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August 2008

The JT-60U experiment was successfully completed at the end of August 2008. We would like to acknowledge all people who have contributed to the JT-60U project.

Neoclassical tearing mode stabilization

Neoclassical tearing mode stabilization by modulated ECCD is considered to be more effective than unmodulated ECCD. In the JT-60U ECRF system, modulation capability has been upgraded to be able to modulate at higher frequency than 5 kHz. In addition, a system to synchronize EC wave with NTM rotation was newly developed. Stabilization of an m/n=2/1 NTM by modulated ECCD was performed. It was found that the stabilization effect by modulated ECCD significantly depends on the phase difference between magnetic perturbation (NTM rotation) and modulated EC wave power: While mode amplitude decreased for nearly O-point ECCD, it increased for nearly X-point ECCD. In addition, it was found that modulated ECCD is more than twice effective than unmodulated ECCD.

Sustainment of high βN above no-wall β limit

To sustain a high beta plasma above the ideal MHD no-wall beta limit with βN>3 for several seconds, plasma rotation and NB injection pattern were optimized so as to avoid resistive wall mode (RWM) and energetic particle driven instability (EWM). This is because it was found that energetic trapped particles can drive EWM and the EWM triggers RWM even if the plasma rotation at q=2 surface was higher than critical rotation speed for RWM stabilization. As a consequence, high βN~3.0 above the no-wall limit was successfully sustained for about 5 s, which corresponds to 3 times longer than the current diffusion time. RWM and EWM were suppressed throughout the discharge by using minimal perpendicular NBs and maximal co-NBs.

JET/JT-60U similarity experiment on the effect of ripple and rotation

In the past similarity experiments in JT-60U after the installation of ferritic steel tiles, pedestal data in co-rotating plasma was only obtained. In order to obtain complete dataset in wide variation of edge Vt, new experiments of rotation scan, NB power scan and density scan were performed in plasmas at δr of ~0.5%. The initial data analysis shows that the achievable pedestal pressure was degraded, when edge Vt became negative (counter rotation). It is noted that the same dependence was found between with and without ferritic steel tiles (δr of ~0.5% and 1.2%).

Zeeman polarimetry measurement by using LiBP

Zeeman polarimetry data were obtained in the H-mode plasma at q95~8 and δ~0.57 (Ip/Bt=0.8 MA/3.8 T). To reduce a systematic uncertainty, identical shots with and without Li-beam injection were performed sequentially, holding all other control parameters fixed. Quasi-stationary H-mode plasma having small ELMs in high q95-δ operational regime (so-called "mixture (grassy and Type-I)" ELMs) was maintained with 4 units of NB injection (PNBI~8 MW), which made it possible to reduce a statistical error with a longer pile-up period. In-situ calibration was performed at L-mode phase (PNBI~2 MW at q95~7.2 and δ~0.19), avoiding a systematic uncertainty. Detailed plasma edge measurements, such as LiBP, CXRS, Dα, etc., were also performed. Change in slope of the field angle [sin(α)] was obtained between in L- and H-mode phases. This seems to indicate existence of localized bootstrap current in H-mode pedestal. Preliminary result for the equilibrium reconstruction using LiBP Zeeman polarimetry data as a constraint has been obtained, indicating the flux-averaged edge current density of ped> ~0.15-0.25 MA/m2 at the H-mode edge region having small ELMs.

Particle balance study in long-pulse H-mode discharges:

At a wall temperature of 150°C, long-pulse H-mode discharges were repeated to evaluate the particle inventory. At a Greenwald density fraction of 0.55, the number of particles retained in the wall in one pulse decreased pulse-by-pulse, and became positive constant after the 5th pulse. Then, at higher density of a Greenwald density fraction of 0.7, the number of particles retained in the wall in one pulse became higher, and was kept approximately constant even with increasing pulse number up to 12 pulses. Thus, no wall saturation was observed at 150°C as already observed at 80°C. These results are in contrast to that at 250°C, where significant outgassing (decrease of the wall inventory) was observed. In the discharges with a Greenwald density fraction of 0.7, 13CH4 was injected around the outer strike point at a rate higher by a factor of approximately two than CD4 flux by the chemical sputtering. The deposited 13C profile will be investigated to understand the transport of carbon generated from the outer divertor.

Toroidal rotation and momentum transport

Beam perturbation techniques with perpendicular NBIs (edge Vt modulation due to fast ion loss modulation induced by toroidal field ripple) were applied in H-mode plasmas with and without ECRF in order to investigate the characteristics of momentum transport with ECRF. The toroidal momentum diffusivity (χφ), the inward convection velocity (-Vconv) and ion heat diffusivity (χi) was found to increase with ECRF. The toroidal rotation velocity (Vt) changed in the co-direction inside the EC deposition radius, and changed in the CTR-direction outside the EC deposition radius.

SOL plasma fluctuations and ELM propagation

SOL fluctuations in the L- and H-mode plasmas and ELM filament propagation were measured using Mach Probes located at the low-field-side (LFS) midplane, X-point and high-field-side (HFS).
  SOL fluctuation characteristics were determined with applying various statistical analysis: probability distribution function (PDF) could describe intermittent (non-diffusion) transport at LFS midplane, i.e. the positive burst events, where the PDF is strongly skewed positively. Conditional averaging analysis of the positive burst signals indicated that the intermittent feature with a rapid increase and a slow decay is similar to that of plasma blobs theoretically predicted. Statistical self-similarity was investigated with Fourier power spectrum, and statistics of waiting-time and duration-time of the fluctuation, showing fractal property of the fluctuations.
  SOL flow measurements for the small gap between the upper main plasma and the first wall (corresponding to the midplane radius of ~2 cm) showed shorter decay lengths in the SOL density and pressure profiles at the HFS than those at the LFS. This result suggested large radial diffusion of the LFS SOL plasma, which is consistent with the in-out asymmetry in the non-diffusion transports.
  Transport of the ELM filaments such as radial propagation and parallel flow was investigated in Type-I ELMy H-mode plasmas with the different toroidal rotation velocities. Radial propagation velocities of the ELM filaments (vr = 0.5 -2.5 km/s) and Mach numbers (M// = 0.5 ~ +0.3) were comparable for the different pedestal toroidal rotation velocities.