Tungsten, the material used in the plasma-facing components (PFCs) of nuclear fusion reactors, develops a fuzz-like surface morphology under typical reactor operating conditions. This fragile ‘fuzz’ surface nanostructure adversely affects reactor performance and operation. Developing predictive models, capable of simulating the spatiotemporal scales relevant to the fuzz formation process is essential for understanding the growth of such extremely complex surface features and improving PFC and reactor performance. Here, we report the development of an atomistically-informed, continuous-domain model for the onset of fuzz formation in helium plasma-irradiated tungsten and validate the model by comparing its predictions with measurements from carefully designed experiments. Our study demonstrates that fuzz forms in response to stress induced in the near-surface region of PFCs as a result of plasma exposure and helium gas implantation. Our model sets the stage for detailed descriptions of this complex fuzz formation phenomenon and similar phenomena observed in other materials.

Herein, we investigate the saturation limits of hydrogen on the (110) and (100) surfaces of tungsten via Density Functional Theory (DFT) and complement our findings with experimental measurements. We present a detailed study of the various stable configurations that hydrogen can adopt upon the surfaces at coverage ratios starting below 1.0, up to the point of their experimental coverage ratios, and beyond. Our findings allow us to estimate that the saturation limit on each surface exists with one monolayer of hydrogen atoms adsorbed. In the case of (110) this corresponds to a coverage ratio of one hydrogen atom per tungsten atom, while in the case of (100) a full monolayer is present at a coverage ratio of 2.0. Preliminary Low Energy Ion Scattering (LEIS) and Direct Recoil Spectroscopy (DRS)measurements complement these results and tend to confirm the findings obtained by DFT. In particular, the preferred adsorption sites on both surfaces at any coverage, the reconstruction of the (100) surface and the saturation limits agree well. We show that depending on the coverage, hydrogen surface binding energies can be of the same magnitude as binding energies to defects like vacancies. As a consequence, surface effects should be included in models aiming to simulate retention and desorption of hydrogen from the bulk.

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The Space Technology 7 Disturbance Reduction System (ST7-DRS) is a NASA technology demonstration payload that operated from January 2016 through July of 2017 on the European Space Agency's LISA Pathfinder spacecraft. The joint goal of the NASA and ESA missions was to validate key technologies for a future space-based gravitational wave observatory targeting the source-rich milliHertz band. The two primary components of ST7-DRS are a micropropulsion system based on colloidal micro-Newton thrusters (CMNTs) and a control system that simultaneously controls the attitude and position of the spacecraft and the two free-flying test masses (TMs). This paper presents our main experimental results and summarizes the overall the performance of the CMNTs and control laws. We find that the CMNT performance to be consistent with pre-flight predictions, with a measured system thrust noise on the order of 100 nN/ √ Hz in the 1 mHz ≤ f ≤ 30 mHz band. The control system maintained the TM-spacecraft separation with an RMS error of less than 2 nm and a noise spectral density of less than 3 nm/ √ Hz in the same band. Thruster calibration measurements yield thrust values consistent with the performance model and ground-based thrust-stand measurements, to within a few percent. We also report a differential acceleration noise between the two test masses with a spectral density of roughly 3 fm/s 2 / √ Hz in the 1 mHz ≤ f ≤ 30 mHz band, slightly less than twice as large as the best performance reported with the baseline LISA Pathfinder configuration and below the current requirements for the Laser Interferometer Space Antenna (LISA) mission.

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