JT-60U MONTHLY SUMMARY

August 1997


OPERATION AND CONFINEMENT PHYSICS

Concerning the threshold heating power for the L-H transition, the initial results showed a slightly (~10%) low threshold power compared to the open divertor cases at a given density.

Toward steady-state high integrated performance, the discharge scenario was optimized at plasma current Ip = 1.5-1.8MA, toroidal field Bt = 3.6T with triangularity ~0.1 and ~0.3. Favorable performance with H-factor =1.8-2, normalized beta =1.8-2.2 was sustained for <4 sec in the ELMy phase with co-tangential NB for current drive and perpendicular NB for heating. The non-inductive current fraction (NB driven current and bootstrap current) is roughly evaluated as 70-80%.

Discharge scenario of reversed shear plasmas were optimized to obtain an H-mode edge in addition to the internal transport barrier using a high triangularity configuration. When triangularity was increased by keeping the plasma volume constant, many collapses were observed due to decrease in both q95 and internal inductance. By expanding the plasma simultaneously with increasing triangularity, a stable discharge (3.5 T, 1.5 MA, q95 = 4.6) was obtained. However, the discharge was finally terminated by a beta collapse with normalized beta of 2.0 and an H factor of 2.7.

CURRENT DRIVE AND HIGH ENERGY PARTICLE PHYSICS

The conditioning of NNB progressed and the injection power of ~2 MW with a pulse length of 700 ms was achieved at 300 keV. The NNB was injected into a target plasma, where the loop voltage and the internal inductance were kept almost constant for ~3 s, and a clear drop in the loop voltage was observed.

Together with optimization of the steady state high performance plasma, establishment of target plasma of NNB injection progressed for non-inductive full current drive of the high performance plasma.

DIVERTOR AND BOUNDARY PHYSICS

Experiments for reducing Zeff was conducted to assess impurity control capability of the W-shaped divertor. The minimum value of Zeff was 1.1 in ohmic discharges and 2 in NBI heated discharges with a high power of 20 MW. Increase in electron density was most effective for reducing Zeff. However, when electron density reached a critical value, X-point MARFE appeared and which limited the further reduction in Zeff.

The divertor pumping characteristics was investigated for NBI heated discharges. The exhausted particle flux was evaluated by comparing gas puff rate necessary to keep the electron density at a given value with or without divertor pumping. The divertor pumping rate was estimated to be about 1% of divertor particle flux at main electron density of 3x1019 m-3, which was as same level as wall pumping rate.

Energy confinement of high density ELMy H-mode was investigated in radiative discharges formed by gas puff. In ELMy H-mode discharges, strong gas puff was needed to increase the electron density, which degraded energy confinement characteristics. The H-factor obtained so far decreased as electron density increased, and the value was about 1.5 at 50% of Greenwald limit. This level of H-factor is almost similar to that obtained in the previous open divertor.