Group Velocity Measurements in Laser-Heated Capillary Discharge Waveguides for Laser-Plasma Accelerators*

To date, the most energetic electron beams from laser-plasma accelerators have been produced using gas-filled capillary discharge waveguides, which increase the acceleration length by mitigating diffraction of the driving laser pulse. [1] To reach higher electron beam energies, lower plasma density is required to reduce bunch dephasing. However, confinement of the driver is reduced for lower plasma density, reducing the acceleration length. A laser-heated capillary discharge waveguide, where the discharge is heated by a coaxial laser pulse, was proposed to create a steeper density gradient at lower density. [2] Here, spectral-interferometric group velocity measurements in laser-heated capillary discharges are presented. Modification of plasma density and matched spot-size by laser-heating is demonstrated. Trends in on-axis plasma density and matched spot-size with discharge and laser parameters are characterized. [1] W.P. Leemans, et al, Physical Review Letters 113, 245002 (2014). [2] N.A. Bobrova, et al., Physics of Plasmas 20, 020703 (2013).