Improvement of collisionless particle confinement in a non-quasi-symmetric stellarator vacuum magnetic field
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Improvement of Collisionless Particle Confinement in a NonQuasiSymmetric Stellarator Vacuum Magnetic Field1 S. V. Kasilova, b, W. Kernbichlerb, M. I. Mikhailovc, V. V. Nemova, b, J. Nührenbergd, and R. Zilled a
Institute of Plasma Physics, National Science Center Kharkov Institute of Physics and Technology, Akademicheskaya ul. 1, Kharkov, 61108 Ukraine b Institut für Theoretische Physik–Computational Physics, Technische Universitat Graz, Association EURATOM–ÖAW, Petersgasse 16, 8010 Graz, Austria c National Research Centre Kurchatov Institute, pl. Kurchatova 1, Moscow, 123182 Russia d MaxPlanckInstitut für Plasmaphysik, EURATOM Assoziation, Teilinstitut Greifswald, Wendelsteinstr. 1, 17491 Greifswald, Germany email: [email protected] Received August 22, 2012; in final form, October 8, 2012
Abstract—A nonquasisymmetric stellarator vacuum magnetic field with an aspect ratio of about 11 is found in which collisionless particles are confined up to about 2/5 of the minor radius. DOI: 10.1134/S1063780X1304003X 1
In conventional stellarator vacuum fields, colli sionless particles are lost through a loss cone. Quasi symmetric stellarators can confine collisionless parti cles well [1]. In quasiisodynamic stellarator vacuum fields a reduced loss cone persisted [2]. Here, by improved computational optimization of collisionless particle confinement, a nonquasisymmetric config uration is found in which the loss cone is eliminated in the core of the confinement region.
The optimization procedure used is essentially the same as in earlier efforts [2]. Additional ingredients here are a weighting procedure which strongly empha sizes early lost particles, an iterative increase of the radius where the particles are started, usage of up to 105 particles and a parallelized optimization proce dure. The initial configuration was an interpolation between a lowβ quasiisodynamic configuration with poloidally closed contours of its field strength with very low bootstrap current [3] and the old case opti mized for collisionless particle confinement [2]. Except for this latter property the optimization was unconstrained. It was terminated when about 0.4 of the minor radius was reached to assess its other prop erties so that modified goals can be formulated. The results are described below.
independent test in a vacuum magnetic field given by a set of harmonic functions [4] satisfying the Neu mann boundary condition at the VMEC boundary. The VMEC result was essentially verified by following αparticles in the vacuum field scaled to fusion dimensions (volume 103 m3, magnetic field 5 T). From the 1000 particles started and followed up to 0.1 s, four particles were lost between 0.003 and 0.01 s. Views of the geometry of the configuration and the structure of its field strength are seen in Figs. 1–3 as obtained by VMEC. Overall, it is close to a quasiiso dynamic configuration with poloidally closed con tours of the field strength [5]. In more detail, the poloidal closure of the contours of
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