The experiments on fast current rampdown in TUMAN-3 device

This paper describes the evolution of the plasma parameters in the TUMAN-3 tokamak (R=52 cm; a=22 cm; BT=5.7 kG; ne=1-1.5 1013cm-3) in the regimes with fast current rampdown. In the described experiments plasma current decreased in 2 times from initial steady state value Ip=85 kA down to 42 kA during the period Δt=5-7.5 ms. The time of current decrease was a factor of 4-6 shorter than the characteristic time for the diffusion of magnetic field τm. The typical energy confinement time τE in steady state before the rampdown did not exceed the values τE=1-1.5 ms. This means, that the proportion τE << Δt << τm took place. An approximate estimations show that more than half of the total value of Ip flows inside plasma radius r=a/2 before the rampdown phase. The rapid decrease of the plasma current must produce an appreciable narrowing of the current density profile and thus help to understand the dependence of the plasma heating and the confinement of energy and particles on the j (r) distribution in ohmic regime. (author) 3 refs. 3 figs