December 2003

  Aiming at performance extension in high beta steady state operation, JT-60U experimental campaign in FY2003 started. In this month, experimental studies for (1) long pulse H-mode, (2) improvement of sustained βN in high βp H-mode, (3) improvement of confinement at high density in high βp H-mode, (4) NTM suppression with ECCD, (5) bootstrap current overdrive, (6) ELM study (toroidal rotation, grassy ELM), (7) transport of fast ions by NNB-induced ALE (Abrupt Large-amplitude Events), and (8) others have been carried out. Results are shown below.

(1) Long pulse H-mode: ELMy H-mode plasmas were successfully sustained for ~30 s by the extension of pulse length from 15 s to 65 s and of heating duration from 10 s to 30 s on JT-60U. Saturation of the first wall was observed in repeated long pulse H-mode discharges with gas puffing. In one of these discharges (Ip=1.0 MA, BT=2.1 T with PNB=10-12 MW and divertor pumping), the line-averaged density was intended to remain at 2.5x1019m-3 (ne/nGW=60%) with feedback control of gas puffing, but it raised up to 3.2x1019m-3 (ne/nGW=77%) without gas puffing in a later phase of heating and H89PL decreased from 1.5 to 1.3 due to the confinement degradation in the high density region.
(2) Improvement of sustained βN in high βp H-mode: A high beta plasma with βN ~3 has been sustained for 6.2 s at triangularity δ =0.4, approaching to the target, 5-8 s sustainment of βN =3-3.5. No large sawteeth or large NTMs appeared in a low q regime, q95 = 2.2-2.8. NTM suppression may be related to reduction of pressure gradient at rational surfaces, q = 1.5 and 2, which were located at the peripheral region in low q plasmas.
(3) Improvement of confinement at high density in high βp H-mode: Discharge scenario was optimized by changing NB heating power, BT (2.1-3.6 T) and configuration (δ=0.55 or δ=0.4). Pellet injection, D2 gas-puffing and Ar gas-puffing were used to raise the density. The high integrated performance with ne/nGW=0.92, HHy2~1 and frad ~ 90% was obtained, exceeding the previous results.
(4) NTM suppression with ECCD: The time for EC injection was scanned and it was observed that the early ECCD (just after the m/n=3/2 NTM onset) was more effective to suppress the NTM than the ECCD at the mode saturation phase.
(5) Bootstrap current overdrive: In several shots with RS plasmas of Ip=0.6 MA, βp ~4 was obtained by the balanced NB injection. Since Vl was negative and the F-coil (ohmic heating coil) current was increasing, which means the recharge to power supply, full or over current drive by bootstrap current was suggested.
(6) ELM study (toroidal rotation, grassy ELM): Optimizing the plasma configuration, density, and toroidal rotation by NB injection, H-mode with no ELM appearance (QH-mode) was successfully sustained for ~3.5 s (~16τE). By using the new ECE transmission line, clear reduction of pedestal temperature at each ELM pulse was firstly observed. Furthermore, it was observed that the counter rotation (or negative Er) was important to stabilize large ELMs even in the grassy ELM regime with high q95 and high δ.
(7) Transport of fast ions by NNB-induced ALE: From the measurements of neutron emission profile, redistribution of fast ions from the center to the peripheral region was indicated during NNB-induced ALE occurrence. The measurements of high energy neutral particle flux with a newly installed diamond detector indicate increase of high energy neutral flux at ALEs, which also suggests radial transport of high energy ions to the peripheral region.
(8) Others: Other experimental studies such as development of weak magnetic shear plasma, divertor local gas-puff, measurements of W transport study using W-divertor-tiles, and monitoring of impurity contents were carried out. The LH conditioning and the adjustment of some diagnostics also progressed.