Effect of shockwave-induced density jump on laser plasma interactions in low-pressure ambient air

The dynamics of laser-produced plasma (LPP) and its shockwave-induced density jump (SIDJ) were investigated in low-pressure ambient air during the laser pulse using an optical interferometer. A tiny SIDJ could be observed clearly in ambient air with pressure as low as 3 Torr even during the rising edge of a 7 ns laser pulse. It was shown that the density jump and ionization could seed laser-induced ambient air breakdown at a relatively low pressure, which could significantly reduce laser energy deposited in the LPP. The ionization of the density jump was confirmed by dynamic emission imaging. (Some figures in this article are in colour only in the electronic version) Laser-produced plasma (LPP) in ambient gas has been investigated widely for a long time, motivated by applications of pulsed laser deposition (PLD), spectroscopic analysis, etc [1]. In recent times, interest has grown in the use of ambient gas with pressures from 10−2 to 100 Torr to mitigate debris from plasma in several LLP applications, such as a soft x-ray source for microscopy [2], an extreme ultraviolet lithography (EUVL) source [3] and laser fusion [4]. During the interaction of an intense laser pulse with a solid target in low-pressure ambient gas, plasma is generated first in the surface of the solid target. The resulting plasma expansion produces a compression wave in the solid material and a strong shockwave in the ambient gas. LPP and its compression wave into solid density material have been studied widely in the fields related to laser fusion [5]. Most of the previous work on LPP and its shockwave in ambient gas have been motivated by research relevant to PLD and mainly focus on the propagation of plasma plume in the ambient gas [6]. Some efforts to characterize LPP and its shockwave showed interesting results only at late time (after laser pulse) [7] and at high pressures [8]. Most of the physical processes of laser plasma interaction, including ionization, energy absorption, radiation transport etc occur during the laser pulse. However shockwaves and other processes, like three-body recombination, charge exchange and radiation heating occurring during the interaction of expanding plasma with ambient gas [9], could significantly modify the interaction of the laser with solid plasma. So it is highly desirable to characterize LPP and its shockwave in ambient gas during the laser pulse to understand the effect of shockwaves on laser plasma interactions. In this report, the dynamics of LPP and its shockwaveinduced density jump (SIDJ) during the laser pulse in background air were investigated using an optical interferometer with high spatial and temporal resolution. The high sensitivity of the interferometer makes it possible to observe the propagation of shockwaves in ambient gas with low pressure at early time. Our efforts focus on the influence of the shockwave on the laser plasma interactions. Experiments were carried out using a Nd:YAG laser (Continuum Surelite II 10), which can produce a laser pulse at 1.064μm with energy of 650 mJ at 10 Hz. The beam diameter is 7 mm. The temporal pulse shape of the laser is close to a Gaussian with a width (FWHM) of 7 ns. The base width, defined by the times when the intensity exceeds 10% of the peak value, is 15 ns. A small part split from the main laser beam is converted into green light (532 nm) by a type II KDP crystal and used as a probe beam. The main laser beam is focused with a lens with a focal length of 100 mm onto the surface of a 2 mm thick Al plate at normal incidence in a vacuum chamber, which can be pumped down to 10−6 Torr. The focus of the laser beam was placed behind the target surface to obtain a diameter (FWHM) of 130μm. The target is moved to maintain a fresh surface for each shot. 0022-3727/06/184027+04$30.00 © 2006 IOP Publishing Ltd Printed in the UK 4027