Magnetic fields with precise quasisymmetry for plasma confinement

Abstract: Quasisymmetry is an unusual symmetry that can be present in toroidal magnetic fields, enabling confinement of charged particles and plasma. Here it is shown that both quasi-axisymmetry and quasi-helical symmetry can be achieved to a much higher precision than previously thought over a significant volume, resulting in exceptional confinement. For a 1 Tesla mean field far from axisymmetry (vacuum rotational transform $>$ 0.4), symmetry-breaking mode amplitudes throughout a volume of aspect ratio 6 can be made as small as the typical $\sim 50$ $\mu$T geomagnetic field.