Mar - Aug 2001

High-βN experiment (High-βp ELMy H-mode plasmas)

JT-60U has improved capability of high-triangularity operations, which allows triangularity (δ) of ~0.6 for 6s or δ of ~0.45 for 9s at the plasma current of 1 MA. The βN value sustainable sufficiently longer than the energy confinement time (> 5τE) increased up to 3.1 in a low-q95 (< 4) regime. At a triangularity of ~0.45, the βN was sustained at 2.7 for 7.4 s; the duration was determined by hardware limitations. Realization of grassy ELMs with small divertor heat load in a low-safety-factor (q95 < 4) regime is one of the urgent issues for ITER. With a high-triangularity (δ= 0.6) shape, grassy ELMy discharges were obtained at the q95 of 3.7.

In a high-βp ELMy H-mode regime, fusion product under full-noninductive current drive has been enhanced up to 3 x 1020 m-3 s keV by NNBI (ENNB = 402 keV, PNNB = 5.7 MW, Ip = 1.8MA, Bt = 4 T, βN ~ 2.4, HHy2 = 1.2). The high-confinement high-βN full-noninductive-current-drive regime was extended to a reactor relevant regime in terms of non-dimensional parameters; low collisionality (~ the collisionality expected in ITER) and small normalized gyroradius (~ 3 times of the normalized gyroradius expected in ITER). This result was obtained in collaborative experiments with the NIFS (National Institute for Fusion Science) team; 13 NIFS scientists participated in the experiment for a week.

High-density experiment (ELMy H-mode plasmas)

In order to extend a high-confinement regime toward high density, Ar injection and pellet injection have been explored. In ELMy H-mode plasmas with Ar injection, confinement time given by the ITER ELMy H-mode scaling (HHy2 ~ 1), electron density of 0.8 nGW and radiation-loss-power fraction of 0.8 were simultaneously achieved with a plasma configuration where the outer separatrix strike point was located at the divertor dome top. By adjusting the Ar and D2 gas puff rates, duration of such high performance plasmas was extended to ~ 2 s (~ 10 x τE). Pellet injection from the high-field-side midplane has become available in addition to pellet injection from the high-field-side top injection. For each pellet, the fueling efficiency in the high-field-side midplane case was higher than that in the high-field-side top injection case, as expected from calculation of radial displacement based on E x B drift and MHD models. However, the electron density achieved in the high-field-side midplane case was lower than that in the high-field-side injection top case, since some pellets were broken due to the small curvature radius of the guide tube in the high-field-side midplane injection.

Steady-state reversed shear plasmas

In 2000, high-performance reversed shear plasmas were almost non-inductively sustained by LHCD and NNBCD with parameters comparable to those expected in an ITER steady-state operational scenario. However, the safety factor was not low (q95 ~ 6.9) enough for the ITER scenario (q95 ~ 4 - 5). In this campaign, optimization has been tried at a low safety factor (q95 ~ 4.9) in other words higher plasma current (up to 1.5 MA), but mainly by LHCD only. Even at the high plasma current, a high confinement (H89L ~ 2.0) was obtained at a high normalized density (0.75 nGW). However, non-inductive current drive has not been as much successful as expected for lack of NNBCD.

Internal transport barrier

Threshold heating power of internal transport barrier (ITB) formation has been investigated for monotonic and reversed magnetic shear plasmas by systematic parameter scans. In addition, profile data of typical ITB plasmas with high confinement (H89L > 2.5) were taken for transport analysis. Perturbation techniques such as gas-puff modulation and pulsed ECH were also applied for study of particle and heat transport. On the other hands, formation of the current hole was investigated, and current drive with ECRF and NNB in the current hole was explored.

Scrape-off layer and divertor

A new reciprocating Mach probe has been installed at the inner baffle, and profile and flow of inner SOL plasmas have been measured for the first time in divertor tokamaks. Small flow reversal was observed only near the separatrix. The flow velocity towards the inner divertor increased at the outer flux surfaces above the inner baffle. Understanding the mechanism to produce this flow pattern is in progress. By changing the vacuum vessel temperature, chemical sputtering yields due to C2Dx production besides CD4 production at the carbon divertor plates have been measured systematically. At the carbon divertor plates, the C2Dx production was determined to be the dominant chemical sputtering process in low-temperature divertor plasmas.