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 (EC H). These are dithering transitions which are induced by sawtooth cras hes and display the typical signatures of H-modes (D-alpha drop, spont aneous density increase, evidence of a transport barrier). However, th ey show only a slight improvement over L-mode energy confinement. The vessel walls are boronized and conditioned prior to experiments to ach ieve low-impurity influx and particle recycling. Discharges which unde rgo transitions are fuelled almost entirely on residual recycling. Tra nsitions are observed when limited on a toroidally localized top or bo ttom limiter and, more often, when the limiter surface is 'fresh', whi ch is achieved by alternating between top and bottom limiters on succe ssive shots. No strong dependence upon the distance from the low-field -side limiter has been found. Transitions are not yet observed when li mited on the high-held-side wall tiles or in the case of TEXT-U divert ed configurations. Preliminary measurements with the 2 MeV heavy-ion b eam probe (HIBP) (in the core) and Langmuir probes (in the edge) indic ate that the plasma potential drops outside the q = 1 radius while onl y small changes are observed in the density fluctuation level.