  Dark matter could have a small electromagnetic charge, provided the
charge-to-mass ratio is much less than that of electrons or protons. This
candidate, commonly known as millicharged dark matter (mDM), would form a
plasma and interact with the interstellar medium and electromagnetic fields
within galaxies. In general, understanding the dynamics of mDM requires
consideration of collective plasma effects. It has been proposed that mDM can
be accelerated in supernova remnants, forming a dark cosmic ray population that
would leave distinct experimental signatures. In this work, we study a
microphysical model where mDM is shocked by a supernova remnant and isotropized
in the frame of the expanding fluid. We find that for $|q_\chi/m_\chi| \gtrsim
10^{-13} e / {\rm MeV}$, the isotropization length for electromagnetic plasma
instabilities is much shorter than the size of the supernova remnant. This is a
necessary, though not sufficient, first step for formation of a
Fermi-accelerated mDM component, and determining the size of this component
requires further study. We discuss additional implications of mDM interactions
in supernova remnants.
