Speaker
Description
Hydrogen plasma chemistry plays crucial role for both power and particle exhaust in low temperature detached divertor plasmas as it can give rise to molecular assisted recombination via $H_2^+$, $H^-$, $H^+_3$. In the present contribution the application of the linear plasma device PSI-2 for validation of the corresponding plasma-chemical models will be discussed.
The effect on which the comparison of experiment with model calculations is based is the generation of excited hydrogen atoms in the processes involving molecules, molecular and negative ions. To make use of this effect as model benchmark the absolutely calibrated spatially resolved imaging measurements of Balmer radiation have been performed. In addition, the shapes of hydrogen lines taken with high resolution spectrometer are analysed. These latter help to isolate the contribution of the excitation channels of ionic (rotating with plasma) origin and deduce the translational temperature of emitting particles. Radial profiles of plasma parameters measured by movable Langmuir probe and pressure gauge measurements serve as input for modelling.
The measurements have been compared with local 0D calculations based on the standard model of $H^+_2$ plasma-chemistry applied in tokamak divertor simulations [1, 2, 3, 4]. Preliminary, this model was found to be sufficient to describe the pure hydrogen dischargesat low gas pressure 0.04 Pa, with electron temperature $T_e$ = 2...10 eV, density $n_e\sim$5$\cdot$10$^{17}$ m$^{-3}$. The calculations suggest that in this regime $H^+_2$ channel is likely to be the dominant source of Balmer radiation. At the same time, the model [1, 2, 3] was clearly seen to be insufficient to describe the discharges with pressure elevated up to 0.5 Pa and with lower $T_e$. Current status of comparison of the experiment with plasma-chemical models will be presented.
References
[1] Sawada K, Eriguchi K and Fujimoto T 1993 J. App. Phys. 73 8122
[2] Sawada K and Fujimoto T 1995 J. App. Phys. 78 2913
[3] Reiter D, May C, Baelmans M and Börner P 1997 J. Nuclear Mater. 241-243 342
[4] Kukushkin A S, Pacher H D, Kotov V, Reiter D, Coster D P and Pacher G W 2005 Nuclear Fusion 45 608