Plasma channels for multi-GeV laser-plasma accelerators using discharges in structured gas cells

Summary form only given. Laser-plasma accelerators use intense laser pulses to produce relativistic plasma density structures that can trap and accelerate electron bunches. The energy gain can be strongly enhanced by extending the acceleration length, where the laser stays focused to high intensity, to a distance much longer than the Rayleigh length. This can be achieved by propagating the laser in a preformed and fully-ionized cylindrical plasma with a parabolic density profile acting as a "graded-index optical plasma fiber". In this work we present a method to produce plasma channels adequate for compact multi-GeV electron acceleration. It uses a high-voltage and high-current discharge between two hollow conical electrodes, through a structured gas cell. This fast discharge produces a plasma line that evolves to a plasma channel by free radial expansion of the plasma inside the gas cell. We also present new experimental results demonstrating a significant improvement of the plasma smoothness and reproducibility over the previous work, by using a low-current simmer discharge to partially pre- ionize the interaction zone. The plasma channels are 2 cm long but this length may be extended to about 10 cm by increasing the length of the plasma cell. The plasma can be made of low Z gases (H₂ or He) so full ionization can be achieved to avoid ionization induced defocusing of the intense laser pulses. Plasma channels with axial electron densities in the range 1 - 5times10¹⁷ cm^-3 can be produced making this a potential target for multi-GeV accelerators with current technology high-power lasers.