H. Tsutsui

K. "Nakamura

K. Kurihara

K. Tokunaga

M. Hasegawa

M. Sueoka

O. Mitarai

X.L." Liu

Y. Kawamata

National Institute for Fusion Science (NIFS-Repository)

In the spherical tokamak QUEST (Bt = 0.25T, R =0.68m, a = 0.40m), as the methods to obtain a steady-state divertor plasma, we have two methods: First, ahigh-density divertor plasma is made by OH (ohmicheating) and the plasma current is planned to be sus-tained by EBW (Electron Bernstein Wave) current drive.Second, a low-density plasma is made by ECRH (Elec-tron Cyclotron Resonance Heating), the plasma currentis ramped up by ECCD (Electron Cyclotron CurrentDrive) and sustained by ECCD.In RF-startup, ramp-up and sustained divertorplasma on QUEST (Fig. 1), plasma current is driveneven in the outside of the LCFS (Large Closed Flux Sur-face) on the low-ﬁeld side. The current is interpretedto be carried by trapped particles in the open magneticsurfaces. The magnetic surfaces are reconstructed by as-suming multi-ﬁlament current in the vacuum chamber1)or assuming truncated current proﬁle, which is a functionof poloidal magnetic ﬂux even in the outside of the closedmagnetic surfaces2). In both cases, the current densityproﬁle is shifted to the low-ﬁeld side from the LCFS.The trapped particles carry current also in the inside ofthe closed magnetic surfaces, and the equilibrium canbe reconstructed by taking into account the anisotropicpressure proﬁle all over the plasma3). In this reconstruc-tion, the perpendicular pressure proﬁle is shifted out-ward reﬂecting the perpendicular velocity component ofthe trapped particles.Plasma equilibrium solution is ﬁtted assuming allplasma current is ﬂowing in the inside of the LCFS. It? ? ? ? ?? ?????????????????????????? ???? ??????????? ? ? ? ?? ???????????????????????????? ??????????? ???????????????????????????????Fig. 1: Time evolution of IP and IPF.Fitted Parameters:βp:2.9382βt:0.95136li:1.6812q95:17.3527δup: 0.62002δdown: 0.62002κ: 0.97384Estor: 94.4815JIP−: −14536.9 AIP+: 295.5822 AR/mZ/mJRZ shot:16499 time: 5000ms ??? ??? ??? ??? ? ??? ???????????????????????????????????Measured Parameters:IP: −9441.9707IPF17: 202.431IPF26: 501.0139IPF35: −295.1984IPF42AB: 348.5172IextraPF: −800IHCUL: −0.29995Fitted Parameters:IP: −9441.0596IPF17: 202.4315IPF26: 501.023IPF35: −295.1969IPF42AB: 348.5172IextraPF: −800IHCUL: −0.29997R/mZ/mJRZ shot:16499 time: 5000ms ??? ??? ??? ??? ? ??? ???????????????????????????????????Fig. 2: Magnetic surfaces (red) and current proﬁle(green). In-vessel poloidal ﬁeld coils are indicated byred squared x markers.is solved within the isotropic pressure proﬁle by EFITcode (Fig. 2a). The overall current density proﬁle isshifted to the low-ﬁeld side from the LCFS and appar-ent poloidal beta becomes larger than unity. Opposite-polarity current density region appeared in the high-ﬁeldregion (negative n-index), where R < 0.56m. The frac-tion of the opposite-polarity current is small to be about2 %. The elongation ratio of the LCFS (separatrix) isabout unity and the aspect ratio is lower than two. Theelongation ratio of the same-polarity region is about 1.4though the aspect ratio becomes larger than two.Plasma equilibrium solution is ﬁtted assumingopen-ﬁeld plasma current ﬂows discretely in in-vesselpoloidal ﬁeld coils in the outside of the LCFS.Anisotropic pressure component is expressed by the in-vessel poloidal ﬁeld coils and isotropic pressure compo-nent is ﬁtted by EFIT code (Fig. 2b). From the re-sult, where the opposite polarity current disappears, theamount of the open ﬁeld current fraction is estimated tobe about 30 % of the total current.Plasma shape was reproduced by CCS (CauchyCondition Surface) method assuming all plasma currentis ﬂowing in the inside of the LCFS. The ill posednesscould not be prevented by reducing the number of ﬁttingparameters. It is reproduced preventing ill-posedness byTikhonov regularization. When Tikhonov regularizationis applied to LSM (Lease Square Method), it is a prob-lem, how to select the Tikhonov matrix and how to deter-mine the regularization parameter. When the Tikhonovregularization is applied to SVD (Singular Value Decom-position) method, the regularization parameter may bedetermined according to the optimal criterion functionfor GCV (Generalized Cross Validation).In preparation for high-temperature wall (reddashed line in Fig. 2) in QUEST in near future, we’dlike to shift inward the current density by shifting inwardthe region with positive n-index. We’d like to increasethe elongation ratio. Such a divertor plasma conﬁgura-tion may be produced by using only inner divertor coil(PF35-1).1) M. Ishiguro, et al.: Physics of Plasmas 19 (2012)062508.2) A. Ejiri, et al.: Nucl. Fusion 49 (2009) 065010.3) T. Maekawa, et al.: Nuclear Fusion 52 (2012) 083008.551§52. Higher-precision Reconstruction and the Feedback Control of Divertor Plasma Shape in QUESTNakamura, K. (RIAM, Kyushu Univ.),  Mitarai, O. (Tokai Univ.),  Tsutsui, H. (Tokyo Tech.),  Kurihara, K., Kawamata, Y., Sueoka, M. (JAEA),  Tokunaga, K., Hasegawa, M. (RIAM, Kyushu Univ.),  Liu, X.L. (IGSES, Kyushu Univ.)

Higher-precision Reconstruction and the Feedback Control of Divertor Plasma Shape in QUEST

Higher-precision Reconstruction and the Feedback Control of Divertor Plasma Shape in QUEST

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