September 2004

  In the middle of this month, the wall conditioning by boronization was carried out to reduce the recycling and impurities. The recycling level (intensity of divertor Dα) was reduced to ~1/4 after the boronization and confinement in H-mode plasmas was improved, which also enhanced the beta values in long pulse discharges with a limited heating power. The experimental studies for (1) stationary sustainment of high βN plasma, (2) long-pulse high-recycling H-mode, (3) high beta full non-inductive current drive, (4) long sustainment of high fBS, (5) high density RS plasma, (6) ELM (edge localized mode) and pedestal, (7) transient heat transport, (8) NTM stabilization with ECCD, (9) real time control of q profile, (10) current hole, (11) CS-less operation, (12) impurity reference, and (13) conditioning of heating systems have been carried out. The results are shown below.

(1) Stationary sustainment of high βN plasma: Conditioning of NB for long-pulse injection and optimization of the NB injection pattern to avoid NTMs were progressed. After the boronization, βN~2.5 was sustained for 15 s with good confinement (H89 =1.9-2.1). In this discharge, N-NB was injected for 16 s and ECRF was for 13 s in series.
(2) Long-pulse high-recycling H-mode: It was attempted to maintain a high density (70% of Greenwald density) H-mode plasma for 30 s with saturated wall/divertor-plates and with the wall temperature of 150C. The gas-puffing was required to maintain the density, which increased the neutral pressure and active divertor pumping. However, the radiation fraction increased and MARFE occurred at a later period of heating, and the H-mode was terminated.
(3) High beta full non-inductive current drive: Current and pressure profiles were optimized to avoid the m/n =2/1 NTM and to obtain longer sustainment of higher βN plasmas with N-NB injection. βN ~2.4 (βp ~1.75) and H89 ~1.8 were sustained for 5.5 s with Bt =2.4 T, Ip =1 MA, q95 ~4.5, and δ ~0.5, where fCD ~80-90% and fBS ~40% were achieved. q(0) was just above unity and the q=1.5 surface was located at r/a ~0.5, and the 3/2 NTM was observed.
(4) Long sustainment of high fBS: Discharge optimization was carried out, based on a previous shot (0.8 MA, 3.4 T, q95 ~8.6) in which fBS ~75% had been maintained for 7.4 s, in a lower q regime (2.7 T, q95 ~6). A wide ITB (r/a~0.8) for high fBS and high beta was formed but a disruptive collapse inhibited long sustainment.
(5) High density RS plasma: The plasma volume was increased from 77 m3 (previous experiment) to 80 m3. The high confinement of HHy2 =1.2 was observed at the Greenwald density with high radiation loss from the main plasma due to accumulation of Cu. The edge density was increased with D2 gas-puffing. However, Cu and C impurities increased with ITB growth, indicating insufficient impurity shielding in the edge plasma.
(6) ELM and pedestal: In JET and JT-60U comparison experiment, ELM activities and pedestal characteristics were investigated by changing the combinations of NBs (P-NB tangential, P-NB perpendicular, N-NB). In the QH (quiescent H) mode experiment, it was found that the distance between the plasma surface and the wall was important to obtain the QH mode. In the grassy ELM experiment, the ELM frequency seemed to decrease, while the ELM amplitude seemed to increase, keeping a similar pedestal pressure, when the counter plasma rotation became small. It was suggested that the ELM size was controlled gradually by changing the plasma rotation.
(7) Transient heat transport: The heat pulse was induced by short (100 ms) off-axis (ρdep~0.4-0.6) ECRF heating. The central Te in the target plasma was varied over a range of 1.5-5.5 keV by changing the NB power (0-10 MW). The amplitude of the heat pulse was decreased with the increase in Te and Te gradient. Abrupt heat pulse propagations were also observed in high power NB plasmas with high Te and Te gradient. Strong nonlinear dependence of χe on Te and/or Te gradient was suggested.
(8) NTM stabilization with ECCD: Effect of second harmonic X-mode ECCD was investigated for high beta plasmas with βN ~2.9 by changing the injection angle at Bt = 1.7T, Ip = 0.85 MA and q95 ~3.2. Amplitude of the m/n= 3/2 mode was decreased during ECCD, and βN and H89 were improved after the NTM stabilization (βN ~2.9, βp ~1.7, H89 ~1.8).
(9) Real time control of q profile: Real time q-profile control system with a real time evaluation of q profile with MSE (motional Stark effect) diagnostics and with control of CD location (ρCD) by parallel refractive index N// of LH waves is under development. A relation between ρCD and phase difference of LH wave (Δφ) was obtained in discharges with pre-programmed Δφ. The real-time control of q profile was attempted to raise q(0) to 1.2-1.3 from <1 in a plasma with Ip = 0.75 MA, Bt = 2.4 T. The increase in q(0) and disappearance of sawteeth were observed, but q(0) did not reach the reference value, which was partly because of trouble in one channel of MSE.
(10) Current hole: Current clamp and asymmetry of Te profile in current hole plasmas were investigated. The current hole was maintained during N-NB injection for 2.5 s. Estimation of accuracy of Vl(r) is required to confirm the existence of current clamp against NBCD. No clear asymmetry for Te profile was observed with the low-field-side ECRF heating.
(11) CS-less operation: Ip ramp-up by bootstrap overdrive was attempted in reversed shear plasmas in a high q95 regime with NB heating without co injection units. Negative loop voltage was observed with feedback control of constant plasma current of 0.5 MA, suggesting bootstrap overdrive, but no stationary conditions were obtained due to frequent beta collapses.
(12) Impurity reference: The effective ionic charge (Zeff) was kept at the lowest level in the database. However, the boronization was done for lowering recycling and covering the metal impurities.
(13) Conditioning of heating systems: Conditioning of the heating systems (N-NB, LHRF and ECRF) have been progressed as follows.
N-NB: 25 s at 350 keV.
LHRF: 14 s at 1.3 MW and 5.8 s at 1.6 MW.
ECRF: #1 (~0.9 MW, ~5.1 s), #2 (~0.9 MW, ~3.8 s), #3 (~1.1 MW, ~4.7 s), #4 (~0.9 MW, ~2.5 s). Each ECRF power is at the gyrotron.