Dependence of alpha-particle-driven Alfv\'en eigenmode linear stability on device magnetic field strength and consequences for next-generation tokamaks

TL;DR

This study analyzes how increasing magnetic field strength in tokamaks affects Alfvén eigenmode stability, revealing potential advantages for next-generation high-field devices in reducing instability growth rates.

Contribution

It provides analytical and numerical insights into the dependence of AE stability on magnetic field strength, highlighting how high-field tokamaks may mitigate alpha-driven instabilities.

Findings

High magnetic fields can suppress AE resonances by making fusion alphas sub-Alfvénic.

Higher magnetic fields lead to higher toroidal mode numbers for unstable modes.

High-field devices may have advantageous AE stability properties at various densities.

Abstract

Recently-proposed tokamak concepts use magnetic fields up to 12 T, far higher than in conventional devices, to reduce size and cost. Theoretical and computational study of trends in plasma behavior with increasing field strength is critical to such proposed devices. This paper considers trends in Alfv\'en eigenmode (AE) stability. Energetic particles, including alphas from D-T fusion, can destabilize AEs, possibly causing loss of alpha heat and damage to the device. AEs are sensitive to device magnetic field via the field dependence of resonances, alpha particle beta, and alpha orbit width. We describe the origin and effect of these dependences analytically and by using recently-developed numerical techniques (Rodrigues et al. 2015 Nucl. Fusion 55 083003). The work suggests high-field machines where fusion-born alphas are sub-Alfv\'enic or nearly sub-Alfv\'enic may partially cut off AE resonances, reducing growth rates of AEs and the energy of alphas interacting with them. High-field burning plasma regimes have non-negligible alpha particle beta and AE drive, but faster slowing down time, provided by high electron density, and higher field strength reduces this drive relative to low-field machines with similar power densities. The toroidal mode number of the most unstable modes will tend to be higher in high magnetic field devices. The work suggests that high magnetic field devices have unique, and potentially advantageous, AE instability properties at both low and high densities.