Experimental studies on NBI and ICRF heated plasmas in the largehelical device

Abstract

In the Large Helical Device (LHD), upgrading of the key hardware systemsince the last EPS conference has led to (1) higher Te (Te(0) = 4.4 keV at ⟨ne⟩ = 5.3×1018 m-3 and Pabs = 1.8 MW); (2) higher confinement (τE = 0.3 s, Te(0) = 1.1 keV at ⟨ne⟩ = 6.5×1019 m-3 and Pabs = 2.0 MW); (3) higher stored energyWpdia = 880 kJ; and (4) the highest β value inhelical devices (2.4% at B = 1.3 T). The energy confinement wassystematically higher than that predicted by the International StellaratorScaling 95 up to a factor of 1.6. Ion cyclotron range of frequencies (ICRF)power up to 1.3 MW was reliably injected into the plasma withoutsignificant impurity contamination and a plasma with stored energy of200 kJ was sustained for 5 s by ICRF alone. Long pulse discharges greaterthan 1 min in duration have been successfully achieved with ICRF poweralone and with neutral beam injection (NBI) power alone. With NBI heatingan 80 s discharge was achieved with a heating power of 0.5 MW at 2.75 T.The electron density was maintained at around 1.6×1019 m-3 by controlled gas puffing. The central electron and iontemperatures were kept around 1.5 keV. With ICRF heating, a similar longpulse discharge was achieved for 68 s with a heating power of 0.85 MW. Thesustained plasma parameters are: Wp~110 kJ, Te(0)~Ti(0) = 2.0 keV and ⟨ne⟩ = 1.0×1019 m-3. During these discharges, no increase in radiationpower has been observed.