  It is generally believed that the recollision mechanism of atomic
nonsequential double ionization is suppressed in circularly polarized laser
fields because the returning electron is unlikely to encounter the core. On the
contrary, we find that recollision can and does significantly enhance double
ionization, even to the extent of forming a "knee", the signature of the
nonsequential process. Using a classical model, we explain two apparently
contradictory experiments, the absence of a knee for helium and its presence
for magnesium.
