Search for a critical electron temperature gradient in DIII-D L-mode discharges

Two experiments on DIII-D have been performed with the purpose of searching for evidence of a critical electron temperature gradient or gradient scale length. Both experiments employed off-axis electron cyclotron heating to vary the local value of inverse temperature scale length while holding the total heating power and thus edge temperatures constant. The incremental or heat pulse thermal diffusivity was monitored as a means of identifying the existence of a threshold or critical value of , k crit , where a large increase in heat pulse diffusivity may occur while going from below to above k crit . No evidence of a critical value was found, but the existence of one cannot be ruled out by the experimental results. The results can be used to place an upper bound on k crit . If k crit exists, the experimental results indicate k crit < 3.8 m −1 at ρ = 0.45 and k crit < 2.5 m −1 at ρ = 0.29 corresponding to temperature scale lengths relative to the plasma minor radius of and 0.65, respectively. Over most of the plasma radius (ρ ⩾ 0.3) the temperature profile in the L-mode discharges studied exceeds these upper bounds on k crit . A transport model based on a critical gradient is consistent with experimental observations but is not unique. Models other than one based on the existence of a critical gradient are also consistent with experimental observations.