Density control in Tore Supra ergodic divertor and multi-pellet injection experiments
Ergodic divertor and multi-pellet injection experiments have shown specific density limits. Ergodic divertor experiments in Tore Supra have shown distinctive features depending on the fuelling gas. When the coils were energised, the density decreased by a large factor for deuterium shots, whereas it remained unchanged for helium shots. At the same time, any attempt to raise the density by means of edge gas fuelling remained almost totally inefficient. When the gas feed rate reached too high a level, the edge radiation was strongly enhanced; subsequently, a radiative collapse could occur, corresponding to a reduction of the density limit by a factor of about 2, compared to the usual ohmic value. The density decrease was very rapid at the edge and led afterwards to a core reduction. This happened on a time scale compatible with an unperturbed diffusion in the plasma bulk. This effect can be interpreted as transient gas pumping by the wall. The modifications of the radiation pattern appear to be related to the large changes of the plasma parameters in the ergodic zone; the lowering of the electronic temperature there from 200 to 50 eV induces an increase of the radiative power loss in the outboard region. Any other attempt to further decrease T(e), such as gas fuelling, can lead to the radiation enhancement. The edge radiation is also related to the impurity production and especially oxygen, which depends strongly on the deuterium flux to the wall.
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: 199110380 / Last updated on: 1994-12-02
Original language: en
Available languages: en