Hydrogenic retention with high-Z plasma facing surfaces in Alcator C-Mod

Abstract

Abstract. The retention of deuterium (D) fuel in the Alcator C-Mod tokamak is studied using a new 'static' gas balance method. C-Mod solely employs high-Z molybdenum (Mo) and tungsten (W) for its plasma-facing materials, with intermittent application of thin boron (B) films. The primarily Mo surfaces are found to retain large fractions, ~20-50%, of the D 2 gas fuelled per quiescent discharge, regardless if the Mo surfaces are cleaned of, or partially covered by, B films. Several experiments and calculations show that it is improbable that B is retaining significant fractions of the fuel. Rather, the retention is occurring in Mo and W surfaces through ion bombardment, implantation and diffusion to trap sites. Roughly 1% D of incident ion fluence, D , to surfaces is retained, and with no indication of the retention rate decreasing after 25 s of integrated plasma exposure. The magnitude of retention is significantly larger than extrapolated from the results of laboratory studies for either Mo or W. The high levels of D/Mo in the near surface, measured directly post-campaign (~ .01) in tiles and inferred from gas balance, are consistent with trapping sites for fuel retention in the Mo being created, or expanded, by high D atom densities in the near surface which arise as a result of high incident ion fluxes. Differences between C-Mod and laboratory retention results may be due to such factors as the multiply-ionized B ions incident on the surface directly creating traps, the condition of the Mo (impurities, annealing), and the high flux densities in the C-Mod divertor which are similar to ITER, but 10-100x those used in laboratory studies. Disruptions produce rapid heating of the surfaces, releasing the trapped hydrogenic species into the vessel for recovery. The measurements of the large amount of gas released in disruptions are consistent with the analysis of tiles removed from the vessel post-campaign -the campaign-integrated retention is very low, of order 1000x less than observed in a single, non-disruptive discharge.

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

“…In short, net deposition rates on the inner wall during LSN were measured at ∼5±3 nm s −1 , consistent with previous estimates of 1-2 nm s −1 [8].…”

“…An exhaustive study of hydrogenic fuel retention in Alcator C-Mod has been published, demonstrating that 20-50% of fueled deuterium gas was consistently retained per quiescent discharge with no saturation evident after ∼25 s of plasma [8]. Unlike hydrogenic retention in tokamaks with carbon (C) PFCs, where the infinite source of eroded C is principally responsible for retention through codeposition, the study showed that the deuterium (D) retention in Alcator C-Mod must occur in the bulk of the molybdenum (Mo) PFCs and not in the thick amorphous thin film layers of boron (B) that accumulate in the machine.…”

“…Ex-situ analysis has shown that the B layers created by BZN are saturated immediately after deposition with D/B atomic ratios of ∼0.4; rapid D outgassing in the approximately 40 following plasma discharges leads to D/B atomic ratios of ∼0.1 in Alcator C-Mod, which are measured via static gas balance and post-mortem analysis [8].…”

“…Lower single null (LSN) diverted plasmas were used since previous experience on Alcator C-Mod has shown that eroded material from the outer divertor migrates in the boundary plasma of Alcator C-Mod and strongly net deposits on the inner wall and inner divertor in this configuration [8,9]. Inner wall limited (IWL) plasma, were chosen since the high-power plasma exhaust in the scrape off layer is directly incident upon the inner wall PFCs accessible to AIMS, leading to conditions on the inner wall PFCs that result in strong net erosion.…”

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

“…The LSN configuration is important because previous experiments on material migration in the boundary plasma have shown that the plasma net deposits material on the inner wall and inner divertor [109,110]; the relative similarity between shots indicates that the deposition will be approximately constant for each shot. Thus, the total deposition for all shots divided by the number of shots provides a representative average of net deposition per shot.…”