Inverse Free Electron Laser Accelerator Research Articles

Electron acceleration in an inverse free electron laser with a tapered wiggler field

Numerical study of inverse free-electron laser acceleration (IFELA) in the presence of a tightly focused laser beam is investigated in vacuum. In this article, we investigate the dynamics of an electron in the inverse free-electron laser (IFEL) with a planer wiggler field geometry. The numerical study on particle acceleration is carried out for tightly focused laser parameters as well as the tapering along the wiggler field. It is observed that the effect of tapering point along the undulator is crucial to maintain the IFEL resonance condition and to maximize the energy gain. The role of normalized laser field and of tapering point of planar wiggler magnetic field is examined on the electron acceleration. The role of pre-energy modulation of real electron-bunch and its acceleration will be planned in our next simulation work.

Burst mode MHz repetition rate inverse free electron laser acceleration

The capability of accelerating electron bunches at high repetition rate is one of the key performance criteria for all high average power particle accelerator applications. High gradient laser-driven acceleration holds the potential for greatly reducing size and costs of future machines, but typically requires very high peak laser powers. On the other hand, MHz pulse trains of TW-class laser beams are much beyond the state of the art, so that laser recycling and recirculation is a necessary step to bridge that gap. In this experiment we demonstrate for the first time an inverse free electron laser accelerator (IFEL) operating within an active optical cavity showing the ability to laser-accelerate electron bunch trains in burst mode at $g20\text{ }\text{ }\mathrm{MHz}$ repetition rate. The experimental setup, synchronization challenges and acceleration results are presented. It is found that careful control of the dispersive properties of the cavity is required in order to sustain high accelerating gradients over many passes in the laser pulse train.

An inverse free electron laser acceleration-driven Compton scattering X-ray source

The generation of X-rays and γ-rays based on synchrotron radiation from free electrons, emitted in magnet arrays such as undulators, forms the basis of much of modern X-ray science. This approach has the drawback of requiring very high energy, up to the multi-GeV-scale, electron beams, to obtain the required photon energy. Due to the limit in accelerating gradients in conventional particle accelerators, reaching high energy typically demands use of instruments exceeding 100’s of meters in length. Compact, less costly, monochromatic X-ray sources based on very high field acceleration and very short period undulators, however, may enable diverse, paradigm-changing X-ray applications ranging from novel X-ray therapy techniques to active interrogation of sensitive materials, by making them accessible in energy reach, cost and size. Such compactness and enhanced energy reach may be obtained by an all-optical approach, which employs a laser-driven high gradient accelerator based on inverse free electron laser (IFEL), followed by a collision point for inverse Compton scattering (ICS), a scheme where a laser is used to provide undulator fields. We present an experimental proof-of-principle of this approach, where a TW-class CO2 laser pulse is split in two, with half used to accelerate a high quality electron beam up to 84 MeV through the IFEL interaction, and the other half acts as an electromagnetic undulator to generate up to 13 keV X-rays via ICS. These results demonstrate the feasibility of this scheme, which can be joined with other techniques such as laser recirculation to yield very compact photon sources, with both high peak and average brilliance, and with energies extending from the keV to MeV scale. Further, use of the IFEL acceleration with the ICS interaction produces a train of high intensity X-ray pulses, thus enabling a unique tool synchronized with a laser pulse for ultra-fast strobe, pump-probe experimental scenarios.

Synchrotron radiation loss of a laser accelerator based on an inverse electron cyclotron resonance maser.

A laser accelerator based on an electron cyclotron resonance maser is described. Important losses that give rise to saturation length and saturation power are included in the analysis. The results are compared with results from an inverse free-electron laser accelerator.

Phase space manipulations for optimization of an inverse free electron laser

The Inverse Free Electron Laser (IFEL) has developed into a mature advanced accelerator with the ability to produce mono energetic high brightness beams accelerated at gradients beyond those of conventional RF technology. By copropagating a relativistic electron beam and a high power laser in a strongly tapered undulator, resonant energy exchange can be maintained over large distances. To optimize the number of electrons accelerated in an IFEL, one must prepare the initial electron beam phase space such that a majority of particles are injected at the accelerating phase of the ponderomotive potential. We present a scheme for optimization of the Rubicon IFEL accelerator trapping by preparing the initial electron beam phase space. Utilizing two cascaded modulator-chicane pre-bunchers simulations predict 95% capture of the injected electron beam.

Electron energy and electron trajectories in an inverse free-electron laser accelerator based on a novel electrostatic wiggler

We expand here a theory of a high-gradient laser-excited electron accelerator based on an inverse free-electron laser (inverse-FEL), but with innovations in the structure and design. The electrostatic wiggler used in our scheme, namely termed the Paul wiggler, is generated by segmented cylindrical electrodes with applied oscillatory voltages over segments. The inverse-FEL interaction can be described by the equations that govern the electron motion in the combined fields of both the laser pulse and Paul wiggler field. A numerical study of electron energy and electron trajectories has been made using the fourth-order Runge–Kutta method. The results indicate that the electron attains a considerable energy at short distances in this device. It is found that if the electron has got sufficient suitable wiggler amplitude intensities, it can not only gain higher energy in longer distances, but also can retain it even after the passing of the laser pulse. In addition, the results reveal that the electron energy gains different peaks for different initial axial velocities, so that a suitable small initial axial velocity of e-beam produces substantially high energy gain. With regard to the transverse confinement of the electron beam in a Paul wiggler, there is no applied axial guide magnetic field in this device.

Ultrashort laser pulse driven inverse free electron laser accelerator experiment

In this paper we discuss the ultrashort pulse high gradient inverse free electron laser accelerator experiment carried out at the Lawrence Livermore National Laboratory which demonstrated gradients exceeding $200\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ using a 4 TW 100 fs long 800 nm $\mathrm{Ti}:\mathrm{Sa}$ laser pulse. Due to the short laser and electron pulse lengths, synchronization was determined to be one of the main challenges in this experiment. This made necessary the implementation of a single-shot, nondestructive, electro-optic sampling based diagnostics to enable time-stamping of each laser accelerator shot with $l100\text{ }\text{ }\mathrm{fs}$ accuracy. The results of this experiment are expected to pave the way towards the development of future GeV-class IFEL accelerators.

Demonstration of inverse free-electron laser seeding in a sub-80 K, short period cryogenic undulator

Short period, high field undulators have been shown to permit operation of x-ray free-electron lasers with short gain-lengths, and at unprecedented short wavelengths. In addition, the reduced beam energy required to reach resonance with a given radiation wavelength in short period undulators could prove useful in advanced inverse free-electron laser (IFEL) seeding schemes for future light sources, such as high-gain harmonic generation and echo-enabled harmonic generation, or in IFEL acceleration. We report here the in situ beam testing of a 9 mm period length cryogenic undulator having undulator parameter near unity in the context appropriate for advanced seeding and acceleration schemes, a linear accelerator. Because of the short period length of the undulator, a 47 MeV high-brightness electron beam could be used to produce near infrared photons via the undulator radiation mechanism. The undulator radiation was observed through a filter and the spectral response of the undulator is compared to simulation. Finally, an 800 nm seed laser was introduced in order to generate an energy modulation via the IFEL mechanism. Resonance between the electron beam and the laser seed was achieved without detailed knowledge of the temperature dependent undulator magnetic field through the observation of the undulator radiation. The energy modulation (and concomitant energy spread increase) of the electron beam was observed both directly in an electron beam spectrometer and indirectly via coherent transition radiation after conversion to a density modulation in a longitudinally dispersive chicane.

Effects of misaligned electron beam on inverse free electron laser acceleration

Abstract In this paper, we discuss the effects of misaligned electron beam on an inverse free electron laser with both an electromagnetic wave wiggler and magnetostatic wiggler acceleration scheme. It is shown analytically that electromagnetic wiggler IFEL energy gain distance is substantially smaller when compared to the standard IFEL i.e. with a magnetostatic wiggler. The analysis further explains a better tolerance of the electromagnetic wiggler IFEL with respect to the misaligned electron beam in comparison to a magnetostatic wiggler IFEL scheme.

High-quality electron beams from a helical inverse free-electron laser accelerator.

Compact, table-top sized accelerators are key to improving access to high-quality beams for use in industry, medicine and academic research. Among laser-based accelerating schemes, the inverse free-electron laser (IFEL) enjoys unique advantages. By using an undulator magnetic field in combination with a laser, GeV m(-1) gradients may be sustained over metre-scale distances using laser intensities several orders of magnitude less than those used in laser wake-field accelerators. Here we show for the first time the capture and high-gradient acceleration of monoenergetic electron beams from a helical IFEL. Using a modest intensity (~10(13) W cm(-2)) laser pulse and strongly tapered 0.5 m long undulator, we demonstrate >100 MV m(-1) accelerating gradient, >50 MeV energy gain and excellent output beam quality. Our results pave the way towards compact, tunable GeV IFEL accelerators for applications such as driving soft X-ray free-electron lasers and producing γ-rays by inverse Compton scattering.

Demonstration of cascaded optical inverse free-electron laser accelerator.

We report on a proof-of-principle demonstration of a two-stage cascaded optical inverse free-electron laser (IFEL) accelerator in which an electron beam is accelerated by a strong laser pulse after being packed into optical microbunches by a weaker initial laser pulse. We show experimentally that injection of precisely prepared optical microbunches into an IFEL allows net acceleration or deceleration of the beam, depending on the relative phase of the two laser pulses. The experimental results are in excellent agreement with simulation. The demonstrated technique holds great promise to significantly improve the beam quality of IFELs and may have a strong impact on emerging laser accelerators driven by high-power optical lasers.

Inverse free electron laser accelerator for advanced light sources

We discuss the inverse free electron laser (IFEL) scheme as a compact high gradient accelerator solution for driving advanced light sources such as a soft x-ray free electron laser amplifier or an inverse Compton scattering based gamma-ray source. In particular, we present a series of new developments aimed at improving the design of future IFEL accelerators. These include a new procedure to optimize the choice of the undulator tapering, a new concept for prebunching which greatly improves the fraction of trapped particles and the final energy spread, and a self-consistent study of beam loading effects which leads to an energy-efficient high laser-to-beam power conversion.

New concept of waveguide for inverse free electron laser accelerator

A segmented waveguide formed by a periodic system of pairs of wedge prisms with a gap between them is proposed. The prisms are made of single-crystal sapphire for which the refraction index ν<1 for CO 2 laser radiation ( λ=10.6 μm ). The attenuation of 0.1 db/m can be estimated for a 5 mm diameter radiation beam for which the Rayleigh distance is approximately 1 m. The gap between adjacent prisms can be useful in FEL-like applications.

Experimental Observation of Femtosecond Electron Beam Microbunching by Inverse Free-Electron-Laser Acceleration

An electron beam microbunched on the optical wavelength scale of $\ensuremath{\approx}2.5\ensuremath{\mu}\mathrm{m}$ by an inverse free electron laser accelerator was observed. The optimum bunching was achieved for a 1% energy modulation of a 32 MeV electron beam with 0.5 GW ${\mathrm{CO}}_{2}$ laser power. The microbunching process was investigated by measuring the coherent transition radiation produced by the energy modulated electron beam. A quadratic dependence of the transition radiation signal on the electron beam charge was observed. The observed shortest wavelength of coherent transition radiation is less than $2.5\ensuremath{\mu}\mathrm{m}$. The debunching process of the microbunched electron beam was experimentally investigated.

Inverse FEL accelerator: experiment and theory

This article has two parts. In the first part of this article, an inverse FEL autoaccelerator experiment which uses the Columbia FEL accelerator system ( V = 800 kV, I = 400 A) is described. High-power 1.5 mm radiation is developed in an optical resonator located along the first section of the undulator (which is 40 cm long and has a period of 1.43 cm and a magnetic field < 1 kG). The electron beam then moves into the second section of the undulator (which is 50 cm long and has its period tapered from 1.8 to 2.4 cm). A fraction of the electrons in the beam is accelerated in the second section by stimulated absorption of the FEL wave. A multimode two-dimensional simulation shows that approximately 50% of the FEL carried power should accelerate a group of trapped electrons to approximately 1200 kV. Measurements of the spectrum of FEL power shows that the stimulated absorption does in fact occur. In the second part of this article, we describe the theory of an inverse FEL beat-wave accelerator (IFELBWA). The essential idea is to use the inverse FEL mechanism to bunch a dense, low-energy electron beam, and then use the electric field generated between the bunches to accelerate a higher-energy electron beam. The phase velocity of the beat wave responsible for acceleration is enhanced by propagating the wave in a waveguide of smaller radius. Accelerating electric fields of the order of 100 MeV/m can be achieved with an electron beam of current density 20 kA/cm 2. The parameters of a proof-of-principle experiment are presented.

Multifrequency effects in an inverse FEL accelerator

The electron dynamics in the inverse FEL accelerator is investigated by a 1D FEL simulation code. The electron bunch length in acceleration buckets and the trapping fraction are found to vary when the gradient of wiggler field tapering is changed. When an infrared laser of 60 GW is focused to a 200 μm diameter laser spot, the maximum acceleration rate reaches 100 MeV/m and the bunch length is about 115 of the laser wavelength. As a realistic case study, FInite bandwidth of the laser field has to be taken into account. The critical bandwidth is found to be approximately a betatron oscillation frequency.

Analysis on inverse free-electron laser accelerator.

The characteristics of the accelerator using the inverse free-electron laser process are investigated by a 1-D FEL simulation code in order to demonstrate the potential of this type of accelerator. When an infrared laser of 60 GW is focused to a 280μm diameter laser spot, the maximum acceleration rate reaches 100 MeV/m and the accelerated electron beam is strongly bunched. The bunch length is about 1/14 of the laser wavelength. For the simulations using the spectrum with sideband components, an unstable phenomenon is found which is due to the resonance between the synchrotron motion and the beat frequencies of the broad band laser radiation. The energy losses by the higher harmonics and by the synchrotron radiation are also studied.

The quest for ultrahigh energies

A categorization is given of all the methods for accelerating particles. It is shown that in principle one can employ the large fields of a laser for this purpose as well as the wake fields of intense low-energy particle beams. Discussion is given of four acceleration schemes which offer the possibility of attaining very high-energy particles; namely, the inverse free-electron laser accelerator, the beat-wave accelerator, the wake-field accelerator, and the two-beam accelerator.

High-energy inverse free-electron-laser accelerator.

We study the inverse free-electron-laser (IFEL) accelerator and show that it can accelerate electrons to the few hundred GeV region with average acceleration rates of the order of 200 MeV/m. Several possible accelerating structures are analyzed, and the effect of synchrotron-radiation losses is studied. The longitudinal phase stability of accelerated particles is also analyzed. A Hamiltonian description, which takes into account the dissipative features of the IFEL accelerator, is introduced to study perturbations from the resonant acceleration. Adiabatic invariants are obtained and used to estimate the change of the electron phase-space density during the acceleration process.

Laser Accelerators

The use of lasers to accelerate particles to ultra-high energies is motivated by the very high electric fields associated with focused laser beams. A number of such schemes have been proposed: for example, microstructures driven by lasers operating in the optical-near infrared frequency range, laser-driven space charge plasma waves, inverse free-electron laser accelerator and the two-beam accelerator. Current status of these schemes is reviewed. Lasers may also find an application in improving the power sources and in focusing very high energy particle beams.