Characterization of cluster/monomer ratio in pulsed supersonic gas jets

Abstract

Zhao, who shared happiness and helped me weather the stress and frustration during my PhD life.Finally, I owe much to my parents, Fangjun Gao and Yadi Gong, and my sister, Yangbo Gao, for their endless love and unconditional support throughout my life, especially in the past seven years when I was studying overseas. Xiaohui GaoThe University of Texas at Austin Cluster mass fraction is an elusive quantity to measure, calculate or estimate accurately for pulsed supersonic gas jets typical of intense laser experiments. The optimization of this parameter is critical for transient phasematched harmonic generation in an ionized cluster jet at high laser intensity. We present an in-depth study of a rapid, noninvasive, single-shot optical method of determining cluster mass fraction f c (r,t) at specified positions r within, and at time t after opening the valve of, a high-pressure pulsed supersonic gas jet. A ∼ 2 mJ fs pump pulse ionizes the monomers, causing an immediate drop in the jet's refractive index n jet proportional to monomer density, while simultaneously initiating hydrodynamic expansion of the clusters.The latter leads to a second drop in n jet that is proportional to cluster density and is delayed by ∼ 1 ps. A temporally stretched probe pulse measures the 2-step index evolution in a single shot by frequency domain holography, enabling recovery of f c . We present the theory behind recovery of f c in detail.vii We also present extensive measurements of spatio-temporal profiles f c (r, t) of cluster mass fraction in a high-pressure supersonic argon jet for various values of backing pressure P , and reservoir temperature T .viii

Ultrafast Laboratory Snapshots and Movies of Intense Laser-Plasma Interactions

“…Main panel: cross sectional profile of cluster fraction in a supersonic gas jet, obtained from single-shot measurements like those shown in inset. Figure 1 shows several examples of FDH images that I will discuss in detail: (a) a plasma wake behind a multiterawatt 30 fs drive laser pulse [1]; (b) a plasma "bubble" that, because it has higher refractive index than the surrounding plasma, traps co-propagating probe light within its profile, thereby illuminating its structure [2]; (c) a 2step ionization front (inset, red curve) in a clustered, supersonic gas jet due to immediate monomer ionization followed by delayed cluster expansion, that enables rapid profiling of cluster fraction within the jet (main panel) [3].…”