Plasma diagnostics of a capillary discharge-produced plmsma to guide an ultrashort, intense laser pulse and its electron acceleration

Summary form only given. Optical guiding of short intense laser pulses over many Rayleigh lengths is important in many applications such as laser-driven plasma accelerators, harmonics generation, and X-ray lasers. Besides the diffraction character of light, the defocus and refraction processes formed by the partially ionized plasma also greatly limit the propagation length of laser. To overcome this limitation, channels in which the refractive index is a function of radial distance from the axis of propagation are required to guide the laser. In the guiding of laser in preformed plasma channel, the guiding properties are strongly depended on the transverse profile and the spot size of the laser pulse, the transverse profile of the plasma and the transverse of the capillary, and the electron density. In gas-filled capillary discharge-produced plasma channel, the waveguide is formed by the discharged ionization of the gas. We developed a plasma waveguide for propagating an intense laser pulse using a capillary discharge-produced plasma. The capillary consisted of a diameter of 300-500 mum and a length of 10-15 mm, respectively. For the present work, the peak discharge voltage and current were 30 kV and 500 A with a pulse width of 100 ns (FWHM), respectively. A electron density and a time-resolved electron temperature were measured to be of the order of 10¹⁷ cm^-3 and a few eV using a laser interferometer and a spectronmeter coupled an ICCD camera, respectively. The width of a plasma channel was 200-300 mum (FWHM). In the guiding experiment, the laser pulse of the central wavelength of 800 nm from a CPA Ti: sapphire laser with a pulse width of 130 fs (FWHM) was used. A peak intensity of the laser pulse was 1 x 10¹⁶ W/cm² with a spot diameter of 30 mum (FWHM) in vacuum. We have demonstrated the guiding of a laser pulse over length of up to 10 mm, which corresponded to 10 times the measured Rayleigh length. Electron acceleration- - experiment was preformed by the capillary. Suprathermal electrons with the energy around 1.6 MeV was measured at the output of the capillary with the blue-shifts of the transmitted laser pulse, which was attributed to the temporal change of the plasma electron density.