Tungsten–Diamond Composite for Extreme Heat Flux Plasma-Facing Components

Tungsten–Diamond Composite for Extreme Heat Flux Plasma-Facing Components

Future fusion reactors such as ITER and EU-DEMO will expose plasma-facing components to extreme thermal loads. In off-normal conditions, divertor materials may experience slow transient heat fluxes on the order of 100 MW·m⁻² over ~1 s, while fast transients associated with ELMs, disruptions, or runaway electrons can reach several GW·m⁻² over millisecond timescales. Although tungsten is currently the reference plasma-facing material due to its high melting point, low erosion rate, and low tritium retention, its relatively low thermal conductivity and intrinsic brittleness make it vulnerable to recrystallization, cracking, and melting under thermal shocks.

Chemical vapor deposition (CVD) diamond offers exceptionally high thermal conductivity (~2000 W·m⁻¹·K⁻¹ at 300 K) and excellent resistance to thermal shock, with low tritium retention. However, its erosion behavior under fusion plasma conditions is comparable to graphite, limiting its direct use. To address these limitations, this work proposes a tungsten–diamond composite consisting of a thin tungsten coating (~10 μm) deposited on a millimeter-thick CVD diamond substrate. In this architecture, tungsten provides surface erosion resistance, while diamond rapidly redistributes heat during transient events.

Several W–diamond configurations were manufactured using single-crystal and polycrystalline CVD diamond substrates with thicknesses of 0.5–1 mm. Coatings included either pure tungsten or tungsten with a thin molybdenum interlayer (~2 μm) intended to improve adhesion and act as a diffusion barrier. Finite element simulations performed with COMSOL Multiphysics predict that, assuming good thermal contact, W–diamond behaves thermally much closer to pure diamond than to bulk tungsten. Under both fast (1 ms, 1 GW·m⁻²) and slow (1 s, 100 MW·m⁻²) transients, simulated surface temperatures of W–diamond are reduced by a factor of 2–3 compared with bulk tungsten.

Thermal shock experiments were conducted using the CHAUCOLASE laser facility, delivering controlled fast, slow, and quasi-steady-state heat loads. W–diamond samples were exposed to up to 10⁴ fast transient cycles at power densities up to 3 GW·m⁻², as well as up to 20 slow transient cycles at 100 MW·m⁻² and continuous loading for 10 s. Post-mortem optical microscopy, scanning electron microscopy, and focused ion beam analyses were performed to assess surface and interfacial damage.

Under fast transients, W–diamond samples remained intact up to ~2–2.5 GW·m⁻², depending on configuration. At higher loads, damage was limited to cracking, partial delamination, or melting of the tungsten coating, primarily attributed to thermal expansion mismatch between tungsten and diamond. Notably, no damage was observed in the diamond substrates themselves. Single-crystal diamond substrates and the use of a molybdenum interlayer showed improved resistance to damage. In contrast, bulk ITER-grade tungsten samples exhibited cracking and melting at all tested fast-transient power levels.

Under slow transient and quasi-steady-state loading, no damage was observed on any W–diamond sample, regardless of substrate type or interlayer, while bulk tungsten consistently showed surface cracking and melting. Surface temperature measurements confirmed that W–diamond operates at significantly lower temperatures than bulk tungsten under identical loading conditions.

Overall, these results demonstrate the strong potential of tungsten–diamond composites for mitigating extreme thermal shocks in fusion plasma-facing components. While further work is required to address erosion behavior, irradiation effects, tritium retention, and large-scale component integration, the present study establishes W–diamond as a promising alternative to bulk tungsten for future high-heat-flux fusion applications.