Current density profile control by programming of gas puffing and plasma current waveform in the JIPP T-II tokamak

Abstract

In the resistive-shell tokamak, JIPP T-II , a control of the current density profile has been attempted by programming both gas puffing and plasma current waveform. A stable high-density plasma has been obtained with the following parameters: the maximum line-average electron density is n̄e = 8.5 × 1013cm−3, the minimum q(a)-value is 2.2, and the relative amplitude of the m/n = 2/l mode is suppressed to an extent less than 10−3. A derivation of the current density profile by solving the magnetic-diffusion equation on the basis of the experimental data shows that the current density profile favourable to the stability of low-m kink and tearing modes is realized by combining the effects of cooling through an increase in density and of heating by a current rise in the outer plasma region. The results of kink and tearing modes analysis agree well with the experimental observations. The criterion that the current density profile is successfully controlled by this method is derived as a function of the ratio of plasma current to electron density in the current-rise phase, i.e. 20 × 10−13 ⪅ Ip/n̄e ⪅ 30 × 10−13 kA·cm3. The major disruption due to the density increase is completely suppressed by the method proposed in this paper. The major disruption due to a reduction of q(a) to less than 2.2 has, however, not yet been suppressed. In future, the current density profile should be maintained more precisely at its optimum shape by using a feedback-control technique and a control of the plasma boundary with titanium gettering, etc.

Simulations of tokamak disruptions including self-consistent temperature evolution

“…With regard to profile control, it is evident from the discussion in Sections 3 and 4 (or even from simply considering the linear stability of the 2/1 and 3/2 modes) that heating outside the q = 2 surface should have a strongly stabilizing influence against disruption, as is experimentally known since a long time [10].…”

“…The most powerful way to induce strong tearing activity and the only way found so far to produce hard disruptions within the reduced MHD model ( 1) is to cool the region outside the q = 2 surface. This could be the result of impurity cooling [10] or increasing plasma current so that the q = 2 surface is near the limiter [5]. In the numerical experiment described here, the current was ramped up slowly, so that q a went from 2.74 to 2.49 in 19000 Alfven times according to q a = q a (0) exp(-t/T q ), with T=2 X 10 s .…”

A density rise experiment on PLT

“…Before gas puffing (300 ms in Fig. 8), the edge-neutral density is n o (a) ~ 2 X 10 9 cm" 3 and the central neutral density is n o (O) = 1 X 10 3 cm" 3 ; at the end of gas puffing (475 ms) these values are: n o (a) ^ 7 X 10 9 cm" 3 and n o (O) ^ 5 X 10 7 cm" 3 . The values of n 0 away from the main limiter and the auxiliary limiter are estimated to be about seven times lower.…”

“…Plasma densities have been raised by gas puffing in a number of tokamak experiments [1][2][3][4][5][6]. In the smaller devices, the evolution of the density profile can be explained by assuming an edge-neutral temperature of 10-50 eV.…”

Carbon dioxide laser with an active medium containing tripropylamine

“…The absolute phase of the SXR components E l (at r = 0. 3) and E2 (at r = 0.4) with respect to the m = 2 component of the coil array signals (at 6 = 0) is shown in The phases E , and e2 thus appear to be both space and time invariant. These results validate the invoked assumption of rigid body rotation and are also consistent with the results of Ref.…”