July 1997


For the real time feedback control of plasma position, optimization of gain in the new coefficient-matrix was completed. Plasma position (vertical and horizontal position of the magnetic axis and height of the X-point ) can be controlled with accuracy within 1cm from the equilibrium calculation. Re-ionization loss of the conventional NB using positive ion source was reduced to <10% after integrated injection time of 50-70 sec. The ratio of Hydrogen / Deuteron in the OH limiter phase was reduced to <10% by 2 weeks-operation after Boronization.

A systematic data set of halo current was taken by forced disruptions where plasma was pushed down toward the divertor area. Total halo current and its toroidal and poloidal distribution were measured through the scan of plasma current and stored energy.

Toward steady-state high integrated performance, optimization of discharge scenario was started at plasma current Ip = 1.5-1.8MA, toroidal field Bt = 3.6T with triangularity ~0.3. Although the confinement enhancement factor (H-factor) was ~ 70-80% of the best cases in 1996, H-factor =1.8-2, normalized beta =1.8-2.2 was sustained for <2 sec in the ELMy phase without counter tangential NB. Based on these results, full non-inductive current drive at Ip=1.5MA is expected in near future with 2-3MW of NNB.

An experimental campaign to get quasi-steady high performance reversed shear discharges with an ELMy H mode edge and high triangularity was started. High confinement discharges (1.5MA, 3.7T, L mode edge) with H factors of 3.1-3.2 and normalized beta of 1.9-2.2 have been obtained in a low triangularity configuration. Optimization in a high triangularity configuration is planned in August.


Conditioning and optimization of NNBI progressed and acceleration current of both upper and lower ion sources increased up to 20 A at 300 keV. This result is promising for early demonstration of the high power injection over 5 MW. The NNB was injected into plasmas for the first time after the divertor modification.

Conditioning of ICRF and LHRF antennas and configuration adjustment in the new divertor configuration have been progressed. The ICRF power of up to 4 MW was successfully coupled with plasmas. It shows that the ICRF power can be coupled with the same power level as that before the Divertor Modification. Concerning the LHRF coupling, plasma configuration in which two of three antennas can couple well with plasmas has been produced successfully. A few mega-watt of LHRF power is expected to be coupled after sufficient conditioning.


In the first two weeks of July, the tokamak operations were mainly concentrated on the wall conditioning for the boronization, planned in the week of July 14, and the NBI port aging to get high NBI heating power and long pulse (PNBI> 20 MW, Tpulse>4 s) as early as possible. The boronization with decaborane B10H14 (70g, the expected boron layer is 250 micro¥m in thickness) was done using He-glow discharge on July 16 and 17. In the third and forth weeks after the boronization, various experiments were started; to get good H-mode at high density , to investigate the divertor functions such as radiating power loss by D2 and Ne puffing, divertor pumping, geometrical effects (dome, inclined targets) and to measure SOL properties.

One of significant changes in divertor operations after the divertor modification was that disruptions did not occur even when the gas puff of 80 Pam3/s x 4 s was injected to the vessel. In the previous open divertor, the maximum gas puff rate was 20 Pam3/s, and disruptions frequently occurred after turning off the NBI power. This clearly shows the effectiveness of the divertor pumping. With combination of gas puffing and divertor pumping, the experiment to reduce Zeff by SOL flow was also tried.

The divertor interferometer with frequency of 183 GHz was found to work well in the OH discharges. However, in ELMy H-mode, the measurement is not still successful because of the occurrence of frequent fringe loss.