The stability of unity beta equilibria in tokamaks

TL;DR

This paper investigates the stability of high beta equilibria in tokamaks, showing that plasmas with beta near unity can be stable, challenging the traditional Troyon limit and highlighting unresolved experimental questions.

Contribution

It demonstrates that high beta (near unity) tokamak plasmas are theoretically stable, questioning the established Troyon limit and addressing a key unresolved issue in plasma physics.

Findings

High beta plasmas with peak beta near unity are stable to ideal MHD modes.

Theoretical stability of unity beta equilibria is confirmed, but experimental realization remains unresolved.

Troyon limit is not an absolute barrier for plasma stability in tokamaks.

Abstract

The plasma beta, ratio of kinetic to magnetic pressure, inside a tokamak should stay below the Troyon limit to avoid major plasma instabilities. However, this paper argues that Troyon limit occurs only when current profiles cannot sustain high beta equilibria. This paper shows that plasmas with peak beta on the order of unity are stable to ideal MHD modes. While the theoretical (ideal-MHD) stability of unity beta equilibrium has been resolved, the experimental existence of high beta equilibria in tokamaks remains an unsolved problem of plasma physics.