PWFA Plasma Source Density and Width Measurement by Robust Optical Methods*

The quality of the accelerated electron beam of an electron beam-driven plasma wakefield accelerator (PWFA) depends significantly on the density profile of the plasma source. Therefore, proper plasma diagnostic methodologies are essential. The PWFA plasma source is a long narrow, laser-ionized gas filament formed ahead of the arrival of the electron beam. It is less than 1 mm in diameter and up to 1 m in length with a typical core density of 10^(16-18) cm^-3. Its geometry and density range make it challenging to diagnose. Laser shadowgraphy and plasma afterglow optical imagery are two easily implemented experimental methods that hold complementary strengths and allow for coarse diagnosis of the PWFA plasma source. The former has an ultrafast response but presents a longitudinally integrated signal. The latter provides fine-scale longitudinal resolution, but its signal is time-integrated. To interpret such time-integrated signals correctly, the temporal evolution of the plasma filament formation and decay processes are modeled. Multiple linear regression is performed on the simulated data to select the heavy-weighted first-order and second-order dependence of the experimental signals on the initial plasma density and width. Experimental results supported by simulations are presented.