Spatial and temporal measurements of plasma/gas densities in a capillary gas-cell for laser-plasma accelerators

Abstract

The measurement of the plasma density is of crucial importance in laser-plasma accelerators. We measured the spatial and the temporal plasma/gas densities in a capillary gas-cell by using the interferometric and the Raman scattering methods. By using transverse interferometry, we were able to measure directly the spatial distribution of the electron density between the entrance and the location of gas injection in the capillary, and we compared the result with that from a computational fluid dynamics (CFD) simulation. By using longitudinal interferometry along the capillary's axis, we were able to obtain the temporal evolution of the density of hydrogen gas. Furthermore, we used the stimulated Raman forward scattering method with a terawatt-level high-power laser beam for real-time measurement of the localized electron density inside the capillary. We found that the interferometry and the Raman scattering methods gave consistent results for the plasma density.