Fast electrons generated by quasistatic electric fields of a fs-laser-pulse-induced plasma

We present a new acceleration mechanism for electrons taking place during the interaction of an ultrashort, nonrelativistic laser pulse with a plasma generated at the surface of a solid density target. In our experiments, the plasma is created by a laser pulse with femtosecond duration and an energy of about 1 mJ focused to intensities of above ${10}^{17}\text{W}/{\mathrm{cm}}^2$. We observe that the electron energies acquired by this mechanism exceed the ponderomotive potential of the laser by an order of magnitude. This result was reproduced and quantitatively confirmed by particle-in-cell simulations, which further revealed that the observed electron acceleration is based on quasistatic electric fields caused by the space charges of ponderomotively preaccelerated electrons. This acceleration process is examined in more detail by a simplified numerical model, which allows a qualitative explanation of the final electron energies.

Figure 1

Experimental setup consisting of the parabolic focusing mirror, aluminum target, electron spectrometer, and the imaging plate. The inset shows the sideview of the spectrometer with typical raw data including an x-ray peak on axis. The electrons are shown as a dark gray beam, the laser is indicated in light gray.

Figure 2

Experimental spectrum recorded in the specular direction. The ponderomotive limit $U_{\text{lim}}=45\text{keV}$ is indicated by a vertical dashed line. Inset: Simulated spectrum in the same direction including an exponential fit.

Figure 3

Angular distribution of the electrons in the experiment (black line, peak around ${60}^{\circ }$) compared with results from the PIC simulation (circles). Maximum kinetic energy for differentemission angles in simulation (gray line). The vertical dashed lines represent the target normal (${45}^{\circ }$) and the specular direction (${90}^{\circ }$).

Figure 4

Kinetic energy of a typical tracked electron as a function of simulation time. At $t=0$ the pulse maximum hits the target. The ponderomotive limit $U_{\text{lim}}$ is marked by the dashed line.

Figure 5

Electrostatic field along the target normal in front of the target for different times as calculated from the charge distribution of the PIC simulation.

Figure 6

Total electrostatic potential (solid lines) and kinetic energy of the electrons (dots) as a function of radial distance for different times according to the simplified model. The inset shows the potential calculated from the charge distribution in the PIC simulation at $t=0$ and the kinetic energy of the electrons at $t=80\mathrm{fs}$.