Lower hybrid current drive at densities required for thermonuclear reactors

For the progress of the thermonuclear fusion energy research based on the tokamak concept, it is essential to drive current non-inductively in high-density plasmas for producing steady state, thermally well insulated, stable and large volume plasmas. ITER (International Tokamak Experiment Reactor), as an example, will have relatively high densities at the periphery of the plasma column (ne_0.8 ≈ 0.8⋅10 m – 1⋅10 m at normalised minor radius r/a ≈ 0.8, where r = a is the plasma minor radius at the last closed magnetic surface). The externally launched lower hybrid wave, producing the lower hybrid current drive (LHCD) effect is potentially the most suitable tool for driving current at large radii of fusion relevant plasmas, consistent with the needs of ITER. However, signs of penetration in the bulk of the coupled LH power didn’t occur for plasma densities at the edge approaching ITER requirements. Wave-wave interactions at the plasma edge would explain the observed experimental behaviour. In LHCD experiments at relatively high plasma edge densities, the parametric instability (PI) produced spectral broadening becomes so strong that penetration into the plasma core is prevented. By numerical investigation, it was also predicted that the PI effect are mitigated when operating with relatively high electron temperatures at the plasma periphery, resulting in better wave penetration into the core of high-density plasmas. In order to test the effect of higher edge temperature (Te_outer) in producing conditions favouring LH power penetration at high density, the flexibility of FTU operations have been exploited, consisting in the lithium-coated vessel, plasma column was leaned on the poloidal outer limiter, which reduces the plasma-wall interactions, consequent to the smaller plasmawall wetting area. Gas fuelling assisted by pellet has been also used, with LH power coupled just after the pellet injection. We refer to these conditions as high Te_outer regime, for distinguishing it from the standard regime described before. With this operation, plasma targets with much higher density (up tone ≈ 2⋅10 m) have been obtained, with higher electron temperature at the periphery, (Te ≈ 300 eV at r/a ≈ 0.8 and Te ≈ 150 eV at r/a ≈ 0.9, to be compared with Te ≈ 100 eV at r/a ≈ 0.8 and Te ≈ 50 eV at r/a ≈ 0.9 of standard conditions). Effects of LHCD have been observed for the first time in FTU at plasma density even higher than that required for ITER.