Dependence of heating efficiency and impurity production on the toroidal wavelength in ion cyclotron resonance heating

The details and results of a new ICRF code are presented. The code is based on a multiple shooting method and evaluates the RF field and power deposition profiles for minority heating in a tokamak plasma. The non-local relation between the RF fields and the plasma currents in the direction parallel to the static magnetic field is taken into account so that the code allows the long toroidal wavelength case to be correctly treated. It is found that the amount of RF power deposited in the scrape-off region increases very rapidly as the toroidal wavelength increases, reaching almost 10% of the total power coupled to the plasma for kϕ = 0. This edge power deposition is almost totally due to cyclotron damping on the minority ions. These results indicate that the increased impurity production observed experimentally when the kϕ spectrum coupled by the antenna is centred around kϕ = 0. may result from poorly confined energetic ions accelerated at the cyclotron resonance in the scrape-off region. The experimental observation that the coupling efficiency increases at low kϕ is also reproduced by the code, which predicts a decreased value of Q for the antenna and decreased fields in the vicinity of the antenna for low kϕ.