Plasma decontamination during ergodic divertor experiments in Tore Supra
A decontamination effect due to the creation of an ergodic boundary zone has been demonstrated in Tore Supra, in both ohmic and lower hybrid heated plasmas. The additionally heated experiments were performed with up to 1 MW of injected lower hybrid power for plasmas limited on either the high- or low-field side. Impurity line brightnesses have been recorded in the range 10 - 300 Angstrom, using an extreme grazing incidence spectrometer. Neutral particle recycling at the inside wall has been studied by visible spectroscopy. Analysis of the CVI Lyman alpha line shows that the carbon concentration in the plasma core is largely reduced during the application of the ergodic divertor, in good agreement with the reduction of the effective charge of the plasma (carbon being the dominant impurity in Tore Supra). However, the decontamination of the plasma bulk is accompanied by an increase of the peripheral line brightnesses (CIV, OIV, OV), correlated to a modification of the neutral particle fluxes at the wall. These results are compared to similar shots with and without lower hybrid, thus comparing the divertor efficiency in both cases. The effect of the impurity source has been investigated by performing nitrogen injection experiments. The nitrogen primary source was kept constant, yet a large reduction of the NVII Lyman alpha line was observed, together with an increase of the NV peripheral lines. All these observations (screening effect and enhanced recycling at the edge) are interpreted as a decrease of the parallel particle confinement time at the very edge of the plasma.
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: 199110382 / Last updated on: 1994-12-02
Original language: en
Available languages: en