Meng Wen

Efficient GeV ion generation by ultraintense circularly polarized laser pulse

The interaction of an ultraintense circularly polarized laser pulse and a solid target is studied by one-dimensional particle-in-cell simulations. Ions at the front of the target are reflected by a moving quasisteady electrostatic field and obtain a relativistic velocity. At a laser intensity of 1022W∕cm2, almost half of the laser energy is transferred to ions and GeV ions are obtained. Effects of laser polarization state and target thickness on the laser energy conversion are investigated. It is found that a circularly polarized laser pulse can accelerate ions more efficiently than a linearly polarized laser pulse at the same laser and target parameters. A monoenergetic ion bunch is obtained for the ultrathin target, which is accelerated as a single entity.

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.

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.

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.

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.

Effect of plasma temperature on electrostatic shock generation and ion acceleration by laser

The effect of plasma temperature on electrostatic shock generated by a circularly polarized laser pulse in overdense plasma is studied by particle-in-cell simulation. Ion reflection and transmission in the collisionless electrostatic shock (CES) are investigated analytically. As the initial ion temperature is varied, a distinct transition from the laser-driven piston scenario with all ions being reflected to the CES scenario with partial ion reflection is found. The results show that at low but finite temperatures the ions are much more accelerated than if they were cold.

Generation of high charged energetic electrons by using multiparallel laser pulses

Large amount of energetic electrons generated in laser wake fields driven by multiparallel laser pulses is investigated with three-dimensional particle-in-cell simulations. By adjusting the distance between the pulses, bubbles with different structure are formed, which results in different injection efficiency. Compared with the single-pulse case, the charge of the energetic electrons could be doubled when the distance between the two pulses is large enough. A characteristic distance between the pulses is obtained, above which the total amount of the energetic electrons increases linearly by applying more laser pulses. There is no limit for the charge increase in our scheme as long as the plasma is wide enough so that more pulses can be applied.

Ultra-intense single attosecond pulse generated from circularly polarized laser interacting with overdense plasma

Few-cycle relativistic circularly polarized (CP) laser pulse reflected from overdense plasma is investigated by analysis and particle-in-cell simulations. It is found that through the laser-induced one-time drastic oscillation of the plasma boundary, an ultra-intense single attosecond light pulse can be generated naturally. An analytical model is proposed to describe the interaction and it agrees well with simulation results. They both indicate that peak intensity of the generated attosecond pulse is higher when the plasma density is closer to the relativistic transparency threshold and/or the pulse duration is closer to plasma oscillating period. Two dimensional simulation shows that a two-cycle 1021 W/cm2 CP laser can generate a single 230 attosecond 2 × 1021 W/cm2 pulse of light at a conversion efficiency greater than 10-2.

Angular distribution of emitted electrons due to intense p-polarized laser foil interaction

The angular distribution of electrons emitting from a foil surface illuminated by p -polarized laser pulses is studied using particle-in-cell simulation for incident angles of θ 1 = 22.5 ° , 45 ° , 67.5 ° and laser amplitudes of a = 0.5 , 1, 2. Theoretical prediction of the emission direction, based on canonical momentum conservation along the target surface, is verified. Surface ablation, the Alfven current limit, as well as self-generated electromagnetic fields on the surface are numerically investigated and found to play important roles in the modulation of the angular distribution of the emitted electrons. The emitted electrons of higher energy are found to be directly accelerated to near the polarization direction of the incident laser light. The simulation results agree very well with the recent experimental results from Al targets irradiated by a 60 fs, 180 mJ laser pulse.