Bindslev, Henrik

Bürger, A.

Delabie, E.

Furtula, Vedran

Kantor, M.Yu.

Korsholm, Søren Bang

Koskela, T.

Leipold, Frank

Meo, Fernando

Michelsen, Poul

Moseev, Dmitry

Nielsen, Stefan Kragh

Salewski, Mirko

Schmitz, O.

Stejner Pedersen, Morten

Uhlemann, R.

Westerhof, E.

Online Research Database In Technology

                       General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.  • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal   If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.     Downloaded from orbit.dtu.dk on: Dec 17, 2017Localized measurements of the fast ion velocity distribution of TEXTOR plasmas usingcollective Thomson scatteringMoseev, Dmitry; Korsholm, Søren Bang; Meo, Fernando; Nielsen, Stefan Kragh; Bindslev, Henrik;Bürger, A.; Delabie, E.; Furtula, Vedran; Kantor, M.Yu.; Koskela, T.; Leipold, Frank; Michelsen, Poul;Salewski, Mirko; Schmitz, O.; Pedersen, Morten Stejner; Uhlemann, R.; Westerhof, E.Published in:ProceedingsPublication date:2010Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Moseev, D., Korsholm, S. B., Meo, F., Nielsen, S. K., Bindslev, H., Bürger, A., ... Westerhof, E. (2010). Localizedmeasurements of the fast ion velocity distribution of TEXTOR plasmas using collective Thomson scattering. InProceedings (pp. P5.123)Localized measurements of the fast ion velocity distribution of TEXTORplasmas using collective Thomson scatteringD. Moseev1, S.B. Korsholm1, F. Meo1, S.K. Nielsen1, H. Bindslev1, A. Bürger2, E.Delabie3, V. Furtula1, M.Yu. Kantor2,3,4, T. Koskela5, F. Leipold1, P.K. Michelsen1, M.Salewski1, O. Schmitz2, M. Stejner1, R. Uhlemann2, E. Westerhof3 and the TEXTOR team21 EURATOM-Risø DTU, Technical University of Denmark, DK-4000 Roskilde, Denmark2 FZ Jülich GmbH, EURATOM, Trilateral Euregio Cluster, D-52425 Jülich, Germany3 FOM-IPP, EURATOM, PO Box 1207, 3430 BE Nieuwegein, The Netherlands4 Ioffe Institute, RAS, Saint Petersburg 194021, Russia5 Aalto University, EURATOM-TEKES, P.O. Box 15100 FI-00076 AALTO, FinlandIntroductionFast ion physics is of importance to nuclear fusion. Therefore measurements of the fast ionvelocity distribution function at different locations in the plasma volume will be of significantvalue. There are a number of diagnostics on present devices which can detect fast ions, e.g. fastion Dα spectroscopy (FIDA) [1], γ-ray spectroscopy [2], scintillator probes [3] and collectiveThomson scattering (CTS) [4]. They do not replace but rather complement each other. For CTS,fast ion information is inferred from probe radiation scattered from electron density fluctuationsin the plasma. When the wavelength of the fluctuations is much larger than the Debye length(LD · kδ ≪ 1), the scattered sigal is mainly due to collective effects in the plasma. Here k⃗δ isa fluctuation wave vector defined by k⃗δ = k⃗s− k⃗i, where k⃗s and k⃗i represent the wave vectorsof scattered and incident radiation respectively. The scattered signal comes from the volumedetermined by the overlap of the probe and receiver beams. The 1D fast ion distribution functionis inferred by fitting the spectrum through a Bayesian procedure using a forward model forscattering [5]. The CTS diagnostic on the TEXTOR tokamak is equipped with a steerable mirrorsystem which enables measurements of 1D projections of the fast ion distribution function alongk⃗δ at different radial positions in the plasma and at different angles ϕ = ̸ (B⃗,⃗kδ ) to the magneticfield. Results presented in this paper are obtained for the plasma center at R≈ 1.8 m and off-axisat R ≈ 2.0 m at two different pitch angles ϕ ≈ 140◦ and ϕ ≈ 110◦.SetupThe experiments presented here were conducted in deuterium plasmas with hydrogen NBI inthe TEXTOR tokamak (R = 1.75 m, a = 0.47 m). Fast ions come from the co-IP neutral beaminjector at 1.2 MW with acceleration voltage of 50 keV. The TEXTOR neutral beam injectors37th EPS Conference on Plasma Physics P5.123(a)1 1.5 2 2.5−0.8−0.6−0.4−0.200.20.40.60.8R, mz, m(b)Figure 1: Geometry of the experiments. (a) Top view showing the plasma center (red) and theLFS limiter (green); (b) Poloidal view showing estimated flux surfaces (red) and limiters andthe vacuum vessel (both in green). The blue and black elipsoids in both figures denote themeasurement volumes, where black corresponds to ϕ ≈ 140◦ and blue to ϕ ≈ 110◦are very tangential. The tangential radius (the radius at which the injected beam is parallel tothe magnetic field) is 1.65 m. A high power microwave beam (180 kW) in ordinary mode at 110GHz was launched into the plasma and scattered radiation is detected by the CTS receiver [6]also in the ordinary mode.ExperimentThe received signal consists of scattered radiation and ECE background. In order to distin-guish between them, the gyrotron is modulated. In this experiment 2 ms bursts of radiation arelaunched periodically every 20 ms. In order to find the antenna position that corresponds tomaximum overlap, we swept the receiver beam across the gyrotron beam. The signal strengthduring such an experiment changes with the overlap and has a bell-like shape as a function oftime. The procedure of overlap sweeps is discussed in detail in [7]. We conducted experimentsat different radial positions and pitch angles. The geometry of the experiments is illustrated inFig. 1.We made measurements during four overlap sweeps in four reproducible discharges with IP= 400 kA, BT = 2.6 T. Such a field is essential to position the fundamental and the second har-monic of electron-cyclotron resonance at the gyrotron frequency outside the plasma in order toavoid absorption of the gyrotron power and to minimize the ECE background. Plasma parame-37th EPS Conference on Plasma Physics P5.123−4 −3 −2 −1 0 1 2 3 4x 10600.511.522.533.5 x 1012Velocity, m/sf1D, s/m4φ = 140°, R = 1.8 m#111508                      φ = 140°, R = 2.0 m#111509                      BULK @ 1.8 m BULK @ 2.0 m (a)−4 −3 −2 −1 0 1 2 3 4x 10600.511.522.533.5 x 1012Velocity, m/sf1D, s/m4φ = 110°, R = 1.8 m  #111506φ = 110°, R = 2.0 m  #111512BULK @ 1.8 m BULK @ 2.0 m (b)Figure 2: 1D projections of fast ions velocity distribution functions to k⃗δ direction at differentradial positions (R = 1.8 m - blue; R = 2.0 m - red). (a) ϕ ≈ 140◦; (b) ϕ ≈ 110◦. The bulkfeatures in both graphs are calculated with CXRS data.ters at the center (ρ = 0) were: ne = 3.5·1019 m−3, Te = 1.7 keV, Ti = 2.7 keV with co-IP NBI at1.2 MW.During the experiments, the plasma was sawtoothing. In order to avoid any influence ofanisotropic fast ion redistribution [8], the gyropulses analyzed were taken right before a saw-tooth crash and still in the time window with sufficient overlap during the sweep.Results and discussionThe measured signal in the CTS diagnostic is the scattered spectrum. The fast ion distributionfunction was inferred through fitting the experimental spectrum with one predicted by a modelfor CTS. We use the model described in [9] and Bayesian least squares fitting procedure whichaccounts for nuiscance parameters (LSN). The nuisance parameters are defined as parameterswhich the CTS spectrum depends on (Te, Ti, scattering angle, etc) but which are not the pa-rameters of interest (i.e. fast ion velocity distribution). The main difference between LSN andthe least squares fitting is that LSN can treat nuisance parameters either as free or fixed andtake uncertainties in them into account. Therefore we can make full use of prior informationabout all relevant model parameters. The inferred 1D velocity distribution functions are shownin Fig. 2. The errorbars in the velocity distribution functions account for one standard deviationof the spectral power density of a single gyrotron burst. These errorbars also take into accountuncertainties of plasma parameters which influence scattered spectrum: Te, Ti, ne, Vrot , etc.37th EPS Conference on Plasma Physics P5.123Positive velocities in the distribution function corresponds to the direction along k⃗δ . Accordingto Fig. 1(a), the toroidal component of k⃗δ is is in the same direction of IP and thus with thedirection of neutral beam injection. One can see strong anisotropy of the distribution functionsat ϕ ≈ 140◦(Fig. 2(a)) which is consistent with our expectations due to very tangential geome-try of the neutral beam injection. Even though the difference in pitch angles in the experimentwas only about 30◦ the anisotropy in the distribution functions at ϕ ≈ 110◦ (Fig. 2(b)) is muchmore weakly pronounced than in the case of ϕ ≈ 140◦ scattering geometry. It is important tonotice that the difference in the distribution function shapes in Fig. 2(a) (ϕ=140◦) is distin-guishable within the diagnostic accuracy. Fig. 2 shows some degree of inhomogeneity, hencehas some information of the fast ion pressure profile which should be compared to Monte Carlosimulations. Future work will seek to improve the accuracy of the inferred fast ion velocity dis-tribution function. Comparison of these measurements with numerical Monte Carlo simulationsis on-going and the results of this comparison will be reported in a separate article.References[1] M. Van Zeeland and et al. PPCF, 51(5):055001, 2009.[2] Kiptily V.G. and et al. Nuclear Fusion, 45(5):L21–L25, 2005.[3] M. Nishiura and et al. RSI, 75(10):3646, 2004.[4] S. Michelsen and et al. RSI, 75(10):3634, 2004.[5] H. Bindslev. RSI, 70:1093–1099, 1999.[6] S. B. Korsholm and et al. RSI, 77, 2006.[7] Nielsen S. K. and et al. Physical Review E, 77:016407, 2008.[8] Nielsen S. K. et al. submitted to PPCF, 2010.[9] H. Bindslev. PhD thesis, Risø National Laboratory, Roskilde, Denmark, 1992.37th EPS Conference on Plasma Physics P5.123

Localized measurements of the fast ion velocity distribution of TEXTOR plasmas using collective Thomson scattering

http://orbit.dtu.dk/ws/files/4903986/article1.pdf

Localized measurements of the fast ion velocity distribution of TEXTOR plasmas using collective Thomson scattering

Abstract

Similar works

Full text

Available Versions

Online Research Database In Technology