Interaction of high density, thin, gas jets with ultrashort laser pulses at 1 kHz repetition rates*

We have investigated the interaction of thin (50 \textasciitilde 150 microns), high-density (10$^{19}$ \textasciitilde 10$^{21}$ cm$^{-3})$ gas jets with 30 fs, \textgreater 5 mJ, 800 nm laser pulses at a 1 kHz repetition rate. Capillary tubes with various diameters (50 \textasciitilde 500 microns) are used to produce dense gas jets in continuous flow at high backing pressure (\textasciitilde 1000 psi) and cryogenic temperature (130 K). The gas/plasma density profiles are characterized by optical interferometry, and Rayleigh/Mie scattering is measured to characterize coexisting atomic clusters. Our result shows a peak plasma density of 10$^{21}$ cm$^{-3}$ near the nozzle orifice, approaching the critical plasma density at 800 nm laser wavelength. This high density plasma efficiently produces X-rays and terahertz radiation, as well as energetic electrons and ions at high-repetition-rates (kHz), without generating unwanted debris as in solid targets.