Release of Hydrogen Atoms and Molecules from Recycling Processes

5 Release of Hydrogen Atoms and Molecules from Recycling Processes 

Only a small fraction of the particles needed to fuel the plasma will be injected from external sources, like gas puff, neutral beams or pellets. The majority enters the plasma via recycling from divertor plates, baffles, limiters and wall. This motivates investigations about the actual recycling mechanisms. In particular, the velocities of the hydrogen particles are of interest in view of the penetration depth, and they are closely connected to the recycling mechanisms. 

A similar measurement, based on molecular spectroscopy, has been made with a pre-heated test limiter in TEXTOR [59,60]. This case, with high flux densities, also shows a transition from molecules to atoms with increasing surface temperature as indicated also in Fig. 1.11. Compared to the beam experiments, the transition temperature is shifted by some 100 K. 

The velocity of the recycling hydrogen atoms is determined by reflection, desorption and ion-induced desorption. The atoms originating from H2 gain their energy from the dissociation process. The velocity distribution contains 

The population of these vibrational states can be described by a collisional-radiative model [66,67], where the population density is mainly a function of Te. It can explain why at low Te (high-density case) the energies of the dissociated atoms can be higher than at higher Te at low Te more molecules are dissociating from the ground state delivering atoms with 2.2 eV, whereas 

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