Laser Acceleration of Electrons, Protons, and Ions VI
A grid of equidistant electron stripes is generated during the collision of two laser pulses unde... more A grid of equidistant electron stripes is generated during the collision of two laser pulses under a small angle in underdense plasma. Due to the oblique incidence, transverse standing wave in plasma is observed, in addition to the longitudinal traveling wave of the compound laser field. This standing wave results in the generation of plasma density grating. The ratio of the peak stripe density to background density can reach the value of 20:1. The grating period is determined by the interaction angle. Analytical theory of the compound electric fields is provided for plane waves. The grating formation is then verified via particle-in-cell simulations for short Gaussian laser pulses with typical experimental parameters. In addition, the interference pattern was also observed during experiments with Diocles laser. The results presented here are relevant for many laser-plasma applications, such as Raman scattering, inertial confinement fusion, plasma photonic crystals and laser wakefield acceleration.
Contemporary ultraintense, short-pulse laser systems provide extremely compact setups for the pro... more Contemporary ultraintense, short-pulse laser systems provide extremely compact setups for the production of high-flux neutron beams, such as those required for nondestructive probing of dense matter, research on neutron-induced damage in fusion devices or laboratory astrophysics studies. Here, by coupling particle-in-cell and Monte Carlo numerical simulations, we examine possible strategies to optimise neutron sources from ion-induced nuclear reactions using 1-PW, 20-fs-class laser systems. To improve the ion acceleration, the laser-irradiated targets are chosen to be ultrathin solid foils, either standing alone or preceded by a plasma layer of near-critical density to enhance the laser focusing. We compare the performance of these single- and double-layer targets, and determine their optimum parameters in terms of energy and angular spectra of the accelerated ions. These are then sent into a converter to generate neutrons via nuclear reactions on beryllium and lead nuclei. Overall,...
We propose a method to use traveling-wave Thomson scattering for spatiotemporally-resolved electr... more We propose a method to use traveling-wave Thomson scattering for spatiotemporally-resolved electron spectroscopy. This can enable ultrafast time-resolved measurements of the dynamics of relativistic electrons in the presence of extremely intense light fields, either in vacuum or in plasma, such as in laser wakefield accelerators. We demonstrate, with test-particle simulation and analysis, the capability of this technique for measurements of various high field phenomena: radiation reaction of electrons due to scattering, dephasing of a laser wakefield accelerator, and acceleration of electrons in multiple buckets by a laser wakefield.
The new generation of multi-petawatt (PW) class laser systems will generally combine several beam... more The new generation of multi-petawatt (PW) class laser systems will generally combine several beamlines. We here investigate how to arrange their irradiation geometry in order to optimize their coupling with solid targets, as well as the yields and beam quality of the produced particles. We first report on a proof-of-principle experiment, performed at the Rutherford Appleton Laboratory Vulcan laser facility, where two intense laser beams were overlapped in a mirror-like configuration onto a solid target, preceded by a long preplasma. We show that when the laser beams were close enough to each other, the generation of hot electrons at the target front was much improved and so was the ion acceleration at the target backside, both in terms of their maximum energy and collimation. The underlying mechanism is pinpointed with multidimensional particle-in-cell simulations, which demonstrate that the magnetic fields self-induced by the electron currents driven by the two laser beams at the target front can reconnect, thereby enhancing the production of hot electrons, and favoring their subsequent magnetic guiding across the target. Our simulations also reveal that the laser coupling with the target can be further improved when overlapping more than two beamlines. This multi-beam scheme would obviously be highly beneficial to the multi-PW laser projects proposed now and in the near future worldwide.
We show the first experiment of a transverse laser interference for electron injection into the l... more We show the first experiment of a transverse laser interference for electron injection into the laser plasma accelerators. Simulations show such an injection is different from previous methods, as electrons are trapped into later acceleration buckets other than the leading ones. With optimal plasma tapering, the dephasing limit of such unprecedented electron beams could be potentially increased by an order of magnitude. In simulations, the interference drives a relativistic plasma grating, which triggers the splitting of relativistic-intensity laser pulses and wakefield. Consequently, spatially dual electron beams are accelerated, as also confirmed by the experiment.
Laser Acceleration of Electrons, Protons, and Ions V, 2019
It is usually assumed that ions are accelerated most efficiently in the case of non-expanded targ... more It is usually assumed that ions are accelerated most efficiently in the case of non-expanded targets irradiated by femtosecond ultra-intense laser pulse, alternatively with only short scale preplasma on their front side. Here, we demonstrate that the ions in an expanded foil with near-critical density plasma before its interaction with the main petawatt pulse may be accelerated to higher energies than that from ultra-thin foils. In order to investigate the mechanisms responsible for the acceleration of the most energetic ions, we used particle tracking in particle-in-cell simulations. It is demonstrated that high-energy ions originate from a small region of the depth below 1 μm and the width about the laser focal spot size (3 - 4 μm) in the case of semi-expanded target (with gradually increasing density up to the maximum density from the front side) and of a thin foil. On the other hand, the length of this region exceeds 5 μm for the expanded target. When the laser pulse propagates through near-critical density targets, a high density electron bunch is formed and travels with the laser pulse behind the target. Behind this electron bunch, a relatively long longitudinal electric field is generated and this field accelerates ions. Longitudinal electric field can be also generated due to expanding transverse magnetic field, which is observed for the expanded target.
An optical injection scheme into the laser wakefield accelerator by preceding injection pulse is ... more An optical injection scheme into the laser wakefield accelerator by preceding injection pulse is investigated by means of 3D numerical particle-in-cell simulations. Quasimonoenergetic hundred-pC electron bunches as short as 6 fs can be generated. Optimal beam separation distance is found at the intersection point of the injection beam bubble with the collection volume for transverse injection into the accelerator beam bubble. It approximately corresponds to the plasma wavelength. The main advantage of this scheme is the localized injection of high charge. This injection mechanism can be useful for applications such as ultrashort and relatively intense X-ray radiation sources such as a betatron radiation or Thomson backscattering, time-resolved electron diffraction or for seeding of further acceleration stages.
This contribution proposes the new scheme of the electron injection into the laser wakefield acce... more This contribution proposes the new scheme of the electron injection into the laser wakefield accelerator [1] using a pair of mutually delayed laser pulses. The ponderomotive force associated with the preceding weak injection prepulse forms an ion cavity with the regions of higher electron densities on its borders. The size of the bubble and the delay between pulses is designed with the intention to place these regions to the collection volume from where the electrons are self-injected into the accelerating wakefield induced by delayed stronger driver pulse [2].
In this paper, we report on development of incoherent secondary X-ray sources at the PALS Researc... more In this paper, we report on development of incoherent secondary X-ray sources at the PALS Research Center and discuss the plan for the ELI Beamlines project. One of the approaches, how to generate ultrashort pulses of incoherent X-ray radiation, is based on the interaction of femtosecond laser pulses with underdense plasma. This method, known as laser wakefield electron acceleration (LWFA ), can produce up to GeV electron beams emitting radiation in the collimated beam with a femtosecond pulse duration. This approach was theoretically and experimentally examined at the PALS Center. The parameters of the PALS Ti:Sapphire laser interaction were studied by extensive particle-in-cell (PIC) simulations with radiation postprocessors in order to evaluate the capabilities of our system in this field. The compressed air, and a mixture of helium and argon were used as accelerating medium. The accelerator was operated in the bubble regime with forced self-injection and resulted in the generati...
Laser-plasma electron accelerators can be used to produce high-intensity x-rays, as electrons acc... more Laser-plasma electron accelerators can be used to produce high-intensity x-rays, as electrons accelerated in wakefields emit radiation due to betatron oscillations. Such x-ray sources inherit the features of the electron beam; sub-femtosecond electron bunches produce betatron sources of the same duration, which in turn allow probing matter on ultrashort time scales. In this paper we show, via Particle-in-Cell simulations, that attosecond electron bunches can be obtained using low-energy, ultra-short laser beams both in the self-injection and the controlled injection regimes at low plasma densities. However, only in the controlled regime does the electron injection lead to a stable, isolated attosecond electron bunch. Such ultrashort electron bunches are shown to emit attosecond x-ray bursts with high brilliance.
Ultrahigh-intensity laser-plasma physics provides unique light and particle beams as well as nove... more Ultrahigh-intensity laser-plasma physics provides unique light and particle beams as well as novel physical phenomena. A recently available regime is based on the interaction between a relativistic intensity few-cycle laser pulse and a sub-wavelength-sized mass-limited plasma target. Here, we investigate the generation of electron bunches under these extreme conditions by means of particle-in-cell simulations. In a first step, up to all electrons are expelled from the nanodroplet and gain relativistic energy from time-dependent local field enhancement at the surface. After this ejection, the electrons are further accelerated as they copropagate with the laser pulse. As a result, a few, or under specific conditions isolated, pC-class relativistic attosecond electron bunches are generated with laser pulse parameters feasible at state-of-the-art laser facilities. This is particularly interesting for some applications, such as generation of attosecond x-ray pulses via Thomson backscatte...
Laser Acceleration of Electrons, Protons, and Ions V, 2019
We examine betatron radiation properties from the bubble regime of laser-wakefield acceleration f... more We examine betatron radiation properties from the bubble regime of laser-wakefield acceleration for a tailored plasma density profile. Previous studies have already discussed enhancement of radiation properties by using various density modifications in later acceleration phases. This paper will focus on a density profile with a short linear up-ramp and compare it with a uniform density case. The process is studied for standard parameters feasible with current sub-100 TW laser systems by means of numerical particle-in-cell simulations. We show here that the critical energy and intensity of radiation increase when the plasma density increases. This enhancement is caused either by electron energy gain in the rear part of the bubble or by oscillation amplitude boost by fields behind the bubble.
High-intensity X-ray sources are essential diagnostic tools for science, technology and medicine.... more High-intensity X-ray sources are essential diagnostic tools for science, technology and medicine. Such X-ray sources can be produced in laser-plasma accelerators, where electrons emit short-wavelength radiation due to their betatron oscillations in the plasma wake of a laser pulse. Contemporary available betatron radiation X-ray sources can deliver a collimated X-ray pulse of duration on the order of several femtoseconds from a source size of the order of several micrometres. In this paper we demonstrate, through particle-in-cell simulations, that the temporal resolution of such a source can be enhanced by an order of magnitude by a spatial modulation of the emitting relativistic electron bunch. The modulation is achieved by the interaction of the that electron bunch with a co-propagating laser beam which results in the generation of a train of equidistant sub-femtosecond X-ray pulses. The distance between the single pulses of a train is tuned by the wavelength of the modulation las...
We demonstrate a novel approach to the generation of femtosecond electron bunch trains via laserd... more We demonstrate a novel approach to the generation of femtosecond electron bunch trains via laserdriven wakefield acceleration. We use two independent high-intensity laser pulses, a drive, and injector, each creating their own plasma wakes. The interaction of the laser pulses and their wakes results in a periodic injection of free electrons in the drive plasma wake via several mechanisms, including ponderomotive drift, wake-wake interference, and pre-acceleration of electrons directly by strong laser fields. Electron trains were generated with up to 4 quasi-monoenergetic bunches, each separated in time by a plasma period. The time profile of the generated trains is deduced from an analysis of beam loading and confirmed using 2D Particle-in-Cell simulations.
The injection process is one of the most crucial attributes that determine the final properties o... more The injection process is one of the most crucial attributes that determine the final properties of the electron bunch in laser wakefield accelerators. Here, a new injection method is proposed and studied via particle-in-cell (PIC) simulations for the typical parameters of the bubble regime. The injection is triggered by the laser beam that reaches the super-Gaussian profile in the focus. Such a beam undergoes rapid variations in its intensity distribution during the diffraction process. If this diffraction occurs in underdense plasma, consequent changes in the bubble structure activate a localized transverse injection process. The generated electron bunch is characterized by the short duration (~ 2 fs) and low transverse emittance (≤ 1 mm mrad), while maintaining relatively high charge (~ 0.2 nC).
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2018
The generation of stable electron beams produced by the laser wakefield acceleration mechanism wi... more The generation of stable electron beams produced by the laser wakefield acceleration mechanism with a fewterawatt laser system (600 mJ, 50 fs) in a supersonic synthetic air jet is reported and the requirements necessary to build such a stable electron source are experimentally investigated in conditions near the bubble regime threshold. The resulting electron beams have stable energies of (17.4 ± 1.1) MeV and an energy spread of (13.5 ± 1.5) MeV (FWHM), which has been achieved by optimizing the properties of the supersonic gas jet target for the given laser system. Due to the availability of few-terawatt laser systems in many laboratories around the world these stable electron beams open possibilities for applications of this type of particle source.
The injection and acceleration dynamics of electron bunches generated by two different optical in... more The injection and acceleration dynamics of electron bunches generated by two different optical injection mechanisms, the injection by an orthogonally crossing pulse with perpendicular polarization and injection by a copropagating preceding pulse, are studied by means of 2D numerical particle-in-cell (PIC) simulations. The effect of the ion cavity (bubble) shape variations induced by injection pulses on the electron bunch formation and observable parameters is explored for early injection and acceleration phases. Even if both schemes have three different injection regions, from which three independent electron sub-bunches emerge, the final merged electron bunch does not exhibit a significant substructure in studied parameters as transverse and longitudinal emittance. The 2D PIC simulations also reveal that the final electron bunch parameters are mainly affected by the spatial charge distribution of individual sub-bunches. Further, the model of the electric and magnetic fields within the slowly evolving ellipsoidal bubble is derived. The electron trajectories in acceleration later stages are analyzed by employing this model for the dynamic changes in the bubble size observed in PIC simulations.
Optical injection of electrons into a laser wakefield accelerator by a low intensity orthogonally... more Optical injection of electrons into a laser wakefield accelerator by a low intensity orthogonally colliding laser pulse is investigated using 2D particle-in-cell simulations. The collision of the main laser pulse driving the plasma wave in the cavitated regime and the low intensity injection pulse affects the trajectories of electrons in the crossing region. As a consequence, some electrons are ejected into the front part of the bubble, and these electrons are subsequently trapped in the rear part of the bubble. The injected and accelerated electron bunch reaches a peak energy of 630 MeV after 8 ps of acceleration being as short as 7.0 fs and is quasimonoenergetic with a low energy spread of 20 MeV (3.8%), having a charge of several dozens of pC and a relatively large emittance of 2.27 p Á mm Á mrad. Two main injection mechanisms-crossing beatwave injection and injection by laser field preacceleration-were identified.
Optical injection of electrons into a laser wakefield accelerator by colliding a strong drive las... more Optical injection of electrons into a laser wakefield accelerator by colliding a strong drive laser pulse and an orthogonally propagating and perpendicularly polarized weaker injection pulse(-s) is investigated using 2D and 3D particle-in-cell simulations. Within this contribution, we will present recent results of improvement in the fundamental scheme with single injection pulse by its specific modifications. Low-intensity injection pulses can generate sub-femtosecond electron bunches. Introduction of a second injection pulse perpendicular to the plasma-wave-driving pulse leads to a decrease in emittance and increase in charge. Moreover, we suggest introducing the negatively chirped pulses to drive the accelerating plasma wave in the configurations with controlled injection. It is demonstrated that the negative chirp effectively suppresses the presence of the dark current which is generally difficult to control. Thus, it amends the electron bunch parameters considerably.
Temporal profile of X-ray betatron radiation was theoretically studied for the parameters availab... more Temporal profile of X-ray betatron radiation was theoretically studied for the parameters available with current laser systems. Characteristics of the betatron radiation were investigated for three different configurations of laser wakefield acceleration: typical self-injection regime and optical injection regime with perpendicularly crossed injection and drive beams, both achievable with 100 TW class laser, and ionization injection regime with sub-10 TW laser system that was experimentally verified. Constructed spectrograms demonstrate that X-ray pulse durations are in order of few tens of femtoseconds and the optical injection case reveals the possibility of generating X-ray pulses as short as 2.6 fs. X-ray pulse duration depends mainly on the length of the trapped electron bunch as the emitted photons copropagate with the bunch with nearly the same velocity. These spectrograms were calculated using novel simplified method based on the theory of Liénard-Wiechert potentials. It takes advantage of the fact that the electron oscillates transversally in the accelerating plasma wave in the wiggler regime and, thus, emits radiation almost exclusively in the turning points of its sine-like trajectory. Therefore there are only few very narrow time intervals, which contribute significantly to the emission of radiation, while the rest can be neglected. These narrow time intervals are determined from the electron trajectories calculated using particle-in-cell simulations and the power spectrum at given point in far field is computed for each electron using the Fourier transform. Spectrograms of the emitted radiation are constructed by summing contributions of individual particles, since the incoherent nature of the electron bunch is assumed.
Laser Acceleration of Electrons, Protons, and Ions VI
A grid of equidistant electron stripes is generated during the collision of two laser pulses unde... more A grid of equidistant electron stripes is generated during the collision of two laser pulses under a small angle in underdense plasma. Due to the oblique incidence, transverse standing wave in plasma is observed, in addition to the longitudinal traveling wave of the compound laser field. This standing wave results in the generation of plasma density grating. The ratio of the peak stripe density to background density can reach the value of 20:1. The grating period is determined by the interaction angle. Analytical theory of the compound electric fields is provided for plane waves. The grating formation is then verified via particle-in-cell simulations for short Gaussian laser pulses with typical experimental parameters. In addition, the interference pattern was also observed during experiments with Diocles laser. The results presented here are relevant for many laser-plasma applications, such as Raman scattering, inertial confinement fusion, plasma photonic crystals and laser wakefield acceleration.
Contemporary ultraintense, short-pulse laser systems provide extremely compact setups for the pro... more Contemporary ultraintense, short-pulse laser systems provide extremely compact setups for the production of high-flux neutron beams, such as those required for nondestructive probing of dense matter, research on neutron-induced damage in fusion devices or laboratory astrophysics studies. Here, by coupling particle-in-cell and Monte Carlo numerical simulations, we examine possible strategies to optimise neutron sources from ion-induced nuclear reactions using 1-PW, 20-fs-class laser systems. To improve the ion acceleration, the laser-irradiated targets are chosen to be ultrathin solid foils, either standing alone or preceded by a plasma layer of near-critical density to enhance the laser focusing. We compare the performance of these single- and double-layer targets, and determine their optimum parameters in terms of energy and angular spectra of the accelerated ions. These are then sent into a converter to generate neutrons via nuclear reactions on beryllium and lead nuclei. Overall,...
We propose a method to use traveling-wave Thomson scattering for spatiotemporally-resolved electr... more We propose a method to use traveling-wave Thomson scattering for spatiotemporally-resolved electron spectroscopy. This can enable ultrafast time-resolved measurements of the dynamics of relativistic electrons in the presence of extremely intense light fields, either in vacuum or in plasma, such as in laser wakefield accelerators. We demonstrate, with test-particle simulation and analysis, the capability of this technique for measurements of various high field phenomena: radiation reaction of electrons due to scattering, dephasing of a laser wakefield accelerator, and acceleration of electrons in multiple buckets by a laser wakefield.
The new generation of multi-petawatt (PW) class laser systems will generally combine several beam... more The new generation of multi-petawatt (PW) class laser systems will generally combine several beamlines. We here investigate how to arrange their irradiation geometry in order to optimize their coupling with solid targets, as well as the yields and beam quality of the produced particles. We first report on a proof-of-principle experiment, performed at the Rutherford Appleton Laboratory Vulcan laser facility, where two intense laser beams were overlapped in a mirror-like configuration onto a solid target, preceded by a long preplasma. We show that when the laser beams were close enough to each other, the generation of hot electrons at the target front was much improved and so was the ion acceleration at the target backside, both in terms of their maximum energy and collimation. The underlying mechanism is pinpointed with multidimensional particle-in-cell simulations, which demonstrate that the magnetic fields self-induced by the electron currents driven by the two laser beams at the target front can reconnect, thereby enhancing the production of hot electrons, and favoring their subsequent magnetic guiding across the target. Our simulations also reveal that the laser coupling with the target can be further improved when overlapping more than two beamlines. This multi-beam scheme would obviously be highly beneficial to the multi-PW laser projects proposed now and in the near future worldwide.
We show the first experiment of a transverse laser interference for electron injection into the l... more We show the first experiment of a transverse laser interference for electron injection into the laser plasma accelerators. Simulations show such an injection is different from previous methods, as electrons are trapped into later acceleration buckets other than the leading ones. With optimal plasma tapering, the dephasing limit of such unprecedented electron beams could be potentially increased by an order of magnitude. In simulations, the interference drives a relativistic plasma grating, which triggers the splitting of relativistic-intensity laser pulses and wakefield. Consequently, spatially dual electron beams are accelerated, as also confirmed by the experiment.
Laser Acceleration of Electrons, Protons, and Ions V, 2019
It is usually assumed that ions are accelerated most efficiently in the case of non-expanded targ... more It is usually assumed that ions are accelerated most efficiently in the case of non-expanded targets irradiated by femtosecond ultra-intense laser pulse, alternatively with only short scale preplasma on their front side. Here, we demonstrate that the ions in an expanded foil with near-critical density plasma before its interaction with the main petawatt pulse may be accelerated to higher energies than that from ultra-thin foils. In order to investigate the mechanisms responsible for the acceleration of the most energetic ions, we used particle tracking in particle-in-cell simulations. It is demonstrated that high-energy ions originate from a small region of the depth below 1 μm and the width about the laser focal spot size (3 - 4 μm) in the case of semi-expanded target (with gradually increasing density up to the maximum density from the front side) and of a thin foil. On the other hand, the length of this region exceeds 5 μm for the expanded target. When the laser pulse propagates through near-critical density targets, a high density electron bunch is formed and travels with the laser pulse behind the target. Behind this electron bunch, a relatively long longitudinal electric field is generated and this field accelerates ions. Longitudinal electric field can be also generated due to expanding transverse magnetic field, which is observed for the expanded target.
An optical injection scheme into the laser wakefield accelerator by preceding injection pulse is ... more An optical injection scheme into the laser wakefield accelerator by preceding injection pulse is investigated by means of 3D numerical particle-in-cell simulations. Quasimonoenergetic hundred-pC electron bunches as short as 6 fs can be generated. Optimal beam separation distance is found at the intersection point of the injection beam bubble with the collection volume for transverse injection into the accelerator beam bubble. It approximately corresponds to the plasma wavelength. The main advantage of this scheme is the localized injection of high charge. This injection mechanism can be useful for applications such as ultrashort and relatively intense X-ray radiation sources such as a betatron radiation or Thomson backscattering, time-resolved electron diffraction or for seeding of further acceleration stages.
This contribution proposes the new scheme of the electron injection into the laser wakefield acce... more This contribution proposes the new scheme of the electron injection into the laser wakefield accelerator [1] using a pair of mutually delayed laser pulses. The ponderomotive force associated with the preceding weak injection prepulse forms an ion cavity with the regions of higher electron densities on its borders. The size of the bubble and the delay between pulses is designed with the intention to place these regions to the collection volume from where the electrons are self-injected into the accelerating wakefield induced by delayed stronger driver pulse [2].
In this paper, we report on development of incoherent secondary X-ray sources at the PALS Researc... more In this paper, we report on development of incoherent secondary X-ray sources at the PALS Research Center and discuss the plan for the ELI Beamlines project. One of the approaches, how to generate ultrashort pulses of incoherent X-ray radiation, is based on the interaction of femtosecond laser pulses with underdense plasma. This method, known as laser wakefield electron acceleration (LWFA ), can produce up to GeV electron beams emitting radiation in the collimated beam with a femtosecond pulse duration. This approach was theoretically and experimentally examined at the PALS Center. The parameters of the PALS Ti:Sapphire laser interaction were studied by extensive particle-in-cell (PIC) simulations with radiation postprocessors in order to evaluate the capabilities of our system in this field. The compressed air, and a mixture of helium and argon were used as accelerating medium. The accelerator was operated in the bubble regime with forced self-injection and resulted in the generati...
Laser-plasma electron accelerators can be used to produce high-intensity x-rays, as electrons acc... more Laser-plasma electron accelerators can be used to produce high-intensity x-rays, as electrons accelerated in wakefields emit radiation due to betatron oscillations. Such x-ray sources inherit the features of the electron beam; sub-femtosecond electron bunches produce betatron sources of the same duration, which in turn allow probing matter on ultrashort time scales. In this paper we show, via Particle-in-Cell simulations, that attosecond electron bunches can be obtained using low-energy, ultra-short laser beams both in the self-injection and the controlled injection regimes at low plasma densities. However, only in the controlled regime does the electron injection lead to a stable, isolated attosecond electron bunch. Such ultrashort electron bunches are shown to emit attosecond x-ray bursts with high brilliance.
Ultrahigh-intensity laser-plasma physics provides unique light and particle beams as well as nove... more Ultrahigh-intensity laser-plasma physics provides unique light and particle beams as well as novel physical phenomena. A recently available regime is based on the interaction between a relativistic intensity few-cycle laser pulse and a sub-wavelength-sized mass-limited plasma target. Here, we investigate the generation of electron bunches under these extreme conditions by means of particle-in-cell simulations. In a first step, up to all electrons are expelled from the nanodroplet and gain relativistic energy from time-dependent local field enhancement at the surface. After this ejection, the electrons are further accelerated as they copropagate with the laser pulse. As a result, a few, or under specific conditions isolated, pC-class relativistic attosecond electron bunches are generated with laser pulse parameters feasible at state-of-the-art laser facilities. This is particularly interesting for some applications, such as generation of attosecond x-ray pulses via Thomson backscatte...
Laser Acceleration of Electrons, Protons, and Ions V, 2019
We examine betatron radiation properties from the bubble regime of laser-wakefield acceleration f... more We examine betatron radiation properties from the bubble regime of laser-wakefield acceleration for a tailored plasma density profile. Previous studies have already discussed enhancement of radiation properties by using various density modifications in later acceleration phases. This paper will focus on a density profile with a short linear up-ramp and compare it with a uniform density case. The process is studied for standard parameters feasible with current sub-100 TW laser systems by means of numerical particle-in-cell simulations. We show here that the critical energy and intensity of radiation increase when the plasma density increases. This enhancement is caused either by electron energy gain in the rear part of the bubble or by oscillation amplitude boost by fields behind the bubble.
High-intensity X-ray sources are essential diagnostic tools for science, technology and medicine.... more High-intensity X-ray sources are essential diagnostic tools for science, technology and medicine. Such X-ray sources can be produced in laser-plasma accelerators, where electrons emit short-wavelength radiation due to their betatron oscillations in the plasma wake of a laser pulse. Contemporary available betatron radiation X-ray sources can deliver a collimated X-ray pulse of duration on the order of several femtoseconds from a source size of the order of several micrometres. In this paper we demonstrate, through particle-in-cell simulations, that the temporal resolution of such a source can be enhanced by an order of magnitude by a spatial modulation of the emitting relativistic electron bunch. The modulation is achieved by the interaction of the that electron bunch with a co-propagating laser beam which results in the generation of a train of equidistant sub-femtosecond X-ray pulses. The distance between the single pulses of a train is tuned by the wavelength of the modulation las...
We demonstrate a novel approach to the generation of femtosecond electron bunch trains via laserd... more We demonstrate a novel approach to the generation of femtosecond electron bunch trains via laserdriven wakefield acceleration. We use two independent high-intensity laser pulses, a drive, and injector, each creating their own plasma wakes. The interaction of the laser pulses and their wakes results in a periodic injection of free electrons in the drive plasma wake via several mechanisms, including ponderomotive drift, wake-wake interference, and pre-acceleration of electrons directly by strong laser fields. Electron trains were generated with up to 4 quasi-monoenergetic bunches, each separated in time by a plasma period. The time profile of the generated trains is deduced from an analysis of beam loading and confirmed using 2D Particle-in-Cell simulations.
The injection process is one of the most crucial attributes that determine the final properties o... more The injection process is one of the most crucial attributes that determine the final properties of the electron bunch in laser wakefield accelerators. Here, a new injection method is proposed and studied via particle-in-cell (PIC) simulations for the typical parameters of the bubble regime. The injection is triggered by the laser beam that reaches the super-Gaussian profile in the focus. Such a beam undergoes rapid variations in its intensity distribution during the diffraction process. If this diffraction occurs in underdense plasma, consequent changes in the bubble structure activate a localized transverse injection process. The generated electron bunch is characterized by the short duration (~ 2 fs) and low transverse emittance (≤ 1 mm mrad), while maintaining relatively high charge (~ 0.2 nC).
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2018
The generation of stable electron beams produced by the laser wakefield acceleration mechanism wi... more The generation of stable electron beams produced by the laser wakefield acceleration mechanism with a fewterawatt laser system (600 mJ, 50 fs) in a supersonic synthetic air jet is reported and the requirements necessary to build such a stable electron source are experimentally investigated in conditions near the bubble regime threshold. The resulting electron beams have stable energies of (17.4 ± 1.1) MeV and an energy spread of (13.5 ± 1.5) MeV (FWHM), which has been achieved by optimizing the properties of the supersonic gas jet target for the given laser system. Due to the availability of few-terawatt laser systems in many laboratories around the world these stable electron beams open possibilities for applications of this type of particle source.
The injection and acceleration dynamics of electron bunches generated by two different optical in... more The injection and acceleration dynamics of electron bunches generated by two different optical injection mechanisms, the injection by an orthogonally crossing pulse with perpendicular polarization and injection by a copropagating preceding pulse, are studied by means of 2D numerical particle-in-cell (PIC) simulations. The effect of the ion cavity (bubble) shape variations induced by injection pulses on the electron bunch formation and observable parameters is explored for early injection and acceleration phases. Even if both schemes have three different injection regions, from which three independent electron sub-bunches emerge, the final merged electron bunch does not exhibit a significant substructure in studied parameters as transverse and longitudinal emittance. The 2D PIC simulations also reveal that the final electron bunch parameters are mainly affected by the spatial charge distribution of individual sub-bunches. Further, the model of the electric and magnetic fields within the slowly evolving ellipsoidal bubble is derived. The electron trajectories in acceleration later stages are analyzed by employing this model for the dynamic changes in the bubble size observed in PIC simulations.
Optical injection of electrons into a laser wakefield accelerator by a low intensity orthogonally... more Optical injection of electrons into a laser wakefield accelerator by a low intensity orthogonally colliding laser pulse is investigated using 2D particle-in-cell simulations. The collision of the main laser pulse driving the plasma wave in the cavitated regime and the low intensity injection pulse affects the trajectories of electrons in the crossing region. As a consequence, some electrons are ejected into the front part of the bubble, and these electrons are subsequently trapped in the rear part of the bubble. The injected and accelerated electron bunch reaches a peak energy of 630 MeV after 8 ps of acceleration being as short as 7.0 fs and is quasimonoenergetic with a low energy spread of 20 MeV (3.8%), having a charge of several dozens of pC and a relatively large emittance of 2.27 p Á mm Á mrad. Two main injection mechanisms-crossing beatwave injection and injection by laser field preacceleration-were identified.
Optical injection of electrons into a laser wakefield accelerator by colliding a strong drive las... more Optical injection of electrons into a laser wakefield accelerator by colliding a strong drive laser pulse and an orthogonally propagating and perpendicularly polarized weaker injection pulse(-s) is investigated using 2D and 3D particle-in-cell simulations. Within this contribution, we will present recent results of improvement in the fundamental scheme with single injection pulse by its specific modifications. Low-intensity injection pulses can generate sub-femtosecond electron bunches. Introduction of a second injection pulse perpendicular to the plasma-wave-driving pulse leads to a decrease in emittance and increase in charge. Moreover, we suggest introducing the negatively chirped pulses to drive the accelerating plasma wave in the configurations with controlled injection. It is demonstrated that the negative chirp effectively suppresses the presence of the dark current which is generally difficult to control. Thus, it amends the electron bunch parameters considerably.
Temporal profile of X-ray betatron radiation was theoretically studied for the parameters availab... more Temporal profile of X-ray betatron radiation was theoretically studied for the parameters available with current laser systems. Characteristics of the betatron radiation were investigated for three different configurations of laser wakefield acceleration: typical self-injection regime and optical injection regime with perpendicularly crossed injection and drive beams, both achievable with 100 TW class laser, and ionization injection regime with sub-10 TW laser system that was experimentally verified. Constructed spectrograms demonstrate that X-ray pulse durations are in order of few tens of femtoseconds and the optical injection case reveals the possibility of generating X-ray pulses as short as 2.6 fs. X-ray pulse duration depends mainly on the length of the trapped electron bunch as the emitted photons copropagate with the bunch with nearly the same velocity. These spectrograms were calculated using novel simplified method based on the theory of Liénard-Wiechert potentials. It takes advantage of the fact that the electron oscillates transversally in the accelerating plasma wave in the wiggler regime and, thus, emits radiation almost exclusively in the turning points of its sine-like trajectory. Therefore there are only few very narrow time intervals, which contribute significantly to the emission of radiation, while the rest can be neglected. These narrow time intervals are determined from the electron trajectories calculated using particle-in-cell simulations and the power spectrum at given point in far field is computed for each electron using the Fourier transform. Spectrograms of the emitted radiation are constructed by summing contributions of individual particles, since the incoherent nature of the electron bunch is assumed.
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Papers by Vojtěch Horný