Angus Wylie

Commercial fusion power plants demand magnet materials that retain structural integrity and thermal conductivity while operating under neutron bombardment at cryogenic temperatures. Understanding how thermo-mechanical properties evolve under these conditions is crucial for selecting materials with high radiation tolerance and predictable failure mechanisms. Presented here is a facility that combines cryogenic transient grating spectroscopy with simultaneous ion irradiation, enabling in situ measurements of thermal diffusivity and surface acoustic wave (SAW) frequencies, allowing inference of microstructural evolution. Using copper as a benchmark material, an irradiation was performed at 30 K with 12.4 MeV Cu 6 + ions producing a fluence of 1.9 × 10 17  ions m⁻². Over the irradiation period, thermal diffusivity nearly halved from an initial value of 1.2 × 10 − 4   m 2 s − 1 while SAW speed did not show significant changes, maintaining a value of 2162 ± 18  m s⁻¹. Given its real-time monitoring capability and the numerous candidate materials that remain under characterized under fusion magnet operating conditions, this facility is poised to deliver new scientific insights into fusion magnet material degradation trends, contributing to improved design criteria and operational certainty for forthcoming fusion power plants.

Transient grating spectroscopy (TGS) is a rapid and non-destructive technique for measuring thermal, acoustic, and elastic properties of solid materials with a multitude of uses across many areas of materials research. Current TGS systems require optics tables and cumbersome amounts of space for an entire setup, restricting TGS to being a lab-based method. This paper presents a new design for TGS systems that rotates the probe laser beams around the axis of the pump beam, allowing for an asymmetric probe, planar, optically 2D setup. This, in turn, allows the setup to be significantly simplified, which enables the setup presented in this paper to be roughly nine times smaller in volume than contemporary setups while being much easier to build, align, and operate. Part of the size reduction was enabled by a mono-homodyne system and the removal of the chopper. This system was benchmarked against an existing TGS system using a single-crystal tungsten sample. This showed that it can produce the same surface acoustic wave frequency data as the existing system. This design enables TGS to be more widely adopted for use in more varied and compact environments because of its smaller size and simplicity.

A facility for the investigation of in situ radiation-materials and plasma-materials interaction is demonstrated with tungsten, using transient grating spectroscopy as a probe of thermal diffusivity and surface acoustic wave speed. Helium plasma exposure at 645 °C to 1.18 × 1018 cm−2 helium, until the growth of tungsten fuzz, showed an increase in surface acoustic wave speed at the near-surface from 2542 ± 1 m s−1 up to 2565 ± 1 m s−1, followed by a greater drop to 2499 ± 7 m s−1. No observable change in thermal diffusivity was present for plasma exposure alone. A separate 10.26 MeV self-ion-irradiation of tungsten to a dose of 7.92 dpa showed a reduction in both thermal diffusivity from 61.4 ± 1.4 mm2 s−1 to 36.0 ± 0.7 mm2 s−1, following trends seen in existing studies, and surface acoustic wave speed from 2647.8 ± 0.6 m s−1 to 2640.0 ± 0.4 m s−1. Facilities like these are poised to rapidly close critical knowledge gaps regarding the coupled effects of plasma and radiation damage for materials in fusion systems.

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.

The speed-up of radiation science development with the advent of ion-irradiation experiments has, until recently, been omitted in the post-irradiation examination technique. This paper reports the results of transient grating spectroscopy—a rapid, non-destructive, in situ photothermal surface technique—of ion-irradiated single-crystals of iron, chromium, vanadium, and tungsten at room temperature. Thermal diffusivity was used to track damage development throughout irradiation, with 5 MeV self-ion irradiated iron, chromium, and vanadium showing little to no change up to damages of the order of 1 dpa. 5 MeV Si3+-ion irradiated tungsten exhibits a reduction of thermal diffusivity from 0.78(7) to 0.29(2) cm2 s−1 with logarithmically increasing dose over a similar damage range. A comparison to literature of transient grating spectroscopy thermal diffusivity values past and present shows good agreement; radiation-induced change can be clearly distinguished from differences between mono- and poly-crystalline tungsten.