Electron heating via mode converted ion Bernstein waves in the Alcator C-Mod tokamak

Highly localized direct electron heating [full width at half-maximum $(\mathrm{FWHM}\text{)≅0.2a}$ ] via mode converted ion Bernstein waves has been observed in the Alcator C-Mod Tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)]. Electron heating at or near the plasma center $\text{(r/a⩾0.3)}$ has been observed in ${\mathrm{H(}}^{\mathrm{3}}\mathrm{He)}$ discharges at $B_0\text{=(6.0–6.5)\hspace{0.17em}}\mathrm{T}$ and $n_e{\text{(0)≅1.8×10}}^{20}\hspace{0.17em}{\mathrm{m}}^{\text{−3}}.$ [Here, the minority ion species is indicated parenthetically.] Off-axis heating $\text{(r/a⩾0.5)}$ has also been observed in ${\mathrm{D(}}^{\mathrm{3}}\mathrm{He)}$ plasmas at $B_0\text{=7.9\hspace{0.17em}}\mathrm{T}.$ The concentration of ${}^3\mathrm{He}$ in these experiments was in the range of $n_{3_{\mathrm{He}}}{\mathrm{/n}}_e\text{≅(0.2–0.3)}$ and the locations of the mode conversion layer and electron heating peak could be controlled by changing the ${}^3\mathrm{He}$ concentration or toroidal magnetic field ${\mathrm{(B}}_0\mathrm{).}$ The electron heating profiles were deduced using a rf modulation technique. Detailed comparisons with one-dimensional and toroidal full-wave models in the ion cyclotron range of frequencies have been carried out. One-dimensional full-wave code predictions were found to be in qualitative agreement with the experimental results. Toroidal full-wave calculations indicated the importance of volumetric and wave focusing effects in the interpretation of the experimental results.