Experiment Group LHD

K. Ida

K. Kamiya

S. Ohdachi

T.E. Evans

Y. Suzuki

National Institute for Fusion Science (NIFS-Repository)

Large Edge-localized modes (ELMs) are a seriousconcern for ITER. In DIII-D as well as in LHD and agrowing number of tokamaks around the world, resonantmagnetic perturbation (RMP) ﬁelds have proven to beeﬀective for mitigating (i.e., signiﬁcantly reducing thesize) of ELMs or even eliminating ELMs completely1).Although ITER is proposing to include RMP coils inits baseline design, there are a number of key physicsquestions that must be understood to assure the bestpossibility of success for RMP ELM mitigation in ITERH-mode plasmas. The goal of NIFS-GA joint experi-ment is to combine the resources of LHD and DIII-Din joint experiments on developing an understanding ofhow the edge magnetic topology aﬀects heat, momentumand particle transport. The direct relationship betweenthe structure of the magnetic island ﬁeld and the edgeradial electric ﬁeld was investigated in LHD in order tounderstand the eﬀect of RMP on the radial electric ﬁeld.Since both the toroidal and poloidal rotation con-tributes to the radial electric ﬁeld, it is import to inves-tigate how the RMP aﬀects the toroidal and poloildalrotations. In order to distinguish the eﬀect of RMP ontoroidal and poloidal rotation, beams are switched fromco-injection to counter-injection in the LHD experiment.Figure 1a shows the radial proﬁles of radial electric ﬁeldwithout the RMP ﬁeld and ﬁgure 1b with RMP ﬁeld(near O-point) in the discharges. Here, the direction ofNBI is switched from co-direction to counter-direction att = 4.3s. The radial radial electric ﬁeld proﬁle becomesstrongly positive outside last closed ﬂux surface (LCFS: R = 4.55m) because electrons escape to the wall alongthe magnetic ﬁeld in the scrape oﬀ layer (SOL)2). Themagnetic island is formed by the RMP at ι =1 rationalsurface (R = 4.32 - 4.42m) as shown by the dashed linesin ﬁgure 1b and in 1a without the island as a reference.When there is no RMP, the radial electric ﬁeld isnegative following the beam switch from co- to counter.This results from a signiﬁcant change in toroidal rota-tion without the island and its contribution to the radialelectric ﬁeld. However, when the RMP is applied with acurrent of 1920 A and phased to place an island O-pointacross the measurement region, the radial electric ﬁeld isapproximately zero across the island and there is almostno change after the beam switch. This is because theboth poloidal and toroidal rotations are damped due tothe formation of the magnetic island created by RMP.This experiment clearly shows the damping of ﬂows dueto the magnetic island, which is consistent with the ob-servation of damping of toroidal ﬂow observed in JT-60Uwhen the neoclassical tearing more grows and reaches tothe mode locking threshold3).Fig. 1: Radial proﬁles of radial electric ﬁeld (a) withoutRMP ﬁeld and (b) with RMP ﬁled (near O-point) in thedischarges, where the direction of NBI is switched fromco-direction to counter-direction.This experiment suggest two interesting eﬀects ofRMP on plasma ﬂow and radial electric ﬁeld, one is re-duction of radial electric ﬁeld and the other is enhance-ment to radial electric ﬁeld when comparing the casewith and without and island O-point. In the RMP ex-periment, the plasma ﬂow and radial electric ﬁeld areexpected to be damped because of the formation of mag-netic island. On the other hand, in the region of theopen ﬁeld outside of LCFS, the existence of a signiﬁ-cant poloidal ﬂow and positive radial electric ﬁeld are ex-pected when the stochastic region appears at the plasmaboundary by RMP. Therefore the larger radial electricﬁeld shear at the LCFS is expected in the plasma withRMP because of the opposite eﬀect of RMP on theplasma ﬂow inside and outside LCFS.1) Evans, T. E., et. al.,Nucl. Fusion 48 (2008) 024002.2) Kamiya, K., et. al.,Nucl. Fusion 53 (2013) 013003.3) Ida, K., et. al., Phys. Rev. Lett. 109 (2012) 065001.39§8. Understanding the Impact of Changes in the Magnetic Topology on Transport in LHDEvans, T.E. (GA), Ida, K., Kamiya, K. (JAEA),  Suzuki, Y., Ohdachi, S., LHD Experiment Group

Understanding the Impact of Changes in the Magnetic Topology on Transport in LHD

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