Multi-scale Full-orbit Analysis on Phase-space Behavior of Runaway Electrons in Tokamak Fields with Synchrotron Radiation

TL;DR

This study uses full-orbit simulations to analyze runaway electron behavior in tokamak fields, revealing how initial conditions and device parameters influence energy limits and phase-space dynamics over multiple timescales.

Contribution

It provides a detailed phase-space analysis of runaway electrons across various timescales, incorporating synchrotron radiation and device parameter effects with a relativistic volume-preserving algorithm.

Findings

Runaway electron trajectories are severely deformed on small timescales.

Initial conditions significantly affect pitch-angle scattering and energy evolution.

Device parameters like magnetic field and loop voltage influence energy limits and scattering strength.

Abstract

In this paper, the secular full-orbit simulations of runaway electrons with synchrotron radiation in tokamak fields are carried out using a relativistic volume-preserving algorithm. Detailed phase-space behaviors of runaway electrons are investigated in different dynamical timescales spanning 11 orders. When looking into the small timescale, i.e., the characteristic timescale imposed by Lorentz force, the severely deformed helical trajectory of energetic runaway electron is witnessed. A qualitative analysis of the neoclassical scattering, a kind of collisionless pitch-angle scattering phenomena, is provided when considering the coupling between the rotation of momentum vector and the background magnetic field. In large timescale up to one second, it is found that the initial condition of runaway electrons in phase space globally influences the pitch-angle scattering, the momentum evolution, and the loss-gain ratio of runaway energy evidently. However, the initial value has little impact on the synchrotron energy limit. It is also discovered that the parameters of tokamak device, such as the toroidal magnetic field, the loop voltage, the safety factor profile, and the major radius, can modify the synchrotron energy limit as well as the strength of neoclassical scattering. The maximum runaway energy is also proved to be lower than the synchrotron limit when the magnetic field ripple is considered.