Fig. 1. The electron diagnostics setup, containing a gas cell, a dipole magnet, and two scintillating screens, DRZ1 and DRZ2. The entire setup is placed inside vacuum chambers. The laser and electron bunches propagate from right to left.

Fig. 2. A drawing of the gas cell. A 532-nm laser is focused through the top window onto the surface of a metal plate and generates the nanoparticles through laser ablation. The nanoparticles mix with the helium gas and fill the volume of the gas cell uniformly. The Texas Petawatt Laser enters the gas cell through a 3-mm-diameter pinhole and generates electrons that exit the gas cell through another 3-mm pinhole.

Fig. 3. A 2D drawing of the setup containing the gas cell and diagnostics. The inset shows the measured magnetic field map of the dipole magnet. The laser and electron bunches propagate from right to left.

Fig. 4. A simplified setup used to calculate the error in the centroid electron energy. The laser and electron bunches propagate from right to left.

Fig. 5. The electron spectrometer total relative error in energy retrieval as a function of the electron energy.

Fig. 6. The parameter space for the DRZ scintillating screens 1 and 2 (Detector 1 and Detector 2, respectively) for various initial electron pointings and energies. The experimental electron spectrum is matched on the two screens, and the corresponding pointings and energies are retrieved for each spectrum feature or each bunch.

Fig. 7. A typical shot recorded without nanoparticles and shown on both DRZ screens. The difference in charge and divergence is due to the different responses of the DRZ screen and the optical system assembly.

Fig. 8. Electron energy spectra of the two most energetic shots recorded by DRZ2. The energy spectra were recorded simultaneously on two consecutive screens to correct any off-axis electron beam pointing. The top spectrum shows a high energy bunch with the centroid at 10.4 ± 1.93 GeV, a 3.4 GeV rms energy spread, a 340 pC electric charge (2.9 nC total charge), and a 0.9 mrad rms divergence. The bottom energy spectrum shows a 4.9 ± 0.39 GeV centroid electron bunch with a tail energy that extends beyond 10.4 GeV and has a 2.2 nC total charge with a 1.4 mrad rms divergence. The energy spread from the electron beam divergence has not been deconvolved, and its value could be lower than estimated.

Fig. 9. Two of the most energetic electron spectra as viewed on DRZ3 (placed 5.855 m away from the exit of the gas cell) with an energy cutoff of ∼2 GeV.

Fig. 10. Data showing the electron energy spectra with energies above 2 GeV recorded by DRZ1 (left column) and DRZ2 (right column). The DRZ1 screen was placed 1.568 m from the exit of the gas cell, and DRZ2 was placed at 2.556 m from the exit. The first two shots show the highest electron energies beyond 10 GeV.

Fig. 11. The dependence of the maximum (or cut-off) electron energy on the position of the laser focal plane in the gas cell. It can be observed that all the shots with electron energies above 3.5 GeV are grouped around 7 ± 1 mm. The red curve is drawn to guide the eye, and the entrance pinhole is at 0 mm where the laser with a vacuum Rayleigh length of ∼1.5 cm is focused.