The inside of a fusion reactor is a place of violence and chaos. A new cold-sprayed coating absorbs heat while also trapping some of the unruly hydrogen particles, potentially making for smaller, better plasma chambers. While fusion energy is still in the experimental stage, the launch of the world's largest and most advanced tokamak fusion reactor in Japan this month shows that the technology is moving from theory to reality.
In a fusion reaction, ionized hydrogen gas, called a plasma, is subjected to pressure and heat equivalent to the pressure and heat at the center of the sun. This would cause atomic nuclei to melt and release huge amounts of clean energy.
Creating chambers that contain the plasma required for nuclear fusion has been a challenge due to the extremely high heat and pressure levels required. Another problem with the process is that sometimes hydrogen atoms become neutralized and escape from the plasma, weakening the plasma's energy.
"These hydrogen neutral particles cause energy losses in the plasma, which makes it very challenging to maintain a hot plasma and have an efficient small fusion reactor," said Mykola Ialovega, a postdoctoral fellow in nuclear engineering and engineering physics at UW-Madison. Ialovega leads research on a coating that has demonstrated the ability to run wires inside fusion reactor cavities and capture this unruly hydrogen.
The coating is made of the metal tantalum and can withstand extremely high temperatures. Tantalum is cold sprayed onto stainless steel and performs exceptionally well under conditions similar to nuclear fusion.
During the cold spray process, tantalum particles are sprayed onto the stainless steel and flattened like pancakes. The researchers found that even when squeezed in this way, there is still a small boundary between each particle, which is an ideal channel for trapping unstable hydrogen particles. When the sprayed steel is exposed to higher temperatures, the trapped hydrogen particles are released, essentially renewing the material so it can be reused.
The team praised the coating not only for its ability to repeatedly capture and release hydrogen while withstanding high temperatures and pressures, but also for its ease of use.
"Another big benefit of the cold spray method is that it allows us to repair reactor parts by applying new coatings on site. Currently, damaged reactor parts often need to be dismantled and replaced with completely new ones, which is expensive and time-consuming," Ialovega said.
The team plans to use the coating on the Wisconsin HTS Axisymmetric Mirror (WHAM), an experimental device that could potentially be used in a next-generation fusion power plant planned by RealtaFusion, a UW-Madison spinout.
Oliver Schmitz said: "Creating a refractory metal composite with good hydrogen handling properties, corrosion resistance and general material elasticity is a breakthrough for the design of plasma devices and fusion energy systems. Changing the alloy and adding other refractory metals to enhance the composite material is particularly exciting for nuclear applications."
Schmitz, a professor of nuclear engineering and engineering physics at UW-Madison, is a co-author of a paper describing the findings, which was published in the journal PhysicaScripta.