Analytic Study of Density Limit Mechanism for Tokamak Lower Hybrid Current Drive Experiments

The dispersion relation of the lower hybrid wave in the tokamak current drive regime(ω>2ωLH,whereωLH is the lower hybrid frequency)is analysed.Toroidicity effects can shift the parallel refractive index upward or downward.The downward shift will lead to a mode conversion from the slow wave to the fast wave.It is indicated that this is the main mechanism limiting the effectiveness of tokamak lower hybrid current drive.Accordingly,a sufficient condition is obtained for the wave penetration,corresponding to a critical density,which is proportional to f^(4/3)B^(2/3)(A)^(4/3),with f being the wave frequency,B the magnetic field strength and A the aspect ratio.Quantitative agreement with experiments is found.