Several types of edge plasma perturbations in the TUMAN-3 tokamak [Proceedings of the 13th International Atomic Energy Agency Conference on Plasma Physics and Controlled Nuclear Fusion Research, 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. 1, p. 509] have been demonstrated to trigger the Ohmic H-mode transition. It is shown that three different methods, (1) radial electric field of either sign imposed by an electrode biased up to 500 V, (2) perturbation of the edge plasma density by strong gas puffing, and (3) LiD pellet injection, bring about the Ohmic H mode. In biasing experiments, the degree of improvement of particle and energy confinement depends on the polarity of the electric field and is higher for negative biasing. The evaporation of a LiD pellet (V∼150 m/sec, size ∼0.3 mm, density perturbation∼50%) in the peripheral region of the plasma column can also lead to the H-mode transition. Experimental results are shown to be in reasonable agreement with the theory of radial electric fields in tokamaks.
The focus of the TUMAN-3 and TUMAN-3M tokamaks programme is on issues of improved confinement. The transition from an ordinary ohmic regime into improved confinement mode has been found in circular limiter configuration in a vessel with all-metallic walls and limiters. The signatures of the H-mode in auxiliary heated tokamaks have been observed in this regime. The crucial role of the radial electric field was found in experiments with internal probe biasing. Other techniques were demonstrated to trigger H-mode: short increase of the working gas puffing rate, minor radius magnetic compression and pellet injection.The scaling of the energy confinement time in ohmic H-mode was obtained, which differs dramatically from the scaling for the ordinary ohmic regime. A strong dependence of τ E on plasma current was found. The τ E scaling for the ohmic H-mode is consistent with the scaling proposed for devices with powerful auxiliary heating (JET/DIII-D H-mode scaling). The result shows that H-mode physics is universal in tokamaks with different geometries and heating methods.In 1994 a new vacuum vessel was installed in the TUMAN-3 tokamak. The modified device, TUMAN-3M, is able to produce higher B T and I p , up to 2 T and 0.2 MA, respectively. During the first operational period a plasma current of 0.15 MA was achieved at B T = 0.8 T, which corresponded to q cyl = 2.6. The impact of the quality of wall coating on confinement was asserted. The longest energy confinement time (30 ms) was observed under the conditions of best boronization.
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