JT-60U MONTHLY SUMMARY

June 1997


OPERATION AND CONFINEMENT PHYSICS

After the major modification of the divertor structure, which involved a large amount of in-vessel work, we proceeded with the discharge cleaning using ohmic and Helium glow plasmas. After 27 ohmic pulses, a nominal 15 second discharge with X-point was successfully established. Following the validation of magnetic probes, capabilities of the feedback equilibrium control were examined. The equilibrium parameters were scanned in a broad range and a favorable controlability was demonstrated with small standard deviations of 2 cm for positions of magnetic axis and X-point, and 0.03 for triangularity. Direct control of the horizontal position of the X-point, which is considered influential to the efficiency of divertor pumping, was also demonstrated. The halo current fraction to the plasma current and its toroidal distribution were systematically studied with forced disruptions at low current. Aging of NB injection duct was also carried out intensively.

Summary of recent analyses of the high performance reversed magnetic shear discharges were presented at the 24th European Physical Society Conference on Controlled Fusion and Plasma Physics, Berchtesgarden, Germany, in which a jump of transport around the internal transport barrier and behavior of low-m/n MHD mode were reported in detail.

CURRENT DRIVE AND HIGH ENERGY PARTICLE PHYSICS

Coupling of LHRF and ICRF waves with plasmas was investigated for the new divertor configuration. The LHRF and ICRF power of 1 MW level was successfully coupled with plasmas in spite of insufficient conditioning of antennas and configuration adjustment. Conditioning of NNBI was started for the high power and long-pulse injection.

Recent progress of NNBI experiments on current drive, TAE modes and multi-step ionization process was presented in the 24th EPS conference. Presentation on the ripple loss of fast particles from reversed magnetic shear plasmas was also given. These progresses contributed to the ITER Physics R&D Expert Group Workshop on Energetic Particles and Heating & Current Drive held in Garching, Germany.

DIVERTOR AND BOUNDARY PHYSICS

Initial divertor experiments after the divertor modification started by focussing on;
1) calibration of the net pumping speed of the newly installed divertor pumping system,
2) calibration of sensitivities of penning gauges and ionization gauges and adjustments of other divertor diagnostic systems,
3) control test of the rearranged gas fueling system, and
4) study of divertor pumping characteristics and divertor characteristics in ohmic plasmas.

The divertor experiments were carried out in ohmic discharges with plasma current of <1.5 MA and electron density of 2.2E19/m3. Although the operational electron density was still low, the nonlinear increase in neutral particle pressure was observed in the pumping duct under the baffle plates. When the pumping system was switched on during a shot, the gas puffing rate necessary to keep the main electron density at constant was found to increase. These results show that the divertor pumping is really active.

Three experts of ITER Physics R&D attended the 6th Joint Workshop ITER Divertor Physics and ITER Divertor Modelling and Database Expert Groups held in ITER Garching JWS from June 16 to June 21. Other two scientists attended the 24th EPS conference and the above ITER workshop as additional experts.