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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-$\rm\beta_p$ 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- $\rm q_{95} (\sim 3)$ 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  $\rm Q_{DT}^{eq} \sim 0.1 (\delta = 0.16)$ was sustained for $\sim$ 9 sec $\rm (\sim 50\tau_E, \sim10\tau_p^*)$ and $\rm Q_{DT}^{eq} \sim 0.16 (\delta = 0.3)$ for 4.5s at $\rm I_p = 1.5MA$. In the latter case, H-factor $\rm (=\tau_E/\tau_E^{\mbox{ITER89PL}}) \sim 2.2$, $\rm\beta_N \sim 1.9$ and $\rm\beta_p \sim 1.6$ were sustained with 60-70% of non-inductive driven current . In the low $\rm q_{95} (\sim 3)$ region, the $\beta$-limit was improved by the high $\rm\delta (\sim 0.46)$ shape where $\rm\beta_N \sim 2.5-2.7$ was sustained for $\sim$ 3.5s with the collisionality close to that of ITER-FDR plasmas. The sustainable $\rm\beta_N$ 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 $\rm T_i$ at the pedestal shoulder and the saturated width (10-15cm at $\rm I_p$ = 1MA) is 2-3 times larger than that in the ELM-free phase. The width $\Delta$-ped is proportional to $\rm (a/R)^{0.5}\rho_{pi}$. 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 $\rm\beta_NH$ increases with increasing $\delta$.

      

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