PFC/JA-83-28 ' ENGINEERING ASPECTS OF LOWER HYBRID MICROWAVE INJECTION INTO THE ALCATOR C TOKAMAK J. J. Schuss, M. Porkolab, D. Griffin, S. Barilovits, M. Besen, C. Bredin, G. Chihoski, H. Israel, N. Pierce, D. Reiser, K. Rice Plasma Fusion Center Massachusetts Institute of Technology Cambridge, MA 02139 June 1983 This work was supported by the U.S. Department of Energy Contract No. DE-AC02-78ET51013. Reproduction, translation, publication, use and disposal, in whole or in part by or for the United States government is permitted. By acceptance of this article, the publisher and/or recipient acknowledges the U.S. Government's right to retain a non-exclusive, royalty-free license in and to any copyright covering this paper. To be published in the proceedings of the Fifth Topical Meeting on the Technology of Fusion Energy, Knoxville, Tennessee, April 26-28, 1983. These proceedings will be published as a special supplement to the September 1983 issue of Nuclear Technology/Fusion. ENGINEERING ASPECTS OF LOWER HYBRID MICROWAVE INJECTION INTO THE ALCATOR C TOKAMAK* J. J. Schuss, M. Porkolab, D. Griffin, S. Barilovitst, M. Besen, C. Bredin, G. Chihoski, H. Israel", N. Pierce, D. Reiser, K. Rice Plasma Fusion Center Massachusetts Institute of Technology Cambridge, Massachusetts (617) 253-8639 02139 arcing and fault ABSTRACT protection for the associated high power RF components. We describe here the RF system currently installed on Alcator C that is being used to inject in excess of 1 MW of net RF power into the tokamak plasma during lower hybrid heating and This system provides current drive studies. for RF power and phase monitoring in each of the individual waveguides of the two 16 wave- guide launching arrays, and also for fault protection both at the waveguide arrays and klystrons. Using this system good waveguide-plasna coupling has been obtained and net RF power densities of 9 kW/cm 2 have been injected by the waveguide array without microwave arcing. The attraction of lower hybrid is waves that they can be used to either heat electrons or ions or drive an electron current in a tokamak. A further advantage of using these waves is that they can be launched by waveguide array couplers, which are better suited to the reactor environment than loop antennas. in However, order to efficiently launch lower hybrid waves, the waveguides of the coupler must be phased to launch slow waves (ikz t/VtI). c/wl > 1, where kz Furthermore, for current drive Z the waves must be launched preferentially in These two the direction of the electron drift. conditions, coupled with the frequency range of lower hybrid waves (1 - 5 GHz), reouire that a large number of waveguides compose the multirow, multicolumn waveguide array. Lower hybrid wave current drive experiments are now being carried out at the I MW power level on the Alcator C tokamak to explore these issues.1-6 Besides having successfully demonstrated current drive and electron heating, these experiments have shown the practicality of using large large, multirow waveguide arrays to couple 2 RF power densities (P f/A - 9 kW cm- ) into Pere we describe the RF the tokamak plasma. antennas and the RF power system used in the Alcator C experiment.' In addition to controlling and monitoring the power and phase in each column of the two 16 waveguide array couplers on Alcator C, this system provides microwave used here can and This sysin prin- .ciple be scaled up to drive the large (> 100 waveguides) lower hybrid coupling structures necessary for a fusion reactor. RF ANTENNA AND POWER SYSTEM Lower hybrid wave heating and current drive studies are being carried out at a frequency f 4.6 GHz on the Alcator C tokamak whose parameters are: major radius R = 64 cm, minor radius a = '6.5 cm, toroidal magnetic field BT = 8 12 T, and line averaged plasma density during 3 At RF experiments Re ~ 0.3 - 3 x 1014 cm- . present INTRODUCTION heating and tem and the techniques array two 16 waveguide arrays are installed on two large side ports of Alcator C that are separated 180* toroidally. Each array consists of 16 individual waveguides in a 4 x 4 mattix, The waveguide array is as shown in Fig. 1. fabricated from 304 stainless steel; the vacuum windows consist of BeO ceramic and are 8located Both 10 cm from the waveguide array mouth. the copper waveguide section of the vacuum window and the 304 stainless steel waveguide into which it is brazed are seamless so as to minimize the chance of a welded seam leaking. Each waveguide of the array has inner dimen- sions of 0.8 cm x 5.75 cm and 1 mm thick walls. These waveguides are joined to an adaptor section which allows each waveguide of the array to be driven by standard C-band waveguide. The entire waveguide array is mounted on bellows so that its radial position can be varied Best coupling to plasma during experiments. waves is obtained with the array mouth positioned near the virtual limiter radius, r = 18.0 cm, where the plasma density is typically me ~ 5 x 1012 cm-3. Each vertical column of the waveguide array is driven by a single klystron. (Each column is oriented perpendicular to the toroidal I field.) The phase and relative power of each column is then independently adjustable. Normal operation involves phasing the columns of the array 0, -x, 0, itfor plasma heating studfor current drive ies, and 0, it/2, n, 31r/2 wftanw 04 STAINLESS /STEEL. ARRAYS BeO WINDOWS P PORT IVIRTUAL 16.5 cr LITERER 19cm I L VACUUM VACUUM VESSEL NITROGEN STRUTS COPPER B T' I WALL Side View Fig. I Geometry of End View the 16 waveguide in a horizontal array port of Alcator C. The stainless steel virtual limiters are located approximately 4.5 cm on each side of the array, whereas the main limiter is located 60* away toroidally from the array. experiments. The RF power spectra launched by the array for these phasings is shown in Fig. 2. Each power spectrum P(n,) is graphed as a func- tion of nz, where nz - k, c/w ; for Inzi < 1, the lower hybrid waves in the plasma are evan- escent and P(nz) - 0. The 0, n/2, R, 31r/2 spectrum is asymmetric in nz with twice as much power flowing in the +nz direction as in the This asymmetry is found to be -nz direction. necessary to sustain RF driven plasma currents About 35% in the absence of a loop voltage. of the RF power where it 1 < nz is located at < 1.3, would be inaccessible to central plasma densities ne > 1014 cm-3 at B - 10 T in deuterium. It has been proposed that this surface can be wave component reduced gations alongside the array. 9 by adding corru- Fig. 3 shows schematically wer RF system. Each 16 the high pow- waveguide array is powered by an RF cart which consists of four 250 kW RF power output klystrons. Each klystron is driven by the same 4.6 GHz oscillator whose 3 mW power output is amplified by a travelling wave tube amplifier after passing through an RF diode switch. This diode switch is used to interrupt the RF power in case of a microwave arc fault, and also is used to delay the onset of RF drive to the klystrons for 1 msec until the klystron beam current has flat-topped. The output of the travelling wave tube amplifier is then split four ways to power the four klystrons. Each RF drive arm has a mechanical attenuator and phase shifter, and an electronic The electronic phase shifter phase shifter. can provide rapid or programmed phase changes for each klystron's RF power output. The final The RF drive leiel at the klystron is ~ 0.7 W. klystron power output passes through both a visible and a reflected power arc detector; if either a visible arc is detected at the klystron output window, or a high reflection occurs in the klystron output waveguide, the RF drive is shut off and the klystron beam voltage The klystron power output then is terminated. passes through a high power low pass filter and a circulator whose isolation is greater than 20 db. This output power is then carried by copper C-band waveguides to the array where it is split vertically to power the four waveThe total power loss beguides of a column. tween the klystron output window and the waveguide array mouth is approximately 25%, which includes a 10% loss in the RF cart output com- ponents and a 9% power loss in the stainless steel waveguide array. This latter loss would be reduced by a factor of 6.5 by silver plating the inside of the waveguides in the array. relative phasing of the waveguide's RF power. An output phase signal, which ranges from 0 to 4 V as the phase varies from 0* to 3600, is This the data system. of two CAMAC 32 channel also routed to system consists data data loggers which communicate with a PDP 11/34 computer over fiber optic data Between links. shots the computer acquires and archives 08 then analyzed, displayed, and hard copied beThis display shows the tween plasma shots. individual forward and reflected power in each waveguide, the forward phase in each waveguide, and the total forward, reflected, and net RF This system presently powers of the array. handles the data of two waveguide arrays and is being modified to acquire and display the data of two more arrays. sing 7r 0.6k P(n. phasing 02 The RF -10 -8 Fig. 2 -6 -4 -2 2 0 nz 4 6 8 K0 Power spectrum P(nz) launched by the 16 waveguide array vs. n, for it(0,x,0,it) relative waveand n/2 (0,,r/2,it,3n/2) Here at the waveguide guide phasing. mouth ne/?ne = .2 cm, and for itphas- ing ne at the waveguide mouth is 10 nc, it is 5 nc. whereas for ir/2 phasing 3 (nc - 2.6 x 1011 cm- ) The RF diagnostic and fault system is schematically illustrated in Fig. 4. A 50 db coupler samples both the forward and reflected power in each of the 16 waveguides of the arsay. The forward power, after an additional 20 db attenuation, is split between a crystal square-law The crystal outputs are detector and a mixer. amplified 50 times and routed to the data system. In adition, these two voltages are compared electronically to detect a VSWR fault. Should the reflected RF power exceed 50% of the forward power in any waveguide the RF power is At < 5 ptsec to prevent arcing shut down in This is accomplished by having 3 db damage. more attenuation in the forward power sampling A 50% than in that of the reflected power. reflection then corresponds to VF = VR, which triggers a comparator circuit. This system has been successful in preventing damage from occurring to the vacuum windows due to microwave arcing. to an RP06 disk storage unit the data taken by the data loggers during the shot; this data is I Or The mixer produces a 1 MHz IF output by beating the 4.6 GHz waveguide signal and a phase locked 4.601 GHz signal provided by the master oscillator. This 1 MHz IF is compared against a phase locked 1 MHz square wave output provided by the oscillator to determine the control system shuts down the RF klystron drive and the klystron beam voltage in the event a VSWR fault is detected at any In addition, the RF waveguide of the array. drive and beam voltage are terminated upon detection of a high klystron body current, a visible arc at a klystron output window, or a high VSWR at a klystron output waveguide. Whereas in most cases the control system terminates the klystron beam voltage by driving the grids of the 4 high voltage modulator tubes negative, in the latter two cases the modulator crowbar is also fired and the vacuum breakers that powThis procedure er the modulator are opened. brings the voltage input to the modulator to zero in less than 100 Wsec. After termination of the RF pulse due to a fault, the control system will not allow another pulse without op- erator intervention. RF SYSTEM OPERATION After installation on the Alcator C device, it was necessary to RF condition the waveguide array into vacuum for approximately 50 - 100 hours before pulsing into plasma. This conditioning consisted of firing RF pulses of 0.1 to 1 maec duration which were repeated as often During this pulsing into as once per second. the vacuum the array was phased 0, 0, 0, 0 so that good coupling was ensured. The condition- ing was continued until 400 kW of RF power could be pulsed into the torus without waveguide arcing with a resulting gas buildup during a After this pulse of less than 1 x 10-7 torr. vacuum conditioning it was found that the net RF power transmitted into plasma without arcing could be raised to 500 kW in the order of 200 In order to reach this power plasma shots. level the waveguide array position had to be adjusted so that the plasma density at the wave- Mechanical Klystron 4.6 GHz 4C shifters ATTf -c -Visible T To Waveguide Array Circulaor Arc Detector TWT pin diode swith ~O.7W Drive 250kW Output from fault circuit Fig. 3 PRlEF (to fault) Gain> 53 db Schematic of the 1 MW RF cart. Only one of the four identical 250 kW klystrons is depicted. 2 3 where 4rn e/m am guide mouth was 10 - 50 nc, , 02 and for f - 4.6 GHz, nc . 2.6 x 1011,c W . RF Couplers (50db) Array P ri Prf PRwn At this radial location the global power reflectivity was R 5 - 10%. It was also found that adjusting the horizontal plasma position helped in controlling this edge density. When the waveguide-plasma coupling was optimized, net powers as high as 650 kW were transmitted into the plasma with no microwave arcing. This corresponds to a power density of 20db PF 20db 4.601 G~z 3db 6db_ ___ RF Box 9 kW/cm 2 at the waveguide mouth and is a record at this frequency range. These results were obtained with the waveguides behind the BeO ceramic vacuum window filled with atmospheric pressure nitrogen. Earlier experiments with a 4 wave- guide array showed that RF breakdown occurred DC at a -.. Block IOdb 1Odb Microwave Components IMHz VR VF Fig. O Electronics V PO6 5(xVR Chmac L- .- F V Doth Loggers - . - -- - - - -- ------- d Fiber Optic POP 11/34 I RP06 I I Disk I Fig. 4 Dato Link VF ULT To Control System IF VF >VR) \OV,IF VS,svl ( 3V, of the RF diagnostic and fault system. Only the circuits of one of the 16 waveguides is shown. Schematic density Prf/A - 1 kW/cm when the part of this region containing the u - uce layer was evacuated. Filling this region with 300 torr of N2 gas prevented arcing by presumably making vc > wce and inhibiting cyclotron resonance. DC Mixer power 2 5 shows an RF current drive shot at a line average density ne . 3 x 1013 cm-3 During current drive operation it was found to be helpful to slowly ramp up the RF power in At ~ 30 msec, as shown in Fig. 5, so that the edge plasma density was maintained for proper coupling. Without RF the plasma current decays with a 150 msec time constant. With RF the plasma current is held constant and the loop voltage is zero. Such flat-top current plasma shots have been produced at plasma densities 3 as high as ne-- 6.6 x 1013 cm- . Up to 200 kA of plasma current has been maintained by the RF alone with zero loop voltage. The best efficiency in hydrogen discharges at BT - 10 T is ne (1014 cm- 3) Ip (A)/Prf(W) - .19 during current drive. At higher plasma densities (8 x 3 3 1013 cm< e < 2 x 1014 cm- ) a transition to electron heating is obtained. At F e - 8 x 1013 cm- 3 in a carbon limiter plasma a 500 eV plasma electron temperature increase was obtained due to an RF power input of 500 kW. Pres- 1014 - * Work supported C has been I suc- search, Brussels, Belgium, Vol. RF I M. PORKOLAB, J. J. SCHUSS et al., "Lower Hybrid Heating Experiments on the Alcator-C and the Versator-II Tokamaks," Proceedings of the 3rd Joint Varenna-Grenoble International Symposium, 22-26 March, 1982, Gre- noble, France, Volume II, 469. I I 4. M. PORKOLAB, J. J. SCHUSS et al., "Lower Hybrid Heating and Current Drive on the Alcator C and Versator II Tokamaks," 9th International Conference on Plasma Physics and I Controlled Nuclear Fusion Research, Balti- more, USA, paper C-4 (1982). 0 0.1 0.2 0.3 0.4 0.5 t(sec) 5 IAEA- 3. 'MO Fig. 1I, 507 (1980). J. J. SCHUSS, M. PORKOLAB, Y. TAKASE and S. TEXTER, "Initial Lower Hybrid Experiments on the Alcator C Tokamak," Bull. An. Pus. Soc. 26, 1023 (1981). I II: VLO of Energy 2. Ip(310kA/iv) I Department M. PORKOLAB, J. SCHUSS et al., "Lower Hybrid Heating and Current Drive in Tokamaks and Related Experiments," Proceedings of the 8th International Conference on ?Pasra Physics and Controlled Nuclear ?usion ReCN38/T-2-1 I "".. S. REFERENCES cessfully utilized to carry out lower hybrid wave current drive and heating experiments at It has demonstrated the the 1 MW power level. I by U. Contract Number DE-AC02-78ET51013. 1. system on Alcator and plasma heating where P.4 >> P 800 kW. CONCLUSIONS The RF 3 t Now at Johns Hopkins University. ttNow at Raytheon Corporation. ently, we are studying plasma heating and current drive using both waveguide arrays at the 1 MW power level. These results have been reported elsewhere.4-G cm- Typical RF current drive shot of 5. J. J. SCHUSS, "Lower Hybrid Heating and Current Drive on the Alcator C Tokamak," Bull. Am. Phys. Soc. 27, 962 (1982). the Alcator C plasma using I of the 2 16 waveguide arrays. The dashed line indicates the plasma current decay in the ife is measabsence of RF injection. ured by a fringe counter, where 1 fringe 3 - 5.5 x 1013 cmline averaged density. During RF the loop voltage VLOOP is nearly zero, the cyclotron emission at u - 2wce increases by an order of magnitude, and the molybdenum radiation IMO stays constant. feasibility of using large, multirow waveguide arrays to launch lower hybrid waves at high power densities in the tokamak environment. This system will be expanded to include two additional waveguide arrays and 1 MW carts in At that time the experiment January 1984. should allow the study of current drive at F e 6. M. PORKOLAB, J. J. SCHUSS, et al., "Lower Hybrid Current Drive and Heating Experiments up to the 1 MW Level in Alcator C," 5th Topical Conference on Radio Frequency Plasma Heating, Madison, Wisconsin, Session B, invited paper. 1983, 7. H. ISRAEL and M. PORKOLAB, "Final Report, Lower Hybrid MDF Project," M.I.T. Plasma Fusion Center Report PFC/RR-80-30 (1980). 8. The window array was fabricated by Mr. P. Spallas of Varian Associates Inc., Palo Alto, California. 9. J. J. SCHUSS and M. PORKOLAB, "Effect of Wall Corrugations on Lower Hybrid Wave Launching and Reflection," Fifth Topical Conference on Radio Frequency Plasma Heating, Madison, Wisconsin, 1983, paper A-L.5.