Plasma Facing Components & Plasma Material Interactions
A plasma facing component (PFC) is an integrated materials system consisting of a sacrificial plasma facing material in the form of tiles or coatings, a strong, actively cooled structural substrate, and a flowing coolant. PFCs are designed to be remotely maintainable and often replaceable. They can cover large areas as in the first wall and divertor, but also provide localized protection as startup limiters and around auxiliary heating components like RF antennas and neutral beam dumps and diagnostics.
The PFC must withstand high temperatures, remove large plasma heat fluxes, and is often subjected to plasma particle erosion and volumetric heating from neutrons. Solid PFCs endure neutron damage, runaway electron damage during disruptions, and tritium permeation, while liquid PFCs must cope with large MHD forces, tritium buildup, and contamination. The solid PFM surface is often profiled to match the plasma contour. It can be angled, castellated and faceted to provide shadowing from high heat fluxes while reducing the leading edges that directly intercept charged particle flux lines.
The PFC must have good thermal contact through all the interfaces or joints between dissimilar materials with similar expansion characteristics, or the composition must be graded throughout to minimize thermal stress and provide the best thermal performance. The PFC structural substrate and mounts also must react to large electromagnetic loads during disruptions and are often segmented to minimize these forces. Pulsed experiments often use inertially-cooled PFCs with no coolant; however, long-pulse confinement devices and fusion reactors will require flowing water, helium or perhaps liquid metal, or even molten salts as coolants. Leaks, corrosion, phase-change leading to critical heat flux, activation, and tritium permeation in the coolant can affect the PFC.
The U.S. VLT eleven-institution PFC group is working on:
1. Modeling/analysis for explaining present plasma/surface interactions in the US and world tokamaks and test devices;
2. In-situ (NSTX, DIII-D, C-MOD, etc.) and lab experiments for scientific understanding of the critical PFC issues; and,
3. PFC design and performance predictions for ITER, DEMO, and the Fusion Nuclear Science Facility. The research focus is on solid plasma-facing PFC surfaces (Be, C, Mo, W) but with a significant program in liquid metal (Li, Sn, Ga) surfaces, and with diverse structural materials and cooling methods.