We present a method for determining the temporal and spatial evolution of a gas jet generated by a pulsed nozzle using high-order harmonics of a titanium–sapphire laser. This radiation in the extreme ultraviolet spectral range (17–40 nm) is transmitted through the gas jet and becomes partially absorbed depending on its wavelength and the gas density. If the absorption in this spectral range shows a sufficiently strong dependence on the wavelength, as is the case for many gases including the noble gases argon, neon, and helium, it is possible to select a proper harmonic exhibiting an absorption strong enough to generate a detectable decrease of the transmitted light but still weak enough to allow a significant amount of radiation to be transmitted through the gas jet. In the case of radial symmetry the density profile can be reconstructed by means of the Abel inversion. We show that this method allows for the determination of argon neutral densities as low as $$10^{17}$$
10
17
cm$$^{-3}$$
-
3
and is also suited for other gases, such as neon and helium.
We present a novel, to the best of our knowledge, design for the spectrally resolved wavefront diagnostics of ultrashort laser pulses. The design uses quasi-self-referenced interferometry (qSRI), is completely achromatic, and avoids dispersion. The qSRI utilizes a perfect reference beam enabling an absolute measurement of the near-field spatial phase distributions for the different spectral components of ultrashort laser pulses. For this, a Mach–Zehnder geometry is coupled with a spatial filter. Combining the qSRI with a reflective grating allows for the measurement of phase fronts in the whole spectrum of a broadband laser pulse without any calibration.
We present a new acceleration mechanism for electrons taking place during the interaction of an ultrashort, nonrelativistic laser pulse with a plasma generated at the surface of a solid density target. In our experiments, the plasma is created by a laser pulse with femtosecond duration and an energy of about 1 mJ focused to intensities of above 10^{17}W/cm^{2}. We observe that the electron energies acquired by this mechanism exceed the ponderomotive potential of the laser by an order of magnitude. This result was reproduced and quantitatively confirmed by particle-in-cell simulations, which further revealed that the observed electron acceleration is based on quasistatic electric fields caused by the space charges of ponderomotively preaccelerated electrons. This acceleration process is examined in more detail by a simplified numerical model, which allows a qualitative explanation of the final electron energies.
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