An in situ accelerator-based diagnostic for plasma-material interactions science on magnetic fusion devices

Abstract

Plasma-material interactions (PMI) in magnetic fusion devices such as fuel retention, material erosion and redeposition, and material mixing present significant scientific and engineering challenges, particularly for the next generation of devices that will move towards reactor-relevant conditions. Achieving an integrated understanding of PMI, however, is severely hindered by a dearth of in-situ diagnosis of the plasma-facing component (PFC) surfaces. To address this critical need, this thesis presents an accelerator-based diagnostic that nondestructively measures the evolution of PFC surfaces in-situ. The diagnostic aims to remotely generate isotopic concentration maps that cover a large fraction of the PFC surfaces on a plasma shot-to-shot timescale.The diagnostic uses a compact, high-current radio-frequency quadrupole accelerator to inject 0.9 MeV deuterons into the Alcator C-Mod tokamak. The tokamak magnetic fields in between plasma shots are used to steer the deuterons to PFCs where the deuterons cause high-Q nuclear reactions with low-Z isotopes ~5 pim into the material. Scintillation detectors measure the induced neutrons and gammas; energy spectra analysis provides quantitative reconstruction of surface concentrations. An overview of the diagnostic technique, known as accelerator-based in-situ materials surveillance (AIMS), and the first AIMS diagnostic on the Alcator C-Mod is given; a description of the complementary simulation tools is also provided. Experimental validation is shown to demonstrate the optimized beam injection into the tokamak, the quantification of PFC surfaces isotopes, and the measurement localization provided by magnetic beam steering. Finally, the first AIMS measurements of fusion fuel retention are presented, demonstrating the local erosion and codeposition of deuterium-saturated boron surface filns. The finding confirms that deuterium codeposition with boron is insufficient to account for the net fuel retention in Alcator C-Mod.

Fuel retention measurements in Alcator C-Mod using accelerator-based in situ materials surveillance

“…The results of AIMS measurements during wall conditioning appear in Figure 5 shown to quantitatively agree with the radially dependent B deposition rates [6,7] that were precisely measured with a quartz microbalance [13]. It should be noted that the data presented in this paper were acquired in only the first three hours of operating the proof-of-principle AIMS diagnostic on Alcator C-Mod.…”

“…Although promising advances have been made with laser-based measurement techniques, such as laser-induced breakdown spectroscopy (LIBS) [3], such approaches have been limited to small PFC surface areas and are intrinsically destructive to surface layers. In contrast, a new diagnostic technique recently developed on the Alcator C-Mod tokamak, known as Accelerator-based In-situ Materials Surveillance (AIMS), is capable of making fuel retention measurementsas well as low-Z erosion/redeposition measurements -over large fractions of PFC surfaces with spatial resolution on the order of 1 cm and a time resolution on a plasma shot-to-shot timescale [4,5,6,7]. A graphical overview of the AIMS technique is presented in Figure 1 for reference.…”

20 years of research on the Alcator C-Mod tokamak

“…7. 54 Early results have proven the principle of the technique and shown that measurements could be routinely made between shots. [55][56][57] C. Divertor regimes and detachment physics Meeting the challenges of divertor power handling and erosion require better understanding of the underlying physics, through which improved designs and operating regimes can be achieved.…”

Alcator C-Mod: research in support of ITER and steps beyond

“…The Accelerator-based In-situ Materials Surveillance (AIMS) system is a novel diagnostic that nondestructively measures the evolution of plasma facing component surfaces (PFC) inside Alcator C-Mod [66]. The diagnostic aims to remotely generate isotopic concentration maps on a plasma shot-to-shot timescale that cover a significant fraction (~1 m2) of the PFC surfaces inside Alcator C-Mod without the need for vacuum breaks or physical access to the PFCs.…”