Analysis of current drive efficiency in mixed OH-LH discharges in Tore Supra
Lower hybrid current drive and heating experiments at 3.7 GHz have been performed in Tore Supra up to central densities of 9.7 x E19/m3. Up to now, a maximum RF current of 0.8 MA has been driven in a mixed OH-LH 1 MA discharge. However, the RF current is defined as the current which is carried by non-thermal electrons and therefore arises from the combined effect of the LH and ohmic powers. The influence of the residual electric field on the current drive efficiency has been analysed in two density regimes, in order to quantify the amount of power taken by suprathermal electrons directly from the OH transformer. The ratio of this to the LH power absorbed by the fast electrons can be evaluated from Fisch-Karney theory as a function of the wave phase velocity, and depends only on Z(eff) when the phase velocity is properly normalised. This ratio can be estimated by fitting the experimental data to the theory. In the low density regime, it is found to be as high as 60% and the extrapolation to a zero loop voltage situation results in an efficiency of 1 x 1.0 E19 A/m2/W.In the high density regime the efficiency rises to 1.1 x 1.0 E19 A/m2/W, but the power ratio drops to about 10% and therefore the correction from the residual electric field contribution is much smaller, giving roughly the same steady-state efficiency. At the highest density, a significant (20%) loop voltage drop together with clear signatures from a non-thermal electron population were still observed with 1.4 MW LH power, indicating that the LHCD density limit was not yet attained.
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: 199110377 / Last updated on: 1994-12-02
Original language: en
Available languages: en