Ultralow Emittance, Multi-MeV Proton Beams from a Laser Virtual-Cathode Plasma Accelerator

The laminarity of high-current multi-MeV proton beams produced by irradiating thin metallic foils with ultraintense lasers has been measured. For proton energies $>10\mathrm{MeV}$, the transverse and longitudinal emittance are, respectively, $<0.004\mathrm{mm}\mathrm{mrad}$ and $<{10}^{-4}\mathrm{eV}\mathrm{s}$, i.e., at least 100-fold and may be as much as ${10}^4$-fold better than conventional accelerator beams. The fast acceleration being electrostatic from an initially cold surface, only collisions with the accelerating fast electrons appear to limit the beam laminarity. The ion beam source size is measured to be $<15\mu \mathrm{m}$ (FWHM) for proton energies $>10\mathrm{MeV}$.

Figure 1

Schematic of experiment. The laser pulse is focused on a foil having a modulated rear surface. Protons are first accelerated normal to the surface by the virtual cathode (VC) producing a modulation of the takeoff angle. The produced beamlets then expand with the quasineutral sheath. This adds an overall near-linear divergence to the beam initial angular modulation. Projected on a film stack far away, this results in a modulation of the dose, allowing one to image the VC stage magnified.

Figure 2

(a)–(c) Angular distributions on RCF of protons accelerated from an $18\mu \mathrm{m}$ thick Al flat target irradiated at ${10}^{19}\mathrm{W}/{\mathrm{cm}}^2$. (d)–(f) Simulated RCF images [same parameters and proton energies as in (a)–(c)] using a 3D PIC effective code.

Figure 3

(a) Image of a $48\mu \mathrm{m}$ thick Au target rear surface. (b) Resulting angular proton distribution at 8 MeV. The phase-space modulations induced by the surface structures produce corresponding intensity modulations, as illustrated in (c) for the case of the three parallel bars. Note that the modulations do not overlap; otherwise, as illustrated in (d), the proton dose between the bars would be different than the one outside, which is not the case.

Figure 4

(a) Transverse phase space plot of the proton beam [from Figs. 2a, 2b, 2c]. Error flags denote the rms angular width ($1\sigma$) of the beamlets. This gives an upper limit on envelope divergence, from which we estimate an upper limit of the emittance ${\epsilon }_N$ at each energy, as marked (in units mm mrad). (b) Same from the effective PIC simulation 600 fs after the interaction. (c) Same as (a) but in a rotated frame. The emittance ellipses using the Twiss parameters are overlaid. The transparencies of the dots encode the value of the dose at each point, following the curves shown in (d).

Figure 5

Structured proton beam at $\sim 6.5\mathrm{MeV}$ produced by a ${10}^{19}\mathrm{W}/{\mathrm{cm}}^2$ pulse incident on a $40\mu \mathrm{m}$ thick Al foil. Observed ripples in the laser focal spot that can deform the bell-shaped sheath [18] are likely to induce the observed distortions in the beam. (a) Ion part of the neutralized beam detected; the RCF is placed 59 mm from the target (b) Non-neutral ion part of the beam.