The polarization of the launched electron cyclotron wave has been optimized for coupling to the X-mode by adjusting the inclination of grooved mirrors located in two consecutive mitre bends of the waveguide. The unwanted O-mode component of the launched beam can be positively identified by the difference in the power deposition profiles between X-mode and O-mode. The optimal polarization for X-mode launch is in good agreement with theoretical expectations

Austin, M. E.

Ikezi, H.

Lohr, J.

Luce, T. C.

Petty, C. C.

Prater, R.

UNT Digital Library

GA-A23208GA PROJECT 30033JULY 1999+ GENERAL ATOMICSPOLARIZATION MEASUREMENTS DURINGELECTRON CYCLOTRON HEATINGEXPERIMENTS IN THE DIII-D TOKAMAKbyC.C. PETTY, T.C. LUCE, M,E, AUSTIN,* H. IKEZI,J. LOHR, AND R. PRATERThis is a preprint of a paper presented at the Twenty-SixthEuropean Physical Society Conference on ControlledFusion and Plasma Physics, June 14-18, 1999 inMaastricht, The Netherlands, and to be published in TheProceedings.*University of Texas at Austin, Austin, Texas.Work supported bythe U.S. Department of Energyunder Contract No. DE-AC03-99ER54463,and Grant No. DE-FG03=97ER54415,DKCLA[MERThis repofi was prepared as an account of work sponsoredby an agency of the United States Government. Neitherthe United States Government nor any agency thereof, norany of their employees, make any warranty, express orimplied, or assumes any legal liability or responsibility forthe accuracy, completeness, or usefulness of anyinformation, apparatus, product, or process disclosed, orrepresents that its use would not infringe privately ownedrights. Reference herein to any specific commercialproduct, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constituteor imply its endorsement, recommendation, or favoring bythe United States Government or any agency thereof. Theviews and opinions of authors expressed herein do notnecessarily state or reflect those of the United StatesGovernment or any agency thereof.DISCLAIMERPortions of this document may be illegiblein electronic image products. Images areproduced from the best available originaldocument.POLARIZATION MEASUREMENTS DURING ELECTRON CYCLOTRONHEATING EXPERIMENTS ON THE D~–D TOKAMAKPOLARIZATION MEASUREMENTS DURINGELECTRON CYCLOTRON HEATING EXPERIMENTSON THE DIII-D TOKAMAKC.C. Petty, T C Lute, ME. Austin, * H Ikezi, J Lohr, and R. PraterGeneral Atomics, P.O. Box 85608, San Diego, CalifomiZ 92186-5608, USA*University of Texas at Austin, Austin, Texas.The polarization of the launched electron cyclotron wave has beenoptimized for coupling to the X–mode by adjusting the inclination ofgrooved mirrors located in two consecutive mitre bends of thewaveguide. The unwanted O-mode component of the launched beamcan be positively identified by the difference in the power depositionprofiles between X-mode and O-mode. The optimal polarization forX-mode launch is in good agreement with theoretical expectations.Electron cyclotron heating (ECH) and current drive (ECCD) experi-ments on the DIII-D tokamak need narrow beams in nearly pure X–modeto achieve the highly localized deposition and current drive required foradvanced tokamak scenarios. The electron cyclotron waves are launchedfrom the low-field-side of the tokamak using two 110 GHz gyrotrons(“Katya” and “Dorothy”), corresponding to the second harmonic of theelectron cyclotron frequency. The two gyrotrons are capable of 1.7 MWof combined power for 1 s pulses. For current drive experiments, atoroidal launch angle of 19° is used, while perpendicular launch is alsopossible for pure heating applications using a different mirror geometry.The ECCD launcher is steerable in poloidal angle which allows thedeposition location to be varied from the plasma center to the edge.The polarization of the launched wave is controlled by adjusting theinclination of grooved mirrors located in two consecutive mitre bends ofthe ejaculated waveguide.1 This type of polarizer produces a polarizationdependent phase delay since the wave is reflected from the top of thegrooves when the electric field is aligned to the groove, whereas the waveis reflected from the bottom of the grooves when the electric field isperpendicular to the groove. Nearly any mixture of X–mode and O–modepower is possible using these polarizers. For perpendicular launch, thedesired polarization for X–mode is linearly polarized (ellipticity = O)withan inclination of 90° relative to the magnetic field at the plasma edge.The power deposition profile is determined experimentally by modu-lating the ECH power at 100 Hz and measuring the electron temperature(Te) response using a 32-channel heterodyne radiometer.2 In the limit ofIinfinite modulation frequency (co), the temperature response (~e iseverywhere proportional to the ECH source term with a 90° phase agC.C. Petty, et al.GENERAL ATOMICS REPORT GA–A23208 1POLARIZATION MEASUREMENTS DURING ELEcTRON CYCLOTRONC.C. Petty, et al. HEATING EXPERIMENTS ON THE DID-D TOKAMAKfrom the injected power. For square wave modulation, the power deposi-tion profile can be found from320) 3‘abs = _— ;nefe , (1)IL Lwhere ne is the electron density. However, this equation places only anupper bound on the deposition profile width for the relatively lowmodulation frequency of 100 Hz used in these experiments.Experimental ResultsRecent experiments on DIII–D have verified the correct operation ofthe polarizers by scanning the mixture of X–mode and O-mode power bychanging the inclination and ellipticity of the launched beam.Perpendicular launch of the ECH is used in this experiment since thisallows the regions of deposition for X-mode power and O-mode power tobe easily separated, as described below and shown in Fig. 1. The X–modecomponent is strongly absorbed off-axis at the location of the secondharmonic ECH resonance, whereas the O–mode component is weaklyabsorbed owing to the low electron temperature off-axis. The remainingO–mode component of the wave reflects off of the graphite inner wall andmakes a second pass through the plasma, this time passing close to theplasma center where the O-mode power is more strongly absorbed due tothe higher electron temperature.ECH/ RayFig. 1. Vacuum ray paths for perpendicularlaunchof electron cyclotron wavesfor theseexperiments.2 GENERAL ATOMICS REPORT GA-A23208POLARIZATION MEASUREMENTS DURING ELECTRON CYCLOTRONHEAmNG EXPERIMENTS ON THE DIII-DTOKAMAKMeasurements of the power deposition profile can clearly resolve theseparate X–mode and O-mode peaks, as shown in Fig. 2. In the case ofFig. 2(a) for the Katya gyrotron, the mixture of X–mode power andO–mode power is varied by changing the inclination of the linearpolarized launched beam, whereas in the case of Fig. 2(b) for the Dorothygyrotron, the ellipticity of the launched beam is changed as well. Figure 2shows that the X–mode component is absorbed near p = 0.55, where p isthe normalized toroidal flux coordinate. After reflecting off the inner wall,the O–mode component is seen to damp near p = 0.15. The relative sizeof the X–mode and O-mode peaks are in good agreement with theexpected X–mode and O-mode powers based on the polarizer settings.Scans of the wave inclination show that the launched X–mode poweris optimized for an inclination near 90°, which agrees with theoreticalexpectations for perpendicular injection. Figure 3(a) shows the measuredpower deposition profiles for Katya during an inclination scan about theoptimal X–mode launch point. The unwanted O–mode component is seento increase as the inclination moves away from 90° in either direction.Conversely, the magnitude of the X–mode component increases as theinclination moves closer to 90°. A similar result is obtained for Dorothy,as shown in Fig. 3(b).Scans of the wave ellipticity show that the unwanted O–mode power isminimized for an ellipticity near O, in agreement with theory forperpendicular launch. This is shown in Fig. 4(a) for Katya and Fig. 4(b) forDorothy. Since the O–mode component in Fig. 4(a) never completelyC.C. Petty, et al.0.06.Incident ~wer = 0.65 MWO-mode x-mode ‘e)11”inclination ~Ol)l#fitinetion0.27 MW .A:$”’+:. ..! .,;::’-,: ... .-...,.: .. ..-j.,.;:‘“..:.”...~“ ‘,...,.3 .:.{ +.”. ..../.. ....”’..%,..,,““....,31% X-mode ““:::p.,..1460 inclination ....~..:,,,,0.28 MW ....-0.PIncident mxver = 0.5 MW0.08-0.060.040.020.00I OQ 0.4 0.6 0.8PFig. 2. Power deposition profilesfor X–mode andO-mode launchfor (a) Katya,and(b) Dorothy.The integmtedabsorbedpowersarealso indicated.I GENERAL ATOMICS REPORT GA–A23208 3POLARIZATIONMEASUREMENTS DURING ELECTRON CYCLOTRONC.C.Petty, et al. HEATING EXPERIMENTSON THE DIII-DTOKAMAKLinear polarization(a.. 98%X-modeA77% X-mode ‘;<” f?&incctio;I;linclction;’ .,;I,/;~’-/ }%,...::“s. ““x,...,,,:. .;“*?,. 97% X-mode ‘S,s’ 101” inctinetion0.49 Mw0.0 0.2 0.4 0.6 0.8Po ~ Linear polarization96% X-mode (b)102” inclination62% X-mode 0.49 MW128° inclination ....%“- ,A,004 0.36 MW “.~-t :,; “’...~-...$ ..,,.,,. ,....- ,, ...:-:-.... ,,....:’ ‘.,..,-,.o#02- ; ;< “’:., ..,..,:80%X-mode<=+.-” 64° inclination ,,“’.....,,,0.47 MWo.OOO~. . 0.4 ~ 0.6 0.8 1Fig. 3. Power deposition profiles for an inclination scan about the optimalX-mode point for (a) Katya, and (b) Dorothy. The integratedabsorbedpowersarealso indicated.0.06~1010 inclination (a)[ ‘“N 1102” inclination (b]o.OOO~ o.oo~0.2 0.4 0.6 0,8 1.0 . . .P P..oFig. 4. Power deposition profiles for an ellipticity scan about the optimalX-mode point for (a) Katya, and (b) Dorothy. The integratedabsorbedpowersarealsoindicated.disappears, even for an ellipticity of O, this would indicate that theinclination has not been set to the ideal value for X–mode launch. There isalso a puzzling asymmetry in the magnitude of the measured O–modepower for ellipticities of &O.5.In Fig. 4(a), the O-mode peak for Katya islower for an ellipticity of 0.5 compared to -0.5, while in Fig. 4(b), thePOLARIZATIONMEAStJR13MENTSDURING ELECTRON CYCLOTRONHEAmNG EXPERIMENTS ON THE DID-D TOKAMAKO–mode peak for Dorothy is higher for art ellipticity of 0.5 compared to-0.5. Although this asymmetry is not understood presently, it may berelated to the slightly different toroidal angular components for the twoECH systems since the launchers actually inject a2° from perpendicular.ConclusionsExperiments on DIII–D have shown that the polarization of thelaunched electron cyclotron waves can be controlled by adjusting theinclination of grooved mirrors located in two consecutive mitre bends ofthe waveguide. The X–mode and O–mode components of the launchedbeam can be identified by their different deposition profiles, determinedby modulating the ECH power and measuring the electron temperatureresponse. For perpendicular injection of the waves, the experimentallydetermined polarization for optimal X–mode launch is in agreement withtheory. This gives confidence that the required polarization for X–modelaunch in the more difficult situation of current drive injection can becorrectly obtained.AcknowledgmentThis is a report of work supported by the U.S. Department of Energyunder Contract No. DE-AC03-99ER54463 and Grant No. DE-FG03-97ER54415.1.2.3.ReferencesDeane, J.L.,Int. J. Infrared Millimeter Waves 13, 1727 (1992).Waug, Z., et al., Proc. of the 9th Int. Workshop on ElectromCyclotron Emission andElectron Cyclotron Heating, Borrego Springs, California, (World Scientific, 1995)p. 427.Murakami, M., et al., Proc. of the 10th Int. Conf. on Radiofrequency Power in Plasmas,Boston, Massachusetts (American Institute of Physics, 1993) p. 48.C.C. Petty, et al.GENERAL ATOMICS REPORT GA–A23208 5

Polarization Measurements During Electron Cyclotron Heating Experiments in the DIII-D Tokamak

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Polarization Measurements During Electron Cyclotron Heating Experiments in the DIII-D Tokamak

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