  Without amplification, magnetic fields in expanding ejecta of young supernova
remnants (SNRs) will be orders of magnitude below those required to shock
accelerate thermal electrons, or ions, to relativistic energies or to produce
radio synchrotron emission at the reverse shock. The reported observations of
such emission give support to the idea that diffusive shock acceleration (DSA)
can amplify magnetic fields by large factors. Furthermore, the uncertain
character of the amplification process leaves open the possibility that ejecta
fields, while large enough to support radio emission and DSA, may be much lower
than typical interstellar medium values. We show that DSA in such low reverse
shock fields is extremely nonlinear and efficient in the production of
cosmic-ray (CR) ions, although CRs greatly in excess of mc^2 are not produced.
These nonlinear effects, which occur at the forward shock as well, are
manifested most importantly in shock compression ratios much greater than four
and cause the interaction region between the forward and reverse shocks to
become narrower, denser, and cooler than would be the case if efficient
cosmic-ray production did not occur. The changes in the SNR structure and
evolution should be clearly observable, if present, and they convey important
information on the nature of DSA and magnetic field amplification with broad
astrophysical implications.
