Multi-GeV energy gain in a plasma-wakefield accelerator

A plasma-wakefield accelerator has accelerated particles by over 2.7 GeV in a 10 cm long plasma module. A 28.5 GeV electron beam with 1.8 x 10(10) electrons is compressed to 20 microm longitudinally and focused to a transverse spot size of 10 microm at the entrance of a 10 cm long column of lithium vapor with density 2.8 x 10(17) atoms/cm3. The electron bunch fully ionizes the lithium vapor to create a plasma and then expels the plasma electrons. These electrons return one-half plasma period later driving a large amplitude plasma wake that in turn accelerates particles in the back of the bunch by more than 2.7 GeV.     

Acceleration of nonmonoenergetic electron bunches injected into a wake wave

The trapping and acceleration of nonmonoenergetic electron bunches in a wake field wave excited by a laser pulse in a plasma channel is studied. Electrons are injected into the region of the wake wave potential maximum at a velocity lower than the phase velocity of the wave. The paper analyzes the grouping of bunch electrons in the energy space emerging in the course of acceleration under certain conditions of their injection into the wake wave and minimizing the energy spread for such electrons. The factors determining the minimal energy spread between bunch electrons are analyzed. The possibility of monoenergetic acceleration of electron bunches generated by modern injectors in a wake wave is analyzed.     

Pulse chirping effect on controlling the transverse cavity oscillations in nonlinear bubble regime

The propagation of an intense laser pulse in an under-dense plasma induces a plasma wake that is suitable for the acceleration of electrons to relativistic energies. For an ultra-intense laser pulse which has a longitudinal size shorter than the plasma wavelength, λp, instead of a periodic plasma wave, a cavity free from cold plasma electrons, called a bubble, is formed behind the laser pulse. An intense charge separation electric field inside the moving bubble can capture the electrons at the base of the bubble and accelerate them with a narrow energy spread. In the nonlinear bubble regime, due to localized depletion at the front of the pulse during its propagation through the plasma, the phase shift between carrier waves and pulse envelope plays an important role in plasma response. The carrier–envelope phase(CEP) breaks down the symmetric transverse ponderomotive force of the laser pulse that makes the bubble structure unstable. Our studies using a series of two-dimensional(2D) particle-in-cell(PIC) simulations show that the frequency-chirped laser pulses are more effective in controlling the pulse depletion rate and consequently the effect of the CEP in the bubble regime. The results indicate that the utilization of a positively chirped laser pulse leads to an increase in rate of erosion of the leading edge of the pulse that rapidly results in the formation of a steep intensity gradient at the front of the pulse. A more unstable bubble structure, the self-injections in different positions, and high dark current are the results of using a positively chirped laser pulse. For a negatively chirped laser pulse, the pulse depletion process is compensated during the propagation of the pulse in plasma in such a way that results in a more stable bubble shape and therefore, a localized electron bunch is produced during the acceleration process. As a result, by the proper choice of chirping, one can tune the number of self-injected electrons, the size of accelerated bunch and its energy spectrum to the values required for practical applications.     

Electron excitation by a multi passage laser beam in a plasma

The limits put by optical guiding, and channel guiding mechanisms on the Laser Wakefield Acceleration (LWFA) technique are imposed on the Resonant Laser Wakefield Acceleration (RLWFA) scheme. Energy gained by the electrons in both schemes are calculated and compared. It has presented that in the RLWFA case, the electrons gain more and more energy after each traversal of the laser pulse and the electrons in a plasma gain about 3 GeV after 10 passages of the laser pulse. They gain 13 GeV when the laser light makes 50 passages and 26 GeV after the laser beam traverses the plasma 100 times. Moreover, the channel guiding mechanism is integrated to the RLWFA scheme and together with diffraction guiding a model for electron acceleration is proposed. Received 13 September 2000 and Received in final form 27 October 2000     

Trapping and Acceleration of Electron Bunches Generated by a Laser Pulse Moving through the Sharp Plasma Boundary

The formation and acceleration of electron bunches resulting from the self-injection of electrons into the wake wave from the laser pulse moving through a sharp plasma boundary are investigated in one-dimensional geometry. It is shown that electron trapping in the accelerating wakefield is governed by the electron energy and has a threshold character. The acceleration of the trapped bunch is numerically simulated.     

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.     

Production of a keV x-ray beam from synchrotron radiation in relativistic laser-plasma interaction

We demonstrate that a beam of x-ray radiation can be generated by simply focusing a single high-intensity laser pulse into a gas jet. A millimeter-scale laser-produced plasma creates, accelerates, and wiggles an ultrashort and relativistic electron bunch. As they propagate in the ion channel produced in the wake of the laser pulse, the accelerated electrons undergo betatron oscillations, generating a femtosecond pulse of synchrotron radiation, which has keV energy and lies within a narrow (50 mrad) cone angle.     

Photo-ionized lithium source for plasma accelerator applications

A photo-ionized lithium source is developed for plasma acceleration applications. A homogeneous column of lithium neutral vapor with a density of 2×10^15-3 is confined by helium gas in a heat-pipe oven. A UV laser pulse ionizes the vapor. In this device, the length of the neutral vapor and plasma column is 25 cm. The plasma density was measured by laser interferometry in the visible on the lithium neutrals and by CO₂ laser interferometry on the plasma electrons. The maximum measured plasma density was 2.9×10 ¹⁴ cm^-3, limited by the available UV fluence (≈83 mJ/cm²), corresponding to a 15% ionization fraction. After ionization, the plasma density decreases by a factor of two in about 12 μs. These results show that such a plasma source is scaleable to lengths of the order of 1 m and should satisfy all the requirements for demonstrating the acceleration of electrons by 1 GeV in a 1-GeV/m amplitude plasma wake     

Electron Acceleration and Bunch Generation by Intense Femtosecond Laser Pulse in Preplasma of a Target

We present analytical studies of electron acceleration in the low-density preplasma of a thin solid target by an intense femtosecond laser pulse. Electrons in the preplasma are trapped and accelerated by the ponderomotive force as well as the wake field. Two-dimensional particle-in-cell simulations show that when the laser pulse is stopped by the target, electrons trapped in the laser pules can be extracted and move forward inertially. The energeticelectron bunch in the bubble is unaffected by the reflected pulse and passes through the target with small energy spread and emittance. There is an optimal preplasma density for the generation of the monoenergetic electron bunch if a laser pulse is given. The maximum electron energy is inverse proportion to the preplasma density.     

超强激光脉冲下等离子体块的整体加速研究

为了克服激光加速中强流离子束空间电荷效应对粒子输运的影响,提出一种利用两块不同密度的固体靶先后和一束强度约为1022 W/cm2、脉冲长度为5T(T为激光周期)的超强脉冲激光相互作用的方案,实现了中性等离子体块的加速。通过一维PIC数值模拟研究发现,在合适的参数下,加速后的电子与质子几乎以相同的速度共同飞行长达60λ(λ为激光波长)的距离,其中质子与电子的能量分别为GeV和100MeV量级。     

Ion acceleration by superintense laser pulses in plasmas

Ion acceleration by petawatt laser radiation in underdense and overdense plasmas is studied with 2D3V-PIC (Particle in Cell) numerical simulations. These simulations show that the laser pulse drills a channel through the plasma slab, and electrons and ions expand in vacuum. Fast electrons escape first from the electron-ion cloud. Later, ions gain a high energy on account of the Coulomb explosion of the cloud and the inductive electric field which appears due to fast change of the magnetic field generated by the laser pulse. Similarly, when a superintense laser pulse interacts with a thin slab of overdense plasma, its ponderomotive pressure blows all the electrons away from a finite-diameter spot on the slab. Then, due to the Coulomb explosion, ions gain an energy as high as 1 GeV. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 2, 80–86 (25 July 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

On the production of flat electron bunches for laser wakefield acceleration

We suggest a novel method for the injection of electrons into the acceleration phase of particle accelerators, producing low-emittance beams appropriate even for the demanding high-energy linear collider specifications. We discuss the injection mechanism into the acceleration phase of the wakefield in a plasma behind a high-intensity laser pulse, which takes advantage of the laser polarization and focusing. The scheme uses the structurally stable regime of transverse wakewave breaking, when the electron trajectory self-intersection leads to the formation of a flat electron bunch. As shown in three-dimensional particle-in-cell simulations of the interaction of a laser pulse elongated in the transverse direction with an underdense plasma, the electrons injected via the transverse wakewave breaking and accelerated by the wakewave perform betatron oscillations with different amplitudes and frequencies along the two transverse coordinates. The polarization and focusing geometry lead to a way to produce relativistic electron bunches with an asymmetric emittance (flat beam). An approach for generating flat laser-accelerated ion beams is briefly discussed. The text was submitted by the authors in English.     

激光等离子体波电子加速器

本文分别用理论分析和粒子模拟方法讨论了等离子体尾波加速器和拍波加速器的物理机制。结果表明,只要激光等离子体波足够强,加上适当强度的横向磁场,就可以把MeV数量级的电子在公尺距离内加速到GeV数量级的能量。另外,还用粒子模拟方法,研究了激光对热等离子体受激向后喇曼散射产生低相速度的等离子体静电波对低能电子加速的问题,探讨了多级或多波加速的可能性。结果表明,利用激光等离子体波加速器,在一般的实验室条件下,就可获得GeV数量级的高能电子。 关键词：     

Particle simulation on electron acceleration process by the laser ponderomotive force in inhomogeneous underdense plasma layers

The mechanism of electron ponderomotive acceleration due to increasing group velocity of laser pulse in inhomogeneous underdense plasma layers is studied by two-dimensional relativistic parallel particle-in-cell code. The electrons within the laser pulse move with it and can be strongly accelerated ponderomotively when the duration of laser pulse is much shorter than the duration of optimum condition for acceleration in the wake. The extra energy gain can be attributed to the change of laser group velocity. More high energy electrons are generated in the plasma layer with descending density profile than that with ascending density profile. The process and character of electron acceleration in three kinds of underdense plasma layers are presented and compared.     

Generation of short electron bunches by a laser pulse crossing a sharp boundary of inhomogeneous plasma

The formation of short electron bunches during the passage of a laser pulse of relativistic intensity through a sharp boundary of semi-bounded plasma has been analytically studied. It is shown in one-dimensional geometry that one physical mechanism that is responsible for the generation of electron bunches is their self-injection into the wake field of a laser pulse, which occurs due to the mixing of electrons during the action of the laser pulse on plasma. Simple analytic relationships are obtained that can be used for estimating the length and charge of an electron bunch and the spread of electron energies in the bunch. The results of the analytical investigation are confirmed by data from numerical simulations.     

Numerical simulation of the creation of a hollow neutral-hydrogen channel by an electron beam

An experimental method is proposed for the creation of plasma optical waveguides at low electron densities. The method consists of creating a hollow neutral-hydrogen channel by means of fast local heating of a hydrogen volume by a needlelike electron beam, followed by laser ionization of the hydrogen to provide the plasma waveguide. Results of numerical simulations are presented which show that guiding with an axial electron density in the range of 10(17) cm-3 can be achieved with a matched spot size of 30 microm. Its application for laser wakefield acceleration of electrons is discussed. The method would enable guiding lengths up to 30 cm at maximal energies of accelerated electrons in the range 10-100 GeV.     

Energy spectra of electrons in plasma accelerators

The acceleration of electrons in the fast (relativistic) plasma wave, generated, e.g., by intense laser pulse in an underdense plasma, is studied theoretically and numerically. The analytical method, developed to describe the energy spectrum of electrons accelerated in one-dimensional (1-D) plasma wave of an arbitrary form, predicts the “bunching” of electrons in the energy space for linear (harmonic) plasma wave in contrast to the nonlinear one. The results of one- and two-dimensional (2-D) numerical simulations of the resonant and nonresonant electron bunch acceleration are presented and discussed     

Forward acceleration and generation of femtosecond megaelectronvolt electron beams by an ultrafast intense laser pulse

We present a new mechanism of energy gain of electrons accelerated by a laser pulse.It is shown thatwhen the intensity of an ultrafast intense laser pulse decreases rapidly along the direction of propagation,electrons leaving the pulse experience an action of ponderomotivc deceleration at the descending part ofa lower-intensity laser field than acceleration at the ascending part of a high-intensity field, thus gain netenergy from the pulse and move directly forward. By means of such a mechanism, a megaelectronvoltelectron beam with a bunch length shorter than 100 fs could be realized with an ultrafast(≤30 fs),intense (>10~(19)W/cm~2)laser pulse.     

Anticorrelation between ion acceleration and nonlinear coherent structures from laser-underdense plasma interaction

In laser-plasma experiments, we observed that ion acceleration from the Coulomb explosion of the plasma channel bored by the laser is prevented when multiple plasma instabilities, such as filamentation and hosing, and nonlinear coherent structures (vortices or postsolitons) appear in the wake of an ultrashort laser pulse. The tailoring of the longitudinal plasma density ramp allows us to control the onset of these instabilities. We deduced that the laser pulse is depleted into these structures in our conditions, when a plasma at about 10% of the critical density exhibits a gradient on the order of 250 μm (Gaussian fit), thus hindering the acceleration. A promising experimental setup with a long pulse is demonstrated enabling the excitation of an isolated coherent structure for polarimetric measurements and, in further perspectives, parametric studies of ion plasma acceleration efficiency.     

Demonstration of a narrow energy spread, ∼0.5 GeV electron beam from a two-stage laser wakefield accelerator

Laser wakefield acceleration of electrons holds great promise for producing ultracompact stages of GeV scale, high-quality electron beams for applications such as x-ray free electron lasers and high-energy colliders. Ultrahigh intensity laser pulses can be self-guided by relativistic plasma waves (the wake) over tens of vacuum diffraction lengths, to give >1 GeV energy in centimeter-scale low density plasmas using ionization-induced injection to inject charge into the wake even at low densities. By restricting electron injection to a distinct short region, the injector stage, energetic electron beams (of the order of 100 MeV) with a relatively large energy spread are generated. Some of these electrons are then further accelerated by a second, longer accelerator stage, which increases their energy to ～0.5 GeV while reducing the relative energy spread to <5% FWHM.