Analysis of D_\alphaH_\alpha spectrum emitted in front of the limiter in HT--7

In order to understand the recycling and emission processes of hydrogen atoms in HT--7, spectral profiles of the D_\alphaH_\alpha line emitted in front of the limiter have been observed with a high-resolution spectrometer and simulated by using the neutral particle transport code DEGAS 2. The results show that four processes are necessary to interpret the D_\alphaH_\alpha line shape: 1) atom desorption, 2) molecular dissociation, 3) particle reflection, and 4) charge-exchange. The products of the first two processes are cold atoms which emit photons near the peak of D_\alphaH_\alpha line shape, and those from the last two are warm atoms contributing to the blue side of the spectrum. For a typical ohmic discharge (shot 68520 n_e (0)≈3x10¹⁹m^-3, these components contribute 32%, 15%, 32% and 21%, respectively. D_\alphaH_\alpha line shapes under different plasma parameters are also discussed in this paper.