Current diffusion and flux consumption in Tore Supra
Development of the internal distribution of current is investigated by analysing Tore Supra discharges along trajectories in the (q(psi)(a),l(i)) plane. A large database has been constructed for which the stationary state of current profile has been reached and maintained for a long time, even at the lowest q(psi)(a). Various current profile evolution patterns, as deduced from the magnetic measurements and numerical code, using resistivity profiles from T(e) experimental profiles, are obtained during the start-up phase. Immediately after breakdown, the current profile is generally rather broad (l(i) less than 0.6), with a hollow temperature profile for 300 ms. The dynamic of the current profile - and consequently the plasma stability - is then entirely determined by its initial internal inductance, the current ramp rate and the flat top current value. By control of these parameters, discharges can be kept inside a safe, MHD stable, operational region. For current ramp rates up to the disruption limit (1.2 MA/s), current diffusion is found to be governed by classical resistivity. This result is obtained even for the case when current penetration is enhanced, in connection with MHD activity development (typically for an initial current ramp greater than 0.7 MA/s). Plasma relax towards a steady-state, characterised by an equilibrium internal inductance, which is mainly determined by the q(psi)(a) flat top value. An optimum is found for plasma surface flux consumption, which scales linearly with plasma current and corresponds to the fastest MHD stable current ramp-up. It must be stressed that the flux consumption to reach a stationary current profile depends only upon the current diffusion time, and is about double the minimum flux to reach the flat top current. Thus, even if ramping the current faster results in a longer current flat top, the steady state phase (constant current profile) is not extended.
Bibliographic Reference: Paper presented: 18th European Conference of the EPS-Plasma Physics Division, Berlin (DE), June 3-7, 1991
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Record Number: 199110383 / Last updated on: 1994-12-02
Original language: en
Available languages: en