Experimental setup. The igniter pulse and delayed Ti:sapphire pulse are focused by the same parabolic mirror to the entrance of the capillary.
(a) Typical radial density profile for a 3 cm long, diameter polyethylene capillary before the arrival of the high intensity pulse. (b) Guiding potentials derived from the profile shown in (a) for three values of the guided beam power. Matched (minimal) guided beam radii are specified for each potential.
Timing window over which good guiding (high signal integrated within a radius of the peak) is achievable. Laser power is accounted for. Delay is defined as timing between arrival of the igniter and high intensity pulses at the capillary entrance. The discharge is triggered by the igniter pulse. All single shots are from a single experiment at 8 TW, which were known to be at optimal alignment and discharge parameters. Timing jitter was 5 ns. Vertical error bars enclose one standard deviation of all shots with identical parameters taken over many experimental runs.
Top images are reference at focus and capillary rear exit without discharge. Rows two, three, and four show guided laser modes at the capillary exit for various powers. Pointing fluctuation is real.
Laser is unaffected until plasma has expanded to fill capillary. Vertical axis is integrated count of laser light, which is higher in frequency than the blue edge of the laser spectrum in vacuum, normalized to the total integrated counts over the entire spectrum. The timing window for guiding is approximately between the green vertical bars. All single shots are from a single experiment at 8 TW. Vertical error bars enclose one standard deviation of all shots with identical parameters taken over many experimental runs. A trace of the discharge current is overlaid.
Optical spectra (individually normalized) of transmitted laser light in vacuum and with discharge at various delay and laser power. At short delay, blueshift is weak and insensitive to delay, with the shift of peak increasing slightly with increasing delay. The peaks from the 8 TW shots obey this order. At larger delay, carbon ions from the wall have reached the laser axis, leading to strong ionization blueshift with shift proportional to delay. For higher power, blueshift is more prevalent.
Quality of guiding is diminished when strong blueshifting is observed. Axes and error bars are the same as in Figs. 3 and 5. All shots are from a single experiment at 8 TW.
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