X. Q. Yan

Radiation-Pressure Acceleration of Ion Beams Driven by Circularly Polarized Laser Pulses

We present experimental studies on ion acceleration from ultrathin diamondlike carbon foils irradiated by ultrahigh contrast laser pulses of energy 0.7 J focused to peak intensities of 5x10(19) W/cm2. A reduction in electron heating is observed when the laser polarization is changed from linear to circular, leading to a pronounced peak in the fully ionized carbon spectrum at the optimum foil thickness of 5.3 nm. Two-dimensional particle-in-cell simulations reveal that those C6+ ions are for the first time dominantly accelerated in a phase-stable way by the laser radiation pressure.

Self-Organizing GeV, Nanocoulomb, Collimated Proton Beam from Laser Foil Interaction at 7 X 1021 W/cm2

We report on a self-organizing, quasistable regime of laser proton acceleration, producing 1 GeV nanocoulomb proton bunches from laser foil interaction at an intensity of 7 x 10;{21} W/cm;{2}. The results are obtained from 2D particle-in-cell simulations, using a circular polarized laser pulse with Gaussian transverse profile, normally incident on a planar, 500 nm thick hydrogen foil. While foil plasma driven in the wings of the driving pulse is dispersed, a stable central clump with 1-2lambda diameter is forming on the axis. The stabilization is related to laser light having passed the transparent parts of the foil in the wing region and enfolding the central clump that is still opaque. Varying laser parameters, it is shown that the results are stable within certain margins and can be obtained both for protons and heavier ions such as He;{2+}.

Theory of laser ion acceleration from a foil target of nanometer thickness

A theory for ion acceleration by ultrashort laser pulses is presented to evaluate the maximum ion energy in the interaction of ultrahigh contrast (UHC) intense laser pulses with a nanometer-scale foil. In this regime, the ion energy may be directly related to the laser intensity and subsequent electron dynamics. This leads to a simple analytical expression for the ion energy gain under the laser irradiation of thin targets. Significantly higher energies for thin targets than for thicker targets are predicted. The theory is concretized with a view to compare with the results and their details of recent experiments.

Efficient and stable proton acceleration by irradiating a two-layer target with a linearly polarized laser pulse

We report an efficient and stable scheme to generate ∼200 MeV proton bunch by irradiating a two-layer targets (near-critical density layer+solid density layer with heavy ions and protons) with a linearly polarized Gaussian pulse at intensity of 6.0×1020 W/cm2. Due to self-focusing of laser and directly accelerated electrons in the near-critical density layer, the proton energy is enhanced by a factor of 3 compared to single-layer solid targets. The energy spread of proton is also remarkably reduced. Such scheme is attractive for applications relevant to tumor therapy.

Self-induced magnetic focusing of proton beams by Weibel-like instability in the laser foil-plasma interactions

In the laser foil-plasma interaction the effects of Weibel-like instability have been explored. The self-induced magnetic fields result in the merging of filaments formed at the earlier stage of the instability and subsequent formation of a plasma clump close to the laser propagation axis. A photon cavity is formed in the laser plasma interactions, which can accelerate and focus the proton bunch efficiently, as identified by multidimensional particle-in-cell simulations. These processes are helpful to realize the stable acceleration of hundreds of MeV proton beams with a very low energy spread with circularly polarized intense laser pulses.

Sub-TeV proton beam generation by ultra-intense laser irradiation of foil-and-gas target

A two-phase proton acceleration scheme using an ultra-intense laser pulse irradiating a proton foil with a tenuous heavier-ion plasma behind it is presented. The foil electrons are compressed and pushed out as a thin dense layer by the radiation pressure and propagate in the plasma behind at near the light speed. The protons are in turn accelerated by the resulting space-charge field and also enter the backside plasma, but without the formation of a quasistationary double layer. The electron layer is rapidly weakened by the space-charge field. However, the laser pulse originally behind it now snowplows the backside-plasma electrons and creates an intense electrostatic wakefield. The latter can stably trap and accelerate the pre-accelerated proton layer there for a very long distance and thus to very high energies. The two-phase scheme is verified by particle-in-cell simulations and analytical modeling, which also suggests that a 0.54 TeV proton beam can be obtained with a 1023 W/cm2 laser pulse.

Bright Subcycle Extreme Ultraviolet Bursts from a Single Dense Relativistic Electron Sheet

Double-foil targets separated by a low density plasma and irradiated by a petawatt-class laser are shown to be a copious source of coherent broadband radiation. Simulations show that a dense sheet of relativistic electrons is formed during the interaction of the laser with the tenuous plasma between the two foils. The coherent motion of the electron sheet as it transits the second foil results in strong broadband emission in the extreme ultraviolet, consistent with our experimental observations.

Collimated proton acceleration in light sail regime with a tailored pinhole target

A scheme for producing collimated protons from laser interactions with a diamond-like-carbon + pinhole target is proposed. The process is based on radiation pressure acceleration in the multi-species light-sail regime [B. Qiao et al., Phys. Rev. Lett. 105, 155002 (2010); T. P. Yu et al., Phys. Rev. Lett. 105, 065002 (2010)]. Particle-in-cell simulations demonstrate that transverse quasistatic electric field at TV/m level can be generated in the pinhole. The transverse electric field suppresses the transverse expansion of protons effectively, resulting in a higher density and more collimated proton beam compared with a single foil target. The dependence of the proton beam divergence on the parameters of the pinhole is also investigated.

Autofocused, enhanced proton acceleration from a nanometer-scale bulged foil

We report an autofocused, enhanced proton acceleration by the interaction of an intense laser pulse with a bulged target. These results are obtained from two-dimensional particle-in-cell simulations using a real Gaussian laser pulse, normally incident on a bulged/planar, 60 nm thick foil (C:H=1:1). When the laser pulse hits the precurved target, energetic protons are converged on the axis automatically. For the bulged foil, due to oblique incidence at the wing region, the efficient vacuum heating at larger incidence angles will result in more energetic hot electrons than from the flat foil. The enhancement of hot electron temperature and density will result in a larger longitudinal field, which contributes to an enhancement of proton energy. The maximum proton energy of 124 MeV is attained from a bulged target irradiated by a linear polarized laser pulse at an intensity of 1.3×1020 W/cm2, which is two times higher than from the planar target (61 MeV).

Theoretical investigation on novel particle beams and radiation sources in relativistic laser-solid interactions

We report the recent theoretical and numerical studies of quasi-monoenergetic electron and proton beams in laser solid interactions, where the quasi-monoenergetic electrons are produce in laser interaction with wire-targets and the quasi-monoenegetic protons are produced by ultra-intense circularly polarized laser pulses in a new acceleration regime called phase-stability acceleration. For the generation of coherent radiation at high frequencies, attention is paid on the emission from electron plasma waves excited in a thin solid target.