Quantum 'spin' could enhance efficiency of fusion fuels – study

Aligning the quantum ‘spin’ property for deuterium-tritium fusion fuels could significantly improve their efficiency and make it easier to generate electricity economically.

In a new study from researchers at the Princeton Plasma Physics Laboratory (PPPL), spin polarised deuterium-tritium (D-T) fuel with more deuterium than tritium was found to increase the tritium burn efficiency by at least a factor of ten without compromising the fusion power output, compared to unpolarised fuel.

Such an approach would have two major impacts – one the need for less tritium, which is rare in nature and needs to be ‘bred’ for fusion, and the other as a consequence that the overall size of the fusion power plant could be reduced, making it easier to license, situate and construct.

Together these would then lower the overall costs of developing and operating a fusion system.

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“Fusion is really, really hard, and nature doesn’t do you many favours,” said Jason Parisi, a staff research physicist at PPPL and lead author on the research paper.

“So, it was surprising how big the [efficiency] improvement was.”

Unlike a ball which can be thrown with any one of multiple spins, there are only a few discrete options for the quantum spin on a particle, such as up and down.

When two fusion fuel atoms have the same quantum spin, they are more likely to fuse.

Existing spin-polarisation methods don’t align every atom but the modelling shows that 100% spin alignment is not required and just modest levels of spin polarisation can substantially improve the efficiency of the tritium burn to improve the overall efficiency and reduce the tritium consumption.

Increasing the D-T mix

Normally a D-T mix for fusion would be around 50:50 and the PPPL researchers find that an ARC-like tokamak – similar to PPPL’s primary fusion device – producing 481MW of fusion power with unpolarised 53:47 D-T fuel requires a minimum startup tritium inventory of 0.69kg.

By spin polarising half of the fuel and using a 60:40 D-T mix, the minimum startup tritium inventory is reduced to 0.08kg, while fully spin polarising the fuel with a 63:37 D-T mix reduces the level further to 0.03kg.

With these findings, the researchers suggest that with advancements in helium divertor pumping efficiency, tritium burn efficiency values of approximately 10%-40% could be achieved using low tritium fraction and spin polarised fuel with minimal power loss.

This would dramatically lower the tritium startup inventory requirements and reduce the amount of on-site tritium, they state.

“More generally than just for spin polarised fuels, increased plasma performance can be used to increase the tritium burn efficiency. This strongly motivates the development of spin polarised fuels and low tritium fraction operation for burning plasmas.”

The research was published in the journal Nuclear Fusion and is declared a first look at how spin polarised fuel could improve tritium burn efficiency.

Further work is needed to investigate the requirements needed to implement the proposed system.

Separate research is under way with DOE funding to investigate the technologies needed to inject spin polarised fuel into the fusion vessel.

Further work also is needed on polarisation methods and for example on the storage of spin polarised fuel.