H-Mode Development In Text-U Limiter Plasmas

H-mode transitions in TEXT-U limiter plasmas have been observed at q(a) approximate to 3 and I-p approximate to 250 kA (P-OH approximate to 300 kW) With at least 300 kW of central electron-cyclotron heating (ECH). These are dithering transitions which are induced by sawtooth crashes and display the typical signatures of H-modes (D-alpha drop, spontaneous density increase, evidence of a transport barrier). However, they show only a slight improvement over L-mode energy confinement. The vessel walls are boronized and conditioned prior to experiments to achieve low-impurity influx and particle recycling. Discharges which undergo transitions are fuelled almost entirely on residual recycling. Transitions are observed when limited on a toroidally localized top or bottom limiter and, more often, when the limiter surface is 'fresh', which is achieved by alternating between top and bottom limiters on successive shots. No strong dependence upon the distance from the low-field-side limiter has been found. Transitions are not yet observed when limited on the high-held-side wall tiles or in the case of TEXT-U diverted configurations. Preliminary measurements with the 2 MeV heavy-ion beam probe (HIBP) (in the core) and Langmuir probes (in the edge) indicate that the plasma potential drops outside the q = 1 radius while only small changes are observed in the density fluctuation level.