic fields required for optimal beamtransport through the interaction cavity were obtained.The FEA gyrotron mm-wave output was measured us-ing a Hewlett-Packard (HP) R8486D calibrated powermeter, which was capable of measuring powers in therange 100 pW to 1 mW. The mm-wave output wascoupled into the power detector via a variable (0–40 dB)in-line attenuator. At a cathode and intermediate an-ode potential of 232 kV and constant beam current of,2 mA, the cyclotron frequency was varied by sweep-ing the central intracavity magnetic field. This was sweptat a rate of 1 mTymin between 1.625 and 1.645 T, andbetween 1.130 and 1.150 T. Within each range, the de-tected mm-wave power increased by up to 5 ordersof magnitude, from ,10 nW to ,1 mW, in the ranges1.630–1.636 and 1.138–1.144 T, Fig. 4. This representsa transition from the incoherent background cyclotronemission, verified as varying linearly with the electronbeam current, to a collective emission process. The cy-clotron frequencies for the magnetic fields at resonancewere calculated using Eq. (1) to be 42.94 and 29.98 GHz,respectively. The gyrotron interaction frequency will beslightly greater due to a small Doppler shift, and will alsobe slightly above the mode cutoff frequencies of the TE03( fco ­ 43.59 GHz) and TE02 ( fco ­ 30.06 GHz) modes,respectively (cavity inner diam ­ 22.27 mm). Therefore,for frequencies closely satisfying the cyclotron resonancecondition, coherent emission due to the CRM instabilitywas observed.The angular distribution of the mm-wave energy radi-ated from the output coupling horn was measured usinga Ka-band crystal detector, scanned through the far-fieldradiation while operating conditions were kept constant.Output modes were identified by comparison with calcu-lated mode patterns to be TE02 and TE01 for cavity fields1.636 and 1.143 T, respectively. It was deduced from themeasured output mode patterns that mm-wave power wascoupling out of the cavity via down conversion of thewaveguide mode, following the pattern TE0n to TE0n21.FIG. 4. Detected microwave power of the FEA gyrotron nearto resonance with the TE03 operating mode. The transition tocoherent CRM emission is signified by an increase in outputpower of 5 orders of magnitude.2322VOLUME 77, NUMBER 11 P HY S I CA L REV I EW LE T T ER S 9 SEPTEMBER 1996An attempt was made to operate the gyrotron in the TE01cavity mode with cutoff frequency 16.42 GHz. However,this mode was cut off in the narrowed section of the wave-guide and, since mode conversion to a lower mode withsimilar angular momentum was not possible, no oscilla-tions could be observed.Measurement of the far-field radiation power densitygave an estimated 1 W absolute microwave output powerfrom this gyrotron, during operation with a 100 W elec-tron beam. This represents a significant conversion effi-ciency for a low current gyrotron of ,1%.These results demonstrate that FEA cathodes can beused not only in high power electron gun designs, but alsoin high power mm-wave devices. Relativistic electronbeams with powers of up to 720 W from FEAs werepropagated through the gyrotron cavity and radiatedcoherently via the CRM mechanism. It was shown thatFEA lifetimes were not unduly affected by high voltageoperation, given adequate shielding. Lifetimes of theorder of several hours of operation were achievable withcurrents of .5 mA per chip under nonideal conditions.Considering the present rate of research and device de-velopment activities throughout the world, it is likely thatthe next ten years should result in a number of prod-ucts based on vacuum microelectronic devices. This workdemonstrates one of the first such exciting applications,the operation of the first millimeter wave device employ-ing an FEA cathode.This work was supported by the SERC, DRA Malvern,and GEC-Marconi Ltd. Encouragement, advice and dis-cussions with N. Cade, D. F. Howell, R. D. Groves, D.M.Parkes, and K. Trafford are gratefully acknowledged.*Present address: University of Maryland, College Park,MD 20742 and Naval Research Laboratory, Washington,DC 20375.†Present address: Defence Research Agency, Malvern,United Kingdom.[1] T. Utsumi, IEEE Trans. Electron Devices 38, 2276 (1991).[2] C. A. Spindt, C. E. Holland, and R. D. Stowell, Appl. Surf.Sci. 16, 268 (1983).[3] C. C. Curtis and K. C. Hsieh, Rev. Sci. Instrum. 57, 989(1986).[4] I. Brodie and C.A. Spindt, Appl. Surf. Sci. 2, 149 (1979).[5] P. N. Lally, Y. Goren, and E.A. Nettesheim, IEEE Trans.Electron. Devices 36, 2738 (1989).[6] C. A. Spindt, C. E. Holland, A. Rosengreen, and I. Brodie,IEEE Trans. Electron Devices 38, 2355 (1991).[7] E. A. Adler et al., IEEE Trans. Electron Devices 38, 2304(1991).[8] C. A. Spindt, J. Appl. Phys. 39, 3504 (1968).[9] A. D. R. Phelps, A. Z. Maatung, and S.N. Spark, Int. J.Electron. 64, 369 (1988).[10] S. N. Spark and A.D. R. Phelps, J. Phys. D 22, 1807(1989).[11] S. N. Spark, A.D. R. Phelps, and P. R. Winning, Int. J.Infrared Millim. Waves 12, 885 (1991).[12] A.W. Cross, Ph.D. thesis, University of Strathclyde, 1993.[13] M. Garven, A. D.R. Phelps, and S. N. Spark, Vacuum 45,513 (1994).2323

Gyrotron experiments employing a field emission array cathode

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Gyrotron experiments employing a field emission array cathode

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