A laser parameter study on enhancing proton generation from microtube foil targets

Abstract

The interaction of an intense laser with a solid foil target can drive [Formula: see text] TV/m electric fields, accelerating ions to MeV energies. In this study, we experimentally observe that structured targets can dramatically enhance proton acceleration in the target normal sheath acceleration regime. At the Texas Petawatt Laser facility, we compared proton acceleration from a [Formula: see text] flat Ag foil, to a fixed microtube structure 3D printed on the front side of the same foil type. A pulse length (140-450 fs) and intensity ((4-10) [Formula: see text] W/cm[Formula: see text]) study found an optimum laser configuration (140 fs, 4 [Formula: see text] W/cm[Formula: see text]), in which microtube targets increase the proton cutoff energy by 50% and the yield of highly energetic protons ([Formula: see text] MeV) by a factor of 8[Formula: see text]. When the laser intensity reaches [Formula: see text] W/cm[Formula: see text], the prepulse shutters the microtubes with an overcritical plasma, damping their performance. 2D particle-in-cell simulations are performed, with and without the preplasma profile imported, to better understand the coupling of laser energy to the microtube targets. The simulations are in qualitative agreement with the experimental results, and show that the prepulse is necessary to account for when the laser intensity is sufficiently high.

Figure 1

(a) Sketch of the experimental setup with the Texas Petawatt Laser, not to scale. After the plasma mirror, the laser is normally incident onto the front surface, either directly onto a 1 $\mathrm{\mu }\text{m}$ flat Ag foil, or the microtube array. A Thomson Parabola (TP) ion spectrometer is placed along with the target normal with the EPPS placed off-axis. For several shots on each configuration, a radiochromic film (RCF) stack was placed 4 cm from the target rear surface in order to capture the full proton beam. This diagnostic is not shown in the setup because it blocks the target line-of sight to the TP and EPPS. (b) Scanning electron microscopy (SEM) image of a $3\times 5$ microtube array 3D printed on a 1 $\mathrm{\mu }\text{m}$ Ag foil. The large array size relative to the laser spot size eases alignment, and guarantees the laser will hit the array.

Figure 2

Proton and electron spectra for each laser configuration. (a–c) Maxwell–Boltzmann fits to the RCF spectra, with good correspondence to the Thomson parabola at $0^{\circ }$ (d–f). The full energy configurations (top and bottom rows) show a minimal difference between proton spectra from flat and microtube targets. The middle row, however, indicates that for the 28 J configuration, microtubes enhance maximum proton energy by 50%, and total proton yield by 3$\times$ (b,e). A corresponding enhancement in electron spectra, namely a 12% temperature increase, is also observed in (h). The width of each spectrum represents the error bands, as averaged over $\sim$ 3–5 shots for RCF, and $\sim$ 10 shots for the TP and EPPS. Key takeaways from the spectra are summarized in Table 2.

Figure 3

Comparison of flat and microtube ($3\times 5$) targets for the optimum laser configuration (28 J, 140 fs). The longitudinal electric field in the flat foil (a) is outperformed by the microtube target (b), shown at approximately laser peak arrival at 233 fs. This stronger electric field is indicative of a dramatic increase in the acceleration of electrons from microtube targets (c). The distribution of forward-accelerated electrons from the foil is similar, regardless of whether a target structure is in place. Microtubes provide an additional source of hot electrons that dominates the energy spectrum. As the simulation progresses, the maximum proton energy doubles relative to flat targets (d). Evolution of extraction from the microtube walls, respectively at 110, 220, and 330 fs (e–g). The modulation of the incident laser field is shown for both target types at 266 fs (h).

Figure 4

The role of the preplasma for structured targets at high drive energy (82 J). (a) FLASH simulations produced a density profile generated by the prepulse, with a contour (dashed line) indicating the overcritical region. These data were extracted 3 ps before the peak of the main pulse arrived. (b,c) Compare the spectra of forward-accelerated electrons without and with the preplasma, respectively, from each target component. Tube structures are the dominant source of hot electrons. The over-critical preplasma in (a) reduces the total number of electrons accelerated > 100 keV by $\sim$ 50% (c). The density profile of (a) was assumed to be cylindrically symmetric for the EPOCH simulation.