JT-60U MONTHLY SUMMARY

September - December 2000

CORE PLASMA

In reversed shear discharges, effort to extend operational regime of the steady state high performance plasmas had mainly been focused on high density, relative to the Greenwald density (nGW), regime. As the results, a full non-inductive current drive was achieved in a reversed shear plasma (Ip = 0.9 MA, Bt = 2.5 T, q95 = 6.9) with HH98(y,2) ~ 1.4 and βN = 2.2 at ne ~ 0.8 nGW for 0.8 s. In the plasma, the fraction of the bootstrap current was about 60% to the total current and the rest was carried by LHCD and NBCD (both positive and negative-NB). In a similar plasma, the electron density was increased up to 1.2 nGW keeping H89p ~ 1.75. In a high βp H-mode plasma (Ip = 1 MA, Bt = 2.1 T, q95 = 4.4) with N-NBI, βN ~ 3 with HH98(y,2) ~ 1.2 was sustained with Vl~ 0 for ~ 1 s at ne ~ 0.6 nGW. By injecting pellets into the same target, ne increased to 0.7 nGW with βN ~ 2.9, however Vl remained positive (0.05 - 0.1 V) and HH98(y,2) ~ 1.1. In ELMy H-mode discharges, high-density operation was also explored at high plasma current. Using pellet injection from the low-field side, the electron density reached ~ 8 x 1019 m-3 at Ip = 2.3 MA.
Pellet injection from the high-field side had higher fueling efficiency than that from the low-field side, and it was consistent with a theoretical model based on an E x B drift effect. Plasma current start-up with low loop voltage and assistance of ECRF was studied from the fundamental to the third harmonic resonant conditions, so far a plasma was successfully built up to the second harmonic resonance. Formation and sustainment of a reversed shear configuration with slow current ramp up, dIp/dt = 0.2 MA/s (0.4 - 0.5 MA/s in a usual scenario), was confirmed with LHCD. The N-NB with the acceleration voltage of 405 keV was successfully injected into a plasma. The injected power was about 5.1 MW.

PERIPHERAL PLASMAS

By the divertor modification that enables us to put the separatrix strike points close to the pumping slots, in ELMy H-mode discharges, the pumping rate for neutral particles has been increased by ~ 70% and helium exhaust efficiency has been increased by ~ 45%. For reversed shear discharges, compatibility of high energy confinement in the core and high recycling in the divertor is an important problem to be solved. Chemical sputtering yield of CFC divertor tiles was derived from CD and C2 band intensity. Besides CD4 production, importance of C2D2,4 production was found. Flux dependence of the sputtering yield, that is expected in ITER, was not observed.
Wall conditionings with toroidal magnetic fields were studied using LH, IC, and EC radio frequencies. Effective wall conditionings using LHRF were demonstrated in baking temperature range of 80 - 300C.

Contributions to International conferences

I. 18th IAEA Fusion Energy Conference
1. "Extended JT-60U Plasma Regimes toward High Integrated Performance", Y. Kamada and the JT-60 Team,
2. "Sustainment of High Confinement in JT-60U Reversed Shear Plasmas", T. Fujita et al.
3. "Reactor Relevant Current Drive and Heating by N-NBI on JT-60U", T. Oikawa et al.
4. "Characteristics of Internal Transport Barrier in JT-60U Reversed Shear Plasmas", Y. Sakamotoet al.
5. "Runaway Current Termination in JT-60U", H. Tamai et al.
6. "Alfven eigenmodes driven by energetic ions in JT-60U", K. Shinohara et al.
7. "Long Sustainment of Quasi-steady State High bp H-mode Discharges in JT-60U". A. Isayama et al.
8. "Resistive Instabilities in Reversed Shear Discharges and Wall Stabilization on JT-60U", S. Takeji et al.
9. "Understanding of the H-mode Pedestal Characteristics using the Multi-machine Pedestal Database", T. Hatae et al.
10. "High-Radiation and High-Density Experiments in JT-60U", H. Kubo et al.
11. "Substantial Reduction of H-mode Transition Threshold Power in JT-60U", K. Tsuchiya et al.
12. "Helium Exhaust and Forced Flow Effects with Both-leg Pumping in W-shaped Divertor of JT-60U", A. Sakasai et al.

II. 42nd APS Division of Plasma Physics Annual Meeting Joint with ICPP-2000
1. "Improved particle control for high integrated plasma performance in Japan Atomic Energy Research Institute Tokamak-60 Upgrade" H. Takenaga and the JT-60 Team
2. "Measurement of sputtering yields of CD4,C2D2 and C2D4 at the carbon divertor plates of JT-60U" T. Nakano et. al.
3. "Local Heating and Current Drive using the fundamental O-mode EC wave on JT-60U" K. Kajiwara et al.

III. 11th Int. Toki Conf. on Potential and Structure in Plasmas
1. "Formation of internal transport barriers and its impact on the JT-60U plasmas", S. Ide and the JT-60 Team
2. "Active Control of Internal Transport Barrier and Confinement Database in JT-60U Reversed Shear Plasma", Y. Sakamoto et. al.
3. "H-mode edge structure in JT-60U high density improved confinement plasmas", T. Fukuda et. al.
4. "Fast plasma shutdown of JT-60U with increased critical electric field for runaway electron generation", M. Bakhtiari
5. "Pedestal Structure and Thermal Energy Confinement of ELMy H-mode in JT-60U", H. Urano et. al.