Externally launched lower hybrid (LH) waves do not propagate into the plasma core during operations of JET with radial profiles with relatively high density even at the periphery, approaching the condition necessary for ITER. Modelling results indicate that this problem would be caused by parametric instability (PI)-induced LH spectral broadening, which is expected to occur in the plasma edge and prevents the coupled LH power penetrating the plasma core. However, operation with relatively high electron temperature at the edge is expected to diminish the PI effect and extend the LH current drive effectiveness to reactor-grade high density plasmas, consistent with results obtained in other experiments.
Spectral broadening of lower hybrid waves produced by parametric instability in current drive experiments of tokamak plasmas