Development of accelerator based spatially resolved ion beam analysis techniques for the study of plasma materials interactions in magnetic fusion devices

Abstract

Plasma-material interactions (PMI) in magnetic fusion devices pose significant scientific and engineering challenges for the development of steady-state fusion power reactors. Understanding PMI is crucial for the develpment of magnetic fusion devices because fusion plasmas can significantly modify plasma facing components (PFC) which can be severely detrimental to material longevity and plasma impurity control. In addition, the retention of tritium (T) fuel in PFCs or plasma co-deposited material can disrupt the fuel cycle of the reactor while contributing to radiological and regulatory issues. The current state of the art for PMI research involves using accelerator based ion beam analysis (IBA) techniques in order to provide quantitative measurement of the modification to plasma-facing surfaces. Accelerated ~MeV ion beams are used to induce nuclear reactions or scattering, and by spectroscopic analysis of the resulting high energy particles (s', p, n, a, etc.), the material composition can be determined. PFCs can be analyzed to observe erosion and deposition patterns along their surfaces which can be measured with spatial resolution down to the -1 mm scale on depth scales of 10 - 100 pim. These techniques however are inherently ex-situ and can only be performed on PFCs that have been removed from tokamaks, thus limiting analysis to the cumulative PMI effects of months or years of plasma experiments. While ex-situ analysis is a powerful tool for studying the net effects of PMI, ex-situ analysis cannot address the fundamental challenge of correlating the plasma conditions of each experiment to the material surface evolution. This therefore motivates the development of the in-situ diagnostics to study surfaces with comparable diagnostic quality to IBA in order resolve the time evolution of these surface conditions. To address this fundamental diagnostic need, the Accelerator-Based In-Situ Materials Surveillance (AIMS) diagnostic [22] was developed to, for the first time, provide in-situ, spatially resolved IBA measurements inside of the Alcator C-Mod tokamak. The work presented in this thesis provided major technical and scientific contributions to the development and first demonstration AIMS. This included accelerator development, advanced simulation methods, and in-situ measurement of PFC surface properties and their evolution. The AIMS diagnostic was successfully implemented on Alcator C-Mod yielding the first spatially resolved and quantitative in-situ measurements of surface properties in a tokamak, with thin boron films on molybdenum PFCs being the analyzed surface in C-Mod. By combining AIMS neutron and gamma measurements, time resolved and spatially resolved measurements of boron were made, spanning the entire AIMS run campaign which included lower single null plasma discharges, inboard limited plasma discharges, a disruption, and C-Mod wall conditioning procedures. These measurements demonstrated the capability to perform inter shot measurements at a single location, and spatially resolved measurements over longer timescales. This demonstration showed the first in-situ measurements of surfaces in a magnetic fusion device with spatial and temporal resolution which constitutes a major step forward in fusion PMI science. In addition, an external ion beam system was implemented to perform ex-situ ion beam analysis (IBA) for components from Alcator C-Mod Tokamak. This project involved the refurbishment of a 1.7 MV tandem linear accelerator and the creation of a linear accelerator facility to provide IBA capabilities for MIT Plasma Science and Fusion Center. The external beam system was used to perform particle induced gamma emission (PIGE) analysis on tile modules removed after the AIMS measurement campaign in order to validate the AIMS using the well established PIGE technique. From these external PIGE measurements, a spatially resolved map of boron areal density was constructed for a section of C-Mod inner wall tiles that overlapped with the AIMS measurement locations. These measurements showed the complexity of the poloidal and toroidal variation of boron areal density between PFC tiles on the inner wall ranging from 0 to 3pm of boron. Using these well characterized ex-situ measurements to corroborate the in-situ measurements, AIMS showed reasonable agreement with PIGE, thus validating the quantitative surface analysis capability of the AIMS technique.