Brightγ-ray source with large orbital angular momentum from the laser near-criticalplasma interaction

We propose a new laser-plasma-based method to generate brightγ-rays carrying large orbital angular momentum by interacting a circularly polarized Laguerre–Gaussian laser pulse with a near-critical hydrogen plasma conflned in an over-dense solid tube.In the flrst stage of the interaction,it is found via fully relativistic three-dimensional particle-in-cell simulations that high-energy helical electron beams with large orbital angular momentum are generated.In the second stage,this electron beam interacts with the laser pulse reflected from the plasma disc behind the solid tube,and helicalγbeams are generated with the same topological structure as the electron beams.The results show that the electrons receive angular momentum from the drive laser,which can be further transferred to theγphotons during the interaction.Theγbeam orbital angular momentum is strongly dependent on the laser topological charge l and laser intensity a_(0),which scales as L_(γ)∝a_(0)~4.A short(duration of 5 fs)isolated helicalγbeam with an angular momentum of-3.3×10^(-14)kg m~2 s^(-1)is generated using the Laguerre–Gaussian laser pulse with l=2.The peak brightness of the helicalγbeam reaches 1.22×10^(24)photons s^(-1)mm^(-2)mrad^(-2)per 0.1%BW(at 10 Me V),and the laser-to-γ-ray angular momentum conversion rate is approximately 2.1%.

Two-stage γ ray emission via ultrahigh intensity laser pulse interaction with a laser wakefield accelerated electron beam

In this study, we investigate the generation of twin γ ray beams in the collision of an ultrahigh intensity laser pulse with a laser wakefield accelerated electron beam using a particle-in-cell simulation. We consider the composed target of a homogeneous underdense preplasma in front of an ultrathin solid foil. The electrons in the preplasma are trapped and accelerated by the wakefield. When the laser pulse is reflected by the thin solid foil, the wakefield accelerated electrons continue to move forward and pass through the foil almost without influence from the reflected laser pulse or foil. Consequently, two groups of γ ray flashes, with tunable time delay and energy, are generated by the wakefield accelerated electron beam interacting with the reflected laser pulse from the foil as well as another counter-propagating petawatt laser pulse behind the foil. Additionally, we study the dependence of the γ photon emission on the preplasma densities, driving laser polarization, and solid foil.

激光等离子体相互作用中量子电动力学效应对激光能量吸收的影响

为了研究强激光与等离子体相互作用中的量子电动力学(QED)效应对激光能量吸收的影响,用理论方法对强度为I>10^22wcm^-2、反方向传播的两束圆极化强激光与等离子体固体靶相互作用过程进行研究。探讨经典和QED情况下的辐射阻尼效应对激光能量吸收的影响。结果表明,在QED情况下,在相互作用过程中能够产生高能电子和高能辐射。这些高能电子集体振荡过程中的辐射阻尼导致激光能量的强吸收。QED情况比经典情况具有更好的实际应用价值。

Enhancement of proton collimation and acceleration by an ultra-intense laser interacting with a cone target followed by a beam collimator

A special method is proposed of a laser-induced cavity pressure acceleration scheme for collimating,accelerating and guiding protons,using a single-cone target with a beam collimator through a target normal sheath acceleration mechanism.In addition,the problems involved are studied by using two-dimensional particle-in-cell simulations.The results show that the proton beam can be collimated,accelerated and guided effectively through this type of target.Theoretically,a formula is derived for the combined electric field of accelerating protons.Compared with a proton beam without a beam collimator,the proton beam density and cut-off energy of protons in the type II are increased by 3.3 times and 10%respectively.Detailed analysis shows that the enhancement is mainly due to the compact and strong sheath electrostatic field,and that the beam collimator plays a role in focusing energy.In addition,the simulation results show that the divergence angle of the proton beam in type II is less than 1.67 times that of type I.The more prominent point is that the proton number of type II is 2.2 times higher than that of type I.This kind of target has important applications in many fields,such as fast ion ignition in inertial fusion,high energy physics and proton therapy.

Ultrabright γ-ray emission from the interaction of an intense laser pulse with a near-critical-density plasma

An efficient scheme for generating ultrabright γ-rays from the interaction of an intense laser pulse with a near-criticaldensity plasma is studied by using the two-dimensional particle-in-cell simulation including quantum electrodynamic effects.We investigate the effects of target shape on γ-ray generation efficiency using three configurations of the solid foils attached behind the near-critical-density plasma:a flat foil without a channel(target 1),a flat foil with a channel(target 2),and a convex foil with a channel(target 3).When an intense laser propagates in a near-critical-density plasma,a large number of electrons are trapped and accelerated to GeV energy,and emit γ-rays via nonlinear betatron oscillation in the first stage.In the second stage,the accelerated electrons collide with the laser pulse reflected from the foil and emit high-energy,high-density γ-rays via nonlinear Compton scattering.The simulation results show that compared with the other two targets,target 3 affords better focusing of the laser field and electrons,which decreases the divergence angle of g-photons.Consequently,denser and brighter γ-rays are emitted when target 3 is used.Specifically,a dense γ-ray pulse with a peak brightness of 4.6×10^(26) photons/s/mm2/mrad2/0.1%BW(at 100 MeV)and 1.8×1023 photons/s/mm2/mrad2/0.1%BW(at 2 GeV)are obtained at a laser intensity of 8.5×10^(22) W/cm2 when the plasma density is equal to the critical plasma density nc.In addition,for target 3,the effects of plasma channel length,foil curvature radius,laser polarization,and laser intensity on the γ-ray emission are discussed,and optimal values based on a series of simulations are proposed.

Photon and positron production by ultrahigh-intensity laser interaction with various plasma foils

The generation ofγphotons and positrons using an ultrahigh-intensity laser pulse interacting with various plasma solid foils is investigated with a series of quantum electrodynamic particlein-cell(PIC)simulations.When ultrahigh-intensity lasers interact with plasma foils,a large amount of the laser energy is converted intoγphoton energy.The simulation results indicate that for a fixed laser intensity with different foil densities,the conversion efficiency of the laser toγphotons and the number of produced photons are highly related to the foil density.We determine the optimal foil density by PIC simulations for high conversion efficiencies as approximately 250 times the critical plasma density,and this result agrees very well with our theoretical assumptions.Four different foil thicknesses are simulated and the effects of foil thickness onγphoton emission and positron production are discussed.The results indicate that optimal foil thickness plays an important role in obtaining the desiredγphoton and positron production according to the foil density and laser intensity.Further,a relation between the laser intensity and conversion efficiency is present for the optimal foil density and thickness.

Fast electrons collimating and focusing by an ultraintense laser interacting with a high density layers

The use of a novel double-cone funnel target with high density layers （HDL） to collimate and focus electrons is investigated by two-dimensional particle-in-cell simulations. The proposed scheme can guide, collimate and focus electron beams to smaller sizes. The collimation reasons are analyzed by the quasi-static magnetic fields generation inside the beam collimator with HDL. It is found that the energy conversion efficiency is increased by a factor of 2.2 in this new scheme in comparison with the that without HDL. Such a target structure has potential for design flexibility and prevents inefficiencies in important applications such as fast ignition, etc.

Fully polarized Compton scattering in plane waves and its polarization transfer

Fully polarized Compton scattering from a beam of spin-polarized electrons is investigated in plane-wave backgrounds in a broad intensity region from the perturbative to the nonperturbative regimes.In the perturbative regime,polarized linear Compton scattering is considered for investigating polarization transfer from a single laser photon to a scattered photon,and in the high-intensity region,the polarized locally monochromatic approximation and locally constant field approximation are established and are employed to study polarization transfer from an incoming electron to a scattered photon.The numerical results suggest an appreciable improvement of about 10%in the scattering probability in the intermediate-intensity region if the electron’s longitudinal spin is parallel to the laser rotation.The longitudinal spin of the incoming electron can be transferred to the scattered photon with an efficiency that increases with laser intensity and collisional energy.For collision between an optical laser with frequency1 eV and a 10 GeV electron,this polarization transfer efficiency can increase from about 20%in the perturbative regime to about 50%in the nonperturbative regime for scattered photons with relatively high energy.