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

In tokamaks, lower hybrid (LH) waves are routinely used for current drive a nd 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 laun ched as slow waves into a tokamak, but during the propagation part of the w ave 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 transfo rmed 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 s mall values of the toroidal refractive index (1.6 less than or similar to n (phi 0) less than or similar to 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 n(phi 0) is large (n(phi 0) greater than or similar to 2.2), the effect of the-mode transformed fast waves is small or negligible. At modest central densities (n(e0) similar to 0.5 x 10(20) m(-3)), the contribution of the fast wave to the power deposition can be mo re 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 de nsities (n(e0) less than or similar to 0.3 x 10(20) m(-3)), practically no absorption of fast waves occurs.