Density measurement in a laser-plasma-accelerator capillary using Raman scattering

Abstract

Laser wakefield accelerators have shown 1 GeV electron beams with some 10 pC charge from centimeter-length gas capillaries. The electrons are accelerated by the field of a plasma wave trailing an intense laser pulse. For improving the stability, electron injection and acceleration should be separated. One possible scheme is self-injection with a plasma density gradient and subsequent acceleration at constant density. This can be realized by embedding a high-density gas jet into a capillary. A critical parameter for this scheme to work is the realization of a specific density gradient, therefore a robust measurement is desirable. A new method utilizing the density dependence of Raman scattering has been used to characterize the high-density region of a neutral gas within a capillary with a few ten micrometer longitudinal resolution. This allowed us to measure a density drop of a factor of 4 within 200 micrometers.