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January 2004

  In this month, the experimental studies for (1) long pulse operation, (2) sustainment of high bootstrap current fraction, (3) JET/JT-60U similarity experiment, (4) CS(Center Solenoid)-less start-up, (5) MHD instabilities in low beta RS plasmas, (6) current hole, (7) conditioning of a modified LH antenna, (8) real time control of q profile, and (9) others have been carried out. Results are shown below.

(1) Long pulse operation: The discharge duration has been extended and finally has reached 65 s, the limit of modified control systems. In this discharge, Ip=0.7 MA, BT=2 T, and the divertor configuration was maintained for 60 s. NB heating for 30 s was employed to save the flux consumption. In long pulse H-mode experiments, βN~2 has been sustained for 20 s (> 100 τE) with good reproducibility by the optimization of discharge to avoid NTMs; βN=2.1 has been sustained for 20 s and βN~1.9 for 24 s. This is a new world record for the sustainment of βN~2, which was restricted below 10 s previously. The injection energy has reached ~360 MJ, which is much larger than the previous record (200 MJ) in JT-60U. When strong gas-puffing of 40-50 Pam3/s (total particles of 3.4x1023) was introduced for ~16 s in a high density ELMy H-mode discharge, the detached plasma for outer divertor was sustained for ~15 s, and the wall temperature remained below ~590°C. Neither baffle nor first wall was saturated yet in this discharge. On the other hand, when an attached condition was maintained by the density control to ne/nGW~65%, the outer divertor tile temperature around the strike point was raised up to ~820°C by the high heat load.
(2) Sustainment of high bootstrap current fraction: Extension of duration of RS plasmas with high bootstrap current fraction fBS was attempted to address the evolution of current and pressure profiles in a long time scale in highly self-organized plasmas. As a result, high fBS>75% and fCD>90% were successfully sustained for 7.4 s in an optimized RS discharge with an ELMy H-mode edge. In this discharge, Ip = 0.8 MA, Bt = 3.4 T, q95 ~ 8.6, βN~1.7, βp ~ 2.25, H89 ~ 3.0 and HHy2 ~ 1.7. The profiles of q and Ti approached stationary conditions in a later phase.
(3) JET/JT-60U similarity experiment: Experiments to obtain database on the H-mode pedestal and ELMs were carried out at Ip = 1.1 MA and 1.8 MA for comparison in a matched shape in JET and JT-60U. Increase of pedestal pressure and global confinement, accompanied by change of toroidal rotation to the co-direction, was observed with NNB injection at 1.1 MA.
(4) CS-less start-up: In previous CS-less start-up experiments in 2002, vertical field coils, VR coil and VT coil, were employed, where the VT coil has some turns near the equatorial plane in the high field side. To demonstrate completely CS-free start-up, the start up with VR only up to 80 kA was achieved by reducing an assist of the VT coil gradually.
(5) MHD instabilities in low beta RS plasmas: In RS plasmas, disruption or mini collapse occurs when qsurf reaches ~4 even with low βN (~1). Profile data for density, temperature and safety factors for a whole plasma volume and Te fluctuation data were newly obtained in this type of discharges to perform stability analysis.
(6) Current hole: Data of responses of electron temperature profile and current density profile to the EC local heating and current drive was obtained to analyze the current clamp and the magnetic island structure in the current hole.
(7) Conditioning of a modified LH antenna: Conditioning of a modified LH antenna with a new carbon antenna mouth has progressed with injection into plasmas, and injection of ~900 kW x 7.1 s x duty 0.84 has been achieved.
(8) Real time control of q profile: Real time q-profile control system has been developed with a real time evaluation of q profile with MSE diagnostics and with control of CD location by parallel refractive index N// of LH waves. It was demonstrated that the q profile evaluated in real-time from local measurements of pitch angle by MSE agrees well with the q profile reconstructed using an MHD equilibrium code.
(9) Others: Other experimental studies, in connection with the improvement of sustained βN in high βp H-mode (NNB and weak shear scenarios), ITB characteristics under dominant electron heating, fast ion transport by ALEs (Abrupt Large-amplitude Events), AIC (Alfven Ion Cyclotron) wave detection, W transport study by using the W-divertor-tiles, and monitoring of impurity contents, have been carried out.