Dependence of Alfvén eigenmode stability on device magnetic field strength and consequences for next-generation tokamaks*

Recently-proposed tokamak concepts use magnetic fields up to 12 T [1,2], 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. We consider trends in Alfvén 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 using numerical techniques reported in [3]. We suggest high-field machines 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 high electron density and field strength reduces this beta relative to low-field machines with similar power densities. 

[1] Sorbom et al., Fus. Eng. and Des. 100 (2015): 378-405.

[2] Greenwald et al., PSFC Rep. RR-18-2 (2018).

[3] Rodrigues et al., Nucl. Fusion 55.8 (2015): 083003.