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Feb - Mar 2000

The experimental campaign of this year started in February after three-month shutdown on schedule. After wall conditionings were conducted, most of the experiment during February-March period was dedicated to study of divertor and SOL physics. Data for the coming PSI conference were obtained. As for the wall conditionings, boronization using B10D14 was conducted for the first time in JT-60U. A centrifugal pellet injector has been installed during the last vent period, and multiple pellets were successfully injected both from outside and inside of the torus.

Divertor and SOL physics

At the end of 1998, the JT-60U divertor was modified; the former divertor had a pumping slot at the inner leg only, and the new one has pumping slots at both the inner and outer legs. The pumping rate was increased from 13 m3/s to 17 m3/s. The material of the private dome was changed from isotropic graphite to carbon fiber composite. Thus, it became possible to set the separatrix close to the dome and to set the strike points to the pumping slots for obtaining high pumping rate. Effects of divertor pumping on detachment, MARFE and helium exhaust were investigated in the both-side pumped divertor. Following results were obtained

The both-side pumping was favorable to maintain partial detachment without appearance of null-point MARFE. By pumping, the electron density at MARFE onset increased while the radiation loss fraction at MARFE onset did not change. In ELMy H-mode discharges, τHe* / τE was decreased down to 3 by setting the strike points close to the pumping slots. Before the divertor modification, τHe* / τE was higher than 4. Therefore, the helium exhaust efficiency became higher by the divertor modification. The results will be presented at the PSI meeting (May 22-26, 2000, Rosenheim).

Multiple pellet injection

In order to obtain high performance plasmas at high density, the pellet injection experiment was started using a centrifugal pellet injector. The new injector can inject pellets horizontally from the low-field side and almost vertically from the high-field side. In the initial experiment, the plasma density was raised up to the Greenwald density limit in OH plasmas (Ip = 0.8 MA) by the low-field-side injection (pellet dimension: 2.1 mm cube, repetition frequency: 10 Hz, speed: 690 m/s).

Boronization using B10D14

In JT-60U, B10H14 was used for boronization until this year. Then, the H/D ratio increased and a week was needed for degassing hydrogen. By introduction of B10D14, oxygen concentration and recycling was reduced without increase in the H/D ratio.