Plasma heating by directly launched ion Bernstein waves is investigated in the Alcator C tokamak at densities ${\overline{n}}_e\gtrsim 1\times {10}^{20}$ ${\mathrm{m}}^{-3\mathrm{}}$. For injected rf powers $P_{\mathrm{rf}}\lesssim 180$ kW, bulk-ion-temperature increases of $\Delta T_i\lesssim 380$ eV, as well as improvements in global particle and central impurity confinement times by a factor of 2-3, are observed. In certain regimes, the energy-confinement time improves relative to the initial Ohmic value. At higher densities (${\overline{n}}_e\gtrsim 2\times {10}^{20}$ ${\mathrm{m}}^{-3\mathrm{}}$), the heating ceases to be efficient.

• Received 3 June 1987

DOI:https://doi.org/10.1103/PhysRevLett.60.298