Resistive Wall Mode (RWM) Study on JT-60U

1. JSPS-CAS Core University Program 2008 in ASIPP Plasma and Nuclear Fusion Feb. 16-21, 2009 in ASIPP Resistive Wall Mode (RWM) Study on JT-60U Go Matsunaga 松永　剛 Japan Atomic Energy Agency, Naka, Japan

2. Outline • Introduction • Current driven RWM in OH plasmas • RWM in high-b plasmas • Recent RWM topics • Summery & Suggestion for EAST experiments G. Matsunaga JAEA, CUP in ASIPP

3. Introduction Toward fusion reactors, the high-bN operation is very attractive and advantageous, because high bootstrap current (fBS) and high fusion output (Pfus)are expected. Device Size • However, achievable bN is limited bylow-n MHD instability. • No-wall bN-limit (bN=bNno-wall ->Cb=0) • Ideal-wall bN-limit (bN=bNideal-wall ->Cb=1) Finite wall resistivity makes another mode, Resistive Wall Mode (RWM) that limits achievable bN. (RWM is characterized by wall diffusion time, tw) Therefore, RWM stabilization is a key issue for high-bN operation in ITER and a fusion reactor. G. Matsunaga JAEA, CUP in ASIPP

4. 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 G. Matsunaga JAEA, CUP in ASIPP

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

6. Current driven RWM in OH plasmas

7. 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. G. Matsunaga JAEA, CUP in ASIPP

8. 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, tw is several milliseconds. Current driven RWM ↑ external kink mode + wall stabilizing effect G. Matsunaga JAEA, CUP in ASIPP

9. Wall location effect for RWM Wall stabilizing of current-driven kink mode on OH plasma G. Matsunaga, PPCF, Vol. 49, p.95(2007) G. Matsunaga JAEA, CUP in ASIPP

10. RWM growth rates vs. wall location • 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=3/1 kink mode m/n=2/1 tearing modes G. Matsunaga et al., PPCF, Vol. 49, pp.95-103 (2007) G. Matsunaga JAEA, CUP in ASIPP

11. RWM in high-b plasmas

12. To identify critical plasma rotation for RWM stabilization, we only changed plasma rotation. At 5.9s : Stored energy FB was started → keeping bN constant At 6.0s : Tang NBs were switched from CTR-NB to CO-NB → slowly reducing Plasma rotation Identification of critical rotation for RWM stabilizing G. Matsunaga JAEA, CUP in ASIPP

13. Just before collapse, n=1 radial magnetic field was growing with ~10ms growth time. → RWM High-b RWM was observed by reducing plasma rotation G. Matsunaga JAEA, CUP in ASIPP

14. Plasma rotation profiles • Since bN 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. G. Matsunaga JAEA, CUP in ASIPP

15. Dependence of critical rotation on Cb • Target value of stored energy FB was changed to get the dependence of the critical plasma rotation. • The dependence of the critical rotation on Cb is weak. • This means that we can sustain the high-βup to the ideal wall limit. G. Matsunaga JAEA, CUP in ASIPP

16. Recent RWM topics

17. Challenge of sustainment of high-b discharge • Previously, on JT-60U, the high-bN plasmas > bNno-wall were transiently obtained. • In this campaign, we have tried to sustain the high-bN plasma > bNno-wall with plasma rotation larger than Vtcri. • We have successfully obtained the high-bN plasma for several seconds. G. Matsunaga JAEA, CUP in ASIPP

18. ~5s (~3tR) Best discharge; bN~3.0, ~5sec On the best discharge, • bN~3.0 (Cb~0.4) was sustained by plasma rotation > Vtcri. • Sustained duration is ~5s, which is ~3 time longer than tR. • Time duration is determined by the increase of bNno-wall due to gradual j(r) penetration. • According to ACCOME, fCD80% and fBS~50% were also achieved. G. Matsunaga JAEA, CUP in ASIPP

19. What limits for high-bN long discharges However, the sustainment of high-bN 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 Vt > Vtcri • RWM Precursor strongly affects Vt-profile at q=2, finally, induces RWM onset G. Matsunaga JAEA, CUP in ASIPP

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

21. 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 G. Matsunaga JAEA, CUP in ASIPP

22. 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. G. Matsunaga JAEA, CUP in ASIPP

23. Hot pressure of PERP-NB seems to drive • EWM is stabilized by reducing PERP-NB injection power while keeping bN constant. →Driving source is trapped energetic particle pressure Dbh/btotal ~ -10% G. Matsunaga JAEA, CUP in ASIPP

24. Cb>0,bN<3.0 Cb>0,bN~3.0 Cb~0,bN~3.0 EWM EWM No EWM bN>bNno-wall (Cb>0) is required to drive EWM • The EWM were observed in high-bN plasmas. • However, the EWM requires Cb>0, NOT only high-bN. G. Matsunaga JAEA, CUP in ASIPP

25. EWM stability domains • If the no-wall b limit is changed by j(r), EWM is always destabilized above the no-wall b limit. • Increasing plasma rotation, EWM boundary seems to follow it. → EWM has a similar stability to RWM G. Matsunaga JAEA, CUP in ASIPP

26. 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-b 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! G. Matsunaga JAEA, CUP in ASIPP

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

28. 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-b RWM • Energetic particle effect • ELM interaction G. Matsunaga JAEA, CUP in ASIPP

29. Plasma Rotation Kinetic Energy Integral Plasma Potential Energy Vacuum Energy with No Wall Complex Growth Rate Kinetic Energy Integral Plasma Potential Energy Vacuum Energy with Resistive Wall Dissipation Energy Integral Vacuum Energy with Ideal Wall Wall Skin Time Vacuum Energy without 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) RWM dispersion relation G. Matsunaga JAEA, CUP in ASIPP

30. 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. G. Matsunaga JAEA, CUP in ASIPP

31. Possible interpretation for EWM EWM is originated from energetic particles and marginally stable RWM. MHD Kinetic contribution of fast particles • m/n=1/1 Internal-kink • Fishbone • RWM • Energetic particle driven Wall mode (EWM) unstable marginally stable marginally stable unstable G. Matsunaga JAEA, CUP in ASIPP

32. This mode is unstable w/o wall, however, stable with ideal wall. The mode structure is localized in the LFS → Kink-Ballooning mode structure Ideal MHD analysis by MARG2D G. Matsunaga JAEA, CUP in ASIPP