Proton acceleration in the electrostatic sheaths of hot electrons governed by strongly relativistic laser-absorption processes

Two different laser energy absorption mechanisms at the front side of a laser-irradiated foil have been found to occur, such that two distinct relativistic electron beams with different properties are produced. One beam arises from the ponderomotively driven electrons propagating in the laser propagation direction, and the other is the result of electrons driven by resonance absorption normal to the target surface. These properties become evident at the rear surface of the target, where they give rise to two spatially separated sources of ions with distinguishable characteristics when ultrashort $\left(40\mathrm{fs}\right)$ high-intensity laser pulses irradiate a foil at $45°$ incidence. The laser pulse intensity and the contrast ratio are crucial. One can establish conditions such that one or the other of the laser energy absorption mechanisms is dominant, and thereby one can control the ion acceleration scenarios. The observations are confirmed by particle-in-cell (PIC) simulations.

Figure 1

(Color) Spatially resolved CCD pictures of Čerenkov radiation (arrows show emission direction) at $45°$ laser incidence resulting from electrons propagating: (a) In the laser direction and perpendicular to it; or (b) perpendicular to the laser direction only; and (c) in the laser direction only. (d) Recorded Čerenkov light distributions as a function of Al-target thickness. With a $3\mathrm{\mu }\mathrm{m}$ target the two components are merged together, while at $6\mathrm{\mu }\mathrm{m}$ they are clearly separated. When the target thickness is $15\mathrm{\mu }\mathrm{m}$, further broadening of the electron beams propagating inside the target occurs and blurs the Čerenkov emission.

Figure 2

CCD picture of a proton spectrum at $45°$ laser incidence on a $6\mathrm{\mu }\mathrm{m}$ Al target, measured in the direction of the target normal. The inset shows proton spectra from a $3\mathrm{\mu }\mathrm{m}$ target thickness at similar irradiation conditions. Two parallel proton parabolas show the existence of two sources of proton emission. The lower trace arises from the source created by electrons driven by resonance absorption, and the upper one from the source created by ponderomotively driven electrons.

Figure 3

(Color) Electron momentum distribution observed in PIC simulation at the target rear surface for the (a) “small” $L=0.3\mathrm{\mu }\mathrm{m}$ scale length preplasma case, and for the (b) $L=3.5\mathrm{\mu }\mathrm{m}$ “large” scale length preplasma case. Laser pulses irradiate the target from the left boundary. In the large preplasma case, resonance absorption plays the dominant role in accelerating electrons in the target-normal direction.

Figure 4

(Color) Ion density distribution for a (a) large $(L=3.5\mathrm{\mu }\mathrm{m})$ preplasma and a (b) small $(L=0.3\mathrm{\mu }\mathrm{m})$ preplasma. Laser irradiation is from the left boundary, where the spot size is $15\mathrm{\mu }\mathrm{m}$, and the spot center corresponds to $y=8.5\mathrm{\mu }\mathrm{m}$ at the left boundary. Two arrows in (b) indicate the trace of the laser spot center and the target-normal projection of the spot center at the critical surface. Magnified figures around the laser focal point are plotted in (c) and (d) for large and small preplasma cases, respectively.