Permissions for Reuse

Inelastic transitions resulting from atomic collisions in the plasma edge (scrape-off layer and divertor) regions of the tokamak plasma can have a major impact on the energy transfer and exhaust from the plasma core.¹ Various inelastic interactions of hydrogen ions and atoms with vibrationally and rotationally excited hydrogen molecules and molecular ions, especially charge transfer, are critical due to their significantly enhanced transition probabilities over the corresponding processes with ground-state molecules (Ref. 2, and references therein). Although vibrationally excited states are produced by recombination, i.e., e⁻+H, it is believed that the main source of rovibrationally excited H₂ is hydrogen molecules sputtered from the hydrogenated carbon surface of the divertor tiles.³

Rovibrationally excited molecules are also important in possible mechanisms for enhancing the volumetric recombination of divertor plasma in formation of the plasma detachment regime,^4,5,6 which is necessary for the reduction in heat loads on divertor plates, one of the most important tasks in today's fusion energy research. Heat load reduction schemes include the use of collision processes involving rovibrationally excited hydrogen molecules and hydrocarbon molecular plasma impurities.⁷ The latter represents an alternative to the standard recombination schemes based on vibrationally excited molecular hydrogen. Assuming that there is a significant fraction of vibrationally excited hydrogen molecules in the divertor region, plasma recombination mechanisms such as molecule-assisted recombination (MAR) and molecule-assisted dissociation (MAD) were proposed.⁸ For example, MAR starts with a charge exchange reaction H₂(j,v)+H⁺H(j,)+H and is followed by dissociative recombination with a plasma electron. The MAR based on vibrationally excited H₂ molecules is a sink for molecules, which are only efficiently regenerated at the wall surface.

The MAD chain of reactions can also start from a vibrationally excited hydrocarbon, C_xH_y, which might be even more effective in the creation of the detached plasma layer than H₂ due to the “recycling” features of the C_xH_y MAR: the products of the MAD with more complex hydrocarbons are often themselves hydrocarbon molecules, ions, or radicals which can again undergo MAR/MAD processes.⁷

A significant weakness in assessments of the importance of MAR, both hydrogen molecule and hydrocarbon based, is in the theoretical models describing the kinetics of vibrational and rotational excitations of hydrogen molecules coupled to transport of H and C_xH_y, and, in particular, a lack of knowledge of the rovibrational distribution of these molecules, both when created at the carbon surface and deeper in the plasma. Here we study only one aspect of these complex atomic processes: the distribution of rovibrational and translational energies of the particles as they are ejected from carbon surfaces upon bombardment by deuterium atoms.

Interest in the MAR processes extends well beyond the fusion community. For example, they are very important for building our basic understanding of the plasma constituents in many technical applications. Moreover, since H, H₂, C_xH_y, and their isotopes are among the dominant species in the universe they involve some of the most important atomic reactions in astrophysics.⁹ For example, the H₂+D⁺HD+H⁺ reaction is a major source of HD found in diffuse stellar clouds and may have played a dominant role in HD formation in the early universe. In addition, hydrocarbon chemistry plays a central role in the gas phase chemistry crucial to our understanding of the structure and evolution of star-forming regions.¹⁰

In most presently operating medium- and large-size fusion tokamak devices the plasma-facing components in the divertor contain carbon-based materials. The interaction of divertor hydrogenic plasma with these materials leads, primarily through chemical erosion, to significant amounts of hydrocarbon molecules which enter the plasma. The composition of these molecules depends on the energy of the impact particles, the temperature of the surface, its microstructure, and the level of hydrogenation. At higher impact energies more complex hydrocarbons are emitted.¹¹ In this paper, using a molecular dynamics (MD) approach,^{12,13,14,15,16} we focus on the states of the molecules sputtered from carbon by deuterium impact, which is important information for realistic modeling of the edge plasma chemistry, but has not been intensively studied

Details of our molecular dynamics approach are presented in Sec. II. The dependence of the average translational and rovibrational energies of sputtered particles on impact energy is studied in Sec. III, while the detailed distributions of the various energy modes, as well as their angular properties, are presented in Secs. IV s5, respectively. Finally, Sec. VI contains our conclusions.