JT-60U Resistive Wall Mode (RWM) Study on JT-60U Go Matsunaga 松永 剛 Japan Atomic Energy Agency, Naka, Japan JSPS-CAS Core University Program 2008 in ASIPP Plasma and Nuclear Fusion Feb , 2009 in ASIPP

JT-60U Feb , 2009G. Matsunaga JAEA, CUP in ASIPP2 Outline  Introduction Current driven RWM in OH plasmas RWM in high-  plasmas Recent RWM topics  Summery & Suggestion for EAST experiments

JT-60U Feb , 2009G. Matsunaga JAEA, CUP in ASIPP3 Introduction Toward fusion reactors, the high-  N operation is very attractive and advantageous, because high bootstrap current (f BS ) and high fusion output (P fus ) are expected. Finite wall resistivity makes another mode, Resistive Wall Mode (RWM) that limits achievable  N. (RWM is characterized by wall diffusion time,  w ) However, achievable  N is limited by low-n MHD instability. No-wall -limit (= no-wall ->C  =0) No-wall  N -limit (  N =  N no-wall ->C  =0) Ideal-wall -limit(= ideal-wall ->C  =1) Ideal-wall  N -limit(  N =  N ideal-wall ->C  =1) Therefore, RWM stabilization is a key issue for high-  N operation in ITER and a fusion reactor. Device Size

JT-60U Feb , 2009G. Matsunaga JAEA, CUP in ASIPP4 What is key for RWM study?  RWM behaviors  Wall location effect  Rotation stabilization effect →Stabilization Mechanisms  Feedback control →Establishment, Mode controllability  Interaction with other instabilities →ELMs, Energetic particle driven modes  Error field effect →Resonant field amplification (RFA), Active sensing

JT-60U Feb , 2009G. Matsunaga JAEA, CUP in ASIPP5 Useful tools for RWM study onJT-60U Plasam-Wall clearance feedback control  Positive ion based NBs (PNB) 4 tangential  CO ~ 4MW  CTR ~ 4MW 7 perpendicular  Negative ion based NBs (NNB) 2 tangential  CO ~ 4MW  Positive ion based NBs (PNB) 4 tangential  CO ~ 4MW  CTR ~ 4MW 7 perpendicular  Negative ion based NBs (NNB) 2 tangential  CO ~ 4MW Various NB injections

JT-60U Current driven RWM in OH plasmas

JT-60U Feb , 2009G. Matsunaga JAEA, CUP in ASIPP7 Current driven RWM experiments  In order to investigate wall location effect on MHD instability, plasma-wall gap scan has been performed in OH plasma. → since only q-profile can determine the stability, wall effect can be clearly measured.  To destabilize current driven external kink mode, surface q was decreasing by plasma current ramping up.

JT-60U Feb , 2009G. Matsunaga JAEA, CUP in ASIPP8 m/n=3/1 Current driven RWM is observed  qeff was just below 3, m/n=3/1 instability appeared and thermal collapse occurred.  The growth time of this mode is about 10ms. → On JT-60U,  w is several milliseconds. Current driven RWM ↑ external kink mode + wall stabilizing effect

JT-60U Feb , 2009G. Matsunaga JAEA, CUP in ASIPP9 Wall location effect for RWM G. Matsunaga, PPCF, Vol. 49, p.95(2007) Wall stabilizing of current-driven kink mode on OH plasma

JT-60U Feb , 2009G. Matsunaga JAEA, CUP in ASIPP10 RWM growth rates vs. wall location G. Matsunaga et al., PPCF, Vol. 49, pp (2007)  Increasing d/a, RWM growth rate increased.  According to AEOLUS-FT with taking into account a resistive wall, m/n=3/1kink and m/n=2/1 tearing modes are unstable.  The dependence qualitatively agrees with RWM dispersion relation without plasma rotation. m/n=2/1 tearing modes m/n=3/1 kink mode

JT-60U RWM in high-  plasmas

JT-60U Feb , 2009G. Matsunaga JAEA, CUP in ASIPP12 Identification of critical rotation for RWM stabilizing  To identify critical plasma rotation for RWM stabilization, we only changed plasma rotation.  At 5.9s : Stored energy FB was started → keeping  N constant  At 6.0s : Tang NBs were switched from CTR-NB to CO-NB → slowly reducing Plasma rotation

JT-60U Feb , 2009G. Matsunaga JAEA, CUP in ASIPP13 High-  RWM was observed by reducing plasma rotation  Just before collapse, n=1 radial magnetic field was growing with ~10ms growth time. → RWM

JT-60U Feb , 2009G. Matsunaga JAEA, CUP in ASIPP14 Plasma rotation profiles  Since  N was kept constant, deceleration of plasma rotation was thought to make the RWM unstable.  Focusing on the plasma rotation at the q=2, critical plasma rotation is less than 1kHz.  This value is corresponding to 0.3% of Alfvén velocity.

JT-60U Feb , 2009G. Matsunaga JAEA, CUP in ASIPP15 Dependence of critical rotation on C  Target value of stored energy FB was changed to get the dependence of the critical plasma rotation. The dependence of the critical rotation on C  is weak. This means that we can sustain the high-βup to the ideal wall limit.

JT-60U Recent RWM topics

JT-60U Feb , 2009G. Matsunaga JAEA, CUP in ASIPP17 Challenge of sustainment of high-  discharge  Previously, on JT-60U, the high-  N plasmas >  N no-wall were transiently obtained.  In this campaign, we have tried to sustain the high-  N plasma >  N no-wall with plasma rotation larger than V t cri.  We have successfully obtained the high-  N plasma for several seconds.

JT-60U Feb , 2009G. Matsunaga JAEA, CUP in ASIPP18 Best discharge;  N ~3.0, ~5sec On the best discharge,  N ~3.0 (C  ~0.4) was sustained by plasma rotation > V t cri. Sustained duration is ~5s, which is ~3 time longer than  R. Time duration is determined by the increase of  N no-wall due to gradual j(r) penetration. According to ACCOME, f CD  80% and f BS ~50% were also achieved. ~5s (~3  R )

JT-60U Feb , 2009G. Matsunaga JAEA, CUP in ASIPP19 What limits for high-  N long discharges However, the sustainment of high-  N is not straightforward. Because almost all discharges were limited by Resistive Wall Mode (RWM) Neoclassical Tearing Mode (NTM) Furthermore, many discharges have been lost by new instabilities: Energetic particle driven Wall Mode (EWM) directly induces RWM despite V t > V t cri RWM Precursor strongly affects V t -profile at q=2, finally, induces RWM onset

JT-60U Feb , 2009G. Matsunaga JAEA, CUP in ASIPP20 EWM can directly induce RWM In the wall-stabilized high-bN region, Energetic particle driven Wall Mode (EWM) is newly observed. At RWM onset, rotation was enough for stabilization. The EWM is dangerous The EWM is dangerous for RWM n=1

JT-60U Feb , 2009G. Matsunaga JAEA, CUP in ASIPP21 Features of EWM Toroidal mode number: n=1 Poloidal mode number: m~3 (Kink Ballooning-like) Radial mode structure: globally-spread Growth time: 1~2ms

JT-60U Feb , 2009G. Matsunaga JAEA, CUP in ASIPP22 Trapped energetic particle by PERP-NBs (85keV)  Mode frequency is chirping down as mode amplitude is increasing.  Initial mode frequency agrees with the precession frequency of the energetic particles from the PERP-NB.

JT-60U Feb , 2009G. Matsunaga JAEA, CUP in ASIPP23 Hot pressure of PERP-NB seems to drive  h /  total ~ -10%  EWM is stabilized by reducing PERP-NB injection power while keeping  N constant. → Driving source is trapped energetic particle pressure

JT-60U Feb , 2009G. Matsunaga JAEA, CUP in ASIPP24  N >  N no-wall (C  >0) is required to drive EWM  The EWM were observed in high-  N plasmas.  However, the EWM requires C  >0, NOT only high-  N. C  >0,  N 0,  N <3.0 EWM C  >0,  N ~3.0 EWM C  ~0,  N ~3.0 No EWM

JT-60U Feb , 2009G. Matsunaga JAEA, CUP in ASIPP25 EWM stability domains  If the no-wall  limit is changed by j(r), EWM is always destabilized above the no-wall  limit.  Increasing plasma rotation, EWM boundary seems to follow it. → EWM has a similar stability to RWM

JT-60U Feb , 2009G. Matsunaga JAEA, CUP in ASIPP26 Summery  RWM is a key issue in an economical aspect for future fusion reactors.  On JT-60U, RWM has been well studied; Current driven RWM → Wall location effect, High-  RWM → Plasma rotation stabilizing, Instability related to RWM → Coupling to energetic particles.  JT-60U has been shut down in last August. We must wait for JT-60SA for further RWM study. Our corroborations become important!

JT-60U Feb , 2009G. Matsunaga JAEA, CUP in ASIPP27 m/n=1/1 Internal-kink Fishbone Possible interpretation for EWM EWM is a coupling mode between energetic particles and marginally stable RWM. Kinetic contribution of fast particles MHD marginally stable unstable unstable RWMRWM Energetic particle driven Wall mode (EWM)

JT-60U Feb , 2009G. Matsunaga JAEA, CUP in ASIPP28 Suggestion for EAST experiment  Current driven RWM by Ip ramping Wall location effect Stabilizing by fast ion tail by ICRF  External coils Feedback control Active sensing (RFA) Rotation control (Error field effect)  Neutral Beam High-  RWM Energetic particle effect ELM interaction

JT-60U Feb , 2009G. Matsunaga JAEA, CUP in ASIPP29 RWM dispersion relation KineticEnergyIntegralPlasmaPotentialEnergyVacuumEnergywith No Wall M. S. Chu et al., Phys. Plasma, Vol. 11, p.2497(2004) M. S. Chu et al., Phys. Plasma, Vol. 2, p.2236(1995) M. S. Chu et al., Phys. Plasma, Vol. 11, p.2497(2004) M. S. Chu et al., Phys. Plasma, Vol. 2, p.2236(1995) WallSkinTime KineticEnergyIntegralPlasmaPotentialEnergy Vacuum Energy with Ideal Wall VacuumEnergywithResistiveWallDissipationEnergyIntegral Vacuum Energy without Wall ComplexGrowthRate Plasma Rotation

JT-60U Feb , 2009G. Matsunaga JAEA, CUP in ASIPP30 Plasma rotation stabilizing effect on RWM  Some models predict that the critical rotation is several % of Alfven speed at the rational surface. → Dissipation and rotation are required for RWM stabilization.  How much is the critical rotation for RWM stabilization? Future devices will have low plasma rotation.

JT-60U Feb , 2009G. Matsunaga JAEA, CUP in ASIPP31 m/n=1/1 Internal-kink FishboneRWM Energetic particle driven Wall mode (EWM) Possible interpretation for EWM EWM is originated from energetic particles and marginally stable RWM. Kinetic contribution of fast particles MHD marginally stable unstable unstable

JT-60U Feb , 2009G. Matsunaga JAEA, CUP in ASIPP32 Ideal MHD analysis by MARG2D  This mode is unstable w/o wall, however, stable with ideal wall.  The mode structure is localized in the LFS → Kink-Ballooning mode structure