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(CD2/EX9/2) Long Sustainment of JT-60U Plasmas with High Integrated Performance

Y. Kamada, A. Isayama, T. Oikawa, Y. Sakamoto, N. Hosogane, H. Takenaga, Y. Kusama, T. Fujita, S. Takeji, T. Ozeki, Y. Ishii, S. Tokuda, K. Ushigusa, O. Naito, S. Ishida, Y. Koide, T. Fukuda, T. Takizuka, H. Shirai, T. Hatae, the JT-60 Team

Naka Fusion Research Establishment, Japan Atomic Energy Research Institute, Naka-machi, Naka-gun, Ibaraki-ken, 311-01, Japan

Abstract.  This paper treats the recent development of quasi-steady ELMy high- $\beta_{\mathrm{p}}^{}$ H-mode discharges with enhanced confinement and high-$\beta$ stability, where i) long sustainment time, ii) increase in absolute fusion performance and iii) expansion of the discharge regime toward low- q₉₅( $\sim$ 3) are emphasized. After modification to the new W-shaped pumped divertor, a long heating time (9sec) with a high total heating energy input of 203MJ became possible without harmful increase in impurity and particle recycling. In addition, optimization of the pressure profile characterized by the double transport barriers, electron density and / or high triangularity $\delta$ enabled us to extend the performances in long pulses ; the DT equivalent fusion gain Q_DT^eq $\sim$ 0.1($\delta$ = 0.16) was sustained for $\sim$ 9 sec ( $\sim$ 50$\tau_{\mathrm{E}}^{}$, $\sim$ 10$\tau_{\mathrm{p}}^{*}$) and Q_DT^eq $\sim$ 0.16($\delta$ = 0.3) for 4.5s at I_p = 1.5MA. In the latter case, H-factor (= $\tau_{\mathrm{E}}^{}$/$\tau_{\mathrm{E}}^{\mathrm{ITER89PL}}$) $\sim$ 2.2, $\beta_{\mathrm{N}}^{}$ $\sim$ 1.9 and $\beta_{\mathrm{p}}^{}$ $\sim$ 1.6 were sustained with 60-70% of non-inductive driven current. In the low q₉₅( $\sim$ 3) region, the $\beta$-limit was improved by the high $\delta$( $\sim$ 0.46) shape where $\beta_{\mathrm{N}}^{}$ $\sim$ 2.5 - 2.7 was sustained for $\sim$ 3.5s with the collisionality close to that of ITER-FDR plasmas. The sustainable $\beta_{\mathrm{N}}^{}$ is limited by onset of the low-n resistive modes. The direct measurement of the island width shows the agreement with the neoclassical tearing mode theory. The edge pedestal width $\Delta$-ped in the ELMy phase evolves gradually with increasing T_i at the pedestal shoulder and the saturated width (10-15cm at I_p = 1MA) is 2-3 times larger than that in the ELM-free phase. The width $\Delta$-ped is proportional to (a/R)^0.5$\rho_{\mathrm{pi}}^{}$. The limit of the edge $\alpha$-parameter increases with increasing $\delta$, which is the main reason of the improved high-$\beta$ stability in a long pulse by high-$\delta$. The sustainable value of $\beta_{\mathrm{N}}^{}$H increases with increasing $\delta$.

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