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Fusion Plasma Research


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August 1996


Conditioning of negative-ion-based neutral beam (N-NB) has been continued. At the end of August, the injection power into plasma was 2 MW and the acceleration voltage was 350 kV. Beam profile inside the N-NB beam line was estimated from the D-alpha emission profile. The thermal image on the target first wall was measured by IRTV. The beam dissipation angle was ~ 10 mrad, so far, which is two times as large as the design value. Preliminary, data of density dependence of the shine-through of N-NB was obtained with 350 kV beam energy. The shine- through fractions were ~25% at ne-line (line averaged electron density) ~0.5x1019 m-3 and ~5% at ne-line ~3x1019 m-3.


Plasma performance of shear reversal plasmas has been progressively enhanced in the high performance campaign for two weeks. Discharge optimization was attempted in the high current region up to 2.6 MA with tuning the beam power, plasma density, current ramp-up rate, beam deposition profile and plasma configuration etc. In particular, the profile optimization making the beam deposition profile slightly peaked during the current ramp-up phase appeared to be so effective that the attainable neutron rate was higher at the same stored energy and the energy confinement time was longer at the same plasma current, than those obtained in June. In the course of the campaign, remarkable parameters were simultaneously achieved as a long energy confinement time of ~1.0 s, a high central density of ne(0)~10x1019 m-3 and H-factor of 3.4 for a high stored energy discharge with Wdia~9.4 MJ at 2.5 MA, in which the dW/dt fraction is ~26% to the total heating power. For this discharge, the fusion triple product reached nD(0)tETi (0)~6.8x1020 m-3skeV and the equivalent QDT value would be ~0.73 (~0.82 if including a-heating effect) under a condition of D beams into a DT plasma with [D]:[T]=50:50%, where the thermal fusion reactivity is dominant as much as 80% to the total DT reactivity at a moderate ion temperature of Ti (0)~15 keV. While such high performance discharges were reproducible, disruptive beta limit at a relatively low Troyon factor less than 2 obstructed further improvement in performance.


An experiment was carried out to obtain radiative divertor plasma in a reversed shear mode by Neon and D2 gas puffing. The discharge condition was IP=1.2 MA, BT=3.5 T (qeff=6.7) and PNB=16 MW. Heat load to the divertor plates was reduced to lower than 10% of NB heating power and the fraction of total radiation power was increased up to 50% of PNB. The inboard target temperature was decreased from 450 degrees C. to 300 degrees C.. However, the radiative divertor plasma with Ti (0)=Te (0)~4 keV and ne (0)~4x1019 m-3 could be sustained only for 0.2 s in a deuterium discharge. Last year, the radiative divertor plasma was successfully obtained for 1.2 s in a hydrogen discharge. More optimization will be required to avoid collapse after Neon and D2 gas puffing. Helium behavior of ELMy H-mode plasma was investigated using new spectroscopic measurement with a fast sampling time (0.01 ms). Large periodic pulses of He I (667.8 nm) spectrum were observed to synchronize with pulses of D-alpha emission at ELM pulses. Helium particle exhaust by MHD relaxation at the plasma peripheral region due to ELMs is expected to reduce the He content in the main plasma.