A high density hydrogen-based capillary plasma source and its potential applications for the plasma wakefield accelerators

Summary form only given. Capillary plasmas are typically generated in insulated tubes with the internal diameter ranging from tens of micrometers to several millimeters. It is convenient to produce a pulsed discharge either by ionizing a gas that fills the capillary or by ablation of the capillary wall and electrodes. We report the generation of variable plasma densities up to 10¹⁹ cm^-3 in hydrogen-filled capillary discharges, and consider their applications as a practical plasma source for particle-beam- driven plasma wakefield accelerators. The capillary consists of a transparent, cylindrical borosilicate glass tube with inner diameter less than 1 mm. Stark broadening of the H_?? line and the ratio of the light emitted under two Balmer lines are used to characterize the plasma density and plasma temperature respectively. The time resolved plasma density is found to decay exponentially with a typical time constant of several hundreds of nanoseconds. The time delay between the discharge and the drive electron beam can therefore be tuned to reach the density appropriate for the maximum acceleration gradient. The dependence of the plasma density on the capillary geometry and gas pressure and implications of the results for beam-driven plasma accelerators are discussed.