Intensity measurements of the forbidden line 975 A<i></i>of Fe xviii: A transition sensitive to proton temperature

Abstract

The intensity of the Fe xviii forbidden line at 975 A\r{} (2${s}^{2}$2${p}^{5}$ ${}^{2}$${P}_{1/2}$\ensuremath{\rightarrow}2${s}^{2}$2${p}^{5}$ ${}^{2}$${P}_{3/2}$) relative to that of an allowed line at 94 A\r{} (2s2${p}^{6}$ ${}^{2}$${S}_{1/2}$\ensuremath{\rightarrow}2${s}^{2}$2${p}^{5}$ ${}^{2}$${P}_{3/2}$) was measured during ion-Bernstein-wave heating in tokamak discharges. The results at low electron densities (${n}_{e}$3\ifmmode\times\else\texttimes\fi{}${10}^{13}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$) indicated a noticeable effect of proton collisions on the intensity of the forbidden line. Calculation also shows the importance of proton collision to the Fe xviii forbidden-line intensity.