Beam-based characterization of plasma density in a capillary-discharge waveguide

Next-generation plasma-based accelerators can push electron bunches to gigaelectronvolt energies within centimeter distances. In these devices, the accelerating force is provided by a driver pulse, either a laser pulse or a particle bunch, that loses its energy into the plasma generating huge electric fields up to tens of GV/m. The stability of such fields strongly depends on plasma density, whose exact value should be precisely known and controlled. However, currently available methods based on spectroscopic or interferometric techniques find it very difficult to measure plasma density lower than 1015–16 cm−3 in capillary-discharge waveguides. Here, we present a novel diagnostic tool that allows us to estimate the average density of a plasma capillary by probing it with an ultra-relativistic electron beam. The plasma density and the generated accelerating field are inferred by analyzing the beam longitudinal phase space after its interaction with the plasma. The results are validated by simulations showing excellent agreement.