Michael P. Short’s research while affiliated with Massachusetts Institute of Technology and other places

Reports of critical current ($\rm I_c$) suppression during cryogenic ion irradiation of REBCO tapes have raised concerns for the operational margins of fusion power plant (FPP) magnets. However, the data remain inconclusive regarding beam heating due to the difficulty of measuring local temperatures with contact probes. This leaves a critical knowledge gap concerning the mechanism behind $\rm I_c$ suppression, and whether the so-called \textit{beam on effect} is to be expected under neutron irradiation during FPP operation. In this paper, we show that $\rm I_c$ suppression is independent of atomic displacement rate in the REBCO layer, the latter of which increases twelve-fold as we reduce the beam energy from 2400 to 800~keV. At fixed power, we observe statistically identical suppression with 150~keV protons, which do not have enough energy to reach the REBCO layer, refuting hypotheses about beam on effects being caused by nuclear displacements or direct ion-Cooper pair interactions. These results show that REBCO temperature rise alone can explain $\rm I_c$ suppression, leaving little to no margin for alternative mechanisms. With this insight, we developed a method to measure beam spot temperature that does not depend on the specific installation of our temperature sensor. With this new method, we measured the temperature gradient across the tape during irradiation and found that thermal resistance at the tape/target interface is the controlling variable in $\rm I_c$ suppression. As such, accelerator-based facilities aiming to reproduce the operation of REBCO magnets in a nuclear fusion environment should find strategies to minimize interface thermal resistance. Most importantly, we find that the dose rates expected in a FPP will not change $\rm I_c$ due to ballistic radiation damage or ion-Cooper pair interactions, allowing us to safely ignore these effects when designing FPP magnets.

We present developments for the mapping of large areas using transient grating spectroscopy (TGS) that allow for smoother, larger, autonomous measurements of material samples. The addition of a precise linear stage in the direction parallel to laser sampling coupled with signal optimizing control allows for hands free, self-correcting measurements. In addition, the simplification of the sample holding design to a form that is small enough to mount directly to the linear stage exhibits a straightforward, low-cost solution for automated TGS applications. This capability is demonstrated by taking large uninterrupted maps of gradient wafers, and the results are validated on calibrated tungsten samples and control TGS samples from gradient wafers.

Superconducting magnets based on Rare Earth Barium Copper Oxides (REBCO) offer transformative capabilities in the fields of fusion energy, high energy physics, and space exploration. A challenge shared by these applications is the limited lifetime of REBCO due to radiation damage sustained during operation. Here we present a new ion-beam facility that enables simultaneous cryogenic irradiation and in situ characterization of commercial REBCO tapes. The ion source provides spatially uniform fluxes up to 10¹⁸ protons/m²s with kinetic energies up to 3.4 MeV, in addition to helium and higher-Z species. Using this facility, we can induce uniform damage profiles in the first 10–20 µm of REBCO tapes with less than 0.25 appm of hydrogen implanted in REBCO after a dose of 10²⁰ protons/m². The tape can be held between 20 and 300 K with an accuracy of ±0.1 K and is connected to a four-point probe measuring the critical current, Ic, and critical temperature, Tc, before, during, and after irradiation with transport current ranging from 100 nA to 100 A, and a typical voltage noise less than 0.1 μV. These capabilities are presently used to study the effect of irradiation temperature on REBCO performance change during and after proton bombardment, to assess the possibility of Ic and Tc recovery after irradiation through thermal annealing, and to explore the instantaneous and recoverable suppression of Ic and Tc observed during irradiation.

Vacancies are generated in semiconductor devices while operating in the space radiation environment, impacting semiconductor carrier concentrations and dynamics. Positron annihilation lifetime spectroscopy (PALS) is used to probe these defect concentrations in bulk grown GaN, GaP, InAs, InP, Si, MgO, and ZnO both as-grown and as a function of 2–4 MeV proton irradiation. All samples were irradiated to yield a common initial damage production and characterized identically. In as-grown samples, PALS reveals vacancy concentrations above the saturation limit in the oxides, disabling further analysis. As a function of dose, of the materials in which defect accumulation could be probed, it is observed that GaN is the most resistant to the accumulation of defects (attributed to the Ga vacancies) and Si is the least. GaP (attributed to the Ga vacancy) and InAs exhibit slightly higher rates of vacancy accumulation than GaN. InP exhibits high defect accumulation rates approaching that of Si. This information is key to understanding the operation of a diverse set of semiconductors in the space radiation environment.