Characterization of a Digital Holography Diagnostic for In Situ Erosion Measurement of Plasma-Facing Components in Fusion Devices

Fusion energy devices, particularly tokamaks, face the challenge of interior surface damage occurring over time from the heat flux of the high-energy plasma they generate. The ability to monitor the rate of surface modification is therefore imperative, but to date no proven technique exists for real-time erosion measurement of planar regions of interest on plasma-facing components in fusion devices. In order to fill this diagnostic gap, a digital holography system has been established at ORNL [Oak Ridge National Laboratory] for the purpose of measuring the erosion effects of plasma-material interaction in situ.

The diagnostic has been designed with the goals of measuring both steady-state and off-normal erosion events. It is capable of operating with either one laser for measuring displacements up to 4.5 microns, or two lasers in tandem to extend the measurement range to an upper limit near 2 millimeters.

The performance of the digital holography system is characterized in this dissertation. Discussion of the impetus and background of this project is provided in Chapter I, along with a description of the components and technique. Chapter II covers initial characterization of the measurement fidelity regarding surface roughness of stainless steel surfaces, surface displacements in various sizes, and image noise reduction via multi-frame averaging. A threshold roughness average of 300 nanometers was defined for satisfactory correlation between holographic and validation measurements, and dual-laser measurements were accurate for displacements exceeding 100 microns. In Chapter III, improvements for dual-laser resolution, increased image signal, and noise reduction are described. Chapter IV presents an extensive characterization of ex situ erosion measurement on target plates exposed to an electrothermal arc, with a measured average erosion of 150 nanometers per exposure which indicated that erosion would be distinguishable from noise in situ; the holographic results are compared to profilometry and scanning electron microscopy for validation. In Chapter V, measurements of dynamic surface translations simulating material modification are analyzed, and displacement detection is shown to be accurate. A project summary is given in Chapter VI, and initial results are shown of a successful in situ proof-of-concept demonstration of holographic measurement of erosion from the electrothermal arc.