Few-Cycle Laser-Driven Electron Acceleration

We report on an electron accelerator based on few-cycle (8 fs full width at half maximum) laser pulses, with only 40 mJ energy per pulse, which constitutes a previously unexplored parameter range in laser-driven electron acceleration. The produced electron spectra are monoenergetic in the tens-of-MeV range and virtually free of low-energy electrons with thermal spectrum. The electron beam has a typical divergence of 5–10 mrad. The accelerator is routinely operated at 10 Hz and constitutes a promising source for several applications. Scalability of the few-cycle driver in repetition rate and energy implies that the present work also represents a step towards user friendly laser-based accelerators.

Figure 1

Schematic illustration of the experiment. The 8-fs laser pulses are split into a driver beam with 40 mJ energy and a weak probe beam with less than 1 mJ energy. The driver beam is focused onto a helium jet with electron densities between ${10}^{18}$ to ${10}^{20}{\mathrm{cm}}^{-3}$. The electron beam emerging from the laser-plasma interaction is characterized by a permanent-magnet electron spectrometer.

Figure 2

Typical spectra of monoenergetic electron beams from the sub-10-fs laser-driven accelerator. They have mean energies of 13.4, 17.8, and 23 MeV. The bunches carry a charge of approximately 10, 3.5, and 1.6 pC, respectively. All three spectra show remarkably few thermal background electrons, an observation that has been confirmed down to energies of 100 keV using a smaller spectrometer. These spectra were obtained with the cylindrical (subsonic) $400\mu \mathrm{m}$ gas jet.

Figure 3

Electron spectrum and transverse beam size obtained with a scintillating screen placed in the electron spectrometer and imaged onto a CCD camera. The false-color plot shows the image on the screen with the right and the bottom panels depicting calibrated line outs along the energy axis (lower plot) and the transversal axis (right side). The displayed result was obtained with a $300\mathrm{\text{−}}\mu \mathrm{m}$ supersonic gas jet and exhibits a monoenergetic peak at 24.6 MeV with 3.3% energy spread (FWHM), a divergence of 6.3 mrad (FWHM), and a charge of 3 pC. Also shown is the simulated electron spectrum (red, dashed line), which is in reasonable agreement with the experimental result.

Figure 4

Simulation results showing the physical state of the system after the laser pulse has travelled a distance of $125\mu \mathrm{m}$ inside the fully preionized plasma. Panel (a) shows the electron density (grey-scale plot) and the instantaneous laser intensity (rainbow-scale false-color diagram). The laser intensity plot is clipped along the contour line where the intensity drops by a factor of $1/e^2$ with respect to its peak value. The red line shows the longitudinal accelerating field caused by charge separation in the bubble, amounting to about $0.45\mathrm{TV}/\mathrm{m}$ at the bunch location. Panel (b) plots line outs of electron density and instantaneous laser intensity along the optical axis of the laser beam. The injected electron bunch (red) contains 4.5 pC charge and has almost an order of magnitude higher density than the background plasma.