A crucial question in accelerating a foil to relativistic velocity is its hydrodynamic stability. In particular the relativistic Rayleigh - Taylor (RT) instability was calculated (Pegoraro and Bulanov, 2007) as described in Equation (2). xNR and xR are accordingly the non-relativistic and relativistic development of the instability for an initial disturbance x0 and lT is the target dimension orthogonal to the x amplitude. For example, if lT ¼ 10 mm, x0 ¼ 10 nm, r0 ¼ 1 g/cm3, l ¼ 0.1 mm, and assuming that the foil breaks for x ¼ 10 (i.e. Dx w l) then I ¼ 1024 W/cm2 will cause the foil to break at 2.5fs for the non-relativistic case while in the relativistic regime the foil is stable 50fs. This behavior is