  While from the energetic point of view SNRs are viable sources of Galactic
CRs, the issue of whether they can accelerate protons up to PeV remains
unsolved. Here we discuss particle acceleration at the forward shock of SN and
discuss the possibility that the escaping particle current may excite a
non-resonant instability that in turn leads to the formation of resonant modes
confining particles close to the shock and increasing the maximum energy. This
mechanism works throughout the expansion of the SN explosion, from the ejecta
dominated (ED) to the Sedov-Taylor (ST) phase. Because of their higher
explosion rate,we focus on type II SNae expanding in the slow, dense red
supergiant wind. When the explosion occurs in such winds, the transition
between the ED and the ST phase is likely to take place within a few tens of
years. As a result, the spectrum of accelerated particles shows a break in the
slope, at the maximum energy (Em) achieved at the beginning of the ST phase.
Above this energy, the spectrum becomes steeper but remains a power law than
developing an exponential cutoff. We show that for type II SNae typical
parameters, proton Em can easily reach PeV energies, confirming that type II
SNRs are the best candidate sources for CRs at the knee. We have tried to fit
KASCADE-Grande, ARGO -YBJ and YAC1-Tibet Array data with our model but we could
not find any parameter combination that could explain all data sets. Indeed the
recent measurement of the proton and helium spectra in the knee region, with
the ARGO-YBJ and YAC1-Tibet Array, has made the situation very confused. These
measurements suggest that the knee in the light component is at 650 TeV,
appreciably below the overall spectrum knee. This finding would resolve the
problem of reaching very high energies in SNae, but, on the other hand, it
would open a critical issue in the transition region between Galactic and
extragalactic CRs.
