April-May 2006
The following are the results on several major topics during this period.
(1) High βp ELMy H-mode experiment at high plasma current
High βp ELMy H-mode experiment was conducted at the plasma current Ip of 1.7MA and the toroidal field of 4.05 T. The neutron production rate reached 3.4x1016/s. At the same time, the plasma stored energy of 7.6 MJ, the normalized beta of 2.6 were obtained without destabilizing neoclassical tearing modes (NTMs). This stored energy is one of the highest value at Ip =1.7 MA. Utilizing this series of discharges, we completed official inspection on facility capacity for radiation shielding.
(2) Toroidal rotation and momentum transport
Beam perturbation techniques with perpendicular NBs were applied in L-mode plasmas in order to investigate the driving mechanism of the counter plasma rotation and the parameter dependence of the momentum diffusivity and the inward convective velocity. The phase delay of modulated part of the toroidal rotation velocity became larger in the inner region. The momentum diffusivity and the inward convective velocity at r/a ~ 0.6 increased with increasing the heating power and decreasing the plasma current.
(3) ELM frequency dependence in a grassy ELM regime
A systematic study on the effect of the toroidal plasma rotation on the ELM characteristics has been performed using combinations of tangential and perpendicular NBs. In the grassy ELM regime at high plasma triangularity δ and high q95, the ELM frequency increased clearly up to ~1400 Hz as the counter plasma rotation increased. The response of ELM frequency was independent of βp from 0.84 to 1.88 at δ > 0.53. Even in the plasma with balanced NBs, which was not rotating, higher ELM frequency of ~400 Hz has been observed without large energy loss. When the plasma rotation frequency became higher than ~1 kHz in the co-direction to the plasma current, type I ELM with the frequency of ~20 Hz was observed. Achieved pedestal pressure and plasma confinement were similar in both plasmas with the type I ELM and the grassy ELM.
(4) High βN exceeding the no-wall stability limit and resistive wall mode (RWM) study
Improving the stability limit to increase βN, which was 3.83 in the previous experiment in February, was attempted by increasing qmin with a larger Ip ramp-up rate and by reducing the surface current by a rapid Ip ramp-down just before the NB heating. As a result, βN of 4.2 was achieved in addition to ~10 discharges with βN larger than 4.
Effects of the plasma rotation on the RWM were studied in weak magnetic shear plasmas with Bt = 1.57 T, Ip = 0.9 MA and qmin ~1.8. The distance between the plasma surface and the first wall of was ~20 cm. A NB unit of counter injection was switched to that of co injection during a period with βN maintained constant (~2.8). An n = 1 RWM, which led to a plasma collapse, was observed when the plasma rotation was reduced.
(5) Study of particle control under saturated wall conditions
At a vacuum-vessel temperature of 300°C, particle behavior has been investigated in long-pulse high-density H-mode discharges. It has been observed that the wall-pumping rate decreased in several seconds and became constant in the discharges. The constant wall-pumping rate in discharges with 65% Greenwald density was negative, in other words outgassing. The outgassing rate was related to a rise of the divertor-plate temperature. The results were presented at the 17th Int. Conf. on Plasma Surface Interactions in Controlled Fusion Devices, Hefei, China, May 22-26, 2006.
(6) Plasma fluctuation and flow measurements at High- and Low-field-side SOLs
A high-field-side reciprocating Mach probe was repaired in 2005, and plasma fluctuation measurement was performed, for the first time, at three poloidal locations over some parameter ranges in L-mode plasmas (Ip = 1 MA, Bt = 3.2 T, PNB= 4-5 MW, ne-bar ~0.4-0.55 nGW). Experiment was planned under collaboration with Nagoya- and Kanazawa-Universities. Fluctuation level at the LFS midplane was generally 5-10 times larger than that at HFS SOL. Fluctuation levels at near-SOL (r<4cm, both at HFS and LFS) were decreased with increasing ne, while fluctuation levels at far-SOL were similar. Fluctuation characteristics only at the LFS midplane, i.e. skewness (S=3>/2>3/2) ~0.5 and flatness (F=/2>2) ~4-5, showed difference from those in Gaussian (S=0, F=3), but they were similar in scanned density and power ranges.