The density limit occurring in the electron heating by LH waves in FT was studied in a wide range of plasma parameters (BT = 4−8 T, Ip = 200−400 kA, ≥ 2.5 × 1013 cm−3) and operating in H and D. This limit is similar to the one occurring in current drive experiments. No significant dependence on any of the above parameters nor on the working gas was found, the interaction with electrons vanishing for ≥ 5 × 1013 cm−3 in all the cases studied, but comparison with results of other experiments at different frequencies indicates an ω2 dependence. An explanation of the density limit is the onset of ion absorption, implying a limit proportional to the ion mass. However, the presence of a hydrogen minority in a D plasma eliminates the mass dependence. Careful measurements of H and D neutral energy spectra in D plasmas of FT actually show that the occurrence of the density limit is due to the onset of strong absorption by the H minority. Another result is the presence of a large broadening of the applied n spectrum which confirms the conclusions of previous experiments.
Scaling of energy confinement with minor radius, current and density in Doublet III Ohmically heated plasmas