J * optimization of small aspect ratio stellarator/tokamak hybrid devices

Abstract

A new class of low aspect ratio toroidal hybrid stellarators is found using a more general plasma confinement optimization criterion than quasi-symmetrization. The plasma current profile and shape of the outer magnetic flux surface are used as control variables to achieve near constancy of the longitudinal invariant J* on internal flux surfaces (quasi-omnigeneity), in addition to a number of other desirable physics target properties. We find that a range of compact (small aspect ratio A), high p (ratio of thermal energy to magnetic field energy), low plasma current devices exist which have significantly improved confinement both for thermal as well as energetic (collisionless) particle components. With reasonable increases in magnetic field and geometric size, such devices can also be scaled to confine 3.5 MeV alpha particle orbits.

COBRA: An Optimized Code for Fast Analysis of Ideal Ballooning Stability of Three-Dimensional Magnetic Equilibria

“…The optimization of such configurations from the standpoint of stability, reduced particle transport, and engineering feasibility is an open problem that has been the object of intense investigations in the last few years [1][2][3]. Several compact stellarator configurations based on the concept of quasiomnigeneity [4] (quasi-omnigeneous stellarator, QOS) and showing good particle transport have recently been obtained using an optimization scheme in which the shape of the outer magnetic surface is varied within a Levenberg-Marquardt optimization loop, using the VMEC 3-D equilibrium code [5] at each iteration to recalculate the magnetic field in the plasma [6,7]. The plasma thermal energy that can be confined in these configurations, characterized in terms of β ≡ 2 p/µ 0 B 2 (the ratio of plasma to magnetic energy), is known to be limited by the onset of unstable kink and ballooning modes [8].…”

New Approach to Transport Improvement in a Helical Magnetic Axis System by Introduction of Effective Toroidal Curvature

“…There are two quasisymmetric approaches, or quasihelical [1,2] and quasitoroidal [3] symmetrizations for the transport optimization of stellarator. In parallel with quasisymmetric approaches, omnigeneity techniques for directly reducing particle drifts away from magnetic surfaces have been developed [4,5]. In this Letter, we present a new approach to transport improvement in the helical magnetic axis stellarator.…”

“…Our new approach would give rise to the possibility of stellarator design study in a wider parameter domain than quasisymmetry approaches. The reduced e T configuration can be seen in the low aspect ratio stellarator͞tokamak hybrid, which is optimized by aligning J k contours with magnetic surfaces [4].…”

Recent advances in the design of quasiaxisymmetric stellarator plasma configurations

“…Configuration optimization is performed using a Levenberg-Marquardt scheme to minimize an "objective function" which is a sum of squares of desired targets. 13,5 Targets incorporated in the optimizer include: several measures of neoclassical transport (including a confinement time calculated by the DKES 14 code and an effective ripple calculated by the Nemov-Kernbichler 15 code); the eigenvalue of the most unstable external kink mode; ballooning eigenvalues calculated by the COBRA 12 code; the deviation of the rotational transform from prescribed values on one or two flux surfaces; the complexity and current density of an external current sheet constructed by the NESCOIL 17 code representing a first approximation to a set of coils. In practice, the configurations obtained in this way represent local optima in configuration space that retain a dependence on the starting point of the optimization calculation.…”