Physics issues are discussed for compact stellarator configurations which achieve good confinement by the fact that the magnetic field modulus |B| in magnetic co-ordinates is dominated by poloidally symmetric components. Two distinct configuration types are considered: (1) those which achieve their drift optimization and rotational transform at low β and low bootstrap current by appropriate plasma shaping; and (2) those which have a greater reliance on plasma β and bootstrap currents for supplying the transform and obtaining quasi-poloidal symmetry. Stability analysis of the latter group of devices against ballooning, kink and vertical displacement modes has indicated that stable β values on the order of 15% are possible. The first class of devices is being considered for a low β near term experiment that could explore some of the confinement features of the high β configurations.

A generic model for advection of a scalar by E 3 B flow with a linearly varying mean shows that the cross phase factor in the transport flux is strongly reduced in the strong shear regime (shearing rate . eddy turnover rate), leading to significant transport suppression. The cross phase scales much more strongly with shear strength than do fluctuation amplitudes, allowing significant transport reduction even if fluctuations increase, or decrease only slightly. Cross-phase suppression thus can be the dominant transport-reduction mechanism in transport barriers.

A new procedure for calculating the nonlinear energy transfer and linear growth/damping rate of fully developed turbulence is derived. It avoids the unphysically large damping rates typically obtained using the predecessor method of Ritz ͓Ch. P. Ritz, E. J. Powers, and R. D. Bengtson, Phys. Fluids B 1, 153 ͑1989͔͒. It enforces stationarity of the turbulence to reduce the effects of noise and fluctuations not described by the basic governing equation, and includes the fourth-order moment to avoid the closure approximation. The new procedure has been implemented and tested on simulated, fully developed two-dimensional ͑2-D͒ turbulence data from a 2-D trapped-particle fluid code, and has been shown to give excellent reconstructions of the input growth rate and nonlinear coupling coefficients with good noise rejection. However, in the experimentally important case where only a one-dimensional ͑1-D͒ averaged representation of the underlying 2-D turbulence is available, this technique does not, in general, give acceptable results. A new 1-D algorithm has thus been developed for analysis of 1-D measurements of intrinsically 2-D turbulence. This new 1-D algorithm includes the nonresonant wave numbers in calculating the bispectra, and generally gives useful results when the width of the radial wave number spectrum is comparable to or less than that of the poloidal spectrum.

No abstract

Nine stellarator configurations, three quasiaxisymmetric, three quasihelically symmetric and three non-quasisymmetric are scaled to ARIES-CS size and analyzed for energetic particle content. The best performing configurations with regard to energetic particle confinement also perform the best on the neoclassical Γc metric, which attempts to align contours of the second adiabatic invariant with flux surfaces. Quasisymmetric configurations that simultaneously perform well on Γc and quasisymmetry have the best overall confinement, with collisional losses under 3%, approaching the performance of ITER with ferritic inserts.