Future fusion reactors could be equipped with liquid metal showers to absorb ten times the amount of heat from high-temperature plasma as current experimental methods.
A research group from Japan has developed a new divertor system that uses a jet stream of liquid tin to evacuate plasma as neutral gas.
Scientists say this could make for an inexpensive way to bear extreme heat loads in fusion reactors, and will prevent damage and the need for frequent maintenance that result from plasma striking the divertor.
High-temperature plasma in a magnetic field confinement fusion reactor is maintained by floating it apart from a vessel, the researchers from the National Institute for Fusion Science explain.
But, this forms a location where the plasma will hit.
So, these devices use a divertor in these locations to receive the heat from the plasma.
Current experimental devices including the Large Helical Device (LHD) at the National Institute for Fusion Science and the International Thermonuclear Experimental Reactor (ITER) use solid divertors made of tungsten blocks, which are cooled by water.
Solid blocks, however, suffer wear from being struck by the high-temperature plasma.
With the three-dimensional helical design at NIFS, there are concerns that the quantity of heat will surpass the 20 megawatts per one square meter seen in the ITER design value, and researchers say divertor maintenance will be difficult.
To work around this challenge, the team developed a new type of liquid metal shower divertor system, which would allow it to withstand the fusion plasma’s high heat flux.
Divertors of this kind have been proposed and considered for more than 40 years, with ideas to use melted lithium, tin, and other metals.
The researchers placed the device at intervals in ten places inside the toroidal confinement device, using high-speed liquid metal flow as a countermeasure against the decreased contact area, and higher heat load, of the plasma.
The plasma moves along the magnetic field lines, and the researchers created an impassable wall by placing the liquid metal at a slant.
When the plasma is neutralized on the surface of the liquid metal shower, it passes through the spaces between the streams, moving toward the rear for effective evacuation.
The researchers say this can bear a heat load of roughly ten times the value of that tolerated by the ITER divertor.
And, using a flow of 4 meters per second, the high heat load can be blocked.
It was previously thought that a liquid metal divertor couldn’t be used in the helical design, but the researchers say their findings show it would be possible.
For this to work, they will need to generate a stable flow over a length of a few meters, as flow is accelerated by gravity and becomes unstable, drips fall, and creates a spray as the diameter becomes thin.
So, they’ve used an object – wire and tape, or a chain – to create resistance against the flow to suppress the speed.
So far, they’ve developed a technology to stabilize a flow exceeding several meters.
The findings were presented at the 26th International Atomic Energy Agency Conference in Japan this past October.