Role of fast waves in the central deposition of lower hybrid power

In tokamaks, lower hybrid (LH) waves are routinely used for current drive and heating of plasmas. The LH waves have two modes of propagation that are called the slow and the fast wave. Usually, the lower hybrid waves are launched as slow waves into a tokamak, but during the propagation part of the wave power can be transformed to fast waves. General characteristics of the mode transformation of slow waves to fast waves are first investigated with a simple quasitoroidal ray-tracing model. Next, the effect of mode transformed LH power on the deposition profiles in a JET-like tokamak is analysed by using the fast ray-tracing code FRTC. When the launched spectrum is at small values of the toroidal refractive index (1.6 . nφ0 . 2.0), the contribution of the fast wave to the deposited power is found to be significant and responsible for most of the absorption at the centre. When nφ0 is large (nφ0 & 2.2), the effect of the mode transformed fast waves is small or negligible. At modest central densities (ne0 ∼ 0.5×1020 m−3), the contribution of the fast wave to the power deposition can be more than 50% in the plasma centre. In consequence, the significant amount of wave energy absorbed in the fast mode must be carefully taken into account in modelling LH current drive experiments in the future. At low central densities (ne0 . 0.3 × 1020 m−3), practically no absorption of fast waves occurs.