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Nonlinear plasma wavelength scalings in a laser wakefield accelerator
摘要： Laser wake?eld acceleration relies on the excitation of a plasma wave due to the ponderomotive force of an intense laser pulse. However, plasma wave trains in the wake of the laser have scarcely been studied directly in experiments. Here we use few-cycle shadowgraphy in conjunction with interferometry to quantify plasma waves excited by the laser within the density range of GeV-scale accelerators, i.e., a few 1018 cm?3. While analytical models suggest a clear dependency between the nonlinear plasma wavelength and the peak potential a0, our study shows that the analytical models are only accurate for driver strength a0 (cid:2) 1. Experimental data and systematic particle-in-cell simulations reveal that nonlinear lengthening of the plasma wave train depends not solely on the laser peak intensity but also on the waist of the focal spot.
关键词： nonlinear plasma wavelength,particle-in-cell simulations,plasma waves,laser wake?eld acceleration
更新于2025-09-16 10:30:52
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Particle-in-cell simulations of density peak formation and ion heating from short pulse laser-driven ponderomotive steepening
摘要： We use two-dimensional particle-in-cell (PIC) simulations and simple analytic models to investigate the laser-plasma interaction known as ponderomotive steepening. When normally incident laser light reflects at the critical surface of a plasma, the resulting standing electromagnetic wave modifies the electron density profile via the ponderomotive force, which creates peaks in the electron density separated by approximately half of the laser wavelength. What is less well studied is how this charge imbalance accelerates ions toward the electron density peaks, modifying the ion density profile of the plasma. Idealized PIC simulations with an extended underdense plasma shelf are used to isolate the dynamics of ion density peak growth for a 42 fs pulse from an 800 nm laser with an intensity of 1018 W cm?2. These simulations exhibit sustained longitudinal electric fields of 200 GV m?1, which produce countersteaming populations of ions reaching a few kilo-electron-volt in energy. We compare these simulations to theoretical models, and we explore how ion energy depends on factors such as the plasma density and the laser wavelength, pulse duration, and intensity. We also provide relations for the strength of longitudinal electric fields and an approximate time scale for the density peaks to develop. These conclusions may be useful for investigating the phenomenon of ponderomotive steepening as advances in laser technology allow shorter and more intense pulses to be produced at various wavelengths. We also discuss the parallels with other work studying the interference from two counterpropagating laser pulses.
关键词： laser-plasma interaction,ion heating,ponderomotive steepening,density peak formation,particle-in-cell simulations
更新于2025-09-12 10:27:22
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Enhancement of target normal sheath acceleration in laser multi-channel target interaction
摘要： Target-normal sheath acceleration (TNSA) of ions by >100-fs relativistic laser pulses irradiating a multichannel target consisting of a row of parallel long wires and a plane back foil is studied. Two-dimensional particle-in-cell simulations show that the laser light pulls out from the wires a large number of dense hot attosecond electron bunches, which are synergetically accelerated forward by the relativistic ponderomotive force of the laser as well as the longitudinal electric field of a transverse magnetic mode that is excited in the vacuum channels between the wires. These electrons are characterized by a distinct two-temperature energy spectrum, with the temperature of the more energetic electrons close to twice the ponderomotive potential energy. After penetrating through the foil, they induce behind its rear surface a sheath electric field that is both stronger and frontally more extended than that without the channels. As a result, the TNSA ions have much higher maximum energy and the laser-to-ion energy conversion efficiency is also much higher. It is found that a laser of intensity 1.37 × 1020 W/cm2, duration 165 fs, and energy 25.6 J can produce 85 MeV protons and 31 MeV/u carbon ions, at 30% laser-to-ion energy conversion efficiency. The effects of the channel size and laser polarization on the TNSA ions are also investigated.
关键词： laser-plasma interaction,ion acceleration,Target-normal sheath acceleration,particle-in-cell simulations,TNSA,laser-to-ion energy conversion efficiency
更新于2025-09-12 10:27:22
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Effect of laser polarization on the electron dynamics and photon emission in near-critical-density plasmas
摘要： When the laser intensity approaches 1022 W=cm2, the recoil force from high-energy photons radiated by relativistic electrons becomes important and may be comparable with the ponderomotive force of the laser pulse. As a result, the electron dynamics is modified greatly. By using particle-in-cell simulations, we show that the polarization of laser pulse has significant influence on the electron behaviors and accompanying radiation in near-critical-density plasmas. Due to the RR force and the pinching effect of self-generated magnetic field, it turns out that much more electrons are trapped in the center of a circularly-polarized (CP) laser pulse than that of a linearly-polarized (LP) laser pulse. Meanwhile, the (cid:13)(cid:0)photon emission is characterized by a two-peak structure in the LP case, while it is dominated in the forward direction in the CP case where electrons keep moving forward along the laser propagating direction. These may serve as a potential signature for radiation trapping effect in forthcoming experiments to be carried out on petawatt (PW) laser facilities.
关键词： laser polarization,photon emission,electron dynamics,radiation reaction force,near-critical-density plasmas,particle-in-cell simulations
更新于2025-09-12 10:27:22
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Interactions of laser speckles due to kinetic stimulated Raman scattering
摘要： We examine the possible interactions of two laser speckles due to the exchange of electrons and waves generated by stimulated Raman scattering (SRS) using two-dimensional particle-in-cell simulations. By controlling the relative polarization, spatial placement, and timing of two laser speckles, one above-threshold and one below-threshold for SRS, we isolate and characterize SRS growth stimulated in below-threshold speckles. SRS in the below-threshold speckle is shown to be triggered by energetic electrons, scattered light waves, and scattered plasma waves generated from the above-threshold speckle. We show that scattered light or electrons alone can be an intermediary trigger for SRS.
关键词： electron plasma waves,particle-in-cell simulations,stimulated Raman scattering,laser speckles
更新于2025-09-12 10:27:22
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Improving the quality of proton beams via double targets driven by an intense circularly polarized laser pulse
摘要： A new scheme is proposed to improve the quality of proton beams via ultra-intense laser pulse interacting with double plasma targets, which consist of a pre-target with relatively low density and a main target with high density. Both one- and two-dimensional Particle-in-Cell simulations show that, the using of an appropriate pre-target can help to obtain a much stronger longitudinal charge separation field in contrast to using only the main target. And proton beam with lower momentum divergence, better monochromaticity and collimation, as well as higher current density is generated. Moreover, due to the strengthened coupling between the laser pulse and targets, the energy conversion from laser pulse to protons is also increased.
关键词： proton beams,charge separation field,Particle-in-Cell simulations,circularly polarized laser pulse,double targets
更新于2025-09-12 10:27:22
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THz Generation from Relativistic Plasmas Driven by Near- to Far-Infrared Laser Pulses
摘要： Terahertz pulse generation by ultraintense two-color laser fields ionizing gases with near- to far-infrared carrier wavelength is studied from particle-in-cell simulations. For a long pump wavelength (10.6 μm) promoting a large ratio of electron density over critical, photoionization is shown to catastrophically enhance the plasma wakefield, causing a net downshift in the optical spectrum and exciting THz fields with tens of GV/m amplitude in the laser direction. This emission is accompanied by coherent transition radiation (CTR) of comparable amplitude due to wakefield-driven electron acceleration. We analytically evaluate the fraction of CTR energy up to 30% of the total radiated emission including the particle self-field and numerically calibrate the efficiency of the matched blowout regime for electron densities varied over three orders of magnitude.
关键词： photoionization,laser-plasma interaction,plasma wakefield,Terahertz,coherent transition radiation,particle-in-cell simulations
更新于2025-09-12 10:27:22
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Photoelectrochemical activity of CdS/Ag/TiO2 nanorod composites: Degradation of nitrobenzene coupled with the concomitant production of molecular hydrogen
摘要： A new dynamical model is built to describe the process of microwave window breakdown at the vacuum/dielectric interface. In this model, the effects of gas desorption and gas diffusion on breakdown are first taken into account. The evolution of the density of electrons and the neutral gas is analyzed. Particle-in-cell simulations are employed to validate this model, and the agreements of comparisons are favorable. The results show that the density of neutral gas decreases exponentially with the distance away from the dielectric surface. It is concluded that the gas diffusion is the main factor in the density reduction of neutral gas. With the influence of gas ionization, the number of electrons striking the dielectric surface increases. As a result, the gas pressure above the dielectric surface increases rapidly. In addition, the discharge formation time tc of microwave window breakdown decreases exponentially with the gas desorption rate. Besides, tc decreases with the energy of the first crossover point of the secondary electron emission curve when the strength of microwave electric field Erf is small. This dynamical model provides a solution to designing the microwave window.
关键词： gas desorption,microwave window breakdown,gas diffusion,vacuum/dielectric interface,particle-in-cell simulations
更新于2025-09-11 14:15:04
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A multihertz, kiloelectronvolt pulsed proton source from a laser irradiated continuous hydrogen cluster target
摘要： A high-repetition rate laser-driven proton source from a continuously operating cryogenic hydrogen cluster target is presented. We demonstrate a debris-free, Coulomb-explosion based acceleration in the 10s of kilo-electron-volt range with a stability of about 10% in a 5 Hz operation. This acceleration mechanism, delivering short pulse proton bursts, represents an ideal acceleration scheme for various applications, for example, in materials science or as an injector source in conventional accelerators. Furthermore, the proton energy can be tuned by varying the laser and/or cluster parameters. 3D numerical particle-in-cell simulations and an analytical model support the experimental results and reveal great potential for further studies, scaling up the proton energies, which can be realized with a simple modification of the target.
关键词： high-repetition rate,laser-driven proton source,Coulomb-explosion,particle-in-cell simulations,cryogenic hydrogen cluster target
更新于2025-09-11 14:15:04
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Characterization of laser-driven proton acceleration from water microdroplets
摘要： We report on a proton acceleration experiment in which high-intensity laser pulses with a wavelength of 0.4 μm and with varying temporal intensity contrast have been used to irradiate water droplets of 20 μm diameter. Such droplets are a reliable and easy-to-implement type of target for proton acceleration experiments with the potential to be used at very high repetition rates. We have investigated the influence of the laser’s angle of incidence by moving the droplet along the laser polarization axis. this position, which is coupled with the angle of incidence, has a crucial impact on the maximum proton energy. Central irradiation leads to an inefficient coupling of the laser energy into hot electrons, resulting in a low maximum proton energy. the introduction of a controlled pre-pulse produces an enhancement of hot electron generation in this geometry and therefore higher proton energies. However, two-dimensional particle-in-cell simulations support our experimental results confirming, that even slightly higher proton energies are achieved under grazing laser incidence when no additional pre-plasma is present. illuminating a droplet under grazing incidence generates a stream of hot electrons that flows along the droplet’s surface due to self-generated electric and magnetic fields and ultimately generates a strong electric field responsible for proton acceleration. The interaction conditions were monitored with the help of an ultra-short optical probe laser, with which the plasma expansion could be observed.
关键词： high-intensity laser pulses,water microdroplets,particle-in-cell simulations,laser-driven,proton acceleration
更新于2025-09-11 14:15:04