Required toroidal confinement for fusion and omnigeneity

TL;DR

This paper explores the conditions for magnetic confinement in fusion devices, emphasizing the importance of omnigeneity in magnetic fields to minimize entropy production and enable self-sustaining deuterium-tritium fusion reactions.

Contribution

It establishes the fundamental role of omnigeneity in magnetic confinement and derives conditions for magnetic field configurations that optimize plasma stability and fusion performance.

Findings

Omnigeneity requires unbounded magnetic field contours in at least one angular coordinate.

Chaotic trajectories due to microturbulence can increase transport beyond ideal predictions.

Higher plasma temperatures demand reduced transport for sustainable DT fusion.

Abstract

Deuterium-tritium (DT) burning requires a long energy confinement times compared to collision times, so the particle distribution functions must approximate local-Maxwellians. Non-equilibrium thermodynamics is applicable, which gives relations among transport, entropy production, the collision frequency, and the deviation from a Maxwellian. The distribution functions are given by the Fokker-Planck equation, which is an advection-diffusion equation. A large hyperbolic operator, the Vlasov operator with the particle trajectories as its characteristics, equals a small diffusive operator, the collision operator. The collisionless particle trajectories would be chaotic in stellarators without careful optimization. This would lead to rapid entropy production and transport -- far beyond what is consistent with a self-sustaining DT burn. Omnigeneity is the weakest general condition that is consistent with a sufficiently small entropy production associated with the thermal particle trajectories. Omnigeneity requires that the contours of constant magnetic field strength be unbounded in at least one of the two angular coordinates in magnetic surfaces and that there be a symmetry in the field-strength wells along the field lines. Even in omnigenous plasmas, fluctuations due to microturbulence can produce chaotic particle trajectories and the gyro-Bohm transport seen in many stellarator and tokamak experiments. The higher the plasma temperature above 10~keV, the smaller the transport must be compared to gyro-Bohm for a self-sustaining DT burn. The hot alphas of DT fusion heat the electrons. When the ion-electron equilibration time is long compared to the ion energy confinement time, a self-sustaining DT burn is not possible, which sets a limit on the electron temperature.