Advanced grazing-emission X-ray fluorescence spectroscopy with small-angle neutron scattering for in-vivo surface composition and defect/morphology surface evolution in tokamak PMI
Jean-Paul Allain, University of Illinois at Urbana-Champaign, (Principal Investigator)
Yang Zhang, University of Illinois at Urbana-Champaign, (Co-Investigator)
The dynamic and extreme conditions of thermonuclear fusion tokamak plasmas render material surfaces almost impossible to examine in real-time, surface-sensitive conditions. However, combining particle-probe techniques that are not charge-state dependent open the possibilities of in-vivo diagnosis of the plasma-material interface. The reconstituted surfaces of plasma-facing components ranging from first-wall to divertor-component regions in the device vary depending on the complex plasma transport characteristics of the device. For example, in ITER it is predicted that fluxes of the order 1022 to 1024 and incident charged-particle energies of 100’s to 1000’s of eV will reach the surface while in the divertor regions within the private-flux and near the inner and outer-strike points would vary from a few eV to 100’s of eV governed by the complex Chodura magnetic sheath dynamics. This means that the penetration range of most charged-particle fluxes would range between 1-100’s of nm (e.g. for fuel particles of D and T and He). Therefore, techniques that are surface-sensitive from few nm’s to 100-nm would be of interest given that the depth of origin of emitted species into the fusion plasma range. This region also dictates other critical surface properties such as chemical sputtering, secondary electron and ion emissions (which can affect the local sheath conditions), near-surface transport mechanisms, defect dynamics, fuel diffusion and permeation towards sub-surface bulk regions, surface composition and morphology. Given the extreme conditions of a tokamak plasma edge for future plasma-burning reactors, conventional surface-sensitive techniques are rendered inoperative. This Category 1 “High-Risk, High-Reward” effort will examine an innovative approach combining two emergent and highly surface-sensitive techniques that would enable in-vivo characterization of the evolving re-constituted surfaces of the plasma-material interface. This will enable development of long-pulse (e.g. large fluence) in-situ plasma-materials interaction (PMI) diagnostics that after development and testing phases under a Category 2 program can be migrated to domestic or international facilities in the future. This effort addresses the “priority research directions” outlined in the 2015 DOE FES Workshop Report on Plasma Materials Interactions where the “evolving plasma facing surfaces” call for research in novel in-situ and in-operando PMI diagnostics.
This work focuses on developing the preliminary design of a robust, and versatile PMI diagnostic for the extreme fusion environment encountered in the plasma edge regions of future plasma-burning reactor systems. The approach will be to test alternative diagnostic schemes that could transform our ability to ultimately diagnose the plasma-material interface during plasma shots in candidate tokamak platforms and therefore enable dynamic measurements of fuel retention and sub-surface defect dynamics in candidate fusion material systems (e.g. TZM, tungsten, low-Z materials, etc…). The research effort is based on the premise that non-charged particle excitation radiation sources in a tokamak device can be used for PMI characterization either inherent in the tokamak plasma or provided externally into the key divertor and first-wall regions of interest. Therefore, the two primary aims will be: 1) examining the use of soft to hard X-rays for grazing emission fluorescence spectroscopy characterization of plasma facing component (PFC) materials surfaces during exposure to tokamak plasmas and 2) testing the use of neutron reflectrometry as a viable in-situ surface-sensitive technique.
