Nonresonant current drive and helicity injection by radio-frequency waves

Current drive via nonresonant interaction between radio-frequency (rf) waves and plasma is studied. The averaged force of rf waves acting on each species of a plasma can be divided into a resonant force and a nonresonant one. A part of the nonresonant force cannot be expressed by a gradient of a scalar potential and remains after integrating along the direction of the force. This force mainly acts as an internal force among plasma species and the net momentum input from the wave to the plasma is usually small. This process is not included in the conventional current drive schemes but is associated with the rf wave, helicity injection. Quantitative analysis using a one-dimensional kinetic wave code is applied to waves in the ion cyclotron range of frequencies and low-frequency Alfvén waves in a large tokamak. The driven current is estimated taking account of the effect of the toroidally trapped particles. The spatial profile of the forces acting on electrons and ions as well as the driven current are obtained. The parameter dependence of the current drive efficiency on the wave number, the plasma density, the temperature, and the toroidal magnetic field is compared with the estimate based on a local analysis.