18th Technical Meeting on Energetic Particles in Magnetic Confinement Systems

Global resonance and power transfer of fast particles with GAEs and CAEs with frequencies comparable to the gyrofrequency

Not scheduled

20m

Effects of Energetic Particles in Magnetic Confinement Fusion Devices

Speaker

Michael Fitzgerald (UKAEA)

Description

M. Fitzgerald1, B.N. Breizman2
1 United Kingdom Atomic Energy Authority, Culham Campus, Culham Science Centre, Abingdon, Oxon, OX14 3DB, UK
2 Institute for Fusion Studies, The University of Texas, Austin, TX 78712, United States of America

Compressional Alfven eigenmodes (CAEs) and global Alfven eigenmodes (GAEs) with frequencies comparable to the ion cyclotron frequency have been observed in current tokamaks such as DIII-D, MAST-U, and NSTX. This frequency range $\Omega_c \sim\omega$ is formally outside the scope of the well-understood drift and gyrokinetic theories routinely deployed for Alfven eigenmodes. On the other hand, local Vlasov theory with gyromagnetic resonance $k_\parallel\ v_\parallel-l\Omega_c-\omega=0$ is not well justified and not a convenient starting point for correction to include drift effects globally. Additionally, shear modes such as the GAE are normally considered to be coupled only to $l=0$. Using Hamiltonian methods similar to Kaufman[1], we have clarified the resonance condition for GAEs and CAEs including all finite Larmor and finite drift contributions. We find fundamental differences in the power transfer expected for GAEs versus CAEs which substantially alters the relative contribution of different resonances $n\omega_ \phi+p\omega_\theta+l\Omega_c-\omega=0$ despite similar frequencies. Whereas for GAEs the generation of poloidal sidebands are known to be governed by orbital drift motion through $\Delta^\ast\ m/r$, CAEs are more sensitive to the change in gyroangle over the poloidal orbit through $\Omega_c/\omega_\theta$ . Some example calculations are given for a published DIII-D case where $\omega \sim\Omega_c/2$ [2] which predict the degree of anisotropy required to drive the observed counter NBI propagating GAEs without also driving co-NBI modes.
[1] Kaufman A N 1972 Quasilinear Diffusion of an Axisymmetric Toroidal Plasma Phys. Fluids 15 1063
[2] DeGrandchamp G H, Lestz J B, Van Zeeland M A, Du X D, Heidbrink W W, Thome K E, Crocker N A and Pinsker R I 2022 Mode structure measurements of ion cyclotron emission and sub-cyclotron modes on DIII-D Nucl. Fusion 62