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Fusion Plasma Research


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April - July 2000


(1) High performance and steady state
The regime of the steady state high performance based on a high βp H-mod plasma is extended. A full non-inductive current drive was achieved by utilizing N-NB at Ip = 1.5 MA and Bt = 3.7 T with HH = 1.3 - 1.4, βN = 2.4 - 2.5 and q95 = 4.5. Analysis using ACCOME code shows that the bootstrap current is about 0.8 MA while the beam driven current is 0.9 MA, in which the current drive by N-NB is 0.6 MA, indicating over drive. The current drive efficiency for N-NB is calculated as high as 1.5 x 1019 A蚓2/W. The efficiency is the highest so far achieved by NBCD in the world. This is owing to higher target Te (~ 13 keV at the plasma center) thanks to ECRF direct electron heating. The result increases credibility to extrapolate the current data-base of N-NBCD to the ITER relevant regime. Also a trial of identification of N-NB driven current profile at higher Te regime was carried out in order to support the result above. The target was L-mode plasma of Ip = 1.2 MA with Te0 ~ 10 keV which is more suitable for profile measurements. The estimated driven current profile was found to agree well with the one theoretically expected.
A trial of sustainment of higher βN (~ 2) by means of LHCD was carried out. The LH waves was succeeded to be coupled to ELMy H-mode RS plasmas of βN ~2.15 and H89P ~ 2. The surface one turn voltage was lowered as low as ~ 0.1 V.

(2) Confinement Physics
In order to investigate confinement characteristics under a condition of Te/Ti > 1, which is expected in a fusion plasma and unfavorable for confinement from a view point of micro-instability, fraction of electron heating power was tried to raise in confinement improved plasmas, such as high βp and RS plasmas, utilizing ECRF and N-NBI heating systems. Both in the high βp H-mode plasma (Ip = 1 MA, Bt = 3.65 T) and the ELMy H-mode RS plasma (Ip = 1.35 MA, Bt = 3.73 T) improved confinement, H89P = 1.9 for the high βp H-mode plasma and 2.4 for and the ELMy H-mode RS plasma, was obtained with Te/Ti ~ 1.2.

(3) MHD and high energy particles
In high βp H-mode discharges, what limits long sustainment of high βN is occurrence of tearing modes, which are expected to be neo-classical tearing modes. Suppression of the tearing modes by local current drive by means of ECRF was tried. In a high βp H-mode plasma of Ip = 1.5 MA, Bt = 3.65 T, an m/n = 3/2 tearing mode was succeeded to be suppressed completely by ECRF injection of about 1.5 MW. Complete suppression of sawtooth activities for 1.5 s was demonstrated by ECRF injection near the inversion radius.


(1) Edge Plasma Control
In order to control heat load to the divertor plates, radiation loss power enhancement has been studied by injecting noble gases in ELMy H-mode and reversed shear plasmas. With a high radiation loss power fraction (~80%), a confinement improvement (HH98(y,2) ~ 1) was obtained at an electron density of 0.65 nGW in Ar seeded ELMy H-mode discharges. The HH-factor was about 50% higher than that in plasmas without Ar injection. In reversed shear plasmas with Ne injection, the inner transport barrier was maintained under conditions of divertor plasma detachment.

(2) Particle Control
Multiple pellet injection has been applied for extension of high-performance plasma region to high density regime. The plasma density reached up to the Greenwald density in OH plasmas by injecting pellets (2.1 mm cube, 10 Hz and 690 m/s) from the low-field side midplane. However, as for high-power NBI heated plasmas, pellet injection from the low-field side midplane was less effective due to deteriorated fuelling efficiency and short density decay time. On the other hand, when pellets were injected from the high-filed side top, fuelling efficiency seems to be better even in hig-power NB heated dischrages.
In order to investigate helium exhaust in reversed shear plasmas, He NB has been injected into long-pulse reversed shear plasmas. The analysis of He exhaust efficiency is now in progress.