Abstract Details
Size distribution of microclusters in laser-irradiated plasmas
Author: Alexey Arefiev
Requested Type: Oral Only
Submitted: 2009-04-24 17:54:02
Co-authors: M. Tushentsov, B. Breizman
Contact Info:
Institute for Fusion Studies, The University of Te
1 University Station (C1500)
Austin, TX 78712-0
USA
Abstract Text:
Laser interactions with a mixture of a gaseous plasma and microclusters exhibit a number of interesting phenomena, including fusion neutron production and high-harmonic generation. Microclusters are generated in laboratory experiments by a supersonic gas jet expanding into a vacuum. Gas condensation produces small liquid density droplets and a high-intensity laser pulse converts them quickly into dense plasmas (microclusters). It is usually difficult to measure the initial cluster-size distribution, which introduces significant uncertainties in quantitative interpretation of laser-cluster experiments. Our new approach to finding the cluster-size distribution is based on the notion that the absorbed energy in a two-pulse pump-probe experiment, measured as a function of the delay between the pulses, carries sufficient information to allow reconstruction of the distribution. The role of the pump pulse is to ionize microclusters and heat the electrons inside them. The resulting hot electron pressure drives cluster expansion. A delayed low-intensity probe pulse, with duration much shorter than the cluster expansion time, follows the pump pulse. The energy lost by the probe pulse in a single cluster is determined by the evolving cluster density profile at the location of the plasma resonance. As a cluster expands, its peak density decreases and the resonance eventually disappears. The corresponding expansion time is shorter for smaller clusters than for larger ones. Therefore, the dependence of the absorbed energy on the time delay can be used to find the cluster-size distribution. The dominant absorption mechanism is resonant absorption in those clusters whose peak density is above the critical density, whereas the contributions from cluster with subcritical density and the low-density ambient plasma are negligible. This feature allows us to recover the cluster-size distribution directly from the measured absorption without any knowledge about the ambient plasma. We demonstrate feasibility of the technique by analyzing the data from recent pump-probe experiments at the University of Texas [see presentation by X. Gao, X. Wang, B. Shim, M. Tushentsov, A. Arefiev, B. Breizman, and M. Downer titled “Cluster mass fraction, size distribution and expansion anisotropy determined by fs-time-resolved optical measurements”].
Comments:
This presentation should follow the presentation by X. Gao et al. titled “Cluster mass fraction, size distribution and expansion anisotropy determined by fs-time-resolved optical measurements”