Physics of Compact Advanced Stellarators

Abstract

Abstract. Compact optimized stellarators offer novel solutions for confining high-beta plasmas and developing magnetic confinement fusion. The 3D plasma shape can be designed to enhance the MHD stability without feedback or nearby conducting structures and provide drift-orbit confinement similar to tokamaks. These configurations offer the possibility of combining the steady-state low-recirculating power, external control, and disruption resilience of previous stellarators with the low-aspect ratio, high beta-limit, and good confinement of advanced tokamaks. Quasi-axisymmetric equilibria have been developed for the proposed National Compact Stellarator Experiment (NCSX) with average aspect ratio 4 -4.4 and average elongation ~1.8. Even with bootstrap-current consistent profiles, they are passively stable to the ballooning, kink, vertical, Mercier, and neoclassical-tearing modes for β > 4%, without the need for external feedback or conducting walls. The bootstrap current generates only 1/4 of the magnetic rotational transform at β=4% (the rest is from the coils), thus the equilibrium is much less non-linear and is more controllable than similar advanced tokamaks. The enhanced stability is a result of 'reversed' global shear, the spatial distribution of local shear, and the large fraction of externally generated transform. Transport simulations show adequate fast-ion confinement and thermal neoclassical transport similar to equivalent tokamaks. Modular coils have been designed which reproduce the physics properties, provide good flux surfaces, and allow flexible variation of the plasma shape to control the predicted MHD stability and transport properties.3

New classes of quasi-helically symmetric stellarators

“…The details of the rotational transform at a finite pressure depend on the temperature and density profiles. The pressure profile used in figure 3 is taken from NCSX [6] in the form…”

Measurements and modeling of plasma flow damping in the Helically Symmetric eXperiment

“…A quasisymmetric stellarator possesses a direction of symmetry in ͉B͉ and the neoclassical viscous damping of flows in this direction is minimal. The ability to sustain plasma flows may be an important ingredient in developing improved confinement regimes, 3 and has been invoked as a potential advantage in the designs of the quasitoroidally symmetric National Compact Stellarator eXperiment 4 ͑NCSX͒ and compact helical system, quasiaxisymmetric ͑CHS-qa͒, 5 and the quasipoloidally symmetric stellarator. 6 It is the purpose of this paper to present measurements and modeling of biased plasma flows in the quasihelically symmetric Helically Symmetric eXperiment ͑HSX͒, 7 which demonstrate the reduction of plasma flow damping with quasisymmetry.…”

Stellarator coil design and plasma sensitivity

“…In this section, we present results from a study in which magnetic field perturbations were introduced to the reference NCSX plasma [15]. The general characteristics of the unperturbed NCSX are summarized in figure 1: the four cross sections of the plasma equally spaced in halfperiod, and in figure 2: the rotational transform.…”