The most favorable situation for proving that the main, hadronic CR
component is accelerated in SNRs is when molecular clouds (MC) surround the
SN shock. We suggest a new mechanism of spectrum formation in this
situation. Using an analytic model of nonlinear diffusive shock
acceleration, we calculate the spectra of protons and estimate the
resulting $\gamma$-ray emission occurring when the SNR shock approaches a
MC. We show that the spectrum develops a break in the TeV range and that
its GeV component is suppressed. Possible applications of the new spectra
to the recent CANGAROO and HESS observations of the SNR RX J1713.7-3946 are
discussed.
We also suggest a physical mechanism whereby the acceleration time of CRs
by shock waves can be significantly reduced with a simultaneousdevelopment of a break (knee) on the spectrum. This creates the possibility
of particle acceleration beyond the galactic CR knee energy at
$10^15$ eV. The acceleration results from a nonlinearmodification of the flow ahead of the shock supported by particles already
accelerated to the knee momentum. The particles gain energy bybouncing off converging magnetic irregularities frozen into the flow in the
shock precursor and not so much by re-crossing the shock itself. The
acceleration rate is thus determined by the gradient of the flow velocity
and turns out to be formally independent of the particle mean free
path. The velocity gradient is, in turn, set bythe knee-particles. The acceleration rate of particles above the knee does
not decrease with energy, unlike in the linear acceleration regime. The
knee forms because particles above it are effectively confined to the shock
while they are within limited domains in the momentum space, while other
particles fall into \"loss-islands\",similar to the \"loss-cone\" of magnetic traps. This also maintains the
steep velocity gradient and high acceleration rate.