A PIC-MCC code RFdinity1d for simulation of discharge initiation by ICRF antenna

Discharges produced and sustained by ion cyclotron range of frequency (ICRF) waves in absence of plasma current will be used on ITER for (ion cyclotron-) wall conditioning (ICWC, T_e = 3-5 eV, n_e < 10¹⁸ m^-3). In this paper, we present the 1D particle-in-cell Monte Carlo collision (PIC-MCC) RFdinity1d for the study the breakdown phase of ICRF discharges, and its dependency on the RF discharge parameters (i) antenna input power P_i, (ii) RF frequency f, (iii) shape of the electric field and (iv) the neutral gas pressure p_H2. The code traces the motion of both electrons and ions in a narrow bundle of magnetic field lines close to the antenna straps. The charged particles are accelerated in the parallel direction with respect to the magnetic field B_T by two electric fields: (i) the vacuum RF field of the ICRF antenna E^RF_z and (ii) the electrostatic field E^P_z determined by the solution of Poisson's equation. The electron density evolution in simulations follows exponential increase, ṅ_e ∼ ν_iont . The ionization rate varies with increasing electron density as different mechanisms become important. The charged particles are affected solely by the antenna RF field E^RF_z at low electron density (n_e < 10¹¹ m^-3, |E^RF_z|Gt;|E^P_z|). At higher densities, when the electrostatic field E^P_z is comparable to the antenna RF field E^RF_z, the ionization frequency reaches the maximum. Plasma oscillations propagating toroidally away from the antenna are observed. The simulated energy distributions of ions and electrons at n^e ∼ 10¹⁵ m^-3 correspond a power-law Kappa energy distribution. This energy distribution was also observed in NPA measurements at ASDEX Upgrade in ICWC experiments.