Pinhole closure measurements

Spatial-filter pinholes and knife-edge samples were irradiated in vacuum by 1053-nm, 5-20 us pulses at intensities to 500 GW/cm(2). The knife-edge samples were fabricated of plastic, carbon, aluminum, stainless steel, molybdenum, tantalum, gold and an absorbing glass. Time-resolved two-beam interferometry with a 40-ns probe pulse was used to observe phase shifts in the expanding laser-induced plasma. For all of these materials, at any time during square-pulse irradiation, the phase shift fell exponentially with distance from the edge of the sample. The expansion was characterized by the propagation velocity V-2 pi of the contour for a 2 pi phase shift To within experimental error; V-2 pi was constant during irradiation at a particular intensity,and it increased linearly with intensity for intensities <300 GW/cm(2). For metal samples, V-2 pi exhibited an approximate M-0.5 dependence where M is the atomic mass. Plasmas of plastic, carbon and absorbing glass produced larger phase shifts, and expanded more rapidly, than plasmas of the heavy metals. The probe beam and interferometer were also used to observe the closing of pinholes. With planar pinholes, accumulation of on-axis plasma was observed along with the advance of plasma away from the edge of the hole. On-axis closure was not observed in square, 4-leaf pinholes.