Plasma density limits during Ohmic L mode and ELMing H mode operation in DIII-D

Abstract

The density limits were determined in Ohmic and L mode discharges over a wide range of parameters: BT = 0.8-2.1 T, Ip = 0.5-1.9 MA, an elongation of 1.09 to 1.92, Zeff < 2, Pbeam ≲ 8 MW and ne,max 1.2 × 1020 m-3. The density limits in ELMing H mode discharges were also determined and found to be comparable with those of the Ohmic and L mode discharges. In Ohmic plasmas, the density limit increased linearly with Ip and decreased with minor radius (~1/a2.4); the density limit showed little, if any, dependence on either BT or plasma elongation. These results are in good qualitative agreement with Greenwald density limit scaling, (ne,max(1020 m-3) ~ Ip (MA)/a2 (m)). In discussing the density limits of detached plasmas, the 'effective' minor radius must be considered. The density limit data for L and H mode discharges were also consistent with Greenwald scaling. The density limits in the L and H modes were insensitive to the heating power. In the Ohmic and L mode cases, rotating MHD modes, typically with m/n=2/1, existed in high density discharges with no perceptible adverse impact on particle confinement until they locked, precipitating global disruptive behaviour. To date, the data show that the ELMing H mode density limit is not disruptive in itself