Plasma-based accelerator diagnostics based upon longitudinalinterferometry with ultrashort optical pulses

All-optical ultrafast time-resolved plasma diagnostics of plasma-based accelerators (PBA's) are described, with emphasis on the laser wakefield accelerator (LWFA). Specifically, the diagnostic techniques involve replacing the trailing particle bunch in the LWFA with a trailing photon bunch: a weak ultrashort laser pulse. Since this photon pulse is derived directly from the intense pump pulse, practical difficulties such as synchronization and dephasing are eliminated. The interaction of the photon bunch with the plasma wake is essentially a simple time-domain shift in optical phase, which can produce both “DC” phase shifts and frequency blue/red-shifting of the probe pulse spectrum. These phase/frequency shifts are recorded in frequency domain interferograms, which are formally equivalent to time-domain holograms. Experimental results of longitudinal plasma density profiling are presented in which plasma density oscillations (Langmuir waves) in the wake of an intense (I_peak~3×10 ¹⁷ W/cm²) laser pulse (~100 fs) were measured with ultrafast time resolution. Phase shifts consistent with large amplitude (~80%) density oscillations at the electron plasma frequency were observed in a fully tunnel-ionized He plasma, corresponding to longitudinal electric fields of ~10 GV/m. Strong radial ponderomotive forces enhance the density oscillations. Finally, proposed single-shot schemes for simultaneous transverse and longitudinal profiling are discussed     

Scaling of accelerating gradients and dephasing effects in channel-guided laser wakefield accelerators

Future wakefield accelerator (LWFA) experiments are expected to operate in the short pulse resonant regime and employ some form of laser guiding, such as a preformed plasma channel. Performance of an LWFA may be characterized by the maximum axial electric field E_m, the dephasing length L_d, and the corresponding dephasing limited energy gain W_d. Dephasing is characterized by the normalized phase slippage rate Δβ_p, of the wakefield relative to a particle moving at the velocity of light. This paper presents analytical models for all of these quantities and compares them with results from simulations of channel-guided LWFAs. The simulations generally confirm the scaling predicted by the analytical models, agreeing within a few percent in most cases. The results show that with the proper choice of laser and channel parameters, the pulse will propagate at a nearly constant spot size r_M over many Rayleigh lengths and generate large accelerating electric fields. The spot size correction to the slippage rate is shown to be important in the LWFA regime, whereas Δβ_p, is essentially independent of laser intensity. An example is presented of a 25-TW, 100-fs laser pulse that produces a dephasing limited energy gain in excess of 1 GeV     

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     

Laser acceleration in novel media

With newly available compact laser technology [1] we are capable of producing 100?PW-class laser pulses with a single-cycle duration on the femtosecond timescale. With this fs intense laser we can produce a coherent X-ray pulse that is also compressed, well into the hard X-ray regime (～10?keV) and with a power up to as much as 10 Exawatts. We suggest utilizing these coherent X-rays to drive the acceleration of particles. Such X-rays are focusable far beyond the diffraction limit of the original laser wavelength and when injected into a crystal it forms a metallic-density electron plasma ideally suited for laser wakefield acceleration. If the X-ray field is limited by the Schwinger field at the focal size of ～100?nm, the achievable energy is 1?PeV over 50?m. (If the X-rays are focused further, much higher energies beyond this are possible). These processes are not limited to only electron acceleration, and if ions are pre-accelerated to beyond GeV they are capable of being further accelerated using a LWFA scheme [2] to similar energies as electrons over the same distance-scales. Such high energy proton (and ion) beams can induce copious neutrons, which can also give rise to intense compact muon beams and neutrino beams that may be portable. High-energy gamma rays can also be efficiently emitted with a bril- liance many orders of magnitude above the brightest X-ray sources by this accelerating process, from both the betatron radiation as well as the dominant radiative-damping dynamics. With the exceptional conditions enabled by this technology we envision a whole scope of new physical phenomena, including: the possibility of laser self-focus in the vacuum, neutron manipulation by the beat of such lasers, zeptosecond spectroscopy of nuclei, etc. Further, we now introduce along with the idea of vacuum as a nonlinear medium, the Schwinger Fiber Accelerator. This is a self-organized vacuum fiber acceleration concept, in which the repeated process of self-focusing and defocusing for the X-ray pulse in vacuum forms a modulated fiber that guides the intense X-rays.     

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     

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.     

Enhanced betatron radiation by a modulating laser pulse in laser wakefield acceleration

We propose a new idea to enhance and control the betatron radiation by using a modulating laser pulse in laser wakefield acceleration. In this scheme, a high-power laser pulse is used for self-trapping and acceleration of the plasma electrons and the accelerated electron beam is modulated by a separately-propagating laser pulse for large amplitude betatron oscillations and microbunching. In this way, the relatively low power modulating laser pulse can enhance the X-ray photon flux and energy significantly. We performed two-dimensional particle-in-cell simulations to demonstrate the idea and the results show that a sub-TW laser pulse is enough for electron beam modulation and it can generate easily-controllable fs X-ray pulses with a wide range of photon energies from soft X-rays to hard X-rays.     

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.     

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     

Effect of the ponderomotive scattering and injection position on electron-bunch injection into a laser wakefield

For the purpose of laser wakefield acceleration, it turns out that the injection of electron bunches longer than the plasma wavelength can also generate accelerated femtosecond bunches with a relatively low energy spread. This is of great interest because such injecting bunches can be provided, e.g., by photo cathode rf linacs. Here we show that when an e-bunch is injected into the wakefield, it is important to take into account the interaction of the injected bunch with the laser pulse in the vacuum region located in front of the plasma. We show that at low energies of the injected bunch, this leads to ponderomotive scattering of the bunch and results in a significant drop of the collection efficiency. For certain injection energies the ponderomotive scattering may result in a smaller energy spread in the accelerated bunch. It is found that the injection position in the laser wakefield plays an important role. Higher collection efficiency can be obtained for certain injection energies, when the bunch is injected in plasma at some distance from the laser pulse; the energy spread, however, is typically larger in this case. We also estimate the minimum trapping energy for the injected electrons and the length of the trapped bunch. PACS 52.38.Kd; 41.75.Jv; 41.85.Ar     

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.     

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。     

Femtosecond fiber lasers with pulse energies above 10 nJ

A series of experiments aimed at determining the maximum pulse energy that can be produced by a femtosecond fiber laser is reported. Exploiting modes of pulse propagation that avoid wave breaking in a Yb fiber laser allows pulse energies up to 14 nJ to be achieved. The pulses can be dechirped to sub-100-fs duration to produce peak powers that reach 100 kW. The limitations to the maximum pulse energy are discussed.     

Valley density profile for highly efficient x-ray lasers with good beam quality

A plasma with a valley density profile at the required electron density for high-gain operation of an x-ray laser could be obtained using a normally incident 300 ps laser pulse followed by a grazing-incident 300 ps laser pulse. Sudden heating of the valley plasma region by another grazing-incident 300 fs laser pulse would then yield a highly efficient x-ray laser beam with a deflecting angle of 2 mrad and a divergence angle of 4 mrad. Saturation operation of the x-ray laser beam at 32.6 nm could be achieved with a total pump energy of less than 100 mJ.     

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.     

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.     

Trapping, compression, and acceleration of an electron beam by the laser wake wave

The scheme of laser wake-field acceleration in plasma is proposed and considered for the case where a relatively rare nonrelativistic or weakly relativistic electron beam is initially situated ahead of the intense laser pulse. It is shown that an electron beam is trapped in the region of the first accelerating wake maximum; then it is strongly compressed and accelerated to ultrarelativistic energies.     

Stochastic heating and acceleration of electrons in colliding laser fields in plasma

We propose a mechanism that leads to efficient acceleration of electrons in plasma by two counterpropagating laser pulses. It is triggered by stochastic motion of electrons when the laser fields exceed some threshold amplitudes, as found in single-electron dynamics. It is further confirmed in particle-in-cell simulations. In vacuum or tenuous plasma, electron acceleration in the case with two colliding laser pulses can be much more efficient than with one laser pulse only. In plasma at moderate densities, such as a few percent of the critical density, the amplitude of the Raman-backscattered wave is high enough to serve as the second counterpropagating pulse to trigger the electron stochastic motion. As a result, even with one intense laser pulse only, electrons can be heated up to a temperature much higher than the corresponding laser ponderomotive potential.     

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.     

Simulation studies of laser wakefield acceleration based on typical 100 TW laser facilities

In this paper,2-D Particle-In-Cell simulations are made for Laser Wakefield Accelerations(LWFA).As in a real experiment,we perform plasma density scanning for typical 100 TW laser facilities.Several basic laws for self-injected acceleration in a bubble regime are presented.According to these laws,we choose a proper plasma density and then obtain a high quality quasi-monoenergetic electron bunch with arms energy of more than 650 MeV and a bunch length of less than 1.5 μn.