Molecule assisted recombination of divertor plasmas

Abstract

The recently observed phenomenon of divertor plasma detachment in all presently operating large fusion devices has strongly promoted the hopes for an adequate reduction of thermal and particle fluxes deposited on divertor plates and for a successful solution of the thermal power exhaust problems in fusion reactors. The plasma detachment regime in tokamak divertors and divertor simulators is characterized by a dramatic drop of plasma particle flux onto the divertor plates and a plasma temperature reduction to the eV and sub-eV level near the plates. Both the experimental evidence and theoretical analyses of this phenomenon lead to the conclusion that its appearance and existence are strongly related to volmnetric plasma recombination in the divertor.The theoretical modeling of divertor plasma recombination usually includes the following "standard" recombination mechanisms in the plasma kinetics (in a pure hydrogenic plasma):(1) radiative recombiintion: e + H+ → H(n) + hv;(2) three-body recombination: e + e + H+ → e + H(n);(3) negative-ion mediated recombination: e + H2(v ⩾ 4) → H2- → H + H-, H+ + H- → H + H (n = 3);(4) ion-conversion mediated recombination: H+ + H2(v ⩾ 4) → H + H2+(v'), e + H2+(v') → H + H (n ⩾ 2).The effective (indirect) electron-proton "recombination" in reaction chains (3) and (4) is made possible via the electron and proton reactions with vibrationally excited hydrogen molecules. These two recombination chains are, therefore, called molecule assisted recombination (MAR) mechanisms. A characteristic feature of these two MAR mechanisms is that they require the existence of vibrationally excited molecules in states with vibrational quantum nunbers higher than v = 3, otherwise their efficiency is very low.Analysis of the temperature dependences of the rate coefficients for the reactions involved in the recombination mechanisms (1)–(4) shows that the radiative and three-body recombinations are effective recombination mechanisms for plasma temperatures below ∼ 0.5 eV (and plasma densities above 5 x 1014 cm-3 for three-body recombination). The MAR mechanisms become dominant for temperatures in the range 0.5–2 eV. Above 2–2.5 eV, MAR mechanisms (3) and (4) become ineffective due to the electron impact destruction of H- (electron detachment) and H2+ (dissociative excitation) ions.The modeling of divertor plasma recombination obviously requires an accurate knowledge of the reaction rate coefficients (or cross sections) of all the processes involved in the recombination mechanisms (1)–(4). However, the full description of plasma recombination kinetics also requires inclusion of many other processes involved in the production and destruction of the reactants and products of the above listed reactions. A specific aspect which contributes substantially to the complexity of divertor plasma recombination kinetics is the necessity to consider all the processes of vibrationally excited molecules and molecular ions.The present Topical Issue is devoted to a detailed analysis of the processes involved in volume recombination of divertor plasmas, with emphasis on the processes involved in MAR kinetics. The contributions to this volume address both the general aspects of MAR (its kinetic structure and effectiveness) and the collisional information for the most important processes involved in MAR kinetics. For several of these processes their isotopic variants are also considered. One of the objectives of the contributions devoted to specific atomic and molecular processes, or to entire classes of such processes, was to present the best available quantitative information on the cross sections and/or reaction rate coefficients of the processes considered. By responding to this objective, the present volume not only elucidates the atomic and molecular physics of divertor plasma recombination, but also provides a source of evaluated cross section information for the needs of divertor plasma modeling.