October 1997
OPERATION AND CONFINEMENT PHYSICS
After Oct.13th, we carried out degassing operation using hydrogen and helium discharges in order to reduce tritium produced by the D-D fusion reaction and absorbed in the first wall. The ratio D/H (deuterium / hydrogen ; the measure of degassing) was successfully reduced below 0.03 (the detection limit of deuterium) by the three-week operation.
Plasma current Ip was increased up to 2.5MA, which was the first trial to increase Ip above 2MA after the divertor modification. By this trial, we confirmed robustness of the divertor structure and controllability of equilibrium feedback in the high current regime.
Concerning the halo current study, we tried a strong gas puff just at the disruption. The results show that the halo current decreases with increasing density, which suggests that increase in resistivity in the halo region is effective to reduce the halo current.
In order to clarify the heating power threshold to produce the internal transport barrier in the reversed shear discharges, we scanned NB heating power and electron density ne with almost the same current profile at Ip=1.2MA and Bt=3.5T. The threshold power has the minimum value of ~6MW at ne~2E19/m3 and increases both in the higher and lower density region.
CURRENT DRIVE AND HIGH ENERGY PARTICLE PHYSICS
Regarding current drive study, the driven current profile by NNB (360 keV, 2 MW) was measured with the MSE diagnostic system. Although the driven current was small (0.1 MA at the total plasma current of 0.6 MA), the results showed a good agreement with the calculation by the ACCOME code.
Continuous TAE modes in frequency range of f=120-210 kHz and toroidal mode number n=1 and 2 were observed with hydrogen NNB injection into helium plasmas. Decrease in the mode frequency was consistent with that expected from increase in density. These TAE modes survived for ~200-300 ms, which was significantly different from bursting TAE modes observed in deuterium NNB injection into deuterium plasmas. Amplitude of magnetic fluctuations of the continuous modes was smaller than that of the bursting modes by one order of magnitude.
The reversed shear configuration with the internal transport barrier were maintained by LH current drive for 2 s with a relatively broad driven current profile.
DIVERTOR AND BOUNDARY PHYSICS
Steady and efficient helium exhaust was demonstrated in the W-shaped pumped divertor with argon frosted cryopumps. Helium beams with fueling efficiency of 1.6E20 atoms/s and pulse length of 6 s (equivalent to 90 MW DT-alpha heating) was injected into an ELMy H-mode discharge (Ip=1.4 MA, Bt=3.5 T, PNB=12 MW). The helium concentration reached 4% of the electron density in the main plasma at 1.2 s after the beam injection and was kept constant for 4 s. The global residence time of He particles tau*He was 0.7 s and tau*He / tauE =4 was achieved. This satisfies the helium exhaust condition considered in ITER. The enrichment factor of helium in the divertor was about 1.1, which was 5 times larger than the value required in ITER. Without He pumping, the He concentration increased linearly up to 8% at 5 s after the helium beam injection.
Helium behavior in the divertor was investigated with helium pumping in L-mode discharges. The following results were obtained. (1) In the higher density discharges, the helium pumping efficiency was higher. (2) Even in the detached divertor, helium removal was observed. (3) High energy helium atoms of about 1 eV were measured with a high-wavelength resolution spectrometer in the new divertor as well as in the open divertor.
The 7th Joint ITER Divertor Physics and Divertor Modeling and Database Expert Group Workshop was held at the Naka Joint Work Site, Naka Fusion Research Establishment, JAERI, Naka, Ibaraki, Japan, on Oct. 13-17, 1997. In the review session of recent experiments, results of the new W-shaped divertor of JT-60U and JFT-2M were presented, focused on geometrical effects and pumping effects.