Observation of High Impurity Ion Temperatures in ECW/EBW Sustained Plasmas on LATE

Abstract

Carbon and oxygen impurity line emissions from LATE spherical tokamak plasmas were measured using a visible spectrometer. A plasma current of approximately 10 kA was sustained by ECW/EBW with a frequency of 5 GHz and a power of 130 kW. Emissions of CIII (464.7 nm), OV (278.1 nm) and CV (227.1 nm) were observed, and the ion temperatures calculated from the Doppler broadening reached 40 ± 8, 110 ± 10 and 130 ± 30 eV, respectively. The high ion temperatures cannot be explained by collisional heating from electrons, suggesting that the ions are directly heated via unidentified mechanisms. In order to realize a compact spherical tokamak (ST) reactor, the elimination of the massive central solenoid is necessary. RF waves such as electron cyclotron waves and lower hybrid waves are often used to start-up and sustain ST plasmas [1,2]. It is believed that these RF sustained plasmas comprise high energy electrons which carry the plasma current and dominate equilibrium [3,4], low temperature bulk electrons [5,6] and very low temperature ions. In order to understand the power flow in these RF sustained plasmas, the measurement of each component's temperature is crucial.The ion temperatures in LATE electron cyclotron wave (ECW: 2.45 GHz/50 kW) sustained plasmas and TST-2 lower hybrid wave (LHW: 200 MHz/20 kW) sustained plasmas were about 10 and 4 eV, respectively [7]. Although the bulk electron temperature of LATE plasmas was not measured, it was probably similar to those in TST-2 (a few tens of electron volts), because both showed similar visible spectra, where CIII (464.7 nm, C 2+ , E i = 47.9 eV) intensity was strong and OV (278.1 nm, O 4+ , E i = 114 eV) and CV (227.1 nm, C 4+ , E i = 392 eV) were too weak to be identified. Here E i denotes the ionization energy of each element. In this study, the ion temperatures of LATE plasmas sustained by ECW and/or electron Bernstein wave (EBW) with a frequency of 5 GHz and a power of up to 130 kW are presented. Figure 1 shows the time evolution of a typical discharge. CIII, OV and CV line intensities and the ion temperatures measured by the same visible spectrometer used in our precious study [7] are displayed. The intensities are appropriately scaled to be visible on the same graph. The author's e-mail: ejiri@k,u-tokyo.ac.jp CIII emission is present from the beginning of a discharge, while the intensity of OV emission grows until t ∼ 90 ms; further, it rapidly increases. The CV emission becomes visible at t ∼ 100 ms. Considering the detector (photomultiplier tube) gain variation due to the applied high voltage, the signal intensity ratio between OV, CIII and CV was 25 : 5 : 1 at t ∼ 110 ms. These time evolutions of OV, CIII and CV emissions suggest a monotonically increasing bulk