Abstract Details
Cluster mass fraction, size distribution and expansion anisotropy determined by fs-time-resolved optical measurements
Author: Xiaohui Gao
Requested Type: Poster Only
Submitted: 2009-04-23 12:36:52
Co-authors: Xiaoming Wang, Bonggu Shim, Mikhail Tushentsov, Alex Arefiev, Boris Breizman, and Mike Downer
Contact Info:
The University of Texas at Austin
1 University Station C1600
Austin, TX 78712
USA
Abstract Text:
Accurate interpretation and control of laser-cluster experiments require knowledge of cluster target parameters, among which the cluster mass fraction and size distribution are usually difficult to measure. Here we study the cluster mass fraction, size distribution and expansion anisotropy with fs-time-resolved measurements of refractive index, absorption and third harmonic generation (THG) of the cluster target after ionization and heating by a pump pulse. The fs-time-resolved refractive index measured with frequency domain interferometer (FDI) shows the contribution of the monomer plasma and cluster plasma in different time scale, enabling separation of cluster and monomer density in the time domain. We find the cluster fraction f=0.20 at 600 psi, consistent with recent gasdynamic simulations [1]. The cluster size distribution is retrieved by a deconvolution procedure from the measured fs-time-resolved absorption with a precalculated single-cluster absorption factor. The fs-time-resolved third harmonic generation (THG) exhibits strong transient polarization anisotropy during the expansion, even though the absorption remains isotropic. We extend our previous cold-core model [2] and find the anisotropy of THG is determined only by angular dependence of the ion density nouniformity and not by the entire ion density profile. Consequently, we are able to estimate the weak anisotropy of the ion density nonuniformity from the observed two-fold stronger THG with perpendicular, compared to parallel, pump-probe polarizations.
[1] A. S. Boldarev et al, Rev. Sci. Instrum. 77, 083112 (2006).
[2] M. V. Fomyts’kyi et al, Phys. Plasmas 11, 3349 (2004)
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