Wenpeng Wang

Generation of intense high-order vortex harmonics.

This Letter presents for the first time a scheme to generate intense high-order optical vortices that carry orbital angular momentum in the extreme ultraviolet region based on relativistic harmonics from the surface of a solid target. In the three-dimensional particle-in-cell simulation, the high-order harmonics of the high-order vortex mode is generated in both reflected and transmitted light beams when a linearly polarized Laguerre-Gaussian laser pulse impinges on a solid foil. The azimuthal mode of the harmonics scales with its order. The intensity of the high-order vortex harmonics is close to the relativistic region, with the pulse duration down to attosecond scale. The obtained intense vortex beam possesses the combined properties of fine transversal structure due to the high-order mode and the fine longitudinal structure due to the short wavelength of the high-order harmonics. In addition to the application in high-resolution detection in both spatial and temporal scales, it also presents new opportunities in the intense vortex required fields, such as the inner shell ionization process and high energy twisted photons generation by Thomson scattering of such an intense vortex beam off relativistic electrons.

Efficient acceleration of monoenergetic proton beam by sharp front laser pulse

Stable acceleration of relativistic ions by the radiation pressure of a superintense, circularly polarized laser pulse with sharp front is investigated by analytical modeling and particle-in-cell simulation. For foils with given density and thickness, the suitable steepness of the laser front is found to suppress instabilities and efficiently drive a stable monoenergetic ion beam. With a laser pulse of peak amplitude a{sub 0}=200, a proton beam of energy about 10 GeV can be generated. The dynamics of the laser-compressed electron layer and the ions in the hole-boring stage are investigated. In the case studied, the ions initially in the middle of the target are found to be accelerated to the back surface of the target ahead of the other ions.

Effect of pulse profile and chirp on a laser wakefield generation

A laser wakefield driven by an asymmetric laser pulse with/without chirp is investigated analytically and through two-dimensional particle-in-cell simulations. For a laser pulse with an appropriate pulse length compared with the plasma wavelength, the wakefield amplitude can be enhanced by using an asymmetric un-chirped laser pulse with a fast rise time; however, the growth is small. On the other hand, the wakefield can be greatly enhanced for both positively chirped laser pulse having a fast rise time and negatively chirped laser pulse having a slow rise time. Simulations show that at the early laser-plasma interaction stage, due to the influence of the fast rise time the wakefield driven by the positively chirped laser pulse is more intense than that driven by the negatively chirped laser pulse, which is in good agreement with analytical results. At a later time, since the laser pulse with positive chirp exhibits opposite evolution to the one with negative chirp when propagating in plasma, the wakefield in the latter case grows more intensely. These effects should be useful in laser wakefield acceleration experiments operating at low plasma densities.

Ultrahigh energy proton generation in sequential radiation pressure and bubble regime

Protons in a microtarget embedded in an underdense high-mass plasma can be accelerated sequentially by the radiation pressure of a short circularly polarized laser pulse and the induced wake bubble field in the background plasma, which has been shown in detail by two-dimensional particle in cell simulations. It has been found that instead of using transverse Gaussian pulses proton energy can be increased dramatically by using a transverse super-Gaussian laser pulse. With a 2.14x10{sup 23} W/cm{sup 2} laser pulse in a tritium plasma of density 1.5x10{sup 20}/cm{sup 3}, 76 GeV high-quality quasimonoenergetic protons have been obtained. The scaling relations between the obtainable proton energy and the laser amplitude as well as the background plasma density have been shown.

Scheme for proton-driven plasma-wakefield acceleration of positively charged particles in a hollow plasma channel

A new scheme for accelerating positively charged particles in a plasma wakefield accelerator is proposed. If the proton drive beam propagates in a hollow plasma channel, and the beam radius is of order of the channel width, the space charge force of the driver causes charge separation at the channel wall, which helps to focus the positively charged witness bunch propagating along the beam axis. In the channel, the acceleration buckets for positively charged particles are much larger than in the blowout regime of the uniform plasma, and stable acceleration over long distances is possible. In addition, phasing of the witness with respect to the wave can be tuned by changing the radius of the channel to ensure the acceleration is optimal. Two dimensional simulations suggest that, for proton drivers likely available in future, positively charged particles can be stably accelerated over 1 km with the average acceleration gradient of 1.3 GeV/m.

Controlled electron acceleration in the bubble regime by optimizing plasma density

Improvement of the quality of the monoenergetic electron bunch generated in the laser wakefield is investigated. The electrostatic field is more intense near the back of the bubble than at other locations in the bubble. By optimizing the density gradient of background plasma, the local dephasing problem can theoretically be overcome and the electron bunch can be stably accelerated at the back of the bubble so that the accelerated electrons experience nearly the same electric field. Three-dimensional simulations were performed. Compared with the standard wakefield acceleration schemes, a better-quality electron bunch, with narrower energy spread and higher energy, is obtained with a shorter acceleration distance.

Hollow screw-like drill in plasma using an intense Laguerre–Gaussian laser

With the development of ultra-intense laser technology, MeV ions can be obtained from laser–foil interactions in the laboratory. These energetic ion beams can be applied in fast ignition for inertial confinement fusion, medical therapy, and proton imaging. However, these ions are mainly accelerated in the laser propagation direction. Ion acceleration in an azimuthal orientation was scarcely studied. In this research, a doughnut Laguerre–Gaussian (LG) laser is used for the first time to examine laser–plasma interaction in the relativistic intensity regime in three-dimensional particle-in-cell simulations. Studies have shown that a novel rotation of the plasma is produced from the hollow screw-like drill of an mode laser. The angular momentum of particles in the longitudinal direction produced by the LG laser is enhanced compared with that produced by the usual laser pulses, such as linearly and circularly polarized Gaussian pulses. Moreover, the particles (including electrons and ions) can be trapped and uniformly compressed in the dark central minimum of the doughnut LG pulse. The hollow-structured LG laser has potential applications in the generation of x-rays with orbital angular momentum, plasma accelerators, fast ignition for inertial confinement fusion, and pulsars in the astrophysical environment.

Generation of a large amount of energetic electrons in complex-structure bubble

By means of particle-in-cell (PIC) simulations, we found that when the focus size of a laser pulse is much larger than the plasma wavelength and when the laser power is hundreds of times larger than the critical power required for relativistic self-focusing, a large complex bubble is formed. The transversal size of the bubble depends on the laser spot size. Owing to the large bubble size, a bunch of electrons with the total charge in the range of a few tens of nano- Coulombs is trapped and accelerated in the bubble. When the plasma density is 2◊10 19 cm 3 , the charge of the energetic electron bunch with energy above 5MeV exceeds 45nC with a laser spot size of 60µm. Electrons continuously self-injected into such a complex bubble serve as an effective source of high- charge electron bunches.

High-energy monoenergetic proton bunch from laser interaction with a complex target

Generation of high-energy proton bunch in the interaction of a high-power laser pulse with a complex target consisting of a front horizontal slice adjoining a conventional heavy ion and proton double-layer slab is investigated using two-dimensional particle-in-cell simulation. The laser pulse propagates along both sides of the slice. A large number of hot electrons are generated and accelerated by the surface ponderomotive force, and transported through the double layer, forming a backside sheath field which is considerably stronger and more localized than that produced by the electrons from a simple double layer. As a result, the protons in the proton layer can be accelerated to energies more than three times, and the energy spread halved, that from the simple double layer.

Positron acceleration in a hollow plasma channel up to TeV regime

Nowadays, humans understanding of the fundamental physics is somehow limited by the energy that our high energy accelerators can afford. Up to 4 TeV protons are realized in the Large Hadron Collider (LHC). Leptons, such as electrons and positrons, however gained energies of about 100 GeV or less. Multi-TeV lepton accelerators are still lacking due to the relatively low acceleration gradient of conventional methods, which may induce unbearable cost. On the other hand, plasmas have shown extraordinary potential in accelerating electrons and ions, providing orders of magnitude higher acceleration fields of 10–100 GV/m. In such context, we propose a plasma-based high-energy lepton accelerator, in which a weakly focusing plasma structure is formed near the beam axis. The structure preserves the emittance of the accelerated beam and produces low radiation losses. Moreover, the structure allows for a considerable decrease of the witness energy spread at the driver depletion stage.