Shock front dynamics in the pulsed laser deposition of YBa2Cu3O7-x

The pulsed laser deposition of YBa2Cu3O7-x targets by excimer laser at fluences of 4-10 J cm-2 in low pressure oxygen backgrounds yields emissive plumes with kinetic energies of 50-200 eV, driving the formation of a shock front with Mach numbers of M = 10-50. The propagation of the shock front is independent of atomic species and adequately characterized by the Sedov-Taylor shock model if the dimensionality of the plume is allowed to deviate from ideal spherical expansion. The ideal efficiency of energy conversion from laser pulse to shock expansion is nearly unity at 1 Torr, but decreases rapidly at lower pressures, where the plume expands beyond the laser footprint during ablation. The low oxygen background pressures, 100-1000 mTorr, typically employed for the production of superconducting films is sufficient for the generation of a strong shock front with shock thickness of 5 mm to less than 0.4 mm, but too low to develop three-dimensional flow. Indeed, dimensionality of the expansion ranges from n = 0.8 to 2.4 over the background oxygen pressure range of 25-1000 mTorr. Shock strength is proportional to the Mach number and inversely dependent on pressure, indicating a thickness limited to approximately the collision mean free path