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We present experimental studies on ion acceleration using an 800-nm circularly polarized laser pulse with a peak intensity of 6.9×10^{19}  W/cm^{2} interacting with an overdense plasma that is produced by a laser prepulse ionizing an initially ultrathin plastic foil. The proton spectra exhibit spectral peaks at energies up to 9 MeV with energy spreads of 30% and fluxes as high as 3×10^{12}  protons/MeV/sr. Two-dimensional particle-in-cell simulations reveal that collisionless shocks are efficiently launched by circularly polarized lasers in exploded plasmas, resulting in the acceleration of quasimonoenergetic proton beams. Furthermore, this scheme predicts the generation of quasimonoenergetic proton beams with peak energies of approximately 150 MeV using current laser technology, representing a significant step toward applications such as proton therapy.

A pulse cleaner based on noncollinear optical-parametric amplification and second-harmonic generation processes is used to improve the contrast of a laser of peak intensity ∼2 × 1019 W/cm2 to ∼1011 at 100 ps before the peak of the main pulse. A 7 MeV proton beam is observed when a 2.5 μm-thick Al foil is irradiated by this high-contrast laser. The maximum proton energy decreases to 2.9 MeV when a low-contrast (∼108) laser is used. Two-dimensional particle-in-cell simulations combined with MULTI simulations show that the maximum proton energy sensitively relies on the detecting direction. The ns-time-scale prepulse can bend a thin target before the main pulse arrives, which reduces maximum proton energy in the target normal sheath acceleration.

We present experimental studies on ion acceleration from diamond-like carbon (DLC) foils irradiated by 800 nm, linearly polarized laser pulses with peak intensity of 1.7 Â 10 19 W/cm 2 to 3.5 Â 10 19 W/cm 2 at oblique incidence. Diamond-like carbon foils are heated by the prepulse of a high-contrast laser pulse and expand to form plasmas of near-critical density caused by thermal effect before the arrival of the main pulse. It is demonstrated that carbon ions are accelerated by a collisionless shock wave in slightly overdense plasma excited by forward-moving hot electrons generated by the main pulse. V C 2015 AIP Publishing LLC. [http://dx.

A double beam image (DBI) technique is coupled in the two-stage accelerating mechanism to simultaneously improve the spectra and maximum energy of the proton beam. A proton beam with a narrow-spectrum center at 5.4 MeV and a long tail up to 14.4 MeV is generated in the experiment. Experimental and simulation results show that spatial collineation, time synchronization, and real-time monitoring are needed for optimum two-stage proton acceleration and are realized by the DBI technique to a certain extent in our experiment. This DBI technique can be used to achieve optimum two-stage acceleration in a feasible manner and will allow precise manipulation of multistage acceleration to improve the energy and spectra of particle beams.

The accelerating gradient of a proton beam is a crucial factor for the stable radiation pressure acceleration, because quickly accelerating protons into the relativistic region may reduce the multidimensional instability grow to a certain extent. In this letter, a shape-tailored laser is designed to accelerate the protons in a controllable high accelerating gradient in theory. Finally, a proton beam in the gigaelectronvolt range with an energy spread of ∼2.4% is obtained in one-dimensional particle-in-cell simulations. With the future development of the high-intense laser, the ability to accelerate a high energy proton beam using a shape-tailored laser will be important for realistic proton applications, such as fast ignition for inertial confinement fusion, medical therapy, and proton imaging.

A micrometer-scale “plasma lens” self-constructed by the prepulse and main pulse of the Laguerre–Gaussian (LG) laser is realized to enhance the collimation and acceleration of proton beams in a target normal sheath field acceleration mechanism. Hydrodynamic FLASH and particle-in-cell simulations are carried out and find that a collimated proton source with beam divergence ∼2.7° is generated by the LG laser, which is smaller than the case driven by the traditional Gaussian laser. It demonstrates that the curved sheath field on the “plasma lens” plays an important role in the beam collimation. Such an approach considerably relaxes the constraints of complex design for the target fabrication and auxiliary laser pulse, opening new doors for high-repetition-rate collimated proton accelerations for innovative applications in upcoming high-repetition-rate petawatt laser systems.