Excitation and Control of Plasma Wakefields by Multiple Laser Pulses

We demonstrate experimentally the resonant excitation of plasma waves by trains of laser pulses. We also take an important first step to achieving an energy recovery plasma accelerator by showing that a plasma wave can be damped by an out-of-resonance trailing laser pulse. The measured laser wakefields are found to be in excellent agreement with analytical and numerical models of wakefield excitation in the linear regime. Our results indicate a promising direction for achieving highly controlled, GeV-scale laser-plasma accelerators operating at multikilohertz repetition rates.

Figure 1

Schematic diagram of the experiment layout. The propagation path of the driving pulse train is shown in red, and that of the probe and reference beams is shown in blue. The laser compressor and the components shown above the darker base are located in the vacuum chamber; all other components are mounted in air.

Figure 2

FDH and TESS analyses of linear plasma wakefields driven by a single laser pulse of energy approximately 270 mJ and pulse duration $(46\pm 7)\mathrm{fs}$. (a) shows an example of the wakefield recovered by FDH for a cell pressure of $(31\pm 1)\mathrm{mbar}$, where $\zeta =0$ corresponds to the center of the pump pulse. In the panel above, the solid line shows the amplitude of the wakefield averaged over the range $|r|\le 6\mu \mathrm{m}$; the ticks on the $y$ axis are at $\delta n_e/n_{e0}=\pm 1\%$. (b) shows a waterfall plot of Fourier transforms of the spectral interferograms, where the magnitude of the Fourier transform is plotted on a logarithmic scale. The solid white line shows the expected position of the satellites calculated from the expected plasma frequency. (c) shows, as a function of the gas pressure, the plasma period determined by the FDH and TESS analyses. The solid curve is the plasma period calculated assuming an electron density equal to twice the density of hydrogen molecules. The error bars are estimated from the uncertainty in determining the satellite separation in (b) and the plasma period in (a).

Figure 3

Relative wakefield amplitudes, as a function of gas cell pressure, measured at delay $\zeta$ between the center of the pulse train and the center of the probe pulse for a driving pulse train comprising $N$ pulses of measured pulse separation $\delta \tau$ and total energy $E$ where (a) $N=1$, $E=270\mathrm{mJ}$, $\zeta =2.2\mathrm{ps}$; (b) $N=2$, $\delta \tau =(420\pm 20)\mathrm{fs}$, $E=160\mathrm{mJ}$, $\zeta =2.5\mathrm{ps}$; and (c) $N\approx 7$, $\delta \tau =(112\pm 6)\mathrm{fs}$, $E=170\mathrm{mJ}$, $\zeta =1.3\mathrm{ps}$. Gray circles show single measurements and red diamonds show the same data averaged over pressure bins of width 4 mbar [(a) and (b)] or 2 mbar (c); the error bars are standard errors and the $y$ axes are the same for all plots. The insets show the measured driving pulse trains. The dashed lines show fits of Eq. (4), and the solid lines show the wake amplitudes calculated for the pulse trains shown in the figure insets.