  H-alpha emission from supernova remnants (SNRs) implies the existence of
neutral hydrogen in the circumstellar medium. Some of the neutral particles
penetrating the shock are ionized by the charge exchange process and make a
cold ion beam in the shock downstream region. We perform linear analyses of
collisionless plasma instabilities between the cold beam and the hot downstream
plasma. We find that, under typical SNR conditions, either the resonant
instability or the Weibel instability is the most unstable. This mechanism may
amplify the magnetic field to more than 100 micro G and changes the shock
structure. As a result, the radio spectrum and the large magnetic field can be
explained, apart from the widely discussed Bell's mechanism.
