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(OV5/3) Steady-state Experiments on High Performance, Current Profile Control and Long Sustainment of LHCD Plasmas on the Superconducting Tokamak TRIAM-1M

H. Zushi¹⁾, S. Itoh¹⁾, K. Nakamura¹⁾, M. Sakamoto¹⁾, K. Hanada¹⁾, E. Jotaki¹⁾, Y. D. Pan²⁾, M. Hasegawa¹⁾, S. Kawasaki¹⁾, H. Nakashima^1)
 
1) Advanced Fusion Research Center Research Institute for Applied Mechanics, Kyushu University, JAPAN
²⁾ Southwestern Institute of Physics, China

Abstract.  TRIAM-1M ( R₀ = 0.8m, a×b = 0.12m×0.18m and B = 8T) has the main mission to study the route toward a high field compact steady state fusion reactor. We have advanced steady state operation (SSO) programme in tokamaks, studied a heating mechanism for the high ion temperature (HIT) mode with an internal transport barrier, obtained an enhanced current drive (ECD) mode in an extended (higher power and higher density) operation regime, performed current density profile control experiments using multi-current drive sys-tems and investigated effects of wall recycling, wall pumping and wall saturation on particle control. In HIT mode a hysteresis relation between T_i and n_e is found to be ascribed to different timescales for T_i and n_e to change. Excitation of plasma waves corresponding to ion heating is studied by both measurements of electromagnetic and electrostatic waves and their analysis. Achieved plasma parameters in ECD are as follows; n_e is 4.3×10¹⁹m^{- 3} , I_LHCD is $\sim$ 70kA, T_e and T_i are 0.8 keV and 0.5 keV, respectively, and the stored energy is 1.9 kJ. The energy confinement time $\tau_{\mathrm{E}}^{}$ of 8-10 ms, H_{ITER89 - P} $\sim$ 1.4, is achieved and the current drive efficiency $\eta_{\mathrm{CD}}^{}$ = $\bar{n}_{e}^{}$I_CDR₀/P_LH reaches $\sim$ 1×10¹⁹Am^{- 2}/W at B = 7 T under the fully non-inductive condition. Power threshold and hysteresis nature are studied. Bi-directional current drive and superposed current drive experiments have been carried out. In the former steady current reduction and peaking of j(r) are observed, but it is noticed that self-organization of j(r) occurs above a certain power ratio. In the latter broadening of j(r) can be obtained by increasing superposed RF power, however, self-organization of j(r) also occurs again at a certain power. Temporal behaviour of the recycling coefficient with two different time constants ($\sim$ 3 s and $\sim$ 30 s) is analysed. The wall pumping rates are evaluated to be $\sim$ 1.5×10¹⁶ atoms/s/ m² for low n_e and $\sim$ 4×10¹⁷ atoms/s/ m² for high n_e, respectively. In the high power and high density experiments, the wall saturation phenomenon affects the density control.

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