Optical reflectivity of dense plasmas produced by laser driven shock waves

Abstract

The time history of the optical reflectivity of a dense plasma at the confined interface of a planar aluminium-plastic target was recorded through the transparent material, during the shock emergence from the aluminium. The shock wave was produced by the irradiation of the aluminium side of the target with a long-pulse (∼2.4 ns FWHM) 1.06 µm laser light at irradiances up to 7 × 10 13 W cm −2 and the interface probed by a low-energy 0.53 µm laser pulse. The reflectivity of the probe laser beam at the interface was recorded across the shocked region. In the central shocked region the reflected probe light decreases in less than 200 ps, as the shock arrives at the rear side, and remains nearly constant at later time. The experimental results are interpreted by employing a simple model. It is shown that the reflection at early times can be explained by Fresnel reflectivity of the aluminium which decreases due to a temperature rise and an absorbing plasma in front of the shock. At later time a WKB approximation is used to calculate the reflectivity from the critical density of the dense plasma at the shock front. This reflectivity is very sensitive to the electron-ion collision frequency which is measured by fitting the model and experimental results.

Real temperature calculation of shock wave driven by sub-nanosecond laser pulses

“…where I [λ, T (x, t)] is Planck's law for blackbody radiation intensity of the shock front at wavelength λ at time t and temperature T , i (λ) is the transmission function for a filter 'i' incorporating the streak camera spectral response, and k b is the absorption coefficient which is a density, temperature, and wavelength dependent parameter. We used an absorption coefficient k b for unloading shock that described in [9,10,14] and given as:…”

“…Therefore, luminosity technique has been used for both temperature and shock velocity measurements [5][6][7][8]. The optical emission, and reflectivity of a confined conductor-insulator interface, compressed and heated by a shock wave was also reported [9,10]. Using stepped targets, simultaneous measurements of the shock velocity and brightness (or colour) temperature of shock front at the time of breakthrough into the vacuum, was performed [11].…”

Dense matter characterization by X-ray Thomson scattering

“…This leads to either partial ionization and hence the complication of neighboring bound states and dominance of electron atom collisions, or the production of a transient over-ionized non-equilibrium state which will quickly recombine by three-body recombination. Surface probing of any overdense plasmas is difficult to interpret [6,[35][36][37][38][39] because density gradient scalelengths of the order of λ/2π dramatically modify observables such as reflectivity and phase modulation [36,40,41].…”

Optical Absorption of Ultra-short Laser Pulse in Multilayer Dense Plasmas

“…The interface will typically expand on the order of the skin depth and the absorption of the laser will be changed by this expansion and cannot be ignored [1]. Using low intensity probing of plasma, many efforts also have been made for understanding the electronic properties of solids in non-equilibrium conditions [9,10]. There have been some attempts on understanding the absorption mechanisms in ultra-short pulse irradiating solid targets.…”