July 1996


Beam conditioning of negative-ion-based neutral beam (N-NB) has been continued. The total injected neutral beam power reached 1.9 MW at an acceleration voltage of 350 kV. The beam pulse duration was up to 0.7 s and the re-ionization losses were lower than 10%. Runaway electron (RE) behaviors have been investigated for high li disruptions produced by fast current ramp down . The REs were usually generated with a large one-turn voltage during disruptions with a short current decay time. No REs were observed during the current quench when the DCW (Disruption Control Winding) coil turns on, where the DCW coil mainly produced 3/2 and 2/1 modes error field. Analysis of the relation between the generation of REs and the error field amplitude is under way.


High performance experiments in high-bp H-mode regime have been conducted at high currents above 2.1MA under the maximum utilization of neural beam power up to 41 MW. High-current high-bp H-mode discharges were established up to 2.7 MA in which the stored energy and nD(0)tETi(0) reached 9.4 MJ at 2.6 MA and ~15E20 m-3skeVat 2.4 MA, respectively. A high performance high-bp H-mode discharge was sustained for ~1.5 times energy confinement times at 2.5 MA with ELMy H-mode where high neutron rate of 4-5E16 /s and large stored energy of 8-9 MJ were maintained. The central q value for the high-bp H-mode discharges was measured to be slightly above unity from MSE spectroscopy as inferred so far. In parallel to the above high-bp H-mode experiments, "remote diagnostics" and "remote analysis and participation in experiment" at JT-60U from over sea institutes have been successfully demonstrated in collaboration with Princeton Plasma Physics Lab. and Los Alamos National Lab. in USA under the IEA Three Large Tokamak and US-Japan Fusion cooperations utilizing Video conference and the computer network systems. In ICRF heating experiments with reversed shear plasmas, second harmonic ICRF minority heating into the deuterium discharges was found to efficiently heat bulk plasmas inside the transport barrier. Tail ion stored energy due to ICRF heating was comparable to that of a normal shear discharge. TAE modes (90-110 kHz, n=5-8) became unstable only for a q-profile with relatively weak reversed shear resulted from sequential partial collapses. "Hysteresis" of the threshold power at the H to L back transition, which is one of the urgent ITER Physics R & D issues, was intensively investigated. Contrary to the theoretical predictions, the net heating power at the back transition was similar to that for the L to H transition. Thereby, apparent hysteresis was not observed, possibly due to the accumulation of the impurities and gradual increase of the edge neutral density during the H-mode.


Lower hybrid current drive (LHCD) experiment was carried out to maintain reversed shear by means of LHCD in deuterium discharges. LH wave injection power was achieved 4.5 MW by continuous LH launcher aging. It was estimated that current drive of 90% to plasma current was successfully maintained by LHCD in the reversed shear discharge of IP=1.2 MA and BT=3 T. A pre-formed reversed shear configuration was sustained for 4 s. Reversed shear was formed during LHCD from a preliminary analysis of q profile. Up to now, reversed shear discharges was obtained by pre-NB heating at current build up phase to avoid penetration of current. On the first trial in this campaign, reversed shear discharges was successfully obtained by LHCD in stead of pre-NB heating. Helium transport of reversed shear plasmas has been studied using He beam and He gas puff in deuterium discharges. Reversed shear discharges were optimized to avoid collapse with 1.2 MA/3.0 T (qeff=6.3), PNB=13, 15, 18 MW and 1.7 MA/3.5 T (qeff=4.7), PNB=22-23 MW. Helium density profiles were peaked inside the internal transport barrier (ITB) in reversed shear plasmas. The He density profiles were clearly different from those in ELMy H-mode and L-mode. The enhancement of the He particle confinement and low diffusivity were found in reversed shear mode. It was observed that He particles inside the ITB was expelled when a mini-collapse occurred like particle exhaust due to ELMs. He flux in the divertor was larger 3 times after a mini-collapse than before a mini-collapse.