Physics design of a high-bbeta quasi-axisymmetric stellarator

Abstract

Key physics issues in the design of a high-β quasi-axisymmetric stellarator configuration are discussed. The goal of the design study is a compact stellarator configuration with aspect ratio comparable to that of tokamaks and good transport and stability properties. Quasiaxisymmetry has been used to provide good drift trajectories. Ballooning stabilization has been accomplished by strong axisymmetric shaping, yielding a stellarator configuration whose core is in the second stability regime for ballooning modes. A combination of externally generated shear and non-axisymmetric corrugation of the plasma boundary provides stability to external kink modes even in the absence of a conducting wall. The resulting configuration is also found to be robustly stable to vertical modes, increasing the freedom to perform axisymmetric shaping. Stability to neoclassical tearing modes is conferred by a monotonically increasing ι profile. A gyrokinetic δf code has been used to confirm the adequacy of the neoclassical confinement. Neutral beam losses have been evaluated with Monte Carlo codes.

Optimizing Stellarators for Turbulent Transport

“…Starting in the 1980s, a number of of approaches to neoclassicaltransport-optimized stellarators were discovered [1][2][3][4][5], in which the neoclassical (nc) transport could be reduced to below the level of turbulent or "anomalous" transport over most of the plasma column, making stellarator confinement comparable to that achievable in tokamaks. In recent years, two powerful numerical tools have been developed, which also make mitigating turbulent transport in stellarators a realistic possibility, namely configuration optimization codes such as Stellopt [6], and gyrokinetic (gk) codes valid for 3D configuations, such as the Gene/Gist code package [7,8]. In this paper, we make use of these two new tools to demonstrate that new stellarator configurations with appreciably diminished turbulent transport levels can be evolved from stellarators designed without this turbulent-transport-optimization, raising the prospect of a new class of stellarators with greatly improved overall confinement.…”

Vertical and Kink Mode Stability Calculations for Current Carrying Quasiaxial Stellarators

“…TERPSICHORE has been used to show that high edge magnetic shear and appropriate boundary shaping can stabilize the kink mode in high bootstrap current QAS [4]. CAS3D has confirmed TERPSICHORE calculations [5] of stability for the kink and periodicity-preserving modes for the three field period, 50% external transform stellarator and extended them, finding stability even without a conducting wall.…”

Robustness and flexibility in compact quasiaxial stellarators: Global ideal MHD stability and energetic particle transport

“…(10,5), (8,4), and (5,2). The largest Fourier components of the N=0 family are (6,3), (12,6), (7,3), (11,6), and (5,3). The flux label s is the edge normalized toroidal flux.…”

“…The baseline case, called QAS3_C82, is the candidate design configuration for NCSX presented at the 1999 EPS meeting [3] and at the 1999 APS meeting [4]. This is a three field period, compact stellarator with major radius 1.6 m, and aspect ratio 3.5.…”

“…An intense effort to achieve a stable and well confined compact quasiaxial stellarator (QAS) [1,2] configuration has led to a promising design for a modest size experiment to be called the National Compact Stellarator Experiment (NCSX) [3,4]. New ideas for symmetric stellarator design have driven the development of advanced computational tools to evaluate and optimize neoclassical plasma transport and magnetohydrodynamic (MHD) stability in fully three-dimensional geometries.…”