# Publikationen von

Dr. Sergey Rykovanov

Alle Publikationen des HI Jena

## 2021

**Conceptual design report for the LUXE experiment**

**230**, 2445 (2021)

**Relativistic modified Bessel-Gaussian beam generated from plasma-based beam braiding**

**104**, 021501 (2021)

**Abstract:** We theoretically and numerically demonstrate the generation of a relativistic modified Bessel-Gaussian beam (MBGB) via plasma-based beam braiding. It is realized by injecting several intense Gaussian pulses with well-designed offsets and angles into an underdense plasma channel which acts as a laser-pulse combiner via refractive coupling. The MBGB propagates stably in the plasma channel with a well-controlled orbital angular momentum of large value, exciting a twisted plasma wave. After leaving the plasma, it becomes unguided and survives in vacuum for at least hundreds of femtoseconds. This method is insensitive to the initial laser injection conditions and thus should be robust for experimental implementation. It provides an alternative approach in generating high-quality tunable intense optical vortex beams which are desired for various applications.

**Signatures of the Carrier Envelope Phase in Nonlinear Thomson Scattering**

**11**, 528 (2021)

**Abstract:** High-energy radiation can be generated by colliding a relativistic electron bunch with a high-intensity laser pulse-a process known as Thomson scattering. In the nonlinear regime the emitted radiation contains harmonics. For a laser pulse whose length is comparable to its wavelength, the carrier envelope phase changes the behavior of the motion of the electron and therefore the radiation spectrum. Here we show theoretically and numerically the dependency of the spectrum on the intensity of the laser and the carrier envelope phase. Additionally, we also discuss what experimental parameters are required to measure the effects for a beamed pulse.

## 2020

**Relativistic slingshot: A source for single circularly polarized attosecond x-ray pulses**

**102**, 061201 (2020)

**Abstract:** We propose a mechanism to generate a single intense circularly polarized attosecond x-ray pulse from the interaction of a circularly polarized relativistic few-cycle laser pulse with an ultrathin foil at normal incidence. Analytical modeling and particle-in-cell simulation demonstrate that a huge charge-separation field can be produced when all the electrons are displaced from the target by the incident laser, resulting in a high-quality relativistic electron mirror that propagates against the tail of the laser pulse. The latter is efficiently reflected as well as compressed into an attosecond pulse that is also circularly polarized.

**Propagation effects in multipass high harmonic generation from plasma surfaces**

**22**, 093048 (2020)

**Abstract:** Multipass high harmonic generation from plasma surfaces is a promising technique to enhance the efficiency of the generation process. In this paper it is shown that there is an optimal distance between two targets where the efficiency is maximized, depending on the laser and plasma parameters. This can be explained by the Gouy phase shift, which leads to the relative phase between the colours being changed with propagation in free space. A simple model is used to mimic the propagation of light from one target to another and to observe this effect in 1D particle-in-cell (PIC) simulations. The results are also verified using 2D PIC simulations.

**Giant Isolated Attosecond Pulses from Two-Color Laser-Plasma Interactions**

**124**, 114802 (2020)

**Abstract:** A new regime in the interaction of a two-color (ω,2ω) laser with a nanometer-scale foil is identified, resulting in the emission of extremely intense, isolated attosecond pulses—even in the case of multicycle lasers. For foils irradiated by lasers exceeding the blow-out field strength (i.e., capable of fully separating electrons from the ion background), the addition of a second harmonic field results in the stabilization of the foil up to the blow-out intensity. This is then followed by a sharp transition to transparency that essentially occurs in a single optical cycle. During the transition cycle, a dense, nanometer-scale electron bunch is accelerated to relativistic velocities and emits a single, strong attosecond pulse with a peak intensity approaching that of the laser field.

## 2019

**Intense attosecond pulses carrying orbital angularmomentum using laser plasma interactions**

**10**, 5554 (2019)

**Abstract:** Light beams with helical phase-fronts are known to carry orbital angular momentum (OAM) and provide an additional degree of freedom to beams of coherent light. While OAM beams can be readily derived from Gaussian laser beams with phase plates or gratings, this is far more challenging in the extreme ultra-violet (XUV), especially for the case of high XUV intensity. Here, we theoretically and numerically demonstrate that intense surface harmonics carrying OAM are naturally produced by the intrinsic dynamics of a relativistically intense circularly-polarized Gaussian beam (i.e. non-vortex) interacting with a target at normal incidence. Relativistic surface oscillations convert the laser pulses to intense XUV harmonic radiation via the well-known relativistic oscillating mirror mechanism. We show that the azimuthal and radial dependence of the harmonic generation process converts the spin angular momentum of the laser beam to orbital angular momentum resulting in an intense attosecond pulse (or pulse train) with OAM.

**Flexible x-ray source with tunable polarization and orbital angular momentum from Hermite-Gaussian laser modes driven plasma channel wakefield**

**22**, 071302 (2019)

**Abstract:** A plasma channel undulator/wiggler may be created through the plasma wakefield excited by the beating

of several Hermite-Gaussian laser modes propagating in a parabolic plasma channel. Control over both the

betatron and undulator forces is conveniently achieved by tuning the amplitude ratios, colors, and order

numbers of the modes. A special structure of the undulator/wiggler field without the focusing force near the propagation axis is generated inside the plasma wakefield by matching the strengths of the fundamental and

first-order Hermite-Gaussian modes. The electron beam only experiences forced undulator oscillations in

such a field, which significantly improves the quality of the emitted radiation. Since the value of the

undulator strength parameter could be in a wide range, less or larger than unity, it is capable of generating

narrow bandwidth x-ray, as well as the synchrotronlike high-energy x/γ-ray, radiation by harmonics.

Additionally, controlling the relative phases between the laser modes allows for polarization control of the

plasma undulator. High-order harmonics produced from a circularly polarized plasma undulator clearly

show the vortex nature and carry well-defined orbital angular momentum.

**Optimizing Laser Pulses for Narrow-Band Inverse Compton Sources in the High-Intensity Regime**

**122**, 204802 (2019)

**Abstract:** Scattering of ultraintense short laser pulses off relativistic electrons allows one to generate a large number of X- or gamma-ray photons with the expense of the spectral width---temporal pulsing of the laser inevitable leads to considerable spectral broadening. In this Letter, we describe a simple method to generate optimized laser pulses that compensate the nonlinear spectrum broadening and can be thought of as a superposition of two oppositely linearly chirped pulses delayed with respect to each other. We develop a simple analytical model that allows us to predict the optimal parameters of such a two-pulse---the delay, amount of chirp, and relative phase---for generation of a narrow-band $\gamma$-ray spectrum. Our predictions are confirmed by numerical optimization and simulations including three-dimensional effects.

## 2018

**Very-long distance propagation of high-energy laser pulse in air**

**25**, 113111 (2018)

**Abstract:** Long distance propagation of an energetic laser pulse with intensity slightly below that for multi-photon ionization in air is considered analytically, by noting that in the process, it is mainly the peak region of the pulse that interacts with the air molecules. Similar to that of much shorter femtosecond laser pulses of similar intensity, the affected air becomes slightly ionized and self-consistently forms a co-propagating thin and low-density plasma filament along the axis. It is found that a hundred-Joule-level laser pulse with a relatively large spot radius and pulse duration can propagate (also in the form of a self-consistent filament) tens of kilometers through the atmosphere. Such laser propagation properties should have applications in many areas.

**Analytical solutions for nonlinear Thomson scattering including radiation reaction**

**51**, 225701 (2018)

**Abstract:** Analytical solutions for the emitted nonlinear Thomson scattering spectrum with radiation reaction (RR) included are provided for a single electron colliding with a high intensity laser pulse. Further expressions are derived for the peak intensity for a given harmonic order and the downshift of the frequency when RR is included. Controlling the spectrum with shaping of the laser pulse frequency (chirp) has been investigated. It is shown that chirping of the laser pulse gives a distinct fingerprint of the effect of RR in the spectrum.

**Ring-like spatial distribution of laser accelerated protons in the ultra-high-contrast TNSA-regime**

**60**, 055010 (2018)

**Abstract:** The spatial distribution of protons accelerated from submicron-thick plastic foil targets using multi-terawatt, frequency-doubled laser pulses with ultra-high temporal contrast has been investigated experimentally. A very stable, ring-like beam profile of the accelerated protons, oriented around the target’s normal direction has been observed. The ring’s opening angle has been found to decrease with increasing foil thicknesses. Two-dimensional particle-in-cell simulations reproduce our results indicating that the ring is formed during the expansion of the proton density distribution into the vacuum as described by the mechanism of target-normal sheath acceleration. Here—in addition to the longitudinal electric fields responsible for the forward acceleration of the protons—a lateral charge separation leads to transverse field components accelerating the protons in the lateral direction.

**A systematic approach to numerical dispersion in Maxwell solvers**

**224**, 273 (2018)

**Abstract:** The finite-difference time-domain (FDTD) method is a well established method for solving the time evolution of Maxwell’s equations. Unfortunately the scheme introduces numerical dispersion and therefore phase and group velocities which deviate from the correct values. The solution to Maxwell’s equations in more than one dimension results in non-physical predictions such as numerical dispersion or numerical Cherenkov radiation emitted by a relativistic electron beam propagating in vacuum. Improved solvers, which keep the staggered Yee-type grid for electric and magnetic fields, generally modify the spatial derivative operator in the Maxwell–Faraday equation by increasing the computational stencil. These modified solvers can be characterized by different sets of coefficients, leading to different dispersion properties. In this work we introduce a norm function to rewrite the choice of coefficients into a minimization problem. We solve this problem numerically and show that the minimization procedure leads to phase and group velocities that are considerably closer to c as compared to schemes with manually set coefficients available in the literature. Depending on a specific problem at hand (e.g. electron beam propagation in plasma, high-order harmonic generation from plasma surfaces, etc.), the norm function can be chosen accordingly, for example, to minimize the numerical dispersion in a certain given propagation direction. Particle-in-cell simulations of an electron beam propagating in vacuum using our solver are provided.

**γ-Ray Generation from Plasma Wakefield Resonant Wiggler**

**120**, 134801 (2018)

**Abstract:** A flexible gamma-ray radiation source based on the resonant laser-plasma wakefield wiggler is proposed. The wiggler is achieved by inducing centroid oscillations of a short laser pulse in a plasma channel. Electrons (self-)injected in such a wakefield experience both oscillations due to the transverse electric fields and energy gain due to the longitudinal electric field. The oscillations are significantly enhanced when the laser pulse centroid oscillations are in resonance with the electron betatron oscillations, extending the radiation spectrum to the gamma-ray range. The polarization of the radiation can be easily controlled by adjusting the injection of the laser pulse into the plasma channel.

**Higher-Dimensional Caustics in Nonlinear Compton Scattering**

**120**, 044802 (2018)

**Abstract:** A description of the spectral and angular distributions of Compton scattered light in collisions of intense laser pulses with high-energy electrons is unwieldy and usually requires numerical simulations. However, due to the large number of parameters affecting the spectra such numerical investigations can become computationally expensive. Using methods of catastrophe theory we predict higher-dimensional caustics in the spectra of the Compton scattered light, which are associated with bright narrow-band spectral lines, and in the simplest case can be controlled by the value of the linear chirp of the pulse. These findings require no full-scale calculations and have direct consequences for the photon yield enhancement of future nonlinear Compton scattering x-ray or gamma-ray sources.

**Plasma Channel Undulator for Narrow-Bandwidth X-Ray Generation**

**Abstract:** A new plasma channel undulator concept based on the wakefields generated by short intense laser pulse undergoing centroid oscillations inside parabolic plasma channel is presented. The period of such an undulator is proportional to the Rayleigh length of the laser pulse and can be in the submillimeter range, while its strength can reach unity. Two-dimensional particle-in-cell simulations of the laser pulse propagation and wakefields are presented. Spontaneous radiation produced by the electron beam inside the plasma undulator is calculated.

## 2017

**Plasma channel undulator excited by high-order laser modes**

**7**, 16884 (2017)

**Abstract:** The possibility of utilizing plasma undulators and plasma accelerators to produce compact ultraviolet and X-ray sources, has attracted considerable interest for a few decades. This interest has been driven by the great potential to decrease the threshold for accessing such sources, which are mainly provided by a few dedicated large-scale synchrotron or free-electron laser (FEL) facilities. However, the broad radiation bandwidth of such plasma devices limits the source brightness and makes it difficult for the FEL instability to develop. Here, using multi-dimensional particle-in-cell (PIC) simulations, we demonstrate that a plasma undulator generated by the beating of a mixture of high-order laser modes propagating inside a plasma channel, leads to a few percent radiation bandwidth. The strength of the undulator can reach unity, the period can be less than a millimeter, and the number of undulator periods can be significantly increased by a phase locking technique based on the longitudinal tapering. Polarization control of such an undulator can be achieved by appropriately choosing the phase of the modes. According to our results, in the fully beam loaded regime, the electron current in the plasma undulator can reach 0.3\^aEUR%0kA level, making such an undulator a potential candidate towards a table-top FEL.

**Strong field electrodynamics of a thin foil**

**1812**, 090001 (2017)

**Abstract:** A new one-dimensional analytical model of a thin double layer foil interaction with a laser pulse is presented. It is based on one-dimensional electrodynamics. This model can be used for the study of high intensity laser pulse interactions with overdense plasmas, leading to frequency upshifting, high order harmonic generation, and ion acceleration in different regimes.

**Strong field electrodynamics of a thin foil**

**Abstract:** A new one-dimensional analytical model of a thin double layer foil interaction with a laser pulse is presented. It is based on one-dimensional electrodynamics. This model can be used for the study of high intensity laser pulse interactions with overdense plasmas, leading to frequency upshifting, high order harmonic generation, and ion acceleration in different regimes.

## 2016

**Experimental observation of attosecond control over relativistic electron bunches with two-colour fields**

**32**, 11 (2016)

**Abstract:** Energy coupling during relativistically intense laser–matter interactions is encoded in the attosecond motion of strongly driven electrons at the pre-formed plasma–vacuum boundary. Studying and controlling this motion can reveal details about the microscopic processes that govern a vast array of light–matter interaction phenomena, including those at the forefront of extreme laser–plasma science such as laser-driven ion acceleration, bright attosecond pulse generation and efficient energy coupling for the generation and study of warm dense matter. Here we experimentally demonstrate that by precisely adjusting the relative phase of an additional laser beam operating at the second harmonic of the driving laser it is possible to control the trajectories of relativistic electron bunches formed during the interaction with a solid target at the attosecond scale. We observe significant enhancements in the resulting high-harmonic yield, suggesting potential applications for sources of ultra-bright, extreme ultraviolet attosecond radiation to be used in atomic and molecular pump–probe experiments.

**Reply to ``Comment on `Controlling the spectral shape of nonlinear Thomson scattering with proper laser chirping'''**

**19**, 098002 (2016)

**Abstract:** We reply to Terzic and Krafft’s forgoing Comment [Phys. Rev. Accel. Beams, Comment on “Controlling the spectral shape of nonlinear Thomson scattering with proper laser chirping” 19 (2016)]. We disagree with the conclusion of the Comment regarding the novelty of solutions and the citations presented in our paper.

**Tunable polarization plasma channel undulator for narrow bandwidth photon emission**

**19**, 090703 (2016)

**Abstract:** The theory of a plasma undulator excited by a short intense laser pulse in a parabolic plasma channel is presented. The undulator fields are generated either by the laser pulse incident off-axis and/or under the angle with respect to the channel axis. Linear plasma theory is used to derive the wakefield structure. It is shown that the electrons injected into the plasma wakefields experience betatron motion and undulator oscillations. Optimal electron beam injection conditions are derived for minimizing the amplitude of the betatron motion, producing narrow-bandwidth undulator radiation. Polarization control is readily achieved by varying the laser pulse injection conditions.

**Temporal laser-pulse-shape effects in nonlinear Thomson scattering**

**93**, 063801 (2016)

**Abstract:** The influence of the laser-pulse temporal shape on the nonlinear Thomson scattering on-axis photon spectrum is analyzed in detail. Using the classical description, analytical expressions for the temporal and spectral structure of the scattered radiation are obtained for the case of symmetric laser-pulse shapes. The possibility of reconstructing the incident laser pulse from the scattered spectrum averaged over interference fringes in the case of high peak intensity and symmetric laser-pulse shape is discussed.

**Analytical results for nonlinear Compton scattering in short intense laser pulses**

**82**, 655820203 (2016)

**Abstract:** We study in detail the strong-field QED process of nonlinear Compton scattering in short intense plane wave laser pulses of circular polarization. Our main focus is placed on how the spectrum of the backscattered laser light depends on the shape and duration of the initial short intense pulse. Although this pulse shape dependence is very complicated and highly nonlinear, and has never been addressed explicitly, our analysis reveals that all the dependence on the laser pulse shape is contained in a class of three-parameter master integrals. Here we present completely analytical expressions for the nonlinear Compton spectrum in terms of these master integrals. Moreover, we analyse the universal behaviour of the shape of the spectrum for very high harmonic lines.

**Controlling the spectral shape of nonlinear Thomson scattering with proper laser chirping**

**19**, 030701 (2016)

**Abstract:** Effects of nonlinearity in Thomson scattering of a high intensity laser pulse from electrons are analyzed. Analytic expressions for laser pulse shaping in frequency (chirping) are obtained which control spectrum broadening for high laser pulse intensities. These analytic solutions allow prediction of the spectral form and required laser parameters to avoid broadening. Results of analytical and numerical calculations agree well. The control over the scattered radiation bandwidth allows narrow bandwidth sources to be produced using high scattering intensities, which in turn greatly improves scattering yield for future x- and gamma-ray sources.

**High-energy-density electron beam from interaction of two successive laser pulses with subcritical-density plasma**

**19**, 021301 (2016)

**Abstract:** It is shown by particle-in-cell simulations that a narrow electron beam with high energy and charge density can be generated in a subcritical-density plasma by two consecutive laser pulses. Although the first laser pulse dissipates rapidly, the second pulse can propagate for a long distance in the thin wake channel created by the first pulse and can further accelerate the preaccelerated electrons therein. Given that the second pulse also self-focuses, the resulting electron beam has a narrow waist and high charge and energy densities. Such beams are useful for enhancing the target-back space-charge field in target normal sheath acceleration of ions and bremsstrahlung sources, among others.

## 2015

**Noncollinear Polarization Gating of Attosecond Pulse Trains in the Relativistic Regime**

**115**, 193903 (2015)

**Abstract:** High order harmonics generated at relativistic intensities have long been recognized as a route to the most powerful extreme ultraviolet pulses. Reliably generating isolated attosecond pulses requires gating to only a single dominant optical cycle, but techniques developed for lower power lasers have not been readily transferable. We present a novel method to temporally gate attosecond pulse trains by combining noncollinear and polarization gating. This scheme uses a split beam configuration which allows pulse gating to be implemented at the high beam fluence typical of multi-TW to PW class laser systems. Scalings for the gate width demonstrate that isolated attosecond pulses are possible even for modest pulse durations achievable for existing and planned future ultrashort high-power laser systems. Experimental results demonstrating the spectral effects of temporal gating on harmonic spectra generated by a relativistic laser plasma interaction are shown.

**Plasma Undulator Based on Laser Excitation of Wakefields in a Plasma Channel**

**114**, 145003 (2015)

**Abstract:** An undulator is proposed based on the plasma wakefields excited by a laser pulse in a plasma channel. Generation of the undulator fields is achieved by inducing centroid oscillations of the laser pulse in the channel. The period of such an undulator is proportional to the Rayleigh length of the laser pulse and can be submillimeter, while preserving high undulator strength. The electron trajectories in the undulator are examined, expressions for the undulator strength are presented, and the spontaneous radiation is calculated. Multimode and multicolor laser pulses are considered for greater tunability of the undulator period and strength.

**Narrowband inverse Compton scattering x-ray sources at high laser intensities**

**91**, 033402 (2015)

**Abstract:** Narrowband x- and γ-ray sources based on the inverse Compton scattering of laser pulses suffer from a limitation of the allowed laser intensity due to the onset of nonlinear effects that increase their bandwidth. It has been suggested that laser pulses with a suitable frequency modulation could compensate this ponderomotive broadening and reduce the bandwidth of the spectral lines, which would allow one to operate narrowband Compton sources in the high-intensity regime. In this paper we therefore present the theory of nonlinear Compton scattering in a frequency-modulated intense laser pulse. We systematically derive the optimal frequency modulation of the laser pulse from the scattering matrix element of nonlinear Compton scattering, taking into account the electron spin and recoil. We show that, for some particular scattering angle, an optimized frequency modulation completely cancels the ponderomotive broadening for all harmonics of the backscattered light. We also explore how sensitively this compensation depends on the electron-beam energy spread and emittance, as well as the laser focusing.

## 2014

**Quasi-monoenergetic femtosecond photon sources from Thomson Scattering using laser plasma accelerators and plasma channels**

**47**, 234013 (2014)

**Abstract:** Narrow bandwidth, high energy photon sources can be generated by Thomson scattering of laser light from energetic electrons, and detailed control of the interaction is needed to produce high quality sources. We present analytic calculations of the energy-angular spectra and photon yield that parametrize the influences of the electron and laser beam parameters to allow source design. These calculations, combined with numerical simulations, are applied to evaluate sources using conventional scattering in vacuum and methods for improving the source via laser waveguides or plasma channels. We show that the photon flux can be greatly increased by using a plasma channel to guide the laser during the interaction. Conversely, we show that to produce a given number of photons, the required laser energy can be reduced by an order of magnitude through the use of a plasma channel. In addition, we show that a plasma can be used as a compact beam dump, in which the electron beam is decelerated in a short distance, thereby greatly reducing radiation shielding. Realistic experimental errors such as transverse jitter are quantitatively shown to be tolerable. Examples of designs relevant to nuclear resonance fluorescence and photofission are provided.

**Dependence of Laser-Driven Coherent Synchrotron Emission Efficiency on Pulse Ellipticity and Implications for Polarization Gating**

**112**, 123902 (2014)

**Abstract:** The polarization dependence of laser-driven coherent synchrotron emission transmitted through thin foils is investigated experimentally. The harmonic generation process is seen to be almost completely suppressed for circular polarization opening up the possibility of producing isolated attosecond pulses via polarization gating. Particle-in-cell simulations suggest that current laser pulses are capable of generating isolated attosecond pulses with high pulse energies.

## 2013

**Relativistic electron mirrors from nanoscale foils for coherent frequency upshift to the extreme ultraviolet**

**4**, 1763 (2013)

**Abstract:** Reflecting light from a mirror moving close to the speed of light has been envisioned as a route towards producing bright X-ray pulses since Einstein’s seminal work on special relativity. For an ideal relativistic mirror, the peak power of the reflected radiation can substantially exceed that of the incident radiation due to the increase in photon energy and accompanying temporal compression. Here we demonstrate for the first time that dense relativistic electron mirrors can be created from the interaction of a high-intensity laser pulse with a freestanding, nanometre-scale thin foil. The mirror structures are shown to shift the frequency of a counter-propagating laser pulse coherently from the infrared to the extreme ultraviolet with an efficiency > 10^4 times higher than in the case of incoherent scattering. Our results elucidate the reflection process of laser-generated electron mirrors and give clear guidance for future developments of a relativistic mirror structure.

## 2012

**Coherent synchrotron emission from electron nanobunches formed in relativistic laser-plasma interactions**

**8**, 804 (2012)

**Abstract:** Extreme ultraviolet (XUV) and X-ray harmonic spectra produced by intense laser - solid interactions have, so far, been consistent with Doppler upshifted reflection from collective relativistic plasma oscillations - the relativistically oscillating mirror mechanism. Recent theoretical work, however, has identified a new interaction regime in which dense electron nanobunches are formed at the plasma–vacuum boundary resulting in coherent XUV radiation by coherent synchrotron emission (CSE). Our experiments enable the isolation of CSE from competing processes, demonstrating that electron nanobunch formation does indeed occur. We observe spectra with the characteristic spectral signature of CSE - a slow decay of intensity, I, with high-harmonic order, n, as I(n) ~ n^(−1.62) before a rapid efficiency rollover. Particle-in-cell code simulations reveal how den se nanobunches of electrons are periodically formed and accelerated during normal-incidence interactions with ultrathin foils and result in CSE in the transmitted direction. This observation of CSE presents a route to high-energy XUV pulses and offers a new window on understanding ultrafast energy coupling during intense laser - solid density interactions.