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Description
Build-up of plasma facing component (PFC) debris, a.k.a. “PFC slag”, is a potentially serious concern for the next generation of the magnetic fusion devices. A PFC slag management experiment on DIII-D tokamak tested the ability of strike point sweeps at cleaning low-Z slag from the outer divertor shelf. Layers of enriched boron isotope B10 were deposited on the outer target with the impurity powder dropper (IPD) to simulate low-Z slag accumulation. The strike point was swept over the layers to redeposit B10 onto the Divertor Material Evaluation System (DiMES) head [1] to study the controlled movement of material with sweeping. Initial visual inspection and scanning electron microscopy analysis of the DiMES heads indicate qualitative differences in deposition patterns before and after sweeping as well as the presence of B dust after the sweeps. Isotopic analysis with laser ablation mass spectroscopy (LAMS) will distinguish deposited B10 from the background B content.
Since DIII-D is a short-pulsed device and thick debris layers do not develop naturally, low-Z B slag can be simulated with high powder injection rates from the IPD. During these injections, the B10 powder was observed to agglomerate, leading to increased delivered quantities. Plasma disruptions further contributed to powder deposition on the divertor shelf. Spectroscopy data still indicates over a 3x increase in B erosion on the outer shelf after the deposition discharges, while fast camera imaging suggests most of the additional loose powder had ablated before the beginning of the sweeping stage. The innovative “clam shell” DiMES design utilized then enables direct comparison of the B deposition on DiMES before, during, and after this stage [1].
Strike point sweeping changes areas of net erosion and deposition at the targets, potentially responsible for the successful slag management during the JET DTE1 campaign [2] compared to the WEST high fluence campaign [3]. This experiment investigates this concept further and whether the sweeps erode the material atomistically or as dust. The initial findings suggest some combination of both.
[1] D. L. Rudakov et al., Fus. Eng. Design 124 (2017)
[2] J. P. Coad et al., Fus. Eng. Design 138 (2019)
[3] J. Gaspar et al., Nucl. Mater. Energy 41 (2024)
Work supported by US DOE under: DE-SC0023378, DE-FC02-04ER54698, DE-AC05-00OR22725, DE-AC02-09CH11466