  The radiation drag in photon-rich environments of cosmic explosions can seed
kinetic instabilities by inducing velocity spreads between relativistically
streaming plasma components. Such microturbulence is likely imprinted on the
breakout signals of radiation-mediated shocks. However, large-scale, transverse
magnetic fields in the deceleration region of the shock transition can suppress
the dominant kinetic instabilities by preventing the development of velocity
separations between electron-positron pairs and a heavy ion species. We use a
one-dimensional (1D) five-fluid radiative transfer code to generate
self-consistent profiles of the radiation drag force and plasma composition in
the deceleration region. For increasing magnetization, our models predict
rapidly growing pair multiplicities and a substantial radiative drag developing
self-similarly throughout the deceleration region. We extract the critical
magnetization parameter $\sigma_{c}$, determining the limiting magnetic field
strength at which a three-species plasma can develop kinetic instabilities
before reaching the isotropized downstream. For a relativistic, single ion
plasma drifting with $\gamma_{u} = 10$ in the upstream of a relativistic
radiation-mediated shock, we find the threshold $\sigma_{c}\approx 10^{-7}$ for
the onset of microturbulence. Suppression of plasma instabilities in the case
of multi-ion composition would likely require much higher values of
$\sigma_{c}$. Identifying high-energy signatures of microturbulence in
shock-breakout signals and combining them with the magnetization limits
provided in this work will allow a deeper understanding of the magnetic
environment of cosmic explosions like supernovae, gamma-ray bursts, and neutron
star binary mergers.
