X. H. Yang

Propagation of attosecond electron bunches along the cone-and-channel target

Generation and propagation of attosecond electron bunches along a cone-and-channel target are investigated by particle-in-cell simulation. The target electrons are pulled out by the oscillating electric field of an intense laser pulse irradiating a cone target and accelerated forward along the cone walls. It is shown that the energetic electrons can be further guided and confined by a channel attached to the cone tip. The propagation of these electrons along the channel induces a strong quasistatic magnetic field as well as a sheathelectric field since a part of the energetic electrons expands into the surrounding vacuum. The electromagnetic field in turn confines the surface currents. With the cone-and-channel target the energetic electrons can be much better collimated and propagate much farther than that from the classical cone target.

Bright tunable femtosecond x-ray emission from laser irradiated micro-droplets

It is demonstrated that bright femtosecond X-rays can be obtained by irradiating a moderate laser onto a helium micro-droplet. The laser ponderomotive force continuously sweeps electrons from the droplets and accelerates them forward. The electrons exposed in the outrunning laser field oscillate transversely and emit photons in the forward direction. The total flux of photons with energies above 1 keV is as high as 109/shot which is about 10-fold enhancement compared with betatron oscillation under similar laser conditions. The maximum achieved peak brightness is up to 1021 photons/s/mm2/mrad2/0.1%BW. By adjusting laser and droplet parameters, we can get tunable X-rays with required brightness and energy.

Generation of high-energy-density ion bunches by ultraintense laser-cone-target interaction

A scheme in which carbon ion bunches are accelerated to a high energy and density by a laser pulse (∼1021 W/cm2) irradiating cone targets is proposed and investigated using particle-in-cell simulations. The laser pulse is focused by the cone and drives forward an ultrathin foil located at the cones tip. In the course of the work, best results were obtained employing target configurations combining a low-Z cone with a multispecies foil transversely shaped to match the laser intensity profile.

Collimated proton beam generation from ultraintense laser-irradiated hole target

Collimated proton beams from laser interaction with a slab having a hole on its backside are investigated using particle-in-cell simulation. The hot target electrons driven by the laser expand rapidly into the hole. However, at the holes corners the electrons are strongly compressed and an intense electron jet is emitted from each corner, tightly followed by the ions. The plasma jets focus and collimate along the axis of the hole and can propagate without divergence within the hole. The effect of the hole diameter on the collimated proton beam is considered.

Propagation of intense laser pulses in strongly magnetized plasmas

Propagation of intense circularly polarized laser pulses in strongly magnetized inhomogeneous plasmas is investigated. It is shown that a left-hand circularly polarized laser pulse propagating up the density gradient of the plasma along the magnetic field is reflected at the left-cutoff density. However, a right-hand circularly polarized laser can penetrate up the density gradient deep into the plasma without cutoff or resonance and turbulently heat the electrons trapped in its wake. Results from particle-in-cell simulations are in good agreement with that from the theory.

Fast-electron self-collimation in a plasma density gradient

A theoretical and numerical study of fast electron transport in solid and compressed fast ignition relevant targets is presented. The principal aim of the study is to assess how localized increases in the target density (e.g., by engineering of the density profile) can enhance magnetic field generation and thus pinching of the fast electron beam through reducing the rate of temperature rise. The extent to which this might benefit fast ignition is discussed.

High-flux low-divergence positron beam generation from ultra-intense laser irradiated a tapered hollow target

By using two-dimensional particle-in-cell simulations, we demonstrate high-flux dense positrons generation by irradiating an ultra-intense laser pulse onto a tapered hollow target. By using a laser with an intensity of 4 × 1023 W/cm2, it is shown that the Breit-Wheeler process dominates the positron production during the laser-target interaction and a positron beam with a total number >1015 is obtained, which is increased by five orders of magnitude than in the previous work at the same laser intensity. Due to the focusing effect of the transverse electric fields formed in the hollow cone wall, the divergence angle of the positron beam effectively decreases to ∼15° with an effective temperature of ∼674 MeV. When the laser intensity is doubled, both the positron flux (>1016) and temperature (963 MeV) increase, while the divergence angle gets smaller (∼13°). The obtained high-flux low-divergence positron beam may have diverse applications in science, medicine, and engineering.

Enhanced electron–positron pair production by ultra intense laser irradiating a compound target

High-energy-density electron–positron pairs play an increasingly important role in many potential applications. Here, we propose a scheme for enhanced positron production by an ultra intense laser irradiating a gas-Al compound target via the multi-photon Breit–Wheeler (BW) process. The laser pulse first ionizes the gas and interacts with a near-critical-density plasma, forming an electron bubble behind the laser pulse. A great deal of electrons are trapped and accelerated in the bubble, while the laser front hole-bores the Al target and deforms its front surface. A part of the laser wave is thus reflected by the inner curved target surface and collides with the accelerated electron bunch. Finally, a large number of γ photons are emitted in the forward direction via the Compton back-scattering process and the BW process is initiated. Dense electron–positron pairs are produced with a maximum density of m−3. Simulation results show that the positron generation is greatly enhanced in the compound target, where the positron yield is two orders of magnitude greater than that in only the solid slab case. The influences of the laser intensity, gas density and length on the positron beam quality are also discussed, which demonstrates the feasibility of the scheme in practice.

Effects of external axial magnetic field on fast electron propagation

A scheme employing an external axial magnetic field is proposed to diagnose the intrinsic divergence of laser-generated fast electron beams, and this is studied numerically with hybrid simulations. The maximum beam radius of fast electrons increases with the initial divergence and decreases with the amplitude of the axial magnetic field. It is indicated that the intrinsic divergence of fast electrons can be inferred from measurements of the beam radius at different depth under the axial field. The proposed scheme here may be useful for future fast ignition experiments and in other applications of laser-generated fast electron beams.

Generation of hemispherical fast electron waves in the presence of preplasma in ultraintense laser-matter interaction

AbstractHemispherical electron plasma waves generated from ultraintense laser interacting with a solid target having a subcriticalpreplasma is studied using particle-in-cell simulation. As the laser pulse propagates inside the preplasma, it becomes self-focused due to the response of the plasma electrons to the ponderomotive force. The electrons are mainly heated viabetatron resonance absorption and their thermal energy can become higher than the ponderomotive energy. The hotelectrons easily penetrate through the thin solid target and appear behind it as periodic hemispherical shell-like layersseparated by the laser wavelength.Keywords: Betatron resonance; Electron plasma waves; Ponderomotive force; Preplasma INTRODUCTIONFast electrons generation in ultraintense laser-solid interactionhave been investigated extensively both theoretically andexperimentally (Sentoku et al., 2006;Kempet al., 2009;Nilson et al., 2011) because of their application in laser-plasma accelerators (Borghesi et al., 2006;Yuet al., 2009;Yang et al., 2010), fast ignition (FI) schemes of inertial con-finement fusion (Tabak et al., 1994; Deutsch & Didelez,2011), bright X-ray sources (Pfeifer et al., 2006), etc. Theenergy spectrum, spatial distribution, and divergence angleof the energetic electrons can be significantly affected by theubiquitous low-density blow-off plasma (the preplasma) cre-ated by the inherent laser prepulse and/or spontaneous emis-sions from the lasing system (Nuteret al., 2008;Davieset al.,2009; MacPhee et al., 2010;Caiet al., 2010;Linet al., 2012;Sakagami et al., 2012). Thus, the effect of the preplasmashould be taken into account in studies of fast electron gener-ation from intense laser-solid target interaction.Interaction of an intense laser pulse with a solid targethaving a large preplasma can involve nonlinear processessuch as self-focusing, filamentation, hole-boring, etc. thatcan play important roles in the generation and propagationof fast electrons (Pukhov & Meyer-ter-Vehn et al., 1996;Friouetal.,2012).Fastelectronsarealsogeneratedinthepre-plasma, and it has been found that they usually have a two-temperature Maxwellian distribution. The temperature of thehigh-energy component is much higher than the ponderomo-tiveenergy(Wilksetal.,1992)anditscaleswiththelengthL