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March 1998


After successful boronization (see the divertor section), we started the high performance reversed shear plasma experiment. The high performance campaign will continue for 7 weeks by the middle of June. The purpose of the initial two weeks in March is to establish a start-up scenario of reversed shear plasmas with a strong internal transport barrier (ITB) under the W-shaped pumped divertor at a medium plasma current Ip ~2MA. Compared to the 1996 high performance plasmas with open divertor, the plasma volume has to be reduced by ~ 10% and the plasma position should be shifted upward by 5-10cm and, consequently, current and pressure profiles should be optimized under a new condition of penetration of plasma current and profiles of heat and momentum input. The optimization of the start-up scenario was good enough to achieve the DT equivalent fusion gain QDT ~0.46 at Ip=2MA with an ITB radius reaching ~60% of minor radius, where the normalized beta value was ~ 1.7 and the confinement improvement factor (H-factor) was ~3. For the optimization, feedback control of neutral beam heating power by D-D neutron production rate was successfully applied to sustain the beta value in the stable operation region.
In the Japan Physical Society Meeting held in Chiba on March 30 - April 2, 1998, we reported the confinement, transport and stability analyses of reversed shear plasmas and characteristics of neoclassical tearing mode.


The conditioning of NNB was progressed, and the drain current of ~50 A at 400 keV was attained. Maximum injection power increased up to 5.2 MW at 350 keV. The NNB of ~4 MW at 350 keV was injected for ~0.4 s into a reversed shear plasma, so that clear increases in stored energy (1.5 MJ), central electron temperature (0.6 keV) and neutron emission rate (8 x 10^15 1/s) were achieved. By extending the NNB pulse length up to 1-2 s, the high power central heating of high performance reversed shear plasmas are planned in the next campaign.
In the Japan Physical Society Meeting, recent results on current drive using NNB, control of reversed shear and internal transport barrier (ITB) by the lower hybrid current drive (LHCD), ICRF heating of reversed shear plasmas and Alfven modes were presented.


The scrape off layer (SOL) profile at the mid plane was measured using a Mach probe in OH and NB-heated L-mode (4MW) discharges. Ion saturation current (Is) and the density at the divertor side were larger than those at the mid plane side (Is(div)/Is(mid)~1.5 in OH and Is(div)/Is(mid)~2 in NBI). In addition, electron temperature at the divertor side was smaller than that at the mid plane side. These results suggest that SOL flow is driven from down to up (along the field line) at the outboard edge. Corresponding Mach number was relatively small: M=0.2-0.3. The same experiment was done when direction of both plasma current (Ip) and toroidal field (Bt) was reversed. In this case, the reversal of the SOL flow at the mid plane was observed. This flow might cause inboard-enhanced asymmetry in ion saturation current at the divertor.
Helium exhaust was investigated by using divertor pump (Argon frost cryopumps).
1) In high density ELMy H-mode plasmas with NB heating power of 12 MW, detached divertor was produced at the plasmas density of 0.8 nGr (Greenwald density). Two helium (He) beams were injected to deposit He in the core plasma. From the time evolution of HeII intensity, it was found that He exhaust capability in the detached divertor was reduced to about a half of that for the attached divertor.
2) When the direction of both Ip and Bt was reversed, He exhaust capability in ELMy H-mode plasmas ( at a half of nGr) was reduced to a half of that for the normal direction. The reduced efficiency of He exhaust might be attributed to the reduction of He and D flux at inner target due to the reversed in-out asymmetry of particle fluxes to the divertor. The He and D neutral pressure under the baffle was reduced by a factor of 2 to 3.
In the two operation weeks with NB heating after the boronization of the wall on March 10-12, oxygen level was observed to be very low. Intensity of Oxygen VIII was below a noise level in OH divertor discharges.