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Ultraviolet laser driven radiation pressure acceleration of Cu crystals is investigated by using particle-in-cell simulations. When an ultrathin Cu crystal is irradiated by a circularly polarized pulse with wavelength λ = 72 nm, waist radius w 0 = 4 λ, and normalized magnitude a 0 = 20 (energy of 85 mJ), a plasma with a lattice structure is generated first. Then, an acceleration field of 14.2 TV/cm can be induced by the radiation pressure of the ultraviolet pulse in the target, which is about one order of magnitude larger than that of optical lasers for the same a0, and the lattice structure exerts effect on the acceleration only in the vicinity of the optimal target thickness. As a result, a quasi-monoenergetic Cu ion beam of energy of 5 GeV (75 MeV/nucleon), a charge of 0.12 nC, and the emittance of 7 × 10 − 9 m rad can be produced, which implies that using ultraviolet lasers instead of optical lasers should turn down the size and emittance of ion sources by orders lower than that of optical lasers. Therefore, a high-performance ion source is produced, which may have potential applications in medical therapy and ion radiography.

We investigated laser proton acceleration in the bubble regime when radiation reaction (RR) effects become non-negligible. By using particle-in-cell simulations, it is shown that in addition to the bubble field, the RR trapped electrons form another charge-separation field, leading to fast acceleration of the background protons at the early stage. However, for insufficient laser intensities, the reduced bubble field and acceleration length originating from the laser depletion by gamma-photon emission do not allow for trapping and further acceleration of the protons. The final proton energy is significantly smaller than the one when RR is not considered. This effect can be mitigated by tuning the peak laser intensity (hence the pulse duration) for certain laser energy and plasma density. By increasing the laser intensity (shortening the pulse duration), we found that protons pre-accelerated by the RR-induced charge separation field gain enough momenta such that they are picked-up by the bubble field and continuously accelerated. In this case, the bubble field is also strong enough to trap protons located in the bubble front. Eventually, two groups of protons are effectively accelerated, resulting in a bump in the proton spectrum. These results can be used to design the future proton acceleration experiments in upcoming 100 PW laser facilities.We investigated laser proton acceleration in the bubble regime when radiation reaction (RR) effects become non-negligible. By using particle-in-cell simulations, it is shown that in addition to the bubble field, the RR trapped electrons form another charge-separation field, leading to fast acceleration of the background protons at the early stage. However, for insufficient laser intensities, the reduced bubble field and acceleration length originating from the laser depletion by gamma-photon emission do not allow for trapping and further acceleration of the protons. The final proton energy is significantly smaller than the one when RR is not considered. This effect can be mitigated by tuning the peak laser intensity (hence the pulse duration) for certain laser energy and plasma density. By increasing the laser intensity (shortening the pulse duration), we found that protons pre-accelerated by the RR-induced charge separation field gain enough momenta such that they are picked-up by the bubble field and co...

An approach is proposed to directly measure the relativistic laser pulse duration. In the scheme, two identical high-intensity laser pulses are irradiated on a thin plasma target symmetrically in V-shape. High order harmonics carrying the information of two incident pulses are generated in different directions during the nonlinear interaction process. The direction of the third harmonic can be predicted from the theoretical analysis and its intensity in this direction can be recorded as an autocorrelation function of the delay time between the incident pulses. Then, the pulse duration which is 70% of the full width at half maximum of the autocorrelation curve can be obtained. This approach has been verified by particle-in-cell simulations and the error is 3.7% for a 30 fs relativistic laser pulse as an example.An approach is proposed to directly measure the relativistic laser pulse duration. In the scheme, two identical high-intensity laser pulses are irradiated on a thin plasma target symmetrically in V-shape. High order harmonics carrying the information of two incident pulses are generated in different directions during the nonlinear interaction process. The direction of the third harmonic can be predicted from the theoretical analysis and its intensity in this direction can be recorded as an autocorrelation function of the delay time between the incident pulses. Then, the pulse duration which is 70% of the full width at half maximum of the autocorrelation curve can be obtained. This approach has been verified by particle-in-cell simulations and the error is 3.7% for a 30 fs relativistic laser pulse as an example.

Antiproton beam generation is investigated based on the ultra-intense femtosecond laser pulse by using two-dimensional particle-in-cell and Geant4 simulations. A high-flux proton beam with an energy of tens of GeV is generated in sequential radiation pressure and bubble regime and then shoots into a high-Z target for producing antiprotons. Both yield and energy of the antiproton beam increase almost linearly with the laser intensity. The generated antiproton beam has a short pulse duration of about 5 ps and its flux reaches 2 × 10 20 s − 1 at the laser intensity of 2.14 × 10 23 W / cm 2. Compared to conventional methods, this new method based on the ultra-intense laser pulse is able to provide a compact, tunable, and ultrafast antiproton source, which is potentially useful for quark-gluon plasma study, all-optical antihydrogen generation, and so on.

We report a proposal to generate an ultra-bright, well-collimated, GeV gamma-ray source in the quantum electrodynamics regime. An ultra-intense laser is used to excite a bubble-like structure in a near-critical-density plasma, where the trapped electrons with overdense density can be effectively accelerated to multi-GeV. These electrons are capable of generating high-quality gamma-ray radiation with high energy-conversion-efficiency under the ultra-intense wakefield. Meanwhile, the radiation reaction effect exerted on the electrons further improves the collimation of the gamma-ray radiation. The validity of our proposal has been confirmed using two- and three-dimensional (2D and 3D) particle-in-cell simulations. 3D simulation shows that a bunch of gamma-ray photons with a density of 3.52 × 1023/cm3, an emittance of 1.04 mm·mrad, a divergence of about 10°, and a maximum energy of about 6 GeV can be realized by a circularly polarized laser with an intensity of 3.8 × 1023 W/cm2. The total number of the photons in the energy range between 1 GeV and 6 GeV is about 2.91 × 1011, which corresponds to a brilliance of 2.34 × 1025 photons/s/mm2/mrad2/GeV.

When laser intensity exceeds 10^22W/cm^2, photons with energy above MeV can be generated from high-order harmonics process in the laser-plasma interaction. We find that under such laser intensity, QED effect plays a dominating role in the radiation pattern. Contrast to the gas and relativistic HHG processes, both the occurrence and energy of gamma-ray emission produced by QED harmonics are random and QED harmonics are usually not coherent, while the property of high intensity and ultra-short duration is conserved. Our simulation shows that the period of gamma-ray train is half of the laser period and the peak intensity is 1.4e22W/cm^2. This new harmonic production with QED effects are crucial to light-matter interaction in strong field and can be verified in experiments by 10PW laser facilities in the near future., Comment: 12 pages, 4 figures

Experimental evidence has accumulated to indicate that wakefield acceleration (WFA) accompanies intense and sometimes coherent emission of radiation such as from betatron radiation. The investigation of this issue has additional impetus nowadays because we are learning (1) there is an additional acceleration process of the ponderomotive acceleration; (2) WFA may become relevant in much higher density regimes; (3) WFA has been proposed as the mechanism for extreme high energy cosmic ray acceleration and gamma ray bursts for active galactic nuclei. These require us to closely examine the radiative mechanisms in WFA anew. We report studies of radiation from wakefield (self-injected betatron) and ponderomotive (laser field) mechanisms in scalings of the frequency and intensity of the driver, as well as the plasma density.

We present a quasi-stationary fluid theory to precisely describe the hole-boring process. The corresponding distributions of the electrostatic field and the particle density are theoretically obtained, which give more details than the previous stationary theory. The theoretical result is confirmed by one-dimensional particle-in-cell simulations. Such quasi-stationary fluid theory may help in understanding the basic mechanisms of ion acceleration in the radiation pressure acceleration.

A cascaded radiation-pressure acceleration scheme is proposed. When an energetic proton beam is injected into an electrostatic field moving at light speed in a foil accelerated by light pressure, protons can be re-accelerated to much higher energy. An initial 3-GeV proton beam can be re-accelerated to 7 GeV while its energy spread is narrowed significantly, indicating a 4-GeV energy gain for one acceleration stage, as shown in one-dimensional simulations and analytical results. The validity of the method is further confirmed by two-dimensional simulations. This scheme provides a way to scale proton energy at the GeV level linearly with laser energy and is promising to obtain proton bunches at tens of gigaelectron-volts.

A new scheme for proton acceleration by cascaded collisionless electrostatic shock (CES) is proposed. By irradiating a foil target with a moderate high-intensity laser beam, a stable CES field can be induced, which is employed as the accelerating field for the booster stage of proton acceleration. The mechanism is studied through simulations and theoretical analysis, showing that a 55 MeV seed proton beam can be further accelerated to 265 MeV while keeping a good energy spread. This scheme offers a feasible approach to produce proton beams with energy of hundreds of MeV by existing available high-intensity laser facilities.

The sequential proton acceleration by two laser pulses of relativistic intensity is proposed to produce high energy protons. In the scheme, a relativistic super-Gaussian (SG) laser pulse followed by a Laguerre-Gaussian (LG) pulse irradiates dense plasma attached by underdense plasma. A proton beam is produced from the target and accelerated in the radiation pressure regime by the short SG pulse and then trapped and re-accelerated in a special bubble driven by the LG pulse in the underdense plasma. The advantages of radiation pressure acceleration and LG transverse structure are combined to achieve the effective trapping and acceleration of protons. In a two-dimensional particle-in-cell simulation, protons of 6.7 GeV are obtained from a 2 × 1022 W/cm2 SG laser pulse and a LG pulse at a lower peak intensity.

CO2 laser-driven electron acceleration is demonstrated with particle-in-cell simulation in low-density plasma. An intense CO2 laser pulse with long wavelength excites wakefield. The bubble behind it has a broad space to sustain a large amount of electrons before reaching its charge saturation limit. A transversely propagating inject pulse is used to induce and control the ambient electron injection. The accelerated electron bunch with total charge up to 10 nC and the average charge per energy interval of more than 0.6 nC/MeV are obtained. Plasma-based electron acceleration driven by intense CO2 laser provides a new potential way to generate high-charge electron bunch with low energy spread, which has broad applications, especially for X-ray generation by table-top FEL and bremsstrahlung.

Light pressure acceleration of ions in the interaction of the frequency-tripled (3ω) laser pulse and foil target is studied, and a promising method to increase accelerated ion energy is shown. Results show that at a constant laser energy, much higher ion energy peak value is obtained for 3ω laser compared with that using the fundamental frequency laser. The effect of energy loss during frequency conversion on ion acceleration is considered, which may slightly decrease the acceleration effect.

An interesting layered structure of multiple high density layers are formed when two counter-propagating circularly polarized laser pulses with the same polarization direction irradiate on an ultra-thin foil. This structure changes periodically. For light atoms most of which electrons may be fully ionized, this layered structure can keep for dozens of laser periods after the laser-foil interaction. This interesting structure may have potential applications.

A cascaded target normal sheath acceleration (TNSA) scheme is proposed to simultaneously increase energy and improve energy spread of a laser-produced mono-energetic proton beam. An optimum condition that uses the maximum sheath field to accelerate the center of the proton beam is theoretically found and verified by two-dimensional particle-in-cell simulations. An initial 10 MeV proton beam is accelerated to 21 MeV with energy spread decreased from 5% to 2% under the optimum condition during the process of the cascaded TNSA. The scheme opens a way to scale proton energy lineally with laser energy.

High harmonic generation (HHG) from relativistic laser-foil interaction is investigated analytically and through particle-in-cell simulations. Previous work has shown that when two counter-propagating circularly polarized (CP) laser pulses interact with a thin foil, electrons can be well confined spatially to form a high density layer. The layer electrons oscillate in certain transversal direction and radiate intense high order harmonics. It is demonstrated here that there is a critical foil thickness, only below which can high harmonics be generated efficiently. Furthermore, to enhance the intensity in higher order region, the third linearly polarized (LP) short-wavelength laser pulse with much lower intensity is introduced. Analysis and simulations both show that the enhancement is determined by the relative phase δϕ between the driving CP laser pulses and LP pulse. The enhancement at high order is quite considerable and very sensitive to the relative phase δϕ, thus offering not only a way to efficientl...

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.

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.

Instabilities in the interaction of a normal intensity circularly polarized pulse and an overdense foil are investigated with two and three dimensional particle-in-cell simulations. Two typical instabilities were shown during the interaction. One is the Weibel-like instability induced by the current far above the Alfven limit, and the other is the boundary instability with ring structures spreading to the center from the boundary which is induced by the transverse boundaries of the target or the laser pulse. These instabilities are important to the proton acceleration by using moderate laser pulses at intensities accessible experimentally with existing laser systems.

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.

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.

The overloading effect of self-injected high-charge electron bunch in the bubble regime of laser wakefield acceleration is studied. When too many electrons are trapped by the bubble, the wakefield can be strongly modified, preventing further injection of the background electrons. This process is directly observed in two-dimensional particle-in-cell simulation and is explained using a one-dimensional wake model. For obtaining significantly more energetic electrons, the use of a decreasing plasma density profile is proposed.

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.

The influence of laser pulse duration and areal density of target in the interaction of a circularly polarized pulse with an ultrathin overdense foil is investigated. One-dimensional particle-in-cell simulation shows that with an appropriate laser-pulse rising front, the light pressure acceleration regime is effective even though the thin foil is transparent. As the laser intensity evolves, three stages in the acceleration process can be identified: at first the total reflection of the laser pulse, followed by partial reflection, and then near total reflection again due to the Doppler effect. The influences of the rising front of laser pulse and areal density of the ultrathin foil are investigated. It is found that an optimal laser pulse rising front exists for obtaining high (saturation) ion energy with the same laser energy within a short time. An optimal areal density also exists for obtaining the highest energy. For the same laser pulse, a higher areal density or a higher density with same areal density is more appropriate for obtaining a stationary state for making light pressure acceleration mechanism more effective.

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.

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.

In laser-target interaction, the effects of laser intensity on plasma oscillation at the front surface of targets have been investigated by one-dimensional particle in cell simulations. The periodical oscillations of the ion density and electrostatic field at the front surface of the targets are reported for the first time, which is considered as an intrinsic property of the target excited by the laser. The oscillation period depends only on initial plasma density and is irrelevant with laser intensity. Flattop structures with curves in ion phase space are found with a more intense laser pulse due to the larger amplitude variation of the electrostatic field. A simple but valid model is proposed to interpret the curves.

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.

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.

Electrons accelerated by a propagating laser pulse of linear or circular polarization in vacuum have been investigated by one-dimensional particle-in-cell simulations and analytical modeling. A stopping target is used to stop the laser pulse and extract the energetic electrons from the laser field. The effect of the reflected light is taken into account. The maximum electron energy depends on the laser intensity and initial electron energy. There is an optimal acceleration length for electrons to gain maximum energy where electrons meet the peak of the laser pulse. The optimal acceleration length depends strongly on the laser pulse duration and amplitude.

A multiple-staged ion acceleration mechanism in the interaction of a circularly polarized laser pulse with a solid target is studied by one-dimensional particle-in-cell simulation. The ions are accelerated from rest to several MeV monoenergetically at the front surface of the target. After all the plasma ions are accelerated, the acceleration process is repeated on the resulting monoenergetic ions. Under suitable conditions multiple repetitions can be realized and a high-energy quasi-monoenergetic ion beam can be obtained.

An alternative fast-ignition method is proposed involving the formation of a hot spot outside the precompressed fusion-fuel core by a series of shocks driven directly by the light pressure of laser pulses of increasing intensities. It is shown that a hot spot, which can be of different material from that of the fuel core, with temperature ∼10keV and density ∼200g∕cm2, can be formed. Being an electrically neutral plasma, the hot spot can easily be sent into the fuel core.