Development of a double-grating differential interferometer for laser-plasma diagnostics*

In laser-plasma acceleration experiments, the characteristics of the accelerated particle beams are strongly influenced by the plasma density, making precise diagnostics of this parameter critically important. Generally, interferometry can be employed to measure the density of such plasmas. However, the reliability of this method can be limited in certain regions due to the noise amplification of Abel inversion, a common issue encountered with this method. Therefore, differential interferometry would be better for diagnostics of a plasma with a high gradient in density as it measures the phase shift derivative directly. For this purpose, a special shearing interferometer with two gratings was developed to apply differential interferometry in laser-produced plasma diagnostics. Compared with other types of interferometers, our design provides a significant advantage in that the shear distance, shear direction, and fringe width can be controlled independently, enabling precise adjustment of these parameters. This double-grating-based differential interferometer was demonstrated for diagnostics of the laser-produced plasma by focusing a 1 TW/35 fs Ti:sapphire laser pulse in a gas jet with a 100 μm orifice diameter. It was verified that our differential interferometer can give more reliable and precise plasma densities, particularly for plasmas having a steep spatial density gradient. In this presentation, detailed results will be given.