Configuration Control Studies of Heliotron J F. Sano, et al. Kyoto University

1. 21st IAEA Fusion Energy Conference, 16-21 October 2006, Chengdu, China EX/5-5Ra Configuration Control Studies of Heliotron JF. Sano, et al.Kyoto University Heliotron J CHS Progress of Confinement Physics Study in Compact Helical System S. Okamura, et al. National Institute for Fusion Science EX/5-5Rb

2. Bilateral collaboration Program of NIFS Opportunities and challenges to experimentally study the key issues of helical system optimization Heliotron J : i(a)/2p and eb Control Studies • Heliotron J (2000 - ) • helical-axis heliotron L=1/M=4 • R=1.2 m, B < 1.5 T • ap=0. 17 m • Ap=7 • - ia/2p ~ 0.56 low shear • ECH/NBI/ICRF systems CHS : Transport Barrier Physics Studies CHS (1988 - 2006) - LHD-type heliotron L=2/M=8 - R=1 m, B < 2 T - ap=0.2 m - Ap=5 low aspect ratio -ia/2p ~ 0.8-1.2 high shear at edge - ECH/NBI/ICRF systems

3. 21st IAEA Fusion Energy Conference, 16-21 October 2006, Chengdu, China FEC2006 F. Sano1, T. Mizuuchi1, K. Kondo2, K. Nagasaki1, H.Okada1, S. Kobayashi1, K. Hanatani1,Y. Nakamura2, Y.Torii1,S. Yamamoto4, M.Yokoyama5, Y. Suzuki5, M. Kaneko2, H. Arimoto2, G. Motojima2, S. Fujikawa2, H. Kitagawa2, H. Nakamura2, T. Tsuji2, M. Uno2, H. Yabutani2, S.Watanabe2, S. Matsuoka2, M. Nosaku2, N. Watanabe2, N. Nishino6, Z. Feng 7, Y.Ijiri1, T.Senju1, K.Yaguchi1, K.Sakamoto1, K.Tohshi1, M.Shibano1 1Institute of Advanced Energy, Kyoto University, Uji, Japan; 2Graduate School of Energy Science, Kyoto University, Kyoto, Japan; 3Graduate School of Engineering, Kyoto University, Kyoto, Japan; 4Graduate School of Engineering, Osaka University, Suita, Japan; 5National Institute for Fusion Science, Toki, Japan; 6Graduate School of Engineering, Hiroshima University, Hiroshima, Japan; 7Southwestern Institute of Physics, Chengdu, China; Configuration Control Studies of Heliotron J Outline 1. Introduction 2. Objective 3. Experimental Setup 4. Results and Discussion about the Bumpiness Control Studies . Plasma Current Control . Fast Ion Confinement . Bulk Plasma Confinement 5. Summary Optimization study of a helical-axis heliotron High-quality H-mode appears to be linked with access to the specific vacuum i(a)/2p values. Objective To extend the understanding of neoclassical transport of 3-D plasmas and the related role of field ripples such as bumpiness in confinement improvement for the quasi-omnigeneous approach of the optimization of a helical-axis heliotron.

4. w0/w=0.499 Bumpiness control studies under the fixed vacuum i(a)/2p=0.56 : (1) Low-eb, (2) Medium-eb, and (3) High-eb configurations. Poster EX/P6-14 “Control of Non-Inductive Current in Heliotron J”, K.Nagasaki, G.Motojima, et al. Configuration Set-up i(a)/2p=0.56 Bootstrap current control ECCD control eb et High-eb Medium-eb eh Low-eb For the fixed i(a)/2p=0.56, the bumpiness only was varied by using the independent control of each toroidal coil current (TA or TB) under almost the same eh and et conditions. Bumpiness plays an important and effective role in the control of the bootstrap current and electron cyclotron current drive (ECCD) in Heliotron J.

5. For NBI heating, the 1/e decay time of CX-flux after NBI turned off suggests that the higher eb configuration is more favorable for the fast ion confinement due to the reduced B drift. For ICRF heating, the high-energy iontail temperature increases with an increase in eb . Bumpiness dependence of 1/e decay time 1/e decay time (ms) Loss rate from orbit calculation ICRF Medium-eb Medium-eb Low-eb High-eb High-eb High-eb NBI Low-eb Medium-eb Low-eb Poster EX/P6-1 “Dependence of the Confinement of Fast Ions Generated by ICRF Heating on the Field Configuration in Heliotron J”, H.Okada, et al.

6. As for bulk plasma confinement, the experimental bumpiness dependence of the volume normalized plasma energy of the 70-GHz, 0.3-MW ECHas a function of density suggests that the medium-eb plasmas provide more favorable thermal confinement properties. Low-eb Medium-eb High-eb `ne=0.4x1019m-3 Medium-eb LCFS Depending on the density evolution, ECH plasma spontaneously develops into H-mode at densities higher than the threshold density, followed by radiation collapse in a time scale of tEexp. Edge/SOL Characteristics in the low-density case

7. For high-eb, a weak (or slow) L-H transition only was observed at this ECH power level. This indicates that the configuration modified with the bumpiness affects the threshold nature of H-mode in Heliotron J. For low-eb, the dithering transitions showed only a modest improvement of Wp as a result of density rise. Calculated neoclassical poloidal viscous damping rate coefficient Cp as a function of radius r (m) for high-eb, medium-eb and low-eb configurations. FEC2006 High- eb Low- eb The difference in Cp between the three configurations considered here are almost negligible, much more work is necessary before comparison with experiment.

8. The reduction in eeff suggests a favorable effect on the confinement of ECH plasma in the L-mode and the transient phase of the H-mode ( including dWp/dt effects) . tEexp =Wp/(habsPECH - dWp/dt) habs= habs1+ href(1- habs1) under the assumption of href=0.3. The reduction of the neoclassical diffusion coefficient depends on the appropriate choice of eb. The results 1) from the DCOM code showed that the medium-eb configuration provides a greater degree of neoclassical optimization in the 1/n regime. However, due to the large data scatter and inherent error bars, further studies are necessary to understand the more statistical and physical trends of anomalous confinement of ECH plasmas. 1) The results (eeff) were recently revised and a factor 2 larger than before.

9. Configuration Control Studies of Heliotron J Summary 1. Bumpiness control experiments have been carried out with special reference to the omnigeneous (isodynamic) optimization of a helical-axis heliotron. 2. The bumpiness was found to effectively control the bootstrap current and the balance of the ECCD mechanisms (EX/P6-14). 3. The NBI and ICRF experiments suggest that the higher-eb configuration provides better fast ion confinement (EX/P6-1). 4. The ECH experiments suggest that the lower ”effective helical ripple, eeff” configuration of medium-eb provides better global energy confinement in the L-mode and also in the transient phase of H-mode. 5. Further studies are necessary to determine what effect (including the plasma electric field, the plasma flow and/or edge/SOL plasma behavior) makes up the observed difference between the bumpiness dependence or the ”effective helical ripple, eeff” dependence. It should be noted here that the effective helical ripple represents the local neoclassical diffusivity in the 1/n regime and that as for fast ion confinement, the drift loss is essentially important.

10. Confinement Physics Study in Compact Helical System CHS Progresses in ITB Physics National Institute for Fusion Science, Toki, Japan • Ion Confinement Improvement • Turbulence Measurement with HIBP S. Okamura, T. Akiyama, A. Fujisawa, K. Ida, H. Iguchi, R. Ikeda, M. Isobe, Y. Jinguji, S. Kado 1, T. Kobuchi, K. Matsuo 2, K. Matsuoka, T. Minami, S. Mizuno, K. Nagaoka, K. Nakamura, H. Nakano, S. Nishimura, T. Oishi, S. Ohshima, A. Shimizu, C. Suzuki, C. Takahashi, M. Takeuchi, K. Toi, N. Tomita 3, S. Tsuji-Iio 3, Y. Yoshimura, M. Yoshinuma and CHS group Progresses in ETB Physics (H-mode) • Edge Turbulence Measurement • Edge Electric Field Measurement • New H-mode with High Density • Edge Harmonic Oscillation Study Poster EX/P8-1 1) High Temperature Plasma Center, The University of Tokyo, Chiba, Japan 2) Fukuoka Institute of Technology, Fukuoka, Japan 3) Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, Tokyo, Japan Progresses in TAE & EPM Study • Local Measurement of Energetic Particles Poster EX/P6-8 1/7

11. Ion Confinement Improvement for ITB Plasma CERC Plasma with NBI Electron Temperature Profile Electron Temperature Foot points of temper-ature gradient (internal transport barrier) appear to be different More precise information is required for transport barrier structure study Ion Temperature Gradient Ion Temperature Ion Temperature Gradiant Measurement dTi(R) insead of Ti(R) dR Electron Density Steep temperature gradiant (13 keV/m) for ions is measured using new TVCXS diagnostic Locations of transport barriers are different for electrons and ions in ITB discharges 2/7

12. 200 150 f (kHz) 100 50 0 40 60 80 100 Time(ms) transition 1 f flux (a.u.) Flux at ~70kHz 0 Measurement of Turbulent Flux by Heavy Ion Beam Probe Turbulent flux is estimated with measured fluctuations of density and potential HIBP measured suppression of turbulent particle flux (in the frequency range of 70 kHz) when the Internal Transport Barrier (ITB) is formed. no ITB with ITB Back-transition HIBP fluctuation measurement 3/7

13. Measurement of Edge Fluctuations for H-mode Plasma Beam Emission Spectroscopy (BES) measures edge pedestal of H-mode BES measures suppression of turbulence at plasma edge Large reduction of fluctuation at (r/a)=0.95 RMS value (0-100 kHz) H-alpha 4/7

14. Measurement of Edge Electric Field using Carbon VI Doppler Shift Negative radial electric field of Er ~ 10 kV/m appeared with ETB formation Electric field shear of ~ 2 MV/m2 is created just inside the last closed magnetic surface and sustained during H-mode Time window of TVCX measurement is 20 msec H-mode transition appears at 80 msec Poloidal flow speed of C6+ Edge ion temperature Electron Diamagnetic Direction V(km/s) Ti (eV) (r/a)=0.9 (r/a)=1 (r/a)=0.9 (r/a)=1 5/7

15. New H-mode Discharge for High Density Plasma High performance H-mode was triggered by stopping strong gas puff High electron temperature and high electron pressure were sustained at plasma edge region Stopping gas puff Edge electron temperature and density at (r/a) = 0.7 Central electron temperature and density at (r/a) = 0.0 High performance H-mode H-mode H-alpha 1st L-H 2nd L-H Back transition 6/7

16. Summary of CHS Transport Barrier Physics Experiment In the internal transport barrier (ITB) experiment, new diagnostic for ion temperature gradient measurement showed a steep gradient of 13 keV/m. ITB locations are different for electrons and ions. In the edge transport barrier (ETB) experiment, negative radial electric field (~ 10 kV/m) was measured at the plasma edge by the charge exchange spectroscopy. Electric field shear of 2 MV/m2 is created, which is strong enough to suppress the turbulence. New high performance H-mode was found for high density plasma (Ne~ 1 x 1020 m-3) with gas puff control. High electron temperature and high electron pressure were created at plasma edge. 7/7

17. Collaboration Research Program between CHS and Heliotron J Groups Overdense Plasma Heating by O-X-B Mode Conversion • Evident increase in stored energy has been observed by applying 54.5 GHz ECH in overdense NBI plasmas on CHS • The electron density exceeds the O-mode cut-off, nec=3.7x1019m-3 • The EC injection angle for max Wp is close to a predicted O-X conversion point

18. Frequency (kHz) m~3/n=2 100 Freq. (kHz) m~2/n=1 1/q m=4/n=2 0 (A.U.) IS (kJ) ,I a dia H I W Collaboration Research Program between CHS and Heliotron J Groups Studies for Fast Ion Transport Induced by MHD modes Comparison of shear Alfvén spectra between CHS and Heliotron J • CHS • Moderate negative magnetic shear. • TAE gaps formed by the poloidal mode coupling exist on. • Heliotron J • Weak magnetic shear. • Shear Alfvén continua cannot couple with each other. TAEs or EPMs (toroidal AEs) GAEs (global AEs) * M. Isobe, et al. EX/P6-8 Heliotron J #21145, eb = 0.16, Bt =1.36T • Bursting GAEs in Heliotron J • Bursting GAEs (m=4/n=2, f = 40~70 kHz) appeared in Co-injected NB plasmas at high eb configuration. • Simultaneous bursts in ion saturation current and Ha signal support the existence of the outward particle flux. • Installation of directional Langmuir probe** for energetic ion measurements is planned. • **K. Nagaoka, et al., PFR Vol. 1 (2006) 005