Heat flux of fast electrons to the limiter in lower hybrid current drive plasma on WT-3

The heat flux of fast electrons to the local limiter in LHCD plasmas in WT-3 has been investigated by thermal measurement of the limiter. The amount of the heat flux (P_FE) is found to be about on third of the net radiofrequency power (P_rf) injected into the plasma for various discharge conditions. The results combined with other measurements show that the confinement of fast electrons deteriorates as P_rf increases. This direct loss of fast electrons is one of the causes of the degradation of the current drive efficiency. Heat transport of the bulk electrons is also found to increase as P_rf increases. Experimental results indicate that a significant part of the remaining RF power (2P_rf/3) flows to the bulk electrons. The slowing down power of fast electrons in the energy range above several tens of keV is estimated to be quite small compared with 2P _rf/3, suggesting that a significant part of the remaining power flows to the bulk electrons via other channels. A plausible channel is the absorption of RF power via lower energy electrons by an upshift of the parallel refractive index of the injected lower hybrid waves. This seems to be another cause of the degradation of the current drive efficiency.