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Collisions of plasma puffs, whose characteristics (density and velocity) vary considerably, are studied in a homogeneous longitudinal magnetic field. It is verified that the puffs meet head on. Collisions of dense (ni? 41014cm?3) and relatively slow (v0? 107cm s?1) puffs are "efficient", that is the initial kinetic energies of the puffs are converted into thermal energy of the ions in the resulting plasma. Faster and less dense puffs interact little (incomplete conversion of kinetic into thermal energy). The limit between efficient and inefficient collision corresponds to that which is expected from the test particle model. The slowing down of our puffs is thus ensured mainly by ion-ion interactions. The use of electric probes and the examination of the evolution of the plasma produced by collision show that this plasma does not exhibit any rapid radial motion.