Current density profile control by programming of gas puffing and plasma current waveform in the jipp t-ii tokamak

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 n –_e = 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 ℒ I_p/n –_e ℒ 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.