From cosmic ray source to the Galactic pool
  The Galactic cosmic ray spectrum is a remarkably straight power law. Our
current understanding is that the dominant sources that accelerate cosmic rays
up to the knee ($3 \times 10^{15}$ eV) or perhaps even the ankle ($3 \times
10^{18}$ eV), are young Galactic supernova remnants. In theory, however, there
are various reasons why the spectrum may be different for different sources,
and may not even be a power law if nonlinear shock acceleration applies during
the most efficient stages of acceleration. We show how the spectrum at the
accelerator translates to the spectrum that make up the escaping cosmic rays
that replenish the Galactic pool of cosmic rays. We assume that cosmic ray
confinement, and thus escape, is linked to the level of magnetic field
amplification, and that the magnetic field is amplified by streaming cosmic
rays according to the non-resonant hybrid or resonant instability. When a fixed
fraction of the energy is transferred to cosmic rays, it turns out that a
source spectrum that is flatter than $E^{-2}$ will result in a $E^{-2}$ escape
spectrum, whereas a steeper source spectrum will result in an escape spectrum
with equal steepening. This alleviates some of the concern that may arise from
expected flat or concave cosmic ray spectra associated with nonlinear shock
modification.
