Coda, S.

Delpech, L.

Goodman, T.P.

Henderson, M.A.

J.-Ph., Hogge

Llobet, X.

Marletaz, B.

Peysson, Y.

Pietrzyk, Z.A.

Pochelon, A.

Pochon, G.

Sauter, O.

Weisen, H.

Infoscience - École polytechnique fédérale de Lausanne

26th EPS Conf. on Contr. Fusion and Plasma Physics, Maastricht, 14 - 18 June 1999      ECA Vol.23J (1999) 1097 - 11001097Measurements of Hard X–Ray Emission Profiles in the TCV Tokamakduring Electron Cyclotron Heating and Current DriveS. Coda, Y. Peysson, L. Delpech, T.P. Goodman,M.A. Henderson, J.-P. Hogge, X. Llobet, B. Marle´taz,Z.A. Pietrzyk, A. Pochelon, G. Pochon, O. Sauter, H. Weisen.Centre de Recherches en Physique des Plasmas,Association EURATOM – Confe´de´ration Suisse,Ecole Polytechnique Fe´de´rale de Lausanne, CH–1015 Lausanne, Switzerland De´partement de Recherches sur la Fusion Controˆle´e, Association EURATOM–CEA,CEA/Cadarache, 13108 Saint Paul-lez-Durance Ce´dex, FranceFig. 1 Geometry of hard X-raycamera viewing chords for a typi-cal EC-heated TCV plasma(R = 88 cm, a = 23 cm). Thechords are partially overlapped.1. IntroductionA multichannel hard X–ray (HXR) diagnostic system de-veloped for the Tore Supra tokamak [1] has been employed onthe TCV tokamak with the aim of characterising the spectraland spatial distribution of fast-electron bremsstrahlung emis-sion during electron cyclotron heating (ECH) and current drive(ECCD). The system consists of a vertically viewing pinholecamera equipped with an array of CdTe detectors. CdTe technol-ogy was chosen for this system in order to satisfy the combinedrequirements of good temporal and spatial resolution, of effi-cient  –ray rejection and of compactness. On TCV, 14 partiallyoverlapped viewing chords span the entire outer minor radius ofthe plasma, with a radial resolution of  2 cm on the midplane(Fig. 1). The intrinsic energy resolution is  5–7 keV. After am-plification, each signal is distributed to 8 discriminator-counterchains, generating spectra in the range 10–150 keV. Count ratesup to 1.5  10  s  can be detected before the onset of pileup.The time resolution, determined by the requirement of a relativestatistical noise  10%, is in the order of 1–5 ms.The ECH and ECCD experiments described in this pa-per have been carried out with up to three 0.5 MW gyrotrons,operating in X-mode at the second harmonic (82.7 GHz) [2].The launching mirrors can be independently rotated in both thepoloidal and the toroidal direction, providing great flexibility inthe choice of heating locations and parallel wave numbers. TheHXR camera constitutes a crucial tool for investigating the loca-tion of the power deposition and the distribution and dynamics ofsuprathermal electrons. Initial results are presented in this paper.2. Suprathermal electron population during ECH and ECCDThe parallel wave number of the electron cyclotron (EC) wave was scanned in a set of simi-lar discharges by varying the toroidal launching angle 	 from -35  to +35  (this angle is defined26th EPS CCFPP 1999 ; S.Coda et al.: Measurements of Hard X-Ray Emission Profiles in the TCV Toka...1098−40 −20 0 20 4002468x 104Toroidal launching angle (degrees)Photon counts (s   )CNT−ECCD CO−ECCD−1Fig. 2 Hard X-ray emission from the plasma center in therange 40–50 keV as a function of the toroidal launching angle,at constant current, density and plasma shape (central heat-ing, 1.5 MW).Central T  (Thomson)       T from fit to HXR spectrum0 0.5 1 1.5 201234567Time (s)T  (keV)eeFig. 3 Comparison between central electron temperature mea-sured by Thomson scattering (solid line) and effective temperatureobtained by a 3-point exponential fit to the hard X-ray spectrum fromthe plasma center (circles), with 1.5 MW of EC heating (off-axis inthe middle part of the discharge, on-axis elsewhere).at the launcher, the 0  case correspond-ing to pure ECH). In these dischargesthe plasma current was 170 kA, thetoroidal field 1.4 T, the peak density 2–2.5  10  cm  , the plasma elongation1.3 and the triangularity +0.3. A totalpower of 1.5 MW was injected near theplasma center.The intensity of hard X-ray brems-strahlung emission increases with 	 ,in both the co- and counter-ECCD di-rections (Fig. 2). In the co-ECCD casethe current-drive efficiency has alsobeen found to increase with 	 , and thelargest non-inductive currents in TCVto date have been generated at the max-imum toroidal angle explored in thisscan (35  ). This is somewhat in con-trast with code predictions, which havegenerally placed the optimum angle be-tween 25 and 30  [3]. This scan has proven fruitful also in allowing us to identify a range ofangles (5–15  ) in the counter-ECCD direction in which very high central electron temperatures(up to  10 keV) are obtained.In the pure ECH case the shape of the measured spectrum is consistent with the emissionfrom a Maxwellian plasma of temperature equal to that measured by the Thomson scatteringdiagnostic; this comparison can bequantified by calculating an effec-tive photon temperature through anexponential fit to the spectrum: anexample is shown in Fig. 3. Roughanalytical estimates of the expectedabsolute photon emission from aMaxwellian plasma are also in goodagreement with the measurement.In the ECCD cases, not onlyis the intensity considerably higherthan in the ECH case at all energies,but the effective photon temperatureis typically in the range from 20 to60 keV (see Fig. 4), clearly reveal-ing the presence of a suprathermaltail in the electron velocity distribu-tion.The dissimilarity between theECH and ECCD cases, further seenin the integrated spatial profiles inFig. 5, is in qualitative agreement26th EPS CCFPP 1999 ; S.Coda et al.: Measurements of Hard X-Ray Emission Profiles in the TCV Toka...1099CO−ECCD CNT−ECCDECH     0 20 40 60103104105106Energy (keV)Emissivity (keV/s/keV)T     = 26.2 keVCOT      = 51.2 keVCNTT      = 5.1 keVECHFig. 4 Central hard X-ray emissivity for similar shots with co-ECCD (  = 21  ), counter-ECCD (  = 21  ), and pure ECH,respectively (central heating, 1.5 MW). In the exponential fitsin the ECCD cases the lowest energy point was ignored, as it isinfluenced by the bulk Maxwellian distribution.CO−ECCD ECH     CNT−ECCD5 10 150123x 109Chord number−1−1Emissivity (cm   sterad   s   )−2Fig. 5 Spatial profiles of line-integrated hardX-ray emissivity (24–32 keV), for the same con-ditions as in Fig. 4. The chords are numberedfrom the plasma edge to the center, with a ra-dial separation of  2 cm.with numerical simulations carried out with the CQL3D Fokker–Planck code [4]. In the pureheating case the effect of the EC wave is to increase the temperature of the bulk plasma withoutappreciable modification of the shape of the distribution function, owing to rapid thermalisationof the heated electrons. By contrast, the preferential heating of suprathermal electrons on thelow-field side of the resonance in the ECCD case permits the generation and sustainment of anon-Maxwellian tail, which carries the non-inductive current.3. Investigation of fast electron dynamicsStudies of fast electron dynamics in large tokamaks have generally indicated [5,6] thatcollisional slowing-down (momentum destruction and pitch-angle scattering) is the dominantrelaxation mechanism, whereas radialdiffusion plays only a secondary role.In the TCV tokamak the charac-teristic times for these phenomena arecomparable with the temporal resolu-tion of the HXR camera. In orderto study the response and relaxationphenomena in detail, we have carriedout an experiment with modulated ECpower. Under stable plasma condi-tions, the excellent repeatability andlocalisation of EC-wave–plasma cou-pling have allowed a substantial en-hancement of the effective time resolu-tion (down to  300  s) through sum-mation of the photon counts over mul-tiple modulation periods. The modula-tion period must of course be a multiple of the sam-pling interval for the HXR diagnostic. In the ex-ample shown in Fig. 6, 1 MW of ECCD powerwas modulated at 100% with a period of 9.36 ms;the HXR sampling time was 585  s and the countswere then summed over 10 periods. The resultingsignal is shown in Fig. 6 for a central chord andfour different energy levels. The relaxation dynam-ics at turn-on and turn-off are clearly adequately re-solved.To extract the essential physics of the supra-thermal electron dynamics, we have employed asimple model consisting of a source (the localised26th EPS CCFPP 1999 ; S.Coda et al.: Measurements of Hard X-Ray Emission Profiles in the TCV Toka...110016−24 keV24−32 keV 40−48 keV32−40 keV012345678x 104Photon counts (s   )−10.436 0.438 0.44 0.442 0.44400.51Time (s)ECH power (MW)Fig. 6 Hard X-ray emission from the plasma center, for four different en-ergies, and EC power vs. time (central co-ECCD,  = 21  ). The photoncounts are summed over 10 successive EC modulation periods.EC power), a Krook collisionoperator, and a radial diffusiv-ity. The characteristic timesare varied to simulate the timehistory of the experimentalHXR emissivity. In a pre-liminary comparison, we havefound that satisfactory agree-ment is obtained only whenthe collisional term is domi-nant, with characteristic relax-ation times of the order of 1–3ms: these values are consistentwith the collisional slowing-down time for the energy rangeunder consideration.Under these conditions,the spatial distribution of theHXR emissivity can be takenas a good measure of the ECpower deposition profile.More detailed studies are cur-rently in progress. Compar-isons between the experimen-tal local (Abel-inverted) emis-sivity profile and the power de-position profile calculated byray tracing are also underway.AcknowledgmentsWe wish to thank the CEA for the loan of the HXR camera and associated electronics tothe CRPP. We are also grateful to the entire TCV team for their invaluable role in executing theexperiments described in this paper. This research was partly supported by the Swiss NationalScience Foundation.References[1] Y. Peysson and R. Arslanbekov, Nucl. Instr. and Methods, 380 (1996) 423.[2] T.P. Goodman, S. Alberti, M.A. Henderson, A. Pochelon, M.Q. Tran, Proc. 19th SOFT,Lisbon (1996), 565.[3] A. Pochelon, S. Alberti, M. Bourqui, et al., Bull. Am. Phys. Soc. 41 (1996) 1514.[4] R.W. Harvey and M.G. McCoy, in Proc. IAEA TCM/Advances in Simulation andModeling in Thermonuclear Plasmas, Montreal (1992).[5] Y. Peysson, Plasma Phys. Control. Fusion, 35 (1993) B253.[6] S.E. Jones, J. Kesner, S. Luckhardt, et al., Phys. Plasmas, 2 (1995) 1548.

Measurements of hard X-ray emission profiles in the TCV tokamak during electron cyclotron heating and current drive

http://infoscience.epfl.ch/record/123833/files/eps_1999.pdf

Measurements of hard X-ray emission profiles in the TCV tokamak during electron cyclotron heating and current drive

Abstract

Similar works

Full text

Available Versions

Infoscience - École polytechnique fédérale de Lausanne