Abstract
Based on the magnetic field configuration of the Chinese First Quasi-axisymmetric Stellarator (CFQS) device, three types of orbits such as the passing orbit, blocked trapped orbit, and localized trapped orbit are simulated using the Boris algorithm. Also, the orbital topology, orbit transition, and loss characteristics of these particles under different initial conditions are studied in this paper. It is found that there exists a transition from blocked trapped orbits to localized trapped orbits due to small continuous helical ripples. This phenomenon is analyzed as follows: we define the angle between the particle drift velocity (v→D) and the radial direction (ρ^) as β. If the blocked-localized transition appears in the region where β>90○, the localized particles will return to the blocked particles and be constrained. However, if the blocked-localized transition happens in the region where β<90○, localized particles will drift out of the last closed flux surface and be lost. The simulation results show that the loss of localized particles in the CFQS is mainly located in some specific regions within the vicinity of the toroidal angle φ=0○,180○ and the poloidal angle θ=90○.
Highlights
A stellarator is a kind of significant device which is a possible candidate for future nuclear fusion.1 Different from tokamaks, the nonaxisymmetric magnetic field in a stellarator is produced by the external coils2,3 rather than depending on the plasma current
As for the study of single-particle orbits, Gibson and Taylor21 identified particles into three groups, namely, passing, blocked, and localized particles. They found that the localized particles would drift out of the last closed flux surface (LCFS) regardless of the strength of the magnetic field
Particle orbits are simulated based on the method stated above; the basic orbital types in the Chinese First Quasi-axisymmetric Stellarator (CFQS) are identified into three categories:21 the Passing particles are those that can pass completely around the poloidal cross section without being reflected, the scitation.org/journal/adv blocked trapped particles are particles that can be reflected before passing completely round the poloidal cross-section they may pass through several toroidal cycles, and the localized trapped particles are particles that remain locally trapped in a local magnetic well, and they can be lost due to the drift induced by the curvature and gradient of the magnetic field strength
Summary
A stellarator is a kind of significant device which is a possible candidate for future nuclear fusion. Different from tokamaks, the nonaxisymmetric magnetic field in a stellarator is produced by the external coils rather than depending on the plasma current. QAS equilibria possess a magnetic field strength that is symmetric in the toroidal Boozer coordinate; the neoclassical transport should be greatly reduced. The configuration design of the CFQS was carried out with an optimization code containing different combinations of control variables for the boundary shape Fourier components to reduce the nonaxisymmetric components of the Boozer spectra and control the rotational transform.. As for the study of single-particle orbits, Gibson and Taylor identified particles into three groups, namely, passing, blocked, and localized particles They found that the localized particles would drift out of the last closed flux surface (LCFS) regardless of the strength of the magnetic field. Since the CFQS is still under construction, it is necessary to further study its orbital characteristics and constraints in detail and examine the single particle constraint performance in the device to provide theoretical basis for its later optimization.
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