CRP(2011-2015): Atomic and Molecular Data for State-Resolved Modelling of Hydrogen and Helium and Their Isotopes in Fusion Plasma

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In the divertor and near-wall region of magnetic confinement fusion plasma experiments processes involving neutral atoms, molecules and molecular ions are important. The primary plasma constituents are hydrogen and helium and their isotopes and the molecules and molecular ions may be in rovibrationally or electronically excited states. For a complete description one needs cross-sections for collisions with electrons, collisions among the heavy particles, photon-induced and radiative processes and processes on the walls, all resolved with respect to excited states. The objective of the present CRP is to evaluate existing data for the relevant atomic and molecular processes of hydrogen and helium, generate new fundamental atomic and molecular data where this is needed, and assemble the information into a knowledge base and numerical databases for use by the fusion community. The knowledge base will include numerical data in fully resolved form and in reduced forms that are directly useful in plasma modelling.

Background Situation Analysis

The divertor of a magnetic confinement fusion experiment is the region where plasma that flows along magnetic field lines interacts with a material boundary. In this region the plasma is relatively cold and dense. Hydrogen molecules are formed on the walls and under some conditions also by volume recombination, and in parts of the divertor region the plasma transitions to a neutral gas (conditions of detached or semi-detached divertor plasma). The divertor design must be optimized for particle control (pumping of impurities and in a reactor also helium ash) and for handling the high heat load. Molecular processes and interaction of the plasma electrons and ions with molecules and molecular ions are critical features of the divertor plasma and their correct treatment, together with plasma-wall interaction, is the preeminent concern of edge and divertor plasma modelling. In numerical models of the edge and divertor plasma the neutral atoms and molecules and the molecular ions are most often treated by Monte Carlo codes such as EIRENE [1], DEGAS-2 [2] or NEUT-2D [3], often in conjunction with a fluid code for the plasma species. These codes are essential tools for interpretation of edge plasma diagnostics and for the design of new experiments. The principal difficulty in modelling the mixture of neutral and ionized species in the plasma boundary region is the need to take into account the population balance of electronically and rovibrationally excited states. To that end one needs cross-sections for collisional, photon-induced and radiative processes that are resolved with respect to excited state of the incoming and outgoing particles. The existing databases have gaps and quite large uncertainties in this area, even for such a relatively simple process as dissociative recombination of hydrogen: e-+H2+→H+H; the incoming H2+ and the outgoing H+H may be in some excited state, and the cross-section for the collision depends strongly on the vibrational state of the H2+. The database is especially sparse for processes involving the molecular ions HeH+ and He2+, which are of much recent interest due to the plans for an extensive helium campaign on ITER before its nuclear phase.

The improved atomic and molecular database for hydrogen and helium will be of special interest for diagnostics and control of the isotopic composition of plasma. Electronic processes don′t vary much between isotopes, but molecular processes depend fully on species mass. In ITER or in a reactor the D/T ratio in the core plasma will be diagnosed primarily by nuclear means (neutron production associated with modulated beam injection of D or T), but in the edge region one will measure the intensity of the Balmer-α and other spectral lines of D and T. The relation between the populations of neutral D and T and the populations of the plasma constituents D+ and T+ is complicated, and in the divertor region, where the production of neutral atoms proceeds primarily through breakup of neutral molecules, the whole range of molecular processes studied in this CRP is of interest. The improved database will also be of interest for the simulation of neutral beam injection systems ranging from the generation and destruction of H- negative ions in the injector box to the charge transfer and ionization processes of the neutral beam in the core plasma. Charge transfer spectroscopy, in particular, is an important diagnostic tool that may also have application for core plasma measurement of the D/T and 3He/4He ratios.

The CRP on ″Atomic and molecular data for state-resolved modelling of hydrogen and helium and their isotopes in fusion plasma″ will bring together atomic physicists and plasma modellers. The CRP will produce new data, but a key task throughout the project will be comparison and critical evaluation of existing data, and of the newly produced data too. It is envisaged that the plasma modellers in the CRP will represent the most widely used codes from around the world for kinetic (generally Monte Carlo) simulation of edge and divertor plasma including atomic and molecular processes. These modellers are themselves actively engaged in the evaluation of A+M data and in the processing of data into the forms most useful for plasma modelling, although not in the production of new A+M data. The atomic physicists in the CRP will be mostly computational scientists; experiment will be represented as well, but for the production of comprehensive state-resolved cross-sections theory and computation have the central role. The emphasis on evaluation demands a strong cooperative mode of work, and due to the limited scope of the CRP we expect a high degree of synergy among the experimental, computational and modelling aspects of the project. The key measure of success will be the recognition of the resulting knowledge base and numerical databases as the most useful source of atomic and molecular data for processes of hydrogen and helium in fusion plasma.

Research Objective

In general: To increase capabilities of Member States to undertake fusion plasma modelling and simulation of present and future experiments and reactor designs through improved data for atomic, molecular and plasma-material interaction processes, and thereby to contribute to the development of fusion energy generation.

Specifically: To assemble, generate and evaluate fundamental and derived data for collisional and radiative processes of H, H+, H-, He, He+, He2+, He-, H2, H2+, H3+, HeH+, He2+ and their isotopic variants, resolved with respect to excited states, in a fusion plasma environment. Fundamental data include cross-sections for collisions with electrons and collisions among themselves, photon-induced processes, lifetimes of excited states, and line shapes of the principal emissions. Derived data are effective cross-sections and rate coefficients for collisional-radiative modelling of the divertor and edge region, beam plasma interaction and the neutral beam heating system of fusion energy experiments.

Expected Research Outputs and Outcomes The output of the research will be an electronically accessible knowledge base and associated numerical databases, and publication of the results in scientific journals. A summary of the research results will be published in the IAEA journal Atomic and Plasma-Material Interaction Data for Fusion (APID). Progress Reports will be prepared by the Research Contract holders each year, Summary Reports will be prepared after each RCM, and in the Summary Report of the 3rd RCM a detailed report of the work and accomplishments will be given and final reports by all CRP participants will be included.

It is intended that data from this research will be used in the interpretation of diagnostics of the divertor and edge plasma in fusion experiments, the control of isotopic composition of the plasma, and in plasma modelling for the design of future fusion reactor experiments.


[1] D. Reiter, M. Baelmans and P. Börner: The EIRENE and B2-EIRENE Codes. Fusion Science and Technology 47 (2005) 172. [2] D. P. Stotler and C. F. F. Karney: Neutral Gas Transport Modeling with DEGAS 2. Contrib. Plasma Phys 34 (1994) 392. [3] K. Shimizu et al.: Kinetic modelling of impurity transport in detached plasma for integrated divertor simulation with SONIC (SOLDOR/NEUT2D/IMPMC/EDDY). Nuclear Fusion 49 (2009) #065028.

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