Simulation of intense laser pulse propagation in capillary discharge plasma channels

Many applications of ultra intense laser pulses require propagation in plasmas over distances of many Rayleigh lengths. Hollow plasma channels such as those produced by a capillary discharge have successfully guided pulses with small spot size (r_s to approximately 30 μm) over distances as long as 6 cm. Recent experiments have extended the capillary discharge technique to laser intensities of 10¹⁷ W/cm³. These experiments use a double capillary design that allows more control over plasma parameters. Simulations of laser propagation in these channels show that the laser pulse radius undergoes oscillations about the expected matched radius r_M at the expected frequency. The pulse may be distorted by several effects, including laser-generated ionization and plasma motion in the intense laser fields. In addition, finite pulse length corrections to the wave equation cause initially the oscillations in the laser beam size to damp in the front of the beam and grow in the back. Eventually, the oscillations are damped by phase mixing effects. Experiments to date have been at relatively high densities (approximately 10¹⁹ cm^-3). For standard laser wakefield accelerator applications, the on-axis channel density is likely to be substantially lower. As expected, simulations in this lower density regime show lower accelerating gradients, larger laser spot sizes, and higher wakefield phase velocities. The dephasing limit on single stage final electron beam energy is thus also much higher. Possible methods for producing lower density plasma channels will also be discussed.