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We present a first 3D magnetohydrodynamic (MHD) simulation of convectiveoxygen and neon shell burning in a non-rotating $18\, M_\odot$ star shortlybefore core collapse to study the generation of magnetic fields in supernovaprogenitors. We also run a purely hydrodynamic control simulation to gauge theimpact of the magnetic fields on the convective flow and on convective boundarymixing. After about 17 convective turnover times, the magnetic field isapproaching saturation levels in the oxygen shell with an average fieldstrength of $\mathord{\sim}10^{10}\, \mathrm{G}$, and does not reach kineticequipartition. The field remains dominated by small to medium scales, and thedipole field strength at the base of the oxygen shell is only $10^{9}\,\mathrm{G}$. The angle-averaged diagonal components of the Maxwell stresstensor mirror those of the Reynolds stress tensor, but are about one order ofmagnitude smaller. The shear flow at the oxygen-neon shell interface createsrelatively strong fields parallel to the convective boundary, which noticeablyinhibit the turbulent entrainment of neon into the oxygen shell. The reducedingestion of neon lowers the nuclear energy generation rate in the oxygen shelland thereby slightly slows down the convective flow. Aside from this indirecteffect, we find that magnetic fields do not appreciably alter the flow insidethe oxygen shell. We discuss the implications of our results for the subsequentcore-collapse supernova and stress the need for longer simulations, resolutionstudies, and an investigation of non-ideal effects for a better understandingof magnetic fields in supernova progenitors.

3D Simulations of Oxygen Shell Burning with and without Magnetic Fields