Enhanced stability of laser wakefield acceleration using dielectric capillary tubes

The stability of beams of laser wakefield accelerated electrons in dielectric capillary tubes is experimentally investigated. These beams are found to be more stable in charge and pointing than the corresponding beams of electrons accelerated in a gas jet. Electron beams with an average charge of 43 pC and a standard deviation of 14% are generated. The fluctuations in charge are partly correlated to fluctuations in laser pulse energy. The pointing scatter of the electron beams is measured to be as low as 0.8 mrad (rms). High laser beam pointing stability improved the stability of the electron beams.

Figure 1

Sketch of the experimental setup. The main laser pulse (red) is focused by an off-axis parabolic mirror (OAP) into a dielectric capillary tube (or a gas jet) filled with hydrogen gas. The accelerated electrons are dispersed by a dipole magnet before reaching a scintillating screen. The leakage from a dielectric mirror in the beam path is used to record spatial and spectral intensity distributions of every laser pulse. A continuous reference beam (marked in green for clarity, but in reality of the same wavelength as the main beam) is used in combination with a position sensitive detector (PSD) for feedback to a system for active pointing stabilization.

Figure 2

Laser pulse energy delivered to the entrance of a 20 mm long, $152\mu \mathrm{m}$ diameter capillary tube (a) and the corresponding accelerated charge with an energy above 40 MeV (b) as a function of shot number. The active stabilization system is on during the first 90 pulses, and thereafter off. The stability of the electron bunch charge is significantly decreased by turning this system off.

Figure 3

False color images of the dispersed electrons on the scintillating screen from 15 consecutive shots are shown in (a), all using the same color scale. The average electron spectrum of all shots in the sequence is plotted as a solid red line in (b). The shaded area, bounded by dashed lines, shows the corresponding standard deviation from the average spectrum. Over the full energy range, the maximum standard deviation from the average spectrum is 27%.

Figure 4

In (a), the charge of the electrons in the beam, with energy above $\sim 40\mathrm{MeV}$, is plotted as a function of laser pulse energy for the first 90 shots in Fig. 2, with the active pointing stabilization system on. The estimated measurement error in laser energy of 1% is marked as a solid red line only in one data point for clarity. This figure shows a clear correlation to laser energy. In (b), a typical beam profile is shown together with a scatter map of the centroids of a sequence of 30 shots with the pointing stabilization system on.