PLASMA-DENSITY LIMITS DURING OHMIC L-MODE AND ELMING H-MODE-OPERATIONIN DIII-D

The density limits were determined in Ohmic and L mode discharges over a wide range of parameters: B-T = 0.8-2.1 T, I-p = 0.5-1.9 MA, an elo ngation of 1.09 to 1.92, Z(eff) < 2, P-beam less than or similar to 8 MW and (n) over bar(e,max) less than or similar to 1.2 x 10(20) m(-3). The density limits in ELMing H mode discharges were also determined a nd found to be comparable with those of the Ohmic and L mode discharge s. In Ohmic plasmas, the density limit increased linearly with I, and decreased with minor radius (similar to 1/a(2.4)); the density limit s howed little, if any, dependence on either B, or plasma elongation. Th ese results are in good qualitative agreement with Greenwald density l imit scaling, ((n) over bar(e,max)(10(20) m(-3)) similar to I-p (MA)/p i a(2) (m)). In discussing the density limits of detached plasmas, the 'effective' minor radius must be considered. The density limit data f or L and H mode discharges were also consistent with Greenwald scaling . The density limits in the L and H modes were insensitive to the heat ing power. In the Ohmic and L mode cases, rotating MHD modes, typicall y with m/n = 2/1, existed in high density discharges with no perceptib le adverse impact on particle confinement until they locked, precipita ting global disruptive behaviour. To date, the data show that the ELMi ng H mode density limit is not disruptive in itself.