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

Abstract

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 (n e = (0.5-1) × 10 12 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 T i = 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.

Characterization of the 20 kHz transient MHD burst at the fast U-3M confinement modification stage

“…It was concluded in [10] that the transition is triggered by the fast ions loss (ion orbit loss). According to previous NPA measurements [23] the edge RF energy deposition onto the ions is expected. We cannot talk with certain level of confidence about fast ion losses from the central plasma volume because they are mainly localized at the edge region However one may expect a strong modification of fast ion production in the FTA region at the plasma edge during the transition stage.…”

“…In a set of different FTA discharge conditions this explanation has been confirmed, however, further discussion on the details of RF heating is out of the scope of this paper. We just should note here that the strong absorption of the RF power generated by the FTA occurs at the edge region [21,23]. A substantial transient increase of the DC part of the emission from the upper channels #4, 5 (with impact parameters near ρ≈0.5-0.6) is observed during the This indicates on the increase in the electron temperature and/or the plasma density in the plasma region with a very poor confinement.…”

“…The oscillating parts of the signals become equal at this stage. Such behavior of the U-3M emission is attributed to the nature of RF plasma heating: the RF power is absorbed in the external part of the cross-section, where the plasma confinement is not sufficient [21,23]. It is not related to any imperfection of the diagnostics.…”

“…The properties of these diagnostics are similar to those described in [12]. The technical details of the vertical neutral particle analyzer (NPA) are described in [23]. The NPA collects the charge exchange (CX) neutral flux from the viewing area of square 3×3 cm cross-section.…”

“…The local heating can cause a three-dimensional structure of the plasma pressure distribution in this stochastic region. Different mechanisms of the RF power absorptions [22] can cause a substantial heating in the vicinity of the RF antenna, including the ion component heating [23].…”