Dense Electron Bunches Research Articles

Electron bunch dynamics and emission in particle-in-cell simulations of relativistic laser–solid interactions: On density artifacts, collisions, and numerical dispersion

Sub-optical-cycle dynamics of dense electron bunches in relativistic-intensity laser–solid interactions lead to the emission of high-order harmonics and attosecond light pulses. The capacity of particle-in-cell simulations to accurately model these dynamics is essential for the prediction of emission properties because the attosecond pulse intensity depends on the electron density distribution at the time of emission and on the temporal distribution of individual electron Lorentz-factors in an emitting electron bunch. Here, we show that in one-dimensional collisionless simulations, the peak density of the emitting electron bunch increases with the increase in the spatial resolution of the simulation grid. When collisions are added to the model, the peak electron density becomes independent of the spatial resolution. Collisions are shown to increase the spread of the peaks of Lorentz-factors of emitting electrons in time, especially in the regimes far from optimum generation conditions, thus leading to lower intensities of attosecond pulses as compared to those obtained in collisionless simulations.

Coherent Spontaneous Emission from Short Electron Bunches: Competition between Different Transverse Waveguide Modes

In this paper, we study spontaneous coherent undulator emission from a short dense electron bunch. A special “negative-mass” undulator ensuring stabilization of the axial size of the bunch is considered. We focus on the situation where the competition between two different transverse waveguide modes takes place, so that the undulator resonance is provided simultaneously for three waves placed on the dispersion diagram symmetrically with respect to electrons (namely, the group velocity of the higher waveguide mode coincides with the electron velocity, whereas two resonance frequencies of the lower transverse mode correspond to wave group velocities, one of which slightly exceeds the translational velocity of electrons, with the other being less than the electron velocity). We show that the higher waveguide mode can win this competition due to its proximity to group synchronization with the electron bunch.

Photoinjector complex in IAP RAS: design parameters and current state of realization

The calculated parameters of the photoinjector electron accelerator currently being implemented at the Institute of Applied Physics are presented, and the current state of work is described. The complex provides a step-by-step increase of the average energy of particles in dense electron bunches up to 20 MeV, which opens possibilities for the implementation on its basis of a wide class of electromagnetic radiation sources.

Energy-chirp compensation of laser-driven ion beams enabled by structured targets

We show using 3D simulations that the challenge of generating dense mono-energetic laser-driven ion beams with low angular divergence can be overcome by utilizing structured targets with a relativistically transparent channel and an overdense wall. In contrast to a uniform target that produces a chirped ion beam, the target structure facilitates formation of a dense electron bunch whose longitudinal electric field reverses the energy chirp. This approach works in conjunction with existing acceleration mechanisms, augmenting the ion spectra. For example, our 3D simulations predict a significant improvement for a 2 PW laser pulse with a peak intensity of $5 \times 10^{22}$ W/cm$^2$. The simulations show a mono-energetic proton peak in a highly desirable energy range of 200 MeV with an unprecedented charge of several nC and relatively low divergence that is below 10$^{\circ}$.

Efficiency enhancement of the reltron oscillator using four-grid asymmetrical RF interaction cavity

In this article, a modified reltron was proposed. Here an asymmetrical radio frequency (RF) modulation cavity interaction structure was used which consists of three coupled cavities instead of two used in the conventional reltron. Hence, instead of three grids in its modulation cavity, the proposed modified reltron structure consists of four grids. This additional grid inside the modulation cavity: distributes the length of the modulation cavity asymmetrically, which helps to form tight and dense electron bunches within the device. The proposed structure was designed and simulated using a CST Studio Suite. For the optimized grid spacing, the results of the PIC simulation showed the modified reltron was able to produce ∼350 MW power at 2.75 GHz frequency which was ∼100 MW more than the reltron. With the typically selected device's electric parameters the device efficiency was also found ∼41.2 % which was ∼11 % higher than previously reported results.

Electron acceleration using twisted laser wavefronts

Using plasma mirror injection we demonstrate, both analytically and numerically, that a circularly polarized helical laser pulse can accelerate highly collimated dense bunches of electrons to several hundred MeV using currently available laser systems. The circular-polarized helical (Laguerre–Gaussian) beam has a unique field structure where the transverse fields have helix-like wave-fronts which tend to zero on-axis where, at focus, there are large on-axis longitudinal magnetic and electric fields. The acceleration of electrons by this type of laser pulse is analyzed as a function of radial mode number and it is shown that the radial mode number has a profound effect on electron acceleration close to the laser axis. Using three-dimensional particle-in-cell simulations a circular-polarized helical laser beam with power of 0.6 PW is shown to produce several dense attosecond bunches. The bunch nearest the peak of the laser envelope has an energy of 0.47 GeV with spread as narrow as 10%, a charge of 26 pC with duration of as, and a very low divergence of 20 mrad. The confinement by longitudinal magnetic fields in the near-axis region allows the longitudinal electric fields to accelerate the electrons over a long period after the initial reflection. Both the longitudinal E and B fields are shown to be essential for electron acceleration in this scheme. This opens up new paths toward attosecond electron beams, or attosecond radiation, at many laser facilities around the world.

Generation in the regime with three resonance frequencies

If the group velocity of the wave is close to the bunch velocity, the bunch placed in the maximumum of the radiated pulse. It provides high effeciency of the electron-wave interaction. However, there are other factors related to particle dynamics, which have strong influence on the radiation process. In this paper the regime with three resonance frequencies is discussed. By varying the phase size of the electron bunch, the generation conditions at each of the frequencies can be changed. There are results for the spontaneous coherent super-radiative undulator emission in the terahertz frequency range from a short (as compared to the wavelength of the radiated wave) dense electron bunch. As a result, an electron bunch radiates two pulses with amplitudes of the radiated fields ∼ 10-70 MV/m.

Generation of ultrashort pulses in the THz frequency range

There are results for the spontaneous coherent super-radiative undulator emission in the terahertz frequency range from a short (as compared to the wavelength of the radiated wave) dense electron bunch. If the group velocity of the wave is close to the bunch velocity, this is a process of spontaneous radiation followed by amplification of a single wave cycle. Despite the Coulomb repulsion of electrons inside the bunch, its compactness is provided by the compression of the bunch under the action of its own radiation fields. As a result, formation of an ultra-short (several cycles long) powerful wave packet occurs when the bunch moves through several undulator periods with high (∼20% in optimized systems) efficiency of extraction of the electron energy and high intensity (∼ 100 MV/m) of the peak wave field.

Coherent super-radiative undulator emission of ultra-short THz wave pulses

We study spontaneous coherent super-radiative undulator emission in the terahertz frequency range from a short (as compared to the wavelength of the radiated wave), dense electron bunch. Since the group velocity of the wave is close to the bunch velocity, this is a process of spontaneous radiation followed by amplification of a single wave cycle. Despite the Coulomb repulsion of electrons inside the bunch, its compactness, which is necessary to ensure the spontaneous coherent character of the radiation process, is provided by the compression of the bunch under the action of its own radiation fields. As a result, formation of an ultra-short (several cycles long) powerful wave packet occurs when the bunch moves through several undulator periods with high (∼20% in optimized profiled systems) efficiency of extraction of the electron energy and high intensity (∼100 MV/m) of the peak wave field.

Efficiency of terahertz undulator radiation from short electron bunches moving in the field of permanently magnetized helices

The motion and radiation of short dense bunches of ultrarelativistic electrons produced by laser-driven accelerators and moving in an undulator in the form of magnetized helices have been studied. Simulations demonstrate the possibility of generating wideband THz pulses with energies of hundreds of microjoules and relatively high efficiency in regimes close to the group synchronism of electrons with the waveguide mode.

Generation of intense coherent electromagnetic radiation during the interaction of a multi-terawatt laser pulse with a nanowire target* *Reported at the 4th International Conference on Ultrafast Optical Science (Russia, Moscow, FIAN, 28 September – 2 October 2020).

Generation of coherent radiation in the IR and terahertz ranges during the propagation of a multi-terawatt laser pulse along a nanowire target is investigated. In the process of interaction, dense electron bunches are displaced from the target and accelerated in the laser field, generating intense electromagnetic radiation. Three regimes of interaction can be realised depending on the duration and shape of the laser pulse. In the first regime, when the laser pulse is long enough (tens and hundreds of femtoseconds), electrons are only partially forced out of the target. The characteristics of the low-frequency part of the spectrum of the generated radiation are determined in this case by the duration of the laser pulse, as well as by its amplitude and target parameters (geometric dimensions and concentration of electrons). In the second regime, the laser pulse has a large amplitude and a steep rising edge (the amplitude of the first half-wave is of the order of the maximum pulse amplitude); as a result, most of the electrons are displaced from the target already at the initial moment of interaction. In this regime, unipolar and bipolar pulses with a duration of tens of laser field periods can be formed. Changing the target length makes it possible to control the period of field oscillations and their number in the generated radiation. In the intermediate regime of short laser pulses with an insufficiently steep rising edge, oscillations of the formed electron bunches can occur in the macroscopic Coulomb attraction field of the charged target, which gives rise to radiation with a frequency several times lower than that of laser radiation. In this case, the pulses of the generated radiation contain a few cycles of the field with decreasing amplitude and increasing frequency. Using numerical simulation in three regimes of interaction, the characteristics of IR and terahertz radiation are found, in particular, the pulse shapes, ranges of generated frequencies, amplitudes and angular distributions of radiation are determined. It is shown that the amplitude of the generated pulse can reach subrelativistic and relativistic values (the field strength is more than 1 TV m−1 at a frequency ten times lower than the laser radiation frequency), and the energy conversion efficiency can be of the order of one percent.

Attosecond γ -ray vortex generation in near-critical-density plasma driven by twisted laser pulses

Ultra-short brilliant γ-rays have many potential applications in astrophysics, nuclear physics, and ultra-fast science. However, attosecond γ-ray beams with energy above 100 MeV are still very challenging. Here, we propose and numerically demonstrate an all-optical scheme to produce sub-GeV attosecond γ-rays in near-critical-density (NCD) plasma. When a left-hand circularly polarized Laguerre–Gaussian (LG) laser pulse irradiates the NCD plasma, dense attosecond electron bunches are produced, trapped by the LG laser, and accelerated to GeV energies. Subsequently, these electrons oscillate in the LG laser electric fields and emit a string of hundreds MeV attosecond (367 as) γ-ray pulses. Three-dimensional particle-in-cell simulations indicate that, at a laser intensity of 1022 W/cm2, the yield of γ-ray pulses with photon energies above 1 MeV is as high as 1013 with a peak angular momentum of 10−15 kg m2/s. This results in γ-ray vortex beams with an unprecedented peak brilliance of 1024 photons s−1 mm−2 mrad−2 per 0.1% bandwith at 1 MeV, which may offer a unique opportunity for diverse applications.

Injection of a short electron bunch into THz radiation section with an undulator and strong guiding magnetic fields

To implement an efficient source of coherent radiation with negative mass longitudinal stabilization, the methods of the formation and injection of a dense electron bunch onto a stationary helical trajectory in a combined helical undulator and a strong uniform magnetic field are studied. Using a magnetic or electric lens permits sending particles almost along the converging lines of the magnetic field (magnetic following) and obtaining a nearly rectilinearly moving compressed bunch inside a solenoid. After that, the bunch can be injected into an adiabatically increasing field of the helical undulator. In this way, it is possible to excite operating undulator oscillations of particles, significantly mitigating the effects of destructive bunch expansion, the excitation of parasitic cyclotron oscillations, and velocity spread, thereby providing stabilization and terahertz radiation of a dense bunch. Due to a significant mode selection for an axis-encircling bunch, as well as due to the long-term interaction of the particles with a dominant mode, which is closest to the group synchronism conditions, radiation with a relatively narrow spectrum and high efficiency can be obtained even in a strongly oversized waveguide. An additional efficiency enhancement can be obtained due to the reduction of the velocity spread caused by the mutual Coulomb repulsion of electrons during injection due to the initial energy chirp of the bunch.

Spontaneous coherent cyclotron emission from photo-injector electron bunches: superradiation and two-frequency regime

Short dense electron bunches are attractive from the viewpoint to use them for the implementation of powerful and compact radiation source based on the spontaneous mechanism of radiation. The regime of spontaneous radiation is realized, when the bunch phase size is smaller than 2π. Modern photo-injectors insure formation of mildly relativistic dense electron bunches with duration less than 1 ps. Such bunches are “ready” to radiate waves at terahertz frequencies in the spontaneous regime. However, the repulsion of particles caused by a strong Coulomb field inside the dense electron bunch strictly limits the duration of the radiation process due to the increase in the bunch length. We propose two mechanism of cyclotron radiation to solve this problem. The first one is the regime of group synchronism of particles with the radiated wave; in this case compensation of Coulomb repulsion in the phase space takes place. The second proposed solution is to use compression of bunch by radiated electromagnetic fields.

Spontaneous super-radiative cascade undulator emission from short dense electron bunches

We propose to use super-radiative self-compression of a short dense electron bunch to provide the cascade two-undulator regime of spontaneous emission from the bunch. At the first stage of this cascade, the spontaneous super-radiative emission of a relatively long-wavelength wave results in compression of the bunch by the radiated field. This results in high-efficiency spontaneous radiation of a short-wavelength wave at the second stage. According to the simulations performed for electron bunches with the parameters typical for modern photoinjectors, the cascade regime ensures radiation in the subterahertz frequency range with efficiencies from 10% (in regular systems) up to 30%–50% (in profiled systems).

Spontaneous superradiant sub-THz coherent cyclotron emission from a short dense electron bunch

Short dense electron bunches produced by modern photoinjectors are attractive from the viewpoint of the realization of powerful and effective sources of subterahertz radiation based on the spontaneous coherent mechanism of emission. This type of emission is realized if the effective phase size of the bunch with respect to the radiated wave is small enough. Therefore, the repulsion of particles caused by a strong Coulomb field inside the dense electron bunch strictly limits the duration of the radiation process due to the increase in the bunch length. We show that this problem can be solved by using the cyclotron mechanism of the spontaneous radiation due to the effect of compensation of the Coulomb repulsion in the phase space.

Dynamics of the interaction of relativistic Laguerre–Gaussian laser pulses with a wire target

By using three-dimensional particle-in-cell simulations, we investigate the dynamics of ultraintense Laguerre–Gaussian (LG) laser pulses interacting with a wire target. When a relativistic LG-mode laser pulse sweeps a wire target, annular electron bunches with attosecond duration are periodically dragged out of the left tip of the wire. Due to the radial laser electric field force exerted on the electrons, the annular bunches are tightly constrained near the target surface and steadily propagate along the wire. A strong return current is thus induced to satisfy the plasma neutralization condition. Once leaving from the right tip of the target, the electron emission angle gradually decreases and each hollow electron bunch is converged into an electron disc. Under the action of the longitudinal electric field, electrons are continuously accelerated to 100s MeV. At the same time, the laser angular momentum is transferred efficiently to the beam angular momentum (BAM) of the bunches. The structure of the dense short electron bunches is stable and keeps intact for more than 300 fs. We can manipulate the quality of the bunches by changing the laser and target parameters, such as the laser handedness and intensity, beam waist radius, wire length and radius. The scheme paves the way for the generation of high-quality attosecond electron bunches with low divergence, high beam charge and large BAM, which will have wide-range applications in various domains.

A Free Electron Laser Based on a Sectional System of RF Undulators

An efficient mode of the multistage capture and deceleration of particles in a sectioned undulator system for use in free electron lasers. This scheme is based on using a series of RF undulators with profiled resonant parameters that are powered by the independent powerful pulsed RF oscillators. The proposed mode allows efficiency to be obtained at a level of several percent in short-wavelength free electron lasers based on using dense bunches of electrons with relatively high energy spreads, formed in state-of-the-art laser plasma accelerators.

Self-compression of dense photo-injector electron bunches

Several different mechanisms of compression of dense electron bunches in self fields are described. It is shown that the spontaneous radiation from a short (shorter than the radiation wavelength) electron bunch can result in a significant axial compression of the bunch under effect of the rf field of the radiated wave. This radiative self-compression can be provided in regimes of both cyclotron and undulator emission. Self-compression in the case of the “negative-mass” regime of the electron motion in an undulator of a special type is also described.

Spontaneous Cyclotron Radiation of a Dense Electron Bunch

Short dense electron bunches generated by today’s photoinjectors [1–5] are advantageous for creating powerful sources of spontaneous coherent radiation in the terahertz frequency range. The regime of spontaneous radiation is achieved when the bunch size in the phase space is well below 2π. Coulomb interaction between particles in the dense bunches increases the axial size, and thus can limit the power and duration of radiation. It is shown this problem can be solved for cyclotron radiation by compensating for the Coulomb repulsion in the phase space.