Rayleigh-Taylor instability of radiation pressure driven foils: 2D effects

Rayleigh-Taylor instability of radiation pressure accelerated ultra-thin foils of different thickness profiles and initial curvature is investigated in two dimensions using numerical simulations. The convex curvature of the foil (when viewed from the rear side of the foil) provides radially inward motion to the off-axis ions countering the radial divergence due to the Gaussian intensity distribution of the laser. Nonuniform foils, having maximum thickness on laser axis have similar effect. When a small ripple is superimposed on the foil on the scale of laser wavelength, the radiation pressure acts nonuniformly on the microscopic scale and the perturbation grows as the foil moves. After a certain distance of travel, the foil crests turn into cusps and the phase of quasi mono-energy ion acceleration terminates. The planar foil with uniform laser has a strong growth of Rayleigh-Taylor instability (RTI). The foils with curvature and nonuniform thickness have marginally lower growth rate. However, under all circumstances, RTI is found to induce additional energy spread, in radiation pressure acceleration (RPA) of thin foils.