A High-Density Hydrogen-Based Capillary Plasma Source for Particle-Beam-Driven Wakefield Accelerator Applications

We report the generation of variable plasma densities up to 10¹⁹ cm^-3 in hydrogen-filled hollow cathode 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. The plasma density is determined as a function of time, using Stark broadening of the H_alpha line, with a resolution of 50 ns, and 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 is discussed, and the results of optical studies of the discharge channel formation process are presented. The implications of the results for beam-driven plasma accelerators araree discussed.