β=1 is not a principal limit for magnetic plasma confinement

What is it about?

Efficiency of magnetic plasma confinement is determined by the parameter β that shows how closely plasma pressure approaches pressure of magnetic field at the plasma-vacuum boundary. In magnetohydrodynamics, β cannot be higher than unity. Using fully kinetic simulations of cylindrical plasma confinement in a uniform magnetic field, we showed that pressure in the center of plasma column can exceed this limit by 15%. It becomes possible due to non-gyrotropy of plasma pressure caused by non-circular orbits of particles.

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Photo by Nishaan ahmed on Unsplash

Photo by Nishaan ahmed on Unsplash

Why is it important?

Reaching the β=1 limit has been a long-standing dream of the fusion community, as it would allow the idea of magnetic plasma confinement to be realized in the most efficient way. Such a regime with the complete exclusion of the magnetic field from the plasma volume (so called diamagnetic bubble regime) is planned to implement in the Gas Dynamic Multiple-mirror Trap (GDMT) [D.Skovorodin et al. Plasma Phys. Rep. 49, 1039 (2023)]. The possibility to exceed the MHD limit on the relative pressure in the long cylindrical plasma of GDMT makes this project even more promising.

Perspectives

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The fact that a uniform vacuum magnetic field can hold plasma with pressure greater than the field pressure is very interesting and fundamental, but the question of how efficiently the non-gyrotropic effect can increase the plasma beta in real cusp and mirror magnetic configurations remains to be answered. To confirm that plasma equilibrium with β>1 in such traps does not only exist, but is also stable, one needs to carry out resource-intensive 3D particle-in-cell simulations.

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