**Time:**** Monday, May 7, at 2pm**

**Location: ****Rm. 413, Bld. 11#, Yuquan Campus**** **

**Title: ** M**agnetic moment of runaway electrons and collisionless pitch-angle scattering**

Chang Liu

Princeton Plasma Physics Laboratory

**Abstract:**

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.

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- The 11th International Conference on Computational Physics November 19-23, 2018, Hangzhou, China
- Talk by Dr. Chang Liu of PPPL at 2pm，May 7th, Monday
- Four Tutorial Lectures from Prof. Allen Boozer (Columbia University)
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- Second Announcement of the Hangzhou International Stellarator Workshop, March 26-28, 2018
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- Progress in direct-drive inertial confinement fusion
- Progress in laboratory exploration of astrophysical phenomena
- First announcement of the Hangzhou International Stellarator Workshop (HISW2018), March 26-28, 2018, Hangzhou, China
- 上交大激光等离子体研究人员受邀到访及学术报告