Intensity measurements of the forbidden line 975 Åof Fe xviii: A transition sensitive to proton temperature

The intensity of the Fe x v i i i forbidden line at 975 Å (2$s^2$2$p^5$ ${}^2$ $P_{1/2}$→2$s^2$2$p^5$ ${}^2$ $P_{3/2}$) relative to that of an allowed line at 94 Å (2s2$p^6$ ${}^2$ $S_{1/2}$→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×${10}^{13}$ ${\mathrm{cm}}^{\mathrm{-}3}$) 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.