Lateral movement of a laser-accelerated proton source on the target's rear surface

The spatial dependence of proton acceleration at the rear surface of a target that is irradiated by high-contrast and ultraintense laser pulses is investigated. Lateral movement of the proton acceleration position at the rear surface is observed; this is tested by a two-pinhole measurement which results in the observation of protons with a narrow energy band. This drifting is only observed when relativistic-intensity laser pulses irradiate targets with a small preplasma at oblique incidence, as is confirmed by two-dimensional particle-in-cell simulations. This scenario of proton acceleration by the fast-moving sheath field leads to energy selection of the accelerated protons as a function of observing position.

Figure 1

(a) Charge-coupled device (CCD) picture of the proton spectrum at 45° laser incidence on a target measured in the direction of the target normal with a magnification imaging ratio of 1:15. The ideal parabola trace is the solid line in the figure inset. (b) CCD picture of the proton spectrum from a Mylar target. (c) CCD picture of the proton spectrum from a target measured in the direction of the target normal when the proton beam passes through two pinholes. The two-pinhole measurement makes it possible to detect the protons emitted only from a $16–18\mu \text{m}$ area of the source. (d) Schematic figure of the geometry of the two-pinhole measurement.

Figure 2

(Color online) Temporal evolution of the electron energy density and the longitudinal electric field. The electron energy density is normalized by $n_{c}m{c}^2$, and observed at (a) 140, (c) 180, and (e) 220 fs. The electric field is normalized by the laser electric field, and observed at (b) 140, (d) 180, and (f) 220 fs. The target width is $13\mu \text{m}$ and is located at $10<x<23\mu \text{m}$. The black line in (b) indicates the laser axis.

Figure 3

(Color online) (a) Temporal evolutions of maxima of sheath field intensity at different positions $y=30$, 41, and $52\mu \text{m}$. (b) Proton energy spectra observed at three positions chosen the same as in (a). The observation timings are delayed for the upper position such that $t=400$, 440, and 480 fs at $y=30$, 41, and $52\mu \text{m}$, respectively, since there is a time delay before the sheath field reaches and starts to accelerate protons.

Figure 4

(Color online) Angular distribution of laser-accelerated electrons for scale length of preplasma (a) 0.5 and (b) $3.5\mu \text{m}$.