Physics studies for helical-axis advanced stellarators

Various Helias configurations (helical advanced stellarator) are known with M = 4 to 6 field periods. Wendelstein VII-X, the future large Garching stellarator experiment, will employ a Helias configuration with M = 5. Helias systems can be typified between the two limiting cases, linked mirror configurations and quasi-helical configurations, according to their different structure of mod B on the magnetic surfaces. Stability calculations have shown that resistive interchange and ideal ballooning modes are stable up to <β> approx. = 5% in Helias configurations with five field periods and aspect ratio A approx. = 10. Confinement times of a finite-β long mean free path (lmfp) quasi-neutral Maxwellian plasma with an assumed radial electric field can be estimated from Monte Carlo calculations. Studies show that Helias stellarators may be found with an equivalent ripple smaller than 2% at half the plasma radius. Monte Carlo simulations have also been used to obtain a preliminary assessment of the bootstrap current in the lmfp regime. Other transport studies using the bounce-averaged Fokker-Planck equation for Helias configurations and heating scenarios obtained in a simple 1D transport code show typical values of <β> = 5% with a power of 20 MW at B = 2-2.5 T, at densities of about 1020m-3 and peak temperatures around 5 keV. Applications of the Neumann problem for vacuum magnetic fields of stellarators have led to improvements in the structure of magnetic surfaces, of the structure of island chains bounding the confinement region, in computing free boundary equilibria, and in the search for modular coils. Engineering studies of superconducting coil systems for Helias configurations have shown that in a case with 4 periods, 12 coils per period, R0 = 5 m, B0 = 4 T, current density in the coils jc = 40 MA/m2, the maximum net coil force amounts to approximately 4 MN. All components of the stress tensor are important and amount to a maximum von Mises stress of about σνM approx. = 110 MPa, and a maximum shear stress of about 1/3 of this value inside the coils. 15 refs, 12 figs, 3 tabs