Bindslev, H.

Furtula, V.

Korsholm, S.

Leipold, F.

Leuterer, F.

Meo, F.

Michelsen, P.

Moseev, D.

Nielsen, S.

Salewski, M.

Stejner, M.

Stober, J.

Tardini, G.

Wagner, D.

Woskov, P.

English

MPG.PuRe

Comparison of Collective Thomson Scattering Diagnostic Results toTRANSP Simulations on ASDEX UpgradeM. Stejner1, F. Meo1, M. Salewski1, H. Bindslev1, V. Furtula1, S. B. Korsholm1, F. Leuterer3,F. Leipold1, P. K. Michelsen1, D. Moseev1, S. K. Nielsen1, J. Stober3,G. Tardini3, D. Wagner3, P. Woskov2, and The ASDEX Upgrade team1Association EURATOM-Risø National Laboratory for Sustainable Energy, TechnicalUniversity of Denmark, DK-4000 Roskilde, Denmark2MIT Plasma Science and Fusion Center, Cambridge, MA 02139,USA3Max-Planck-Institut für Plasmaphysik, EURATOM Association, D-85748 Garching, GermanyIntroductionThe Collective Thomson Scattering (CTS) technique offers the ability to measure projectionsof the confined ion velocity distributions in tokamak plasmas. The ion velocity distribution canbe inferred from the spectrum of received scattered radiation when a powerful probe beam isincident on plasma fluctuations driven by the ion motion [1, 2]. Fluctuations with wave vectorkδ = ks−ki will scatter probe radiation to frequencies shifted by approximatelyωδ = vion ·kδ ,and the component of the ion velocityvion in the direction ofkδ can be inferred. Hereksandki are the wave vectors of the received scattered radiation and the incident probing beamrespectively. It is of particular interest to obtain the velocity distributions of energetic non-thermal ions resulting from fusion processes or externally applied heating. These fast ions maycontain a large fraction of the plasma energy which must be channeled into heating the bulkplasma. However, they also constitute a source of free energy which can excite plasma wavesand turbulence which in turn affect the confinement of both fast ions and bulk plasma.First scattering results of the CTS system installed on ASDEX Upgrade (AUG) have beenachieved using gyrotron radiation at 105 GHz as the probing beam [3]. This paper presentsthe methodology of comparing experimental CTS results to TRANSP/NUBEAM simulationsof the fast ion velocity distribution in AUG. We use plasma heating by neutral beam injection(NBI) as an example. A detailed account of the results is also presented at this conference in [4].CTS measurements in the NBI heated AUG discharge 24089In AUG discharge 24089 an H-mode plasma was generated with a magnetic field of 2.6 T onaxis with two phases of NBI heating, i.e. with one and two neutral beams respectively. The twobeams deposited deuterium ions with similar injection angles, beam energies of respectively 93keV and 60 keV and a power of 2.5 MW each. The fast ion population was measured by CTSin each phase with the scattering geometry illustrated in Fig. 1(a) where the beams and relevant36th EPS Conference on Plasma Phys. Sofia, June 29 - July 3, 2009 ECA Vol.33E, P-1.163  (2009)−2.5 −2 −1.5 −1 −0.5 000.511.522.5x [m]y [m]B p I ik k k t s δ(a) Scattering geometry−3 −2 −1 0 1 2 300.511.522.53kδBv|| [106 m/s]v ⊥ [106  m/s](b) TRANSP/NUBEAM simulationFigure 1:(a) Sketch of the scattering geometry in discharge 24089 projected onto the toroidal plane. The probebeam is shown in blue, the receiver beam in green. The scattering volume lies at the intersection of probe andreceiver beams and is shown in magenta. The direction of the toroidal magnetic field, the plasma current, and theprojected wave vectors are indicated. (b) Equally spaced contours of the fast ion velocity distribution in the scatter-ing volume calculated by TRANSP/NUBEAM for the phase with two neutral beams. The bulk ion distribution isnot included which leads to low densities near the origin. The directions of the magnetic field andkδ re indicated.wave vectors are shown projected onto the toroidal plane. The angle betweenkδ and the mag-netic field – which is also the angle between the resolved velocity component and the magneticfield – was 122±1◦. The measurement is spatially resolved with a resolution of approximately10 cm at the location of the probe and receiver beam intersection which was located at the centerof the plasma. In order to subtract background ECE radiation emanating from the plasma, thegyrotron was modulated. For the present purpose of comparing heating regimes we shall presentdata averaged over 15 gyropulses spanning 100 ms. A more detailed account of the AUG CTSsystem and its capabilities can be found in [3].TRANSP/NUBEAM simulationsTheoretical two-dimensional fast ion velocity distributions were calculated for each NBI config-uration at the location of the CTS beam overlap in the plasma center using the plasma analysiscode TRANSP coupled with the neutral beam module NUBEAM [5, 6]. The NUBEAM mod-ule is a comprehensive computational model for neutral beam injection in tokamaks. NUBEAMcomputes the time-dependent deposition and slowing down of the fast ions produced by NBI,taking into consideration beam geometry and composition, ion-neutral interactions (atomicphysics), anomalous diffusion of fast ions, the effects of large scale instabilities, the effect ofmagnetic ripple, and finite Larmor radius effects. Fig. 1(b) shows the resulting velocity distribu-tion for the two-beam heating regime as well as vectors indicating the directions of the magnetic36th EPS 2009; M.Stejner et al. : Comparison of Collective Thomson Scattering Diagnostic Results to TRANSP... 2 of 4field and the resolved velocity component. The largest densiti s for both beams are found at apitch angle of 120◦ which is the same velocity component measured by the CTS receiver. Forcomparison with the measured CTS spectrum we use the projection of the ion velocity distribu-tion along the direction of the resolvedkδ vector.Comparison and interpretation of experimental and theoretical resultsThe CTS spectra measured during each heating regime are shown in Fig. 2(a). The contributionfrom bulk ions dominates at frequencies in the range 104.3–105.5 GHz which correspondsto ion velocities below two times the ion thermal velocity. The beam ions have a preferredvelocity direction, so the spectrum is asymmetric with the beam ions contributing mainly athigh frequencies. The decrease in the fast ion population after the 60 keV beam is switched offcan be clearly seen as a decrease in signal strength at high frequencies – the magnitude of whichis outside the error bars.Experimental CTS results can be compared to theoretical expectations in two complementaryways. The comparison can either be done in frequency space if a synthetic CTS spectrum iscalculated from a theoretical ion velocity distribution, or it can be done in velocity space if theion velocity distribution can be inferred from the experimental CTS spectra. A forward modelexists which – given an ion velocity distribution and estimates of relevant plasma parameters –can calculate synthetic CTS spectra based on a kinetic and fully electromagnetic description ofthe scattering process [1, 2]. However, no direct mapping from frequency space to velocity spacehas been formulated, so the inference of ion velocity distributions from measured CTS spectrarequires the solution to an inverse problem based on forward modeling. This problem can bereliably solved using a least squares type method of inference based on Bayesian analysis whichfully takes into account the uncertainties of all relevant plasma parameters as well as those ofthe measured CTS spectrum [7].Fig. 2(a) shows the measured spectra and the synthetic spectra calculated by forward model-ing based on the fast ion velocity distribution functions from TRANSP/NUBEAM and assumingMaxwellian velocity distributions for all other plasma components. Fig. 2(b) shows the theoreti-cal fast ion velocity distribution and the fast ion velocity distribution inferred from the measuredCTS spectra. In the experimental data both the asymmetry induced by the neutral beams andthe decrease in signal strength and fast ion density when one beam is turned off can be clearlyseen and are outside the error bars in both figures. However, although the spectral shape of theexperimental data is well reproduced by the synthetic data some disagreement can be seen in theoverall signal strength. The significance and origin of the disagreement is a subject of ongoinginvestigation. In this respect it may be noted that the simulated spectra shown in Fig. 2(a) donot account for uncertainties in the parameters used to generate them. This includes an overall36th EPS 2009; M.Stejner et al. : Comparison of Collective Thomson Scattering Diagnostic Results to TRANSP... 3 of 4103 104 105 106 1075101520ν [GHz]Spectral power density [eV]2 NBIs1 NBI(a) Measured and simulated spectra.−3 −2 −1 0 1 2 3x 1060123x 1012u [m/s]f [s/m4 ]2 NBIs1 NBI(b) Measured and simulated fast ion veloc-ity distributionsFigure 2:(a) Comparison of measured and simulated spectra. The two heating regimes are shown respectively inblue (one beam) and red (two beams).−−NUBEAM, ◦ - measurement,· · · - gyrotron frequency. (b) Comparisonof measured and simulated fast ion velocity distributions alongkδ . The measured distribution is restricted to therange outside twice the thermal velocity: 2·vT ∼ 106 m/s.scaling factor which is poorly known. Such uncertainties are, however, taken into account bythe Bayesian method used to infer the fast ion velocity distribution shown in Fig. 2(b).ConclusionWe have performed CTS measurements to diagnose the fast ion velocity distribution for an H-mode discharge with different NBI configurations. The results demonstrate the ability of theCTS system to detect fast ions from neutral beam injection and to distinguish between thedifferent heating regimes. The experimental results were compared to theoretical expectationsfor the fast ion velocity distribution based on TRANSP/NUBEAM calculations. Within theuncertainties of both theory and experiment this first comparison show a reasonable level ofagreement.References[1] Bindslev H., Plasma Phys. Control Fusion,35 1615 (1993)[2] Bindslev H., J Atmos. Terr. Phys.,58, 983 (1996)[3] Meo F.et al, Rev. Sci. Instrum.,79, 10E501 (2008)[4] Meo F.et al, this conference.[5] Budny R. V. et al, Nucl. Fusion,35, 1497 (1995)[6] Pankin A., McCune D., Andre R. and Kritz A., Comp. Phys. Communications,159, 157 (2004)[7] Bindslev H., Rev. Sci. Instrum.,70, 1093 (1999)36th EPS 2009; M.Stejner et al. : Comparison of Collective Thomson Scattering Diagnostic Results to TRANSP... 4 of 4

Comparison of Collective Thomson Scattering Diagnostic Results to TRANSP Simulations on ASDEX Upgrade

http://hdl.handle.net/21.11116/0000-000A-2D05-F

Comparison of Collective Thomson Scattering Diagnostic Results to TRANSP Simulations on ASDEX Upgrade

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