In the magnetic confinement approach to fusion a deuterium-tritium (D-T) plasma at a temperature of around 15 keV (about 17 million K) is trapped in a toroidal magnetic field inside a vacuum vessel. The confinement is not perfect, and in many experiments energy is deposited onto the walls through small or large bursts known as edge-localized modes (ELMs). Besides, it may happen that vertical stability of the plasma is lost and then the plasma energy gets released through a vertical displacement event (VDE) or a disruption. Depending on plasma conditions and on the wall material these ELMs or VDEs can lead to evaporation or ablation and they can cause significant damage to the confinement vessel.
When wall material is rapidly evaporated or ablated a dense expanding plasma cloud is formed in front of the surface. In this dense plasma the incoming energy may largely be converted from fast particle kinetic energy into radiation energy, and gradient effects may cause this radiation to be largely directed away from the wall back into the plasma. Energy in photons is more benign to the wall than energy in fast particles, and energy that is reflected back into the plasma is most benign to the wall. This conversion of energy into radiation largely directed away from the material wall is referred to as vapour shielding. Vapour shielding also reduces the sheath potential and thereby it reduces the damage by fast heavy particles. It may protect against runaway self-sputtering. The reduction in energy to the surface can be very large; much larger than a simple factor of two that would be expected for isotropic emission of radiation.Vapour shielding is therefore of interest for estimating surface erosion and the lifetime of plasma-facing materials subjected to pulsed heat loads. Vapour shielding also influences the penetration of hydrogen fuelling pellets or of "killer" impurity pellets into the hot plasma.
Simulation of vapour shielding involves radiation hydrodynamics and it requires atomic data such as are used for simulations of hot dense matter: collisional-radiative (CR) data, non-local thermodynamic equilibrium (non-LTE) atomic kinetics, line shapes, opacities and emissivities. Following advice from the IFRC Subcommittee on Atomic and Molecular (A+M) Data the A+M Data Unit is planning to start a Coordinated Research Project (CRP) on atomic data that are needed to simulate vapour shielding or needed to interpret spectroscopic measurements of ablating material under pulsed heat loads when vapour shielding is happening. A Consultancy Meeting (CM) is foreseen for some time early in 2018 to develop the scope and objectives of the proposed project. Then a detailed proposal for the CRP would be prepared and reviewed, and the CRP would be expected to start later in 2018.
An activity by the A+M Data Unit on vapour shielding would be concerned with relevant first wall and divertor plasma-facing materials, maybe also with hydrogen pellets and maybe with impurity pellets. For easily ablating materials such as lithium the process sets in at relatively low heat loads; there is even a notion of continuous vapour shielding in a box enclosure. For tungsten the process is really associated with very high pulsed loads that occur on fusion experiments but are difficult to simulate in smaller laboratory experiments. Other relevant materials include beryllium, carbon and iron as possible wall materials, gallium and tin in connection with liquid metal walls, aluminum primarily because of its use in laboratory experiments (as a surrogate for beryllium or just for basic well-diagnosed studies); also hydrogen for pellet ablation and maybe neon, argon or other for impurity pellet injection.
There are many experimental studies in the fusion community for lithium and tungsten. Theoretical studies and modelling activities tend to focus on plasma effects, for example associated with the electrostatic sheath, and to use rather simple models for the atomic physics. A CRP on atomic data for vapour shielding would involve people from radiative properties of dense plasmas along with people from fusion energy research.
Consultancy Meeting on Atomic Data for Vapour Shielding in Fusion Devices: At IAEA in Vienna, planned for Q1, 2018.
Further events: Information to follow.
CRP on Plasma-wall Interaction with Reduced-activation Steel Surfaces in Fusion Devices (2015-2020). This CRP is intended to enhance the knowledge base on erosion, tritium deposition and tritium migration processes involving fusion relevant (reduced activation) steel surfaces. The plasma-wall interaction processes include sputtering by H and He and plasma impurities, trapping of hydrogen (H, D, T) in surfaces exposed to plasma, transport of hydrogen in the steel and means to extract trapped tritium.
CRP on Plasma-Wall Interaction with Irradiated Tungsten and Tungsten Alloys in Fusion Devices (2013-2018). This CRP was established to improve the knowledge base and databases on properties of irradiated tungsten. The most important topic is to understand how tritium retention, tritium migration and ways to extract trapped tritium are affected by radiation damage.
CRP on Data for Erosion and Tritium Retention in Beryllium Plasma-Facing Materials (2012-2016). This CRP is intended to enhance the knowledge base on fundamental particle-material interaction processes involving beryllium in the fusion plasma environment. The key processes to be studied in the CRP are physical and chemical sputtering by H, He and Be, which release beryllium impurities into the plasma, trapping and reflection of hydrogen (H, D, T) on beryllium surfaces, the transport of hydrogen in beryllium and means to extract trapped tritium.