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 in a fusion reactor environment where hydrogen isotope transport dictates fusion reactor operation. 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 m^-2s^-1 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).  Although diffusion-driven mechanisms will result in much deeper penetration of hydrogen in candidate materials, the dynamic accumulation and transport to and from the plasma-facing material interface will be critical in determining the burning plasma fusion operation window.  Moreover, the surface chemistry and its impact on hydrogen transport recycled to the plasma or its transport to the bulk via permeation will also impact both fueling requirements and safety margins.

Therefore, innovation in measurements that diagnose PMI in a few nm’s to 100’s of nm length scales with high resolution and sensitivity would be of interest given that the depth of origin of emitted species into the fusion plasma range between a monolayer to 50-100 nm and the characteristic scale of re-deposited surface films are from a few nm to 10’s of microns.  This “High-Risk, High-Reward” proposal will develop an innovative measurement approach combining three complimentary surface-sensitive techniques that enable in-vivo and in-situ characterization of the evolving re-constituted surfaces at the plasma-material interface. 

The proposed PMI innovative measurements suite consists of three novel complementary PMI techniques that would be implemented in a compact design and tested under simulated conditions to a tokamak plasma: 1) grazing emission X-ray fluorescence spectroscopy (which uses photons for pump-probe surface spectroscopy) combined with an optical surface-stress probe, 2) a dual magnetic spectrometer for high-resolution RBS system and 3) Small-Angle Neutron Scattering with both transmission (SANS) and reflection (Neutron Reflectometry, NR) geometry setups.  This powerful combination would yield a surface-sensitive diagnostic suite unequal in its ability to measure in-vivo the surface composition/chemistry with a probing depth of a few nm to 100’nm and surface morphology and sub-surface defect dynamics of a few 10’s nm to 100’s nm.  The suite would also intrinsically contain time-resolution in the order of a few microseconds to nanoseconds yielding critical information on the evolution of fuel retention and impurity-driven surface morphology and composition.  Since X-rays and neutrons would not be influenced by the strong magnetic field gradients in a tokamak, the possibility for development of an in-vivo PMI diagnostic would be realized.  These measurements could optimally guide wall conditioning approaches as well as novel PMI materials designs that would guide large-fluence, long-term materials operation and handling.  Ultimately the long-term goal of this study is to establish the science and technology of using energy-disperse X-ray spectrum and neutrons for in-vivo surface characterization of the plasma-material interface in tokamak plasmas.  These innovative measurements would also be critical to computational modeling validation.