Laser wake field acceleration: the highly non-linear broken-wave regime

We use three-dimensional particle-in-cell simulations to study laser wake field acceleration (LWFA) at highly relativistic laser intensities. We observe ultra-short electron bunches emerging from laser wake fields driven above the wave-breaking threshold by few-cycle laser pulses shorter than the plasma wavelength. We find a new regime in which the laser wake takes the shape of a solitary plasma cavity. It traps background electrons continuously and accelerates them. We show that 12-J, 33-fs laser pulses may produce bunches of 3×10¹⁰ electrons with energy sharply peaked around 300 MeV. These electrons emerge as low-emittance beams from plasma layers just 700-μm thick. We also address a regime intermediate between direct laser acceleration and LWFA, when the laser-pulse duration is comparable with the plasma period. Received: 12 December 2001 / Published online: 14 March 2002     

Neutron Generation and Kinetic Energy of Expanding Laser Plasmas

We investigate the kinetic energy of expanding plasma of a solid target heated by a ultra-short and ultra-intense laser pulse and the efllciency of energy coupling between the ultra-intense laser pulse and the solid target, in order to increase the utilization ratio of laser energy and to raise the neutron generation farther. Some new ideas about improving the energy utilization by head-on collision~, between the expanding plasmas are proposed. The significance is the raise of generation of shorter duration neutron, of the order of picoseconds, which allows for an increase of energy resolution in time-of-flight experiments and also for the investigation of the dynamics of nuclear processes with high temporal resolution.     

Intensity threshold in vacuum laser acceleration

The dependence of the electron-energy gain on the on-axis laser intensity of a TEM₀₀ light wave in vacuum, called the capture and acceleration scenario (CAS), has been studied. We found that there exists a laser intensity threshold for the CAS scheme to work. The physical meaning of the intensity threshold is that, when the intensity is strong enough, fast electrons injected into the Rayleigh zone where the phase velocity of the light wave is subluminous can be accelerated until they catch up with the phase velocity before they slip out. Thereby these electrons can receive a considerable amount of energy from the laser field. Analytical calculations based on this situation and simulation results show similar features in that the intensity threshold value, (a₀ ^T)², is strongly dependent on the laser-beam width at focus, kw₀. For example, kw₀=40 corresponds to a₀ ^T∼5, which is available by present laser systems. Also, it has been proved that the maximal electron-energy gain in the CAS regime is linearly proportional to the laser intensity as well as to kw₀. Received: 20 January 2003 / Revised version: 6 March 2003 / Published online: 23 May 2003 RID="*" ID="*"Corresponding author. Fax: +86-21/6564-3815, E-mail: hoyk@fudan.ac.cn     

Diffraction of laser-plasma-generated electron pulses

We report the observation of the Debye–Scherrer diffraction using electron pulses emitted from a fs-laser plasma. Titanium sapphire laser pulses with 1.6 mJ/45 fs at 1 kHz are focused on a moving steel tape at close to normal incidence. The laser plasma generated ejects a large number of electrons in the direction of polarization, with a continuous energy spectrum extending up to 100 keV. Selecting an energy range of these electrons and scattering them on a thin aluminium sample generates a “streaked” diffraction pattern with unique features.     

Effect of spontaneous magnetic fields on energy transfer by fast electrons in spherical laser targets

It is shown that the spontaneous magnetic fields generated due to development of hydrodynamic instability in the process of compression of spherical laser targets can substantially affect the energy transfer by fast electrons. The analytical solutions to the kinetic equation for the distribution function of fast electrons are obtained taking into account the reflection from the region of the magnetic-field localization. On the basis of these solutions, the effect of the spontaneous magnetic fields on the specific energy released by the fast electrons per unit mass and on preheating of the target material being compressed is investigated.     

Effect of Cluster Coulomb Fields on Electron Acceleration in Laser-Cluster Interaction

Single particle simulations are used to investigate electron acceleration in the laser-cluster interaction, taking into account the Coulomb fields around individual clusters. These Coulomb fields are induced from the cluster cores with positive charge when electrons escape from the cluster cores through ponderomotive push from the laser field. These Coulomb fields enable some stripped electrons to be stochastically in phases with the laser fields so that they can gain net energy from the laser efficiently. In this heating mechanism, circularly polarized lasers, larger cluster size and higher cluster densities make the acceleration more efficient.     

Electron acceleration to GeV energy by a radially polarized laser

We present a relativistic single particle simulation of vacuum acceleration of an electron by a high-intensity radially polarized laser beam. The inherent complete symmetry of radially polarized laser beam leads to improvement in the trapping and acceleration of an electron so that an electron can be accelerated to the level of GeV. In addition, the external magnetic field further enhances the electron acceleration. Hence, an electron of ultrahigh energy was observed. The strong correlation between final electron energy and scattering angle is discussed.     

激光等离子体相互作用的局域振荡电子加热机制

 用2.5维粒子模拟程序模拟了超强激光与等离子体的相互作用过程，发现超强激光可以通过J×B加热机制加速电子并引起电荷分离，从而产生很强的静电场并形成电场势阱，电子在静电场势阱中振荡，被多次加速，使得高速电子被甩出势阱，进而增强电荷分离，然后静电场结构被破坏，静电势能传给电子。在此过程中，电子在此阱中作局域振荡，并且被J×B机制多次加速，激光的能量会有效地传给电子，使电子能量高达10MeV。这是一种新的电子加热机制，称之为局域振荡电子加热机制。     

A simple non-perturbative approach of atom ionisation by intense and ultra-short laser pulses

A simple theoretical approach based on Coulomb-Volkov states is introduced to predict ionisation of atoms by intense laser pulses in cases where the effective interaction time does not exceed one or two optical cycles [M. Nisoli et al., Opt. Lett. 22, 522 (1997)]. Under these conditions, the energy distributions of ejected electrons predicted by this non-perturbative approach are in very good agreement with “exact" results obtained by a full numerical treatment. The agreement is all the better that the principal quantum number of the initial state is high. For very strong fields, most electrons are ejected at an energy which is close to the classical kinetic energy that would be transferred to free electrons by the electromagnetic field during the pulse. The power of the present approach appears when keV. In this region, full numerical treatments become very lengthy and finally do not converge. However, the present Coulomb-Volkov theory still makes reliable predictions in very short computer times. Received 19 November 1999 and Received in final form 19 January 2000     

Thermal emission of electrons from highly excited sodium clusters

Positively charged sodium clusters can be easily ionized by a fs laser pulse of relatively low intensity (<10¹⁰ W/cm²), if the laser is in resonance with the plasmon excitation of the cluster. This ionization process was investigated in detail by measuring the kinetic energy distribution of electrons emitted from a size-selected Na₉₃ ⁺ as a function of the fs laser intensity. In all cases pure Boltzmann-like energy distributions were observed. A comparison with statistical theory shows that the emission is a purely thermal process. It is different to normal thermionic emission insofar as the electrons are emitted from a hot electron system which is only weakly coupled to a cold ionic background. The results demonstrate purely statistical behaviour of a small fermionic system even for very high excitation energy. Received: 25 May 2000 / Accepted: 6 November 2000 / Published online: 9 February 2001     

Motion and acceleration of electrons in high-intensity laser standing waves

The motion and the energy of electrons driven by the ponderomotive force in linearly polarized high-intensity laser standing wave fields are considered. The results show that there exists a threshold laser intensity, above which the motion of electrons incident parallel to the electric field of the laser standing waves undergoes a transition from regulation to chaos. We propose that the huge energy exchange between the electrons and the strong laser standing waves is triggered by inelastic scattering, which is related to the chaos patterns. It is shown that an electron's energy gain of tens of MeV can be realized for a laser intensity of 10²⁰ W/cm².     

Theory of single-mode lasing in coherent quantum cascade lasers

A theory is developed for steady-state single-mode lasing in coherent quantum-well cascade lasers. This laser model is an example of a strictly quantum mechanical problem in which approximate kinetic approaches are not used to account for dissipative scattering processes. Exact wave functions are found for the system in weak and strong electromagnetic fields, so that the output power and frequency can be determined as functions of the coherent pump current and system parameters. It is shown that for pumping by monoenergetic electrons the power has a nonlinear (root) dependence and tends to saturate in strong fields. It is predicted that the coherent pumping efficiency may be increased by adjusting the energy of the pump electrons, which will lead to a linear power dependence, a high efficiency, and low threshold currents. A population inversion is found not be a necessary condition for lasing in the coherent laser. In particular, in the high field regime the population of the lower level exceeds that of the upper, while in the optimally adjusted regime they are the same. Zh. éksp. Teor. Fiz. 112, 483–498 (August 1997)     

电子在激光驻波场中运动产生的太赫兹及X射线辐射研究

采用单电子模型和经典辐射理论分别对低能和高能电子在线偏振激光驻波场中的运动和辐射过程进行了研究. 结果表明: 垂直于激光电场方向入射的低速电子在激光驻波场中随着光强的增大, 逐渐从一维近周期运动演变为二维折叠运动, 并产生强的微米量级波长的太赫兹辐射; 高能电子垂直或者平行于激光电场方向入射到激光驻波场中, 都会产生波长在几个纳米的高频辐射; 低能电子与激光驻波场作用中, 激光强度影响着电子的运动形式、辐射频率以及辐射强度; 高能电子入射时, 激光强度影响了电子高频辐射的强度, 电子初始能量影响着辐射的频率; 电子能量越高, 产生的辐射频率越大. 研究表明可以由激光加速电子的方式得到不同能量的电子束, 并利用电子束在激光驻波场的辐射使之成为太赫兹和X射线波段的小型辐射源. 研究结果可以为实验研究和利用激光驻波场中的电子辐射提供依据.     

Effects of inverse bremsstrahlung in laser-induced plasmas from a graphite surface

The kinetic energy of electrons emitted due to laser interaction with a graphite surface was studied with a time-of-flight spectrometer. In addition the yields of carbon atomic and molecular ions were measured as a function of laser pulse energy. Pulse energy thresholds for ion emission are observed to correlate with the observed maximum electron energies. Furthermore, the data suggest that ionic carbon clusters can be dissociated by energetic electrons or photons created in the plasma. We believe that initially photoemitted electrons are accelerated by inverse bremsstrahlung to the energies required for electron impact ionization and dissociation     

Kinetic Thomas-Fermi solutions of the Gross-Pitaevskii equation

Approximate solutions of the Gross-Pitaevskii (GP) equation, obtained upon neglection of the kinetic energy, are well known as Thomas-Fermi solutions. They are characterized by the compensation of the local potential by the collisional energy. In this article we consider exact solutions of the GP-equation with this property and definite values of the kinetic energy, which suggests the term “kinetic Thomas-Fermi” (KTF) solutions. Despite their formal simplicity, KTF-solutions can possess complex current density fields with unconventional topology. We point out that a large class of light-shift potentials gives rise to KTF-solutions. As elementary examples, we consider one-dimensional and two-dimensional optical lattice scenarios, obtained by means of the superposition of two, three and four laser beams, and discuss the stability properties of the corresponding KTF-solutions. A general method is proposed to excite two-dimensional KTF-solutions in experiments by means of time-modulated light-shift potentials.     

Electron dynamics characteristics in high-intensity laser fields

This paper addresses the conditions under which the vacuum laser acceleration scheme CAS (capture and acceleration scenario), newly proposed by the authors (see, e.g., P.X. Wang et al., Appl. Phys. Lett. 78, 2253 (2001)), can be observed. Specifically, the laser intensity threshold (a₀)_th and the range of the electron incident momentum for the CAS scheme to emerge are examined. We found that (a₀)_th is critically dependent on the laser beam width w₀. At kw₀=60, (a₀)_th=8, which is an intensity obtainable using present laser systems. The required energy of the incident electron is in the range 5–15 MeV. This study is of significance in designing an experimental setup to test CAS and helpful in understanding the basic physics of CAS. Received: 4 March 2002 / Revised version: 25 March 2002 / Published online: 8 May 2002     

Non-equilibrium electron and phonon dynamics in metals under femtosecond laser pulses

Different models for relaxation dynamics of electrons and phonons in a thin metal film heated by femto-pico second laser pulses have been discussed. The traditional two-temperature approach reveals to be inaccurate due to deviations of electrons and phonons from Fermi-Dirac and Bose-Einstein distributions, respectively. Coupled Boltzmann kinetic equations have been adapted for the quantum statistics to study the energy distribution of electrons and phonons in metals. Theoretical details have been discussed and a new solution method has been proposed overcoming numerical problems and improving stability, allowing the study of the dynamics until the complete relaxation. Numerical results have been compared with photoemission spectroscopy experimental data.     

Interaction of a high-intensity ultrashort laser pulse with extended nanofilaments of dense plasma

Generation and propagation of fast electrons in laser targets consisting of thin nanofilaments are studied numerically and analytically. Such targets completely absorb laser radiation and exhibit a large coefficient of laser-energy conversion to kinetic energy of a flow of fast electrons. Analytical estimates show that the optimal thickness of the filament is on the order of the skin depth of the laser plasma, while an optimal distance between filaments is on the order of the Debye radius of hot electrons. A bunch of relativistic electrons can propagate as far as several hundred micrometers in such targets, while the fastest electrons can propagate several millimeters. Upon bending of filaments, the flow of electrons propagates along the filaments and can be focused by bringing the filaments together. Laser targets of the discussed composition are used as sources of dense bunches of relativistic electrons and subsequent generation of high-intensity X-ray radiation with their help.     

通道靶对超强激光加速质子束的聚焦效应

发散角过大是制约超强激光与固体靶相互作用加速产生高能质子束应用的一个重大物理难题.本文提出了一种结构化的通道靶型,与超强激光相互作用可提高质子束的发散特性,通道壁上产生的横向电荷分离静电场可对质子有效聚焦.采用二维particle-in-cell粒子模拟程序对激光通道靶相互作用过程进行了研究,分析了加速质子束的性能特点.模拟结果表明,与传统平面靶相比,通道靶可以在不过多损失能量的情况下产生具有更好准直性的质子束,尤其当通道靶的直径与激光焦斑尺寸和质子源尺寸相当时,横向静电场能够有效聚焦质子束,并且可保证相对较高的激光能量利用率.     

Additional acceleration and collimation of relativistic electron beams by magnetic field resonance at very high intensity laser interaction

For the interpretation of experiments for acceleration of electrons at interaction up to nearly GeV energy in laser produced plasmas, we present a new model using interaction magnetic fields. In addition to the ponderomotive acceleration of highly relativistic electrons at the interaction of very short and very intense laser pulses, a further acceleration is derived from the interaction of these electron beams with the spontaneous magnetic fields of about 100 MG. This additional acceleration is the result of a laser-magnetic resonance acceleration (LMRA) around the peak of the azimuthal magnetic field. This causes the electrons to gain energy within a laser period. Using a Gaussian laser pulse, the LMRA acceleration of the electrons depends on the laser polarization. Since this is in the resonance regime, the strong magnetic fields affect the electron acceleration considerably. The mechanism results in good collimated high energetic electrons propagating along the center axis of the laser beam as has been observed by experiments and is reproduced by our numerical simulations. PACS 41.75.Jv; 52.38.Kd; 52.65.Cc