Experimental Conditions for Improved Confinement Modes in Heliotron J

抄録

ECH/NBI experiments in Heliotron J have revealed the existence of a spontaneous transition to improved confinement modes. Based on the experimental database obtained up to now, the characteristics of the transition and the density and power thresholds are discussed paying attention to the configuration effects. For ECH+NBI discharges with injection powers of P_ECH ˜ 0.29 MW and P_NBI ˜ 0.57 MW, transition phenomena were observed in almost all ι(a)/2π configurations. The global plasma confinements before and after the transition are affected by the value of the edge rotational transform ι(a)/2π. During the improved mode, the ι(a)/2π-dependence of the plasma stored energy is mitigated compared to the pre-transition phase. The database indicates the existence of a critical value in the line-averaged electron density under which the transition cannot be observed. This threshold line-averaged density is in the range of 1.1-2.0×10¹⁹ m³ in most configurations, indicating that it is not a strong function of the injection power and/or the heating method. The existence of an ι(a)/2π value where the transition was not observed for ECH-only discharges but observed for NBI-only or ECH+NBI discharges suggests the existence of a configuration effect and/or heating-method effect on the threshold power.