Fully kinetic simulations of the stabilization of the AIC mode through short plamas

The Alfven-ion cyclotron (AIC) mode can affect magnetized plasmas with anisotropic pressure. The mode acts to isotropize the plasma pressure, potentially leading to significant pitch-angle scattering of ions. It has been observed on several mirror devices in the past [1] but has not yet been observed on field-reversed configuration (FRC) devices developed by TAE Technologies [2]. Previous theoretical work [3] found that stabilization of the AIC mode could occur through axial inhomogeneity and derived a characteristic critical inhomogeneity length scale. This work examines this stabilization mechanism with fully kinetic particle-in-cell (PIC) simulations of a high β (β ~ 1) plasma. A fast ion population with various initial pitch angles is used as the source of pressure anisotropy. In each case of fast-ion pitch angle, the AIC activity is measured over a scan of plasma lengths. The resulting empirical critical lengths from this scan agree well with those predicted from past theory. [4]

[1] A. Anikeev et al., Fusion Sci. Technol. 59, 104 (2011)

[2] H. Gota et al., Nucl. Fusion 59, 112009 (2019)

[3] T. Tajima, K. Mima, and J. M. Dawson, Phys. Rev. Lett. 39, 201 (1977)

[4] Submitted, B. S. Nicks, S. Putvinski, and T. Tajima, Phys. Plasmas, 2023