Kinematics of femtosecond laser-generated plasma expansion : determination of sub-micron density-gradient and collisionality evolution of over-critical laser plasmas

Scott, G. G. and Indorf, G.F.H. and Ennen, M.A. and Forestier-Colleoni, P. and Hawkes, S. and Scaife, L. and Sedov, M. and Symes, D. R. and Thornton, C. and Beg, F. and Ma, T. and McKenna, P. and Andreev, A. A. and Teubner, U. and Neely, D. (2021) Kinematics of femtosecond laser-generated plasma expansion : determination of sub-micron density-gradient and collisionality evolution of over-critical laser plasmas. Physics of Plasmas, 28 (9). 093109. ISSN 1070-664X

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    Abstract

    An optical diagnostic based on resonant absorption of laser light in a plasma is introduced and is used for the determination of density scale lengths in the range of 10 nm to >1 μm at the critical surface of an overdense plasma. This diagnostic is also used to extract the plasma collisional frequency, allowing inference of the temporally evolving plasma composition on the tens of femtosecond timescale. This is found to be characterized by two eras: the early time and short scale length expansion (L < 0.1λ), where the interaction is highly collisional and target material dependent, followed by a period of material independent plasma expansion for longer scale lengths (L > 0.1λ); this is consistent with a hydrogen plasma decoupling from the bulk target material. Density gradients and plasma parameters on this scale are of importance to plasma mirror optical performance and comment is made on this theme.

    ORCID iDs

    Scott, G. G., Indorf, G.F.H., Ennen, M.A., Forestier-Colleoni, P., Hawkes, S., Scaife, L., Sedov, M., Symes, D. R., Thornton, C., Beg, F., Ma, T., McKenna, P. ORCID logoORCID: https://orcid.org/0000-0001-8061-7091, Andreev, A. A., Teubner, U. and Neely, D.;