smoothed out on this scale.

We show that diffusive shock acceleration (DSA) in systems with colliding shock flows (CSFs) can provide maximum particle energies, CR fluxes, and energy conversion efficiencies well above those produced in an isolated SNR shock of the same velocity [@MNRAS_BGO13]. While CSFs are expected to occur in colliding stellar winds, the most powerful events should happen when a supernova shock impacts the extended fast wind of a nearby young massive star cluster.

The important features of shock acceleration in are: (i) the production of a piece-wise power-law particle distribution with a very hard spectrum of confined particles at the high-energy end just before a break, and (ii) an increase in the maximum energy of the accelerated particles, and the acceleration efficiency, compared to that obtained with DSA at an isolated SNR shock of the same speed. These two properties imply that a substantial fraction of the flow ram pressure is converted into relativistic particle pressure. The high-energy CRs, therefore, must modify the dynamics of the CSF system. To model the spectra of accelerated particles in CSFs, a [nonlinear]{}, time-dependent model was constructed in @MNRAS_BGO13. The maximum energy and absolute fluxes of the accelerated particles, both inside 