Equilibrium conditions of radiofrequency-heated plasma cylinders

The equilibrium conditions of radiofrequency heated plasma cylinders have been calculated by solving the two coupled problems of the electromagnetic power deposition and the macroscopic transport of charged and neutral species. The two Maxwell wave equations have been finite-differenced along the radius of the cylinder (SPIREs code), providing the input source terms for the transport problem. The continuity and momentum equations of a singleionized cold plasma, together with energy conservation, have been solved along the same radial direction (EQM code). An iterative procedure has been used between the two problems in order to evaluate the profiles at equilibrium. The method allows the prediction of the profiles of plasma density, electron temperature and neutral density in RF heated plasma cylinders. Calculations have been done for typical conditions encountered in low-pressure helicon discharges, where the plasma is magnetized with an external field directed along the axis of the cylinder. The plasma density profiles are peaked at the center, even if most of the RF deposition occurs at the edge of the discharge. The increase of plasma density with the RF power has been calculated for a direct comparison with Langmuir probe measurements in helicon experiments. The electron temperature remains constant with the increasing power, but increases at the edge of the cylinder where the RF wave is strongly dumped via the Trivelpiece-Gould mode. Neutrals are depleted at the center of the discharge, where the plasma density reaches its maximum.