Experimental and numerical results establishing the limits at which the Raman backscattered (RBS) radiation can be used as a diagnostic tool for real laser-plasma accelerators are presented. As an example of the acceleration structure, we consider the down-ramp plasma density electron injector operated in the plasma cavitation (bubble) regime. Transition from linear to nonlinear RBS, and to wave breaking is demonstrated experimentally and analyzed numerically and analytically.
Raman backscattering, the reflection of laser light from electron plasma waves, can provide a valuable diagnostic of the plasma density. In the simplest picture, the frequency of the backscattered light is downshifted by the plasma frequency, which is proportional to the square root of the electron density. Ultra-high power lasers introduce nonlinear and relativistic corrections to this simple relationship between the frequency shift and plasma density. In laser wakefield accelerators, the picture is further complicated by the relativistic effects associated with the ultra-short pulse duration, structured targets, bubble and sheath formation, wave breaking and phase-mixing processes that destroy the periodic structure of the plasma wave. Here we present experimental and numerical results establishing the limits at which the Raman backscattered radiation can be used as a diagnostic for real laser-plasma accelerators. As an example, we consider the down-ramp plasma density electron injector operated in the plasma cavitation (bubble) regime.
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