a spin period of only $\sim$$1\,\mathrm{millisecond}$. Its core is expected to be spinning uniformly, but its outer regions will be extremely differentially rotating. These are ideal conditions for the *magnetorotational instability* (MRI,[@balbus:91]) to operate, amplify any seed magnetic field, and drive magnetohydrodynamic (MHD) turbulence. If a dynamo process is present, an ultra-strong large-scale (globally ordered) magnetic field is built up. This makes the protoneutron star a *protomagnetar*. Provided this occurs, magnetic pressure gradients and hoop stresses could lead to outflows along the axis of rotation. The MRI’s fastest growing mode has a small wavelength and is extremely difficult to resolve numerically.

Because of this, all simulations of the magnetorotational mechanism to date have simply made the assumption that a combination of MRI and dynamo is operating. They then ad-hoc imposed a strong large-scale field as an initial condition. In 2D simulations, collimated jets develop along the axis of rotation. In 3D, the jets are unstable and a more complicated explosion geometry develops [@moesta:14b], as shown in Figure \[fig:mhd2014\]. Nevertheless, even in 3D, an energetic explosion could potentially be powered.

The magnetorotational mechanism requires one special property of the progenitor star: rapid core rotation. Presently, stellar evolution theory suggests that the 