2024 – Exposure and characterization of novel ceramic materials in DiMES

Exposure and characterization of novel ceramic materials in DiMES

2024 Research Campaign, Thrust: FPP Candidate Walls

Purpose of Experiment

The walls of fusion devices are subjected to extreme heat fluxes, more than a reentry vehicle from space. Unlike reentry vehicles, these materials must be able to withstand the heat fluxes for extended periods of time because it takes days to months to swap out wall tiles. This is well-handled in today’s experiments, but future reactors will subject the wall to even more extreme heat fluxes. Even worse, future reactor walls will be subjected to an additional neutron flux from the nuclear fusion occurring in the core of the plasma. Neutron damage in materials can be extremely severe, sometimes more devastating than the impinging heat flux from the plasma. We will not be able to test materials in intense fusion neutron fluxes until these future reactors are built, but we have a decent understanding of which materials should be the most resistant to neutron damage by using results from today’s nuclear fission plants and experiments. Therefore, it is vital we test materials that are not only capable of handling the extreme heat flux in today/tomorrow’s fusion devices, but also resistant to the anticipated neutron damage that will occur. Advanced ceramics are a clear contender in this realm. A set of advanced ceramics has been identified that could be used in fusion devices: thermal plasma spray SiC, SiC-fiber reinforced SiC, TiB2 and ZrB2. The goal of this experiment is to “stress-test” these ceramics to demonstrate their ability to handle the intense heat fluxes in DIII-D, supporting the case for their inclusion in the fusion devices of today and tomorrow.

Experimental Approach

This experiment will expose our set of ceramics to the intense heat flux in the divertor of DIII-D using a material exposure stations called DiMES. DiMES enables rapid exposure and extraction of small (~ cm’s) material samples. We will characterize our materials both before and after the experiment to see how well the ceramics hold up under plasma exposure. High powered H-mode plasmas will be used to study the performance of the ceramics under the most intense conditions they would need to survive in. Demonstrating that the material survives will provide a huge boost for these materials to be used for the wall in both DIII-D as well as future reactors. A separate set of exposures using a lower powered L-mode plasma will provide more uniform (in time and space) conditions across the samples with the purpose of allowing the ceramics to absorb deuterium. We want to prove that the deuterium uptake (or retention) is low. If too much deuterium – a proxy for radioactive tritium used in future reactors – is retained, then the walls of future reactors made out of this material may become too radioactive and limit the lifetime of it. A key aspect of this experiment is the pre- and post-characterization of the ceramics with various techniques. For instance, a scanning electron microscope will visually inspect the surface of the ceramics down to the micron scale or smaller. This will enable us to look for small cracks in the material that could eventually grow into larger ones, any flaking of the samples that could lead to excessive erosion over longer timescales, and even the elemental composition of the material to test if the ratio of elements (e.g., 50/50 Si/C in SiC) remains the same. Another example uses a technique called thermal desorption spectrometry. This involves heating the sample up and causing all the retained deuterium to desorb. Measuring the amount of desorbed deuterium allows us to extrapolate to future reactors to determine if the retained tritium will limit the lifetime of the reactor. All together, a successful result – the ceramics survive exposure and do not retain too much deuterium – will give a large boost towards incorporating these materials into nuclear fusion experiments. DIII-D will soon change it’s wall from graphite to a to-be-decided material and these materials are currently under consideration. Therefore these experiments will provide some much needed information on what the next generation of DIII-D’s walls, and one day reactor walls, may look like.

Interested in a behind-the-scenes look at DIII-D? Join us for a virtual OR in-person tour during Fusion Energy Week (May 5-9)! Sign up for a tour here.

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