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Shear Alfvén continua have been calculated for stellarators over a range of shapes and aspect ratios as a first step toward understanding Alfvén instability induced fast ion losses in such systems and possible means for minimizing these losses. Stellarators introduce strong poloidal/toroidal couplings in both I BI and the g[sup ρρ] metric coefficient that can induce new continuum gap structures not present in axisymmetric tokamaks. Low field period (N[sub fp] = 2-3), low aspect ratio devices result in strongly coupled toroidal mode families (n = ±n[sub 0],±n[sub 0] ± N[sub fp],±n[sub 0]±2N[sub fp], etc.) that lead to helically coupled Alfvén gaps at lower frequencies and with wider gap structures than are the case for larger aspect ratio, higher field period stellarator devices. [ABSTRACT FROM AUTHOR]

The application of non-axisymmetric magnetic perturbations has been demonstrated to destabilize edge-localized modes (ELMs) in the National Spherical Torus Experiment. A model 3D equilibrium has been calculated for these experiments using the VMEC code, which assumes nested flux surfaces and therefore that resonant perturbations are shielded. First, a free-boundary equilibrium is calculated using the NSTX coil set, with pressure and current profiles matched to a standard 2D reconstruction, but with up-down symmetry enforced. A new equilibrium is then calculated with the n = 3 field applied at a level consistent with experiment. This equilibrium is then used as the basis of further calculations using codes developed for analysis of stellarator plasmas. The neoclassical transport due to the 3D fields is calculated to be small compared with the experimental transport rates. Initial stability analysis has been performed, and indicates a modest degradation in ballooning stability with 3D fields applied. A new 3D equilibrium is also calculated using the SIESTA code, which allows for the formation of islands and stochastic regions. A comparison of the field structure between the SIESTA calculation and the assumption of fully penetrated vacuum perturbation indicates smaller island sizes and very small stochastic transport in the SIESTA case. [ABSTRACT FROM AUTHOR]

Quasi-symmetric configurations have a better neoclassical confinement compared to that of standard stellarators. The reduction of the neoclassical viscosity along the direction of quasi-symmetry should facilitate the self-generation of zonal flows and, consequently, the mitigation of turbulent fluctuations and the ensuing radial transport. Therefore, it is expected that quasi-symmetries should also result in better confinement properties regarding radial turbulent transport. In this paper we show that, at least for quasi-poloidal configurations, the influence of quasi-symmetry on radial transport exceeds the expected reduction of fluctuation levels and associated effective transport coefficients, and that the intimate nature of transport itself is affected. In particular, radial turbulent transport becomes increasingly subdiffusive as the degree of quasi-symmetry becomes larger. This behavior is somewhat reminiscent of what has been previously reported in tokamaks with strong radially sheared zonal flows. [ABSTRACT FROM AUTHOR]

RFX-mod self-organized single helical axis (SHAx) states provide a unique opportunity to advance 3D fusion physics and establish a common knowledge basis in a parameter region not covered by stellarators and tokamaks. The VMEC code has been adapted to the reversed-field pinch (RFP) to model SHAx equilibria in fixed boundary mode with experimental measurements as constraint. The averaged particle diffusivity over the helical volume, estimated with the Monte Carlo code ORBIT, has a neoclassical-like dependence on collisionality and does not show the 1/ν trend of un-optimized stellarators. In particular, the helical region boundary, corresponding to an electron transport barrier with zero magnetic shear and improved confinement, has been investigated using numerical codes common to the stellarator community. In fact, the DKES/PENTA codes have been applied to RFP for local neoclassical transport computations, including radial electric field, to estimate thermal diffusion coefficients in the barrier region for typical RFX-mod temperature and density profiles. A comparison with power balance estimates shows that residual chaos due to secondary tearing modes and small-scale turbulence still contribute to drive anomalous transport in the barrier region. [ABSTRACT FROM AUTHOR]

Second ballooning stability is examined in quasipoloidally symmetric, compact stellarator configurations. These high-β(volume-average β>4%) free-boundary equilibria are calculated using a reference Quasi-Poloidal Stellarator (QPS) configuration. QPS plasmas have low-shear, stellarator-like rotational transform profile with |B| that is approximately poloidally symmetric. The high-β QPS equilibria are similar in their magnetic configuration to previously studied tokamak-stellarator hybrids which have a high-shear, tokamak-like rotational transform profile. Both types of configurations have strong magnetic wells and consequently high interchange stability β limits. Free-boundary QPS equilibria have regions of second stability at high β. For infinite-n ballooning modes in QPS plasmas, the boundary for first instability is 〈b〉∼2% and the boundary for second stability is 〈bβ∼6%. Finite-n ballooning mode calculations show higher β limits, 〈b〉>5%. Increasing plasma current (for fixed plasma pressure) can lower the finite-n ballooning mode β-limit to 〈b〉=3% by reducing magnetic shear. QPS plasmas with Ohmic current profiles (peaked on-axis) have both a lower infinite-n ballooning β-limit for the onset of first instability and a higher β-limit for the onset of second stability relative to QPS plasma with bootstrap current profiles (peaked off-axis). QPS plasmas are stable to low-n ideal magnetohydrodynamic kink modes and vertical modes for values of β in this range (〈β〉∼6%) due to the low level of plasma current in QPS relative to an equivalent tokamak. [ABSTRACT FROM AUTHOR]

Presents the magnetohydrodynamics stability of external kink modes and vertical modes in compact stellarators. Stabilization of vertical mode; Derivation of a simple stability criterion; Results of the systematic parameter study.

Recent progress in the theoretical understanding and design of compact stellarators is described. Hybrid devices, which depart from canonical stellarators by deriving benefits from the bootstrap current which flows at finite beta, comprise a class of low aspect ratio A<4 stellarators. They possess external kink stability (at moderate beta) in the absence of a conducting wall, possible immunity to disruptions through external control of the transform and magnetic shear, and they achieve volume-averaged ballooning beta limits (4%-6%) similar to those in tokamaks. In addition, bootstrap currents can reduce the effects of magnetic islands (self-healing effect) and lead to simpler stellarator coils by reducing the required external transform. Powerful physics and coil optimization codes have been developed and integrated to design experiments aimed at exploring compact stellarators. The physics basis for designing the national compact stellarator will be discussed. [ABSTRACT FROM AUTHOR]