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Fusion Plasma Research


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May - July 1999


High performance reversed shear (HPRS) operation with high Ip (2.4 MA) was optimized towards the long sustainment of high QDTeq (the equivalent fusion power gain for an assumed D-T fuel), obtaining discharges persisting for 0.8 s at QDTeq ~ 0.5 and for 1 s at QDTeq ~ 0.4. The sustainment of HPRS was limited by a collapse which occurred at qmin = 2. Experimentally, toroidal rotation was found to mitigate the collapse. Furthermore, the optimization of steady-state RS with high bootstrap (BS) current fraction produced a discharge with the H-factor of 4.1 (no correction of ripple loss) and a plasma lasting for 2 s with βN = 1.9-2.1, HITER-89L = 2.8-3.4 at the BS current fraction of 70%. Steady-state RS discharges with LHCD were extended to higher βN. In order to keep good coupling with LHW and the plasma, the distance between the plasma low-field-side edge and the LHRF launcher was controlled. The plasmas demonstrated higher performance with βN > 1.5 for 2 s, exceeding the previous performance (βN ~ 0.95-1.2). The confinement of NBI ions was estimated from neutron emission in quasi-steady-state RS plasmas (0.85 MA, 2.1 T). The beam ion loss for tangential NBI was about 46%, probably explained by ripple loss. In addition, the effect of plasma rotation on ITB in RS plasmas was investigated. It was found that ITB survived when no plasma rotation or co-current rotation was observed. For counter-current rotation, the ITB diminished with time.
In pursuit of higher βN with the use of wall stabilization effects, large volume H-mode experiment was carried out. The best shot reached βN = 3.42, which possibly exceeded the ideal stability limit for a free-boundary plasma.
ECRF waves with the frequency of 110 GHz were injected to a low-density plasma at 0.25 MW for 5 s. During the ECRF injection, a clear Te rise was observed. Such a long pulse injection is a world-leading achievement in the frequency range of ECH. We heated for the first time RS plasmas with ITB by the 0.6-0.65 MW ECRF waves (O mode), and observed a electron temperature rise: Te (0) from 5 to 6.2 keV. When ECRF waves were injected into a plasma with m/n = 1/1 mode, a decrease in the mode amplitude was observed.


Argon seed H-mode experiment was conducted in order to produce high density ELMy H-mode plasma with good confinement, and to increase radiative loss at the main plasma and divertor region. The confinement performance of HITER-89L = 1.6 at ne/nGr = 0.6 was sustained for 1.5 s in an Ar seed H-mode plasma after Ip ramp-down from 1.7 MA to 1.2 MA at Bt = 3.5 T. In this discharge, the fraction of total radiative loss to the absorbed heating power was estimated to be 70%. Argon seed reversed shear (RS) experiment has just started to also aim at high confinement at high densities. The optimization of Ar gas puff rate, its timing and the stored energy feedback control of RS discharges is required in the next experiments.
Helium exhaust experiment in RS plasmas was conducted to obtain good He exhaust efficiency in RS plasmas with divertor pump (Argon frosted cryopumps). Helium beam was injected for 1.2 sec into a RS plasma (Ip = 1.7 MA /Bt = 3.7 T) with the ITB formation. Actually, the residual time of He density inside the ITB seems to be a few times longer than the apparent decay time of the He II intensity from CXRS measurement. As the result, helium removal inside the ITB is probably difficult as compared with that outside the ITB in RS discharges in the previous experiment from He gas puff experiment. Helium exhaust experiment in ELMy H-mode plasmas was also performed. The helium residual time was found to be 0.4 sec in the experiment. He exhaust efficiency in attached divertor was improved by 40 - 50% by the addition of the outside pumping and the narrow gaps between the inner/outer separatrix and the inner/outer pumping slots.
The main gas puff and pumping in the low Xp discharges was very effective for the reduction of impurity level and the enhancement of impurity shielding. The MARFE onset electron density increased by about 20% with the divertor closure (the narrow gaps) in the both leg divertor, reaching ne/nGr = 0.7.