Developments in laser wakefield accelerators:From single-stage to two-stage

Laser wakefield accelerators(LWFAs)are compact accelerators which can produce femtosecond high-energy electron beams on a much smaller scale than the conventional radiofrequency accelerators.It is attributed to their high acceleration gradient which is about 3 orders of magnitude larger than the traditional ones.The past decade has witnessed the major breakthroughs and progress in developing the laser wakfield accelerators.To achieve the LWFAs suitable for applications,more and more attention has been paid to optimize the LWFAs for high-quality electron beams.A single-staged LWFA does not favor generating controllable electron beams beyond 1 Ge V since electron injection and acceleration are coupled and cannot be independently controlled.Staged LWFAs provide a promising route to overcome this disadvantage by decoupling injection from acceleration and thus the electron-beam quality as well as the stability can be greatly improved.This paper provides an overview of the physical conceptions of the LWFA,as well as the major breakthroughs and progress in developing LWFAs from single-stage to two-stage LWFAs.     

激光等离子体尾波加速器的发展和展望

超强激光在气体等离子体中传输时可以激发出大振幅的电子等离子体尾波。激光等离子体尾波加速器是利用该尾波对带电粒子（特别是电子和正电子）进行加速的一种新型装置。由于其加速梯度相较于现有的常规加速器可以提升1000倍,为建造超紧凑型的加速器和辐射源奠定了基础,也为将来建造基于等离子体的超高能正负电子对撞机和自由电子激光装置提供了可能。对该新型加速器的原理、特点、发展历程,尤其是近十年来的主要进展和未来发展趋势及面临的主要挑战进行简要梳理和介绍。     

实时单发测量的汤姆逊离子谱仪

介绍一种使用闪烁体耦合电子倍增电荷耦合器件(EMCCD)的方式对离子进行记录的汤姆逊能谱仪,可实现对离子能谱的实时单发测量。同时,该谱仪利用倾斜电极板对离子进行偏转,可减少由于离子打在电极板上产生的电磁噪声,能够提高实验结果的信噪比。该谱仪在北京大学4.5 MV静电加速器和2×6MV串列加速器上进行了标定实验,测量了闪烁体将离子转化成光子后的探测效率,实验结果也验证了该谱仪的可行性和稳定性。该汤姆逊谱仪将用于北京大学激光加速器CLAPA对离子束流的测量研究。     

Production of high-energy particles in laser and Coulomb fields and the e+e- antenna

A strong laser field and the Coulomb field of a nucleus can produce e(+) e(-) pairs. It is shown for the first time that there is a large probability that electrons and positrons created in this process collide after one or several oscillations of the laser field. These collisions can take place at high energy, resulting in several phenomena. The quasielastic collision e(+) e(-) --> e(+) e(-) allows acceleration of leptons in the laser field to higher energies. The inelastic collisions allow production of high-energy photons e(+) e(-) --> 2 gamma and muons e(+) e(-) --> micro(+) micro(-). The yield of high-energy photons and muons produced via this mechanism exceeds exponentially their production through conventional direct creation in laser and Coulomb fields. A relation of the phenomena considered with the antenna mechanism of multiphoton absorption in atoms is discussed.     

Distribution uniformity of laser-accelerated proton beams

Compared with conventional accelerators,laser plasma accelerators can generate high energy ions at a greatly reduced scale,due to their TV/m acceleration gradient.A compact laser plasma accelerator(CLAPA) has been built at the Institute of Heavy Ion Physics at Peking University.It will be used for applied research like biological irradiation,astrophysics simulations,etc.A beamline system with multiple quadrupoles and an analyzing magnet for laser-accelerated ions is proposed here.Since laser-accelerated ion beams have broad energy spectra and large angular divergence,the parameters(beam waist position in the Y direction,beam line layout,drift distance,magnet angles etc.) of the beamline system are carefully designed and optimised to obtain a radially symmetric proton distribution at the irradiation platform.Requirements of energy selection and differences in focusing or defocusing in application systems greatly influence the evolution of proton distributions.With optimal parameters,radially symmetric proton distributions can be achieved and protons with different energy spread within ±5% have similar transverse areas at the experiment target.     

Demonstration of high-trapping efficiency and narrow energy spread in a laser-driven accelerator

Laser-driven electron accelerators (laser linacs) offer the potential for enabling much more economical and compact devices. However, the development of practical and efficient laser linacs requires accelerating a large ensemble of electrons together ("trapping") while keeping their energy spread small. This has never been realized before for any laser acceleration system. We present here the first demonstration of high-trapping efficiency and narrow energy spread via laser acceleration. Trapping efficiencies of up to 80% and energy spreads down to 0.36% (1 sigma) were demonstrated.     

激光-等离子体加速器研究综述

综述了有关激光尾场加速器、等离子体拍频波加速器、多束激光脉冲驱动的尾场加速器以及自调制激光尾场加速器的概念及其基本特性,概述了近期的实验结果。介绍了等离子体波的产生机理及等离子体波中电子的俘获和加速,并讨论了存在于激光等离子体加速器中的一些限制和今后发展前景。     

激光等离子体加速器研究综述

 综述了有关激光尾场加速器、等离子体拍频波加速器、多束激光脉冲驱动的尾场加速器以及自调制激光尾场加速器的概念及其基本特性,概述了近期的实验结果。介绍了等离子体波的产生机理及等离子体波中电子的俘获和加速,并讨论了存在于激光等离子体加速器中的一些限制和今后发展前景。     

Laser electron acceleration beyond 100 GeV

Nowadays there is great progress on laser-driven plasma-based accelerators by exploiting petawatt-class lasers, where for one aspect electron beams can be accelerated to multi-GeV energy in a centimeter-scale plasma due to laser wakefield acceleration mechanism. While to date, worldwide researches on laser-plasma accelerators are focused to create compact particle and radiation sources for applications in a wide range of sciences, including basic, medical and industrial sciences, there are great interests in applications for high energy physics and astrophysics that explore unprecedented high-energy frontier phenomena, for which laser plasma accelerator concepts provide us with promising tools. Here, our endeavors toward “extreme light” in the IZEST are envisaged for the next 30 years perspective and issues on laser plasma electron acceleration beyond 100 GeV and furthermore toward the TeV regime, aiming at high energy physics applications.     

High-intensity lasers as radiation sources

In the last decade or so, an evolution in experimental relativistic laser-plasma physics has led to highly sophisticated lasers which are now capable of generating ultra-short pulses and can be focused to intensities in excess of 10²¹ W cm^-2. The laser interaction with solid or gas targets can generate collimated beams of highly energetic electrons, protons and ions. These high-intensity laser systems, therefore, turn out to be versatile and powerful sources of radiation and high-energy particles, without recourse to large-scale facilities such as nuclear reactors or particle accelerators. The potential to induce various kinds of nuclear reactions with laser-induced radiation fields has been demonstrated at several laboratories in recent years. The present paper lays out a comprehensive overview of nuclear reactions induced by high-intensity laser matter interactions. Mechanisms for electron, proton and ion acceleration, in addition to secondary bremsstrahlung, positron and neutron production, are addressed, with a focus on the types of nuclear reactions that are possible and potential applications. Discussion of the extrapolation of these processes and applications to the next generation of table-top lasers under construction is also presented.     

Self-Injection and Acceleration of Monoenergetic Electron Beams from Laser Wakefield Accelerators in a Highly Relativistic Regime

Self-injection and acceleration of monoenergetic electron beams from laser wakefield accelerators are first investigated in the highly relativistic regime, using 100 TW class, 27 fs laser pulses. Quasi-monoenergetic multi- bunched beams with energies as high as multi-hundredMeV are observed with simultaneous measurements of side-scattering emissions that indicate the formation of self-channelfing and self-injection of electrons into a plasma wake, referred to as a ＇bubble＇. The three-dimensional particle-in-cell simulations confirmed multiple self-injection of electron bunches into the bubble and their beam acceleration with gradient of 1.5 GeV/cm.     

高品质激光尾波场电子加速器

激光尾波场电子加速的加速梯度相比于传统直线加速器高了3—4个量级,对于小型化粒子加速器与辐射源的研制具有重要的意义,成为当今国内外的研究热点.台式化辐射源应用需求的提高,特别是自由电子激光装置的快速发展,对电子束流品质提出了更高的要求,激光尾波场电子加速的束流品质和稳定性是目前实现新型辐射源的首要障碍.本文归纳整理了中国科学院上海光学精密机械研究所电子加速研究团队十年来在研制台式化激光尾波场电子加速器过程中采取的方案和取得的进展.例如率先提出了注入级和加速级分离的级联加速方案,通过实验获得了GeV量级的电子束能量;基于级联加速方式利用能量啁啾控制,实验获得世界最高品质的电子束流;通过优化激光系统稳定性和特殊的气体喷流结构,获得稳定的高品质电子束流输出等.这一系列实验结果有利于进一步推进激光尾波场电子加速器的应用.     

Mechanisms of electron injection into laser wakefields by a weak counter-propagating pulse

Numerical studies are conducted on the electron injection into the first acceleration bucket of a laser wakefield by a weak counter-propagating laser pulse. It is shown that there are two injection mechanisms involved during the colliding laser interaction, the collective injection and stochastic injection. They are caused by the time-averaged ponderomotive force push and stochastic acceleration in the interfering fields, respectively. The threshold amplitude of the injection laser pulse is estimated for the occurrence of electron injection, which is close to that for stochastic acceleration and depends weakly upon the plasma density. The trapping of a large number of injection electrons can result in significant decay of the laser wakefield behind the first wave bucket.     

Electron acceleration in quasi-stationary electromagnetic fields during the self-channeling of intense light pulses

We theoretically investigate the possibility of electron acceleration during the self-channeled propagation of laser radiation. We consider a new acceleration mechanism associated with the formation of an ion cloud in material (under the ponderomotive force of the laser radiation) that moves together with the laser pulse. We show that the quasi-stationary electric and magnetic fields generated by the moving ion cloud can lead to the acceleration of electrons up to energies of several dozen MeV and to the formation of an electron beam propagating forward coaxially with the laser pulse. The calculated angular distribution of the accelerated electrons is in satisfactory agreement with published experimental results.     

Production of energetic electrons during magnetic reconnection

The production of energetic electrons during magnetic reconnection is explored with full particle simulations and analytic analysis. Density cavities generated along separatrices bounding growing magnetic islands support parallel electric fields that act as plasma accelerators. Electrons because of their low mass are fast enough to make multiple passes through these acceleration cavities and are therefore capable of reaching relativistic energies.     

Ion acceleration regimes in underdense plasmas

Acceleration of large populations of ions up to high (relativistic) energies may represent one of the most important and interesting tools that can be provided by the interaction of petawatt laser pulses with matter. In this paper, the basic mechanisms of ion acceleration by short laser pulses are studied in underdense plasmas. The ion acceleration does not originate directly from the pulse fields, but it is mediated by the electrons in the form of electrostatic fields originating from channeling, double layer formation and Coulomb explosion     

Laser-driven electron beam and radiation sources for basic,medical and industrial sciences

To date active research on laser-driven plasma-based accelerators have achieved great progress on production of high-energy, high-quality electron and photon beams in a compact scale. Such laser plasma accelerators have been envisaged bringing a wide range of applications in basic, medical and industrial sciences. Here inheriting the groundbreaker’s review article on “Laser Acceleration and its future” [Toshiki Tajima, (2010)],¹⁾ we would like to review recent progress of producing such electron beams due to relativistic laser-plasma interactions followed by laser wakefield acceleration and lead to the scaling formulas that are useful to design laser plasma accelerators with controllability of beam energy and charge. Lastly specific examples of such laser-driven electron/photon beam sources are illustrated.     

Development of a laser-driven plasma cathode for medical applications

In this article, the present status of radiation therapy in Japan and updated medical accelerators are reviewed. In addition, the potential of laser plasma acceleration as a future medical accelerator is discussed. The updated results of laser plasma cathode experiment by the University of Tokyo are described.     

All-optical cascaded laser wakefield accelerator using ionization-induced injection

We report on near-GeV electron beam generation from an all-optical cascaded laser wakefield accelerator (LWFA). Electron injection and acceleration are successfully separated and controlled in different LWFA stages by employing two gas cells filled with a He/O2 mixture and pure He gas, respectively. Electrons with a Maxwellian spectrum, generated from the first LWFA assisted by ionization-induced injection, were seeded into the second LWFA with a 3-mm-thick gas cell and accelerated to be a 0.8-GeV quasimonoenergetic electron beam, corresponding to an acceleration gradient of 187 GV/m. The demonstrated scheme paves the way towards the multi-GeV laser accelerators.     

Traveling-wave Thomson scattering and optical undulators for high-yield EUV and X-ray sources

We present a novel high-yield Thomson scattering geometry that takes advantage of compact electron bunches, as available in advanced, low-emittance linear accelerators or laser wakefield accelerators. In order to avoid the restrictions on the X-ray photon yield imposed by the Rayleigh limit, we use ultrashort, pulse-front tilted laser pulses in a side-scattering geometry. Such a traveling-wave setup allows an overlap of electron and laser beams, even after propagating over distances much longer than the Rayleigh length. Experimental designs are discussed and optimized for different scattering angles. Specifically, to minimize group delay dispersion at large scattering angles >10°, we propose the use of varied-line spacing (VLS) gratings for spatio-temporal laser pulse shaping. Compared to head-on (180°) Thomson scattering, interaction lengths are in the centimeter to meter range and photon numbers for ultrashort X-ray pulses can increase by several orders of magnitudes.