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.     

Electron acceleration in the inverse free electron laser with a helical wiggler by axial magnetic field and ion-channel guiding

Electron acceleration in the inverse free electron laser (IFEL) with a helical wiggler in the presence of ion-channel guiding and axial magnetic field is investigated in this article. The effects of tapering wiggler amplitude and axial magnetic field are calculated for the electron acceleration. In free electron lasers, electron beams lose energy through radiation while in IFEL electron beams gain energy from the laser. The equation of electron motion and the equation of energy exchange between a single electron and electromagnetic waves are derived and then solved numerically using the fourth order Runge-Kutta method. The tapering effects of a wiggler magnetic field on electron acceleration are investigated and the results show that the electron acceleration increases in the case of a tapered wiggler magnetic field with a proper taper constant.     

Plasma expansions with a time-dependent electron temperature

The fundamental of ion acceleration from the laser-solid interactions is the theory for describing plasma expansions into a vacuum. In the time interval of tens of femtoseconds after the laser pulse reaches the target, the plasma reaches local-thermal equilibrium and then hydrodynamic equations are available. However, it cannot reach macro-thermal equilibrium and does not satisfy Boltzmann distribution. The plasma is non-quasi-neutral and a strong charge separation exists. In fact, the electron temperature is time-dependent and the changing law is governed by the energy transfer between electrons and ions. In this Letter, an analytical solution is obtained for plasma expansions into a vacuum to describe the ion acceleration with a time-dependent electron temperature. It is obtained that the dependence of the plasma density on the potential is different from Boltzmann distribution in the time-dependent case. The time-dependent electron temperature also induces the unsteady ablation rate of the ablation plane, which is defined as the interface between the plasma and the solid (or liquid or gas). The electric field is also obtained and discussed in detail. The particular solution is given to show the influence of the time-dependent electron temperature on the laser-ion acceleration in a particular case: the electron temperature is proportional to the square of time. From that, it is concluded that laser-ion acceleration is more efficient in the time-increasing electron-temperature case than that in the isothermal case. The time-dependence of electron temperature comes from the time-dependence of laser intensity and induces the different efficiency of the laser-ion acceleration. At the ablation plane, the electron density and velocity are also predicted and explained reasonably.     

Electret mechanism of electron beam capture by the magnetic field of a betatron

The mechanism of electron capture into acceleration that takes into account the electret properties of the accelerating chamber shell is described. The electron capture into acceleration is a self-consistent problem. It is demonstrated that the electron capture into acceleration is caused by the interaction of the injected electrons with the electric field of the charge created on the side interior wall of the chamber by electrons dropped out of the acceleration. The spectrum of the captured electrons is not normal. A large number of low-energy electrons are presented in the spectrum. Two and more peaks previously unknown are revealed in the dependence of the captured charge on the injected charge for large values of the injected charge. The results obtained are in agreement with the data of previous experimental studies. The captured charge and the dose rate of bremsstrahlung from a target correspond to their actual values for betatrons with accelerated electron energies of 6 and 10 MeV. Results of simulation can be used to design accelerating chambers and electron injection systems of betatrons and other cyclic accelerators. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 35–45, December, 2006.     

锥形激光等离子体中Compton 散射对电子的加速

应用相对论性电子与光子非弹性碰撞模型和经典相对论电动力学理论,结合锥形飞秒强激光等离子体中的光场特性和静电场能,分析、计算了入射的高能电子束与等离子体中的光子发生多光子非线性Compton散射时对电子的加速效应,发现等离子体中的光场会引起电子加速能量的振荡;等离子体中的静电场降低电子的加速效应。用高能电子束与锥形飞秒强激光等离子体中的光子发生双光子非线性Compton散射,是加速电子最为理想的情况。     

驻波驱动的等离子体电子相对论随机加速

就电子在等离子体中被驻波随机加速的可能性和条件进行了分析和数值计算.分析结果表明:等离子体中的电子在驻波场中的两个共振区的叠加导致电子的随机行为的产生.并估算了随机加速的阈值.数值计算了电子的随机加速的能量和随机加速的阈值.采用Poincare截面图技术,跟踪了电子的随机运动的轨迹. 关键词：     

Parameters of the electron beam in a betatron with radial comb-type poles

Expressions for the vector potential and components of the magnetic field induction vector of a betatron with radial comb-type poles are derived. The dynamics of the electron beam in the electromagnetic betatron field is investigated in the process of electron injection and acceleration. It is demonstrated that the azimuthally varying field engender beam beats. However, the amplitudes of beam particle oscillations during acceleration do not exceed their values estimated from the symmetric azimuthal component of the betatron magnetic field induction. The energy spectrum of accelerated electrons is not described by a normal law. In the electron energy spectrum, the relative number of electrons whose energy exceeds the average value is large. Application of poles with radial combs improves the efficiency of electron capture in acceleration. Results of investigations can find application in the development and adjustment of electron beam accelerating systems. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 27–34, October, 2005.     

Laser acceleration of light impurity from an ultrathin foil of complex ionic composition

The model of acceleration of light impurity particles from a planar ultrathin foil of complex ionic composition under an ultrashort high-power high-contrast laser pulse is proposed. Both the mode of pure Coulomb acceleration of ions, characteristic of extremely high electron energies, and acceleration under conditions of spatial charge separation controlled by a finite characteristic electron temperature are studied. Accurate and approximate analytical approaches for describing impurity particle acceleration are formulated. Spatial and spectral characteristics of accelerated particles are determined. Particle dynamics is studied in both the approximation of test impurity particles and taking into account their intrinsic electrostatic field, depending on the relative charge density of light particles.     

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.     

Generation of Electron Beams in the Range of Forevacuum Pressures

This paper presents the results of a study on the generation of electron beams at gas pressures ranging from 0.01 to 0.1 Torr. The fact that this range of pressures is attainable with mechanical pumps only has provoked interest in this problem. To generate an electron beam, use is made of a plasma source based on a hollow-cathode discharge in combination with a plane-parallel acceleration gap. In the given range of pressures, the peculiarities of emission and acceleration of electrons are related to the high probability of ionization of the gas in the acceleration gap and to the formation of an ion flow propagating toward the electron beam. This causes a decrease in discharge operating voltage and also an increase in plasma density in the emission region. Two types of breakdown are observed in the acceleration gap: an interelectrode breakdown and a breakdown in the plasma–electrode system. The designed electron source allows one to obtain beams of cylindrical cross section with currents of up to 1 A and energies of up to 10 keV.     

Dynamics of an ion flow in an electron layer

We study the acceleration of an ion flow in the electron layer formed by an electron flow moving in a transverse electric field and confined by the intrinsic magnetic field. The possibility of extraction of heavy ions with velocities lower than the ion sound velocity from the plasma, and the feasibility of their further acceleration by an external field is demonstrated.     

Dynamics of an electron beam in the magnetic field of a betatron

Expressions for the vector potential and magnetic induction vector components have been obtained for a vertically asymmetric magnetic field of a betatron. The dynamics of the electron beam in the process of injection and acceleration in the electromagnetic field of the betatron has been investigated. It has been shown that the asymmetry of the magnetic field decreases the efficiency of the electron involvement in acceleration. The mutually related radial-vertical asymmetric oscillations of the electron beam in the asymmetric field lead to considerable losses of the beam particles on the walls and injector of the acceleration chamber. The results of these investigations may be useful in developing and tuning electron beam acceleration systems.__________Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 42–46, December, 2004.     

Controlled collective acceleration of electron-ion bunches

The fundamental possibility of a new method for controlled collective ion acceleration by electron bunches of high-current relativistic picosecond beams has been proved. Dense relativistically rotating electron bunches are formed using a cusp magnetic system by their capture in a special magnetic trap. An electron bunch is filled with ions when it interacts with a preliminarily prepared plasma bunch with a certain density. Then, the effective potential well of the magnetic trap is stepwise shifted synchronously with the motion of ions by means of a system of turns with controlled currents. This ensures the displacement and confinement of electrons in the direction of acceleration. The shift of the center of the well at each step is chosen such that ions are in the region of acceleration by a high self electric field of the electron bunch. In contrast to the known methods for collective acceleration, the proposed method makes it possible to avoid the mismatch of the electron and ion components of bunches, disruption of the acceleration of ions, and development of numerous instabilities, because the duration of the acceleration cycle is in the nanosecond range.     

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     

Mechanism of electron violent acceleration by extra-intense lasers in vacuum

The mechanism underlying the electron capture acceleration (Phys. Rev. E 58 (1998) 6575) is studied. It is found that the longitudinal electric field, resulting from the transverse spatial gradient of the laser fields, is responsible for the violent electron acceleration while the transverse fields play the role of confining the electron inside the laser beam.     

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.     

Electron, positron, and photon wakefield acceleration: trapping, wake overtaking, and ponderomotive acceleration

The electron, positron, and photon acceleration in the first cycle of a laser-driven wakefield is investigated. Separatrices between different types of the particle motion (trapped, reflected by the wakefield and ponderomotive potential, and transient) are demonstrated. The ponderomotive acceleration of electrons can be largely compensated by the wakefield action, in contrast to positrons and positively charged mesons. The electron bunch energy spectrum is analyzed. The maximum upshift of an electromagnetic wave frequency during reflection from the wakefield is obtained.     

Maximal acceleration,Mach's principle,and the mass of the electron

Recent arguments for an upper limit to the proper acceleration of extended massive bodies are briefly reviewed. A transient mass shift in accelerated objects with non-constant proper mass density, expected in all locally Lorentz-invariant theories of gravitation which satisfy Mach's principle, is considered. This effect affects arguments for a maximal proper acceleration. It is shown that, while the widely discussed upper limit to proper acceleration obtains for rigid bodies with constant proper mass density, the limit ceases to obtain generally if this effect is taken into consideration. Applicability of maximal acceleration arguments to elementary particles is briefly considered in the context of a plausible classical model of the electron (one where the mass of the electron follows from the electronic charge).     

Cavity generation and quasi-monoenergetic electron generation in laser-plasma interaction

Electrons cavity acceleration is one the relativistic regime to describe the monoenergetic electron acceleration. In this work, we introduce a new ellipsoid model that could be improved the quality of the electron beam in contrast to other methods such as that using periodic plasma wake field, spherical cavity regime and plasma channel guided acceleration. The trajectory of the electron motion can be described as hyperbola, parabola or ellipsoid path. It is influenced by the position and energy of the electrons and the electrostatic potential of the cavity. We have noticed that the electron output energy is not affected by the elongation of the transverse cavity radius in the ellipsoid regime.     

Reformed Solitary Kinetic Alfven Waves due to Dissipations and Auroral Electron Acceleration

The physical nature of the auroral electron acceleration has been an outstanding problem in space physics for decades. Some recent observations from the auroral orbit satellites, FREJA and FAST, showed that large amplitude solitary kinetic Alfvén waves (SKAWs) are a common electromagnetic active phenomenon in the auroral magnetosphere. In a low-β (i.e., β/2<<m_e/m_i<<1) plasma, the drift velocity of electrons relative to ions within SKAWs is much larger than thermal velocities of both electrons and ions. This leads to instabilities and causes dissipations of SKAWs. In the present work, based on the analogy of classical particle motion in a potential well, it is shown that a shock-like structure can be formed from SKAWs if dissipation effects are included. The reformed SKAWs with a shock-like structure have a local density jump and a net field-aligned electric potential drop of order of m_ev_A²/e over a characteristic width of several λ_e. As a consequence, the reformed SKAWs can efficiently accelerate electrons field-aligned to the order of the local Alfvén velocity. In particular, we argue that this electron acceleration mechanism by reformed SKAWs can play an important role in the auroral electron acceleration problem. The result shows that not only the location of acceleration regions predicted by this model is well consistent with the observed auroral electron acceleration region of 1—2 R_E above the auroral ionosphere, but also the accelerated electrons from this region can obtain an energy of several keV and carry a field-aligned current of several μA/m² which are comparable to the observations of auroral electrons.