Mixing experiments have been performed using Si/Al/Au trilayer targets accelerated with laser beams smoothed by random phase plates. The Al-Au mixing is detected by heating the rear of the target with a probe beam and observing the Al line emissions. A null-mix experiment has been performed with Si/Al/Si, which is stable according to Rayleigh and Taylor, in order to check that the probe beam does not induce a specific mixing. Simulations with a hydrocode have been made to characterize the main features of the target when the instabilities occur. In the case of Si/Al/Au targets, the Al emission indicates a mixing of Al and Au for a driver irradiance as low as 1013 W/cm2 and that most of the Au is mixed with Al