Effect of magnetic field strength on Alfvén eigenmode stability

Libby Tolman


Tuesday, April 10, 2018



PSFC Student Seminars

Next-generation fusion experiments will attempt to reduce the size and cost of fusion power plants by increasing volumetric fusion power density, which depends strongly on magnetic field. The recent development of high-temperature superconducting (HTS) technology has allowed the exploitation of this magnetic field dependence in designs for next-generation fusion experiments with large Q that remain compact and cost-efficient by using magnetic fields of greater than 9 T, in excess of the operating range of previous tokamak experiments. Developing theoretical and computational understanding of trends with increasing magnetic field is of critical importance for the new era of very-high-field HTS machines.  This talk introduces one area of particular significance to high-magnetic-field experiments which hope to demonstrate net energy: the stability behavior of Alfvén eigenmodes (AEs).  In a tokamak, energetic particles, including alpha particles produced in D-T fusion reactions, can drive these modes unstable, leading to increased energetic particle transport. This behavior is detrimental to device operation; it can lead both to loss of alpha power needed to heat the plasma and to damage to device walls. AE physics is strongly sensitive to background magnetic field strength through the magnetic field dependence of AE resonances. This talk describes the origin of this dependence, and considers one of its effects, the loss of one of the AE resonances when device Alfvén speed reaches the alpha particle birth velocity. This effect could allow more flexibility in management of AE behavior for high-magnetic field machines.