Time: Monday, May 7, at 2pm
Location: Rm. 413, Bld. 11#, Yuquan Campus
Title: Magnetic moment of runaway electrons and collisionless pitch-angle scattering
Princeton Plasma Physics Laboratory
The conservation of magnetic moment is one of the fundamental principles in the theory of charged particle motion in magnetic fields, but it was recently challenged by the results of full-orbit simulation of runaway electrons. The phenomenon is called “collisionless pitch-angle scattering”, which claims that for highly-energetic electrons moving in curved magnetic field geometry like tokamaks, the parallel moment can be transferred to perpendicular momentum and thus the magnetic moment is not conserved. In this work, we manage to address this issue by revisiting the standard guiding-center theory and its high-order corrections. In addition to the classical guiding-center ordering, we introduce a new assumption that for runaway electrons, perpendicular momentum is much smaller than the parallel component. With the help of this assumption, we can easily derive the Lagrangian and Hamiltonian of runaway electrons using Lie-perturbation method to the second order. The new magnetic moment derived from this theory now depends on both the perpendicular and the parallel momentum. Using this magnetic moment, we can explain the collisionless pitch-angle scattering found in full-orbit simulations. Benchmarking with the simulation results, it is found that the new magnetic moment has a much better conservation property than the standard one. Our work points out the correct way to apply guiding-center approximation for energetic runaway electrons and paves the road for the development of a new guiding-center simulation framework.