OPTIMIZATION OF A QUASI-OMNIGENEOUS STELLARATOR

A low-aspect-ratio quasi-omnigeneous (QO) stellarator could combine the disruption immunity and low recycled power of currentless high-aspect-ratio stellarators with the low aspect ratio and good performance of high-current tokamaks. The QO approach approximately aligns bounce-averaged drift orbits with magnetic surfaces, which reduces energetic particle losses and neoclassical energy transport, and aims at a bootstrap current ~1/10 that in a comparable tokamak, which should lead to configurations that are relatively insensitive to β and robust against current- driven modes. QO-optimized configurations with field periods Nfp = 3 and 4 and plasma aspect ratio Ap = R0/ap from 3 to 4.8 were optimized with respect to trapped particle confinement, ballooning β limits, amount of self-consistent bootstrap current, magnetic field ripple, outer surface curvature, and ι(r). Development of improved QO configurations is due to incorporation of fast calculations of the self-consistent bootstrap current and the ballooning stability limit within the iterative optimization procedure. The reference Nfp = 3, Ap ∪ 3.6 QOS plasma configuration features a large helical axis excursion, high rotational transform, a mirror component of the magnetic field that varies strongly with radius, and low bootstrap current, which leads to good neoclassical confinement and a ballooning stability β limit of 3-4%. 1. Compact Stellarators