Benjamin Dacus

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.

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.