Compared to their conventional counterpart, laser plasma-based ion accelerators offer considerably higher acceleration gradients, opening the way to more compact laser-driven accelerator facilities. In this context, deuteron acceleration has been used for laser-based neutron sources, as deuterons at relatively low kinetic energy can efficiently generate neutrons. While double-pulse and chirp effects have been explored for proton acceleration under radiation pressure and target-normal sheath acceleration schemes, their role in ion acceleration remains unconfirmed, limiting the optimization strategies for neutron sources. Here we clarify the influence of laser pulse temporal shape on ion acceleration from ultrathin targets. We systematically change the temporal structure of the pulse by controlling the group delay dispersion and third-order dispersion (TOD), while pulse energy and focal spot size remain unchanged. The experiments are supported by 2D PIC simulations and analytical modeling. We find that the effect of optimum temporal shape is considerably larger than previously reported: the TOD-induced post-pulses increase the efficiency by 50

Laser-driven deuterons generate neutrons with a mean energy of 2.5 MeV, through the 2H(d,n) fusion reaction in a deuterated polyethylene (dPE) tablet. The deuterium ions are accelerated by 12 fs, 21 mJ laser pulses interacting with a 0.2 µm thin dPE foil at a peak intensity of 1018 W/cm2. The laser was operated at 1 Hz repetition rate in bursts of 75 shots. The interaction was characterized and recorded for each laser shot. The ion spectra were measured in the forward and backward directions by Thomson ion spectrometers. Neutron events were detected by a time-of-flight (ToF) system consisting of four plastic scintillators positioned at various angles around the experimental chamber. The maximum cut-off energy of the forward accelerated protons and deuterons was close to 1.4 MeV and 1 MeV, while the mean values are 428 ± 63 keV and 433 ± 80 keV, respectively. Analysis of ToF distributions from 3128 shots resulted in an average yield of 1142 ± 59 neutrons per shot in the energy range of 1.5-4 MeV. The energy distribution of forward-directed neutrons peaks between 3 and 3.5 MeV. Angular dependence analysis showed a perpendicular minimum and a maximum along the deuteron beam, consistent with the expected distribution from the literature and our simulation results.

Ion acceleration resulting from the interaction of 11 fs laser pulses of ∼ 35 mJ energy with ultrahigh contrast (<10 −10 ) and 10 19 W cm −2 peak intensity with foil targets made of various materials and thicknesses at normal (0°) and 45° laser incidence is investigated. The maximum energy of the protons reached ∼1.4 MeV accelerated in the laser propagation direction and ∼1.2 MeV in the opposite direction from a formvar target. The energy conversion efficiency from the laser to the proton beam is estimated to be as high as ∼1.4% at 45° laser incidence using a 51 nm thick Al target. The high laser contrast indicates the predominance of vacuum heating via Brunel’s effect as an absorption mechanism involving a tiny pre-plasma at the target front. The experimental results are in reasonable agreement with theoretical estimates, where proton acceleration from the target front side in the backward direction is well explained by the Coulomb explosion of a charged cavity formed in a tiny pre-plasma, while forward proton acceleration is likely to be a two-step process: protons are first accelerated in the target front-side cavity and then further boosted in energy through the target back side via the target normal sheath acceleration (TNSA) mechanism.

Spatial characterization of proton beam, driven by 12fs, 35mJ, 1019 W/cm2 intense laser-foil interaction is presented. Measurement shows low divergence (3.8-degree), small source size (< 4.5 µm) and 0.00038 π-mm-mrad normalized emittance of proton beam.

Spatial characterization of 0.5 MeV proton beam, driven by 12 fs, 35 mJ, 1019 W/cm2 intense laser-foil interaction is presented. The accelerated proton beam has been applied to obtain a high-resolution, point-projection static radiograph of a fine mesh using a CR-39 plate. The reconstruction of mesh edge blurring and particle ray tracing suggests that these protons have an effective source size (FWHM) of just 3.3 ± 0.3 µm. Furthermore, the spatial distribution of the proton beam recorded on the CR-39 showed that the divergence of these particles is less than 5-degree (FWHM). The low divergence and small source size of the proton beam resulted in an ultralow transverse emittance of 0.00032 π-mm-mrad, which is several orders of magnitude smaller than that of a conventional accelerator beam.

The calibration of an ion detection system was carried out for protons and carbon ions from a few tens of keV up to about 1 MeV energies. A Thomson spectrometer deflecting the particle beam accelerated from a laser plasma creates the ion spectra on a phosphor screen behind a micro-channel plate (MCP), which are recorded by a camera. During calibration, the ion spectra simultaneously hit the slotted CR-39 track detector installed in front of the MCP and, passing through the adjacent CR-39 stripes, the MCP. The calibration provides the ratio of the interpolated values between two consecutive stripes of the camera signal and the total number of particles recorded on the corresponding stripe of CR-39. The efficiency of proton detection by CR-39 was also measured in a conventional accelerator beam and found to drop by 20% below 100 keV.

The interaction of high-intensity laser pulses with homo-nuclear aluminum clusters carried out by 2D3V UMPIC code has been investigated. The qualitative picture of the ion acceleration depends on the strength of the laser field and the cluster radius. In this study, the radius of the cluster and the laser dimensionless amplitude has been changed, and the effect of laser polarization on ion acceleration by these variations has been considered. Increasing cluster radius (from 10 to 200 nm), simulations show transferred laser energy to the ions increases for both linear and circular polarization, but the magnitude of this growth for circular polarization is larger than that of linear polarization in a given laser intensity. Also, the effect of laser dimensionless amplitude on ion acceleration has been studied for both laser polarizaton. Simulation reveals that aluminum ions cut-off energy and their energy spread increase rapidly up to a0∼2.5, and then saturate at larger amplitudes, for both polarizations. However, by increasing laser amplitude (a0>5), energy spread of linear polarization is less than that of circular polarization.

In the interaction of short-laser pulses with a solid density target, pre-plasma can play a major role in ion acceleration processes. So far, complete analysis of pre-plasma effect on the ion acceleration by ultra-short laser pulses in the radiation pressure acceleration (RPA) regime has been unknown. Then the effect of pre-plasma on the ion acceleration efficiency is analyzed by numerical results of the particle-in-cell simulation in the RPA regime. It is shown that, for long-laser pulses (τp > 50 fs), the presence of pre-plasma makes a destructive effect on ion acceleration while it may have a contributing effect for short-laser pulses (τp < 50 fs). Therefore, the 35 fs (20 fs) laser pulse can accelerate ions up to 40 MeV (55 eV), which is almost two (three) times larger in energy rather than use of a 100 fs pulse with the same pre-plasma scale length.