Investigating Magnetic Moment Conservation in Time and Spatially Varying Electromagnetic Fields

Gyrokinetic models have been extremely useful in simulating plasmas with slowly

varying external magnetic fields. Gyrokinetic models have less dimensions than fully kinetic

ones, and they have been used to efficiently model plasmas within fusion devices, guide field

reconnection, and solar wind turbulence [1] [2] [3]. One of the most fundamental assumptions of

gyrokinetics is magnetic moment invariance, stemming from its adiabatic invariance in single

charged particle trajectories [4]. Work has been done to quantify the degree to which the

magnetic moment is not conserved in several basic electromagnetic field configurations [5]. We

expand on this by investigating moment invariance in more complicated field combinations such

as electromagnetic waves in the presence of a strong magnetic field; we employ both analytical

and numerical methods such as perturbation theory and Runge-Kutta Simulations. We also

propose alternative definitions for the magnetic moment that are calculated by considering

guiding center drifts; It has been shown that these alternative definitions are conserved to a

higher order in these new field combinations.

References

[1] K. Germaschewski, B. Allen, T. Dannert, M. Hrywniak, J. Donaghy, G. Merlo, S. Ethier,

E. D'Azevedo, F. Jenko, and A. Bhattacharjee, Phys. Plasmas 28, 062501 (2021)

[2] Pueschel M.J., Jenko F., Told D. and Buchner J., Phys. Plasmas 18, 112102 (2011)

[3] M. J. Pueschel, D. Told, P. W. Terry, F. Jenko, E. G. Zweibel, V. Zhdankin, and H.

Lesch., ApJS 213, 30 (2014)

[4] A. J. Brizard and T. S. Hahm, Rev. Mod. Phys. 79, 421 (2007)

[5] C. D. Stephens, R. W. Brzozowski III, and F. Jenko, Phys. Plasmas 24, 102517 (2017)