  Observations show that the magnetic field in young supernova remnants (SNRs)
is significantly stronger than can be expected from the compression of the
circumstellar medium (CSM) by a factor of four expected for strong blast waves.
Additionally, the polarization is mainly radial, which is also contrary to
expectation from compression of the CSM magnetic field. Cosmic rays (CRs) may
help to explain these two observed features. They can increase the compression
ratio to factors well over those of regular strong shocks by adding a
relativistic plasma component to the pressure, and by draining the shock of
energy when CRs escape from the region. The higher compression ratio will also
allow for the contact discontinuity, which is subject to the Rayleigh-Taylor
(R-T) instability, to reach much further out to the forward shock. This could
create a preferred radial polarization of the magnetic field. With an adaptive
mesh refinement MHD code (AMRVAC), we simulate the evolution of SNRs with three
different configurations of the initial CSM magnetic field, and look at two
different equations of state in order to look at the possible influence of a CR
plasma component. The spectrum of CRs can be simulated using test particles, of
which we also show some preliminary results that agree well with available
analytical solutions.
