Cold electrons acceleration in TNSA laser-generated plasma using a low-contrast fs laser

The fs laser facility in Bordeaux, delivering an intensity of 10¹⁸ W/cm² at normal incidence on thin foils, has been used to induce forward electron and ion acceleration in target-normal-sheath-acceleration (TNSA) regime. Micrometric thin foils with different composition, thickness, and electron density, were prepared to promote the charge particle acceleration in the forward direction. The plasma electron and ion emission monitoring were performed on-line using SiC semiconductor detectors in time-of-flight (TOF) configuration and gaf-chromics films both covered by thin absorber filters. The experiment has permitted to accelerate electrons and protons. A special attention was placed to detect relativistic hot electrons escaping from the plasma and cold electrons returning to the target position. The electron energies of the order of 100 keV and of about 1 keV were detected as representative of hot and cold electrons, respectively. A high cold electron contribution was measured using low-contrast fs laser, while it is less evident using high-contrast fs lasers. The charge particle acceleration depends on the laser parameters, irradiation conditions, and target properties, as will be presented and discussed.     

Simulation study of electron injection into plasma wake fields by colliding laser pulses using OOPIC

An electron injector concept for a laser-plasma accelerator has been developed which relies on the use of counter propagating ultrashort laser pulses. In this paper, we use OOPIC the fully self-consistent, twodimensional, particle-in-cell code to make a parameter study to determine the bunches that can be obtained through collisions of two collinear laser pulses in uniform plasma. A series of simulations show that one can obtain a short (<10fs) bunch with its charge of about 15pC, and energy spread of about 15%. We also discussed the variation of the transverse spot size of the electron bunch and found the bunch would undergo the betatron oscillations.     

Simulation study of electron injection into plasma wake fields by colliding laser pulses using OOPIC

An electron injector concept for a laser-plasma accelerator has been developed which relies on the use of counter propagating ultrashort laser pulses. In this paper, we use OOPIC the fully self-consistent, two-dimensional, particle-in-cell code to make a parameter study to determine the bunches that can be obtained through collisions of two collinear laser pulses in uniform plasma. A series of simulations show that one can obtain a short (<10fs) bunch with its charge of about 15pC, and energy spread of about 15%. We also discussed the variation of the transverse spot size of the electron bunch and found the bunch would undergo the betatron oscillations.     

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.     

Cohesive acceleration and focusing of relativistic electrons in overdense plasma

We describe our studies of the generation of plasma wake fields by a relativistic electron bunch and of phasing between the longitudinal and transverse fields in the wake. The leading edge of the electron bunch excites a high-amplitude plasma wake inside the overdense plasma column, and the acceleration and focusing wake fields are probed by the bunch tail. By monitoring the dependence of the acceleration upon the plasma's density, we approached the beam-matching condition and achieved an energy gain of 0.6 MeV over the 17 mm plasma length, corresponding to an average acceleration gradient of 35 MeV/m. Wake-induced modulation in energy and angular divergence of the electron bunch are mapped within a wide range of plasma density. We confirm a theoretical prediction about the phase offset between the accelerating and focusing components of plasma wake.     

交叉传播相对论强激光脉冲在等离子体中相互作用及其对电子加热和加速的作用

文章介绍了相干交叉传播的相对论强激光在与等离子体相互作用中产生的能量交换、瞬态电子密度调制和激光加速电子，这些被加速的电子先在交叉光场中被捕获，随后又注入到等离子体波中，获得进一步的加速．这些现象最近在作者的实验研究和数值模拟中被观察到．     

Direct acceleration of electrons in a corrugated plasma waveguide

Historically, direct acceleration of charged particles by electromagnetic fields has been limited by diffraction, phase matching, and material damage thresholds. A recently developed plasma micro-optic [B. Layer, Phys. Rev. Lett. 99, 035001 (2007)] removes these limitations and promises to allow high-field acceleration of electrons over many centimeters using relatively small femtosecond lasers. We present simulations that show a laser pulse power of 1.9 TW should allow an acceleration gradient larger than 80 MV/cm. A modest power of only 30 GW would still allow acceleration gradients in excess of 10 MV/cm.     

激光等离子体稳相加速机制研究

现有激光等离子体加速机制中纵向电场对离子的有效加速长度很短（微米量级）,且束流能散大,得到的离子能量较低.当采用圆偏振激光和固体靶相互作用时,如果激光的归一化光强矢量a与靶的电子面密度n0〖〗ncD〖〗λL相当时,则存在一种稳相加速机制.此时激光和等离子相互作用产生的静电场不仅可以用于加速离子,而且还可以在纵向对离子进行聚束,从而可以有效地降低束流能散.数值模拟结果表明,利用激光加速可以得到能散小于5％的单能离子束,这对激光加速器走向实际应用有着重要意义.     

激光等离子体稳相加速机制研究

现有激光等离子体加速机制中纵向电场对离子的有效加速长度很短(微米量级),且束流能散大,得到的离子能量较低.当采用圆偏振激光和固体靶相互作用时,如果激光的归一化光强矢量α与靶的电子面密度no/nC D/kL相当时,则存在一种稳相加速机制.此时激光和等离子相瓦作用产生的静电场不仅可以用于加速离子,而且还可以在纵向对离子进行聚束,从而可以有效地降低束流能散.数值模拟结果表明,利用激光加速可以得到能散小于5%的单能离子束,这对激光加速器走向实际应用有着重要意义.     

Direct acceleration of electrons by radially polarized laser pulse

Direct electron acceleration by highly focused ultrahigh-power laser pulses of radial polarization in the ultrarelativistic mode was studied. The mode at which the focusing spot size appears of the same order as the laser radiation wavelength was considered. Electromagnetic fields were calculated using exact Stratton-Chu diffraction integrals. Calculations showed that, as for the case of linear polarization, too sharp focusing (in the diffraction limit) is not optimum for electron acceleration, despite the strong axial field namely in the case of a submicrometer laser spot. At the same time, the case of moderate focusing is more attractive for electron acceleration.     

Ponderomotive acceleration of electrons by an ultrashort laser pulse

By using the corrected solutions for an ultrashort laser pulse, we study the laser-driven electron violent acceleration in vacuum. Our simulations demonstrate that an ultrashort laser pulse with an intensity a₀≡eE₀/m_eωc=3 can accelerate electrons to an energy more than 0.5 GeV. The scaling laws for the net energy gain in different pulse length and laser radius at focus are also studied. Its acceleration mechanism is found to be ponderomotive acceleration.     

Laser-driven acceleration of electrons in a partially ionized plasma channel

The generation of quasimonoenergetic electron beams, with energies up to 200 MeV, by a laser-plasma accelerator driven in a hydrogen-filled capillary discharge waveguide is investigated. Injection and acceleration of electrons is found to depend sensitively on the delay between the onset of the discharge current and the arrival of the laser pulse. A comparison of spectroscopic and interferometric measurements suggests that injection is assisted by laser ionization of atoms or ions within the channel.     

Enhanced acceleration of injected electrons in a laser-beat-wave-induced plasma channel

Enhanced energy gain of externally injected electrons by a approximately 3 cm long, high-gradient relativistic plasma wave (RPW) is demonstrated. Using a CO2 laser beat wave of duration longer than the ion motion time across the laser spot size, a laser self-guiding process is initiated in a plasma channel. Guiding compensates for ionization-induced defocusing (IID) creating a longer plasma, which extends the interaction length between electrons and the RPW. In contrast to a maximum energy gain of 10 MeV when IID is dominant, the electrons gain up to 38 MeV energy in a laser-beat-wave-induced plasma channel.     

用拉盖尔-高斯激光对真空中电子直接加速

对用拉盖尔-高斯(LG)光束加速电子作了研究.结果表明，仅模指数为p和l=1的LG光束的纵向电场可用于加速电子.模指数为p和l=1的线偏振和圆偏振LG光束都对电子有加速效果.对轴上光场的相速度和群速度以及电子能量增益等物理特征作了讨论.给出了轴上光场的相速度和群速度、加速电位及电子能量增益等的解析表达式，并作了数值计算和分析. 关键词： 激光电子加速 拉盖尔-高斯光束 线偏振和圆偏振 能量增益     

Particle acceleration in ultra-intense laser plasma interaction

The acceleration of electrons in laser plasma interaction has been observed since the seventies, when it was initially considered as a deleterious effect, as, in the inertial fusion context, the so-called suprathermal electrons preheat the target. However, it has been quickly observed that a large benefit could be taken from these electrons. Two main directions are now followed. In the first direction, one tries to accelerate electrons to high energy, presently in the GeV range. The electrons may originate from a pre-accelerated beam, or directly from a gas target instantaneously transformed in a plasma by the ultra-intense laser pulse. In the second direction, one tries to transfer the energy of the electrons to fast ions, especially protons, presently in the few tens of MeV range. Thin targets are used for this transformation, the electrons being accelerated at the front of the target, while the ions may originate from the front part or the back part of the target, or from inside the target, depending on the parameters of the experiment. While the maximum energy was the initial goal of the pioneer experiments, there are now strong experimental efforts to improve the quality of the beams, in terms of luminosity, emittance, and energy spectrum. In the recent years, quasi-monoenergetic beams were obtained both for electrons and for ions.     

On the theory of the acceleration of plasma electrons during stimulated scattering of an intense laser wave

The mechanism of electron acceleration in the stimulated Raman forward scattering of a monochromatic laser wave in a cold plasma is investigated theoretically. It is shown that as a result of the stochastic interaction of the electrons with the ponderomotive wave and with plasma waves excited in the scattering process, some of the electrons are accelerated to relativistic energies. Zh. Tekh. Fiz. 69, 3–8 (January 1999)     

Surface acceleration of fast electrons with relativistic self-focusing in preformed plasma

We report an observation of surface acceleration of fast electrons in intense laser-plasma interactions. When a preformed plasma is presented in front of a solid target with a higher laser intensity, the emission direction of fast electrons is changed to the target surface direction from the laser and specular directions. This feature could be caused by the formation of a strong static magnetic field along the target surface which traps and holds fast electrons on the surface. In our experiment, the increase in the laser intensity due to relativistic self-focusing in plasma plays an important role for the formation. The strength of the magnetic field is calculated from the bent angle of the electrons, resulting in tens of percent of laser magnetic field, which agrees well with a two-dimensional particle-in-cell calculation. The strong surface current explains the high conversion efficiency on the cone-guided fast ignitor experiments.     

Near-GeV-energy laser-wakefield acceleration of self-injected electrons in a centimeter-scale plasma channel

The first three-dimensional, particle-in-cell (PIC) simulations of laser-wakefield acceleration of self-injected electrons in a 0.84 cm long plasma channel are reported. The frequency evolution of the initially 50 fs (FWHM) long laser pulse by photon interaction with the wake followed by plasma dispersion enhances the wake which eventually leads to self-injection of electrons from the channel wall. This first bunch of electrons remains spatially highly localized. Its phase space rotation due to slippage with respect to the wake leads to a monoenergetic bunch of electrons with a central energy of 0.26 GeV after 0.55 cm propagation. At later times, spatial bunching of the laser enhances the acceleration of a second bunch of electrons to energies up to 0.84 GeV before the laser pulse intensity is significantly reduced.