External proton beam analysis of plasma facing materials for magnetic confinement fusion applications

Abstract

A 1.7MV tandem accelerator was reconstructed and refurbished for this thesis and for surface science applications at the Cambridge laboratory for accelerator study of surfaces (CLASS). At CLASS, an external proton beam set-up was designed and constructed to perform in-air ion beam analysis on plasma facing divertor tiles from the Alcator C-Mod tokamak. A Particle Induced Gamma Emission (PIGE) technique was developed for boron depth profiling. In addition, Particle Induced X-ray Emission (PIXE) was implemented and used for a comprehensive study of poloidal tungsten migration in the C-Mod divertor. A novel PIGE technique was developed for measuring depth profiles of boron deposition on C-Mod tile surfaces. Boron (B) is regularly deposited on C-Mod tiles to improve plasma performance. This technique is therefore useful for studying the interaction of B with plasma facing components (PFC) to develop a better understanding of the effects of B in Alcator C-Mod. The technique involves taking multiple PIGE yield measurements of a single sample while changing the beams path-length through the air to vary the energy of the beam incident on the sample. A numerical code was written to deconvolve boron depth profiles from these gamma yields by exploiting the sharply peaked cross section of the '0B(p, ay)7Be resonance reaction. Simulations demonstrate that this code converges to the expected results. Preliminary measurements of C-Mod tiles were performed using the external proton beam to induce 429keV gamma emission from the 10B(p, ay)7Be reaction which was measured, using a Sodium Iodide (Nal) scintillation detector.

(cont.) These preliminary results verified the feasibility of this technique. An external PIXE ion beam analysis study was conducted to measure campaign integrated, poloidal tungsten (W) migration patterns in the C-Mod divertor. Eroded W from a toroidally continuous row of W tiles near the outer divertor strike point was used as a tracer to map W erosion and redeposition onto a set of Mo and W tiles that covered the poloidal extent of the C-Mod lower divertor which were removed following the 2008 experimental campaign. These tiles were examined for W using external Particle Induced X-ray emission (X-PIXE) analysis; a highly W sensitive ion beam analysis (IBA) technique in which a characteristic x-ray emission is induced from a material surface as it is exposed to an external proton beam, produced by the electrostatic tandem accelerator. With a set of systematic high spacial resolution measurements (~ 3mm resolution), complete poloidal profiles of W redeposition have been constructed. These profiles indicate W transport and redeposition of up to 1.5 x 102 atoms/m 2 (14nm of equivalent W thickness) in several regions including the outer divertor, the inner divertor, and inside the private flux region. In addition to the W results, PIXE allowed for indirect measurements of spatially resolved boron profiles and direct measurements of titanium, chromium, and iron. A comprehensive description and explanation these PIGE and PIXE studies and their results are presented.