[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
[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.
[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.
Physical Review Special Topics - Accelerators and Beams 11/2013; 16(11). · 1.57 Impact Factor
[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.
[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.
[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. The successful operation of the Linac Coherent Light Source  and other free-electron laser (FEL) facilities around the world [2,3] has established the FEL as by far the most brilliant current source of coherent x rays. In a high-gain FEL , a high-brightness electron beam travels in an undulator magnet and amplifies to saturation a cop-ropagating resonant radiation pulse. The main features of FEL light sources are the very high power (up to several tens of gigawatts ), transverse coherence , narrow bandwidth, and tunability over a continuous range of wave-lengths (see, e.g., Ref. ). The generation of coherent hard x rays enables new methods, such as diffraction imaging, that may examine atomic and molecular systems at their characteristic length scale (angstrom). Further, FEL pulse lengths are now obtained at the femtosecond level, thus resolving much of the dynamics of such systems. While this is an impressive achievement, there is demand for generating yet shorter x-ray pulses for pump-probe experiments. In this case the narrow bandwidth of an XFEL, a highly desirable feature for many applications, limits the capability of the FEL to achieve amplification in ultrashort pulses. Thus, in this Letter we discuss an alternative amplifica-tion scheme based on a relativistic two-stream instability driven by longitudinal space-charge forces. As we shall see, the two stream instability is a broadband exponential amplification process and may represent an important alternative to the FEL amplifier in cases in which broad-band operation is needed. Indeed, it may allow the genera-tion and amplification of few cycle pulses at x-ray wavelengths. The two-stream instability is a well-known physical effect in the context of fusion plasmas, space plasmas, and high-energy accelerators. The instability is driven by the longitudinal Coulomb field generated by a plasma with two distinct peaks in the longitudinal velocity distribution (see, e.g., Ref. ). This type of velocity distribution can present itself in a wide variety of forms, such as, for example, a particle beam being injected in a fusion plasma  or an ion beam propagating in the presence of a background plasma that is employed for transverse focus-ing and stabilization . In the current case of interest, we study the the two-stream instability in a relativistic electron beam which has an energy distribution with two narrow peaks, repre-senting a beam with two distinct energy strata or bands. This type of scenario was examined in a different context having much different physical goals by Bekefi and Jacobs , to explore use of the two-stream instability to enhance the gain and efficiency of low-energy, mm-wave and sub-mm-wave FELs. In contrast, in this Letter we discuss the exploitation of the broadband nature of the two-stream instability in a modern FEL in the vacuum ultraviolet (VUV) and soft x-ray regions, in order to allow the generation and amplification of attosecond pulses. Figure 1 shows a schematic layout of the proposed amplification scheme. A premodulated (meaning weakly microbunched by inverse FEL-derived seeding or shot noise) electron beam with two distinct energy bands prop-agates in a focusing channel. This bifurcated energy dis-tribution may be created in many beam pulse compression processes (e.g., Ref. ), as is discussed further below. The two-stream instability then serves to amplify the initial density modulation. After saturation of the instability, the beam is sent to a broadband radiator such as a short undulator or a transition radiation screen and the strong microbunching obtained induces the emission of coherent
[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.
[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.
[Show abstract][Hide abstract] ABSTRACT: A sequential arrangement of three pairs of modulators and dispersive sections that performs precise manipulation of a relativistic electron beam’s longitudinal phase space is described. We show that using only a single laser wavelength, this scheme acts as a waveform synthesizer through linearization of local regions of phase space to generate sawtooth, triangular, and square wave-type distributions. It also acts as an optical analog of an rf function generator to generate intense coherent radiation that has periodic triangle and square field profiles at the optical wavelength. The same setup can also be used to improve the high-harmonic bunching factors in echo-enabled harmonic generation schemes up to 25% and to produce a bunching factor above 90% at the laser fundamental wavelength for high-efficiency capture in inverse free electron laser acceleration applications.
Review of Modern Physics 02/2013; 16(1). · 44.98 Impact Factor
[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. X-ray free-electron lasers (XFELs) [1,2] are a unique tool for the investigation of ultra-small and ultra-fast systems, permitting unprecedented studies of atomic-molecular structure at the angstrom length and femtosec-ond time scale. The XFEL is an example of a new class of intense, coherent electromagnetic sources, which can be fully exploited in measurements by the introduction of innovative, diffraction imaging-based techniques [3,4]. Diffraction imaging requires the use of sophisticated phase-retrieval methods that indeed permit detailed inves-tigations of spatial structures down to the x-ray diffraction limit. This new approach to imaging, stimulated by the burgeoning availability of coherent sources, is rapidly diffusing into a wide range of different applications. In this vein, we extend diffraction imaging techniques to a new frontier application in the physics of intense electron beams and provide a first demonstration of the newly proposed method. An XFEL is a complex system that may be described as a controlled beam-radiation instability. The successful op-eration of an XFEL requires use of a low-emittance, high peak current electron beam. The generation, compression, and transport of such high-brightness relativistic electron beams poses many challenges, due particularly to parasitic beam instabilities that amplify the beam's shot-noise-derived microbunching during beam compression. This type of collective effect may be broadly identified as the microbunching instability (MBI) [5–9]. The MBI may generate strong perturbations in the beam's longitudinal phase space which serve to reduce the efficiency of the downstream FEL [6,10]. Most importantly, MBI may also induce the emission of coherent optical transition radiation (coherent OTR, or COTR) in beam diagnostics [11–14], severely compromising the utility of optical transition radiation-based measurements. While the effect of the microbunching instability on the FEL performance per se can be mitigated using a laser heater , this approach does not effectively suppress COTR emission in diagnos-tics . This situation renders conventional OTR-based diagnostics ineffective for compressed high-brightness electron beams. In this Letter, utilizing methods originally employed in coherent x-ray imaging, we propose and experimentally test a method that exploits the coherent radiation rather than attempting to avoid or eliminate coherence effects in beam diagnostics. This approach, which uses the micro-bunching present in an electron beam to give a single-shot, far-field COTR image, yields a robust path for the recon-struction of the transverse spatial structure of the beam microbunching. We report on the experimental demonstra-tion of this technique at the Next Linear Collider Test Accelerator (NLCTA), located at the SLAC National Accelerator Laboratory. The coherent imaging technique proposed provides a general method for the reconstruction of the beam micro-bunching profile from the far-field COTR image. This technique has a number of important applications that depend on how the microbunching arises in the electron beam. For example, it can be applied as an advanced diagnostic for the FEL interaction, in which the entire electron beam transverse profile contributes to the for-mation of microbunching. It can also be applied to yet more complex cases, as typified by the space-charge induced optical microbunching, in which the beam den-sity modulation may be transversely incoherent [8,9], or to novel types of microbunching with more complex topological dependencies. An example of the latter case is found in the helical microbunching structure used to
[Show abstract][Hide abstract] ABSTRACT: A technique to generate high-brightness electromagnetic vortices with tunable topological charge at extreme ultraviolet and x-ray wavelengths is described. Based on a modified version of echo-enabled harmonic generation for free-electron lasers, the technique uses two lasers and two chicanes to produce high-harmonic microbunching of a relativistic electron beam with a corkscrew distribution that matches the instantaneous helical phase structure of the x-ray vortex. The strongly correlated electron distribution emerges from an efficient three-dimensional recoherence effect in the echo-enabled harmonic generation transport line and can emit fully coherent vortices in a downstream radiator for access to new research in x-ray science.
[Show abstract][Hide abstract] ABSTRACT: We report generation of density modulation at terahertz (THz)
frequencies in a relativistic electron beam through laser modulation of
the beam longitudinal phase space. We show that by modulating the energy
distribution of the beam with two lasers, density modulation at the
difference frequency of the two lasers can be generated after the beam
passes through a chicane. In this experiment, density modulation around
10 THz was generated by down-converting the frequencies of an 800 nm
laser and a 1550 nm laser. The central frequency of the density
modulation can be tuned by varying the laser wavelengths, beam energy
chirp, or momentum compaction of the chicane. This technique can be
applied to accelerator-based light sources for generation of coherent
THz radiation and marks a significant advance toward tunable narrow band
[Show abstract][Hide abstract] ABSTRACT: A chicane compressor developed by UCLA for the production of ultra-short, 60 MeV electron beams at the Brookhaven National Laboratory Accelerator Test Facility has been commissioned, and initial beam physics experiments have been performed. These measurements have established the compression of electron beams to the 100 femtosecond (1 kA peak current) regime, via coherent transition radiation (CTR) based measurements. Investigations of coherent edge radiation (CER) include signatures that differentiate it from coherent synchrotron radiation (CSR), such as polarization and far-field angular distribution. Additionally, the radiation wavelength spectrum is determined from autocorrelation measurements. Radiation properties are compared to detailed start-to-end simulations derived from PARMELA and QUINDI (a Lienard-Wiechert code developed at UCLA). Plans for future experiments which further explore the observed wavelength spectra are presented.
International Journal of Modern Physics A 01/2012; 22(23). · 1.13 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Experimental observation of the microbunching of a relativistic electron beam at the second harmonic interaction frequency of a helical undulator is presented. The microbunching signal is observed from the coherent transition radiation of the electron beam and indicates experimental evidence of a dominantly helical electron beam density distribution. This result is in agreement with theoretical and numerical predictions and provides a proof-of-principle demonstration of proposed schemes designed to generate light with orbital angular momentum in high-gain free-electron lasers. V C 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3690900] Free-electron lasers (FELs) rely on highly relativistic electron beams (e-beams) to generate coherent, high-brightness light pulses. FELs have enormous flexibility in that the character of the emitted light is determined by the properties of the e-beam: the frequency is tunable via its quadratic dependence on the e-beam energy, and the trans-verse optical mode structure is governed both by the har-monic frequency content 1,2 and by the correlated spatial distribution of electrons. In Ref. 3, a scenario referred to as high-gain high-mode generation (HGHMG) was described, where the FEL is tailored to emit light that carries an effec-tive component of orbital angular momentum (OAM) (Ref. 4) using an e-beam with a helical density modulation about its propagation axis. The high power coherent FEL light has a corresponding helical transverse phase that is useful for a broad scope of research. 5–7 Experimental realization of the HGHMG OAM scheme relies on the ability to generate the required helical e-beam density distribution. The electrons must be arranged into a spiral staircase structure with the same longitudinal periodic-ity as the wavelength of the FEL light (helical microbunch-ing). A simple method was proposed in Ref. 8, where the e-beam is naturally helically modulated by a laser tuned to one of the harmonics, h, of the resonant interaction in a heli-cal undulator (inverse FEL, or IFEL). The helical e-beam ge-ometry emerges because the energy kick given to an electron depends on the field gradient of the transversely Gaussian laser profile, which varies azimuthally. The excited azi-muthal e-beam mode l depends on the harmonic number via the relation jlj ¼ h À 1. Thus, a simple Gaussian laser pulse profile interacting at the second harmonic is predicted to gen-erate the required helical microstructure as a seed for jlj ¼ 1 OAM light in a downstream FEL. Here we present experimental evidence of induced heli-cal microbunching in the helical interaction experiment (HE-LiX) at the Neptune Laboratory at UCLA. Results indicate a dominant l ¼ À1 helical distribution imposed on the e-beam density structure at the second harmonic by the transversely Gaussian seed laser. The primary components of the HELiX are illustrated in Fig. 1. A short, r z =c ¼ 3 ps rms bunch length, 12–12.5 MeV (c ¼ 24–25), initially unmodulated e-beam propagates through a k w ¼ 2p=k w ¼ 1.9 cm period helical undulator co-axially with a k ¼ 2p=k ¼ 10.6 lm wavelength, 100 ps long CO 2 laser pulse. The interaction is tuned to be near the h ¼ 2 (second harmonic) resonance k ¼ k w 2hc 2 ð1 þ K 2 Þ, where K ¼ eBk w =2pmc ¼ 0.58 is the nor-malized field strength of the undulator magnetic field B, Àe and m are the electron charge and mass, and c is the speed of light. The laser pulse reaches up to 30 MW (3 mJ) of peak power and comes to a measured w 0 ¼ 400 lm axisymmetric waist 10 cm inside the N w ¼ 12 period undulator. The e-beam had a fixed charge of eN e ¼ 24069 pC, 0.035% slice RMS energy spread, and 1.3% FWHM relative energy spread due to the correlations imposed by the s-band injector. Microbunching was diagnosed by measuring the total coherent transition radiation (CTR) emitted by the e-beam as it passed through a 10 lm thick, 1.5 cm square aluminum foil positioned flush against the 1 cm round undulator exit aper-ture to minimize the large laser background. The helical e-beam bunching factor is defined as b l ¼ N À1
[Show abstract][Hide abstract] ABSTRACT: We report first evidence of wakefield acceleration of a relativistic electron beam in a dielectric-lined slab-symmetric structure. The high energy tail of a $60 MeV electron beam was accelerated by $150 keV in a 2 cm-long, slab-symmetric SiO 2 waveguide, with the acceleration or deceleration clearly visible due to the use of a beam with a bifurcated longitudinal distribution that serves to approximate a driver-witness beam pair. This split-bunch distribution is verified by longitudinal recon-struction analysis of the emitted coherent transition radiation. The dielectric waveguide structure is further characterized by spectral analysis of the emitted coherent Cherenkov radiation at THz frequencies, from a single electron bunch, and from a relativistic bunch train with spacing selectively tuned to the second longitudinal mode (TM 02). Start-to-end simulation results reproduce aspects of the electron beam bifurcation dynamics, emitted THz radiation properties, and the observation of acceleration in the dielectric-lined, slab-symmetric waveguide.
[Show abstract][Hide abstract] ABSTRACT: We develop a three-dimensional analysis of longitudinal space-charge
waves in a relativistic electron beam. Our analysis includes the effects
of longitudinal thermal motion due to energy spread and emittance,
transverse betatron oscillations and edge effects due to the finite
transverse size of the beam. We reduce the system of coupled
Vlasov/Maxwell equations to an integro-differential eigenvalue equation
which represents the three-dimensional dispersion relation for the
plasma oscillation eigenmodes. The dispersion relation can be expressed
in terms of four dimensionless scaling parameters. We solve the
dispersion relation by means of an approximate variational method as
well as a numerical discretization method and use the universally scaled
solutions to explain the main physical features of the problem. Finally,
the initial value problem is solved by means of a bi-orthogonal mode
expansion of an arbitrary initial perturbation in six-dimensional phase
space and used to describe space-charge induced amplification and
suppression of beam microbunching. Possible experimental applications of
this analysis in the context of free-electron laser injectors are also
[Show abstract][Hide abstract] ABSTRACT: A scheme to generate intense coherent light that carries orbital angular momentum (OAM) at the fundamental wavelength of an x-ray free-electron laser (FEL) is described. The OAM light is emitted as the dominant mode of the system until saturation provided that the helical microbunching imposed on the electron beam is larger than the shot-noise bunching that leads to self-amplified emission. Operating at the fundamental, this scheme is more efficient than alternate schemes that rely on harmonic emission, and can be applied to x-ray FELs without using external optical mode conversion elements.