(EX/P8-8) Density Limit in Discharges with High Internal Inductance on JT-60U

H. Yamada1), H. Takenaga2), T. Suzuki2), T. Fujita2), T. Takizuka2), Y. Kamada2), N. Asakura2), T. Tuda2), M. Takechi2), G. Matsunaga2), Y. Miura2)
 
1) National Institute for Fusion Science, Toki, Gifu-ken, 509-5292 Japan
2) Japan Atomic Energy Agency, Naka, Ibaraki-ken, 311-0193 Japan

Abstract.  High densities exceeding the Greenwald limit by a factor of 1.7 have been obtained in discharges with high internal inductances of li as high as 2.8 in JT-60U. The internal inductance is controlled by ramping down the plasma current. While the density is beyond the Greenwald limit, confinement performance is kept as good as H89PL and HHy2 factors of 1.5 and 1, respectively. In NBI heated discharges in JT-60U, the operational density limit has been surveyed by changing the current ramp down rate (0.175-0.75 MA/s) from a flat top (plasma current of 1 MA). Internal inductance $ \ell_{i}^{}$ goes up in this phase, which suggests a peaked current profile with enhanced magnetic shear in the edge region. In a scheme of monotonic current ramp down, $ \ell_{i}^{}$ (magnetic shear) and q have co-linearity. In order to separate these two factors, the current ramp down has been paused at the plasma current of 0.65 MA and the phase with decreasing $ \ell_{i}^{}$ (magnetic shear) at the constant q also has been investigated. In the case of no disruption, the plasma has good confinement performance even beyond the Greenwald limit and the density has reached 1.73 times the Greenwald limit. The confinement performance is better than the 89PL scaling and is consistent with the earlier work on high $ \ell_{i}^{}$ plasmas. A slight increase of the density has resulted in a disruption in the earlier phase of the current ramp down. The density normalized by the Greenwald limit of the case with no disruption is even higher than the case with a disruption in particular in the periphery. The temperature in the periphery is higher in the discharge without disruption than that of the discharge terminated by a disruption. The normalized density when the detachment characterized by the decrease in a Dα signal at the divertor occurs is even higher in the case with no disruption compared with the case with a disruption. This comparison suggests that the improvement in thermal and particle transport does exist in the periphery and the edge, and mitigation of density limit is observed coincidently. Although the high $ \ell_{i}^{}$ discharge studied here lies outside of the usual parameter space for tokamak operation, demonstration of a stable discharge with good confinement beyond the Greenwald limit uncovers its physical element.

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