Transverse emittance growth of proton sources from laser-irradiated sub-$\mu$m-thin planar targets

Proton bunches with maximum energies between 12 and 22 MeV were emitted from submicrometer-thin plastic foils upon irradiation by laser pulses with peak intensity of $4\times {10}^{20}\mathrm{W}/{\mathrm{cm}}^2$. The images of the protons by a magnetic quadrupole doublet on a screen remained consistently larger by a factor of 10 compared to expectations drawn from the ultralow transverse emittance values reported for thick foil targets. Analytic estimates and particle-in-cell simulations attribute this drastically increased emittance to formerly excluded Coulomb collisions between charged particles. The presence of carbon ions and significant transparency likely play a decisive role. This observation is highly relevant because such thin, partially transparent foils are considered ideal for optimizing maximum proton energies.

Figure 1

(a) Experiment setup. Protons accelerated from a sub-µm-thin foil are focused by a pair of permanent magnet quadrupoles onto a scintillator outside the vacuum chamber. (b) Differential energy distribution of protons measured with magnetic spectrometer without quadrupoles in vacuum.

Figure 2

(a) Exemplary 12-MeV proton focus with optimized focusing geometry. Optimized proton spot size for (b) 12 MeV in 31 consecutive shots and (c) varied focused energy between 12 and 22 MeV.

Figure 3

Calculated proton fluence for 12 MeV with geometrical emittance 0 (a), 0.03 (b), and 0.3 mm mrad (c). (d) Calculated proton spectra that would be expected behind an aperture with 1 mm diameter placed in the proton focus for the three cases (a)–(c). (e) Calculated FWHM (a)–(c) and measured spot size margins (shaded area).

Figure 4

Temporal evolution of the geometric emittance of protons with energies between 11 and 13 MeV as observed in 2D PIC simulations, without binary Coulomb collisions (red dash-dotted), Coulomb collisions between all particles with $p$ polarization (blue solid), collisions with carbon ions deactivated (green dotted), and laser $s$ polarization vertical to the simulation plane (black dashed). In all cases field ionization is activated.