Laser-electron Interaction Research Articles

Implementation of a gauge-invariant time-dependent configuration-interaction-singles method for three-dimensional atoms

We present a numerical implementation of the gauge-invariant time-dependent configuration interaction singles (TDCIS) method [Appl. Sci. 8, 433 (2018)] for three-dimensional atoms. In our implementation, orbital-like quantity called channel orbital [Phys. Rev. A 74, 043420 (2006)] is propagated instead of configuration-interaction (CI) coefficients, which removes a computational bottleneck of explicitly calculating and storing numerous virtual orbitals. Furthermore, besides its physical consistency, the gauge-invariant formulation allows to take advantages of the velocity gauge treatment of the laser-electron interaction over the length gauge one in the simulation of high-field phenomena. We apply the present implementation to high-harmonic generation from helium and neon atoms, and numerically confirms the gauge invariance and demonstrates the effectiveness of the rotated velocity gauge treatment.

Obliquely incident laser and electron beam interaction in an undulator

The angular drift of a laser beam is of particular concern in applications such as seeded free-electron lasers. A systematical study of the obliquely incident laser and electron beam interaction in an undulator is presented in this paper. Theoretical analysis and numerical simulations demonstrate that the interaction would imprint energy and angular modulations on the electron beam simultaneously. Compared with the normally incident pattern, the obliquely incident laser-electron interaction leads to reductions in the bunching factors of HGHG and EEHG. In the meanwhile, proactive applications of this multi-dimensional modulation technique may bring vitality to the field of laser-electron manipulation

Magnetic field assisted enhanced electron acceleration due to a chirped echelon phase modulated laser in vacuum

Electron acceleration employing chirped echelon phase modulated (EPM) linearly polarised (LP) laser pulse in vacuum under the influence of axial magnetic field has been explored. Echelon supported laser pulse with phase modulation sustains the trapping of pre-accelerated electron in the most favourable phase. Furthermore, the addition of linear frequency chirp enlarges the electron laser interaction duration. Here, application of axial magnetic field serves a critical role in boosting the electron acceleration upto larger distances in the laser propagation direction. So, few MeV electron can be expedited up to GeV energy and retains it upto longer distances due to the collective influence of the frequency chirp and phase modulation. Effect of laser initial intensity, pulse duration, beam waist and electron injection is taken into consideration and optimized for enhanced energy gain. It is observed that the electron of initial energy ˜0.5 MeV can achieve energy of ˜2.5 GeV with I∼1.38 × 1020 W/cm2 due to chirped LP EPM laser pulse in the existence of suitable axial magnetic field (˜41.7 MG) at most favourable values of electron injection angle and EPM gradient parameter.

Transverse electromagnetic Hermite–Gaussian mode-driven direct laser acceleration of electron under the influence of axial magnetic field

AbstractHermite–Gaussian (HG) laser beam with transverse electromagnetic (TEM) mode indices (m, n) of distinct values (0, 1), (0, 2), (0, 3), and (0, 4) has been analyzed theoretically for direct laser acceleration (DLA) of electron under the influence of an externally applied axial magnetic field. The propagation characteristics of a TEM HG beam in vacuum control the dynamics of electron during laser–electron interaction. The applied magnetic field strengthens the $\vec v \times \vec B$ force component of the fields acting on electron for the occurrence of strong betatron resonance. An axially confined enhanced acceleration is observed due to axial magnetic field. The electron energy gain is sensitive not only to mode indices of TEM HG laser beam but also to applied magnetic field. Higher energy gain in GeV range is seen with higher mode indices in the presence of applied magnetic field. The obtained results with distinct TEM modes would be helpful in the development of better table top accelerators of diverse needs.

The suppression of radiation reaction and laser field depletion in laser-electron beam interaction

The effects of radiation reaction (RR) have been studied extensively by using the interaction of ultraintense lasers with a counter-propagating relativistic electron. At the laser intensity at the order of 1023 W/cm2, the effects of RR are significant in a few laser periods for a relativistic electron. However, a laser at such intensity is tightly focused and the laser energy is usually assumed to be fixed. Then, the signal of RR and energy conservation cannot be guaranteed. To assess the effects of RR in a tightly focused laser pulse and the evolution of the laser energy, we simulated this interaction with a beam of 109 electrons by means of a Particle-In-Cell method. We observe that the effects of RR are suppressed due to the ponderomotive force and accompanied by a non-negligible amount of laser field energy reduction. This is because the ponderomotive force prevents the electrons from approaching the center of the laser pulse and leads to an interaction at the weaker field region. At the same time, the laser energy is absorbed through ponderomotive acceleration. Thus, the kinetic energy of the electron beam has to be carefully selected such that the effects of RR become obvious.

Optical design for increased interaction length in a high gradient dielectric laser accelerator

We present a methodology for designing and measuring pulse front tilt in an ultrafast laser for use in dielectric laser acceleration. Previous research into dielectric laser accelerating modules has focused on measuring high accelerating gradients in novel structures, but has done so only for short electron–laser coupling lengths. Here we demonstrate an optical design to extend the laser–electroninteraction to 1 mm.

Electron acceleration from rest to GeV energy by chirped axicon Gaussian laser pulse in vacuum in the presence of wiggler magnetic field

This paper presents a scheme of electron energy enhancement by employing frequency - chirped lowest order axicon focussed radially polarised (RP) laser pulse in vacuum under the influence of wiggler magnetic field. Terawatt RP laser can be focussed down to ∼5μm by an axicon optical element, which produces an intense longitudinal electric field. This unique property of axicon focused Gaussian RP laser pulse is employed for direct electron acceleration in vacuum. A linear frequency chirp increases the time duration of laser-electron interaction, whereas, the applied magnetic wiggler helps in improving the strength of ponderomotive force v→×B→ and periodically deflects electron in order to keep it traversing in the accelerating phase up to longer distance. Numerical simulations have been carried out to investigate the influence of laser, frequency chirp and magnetic field parameters on electron energy enhancement. It is noticed that an electron from rest can be accelerated up to GeV energy under optimized laser and magnetic field parameters. Significant enhancement in the electron energy gain of the order of 11.2 GeV is observed with intense chirped laser pulse in the presence of wiggler magnetic field of strength 96.2 kG.

Angle-resolved stochastic photon emission in the quantum radiation-dominated regime

Signatures of stochastic effects in the radiation of a relativistic electron beam interacting with a counterpropagating superstrong short focused laser pulse are investigated in a quantum regime when the electron’s radiation dominates its dynamics. We consider the electron-laser interaction at near-reflection conditions when pronounced high-energy gamma-ray bursts arise in the backward-emission direction with respect to the initial motion of the electrons. The quantum stochastic nature of the gamma-photon emission is exhibited in the angular distributions of the radiation and explained in an intuitive picture. Although, the visibility of the stochasticity signatures depends on the laser and electron beam parameters, the signatures are of a qualitative nature and robust. The stochasticity, a fundamental quantum property of photon emission, should thus be measurable rather straightforwardly with laser technology available in near future.

Reversible beam heater for suppression of microbunching instability by transverse gradient undulators

The microbunching instability driven by beam collective effects in a linear accelerator of a free-electron laser (FEL) facility significantly degrades the electron beam quality and FEL performance. A conventional method to suppress this instability is to introduce an additional uncorrelated energy spread by laser-electron interaction, which has been successfully operated in the Linac Coherent Light Source and Fermi@Elettra, etc. Some other ideas are recently proposed to suppress the instability without increasing energy spread, which could benefit the seeded FEL schemes. In this paper, we propose a reversible electron beam heater using two transverse gradient undulators to suppress the microbunching instability. This scheme introduces both an energy spread increase and a transverse-to-longitudinal phase space coupling, which suppress the microbunching instabilities driven by both longitudinal space charge and coherent synchrotron radiation before and within the system. Finally the induced energy spread increase and emittance growth are reversed. Theoretical analysis and numerical simulations are presented to verify the feasibility of the scheme and indicate the capability to improve the seeded FEL radiation performance.

Laser-electron interaction in plasma channel with dispersion effect

SynopsisLaser-electron interaction in a plasma channel is presented, taking into the action of superluminosity of the laser phase velocity induced by the dispersion of the plasma and the channel.

Continuously tunable narrowband pulses in the THz gap from laser-modulated electron bunches in a storage ring

This article reports on the generation of narrowband coherent synchrotron radiation from an electron storage ring. For the first time, this kind of radiation was now produced with continuously tunable frequencies in the so-called “THz gap” (between 1.2 and 5.6 THz), whereas previous experiments were limited to below 750 GHz. The experiment was performed at the DELTA storage ring in Dortmund, Germany, employing the interaction of external intensity-modulated laser pulses with an electron bunch, which causes a periodic longitudinal modulation of the charge density on a sub-millimeter scale. Furthermore, a strong influence of third-order dispersion in the laser pulses on the bandwidth and peak intensity of the THz radiation was observed. This effect is discussed in detail based on numerical simulations of the laser pulse generation and laser-electron interaction, and a modification of the laser system for compensating third-order dispersion is proposed.

THz-driven zero-slippage IFEL scheme for phase space manipulation**Work supported by DOE grant DE-SC0009914 and NSF grant PHY-1415583.

We describe an inverse free electron laser (IFEL) interaction driven by a near single-cycle THz pulse that is group velocity-matched to an electron bunch inside a waveguide, allowing for a sustained interaction in a magnetic undulator. We discuss the application of this guided-THz IFEL technique for compression of a relativistic electron bunch and synchronization with the external laser pulse used to generate the THz pulse via optical rectification, as well as a laser-driven THz streaking diagnostic with the potential for femtosecond scale temporal resolution. Initial measurements of the THz waveform via an electro-optic sampling based technique confirm the predicted reduction of the group velocity, using a curved parallel plate waveguide, as a function of the varying aperture size of the guide. We also present the design of a proof-of-principle experiment based on the bunch parameters available at the UCLA PEGASUS laboratory. With a THz peak field, our simulation model predicts compression of a electron beam by nearly an order of magnitude and a significant reduction of its initial timing jitter.

Quantum radiation reaction in head-on laser-electron beam interaction

In this paper, we investigate the evolution of the energy spread and the divergence of electron beams while they interact with different laser pulses at intensities where quantum effects and radiation reaction are of relevance. The interaction is modelled with a quantum electrodynamic (QED)-PIC code and the results are compared with those obtained using a standard PIC code with a classical radiation reaction module. In addition, an analytical model is presented that estimates the value of the final electron energy spread after the interaction with the laser has finished. While classical radiation reaction is a continuous process, in QED, radiation emission is stochastic. The two pictures reconcile in the limit when the emitted photons energy is small compared to the energy of the emitting electrons. The energy spread of the electron distribution function always tends to decrease with classical radiation reaction, whereas the stochastic QED emission can also enlarge it. These two tendencies compete in the QED-dominated regime. Our analysis, supported by the QED module, reveals an upper limit to the maximal attainable energy spread due to stochasticity that depends on laser intensity and the electron beam average energy. Beyond this limit, the energy spread decreases. These findings are verified for different laser pulse lengths ranging from short ∼30 fs pulses presently available to the long ∼150 fs pulses expected in the near-future laser facilities, and compared with a theoretical model. Our results also show that near future experiments will be able to probe this transition and to demonstrate the competition between enhanced QED induced energy spread and energy spectrum narrowing from classical radiation reaction.

TEM modes influenced electron acceleration by Hermite–Gaussian laser beam in plasma

AbstractElectron acceleration by a circularly polarized Hermite–Gaussian (HG) laser beam in the plasma has been investigated theoretically for the different transverse electromagnetic (TEM) mode indices (m, n) as (0, 1), (0, 2), (0, 3), and (0, 4). HG laser beam possesses higher trapping force compared with a standard Gaussian beam owing to its propagation characteristics during laser–electron interaction. A single-particle simulation indicates a resonant enhancement in the electron acceleration with HG laser beam. We present the intensity distribution for different TEM modes. We also analyze the dependence of beam width parameter on electron acceleration distance, which effectively influences the electron dynamics. Electron acceleration up to longer distance is observed with the lower modes. However, the higher electron energy gain is observed with higher modes at shorter distance of propagation.

Electron injection for enhanced energy gain by a radially polarized laser pulse in vacuum in the presence of magnetic wiggler

We present a scheme of electron injection for enhanced electron energy gain by using a radially polarized (RP) laser pulse in vacuum under the influence of magnetic wiggler. The inherent symmetry of an RP laser pulse enforces the trapping and acceleration of electrons in the direction of propagation of laser pulse during laser electron interaction. A magnetic wiggler encircles the trajectory of accelerated electron and improves the strength of v→×B→ force which supports the retaining of betatron resonance for longer duration and leads to enhance electron acceleration. Four times higher electron energy is observed with a RP laser pulse of peak intensity 8.5×1020 W/cm2 in the presence of magnetic wiggler of 10.69 kG than that in the absence of magnetic wiggler. We have also analyzed the electron injection for enhanced energy gain and observe that the electron energy gain is relatively higher with a sideway injection than that of axial injection of electron. Injection angle δ is optimized and found that at δ=10° to the direction of propagation of laser pulse, maximum energy is obtained.

Polarization effect of a Gaussian laser pulse on magnetic field influenced electron acceleration in vacuum

Electron acceleration by a laser pulse in the presence of azimuthal magnetic field in vacuum has been analyzed. The azimuthal magnetic field influences the trajectory of an accelerated electron during the laser electron interaction in vacuum. The electron trajectory in the absence and presence of azimuthal magnetic field with a linearly polarized (LP) and circularly polarized (CP) laser pulses is analyzed. Due to the presence of azimuthal magnetic field, a confined trajectory of accelerated electron is observed in the direction of propagation of laser pulse. Resonance between the electron and the laser field occurs at optimum values of magnetic field, electron gains high energy from the laser and gets accelerated in the direction of propagation of laser pulse. The azimuthal magnetic field keeps the electron motion close to the axis parallel to the direction of propagation due to which the electron gains and retains high energy for longer distances. The electron energy gain is relatively higher with a CP laser pulse than that with LP laser pulse. The high energy gain of about 2 GeV is observed with a CP laser pulse of peak intensity 2.74×1020 W/cm2 in the presence of azimuthal magnetic field of 534 kG.

Simulation of dark current and dark current-induced background photons in the Thomson scattering X-ray source

A model of dark current generation in the photocathode radio-frequency (RF) gun is established in the Thomson scattering X-ray source, and dark current transport and losses along the beamline are simulated. A velocity bunching cavity is added between the RF gun and the first linac to achieve the longitudinal compression of the photoelectron bunches. Given the longitudinal acceleration and the transverse focusing of the bunching cavity, the dark current electrons with bunching are approximately three times more than those without bunching, and this condition aggravates the harm to the operation of the photoinjector. Numerous dark current electrons around the electron–laser interaction section hit against the pipe inner wall and two laser focusing mirrors, producing a large number of background photons. A simulation of the bremsstrahlung process using an MCNP code is presented, showing that the background photon yield is less than 2.1% of the scattering photon yield, which is acceptable for our application.

Electron acceleration by a chirped laser pulse in vacuum under the influence of magnetic field

A linearly polarized (LP) chirped laser pulse is employed for electron acceleration to GeV energy under the influence of azimuthal magnetic field in vacuum. LP laser pulse supports the trapping of pre-accelerated electron during laser–electron interaction in vacuum. The electron gains energy from LP laser pulse and gets accelerated in the direction of propagation of laser pulse. Additionally, the chirping increases the electron laser interaction for longer duration while the azimuthal magnetic field having pinching effect keeps the motion of electron parallel to the direction of propagation of laser pulse leads to enhance the electron acceleration. The combined effect of chirping of laser pulse and pinching of azimuthal magnetic field not only enhances the electron energy gain but also supports in retaining of gained energy by the electron for longer distances. The accelerating distance is observed to be of three times the Rayleigh length where the Rayleigh length is about 6.78 μm. We observe electron energy gain of about 1.47 GeV in the presence of azimuthal magnetic field of about 438 kG with an intense LP laser pulse of peak intensity of about 3.4 × 1021 W/cm2. Higher electron energy gain may be obtained with highly intense laser pulse.

Ionization rate coefficients in warm dense plasmas.

We recast the atomic processes in a warm, dense plasma using Fermi-Dirac statistics and compare them to the rates of the usual Maxwell-Boltzmann approach of many collisional-radiative models. Population calculations show insignificant differences to calculations assuming nondegenerate free electrons of plasmas at solid density close to local thermodynamic equilibrium, but show departures in average ionization in the presence of strong photoionization. For example, we show that electron degeneracy affects the evolution of plasmas created by ultraviolet free electron laser interaction with solid targets.

Observation of redshifting and harmonic radiation in inverse Compton scattering

Inverse Compton scattering of laser photons by ultrarelativistic electron beam provides polarized x- to $\ensuremath{\gamma}$-ray pulses due to the Doppler blueshifting. Nonlinear electrodynamics in the relativistically intense linearly polarized laser field changes the radiation kinetics established during the Compton interaction. These are due to the induced figure-8 motion, which introduces an overall redshift in the radiation spectrum, with the concomitant emission of higher order harmonics. To experimentally analyze the strong field physics associated with the nonlinear electron-laser interaction, clear modifications to the angular and wavelength distributions of x rays are observed. The relativistic photon wave field is provided by the ps ${\mathrm{CO}}_{2}$ laser of peak normalized vector potential of $0.5l{a}_{L}l0.7$, which due to the quadratic dependence of the strength of nonlinear phenomena on ${a}_{L}$ permits sufficient effects not observed in past 2nd harmonic study with ${a}_{L}\ensuremath{\approx}0.3$ laser [, Phys. Rev. Lett. 96, 054802 (2006)]. The angular spectral characteristics are revealed using $K$-, $L$-edge, and high energy attenuation filters. The observation indicates existence of the electrons' longitudinal motion through frequency redshifting understood as the mass shift effect. Thus, the 3rd harmonic radiation has been observed containing on-axis x-ray component that is directly associated with the induced figure-8 motion. These are further supported by an initial evidence of off-axis 2nd harmonic radiation produced in a circularly polarized laser wave field. Total x-ray photon number per pulse, scattered by 65 MeV electron beam of 0.3 nC, at the interaction point is measured to be approximately ${10}^{9}$.