U-3M ion energy distribution measurements during frame antenna plasma production and heating in the ICRF range

An energy sweeping technique is developed and applied for charge exchange (CX) neutral particle diagnostics in the URAGAN-3M (U-3M) torsatron. Measurements are performed during low-density (ne = (0.5–1) × 1012 cm−3) frame antenna radio frequency (RF) plasma discharges in the RF frequency range close to the ion cyclotron frequency (ICRF). The presence of ions with energies up to 4 keV is established experimentally. According to the experimental data the ion energy distribution is close to Maxwellian in the energy range 0.4–2.5 keV and can be described by the ion temperature Ti = 300–600 eV. This is an indication of direct RF energy deposition onto the ions due to the negligible ion–electron energy exchange. The radial dependences of CX neutral flux and ion temperature under consideration are investigated using a set of similar discharges. The radial distribution of the studied temperature has a flat profile in the range ρ = 0.5–1. This is an indication of the ion energy deposition localization close to the plasma edge. The possible mechanisms of ion heating are briefly discussed.