Applications to ASDEX Upgrade

Probably the most systematic predictive and scaling studies with emphasis on an as complete as possible implementation of atomic, molecular and surface effects for a divertor configuration have been carried out for the ASDEX Upgrade tokamak. Making connection here in particular to the chapter by U. Fantz in this volume (and references therein) we discuss here the initially unexpected role played by the molecular chemistry in dense cold divertor plasmas. 

The chain of reactions, which played the key role in these arguments, was, firstly, a vibrational excitation of molecules by electron impact (through resonant H2 levels), then, secondly, an ion conversion p + Ho(v) — II If), followed by, thirdly, an immediate dissociative recombination e + H. — H + H. The excited atom decays by spontaneous emission. At the end of this chain, one electron-ion pair has recombined into an H-atom, and the H2 molecule has dissociated into H + H. 

The second step in this reaction chain is resonant (comparable to resonant charge exchange between H-atoms and protons), if the molecules are vibrationally excited in the v = 4 level. 

Under typical high recycling divertor conditions, when molecules travel in a bath of 7-8 eV (or hotter) electrons, the molecules are destroyed before they reach such vibrational levels. Hence this chain of reactions is irrelevant there. For detached divertor plasma conditions this is not necessarily so. 

From inspecting the atomic database of the EIRENE code [31], which is used in many applications to a large number of different tokamaks, including for the ITER design, in particular its collisional-radiative models for molecules, it was clear that matters can be more complicated. The relaxation time for establishing a vibrational distribution of the ground state molecule is comparable to the transport time of the molecule, hence the applicability of local collisional-radiative approximations is questionable. Furthermore, one of the two atoms created in dissociative recombination is electronically excited, and, hence, can be ionized very effectively even at low divertor plasma temperatures (instead of radiative decay). In this case, the whole chain of reactions would be just an enhanced ( molecular activated ) dissociation (MAD, i.e., dissociative excitation of those H]] , which have been produced 

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