Time-resolved measurements of nonthermal x-ray emission of Ti xxi lines in electron-cyclotron-heated tokamak plasmas

Observations of the heliumlike titanium spectra with 20-ms temporal resolution during electron cyclotron heating (ECH) of Doublet III tokamak plasma have been made. Several prominent lines in the heliumlike spectra are observed: 1s² ¹{S}₀←1s2p¹P₁$ (labeled w), 1${s}²$ ${}¹S₀{←}1s2p ³P₁,2 (y,x), and 1s² ¹{S}₀←1s2s³S₁$ (z). In addition, the dielectronic satellite lines from the autoionizing lithiumlike ionization states are also observed, e.g., 1${s}²$2p $²^∘{}3/2←1s2{p}²$ $²D5/2$ (j), 1${s}²$2s $²S1/2←1s2p2s²^∘{}3/2$ (q), etc. The intensity of the resonance line w (i.e., 1${s}²$ $¹S₀←1s2p¹P₁) is found to be extremely sensitive to the ECH power. Ion temperatures rise from the Ohmic phase to steady-state ECH phase in 20--40 ms at an electron density of 6×{10}¹³$ ${cm}^{{{-}}3}$. Time-resolved line ratios G=(x+y+z)/w, R=z/(x+y), and j/w are also presented. Detailed analysis of the observed spectra is carried out, based on the relevant atomic and plasma effects, and it is concluded that there exists a significant departure from a Maxwellian distribution of electrons in the present ECH plasmas, which are also dominated by nonequilibrium ionization.