Anderson, James

Shapiro, Michael

Temkin, Richard

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aPSFC/JA-05-1Experimental Studies of Local and GlobalEmission Uniformity for a MagnetronInjection GunJames P. Anderson, Richard J. Temkin, and Michael A. ShapiroJanuary 2005Plasma Science and Fusion CenterMassachusetts Institute of TechnologyCambridge, MA 02139 USASubmitted to IEEE Trans. on Electron Devices.11 l1Experimental Studies of Local and Global EmissionUniformity for a Magnetron Injection GunJames P. Anderson, Richard J. Temkin, and Michael A. ShapiroPlasma Science and Fusion Center, Massachusetts Institute of Technology,Cambridge, MA 02139AbstractWe report measurements of the azimuthal emission nonuniformity of a gyrotron's thermionic cathode as determinedfrom two experiments, one which determines the total emission and one which scans the emitted beam using a rotatingprobe. In the total emission experiment, which measured the total collector current over a range of voltages, the cathode'stotal work function spread was found to be 0.033 eV at T = 1000 'C, centered around 1.88 eV. In the subsequentexperiment, which used a mechanically rotatable current probe, global and local effects were investigated. Voltage wasvaried to examine emission nonuniformities in different regions of cathode operation. While emission became almostcompletely uniform in the low-voltage space-charge limited regime, emission varied as much as 50 % at high voltages. Inanother part of the current probe experiment, current-voltage (I-V) curves were measured at azimuthal locations in 300increments for several cathode temperatures. From this extensive set of data the work function distribution parameterswere identified over small sections of the cathode for the entire cathode surface.The efficiency of microwave radiation produced by high power microwave sources is particularlysensitive to the electron beam quality. The beam for these devices must have low momentum andenergy spread along with high spatial emission uniformity to ensure the best possible efficiency. Highpower gyrotrons extract microwave energy from an annular electron beam produced from a thermioniccathode in a magnetron injection gun (MIG). Theoretical studies [1] and experimental research [2], [3]of gyrotrons have shown that the emission uniformity of the annular beam can have a significant effecton the amount of energy available for exciting a particular electromagnetic mode. Mode competitiondue to an electron beam of poor quality may severely limit the efficiency of a gyrotron.The electron beam quality is largely dependent on the quality of the cathode. For a gyrotroncathode the quality is determined by both its temperature uniformity and its work function uniformity.Cathode emission is generally described by two equations, the Child-Langmuir law, in the low-voltagespace-charge limited regime, and the Richardson-Dushman equation, in the high-voltage temperaturelimited regime. Numerous studies have attempted to model the transition between these two regimesof operation [4][5]. One approach, which is used in this study, assumes distributed parameters tocomine the two equations into one expression [6]. For a work function distribution which is Gaussian,with a central work function value 0, and work function spread o, the emission current density JV ata particular voltage value V may be expanded to [7]:KV I/2 21 # o 2 '4 4Jv (00, a) = 2 +1erf + O - erf + x2 1 (uV 2 a v[-e eE_ e uexp -T 0- (1)kT41rEo 2kTwhere K is the perveance of the beam (dependent on cathode geometry), A0 is the Richardson constant,120 A/cm2 deg2 , T is the cathode temperature, e is the electron charge, Eo the free-space permittivity,and k is Boltzmann's constant. E is the electric field which contributes to the Schottky effect. Thetransitional work function OT is the work function at the threshold voltage where the space-chargelimited current equals the temperature-limited current, as described in [7]. Note this expression isderived assuming the cathode has a spread only in its work function, not in temperature. EmissionGate SuperconductingValve MagnetMIGElectron GunBcam Tunnel Anmi. Cwratpla *-Fig. 1. The diagnostic used for measuring the beam current at a particular azimuthal angle consists of a copper cylinder,which the electron beam enters, and a narrow beam slot, through which a small angular section of the beam exits.This portion of the beam current is measured by a current plate, fixed above the beam slot. The probe structure isrotated around the azimuth to sample the beam current at various angles.nonuniformity due to a temperature distribution for a gyrotron cathode has been shown, both fromtheory and direct measurement [7], to be small compared to nonuniformities caused by a spread in thecathode's work function for most MIG guns. The temperature spread therefore will be ignored.As discussed in previous papers [7]-[9], a distinction may be made between global and local workfunction spreads. The global spread aglobal is due to macroscopic variations over the entire surfaceof the cathode. On a much smaller spatial scale, a work function distribution, independent of theglobal spread, may also be measured. This local spread 0Xocal may be caused by the amount of bariumsurface coverage at the microscopic level, which in turn may be influenced by the pore sizes, which areless than ten microns. The total spread for the cathode's work function is a summation of these twoindependent effects:UTotal = aglobal + 0 locat (2)An experiment was designed at MIT to measure the local and global work function spread of acathode used in a 96 kV, 40 A gyrotron magnetron injection gun (Fig. 1). The experiment included acurrent probe consisting of a cylindrical copper collector with a narrow 10' slot cut along the side. Ametal collector plate was located above the slot to measure the amount of beam current which exits theslot. The probe was then mechanically rotated to sample the beam current at various angles aroundthe azimuth. The gun was operated at high voltages in short (3 ts) pulses.Initial results using an earlier gyrotron cathode were reported in [7]. Due to activation difficulty,that cathode has since been replaced by a new cathode, identical in design but with a slightly differentsurface coating and mixture (see Table I). Extensive measurements were taken of the new cathodeusing the same experimental setup (Fig. 1). The data points shown in Fig. 2 were recorded by guidinga beam to the current probe and measuring the total current, the sum of the collector current andslot current, over a range of voltages. The cathode temperature was also varied. The data set wasfit using Equation 1 to determine the central work function value 00 and spread U for the cathode ateach temperature. In this case, the total work function spreads OTotal are represented, since the entirecollector current was measured and used in the calculations. The central work function value 00 forthis M-type tungsten cathode was around 1.88 eV for each temperature. For the data shown in Fig. 2,the fit using Equation 1 gave a work function spread of 0.046 eV at T = 965 'C, 0.042 eV at T = 975'C, and 0.033 eV at T = 1000 'C. This decrease in work function spread with increasing temperatureagrees with previous observations [6].501000 *C40975 *CF 3020 .- s-~~ 965 *C1001 2x107V3/2 (Volts3/2)Fig. 2. I-V curves and their best fit for total collector current at various temperatures. For the data shown, # = 1.88eV for all temperatures. The spread, UTOtal was 0.046 eV at T = 965 *C, and 0.042 eV at T = 975 OC. At thehighest temperture, T = 1000 *C, the spread decreased to 0.033 eV.Old Cathode [7] New CathodeEmitter Surface Finish 16 microin. RMS 32-64 microin. RMSImpregnate Ratio 5:3:2 4:1:1Coating TA MTABLE IComparison of old and new cathodes.Next, to examine local and global work function variations, the probe was mechanically rotated,and the azimuthal change in probe current was measured. In the first part of this experiment, thetemperature was fixed at T = 1000 'C, while the voltage was varied for each scan. Fig. 3 shows theresults of these measurements. Note each scan was performed multiple times to verify the repeatabilityof the experiment. The voltage was not increased past 70 kV, mainly due to the noise present in themeasurement system at higher voltages. As the results show, the voltage, above 50 kV, did not affectthe azimuthal uniformity of the beam. This was because the cathode at those voltages was fullytemperature-limited. When the voltage decreased below 50 kV, emission became more uniform. Inthis case, more sections of the cathode had become space-charge limited. Neither the cathode's workfunction nor its work function spread have an effect on current emission in the space-charge limitedregime. When the voltage became very small, ~2.5 kV, the current azimuthal distribution was almostcompletely uniform. A small sinusoidal variation was observed, which may be explained by an offseterror of 0.3 mm in the transverse positioning of the cathode with respect to the anode.The high-voltage data in Fig. 3 may be used to estimate the cathode's global work function spread.As described in [7], each normalized current probe value I/max may be converted into a work functiondifference #-#min by applying the Richardson-Dushman equation. The minimum work function valueoccurs at the maximum current Imax. A distribution in these work function differences is thengenerated and a Gaussian spread Ugla;b is estimated from the distribution. Here the work function.- ft410.80.60.4 -|-2.5 kV-- 5 kV0.2 10 kV-- 50 kV-A- 7OkV aUiai=0.2 eV00 90 180 270 360A 2imuthal angle (deg.)Fig. 3. Probe current uniformity for various applied voltages at T = 1000 0C. Below 50 kV the emission is moreuniform since the cathode becomes progressively space-charge limited. The global work function spread may alsobe determined from the data. For this scan, Oglobal = 0.2 eV.spread is global since the distribution represents variations over the entire cathode surface. For thisparticular cathode, the global work function spread was around 0.02 eV at T = 1000 'C. Recallingthat the total spread was 0.033 eV, Equation 2 was applied to determine the local spread, 0.026 eV.An extensive set of data was taken to generate I-V curves of the current probe at each azimuthal anglein 300 increments. Note that this measurement is a new type of analysis not used in the earlier studyof the old cathode in [7], and represents a significant advance in understanding local nonuniformities.A sampling of the data is shown in Fig. 4(a) at T = 1000 'C. Similar measurements were taken atT = 975 'C and T = 965 'C. The azimuthal variations shown in Fig. 4(b) were measured by the probewhen the cathode was fully temperature-limited for each of the three temperatures. The work functiondistribution parameters q0 and a at each location were then determined by using Equation 1 to fiteach curve, using the same procedure as for the total work function distribution parameters. Sincethe slot width of the probe was 100, the current could not be measured over microscopic dimensions.Therefore the spread in this case was not precisely . However, the analysis allowed a closer lookat the variation in work function and spread of small sections around the cathode. A plot may begenerated to illustrate how the work function 0, changed over the cathode surface (Fig. 5). Thesework function variations mirrored the azimuthal current probe data shown in Fig. 4(a). The workfunction values were largely independent of temperature.As has been documented in [10] and [11], the work function is a function of barium surface coverage,or 0 (0), where 9 is the percentage of barium coverage, typically expressed in terms of barium-oxide(Ba-0) density and the moments of the individual Ba-O dipoles[12]. By extension, the work functiondistribution parameters must also have the same dependence. Furthermore, as these measurementshave indicated and previous experiments have demonstrated [6], the spread - is also dependent ontemperature. It would be useful in the future to investigate the relationship between work functionspread and barium coverage as well as temperature for these cathodes.In conclusion, the beam current azimuthal nonuniformities have been measured and analyzed fora thermionic gyrotron cathode at MIT in terms of global and local work function spread. The totalr2.52.01.5-e- Agle = 00.5 -6- Angle = 90*-9- Angle= 180*0 )- Ange = 270*0 1 2x10'j0.60.40.20(a) Probe IV data.0 90 180 270 360A2Imuthal angle (deg.)(b) Probe azimuthal variations.Fig. 4. Sampling of the probe current at various azimuthal angles. I-V measurements were taken every 30 degrees ofrotation. For the data shown in (a), T = 1000 *C. In (b), the azimuthal variations as measured by the probe arerepresented for all three temperatures. The cathode was fully temperature-limited for all scans in (b).2.01.9C1.80 90 180 270 360A imuthal angle (deg.)Fig. 5. Work function variations (in eV) at each 30' of rotation for various cathode temperatures.work function spread, determined from the total collector current, was 0.033 eV at T = 1000 'C. Theglobal spread, as measured by the current probe experiment was 0.02 eV. From these two values, thelocal spread was calculated as 0.026 eV. These results add another data point of comparison for workfunction spreads in various gyrotron cathodes. More significantly, a new analysis was used to determinechanges in the work function distribution parameters over the cathode surface (Fig. 5). It is possible toimport this distribution data into a 3-D electron particle tracking code (such as OMNITRAK[13]), oreven a gyrotron simulation code (such as MAGY[14]). Therefore the azimuthal beam current variationsmay be accounted for in modeling the electron beam quality and gyrotron efficiency. This will allow formore accurate prediction of existing gyrotron behavior, and the opportunity to design higher efficiencygyrotrons in the future.5-G- 1000 *C-V- 975 *C-0- 965 *C-G- 1000 0C-- 975 4C-E- 965 0C6REFERENCES[1] C. S. Nusinovich, A. N. Vlasov, M. Botton, T. M. Antonsen, Jr., S. Cauffman, and K. Felch, "Effect of the azimuthalinhomogeneity of electron emission on gyrotron operation," Phys. Plasmas, vol. 8, no. 7, pp. 3473-3479, 2001.[2] M. Yu Glyavin, A. L. Goldenberg, A. N. Kuftin, V. K. Lygin, A. S. Postnikova, and V. E. Zapevalov, "Experimental studiesof gyrotron electron beam systems," IEEE Tans. Plasma Sci., vol. 27, pp. 474-483, Apr. 1999.[3] R. Advani, J. P. Hogge, K. E. Kreischer, M. Pedrozzi, M. E. Read, J. R. Sirigiri, and R. J. Temkin, "Experimental investigationof a 140-GHz coaxial gyrotron oscillator," IEEE Trans. Plasma Sci., vol. 29, pp. 943-950, Dec. 2001.[4] R. T. Longo, "Physics of thermionic dispenser cathode aging," J. Appl. Phys., vol. 94, no. 10, pp. 6966-6975, Nov. 2003.[5] M. Cattelino and C. Miram, "Predicting cathode life expectancy and emission quality from PWFD measurements," Appl.Surf. Sci., vol. 111, pp. 90-95, Feb. 1997.[6] J. C. Tonnerre, D. Brion, P. Palluel, and A. M. Schroff, "Evaluation of the work function distribution of impregnatedcathodes," Appl. Surf. Sci., vol. 16, pp. 238-249, 1983.[7] J. P. Anderson, S. E. Korbly, R. J. Temkin, M. A. Shapiro, K. L. Felch, and S. Cauffman, "Design and emission uniformitystudies of a 1.5-MW gyrotron electron gun," IEEE Trans. Plasma Sci., vol. 30, no. 6, pp. 2117-2123, Dec. 2002.[8] C. M. Fortgang, "Measuring current emission and work functions of large thermionic cathodes," in 3rd Proc. IEEE Int.Vacuum Electronics Conf. IVEC. Piscataway, NJ: IEEE, 2002, p. 298.[9] K. Wandelt, "The local work function: Concept and implications," Appl. Surf. Sci., vol. 111, pp. 1-10, 1997.[10] D. Dieumegard, J. C. Tonnerre, D. Brion, and A. M. Schroff, "Life test performance of thermionic cathodes," Appl. Surf.Sci., vol. 111, pp. 84-89, 1997.[11] S. Yu. Davydov, "Calculation of temperature dependence of the work function of an adsorption system," Phys. of the SolidState, vol. 45, no. 5, pp. 972-975, 2003.[12] R. Cortenraad, A. W. Denier van der Con, H. H. Brongersma, G. Gartner and A. Manenschijin, "Surface analysis ofthermionic dispenser cathodes," Appl. Surf. Sci., vol. 191, pp. 153-165, 2002.[13] OMNITRAK Users Manual, Field Precision Inc., Albuquerque, NM, 2004.[14] M. Botton, T. M. Antonsen Jr., B. Levush, A. Vlasov, and K. Nguyen, "MAGY: A time dependent code for simulation ofelectron beam devices," IEEE Trans. Plasma Sci., vol. 26, no. 3, pp. 882-892, June, 1998.--- 7-

Experimental Studies of Local and Global Emission Uniformity for a Magnetron Injection Gun

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Experimental Studies of Local and Global Emission Uniformity for a Magnetron Injection Gun

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