Local axisymmetry-breaking–induced transition of trapped-particle orbit and loss channels in quasi-axisymmetric stellarators

¹ Institute of Fusion Science, School of Physical Science and Technology, Southwest Jiaotong University Chengdu 610031, China
² National Institute for Fusion Science, National Institutes of Natural Sciences - Toki 509-5292, Japan
³ The Graduate University for Advanced Study, SOKENDAI - Toki 509-5292, Japan
⁴ College of Physics, Sichuan University - Chengdu 610065, China
⁵ Hefei Keye Electro Physical Equipment Manufacturing Co., Ltd. - Hefei 230000, China

^(a) hfliu@swjtu.edu.cn

Received: 17 December 2019
Accepted: 11 February 2020

Abstract

The transition of trapped-particle orbit topologies has been investigated in quasi-axisymmetric (QA) configurations, such as the Chinese First Quasi-axisymmetric Stellarator (CFQS). It is found that the axisymmetry-breaking phenomenon in QA configurations is of great significance at some specific locations, which could easily induce blocked particles to transit into localized particles. A novel aspect is presented to interpret the transition mechanism of trapped-particle orbit topologies in this paper, i.e., as the amplitudes of non-axisymmetric field increase along the radius direction, the region of large toroidal inhomogeneity is gradually generated, which makes the length of the trapped-particle trajectory substantially short, and hence, may restrict particles to a single helical field period. Meanwhile, at such locations the “pseudo-axisymmetric” field results in coupling of the maximum radial drift and the minimum poloidal drift, which enables the transition of trapped-particle orbit topologies considerably and forms specific loss channels, degrading plasma confinement. These results may shed light on the optimization of QA configurations via avoidance of such coupling with respect to energetic particle confinement. Moreover, this work is also relevant to the generation of inhomogeneity of particle flux deposition on the devertor plates.