Topical issue on collision processes of Be and B atoms and ions in fusion plasmas

Abstract

The present topical issue of Physica Scripta contains the results of a concerted effort to review and expand the cross section data information for collision processes of beryllium and boron atoms and ions in a thermonuclear fusion plasma. These two chemical species are present as dominant impurities in the plasmas of several presently operating large tokamak devices and are anticipated to be among the most abundant impurities in the next generation fusion reactor experiments. Beryllium is attractive as a plasma facing reactor material because of its low atomic number (i.e. low potential for radiative plasma power losses), excellent gettering properties with respect to oxygen (unavoidably present in any fusion plasma) and adequate thermo-mechanical and erosion properties when exposed to plasma energy and particle fluxes. Boron, on the other hand, is frequently used as a dopand in carbon-based plasma facing materials to improve their erosion, hydrogen retention and (some) thermo-mechanical properties. The beryllium and boron atoms enter the plasma edge as a result of plasma-wall interaction and in the course of their penetration into the hot plasma regions become progressively ionized. The variety of collision processes of beryllium and boron atoms and ions with other plasma constituents is very large. In the hot plasma regions, the most important among them are the electron-impact excitation, ionization and recombination. In the cold plasma edge regions which support thermodynamical coexistence of both neutral and ionized species, apart from electron-impact excitation and ionization of neutral Be and B atoms, coffision processes of low charged Be and B ions with the neutral atomic and molecular hydrogen and helium (such as momentum transfer, excitation, ionization and charge exchange) are also important. The inelastic collision processes between Beq+, Bq+ ions and H, He neutrals are particularly important when energetic neutral hydrogen and helium beams are injected into the plasma for heating and diagnostic purposes.The above mentioned collision processes between Be and B atoms and ions and other dominant plasma constituents play a prominent role in determining the transport of these impurity ions in the plasma and their radiating properties. The cross section information on these processes is also essential in the spectroscopic plasma diagnostics based on the analysis of observed radiation from Be and B species.All the important electron and heavy-particle collision processes involving Be and B atoms and ions in a fusion plasma (except the electron-ion recombination) are being addressed in the present volume. While a number of contributions to this volume provide a critical review of already existing cross section data for certain classes of processes, most of the articles report new, previously unavailable cross section information for the studied processes. The present volume of Physica Scripta is, therefore, a significant step towards a better understanding of the collision physics of Be and B ions in fusion plasmas and towards establishing a comprehensive cross section database for their collision processes.It is a great pleasure to express my deep appreciation to all contributors to the present volume for their dedicated effort and friendly cooperation during all stages of its preparation.