Improved confinement studies in TUMAN-3 tokamak

Unusual methods of H-mode initiation were observed in the small tokamak TUMAN-3 in circular limiter configuration. Transitions from ordinary ohmic confinement into the H-mode were caused by means of (1) short increase of puffing rate, (2) minor radius magnetic compression, (3) biasing of the plasma boundary and (4) LiD pellet injection. In some cases transition into the improved confinement regimes arises spontaneously without any external disturbances. H-mode in TUMAN-3has typical signatures of the auxiliary heated divertor H-mode in bigger devices: reduction of the transport near the boundary, suppression of the fluctuations in this region, increase of the stored energy and of the number of particles in the plasma, drop of Dα radiation. Some peculiarities of the Ohmic H-mode were found: (1) noticeable improvement in the core confinement; (2) transient drop in the boundary electron temperature; (3) possibility to trigger H-mode by perturbation of the density gradient. (author). 22 refs, 11 figs, 1 tab