A new optical probing method with increased interferometric sensitivity for a low-density gas jet characterization is presented. The proposed technique employs a Michelson interferometer with a self-imaging object arm, which enables the relay imaging of the object on itself, and in this way, the phase sensitivity of the device is increased by a double propagation of the object laser beam through the gas jet. The wavelength of 405 nm was chosen to further increase the sensitivity by increasing the probe wavenumber. A low-density argon gas jet with various backing pressures was characterized by our method and by a commonly used Mach-Zehnder interferometer setup showing the expected twofold increase in the signal to noise ratio in the double pass configuration.

This paper presents the response calibration of Imaging Plates (IPs) for electrons in the 40-180 MeV range using laser-accelerated electrons at Laboratoire d'Optique Appliquée (LOA), Palaiseau, France. In the calibration process, the energy spectrum and charge of electron beams are measured by an independent system composed of a magnetic spectrometer and a Lanex scintillator screen used as a calibrated reference detector. It is possible to insert IPs of different types or stacks of IPs in this spectrometer in order to detect dispersed electrons simultaneously. The response values are inferred from the signal on the IPs, due to an appropriate charge calibration of the reference detector. The effect of thin layers of tungsten in front and/or behind IPs is studied in detail. GEANT4 simulations are used in order to analyze our measurements.

Temporal profile of X-ray betatron radiation was theoretically studied for the parameters available with current laser systems. Characteristics of the betatron radiation were investigated for three different configurations of laser wakefield acceleration: typical self-injection regime and optical injection regime with perpendicularly crossed injection and drive beams, both achievable with 100 TW class laser, and ionization injection regime with sub-10 TW laser system that was experimentally verified. Constructed spectrograms demonstrate that X-ray pulse durations are in order of few tens of femtoseconds and the optical injection case reveals the possibility of generating X-ray pulses as short as 2.6 fs. X-ray pulse duration depends mainly on the length of the trapped electron bunch as the emitted photons copropagate with the bunch with nearly the same velocity. These spectrograms were calculated using novel simplified method based on the theory of Liénard-Wiechert potentials. It takes advantage of the fact that the electron oscillates transversally in the accelerating plasma wave in the wiggler regime and, thus, emits radiation almost exclusively in the turning points of its sine-like trajectory. Therefore there are only few very narrow time intervals, which contribute significantly to the emission of radiation, while the rest can be neglected. These narrow time intervals are determined from the electron trajectories calculated using particle-in-cell simulations and the power spectrum at given point in far field is computed for each electron using the Fourier transform. Spectrograms of the emitted radiation are constructed by summing contributions of individual particles, since the incoherent nature of the electron bunch is assumed.