Experimental investigation and validation of neutral beam current drive for ITER through ITPA Joint Experiments

摘要

Joint experiments investigating the off-axis neutral beam current drive (NBCD) capability to be utilized for advanced operation scenario development in ITER were conducted in four tokamaks (ASDEX Upgrade (AUG), DIII-D, JT-60U and MAST) through the international tokamak physics activity (ITPA). The following results were obtained in the joint experiments, where the toroidal field, B_t, covered 0.4-3.7T, the plasma current, I_P, 0.5-1.2 MA, and the beam energy, E_b 65-350 keV. A current profile broadened by off-axis NBCD was observed in MAST. In DIII-D and JT-60U, the NB driven current profile has been evaluated using motional Stark effect diagnostics and good agreement between the measured and calculated NB driven current profile was observed. In AUG (at low S ～ 0.2) and DIII-D, introduction of a fast-ion diffusion coefficient of D_b ～ 0.3-0.5 m~2 s~(-1) in the calculation gave better agreement at high heating power (5 MW and 7.2 MW, respectively), suggesting anomalous transport of fast ions by turbulence. It was found through these ITPA joint experiments that NBCD related physics quantities reasonably agree with calculations (with D_b = 0-0.5 m~2 s~(-1)) in all devices when there is no magnetohydrodynamic (MHD) activity except ELMs. Proximity of measured off-axis beam driven current to the corresponding calculation with D_b = 0 has been discussed for ITER in terms of a theoretically predicted scaling of fast-ion diffusion that depends on E_b/T_e for electrostatic turbulence or ß_t for electromagnetic turbulence.