Self-amplified Spontaneous Emission Regime Research Articles

Generation of ultrahigh-brightness pre-bunched beams from a plasma cathode for X-ray free-electron lasers

The longitudinal coherence of X-ray free-electron lasers (XFELs) in the self-amplified spontaneous emission regime could be substantially improved if the high brightness electron beam could be pre-bunched on the radiated wavelength-scale. Here, we show that it is indeed possible to realize such current modulated electron beam at angstrom scale by exciting a nonlinear wake across a periodically modulated plasma-density downramp/plasma cathode. The density modulation turns on and off the injection of electrons in the wake while downramp provides a unique longitudinal mapping between the electrons’ initial injection positions and their final trapped positions inside the wake. The combined use of a downramp and periodic modulation of micrometers is shown to be able to produces a train of high peak current (17 kA) electron bunches with a modulation wavelength of 10’s of angstroms - orders of magnitude shorter than the plasma density modulation. The peak brightness of the nano-bunched beam can be O(1021A/m2/rad2) orders of magnitude higher than current XFEL beams. Such prebunched, high brightness electron beams hold the promise for compact and lower cost XEFLs that can produce nanometer radiation with hundreds of GW power in a 10s of centimeter long undulator.

Compact LWFA-Based Extreme Ultraviolet Free Electron Laser: Design Constraints

The combination of advanced high-power laser technology, new acceleration methods and achievements in undulator development offers the opportunity to build compact, high-brilliance free electron lasers driven by a laser wakefield accelerator. Here, we present a simulation study outlining the main requirements for the laser–plasma-based extreme ultraviolet free electron laser setup with the aim to reach saturation of the photon pulse energy in a single unit of a commercially available undulator with the deflection parameter K0 in the range of 1–1.5. A dedicated electron beam transport strategy that allows control of the electron beam slice parameters, including collective effects, required by the self-amplified spontaneous emission regime is proposed. Finally, a set of coherent photon radiation parameters achievable in the undulator section utilizing the best experimentally demonstrated electron beam parameters are analyzed. As a result, we demonstrate that the ultra-short, few-fs-level pulse of the photon radiation with the wavelength in the extreme ultraviolet range can be obtained with the peak brilliance of ∼7×1028 photons/pulse/mm2/mrad2/0.1%bw.

On the possibilities of amplification and radiation of harmonics in modern X-ray free electron lasers

X-ray free electron lasers (FELs) produce ultra short bursts of coherent radiation at the wavelengths ∼1–100 Å. In the absence of ready seed in this range, the fundamental tone is usually radiated in self-amplified spontaneous emission (SASE) FELs. During the last decade, the quality of modern FELs has improved in all aspects, and this offers possibility to use the harmonic radiation more effectively. With the goal to achieve maximum performance in minimum size and cost of a FEL, we study the harmonic multiplication in resonating undulator cascades as viable alternative to the SASE fundamental radiation. The harmonic power evolution along FELs is analyzed for the SASE and the harmonic self-seed. The analytical results are compared with available data for soft X-rays FLASH2. The possibility of harmonic radiation is studied for FELs with variable deflection parameter k: FLASH2, European FEL, SACLA, SwissFEL and currently built LCLS-II. We demonstrate earlier growth of the harmonic power in a buncher as compared with the SASE regime at the same wavelength. The difference is explained theoretically by the bunching. We find it impossible to use harmonics at the SACLA due to high initial energy spread. Excellent characteristics of the SwissFEL allow amplification of the third FEL harmonic. We propose the harmonic self-seed for it, which would allow 10 m shorter undulator length for saturation at the same power, and up to 30% shorter radiation wavelength with the existing electron beam. The amplification of the third harmonic is possible also at the European FEL; theoretical calculations show for it ∼10 m shorter saturation length, while the saturated power does not change. At the LCLS-II, good bunching at the third harmonic wavelength can be achieved in a dedicated buncher with minimal gap of the undulators. However, further amplification and radiation of the third harmonic in the undulators with small parameter k appear less efficient than its direct amplification in the FEL, where the bunching of the fundamental is disrupted between the undulator cascades.

Об излучении гармоник в рентгеновских лазерах на свободных электронах с изменяемым параметром дипольности ондуляторов

X-ray free electron lasers (FELs) produce ultra short bursts of coherent radiation at the wavelengths ~1–100 . In the absence of ready seed in this range, the fundamental tone is usually radiated in self-amplified spontaneous emission (SASE) FELs. With the goal to achieve maximum performance in minimum size and cost of a FEL, we study the harmonic multiplication in resonating undulator cascades as viable alternative to the SASE fundamental radiation. The harmonic power evolution along FELs is analyzed for the SASE and the harmonic self-seed. The analytical results are compared with available data for soft X-rays FLASH2. The possibility of harmonic radiation is studied for FELs with variable deflection parameter k: FLASH2, SwissFEL and currently built LCLS-II. We demonstrate earlier growth of the harmonic power in a buncher as compared with the SASE regime at the same wavelength. The difference is explained theoretically by the harmonic bunching growth in the buncher, as compared with that in the SASE FEL at the same wavelength. Excellent characteristics of the SwissFEL allow amplification of its third harmonic. We propose the harmonic self-seed, which would allow 10 m shorter undulator length for saturation at the same power, and up to 30% shorter radiation wavelength with the existing electron beam. At the LCLS-II, good bunching at the third harmonic wavelength in a dedicated buncher allows early and strong third harmonic growth. However, further resonating amplifier requires small value of k in LCLS-II undulators, which makes the amplification slow. We propose using the full amplification of the third FEL harmonic in LCLS-II undulators with bunching disruption between them at the wavelength of the fundamental. We show that this scheme allows >1 GW FEL power at λ=0.25 nm wavelength achievable already at ~40 m of the undulators with the electron beam energy E=4 GeV.

The regime of multi-stage trapping in free-electron lasers operating in the super-radiant and SASE regimes

The use of electron–wave interaction systems consisting of several tapered sections is considered as a method of efficiency enhancement of free-electron lasers (FELs) operating in different regimes of emission of a short wave pulse from a short electron bunch. These regimes are the principally multi-frequency self-amplified spontaneous emission regime traditionally used in short-wavelength FELs and the regime of electron–wave group synchronism, in which super-radiation of a quasi-monochromatic wave packet propagating together with the electron bunch occurs. In both the cases, the use of multi-stage trapping of electrons in the bunch by the radiated wave provides a significant (at least by an order of magnitude) increase in efficiency as compared to the saturated-stage efficiency in regular systems.

Coherence and pulse duration characterization of the PAL-XFEL in the hard X-ray regime

We characterize the spatial and temporal coherence properties of hard X-ray pulses from the Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL, Pohang, Korea). The measurement of the single-shot speckle contrast, together with the introduction of corrections considering experimental conditions, allows obtaining an intrinsic degree of transverse coherence of 0.85 ± 0.06. In the Self-Amplified Spontaneous Emission regime, the analysis of the intensity distribution of X-ray pulses also provides an estimate for the number of longitudinal modes. For monochromatic and pink (i.e. natural bandwidth provided by the first harmonic of the undulator) beams, we observe that the number of temporal modes is 6.0 ± 0.4 and 90.0 ± 7.2, respectively. Assuming a coherence time of 2.06 fs and 0.14 fs for the monochromatic and pink beam respectively, we estimate an average X-ray pulse duration of 12.6 ± 1.0 fs.

The quantum free electron laser: A new source of coherent, short‐wavelength radiation

AbstractA Free Electron Laser (FEL) operating in the quantum regime can provide a compact and monochromatic X‐ray source. Here we review the basic principles of a high‐gain quantum FEL starting from noise, with special emphasis on the self‐amplified spontaneous emission (SASE) mode of operation. In the first part of the paper, a condition for the neglect of the fermionic character of the electrons is derived and the full quantum theory of the N‐particle and single‐radiation‐mode FEL Hamiltonian is presented. Quantum effects such as cooperative gain, discrete spectrum and line narrowing are described, both in the multi‐particle and in the second quantization formalism. In the second part, propagation effects (i.e. slippage) are described and the main features of the quantum SASE regime are discussed. The broad and spiky radiation spectrum observed in classical SASE reduces in the quantum regime to a series of narrow lines, associated with sequential transitions between adjacent momentum states. A simple interpretation of the discrete nature of the spectrum and of the linewidth of the single spike observed in the quantum regime is presented.

Transient self-amplified Cerenkov radiation with a short pulse electron beam

An analytic and numerical examination of the slow wave Cerenkov free electron maser is presented. We consider the steady state amplifier configuration as well as operation in the selfamplified spontaneous emission (SASE) regime. The linear theory is extended to include electron beams that have a parabolic radial density inhomogeneity. Closed form solutions for the dispersion relation and modal structure of the electromagnetic field are determined in this inhomogeneous case. To determine the steady state response, a macro-particle approach is used to develop a set of coupled nonlinear ordinary differential equations for the amplitude and phase of the electromagnetic wave, which are solved in conjunction with the particle dynamical equations to determine the response when the system is driven as an amplifier with a time harmonic source. We then consider the case in which a fast rise time electron beam is injected into a dielectric loaded waveguide. In this case, radiation is generated by SASE, with the instability seeded by the leading edge of the electron beam. A pulse of radiation is produced, slipping behind the leading edge of the beam due to the disparity between the group velocity of the radiation and the beam velocity. Short pulses of microwave radiationmore » are generated in the SASE regime and are investigated using particle-in-cell (PIC) simulations. The nonlinear dynamics are significantly more complicated in the transient SASE regime when compared with the steady state amplifier model due to the slippage of the radiation with respect to the beam. As strong self-bunching of the electron beam develops due to SASE, short pulses of superradiant emission develop with peak powers significantly larger than the predicted saturated power based on the steady state amplifier model. As these superradiant pulses grow, their pulse length decreases and forms a series of soliton-like pulses. Comparisons between the linear theory, macro-particle model, and PIC simulations are made in the appropriate regimes.« less

The quantum free-electron laser

A Free-Electron Laser (FEL) operating in the quantum regime can provide a compact and monochromatic X-ray source. Here we review the basic principles of a high-gain quantum FEL starting from noise, with special emphasis on the self-amplified spontaneous emission (SASE) mode operation. In the first part, the full quantum theory of the N-particle and single-radiation-mode FEL Hamiltonian is presented. Quantum effects such as cooperative gain, discrete spectrum and line narrowing are described, both in the multi-particle and in the second quantization formalism. In the second part, propagation effects (i.e. slippage) are described and the main features of the quantum SASE regime are discussed. The broad and spiky radiation spectrum observed in the classical SASE reduces in the quantum regime to a series of narrow lines, associated to sequential transitions between adjacent momentum states. A simple interpretation of the discrete nature of the spectrum and of the line width of the single spike observed in the quantum regime is presented.

Quantum self-amplified spontaneous emission regime of the x-ray Compton laser

In this work we consider in general the high-gain SASE x-ray Compton FEL in the quantum regime of amplification arising from the self-consistent set of the Maxwell and relativistic quantum kinetic equations. The analytical solutions are obtained which are valid before the saturation. The quantum seed-shot noise and subsequent SASE power are calculated. A considerable increase in start-up and narrowing of the spectrum of the SASE power for amplifying x-ray frequencies is shown compared with the classical SASE regime.

Theory of high gain free-electron lasers operating with segmented undulators

The dynamical behavior of high gain free-electron lasers, operating with segmented undulators in the self-amplified spontaneous emission regime, is studied using a set of semianalytical formulas, which reproduce the induced energy spread, the bunching coefficients, and the field evolution along the longitudinal coordinate. The proposed technique is applied to the design of segmented free-electron laser devices, employing undulators with helical or linear polarization. The case of biharmonic sections is discussed too, as well as the possibility of extending the analysis to this “exotic” case.

Semi-analytical model of self-amplified spontaneous-emission free-electron lasers, including diffraction and pulse-propagation effects

We use the high-gain free-electron laser logistic equation to describe the field evolution from spontaneous emission to saturation in the self-amplified spontaneous-emission regime. The model we present is capable of including the effects of diffraction, beam qualities, and pulse propagation.

Unified model of electron beam shot noise and coherent spontaneous emission in the helical wiggler free electron laser

A numerical algorithm for simulating electron beam shot noise in free electron lasers (FELs) is presented. Shot noise is a source of spontaneous emission that may be amplified in the self-amplified spontaneous emission regime of operation. This regime is of great importance to XUV and x-ray FELs where the spontaneous emission is currently the only effective source available for amplification. The algorithm uses a quasiuniform phase-space distribution of appropriately charge weighted macroparticles. The statistical properties of the macroparticles are derived directly from the temporal Poisson statistical properties of the real electron distribution. Unlike previous algorithms, ours does not rely upon any averaging over a resonant radiation period time scale and so more correctly describes the underlying physics. The algorithm also allows shot noise to be modeled self-consistently in unaveraged FEL models which are able to describe subwavelength phenomena such as coherent spontaneous emission (CSE). The algorithm is used in the unaveraged 1D FEL numerical simulation code FemFel and demonstrates spontaneous emission due to shot noise and CSE in both rectangular and Gaussian electron pulse current profiles. The preliminary results show good qualitative and quantitative agreement with theory.