February 2008

  The following experiments were carried out in this month.

(1) Long duration sustainment of high integrated performance plasma
Achievement of long sustainment of high integrated performance, where high βN=2.5-3.0 with high confinement of HH98>1 and high bootstrap fraction of fBS>0.4 is one of the major targets of this campaign in 2007-2008. Making use of long-pulse injection of central heating of perpendicular NBs together with dedicated wall conditioning to reduce the wall recycling, a high integrated performance plasma having βN=2.6, HH98~1 and fBS>0.4 was sustained for 25 s.

(2) Real-time control of ion temperature and toroidal rotation
In order to control ion temperature (Ti) and toroidal rotation velocity (Vt) in real-time, the charge exchange recombination spectroscopy with high time resolution, the real-time processor system, and the real-time control system have been developed. Utilizing this system, real-time control of the Ti gradient has been demonstrated with neutral beams at high beta plasmas (normalized beta βN~2-2.9). The strength of internal transport barrier was successfully controlled. Moreover, real-time control of Vt has been demonstrated from counter- to co-direction. Change in the behavior of edge localized mode (ELM) was observed during the control.

(3) Study of ITB formation and structures
The objective of this experiment is to measure the detail ITB structures in reversed shear plasmas by a newly installed modulation CXRS system that can measure ion temperature and toroidal rotation at 300 radial points every 50 ms. By changing NB power, various ITB structures were observed in weak RS, strong RS and current-hall plasmas.

(4) Development of full current drive plasma
Real-time current profile control was demonstrated at Ip=0.8 MA, Bt=2.2 T, q95=5.2. In the plasma, the minimum value of safety factor qmin was raised from 1.3 to 2.1, and was sustained at 2.1 for about 4 s. In this control, PI (proportional-integral) control was employed by controlling LHCD power to minimize the difference between measured qmin and its reference. The value of qmin was evaluated in real time using motional Stark effect (MSE) diagnostics. When qmin was raised above 2, an m/n=2/1 NTM was stabilized, and then, increase in confinement was observed.

(5) Burning plasma simulation experiments
By using the burning plasma simulation scheme, responses of burning plasmas to fuelling were investigated. Here, 2 groups of NBs for the simulation of α-particle heating (Pα) and for the simulation of external heating (PEX) were used. The value of PEX was determined using feedback control system for the stored energy. The stored energy was sustained at constant value by increasing/decreasing PEX against decrease/increase in Pα. SMBI reduced the simulated fusion gain due to confinement degradation and flattening of pressure profile. Feedback control of decrease in Pα was demonstrated using gas-puffing.

(6) Sustainment of high-βN above no-wall β limit
The goal of this experiment is to sustain a high-beta plasma above the no-wall beta limit for a time comparable to the current diffusion time by utilizing the wall stabilization effect. To achieve this, discharge scenario was optimized to reduce metal impurities for better confinement and increase rotation velocity for better stability. Higher rotation velocity in the co-direction was obtained by reducing perpendicular NBs. In these discharges, βN > βNno-wall was achieved, where the no-wall βN was ~2.3. However, an n=1 bursting mode with the frequency of ~2 kHz and the growing and damping time of ~1 ms was often observed. These modes seemed to reduce plasma rotation and finally cause RWM.