[Show abstract][Hide abstract] ABSTRACT: The phenomenon of Dicke's subradiance, in which the collective properties of a system suppress radiation, has received broad interest in atomic physics. Recent theoretical papers in the field of relativistic electron beams have proposed schemes to achieve subradiance through suppression of shot noise current fluctuations. The resulting "quiet" beam generates less spontaneous radiation than emitted even by a shot noise beam when oscillating in an undulator. Quiet beams could have diverse accelerator applications, including lowering power requirements for seeded free-electron lasers and improving efficiency of hadron cooling. In this paper we present experimental observation of a strong reduction in undulator radiation, demonstrating the feasibility of noise suppression as a practical tool in accelerator physics.
Full-text · Article · May 2015 · Physical Review Special Topics - Accelerators and Beams
[Show abstract][Hide abstract] ABSTRACT: To measure radiation dose in a water-equivalent medium from very high-energy electron (VHEE) beams and make comparisons to Monte Carlo (MC) simulation results.
Dose in a polystyrene phantom delivered by an experimental VHEE beam line was measured with Gafchromic films for three 50 MeV and two 70 MeV Gaussian beams of 4.0-6.9 mm FWHM and compared to corresponding MC-simulated dose distributions. MC dose in the polystyrene phantom was calculated with the EGSnrc/BEAMnrc and DOSXYZnrc codes based on the experimental setup. Additionally, the effect of 2% beam energy measurement uncertainty and possible non-zero beam angular spread on MC dose distributions was evaluated.
MC simulated percentage depth dose (PDD) curves agreed with measurements within 4% for all beam sizes at both 50 and 70 MeV VHEE beams. Central axis PDD at 8 cm depth ranged from 14% to 19% for the 5.4-6.9 mm 50 MeV beams and it ranged from 14% to 18% for the 4.0-4.5 mm 70 MeV beams. MC simulated relative beam profiles of regularly shaped Gaussian beams evaluated at depths of 0.64 to 7.46 cm agreed with measurements to within 5%. A 2% beam energy uncertainty and 0.286° beam angular spread corresponded to a maximum 3.0% and 3.8% difference in depth dose curves of the 50 and 70 MeV electron beams, respectively. Absolute dose differences between MC simulations and film measurements of regularly shaped Gaussian beams were between 10% and 42%.
The authors demonstrate that relative dose distributions for VHEE beams of 50-70 MeV can be measured with Gafchromic films and modeled with Monte Carlo simulations to an accuracy of 5%. The reported absolute dose differences likely caused by imperfect beam steering and subsequent charge loss revealed the importance of accurate VHEE beam control and diagnostics.
[Show abstract][Hide abstract] ABSTRACT: Short period, high field undulators have been shown to permit operation of x-ray free-electron lasers with short gain-lengths, and at unprecedented short wavelengths. In addition, the reduced beam energy required to reach resonance with a given radiation wavelength in short period undulators could prove useful in advanced inverse free-electron laser (IFEL) seeding schemes for future light sources, such as high-gain harmonic generation and echo-enabled harmonic generation, or in IFEL acceleration. We report here the in situ beam testing of a 9 mm period length cryogenic undulator having undulator parameter near unity in the context appropriate for advanced seeding and acceleration schemes, a linear accelerator. Because of the short period length of the undulator, a 47 MeV high-brightness electron beam could be used to produce near infrared photons via the undulator radiation mechanism. The undulator radiation was observed through a filter and the spectral response of the undulator is compared to simulation. Finally, an 800 nm seed laser was introduced in order to generate an energy modulation via the IFEL mechanism. Resonance between the electron beam and the laser seed was achieved without detailed knowledge of the temperature dependent undulator magnetic field through the observation of the undulator radiation. The energy modulation (and concomitant energy spread increase) of the electron beam was observed both directly in an electron beam spectrometer and indirectly via coherent transition radiation after conversion to a density modulation in a longitudinally dispersive chicane.
No preview · Article · Dec 2014 · Journal of Physics B Atomic Molecular and Optical Physics
[Show abstract][Hide abstract] ABSTRACT: It has long been known that lasers can interact with relativistic electrons in magnetic undulators to imprint sinusoidal modulations that can be used to slice electrons into microbunches equally separated at the laser wavelength. Here we report on the first direct measurement of laser-induced microbunching of a relativistic electron beam with femtosecond resolution in the time domain. Using a modified zero-phasing technique to map the electron beam's temporal structures into the energy space, we show that this method can be used to directly quantify the time and spectral content of coherent current modulations imprinted on the beam for harmonic and multicolor lasing applications in accelerator-based light sources.
No preview · Article · Oct 2014 · Physical Review Letters
[Show abstract][Hide abstract] ABSTRACT: We study the effect of longitudinal space charge on the correlated energy spread of a relativistic high-brightness electron beam that has been density modulated for the emission of coherent, high-harmonic radiation. We show that, in the case of electron bunching induced by a laser modulator followed by a dispersive chicane, longitudinal space charge forces can act to strongly reduce the induced energy modulation of the beam without a significant reduction in the harmonic bunching content. This effect may be optimized to enhance the output power and overall performance of free-electron lasers that produce coherent light through high-gain harmonic generation. It also increases the harmonic number achievable in these devices, which are otherwise gain-limited by the induced energy modulation from the laser.
[Show abstract][Hide abstract] ABSTRACT: We describe the experimental generation and measurement of coherent light that carries orbital angular momentum from a relativistic electron beam radiating at the second harmonic of a helical undulator. The measured helical phase of the light is shown to be in agreement with predictions of the sign and magnitude of the phase singularity and is more than 2 orders of magnitude greater than the incoherent signal. Our setup demonstrates that such optical vortices can be produced in modern free-electron lasers in a simple afterburner arrangement for novel two-mode pump-probe experiments.
[Show abstract][Hide abstract] ABSTRACT: X-ray free-electron lasers are enabling access to new science by producing ultrafast and intense x rays that give researchers unparalleled power and precision in examining the fundamental nature of matter. In the quest for fully coherent x rays, the echo-enabled harmonic generation technique is one of the most promising methods. In this technique, coherent radiation at the high harmonic frequencies of two seed lasers is generated from the recoherence of electron beam phase space memory. Here we report on the generation of highly coherent and stable vacuum ultraviolet radiation at the 15th harmonic of an infrared seed laser with this technique. The experiment demonstrates two distinct advantages that are intrinsic to the highly nonlinear phase space gymnastics of echo-enabled harmonic generation in a new regime, i.e., high frequency up-conversion efficiency and insensitivity to electron beam phase space imperfections. Our results allow comparison and confirmation of predictive models and scaling laws, and mark a significant step towards fully coherent x-ray free-electron lasers that will open new scientific research.
No preview · Article · Jul 2014 · Physical Review Special Topics - Accelerators and Beams
[Show abstract][Hide abstract] ABSTRACT: Static magnetic undulators used by x-ray light sources are fundamentally too limited to achieve shorter undulator periods and dynamic control. To overcome these limitations, we report experimental demonstration of a novel short-period microwave undulator, essentially a Thomson scattering device, that has yielded tunable spontaneous emission and seeded coherent radiation. Its equivalent undulator period (λu) is 13.9 mm while it has achieved an equivalent magnetic field of 0.65 T. For future-generation light sources, this device promises a shorter undulator period, a large aperture, and fast dynamic control.
[Show abstract][Hide abstract] ABSTRACT: Accelerator-based light sources such as storage rings and free-electron
lasers use relativistic electron beams to produce intense radiation over a wide
spectral range for fundamental research in physics, chemistry, materials
science, biology and medicine. More than a dozen such sources operate
worldwide, and new sources are being built to deliver radiation that meets with
the ever increasing sophistication and depth of new research. Even so,
conventional accelerator techniques often cannot keep pace with new demands
and, thus, new approaches continue to emerge. In this article, we review a
variety of recently developed and promising techniques that rely on lasers to
manipulate and rearrange the electron distribution in order to tailor the
properties of the radiation. Basic theories of electron-laser interactions,
techniques to create micro- and nano-structures in electron beams, and
techniques to produce radiation with customizable waveforms are reviewed. We
overview laser-based techniques for the generation of fully coherent x-rays,
mode-locked x-ray pulse trains, light with orbital angular momentum, and
attosecond or even zeptosecond long coherent pulses in free-electron lasers.
Several methods to generate femtosecond pulses in storage rings are also
discussed. Additionally, we describe various schemes designed to enhance the
performance of light sources through precision beam preparation including beam
conditioning, laser heating, emittance exchange, and various laser-based
diagnostics. Together these techniques represent a new emerging concept of
"beam by design" in modern accelerators, which is the primary focus of this
Full-text · Article · Apr 2014 · Review of Modern Physics
[Show abstract][Hide abstract] ABSTRACT: We describe the experimental observation of highly nonlinear energy striations generated by two lasers in a relativistic electron beam in an echo-enabled harmonic generation (EEHG) setup. The rich energy banding structure results from strong dispersion of the sinusoidally modulated beam, and measurements of the banding spectrum enable benchmarking, optimization, and characterization of the concomitant EEHG process. Results are found to be in good agreement with theory, and suggest that the presented technique can facilitate the practical implementation of EEHG to generate intense, fully coherent light in future advanced accelerator-based light sources.
No preview · Article · Dec 2013 · Physical Review Special Topics - Accelerators and Beams
[Show abstract][Hide abstract] ABSTRACT: We report on experimental studies on the harmonic interaction between an optical laser and a relativistic electron beam in an undulator up to the 15th order. In this experiment, a significant energy modulation is imprinted on the beam longitudinal phase space through the electron-laser interaction when the laser frequency is the 3rd, 5th, 7th, 9th or 15th harmonic of the fundamental resonant frequency of the undulator. The experimental results are in good agreement with theory, and indicate that high harmonic interactions in undulators with large K values and small phase errors can be quite efficient. The results confirm the basic physics of harmonic interaction with a goal toward ushering forward the development of many high harmonic based applications in free-electron lasers.
No preview · Article · Nov 2013 · Physical Review Special Topics - Accelerators and Beams
[Show abstract][Hide abstract] ABSTRACT: Recent advances in the production and control of high-brightness
electron beams (e-beams) have enabled a new class of intense light
sources based on the free electron laser (FEL) that can examine matter
atångstrom length and femtosecond time scales. The free, or
unbound, electrons act as the lasing medium, which provides unique
opportunities to exquisitely control the spatial and temporal structure
of the emitted light through precision manipulation of the electron
distribution. We present an experimental demonstration of light with
orbital angular momentum (OAM; ref. ) generated from a relativistic
e-beam rearranged into an optical scale helix by a laser. With this
technique, we show that a Gaussian laser mode can be effectively
up-converted to an OAM mode in an FEL using only the e-beam as a
mode-converter. Results confirm theoretical predictions, and pave the
way for the production of coherent OAM light with unprecedented
brightness down to hard X-ray wavelengths for wide ranging applications
in modern light sources.
[Show abstract][Hide abstract] ABSTRACT: The longitudinal space-charge amplifier has been recently proposed by Schneidmiller and Yurkov as an alternative to the free-electron laser instability for the generation of intense broadband radiation pulses [Phys. Rev. ST Accel. Beams 13, 110701 (2010)]. In this Letter, we report on the experimental demonstration of a cascaded longitudinal space-charge amplifier at optical wavelengths. Although seeded by electron beam shot noise, the strong compression of the electron beam along the three amplification stages leads to emission of coherent undulator radiation pulses exhibiting a single spectral spike and a single transverse mode. The on-axis gain is estimated to exceed 4 orders of magnitude with respect to spontaneous emission.
Full-text · Article · Jun 2013 · Physical Review Letters
[Show abstract][Hide abstract] ABSTRACT: The longitudinal space-charge amplifier has been recently proposed by Schneidmiller and Yurkov as an alternative to the free-electron laser instability for the generation of intense broadband radiation pulses [Phys. Rev. ST Accel. Beams 13, 110701 (2010)]. In this Letter, we report on the experimental demonstration of a cascaded longitudinal space-charge amplifier at optical wavelengths. Although seeded by electron beam shot noise, the strong compression of the electron beam along the three amplification stages leads to emission of coherent undulator radiation pulses exhibiting a single spectral spike and a single transverse mode. The on-axis gain is estimated to exceed 4 orders of magnitude with respect to spontaneous emission. The successful lasing of the Linac Coherent Light Source  and the Spring-8 Angstrom Compact free-electron LAser  has established the high-gain free-electron laser (FEL) as the brightest source of mono-chromatic, hard x-rays, allowing the exploration of nature with unprecedented temporal and spatial resolution [3,4]. While the narrow bandwidth of FELs is a desirable feature in applications such as imaging and spectroscopy, it ulti-mately limits the ability of the FEL to generate few-cycle pulses for ultrafast experiments. As an alternative, the longitudinal space-charge amplifier (LSCA) has recently been proposed as a powerful broadband coherent radia-tion source . In a LSCA, a relativistic electron beam (e beam) becomes modulated in density (i.e., microbun-ched) by the interaction with its own collective space-charge forces, combined with longitudinal dispersion in transport. This microbunching instability process was first identified as a detrimental effect in the context of FEL injectors [6–12]. However, as pointed out in , it can be optimized and cascaded through several amplification stages to yield strong microbunching for the emission of intense broadband coherent light. Because of its unique spectral properties, the LSCA is a natural candidate for the generation of intense attosecond radiation pulses . Furthermore, the LSCA presents several advantages in terms of compactness and robustness to nonideal beam conditions. In this Letter, we report on the experimental demons-tration of the LSCA as a new type of broadband, fully coherent radiation source at the Next Linear Collider Test Accelerator (NLCTA) of the SLAC National Accelerator Laboratory. Our experimental setup is shown in Fig. 1, and exploits the existing three-chicane echo-enabled harmonic generation seeding beam line [14,15] as a cascaded three-stage LSCA seeded by shot noise. Through the proper tuning of the bunch compression, we demonstrate the generation of an intense, single mode pulse with an inten-sity gain of 4 orders of magnitude over the spontaneous emission level. The physical mechanism of the space-charge instability can be modeled as a two-step process. An e beam with an initial density perturbation at the longitudinal spatial frequency k travels through a transport channel (drift) of length L d . During transport, the modulated longitudinal space-charge fields induce a corresponding energy modula-tion in the e beam. Afterwards, the electrons travel through a longitudinally dispersive transport element (e.g., a magnetic bending chicane) which transforms the energy modulation back into a density modulation, but with an amplitude larger than the initial value. This process can start from shot noise or from a coherent microbunching induced by interaction with an external laser, and can be repeated in several ampli-fication stages to enhance the density modulation amplitude. The microbunching instability has been investigated in detail elsewhere [9–12,16,17]. To provide a dynamical description of the cascaded LSCA setup explored experi-mentally here, it is useful to follow the matrix formalism of Gover et al.  for a simple one-dimensional (1D), cold beam model. The beam density modulation can be quanti-fied by the beam bunching factor, given by b ¼ P n expðÀikz n Þ=N, where z n is the longitudinal position of
[Show abstract][Hide abstract] ABSTRACT: We report on a proof-of-principle demonstration of a two-stage cascaded
optical inverse free-electron laser (IFEL) accelerator in which an
electron beam is accelerated by a strong laser pulse after being packed
into optical microbunches by a weaker initial laser pulse. We show
experimentally that injection of precisely prepared optical microbunches
into an IFEL allows net acceleration or deceleration of the beam,
depending on the relative phase of the two laser pulses. The
experimental results are in excellent agreement with simulation. The
demonstrated technique holds great promise to significantly improve the
beam quality of IFELs and may have a strong impact on emerging laser
accelerators driven by high-power optical lasers.
No preview · Article · Jun 2013 · Physical Review Letters
[Show abstract][Hide abstract] ABSTRACT: In this Letter we discuss a novel method for generating ultrashort radiation pulses using a broadband two-stream instability in an intense relativistic electron beam. This method relies on an electron beam having two distinct two-energy bands. The use of this new high brightness electron beam scenario, in combination with ultrashort soft x-ray pulses from high harmonic generation in gas, allows the production of high power attosecond pulses for ultrafast pump and probe experiments. DOI: 10.1103/PhysRevLett.110.064804
Full-text · Article · Mar 2013 · Physical Review Letters
[Show abstract][Hide abstract] ABSTRACT: With the advent of coherent x rays provided by the x-ray free-electron laser (FEL), strong interest has been kindled in sophisticated diffraction imaging techniques. In this Letter, we exploit such techniques for the diagnosis of the density distribution of the intense electron beams typically utilized in an x-ray FEL itself. We have implemented this method by analyzing the far-field coherent transition radiation emitted by an inverse-FEL microbunched electron beam. This analysis utilizes an oversampling phase retrieval method on the transition radiation angular spectrum to reconstruct the transverse spatial distribution of the electron beam. This application of diffraction imaging represents a significant advance in electron beam physics, having critical applications to the diagnosis of high-brightness beams, as well as the collective microbunching instabilities afflicting these systems.
No preview · Article · Mar 2013 · Physical Review Letters
[Show abstract][Hide abstract] ABSTRACT: In this Letter we discuss a novel method for generating ultrashort radiation pulses using a broadband two-stream instability in an intense relativistic electron beam. This method relies on an electron beam having two distinct two-energy bands. The use of this new high brightness electron beam scenario, in combination with ultrashort soft x-ray pulses from high harmonic generation in gas, allows the production of high power attosecond pulses for ultrafast pump and probe experiments.
Full-text · Article · Feb 2013 · Physical Review Letters