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The theory of diffusive shock acceleration (DSA) operating in
strong, nonrelativistic shocks, clearly allows for the mechanism to be
highly efficient. However, it is still an open question if actual
collisionless shocks, such as those in young supernova remnants (SNRs), are
as efficient as predicted. Fortunately, the nonlinear feedback effects
resulting from efficient DSA should produce observable consequences
including changes in the morphology of the remnant and a strongly amplified
magnetic field. I will outline the theory for nonlinear DSA and apply it to
recent observations of young SNRs. Preliminary work on magnetic field
amplification will also be presented. This work, which employs a
well-tested Monte Carlo simulation, will self-consistently determine the
plasma flow, magnetic field, and particle spectrum. Amplified magnetic
fields may result in large increases in the maximum particle energy
produced in a given shock.
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