Petawatt Laser Guiding and Electron Beam Acceleration to 8 GeV in a Laser-Heated Capillary Discharge Waveguide

Guiding of relativistically intense laser pulses with peak power of 0.85 PW over 15 diffraction lengths was demonstrated by increasing the focusing strength of a capillary discharge waveguide using laser inverse bremsstrahlung heating. This allowed for the production of electron beams with quasimonoenergetic peaks up to 7.8 GeV, double the energy that was previously demonstrated. Charge was 5 pC at 7.8 GeV and up to 62 pC in 6 GeV peaks, and typical beam divergence was 0.2 mrad.

Figure 1

(a) The measured discharge current temporal profile (green), along with the simulated on-axis temperature (black), and density (magenta). The heater pulse arrived at time 300 ns, causing the short-duration temperature rise and density reduction. (b) The matched spot size vs density for the $800\mu \mathrm{m}$-diameter capillary with discharge alone (black), with the heater pulse arrival at the peak of discharge current (red squares), and with the heater pulse arrival 300 ns (blue circles) and 420 ns (green triangles) after the peak of the discharge current. The matched spot size is measured at the peak of the heater laser pulse and the error bars correspond to the standard deviation of the data.

Figure 2

Schematic layout of the BELLA LPA, including the heater laser system for enhancing the capillary discharge waveguide.

Figure 3

Fluence profiles of 850 TW laser pulses: (a) measured at the entrance of the capillary (laser focal plane); (b) measured at the exit of the 20 cm-long capillary for $n_0=3.4\times {10}^{17}{\mathrm{cm}}^{-3}$ and $r_m=69\mu \mathrm{m}$; (c) retrieved from simulation at the same location as (b), and; (d) measured at 5.4 cm past the focal location with the capillary removed from the beam path. Each image has side equal to the capillary diameter of 0.8 mm. The measured results are from the driver mode imager (see Fig. 2).

Figure 4

(a)–(e): Electron beams measured by the magnetic spectrometer for $n_0=3.4\times {10}^{17}{\mathrm{cm}}^{-3}$, $r_m=69\mu \mathrm{m}$ and laser power 850 TW. The driver laser pulse arrival was timed with the peak of the heater pulse. The heater pulse arrived 300 ns after the peak of the discharge current, except for (e), where the delay was 420 ns, and the heater-induced density reduction was measured to be larger, with $n_0=2.7\times {10}^{17}{\mathrm{cm}}^{-3}$ and $r_m=61\mu \mathrm{m}$. The white dashed lines show the regions that are plotted in the right hand column, which shows the detailed spectrum of the highest energy peaks. The electron beam spectrum simulated by inf&rno using the marple-retrieved density profile (with $n_0=3.4\times {10}^{17}{\mathrm{cm}}^{-3}$) is shown in (f). In (g) a simulation is shown for the parameters of (e) using a transversely parabolic and longitudinally uniform density profile.