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June 2008

Neoclassical tearing mode stabilization

The minimum required EC wave power (i.e. EC-driven current) for complete stabilization of an m/n=2/1 NTM had been identified in a low toroidal field regime in December (See report in December 2007). Similar experiments were done in a high toroidal field regime (Ip=1.5 MA, Bt=3.7 T, q95~4) to obtain data with different value of marginal island width below which an NTM decays spontaneously. The range of the minimum required EC wave power was identified: while a 2/1 NTM was stabilized with 1.3 MW EC wave, it was not completely stabilized with 1.0 MW EC wave.

ELM dynamics and L-H transition studies

Dynamics of edge impurity ion temperature and rotation during an ELM cycle for different rotating plasmas was investigated by using fast-CXRS. In the experiment, combination of co-, balance-, counter- and perpendicular-NBI was changed at Ip/Bt=1.6 MA/3.9 T (q95〜4.2). Perturbation of Ti was observed in wider spatial region for co-NBI than that for counter-NBI. For balanced NBI, the spatial extent was between them. The pedestal height/width were higher/wider in co-NBI than that seen in counter-NBI, suggesting a difference in the edge stability, which seems to be consistent with the difference in the ELM-affected area. Preliminary analysis shows that the Vp and its shear seem to be higher and steeper in the co-NBI, which corresponds to the deeper Er well and steeper Er shear. At the ELM-onset, Vp at the edge changed toward the ion-diamagnetic direction, and Vt at the edge changed toward zero, resulting in reduction of the Er after ELM. ELM characteristics in this campaign seem to be similar to lower Ip/Bt~1.4 MA/2.5 T campaign performed in January. At the L-H transition, the Er-well structure was formed at the plasma edge region. At higher Bt condition (~4 T), a transition phenomena in the Er structure was observed by the upgraded CXRS diagnostics, which shows a rapid change in the Er during ELM-free H-mode phase and associated further confinement improvement (so-called 2nd transition).

Collisionality dependence of grassy ELMs

In JT-60U, νe* scan (toward νe*>1) in grassy ELM regime has been performed with remote participation of Dr. L. Horton from IPP, Garching. The experiments were performed while keeping several important parameters to obtain grassy ELM (plasma shape, βp and Vt) constant. In high q region (q95>6), pure grassy ELMs were remained during νe* scan. But, the amplitude of divertor Dα signal increased with increasing νe* together with the reduction of ELM frequency. No detectable loss was observed in Wdia and smaller losses of pedestal density and temperature than those for type I ELM were observed, suggesting these plasmas with larger ELMs (increased Dα signal) were still in the grassy ELM regime. In lower q region (q95~4.2) with mixture of grassy and type I ELMs, the frequency of type I ELM became higher and the amplitude of grassy ELMs became larger by a factor of 2, when νe* increased. These results suggest that the amplitude of grassy ELM decreases as edge νe* decreases, which is opposite νe* dependence from that for type I ELM.

Toroidal rotation and momentum transport

Beam perturbation techniques with perpendicular-NBIs (edge Vt modulation due to fast ion loss modulation induced by toroidal field ripple) were applied in H-mode plasmas in order to investigate the characteristics of momentum transport and toroidal rotation. The electron density (ne) and the toroidal field ripple were scanned individually by changing the gas-puffing rate and the toroidal magnetic field (Bt), respectively. The toroidal momentum diffusivity ( χφ) and the inward convection velocity (-Vconv) decreased with increasing ne. By increasing Bt, rotation velocity increased in the counter direction in not only the edge region but also in the core region. Such significant change was not observed in Ti and ne profiles.

High radiation fraction and good confinement H-mode plasmas

Neon (Ne), argon (Ar) and the combination gas seeding into the ITB plasma with type I ELMs has been investigated in order to sustain good energy confinement plasma with large radiation fraction of frad > 0.7. Radiation regions (main edge and divertor) could be controlled by the different radiators: Ne seeding increased the radiation power at the divertor, while Ar seeding increased the radiation power both at the main and at the divertor. Combination gas seeding, Ar to the main plasma and Ne to the divertor, could produce the Type-I ELMy H-mode plasma with largely increasing the divertor radiation. A better energy confinement (HH98(y,2) = 0.95-0.8) with the large frad (> 0.8), compared to Ne or Ar simple seeding cases, was sustained continuously for 11 s until a large breakdown of NBI. Active control of the main radiation by Ar feedback has been developed, and feedback control of the Ne seeding in the divertor will be necessary for sustainment of the divertor detachment. Using the feedback of Ar seeding, long-duration sustainment of the Type-III ELM plasma (HH98(y,2) = 0.87-0.75 and frad > 0.8 continuously for 13 s) was also achieved with the divertor detachment as well as large reduction of the ELM activity (WELM/Wdia of 0.15% acceptable for ITER).

Triton burn-up experiment

The DT neutron brings the information of triton confinement. Improvement of the neutron profile diagnostic made it possible to discriminate DT neutron. The time scale of the decay of DT neutron emission after intense NB injection was investigated (Ip/Bt~1.5 MA/3.7 T). The time scale was about half of that for the previous experiment performed around 1995, where the experimental condition was Ip/Bt~2 MA/4 T with the open divertor. In addition, the time scale increased when the X-point height was raised, suggesting some tritons were lost in the divertor region due to large orbit.

Experiment on pedestal and transport in H-mode

The change in temperature profile was examined by the variation of Ip. The pedestal pressure (pped) in standard H-mode plasmas at Ip=0.95 MA did not reach that obtained at Ip=1.15 MA (pped ~ Ip1.5). While H-mode confinement depends on Ip (Wth ~ Ip0.9), pped has a stronger Ip dependence. Thus, even if the total stored energy is the same, it is hard to match the whole plasma profiles when Ip is changed and the dTi/dr inside the pedestal shoulder at low Ip is steeper than that at high Ip. The pped in high βp H-mode at Ip=0.95 MA with ITB was similar to that in standard H-mode at Ip=1.15 MA. It will be investigated whether this difference is attributed to the effect of magnetic shear or increased heat flux.