Showing papers in "Physical Review Special Topics-accelerators and Beams in 2011"
TL;DR: In this article, the results of Monte-Carlo simulations of electron-positron-photon cascades initiated by slow electrons in circularly polarized fields of ultrahigh strength are presented and discussed.
Abstract: The results of Monte-Carlo simulations of electron-positron-photon cascades initiated by slow electrons in circularly polarized fields of ultrahigh strength are presented and discussed. Our results confirm previous qualitative estimations [A. M. Fedotov et al., Phys. Rev. Lett. 105, 080402 (2010)] of the formation of cascades. This sort of cascade has revealed a new property of restoration of energy and dynamical quantum parameter due to acceleration of electrons and positrons by the field. This may become a dominating feature of laser-matter interactions at ultrahigh intensities. Our approach incorporates radiation friction acting on individual electrons and positrons.
289 citations
CERN1
TL;DR: In this article, the authors summarized the experimental setups for electron cloud monitoring, coating procedures, and recent measurements performed with amorphous carbon coated vacuum chambers in the CERN Super Proton Synchrotron (SPS).
Abstract: Electron cloud buildup is a major limitation for high-energy particle accelerators such as the CERN Super Proton Synchrotron (SPS). Amorphous carbon thin films with low initial secondary electron yield (SEY ffi 1:0) have been applied as a mitigation material in the SPS vacuum chambers. This paper summarizes the experimental setups for electron cloud monitoring, coating procedures, and recent measurements performed with amorphous carbon coated vacuum chambers in the SPS. The electron cloud measured by dedicated monitors is completely suppressed for LHC-type beams. Even after more than one year’s exposure in the SPS with the machine in operation, the coating does not show any increase in the secondary electron yield. The study of coated vacuum chambers for the SPS dipole magnets is in progress; the correlation between electron cloud reduction and pressure rises is not yet fully understood. Some prototypes have already been installed in the accelerator and plans for the implementation of an optimized coating technique are under development.
103 citations
TL;DR: The Metrology Light Source is a recently constructed 630 MeV electron storage ring, operating as a synchrotron radiation source for the THz to extreme UV spectral range as discussed by the authors.
Abstract: The Metrology Light Source is a recently constructed 630 MeV electron storage ring, operating as a synchrotron radiation source for the THz to extreme UV spectral range. It is the first storage ring optimized for generating intense, broadband, coherent THz radiation, based on a bunch shortening mode. Stable (``steady state'') or bursting THz radiation up to an average power of about 60 mW can be obtained. The applied machine operation mode is achieved by manipulating the momentum compaction factor $\ensuremath{\alpha}$ by a novel tuning scheme. The underlying low-$\ensuremath{\alpha}$ scheme is of general interest for operating a storage ring in a short bunch mode and is the main subject of this paper.
91 citations
TL;DR: In this paper, the potential of these materials for tolerating cyclic thermal fatigue due to rf magnetic fields was evaluated and the results showed the possibility of pushing the gradient limits due to pulsed heating fatigue by the use of copper zirconium and copper chromium alloys.
Abstract: Cyclic thermal stresses produced by rf pulsed heating can be the limiting factor on the attainable reliable gradients for room temperature linear accelerators. This is especially true for structures that have complicated features for wakefield damping. These limits could be pushed higher by using special types of copper, copper alloys, or other conducting metals in constructing partial or complete accelerator structures. Here we present an experimental study aimed at determining the potential of these materials for tolerating cyclic thermal fatigue due to rf magnetic fields. A special cavity that has no electric field on the surface was employed in these studies. The cavity shape concentrates the magnetic field on one flat surface where the test material is placed. The materials tested in this study have included oxygen free electronic grade copper, copper zirconium, copper chromium, hot isostatically pressed copper, single crystal copper, electroplated copper, Glidcop\textregistered{}, copper silver, and silver plated copper. The samples were exposed to different machining and heat treatment processes prior to rf processing. Each sample was tested to a peak pulsed heating temperature of approximately $110\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ and remained at this temperature for approximately $10\ifmmode\times\else\texttimes\fi{}{10}^{6}$ rf pulses. In general, the results showed the possibility of pushing the gradient limits due to pulsed heating fatigue by the use of copper zirconium and copper chromium alloys.
86 citations
TL;DR: In this paper, two approaches, one based upon analytical calculations and the other based upon Monte Carlo simulations, were proposed to study the Compton scattering process for various electron and laser-beam parameters as well as different gamma-beam collimation conditions.
Abstract: Compton scattering of a laser beam with a relativistic electron beam has been used to generate intense, highly polarized and nearly monoenergetic x-ray or gamma-ray beams at many facilities. The ability to predict the spatial, spectral, and temporal characteristics of a Compton gamma-ray beam is crucial for the optimization of the operation of a Compton light source as well as for the applications utilizing the Compton beam. In this paper, we present two approaches, one based upon analytical calculations and the other based upon Monte Carlo simulations, to study the Compton scattering process for various electron and laser-beam parameters as well as different gamma-beam collimation conditions. These approaches have been successfully applied to characterize Compton gamma-ray beams, after being benchmarked against experimental results at the High Intensity Gamma-ray Source ($\mathrm{HI}\ensuremath{\gamma}\mathrm{S}$) facility at Duke University.
85 citations
TL;DR: In this article, the average temporal and spatial coherence of free-electron-laser pulses in the extreme ultraviolet at FLASH is measured by interfering two time-delayed partial beams directly on a CCD camera.
Abstract: The average temporal (longitudinal) and spatial (transverse) coherence of free-electron-laser pulses in the extreme ultraviolet at FLASH is measured by interfering two time-delayed partial beams directly on a CCD camera. Wavelengths between $\ensuremath{\lambda}=32\text{ }\text{ }\mathrm{nm}$ and $\ensuremath{\lambda}=8\text{ }\text{ }\mathrm{nm}$ are examined. A decrease of the coherence time for the fundamental wavelengths from ${\ensuremath{\tau}}_{c}=(6\ifmmode\pm\else\textpm\fi{}0.5)\text{ }\text{ }\mathrm{fs}$ at 32 nm to ${\ensuremath{\tau}}_{c}=(2.9\ifmmode\pm\else\textpm\fi{}0.5)\text{ }\text{ }\mathrm{fs}$ at 8 nm is measured. At $\ensuremath{\lambda}=8\text{ }\text{ }\mathrm{nm}$ the fundamental wavelength and the third harmonic of 24 nm are compared to each other. For 8 nm radiation as third harmonic of 24 nm a coherence time of ${\ensuremath{\tau}}_{c}=(2.5\ifmmode\pm\else\textpm\fi{}0.5)\text{ }\text{ }\mathrm{fs}$ is observed. The spatial coherence of 24 and 8 nm fundamental pulses are found to be very similar. The visibility decreases to 50% of the maximum visibility at about 3.2 mm overlap of the partial beams, which corresponds to 42% of the beam diameter at a distance of 90 m from the exit of the undulator. These results are analyzed in terms of the Gaussian Schell model resulting in six contributing modes to the total radiation. In addition, the correlation of the visibility between the fundamental radiation at 24 nm and its third harmonic at $\ensuremath{\lambda}=8\text{ }\text{ }\mathrm{nm}$ is investigated for identical shots.
66 citations
TL;DR: In this article, a transverse radio-frequency deflector located after the undulator is used to measure the time-resolved electron-beam energy loss and energy spread induced by the FEL process.
Abstract: We propose a novel method to characterize the temporal duration and shape of femtosecond x-ray pulses in a free-electron laser (FEL) by measuring the time-resolved electron-beam energy loss and energy spread induced by the FEL process, with a transverse radio-frequency deflector located after the undulator Its merits are simplicity, high resolution, wide diagnostic range, and non-invasive to user operation When the system is applied to the Linac Coherent Light Source, the first hard x-ray free-electron laser in the world, it can provide single-shot measurements on the electron beam and x-ray pulses with a resolution on the order of 1-2 femtoseconds rms
66 citations
TL;DR: The success of most of the proposed energy recovery linac (ERL) based electron accelerator projects for future storage ring replacements (SRR) and high power IR-free-electron lasers (FELs) largely depends on the development of an appropriate source as discussed by the authors.
Abstract: The success of most of the proposed energy recovery linac (ERL) based electron accelerator projects for future storage ring replacements (SRR) and high power IR--free-electron lasers (FELs) largely depends on the development of an appropriate source. For example, to meet the FEL specifications [J. W. Lewellen, Proc. SPIE Int. Soc. Opt. Eng. 5534, 22 (2004)] electron beams with an unprecedented combination of high brightness, low emittance ($0.1\text{ }\text{ }\ensuremath{\mu}\mathrm{mrad}$), and high average current (hundreds of mA) are required. An elegant way to create a beam of such quality is to combine the high beam quality of a normal conducting rf photoinjector with the superconducting technology, i.e., to build a superconducting rf photoinjector (SRF gun). SRF gun R programs based on different approaches have been launched at a growing number of institutes and companies (AES, Beijing University, BESSY, BNL, DESY, FZD, TJNAF, Niowave, NPS, Wisconsin University). Substantial progress was achieved in recent years and the first long term operation was demonstrated at FZD [R. Xiang et al., in Proceedings of the 31st International Free Electron Laser Conference (FEL 09), Liverpool, UK (STFC Daresbury Laboratory, Warrington, 2009), p. 488]. In the near future SRF guns are expected to play an important role for linac-driven FEL facilities. In this paper we will review the concepts, the design parameters, and the status of the major SRF gun projects.
64 citations
TL;DR: In this paper, a self-amplified spontaneous emission has been observed driving the amplifier with the high-brightness beam of the SPARC linac, which is a single pass free-electron laser designed to obtain high gain at a radiation wavelength of 500 nm.
Abstract: SPARC (acronym of ‘‘Sorgente Pulsata ed Amplificata di Radiazione Coerente’’, i.e. Pulsed and
Amplified Source of Coherent Radiation) is a single pass free-electron laser designed to obtain high gain
amplification at a radiation wavelength of 500 nm. Self-amplified spontaneous emission has been
observed driving the amplifier with the high-brightness beam of the SPARC linac. We report measurements
of energy, spectra, and exponential gain. Experimental results are compared with simulations from
several numerical codes.
61 citations
TL;DR: In this article, the authors discuss the use of multiobjective optimization for dynamic aperture (DA) in a storage ring and show that using objective functions based only on numerical tracking results can result in faster convergence.
Abstract: Dynamic aperture (DA) is one of the key nonlinear properties for a storage ring. Although there have been both analytical and numerical methods to find the aperture, the reverse problem of how to optimize it is still a challenging problem. A general and flexible way of optimizing the DA is highly demanded in accelerator design and operation. In this paper, we discuss the use of multiobjective optimization for DA. First we consider using objective functions based only on numerical tracking results. Data mining of these results demonstrated a correlation between DA and low-order nonlinear driving terms. Next we considered using objective functions which included both numerical tracking results and analytical estimates of low-order nonlinear driving terms. This resulted in faster convergence. The National Synchrotron Light Source II (NSLS-II) lattice was taken as an example to illustrate this method. This multiobjective approach is not limited by particular linear or nonlinear lattice settings, and can also be applied for optimizing other properties of a storage ring.
59 citations
TL;DR: In this article, a pulse power solenoid was used to capture laser accelerated proton beams and the ability to control their large divergence angles and broad energy range was demonstrated using a scaled target normal sheath acceleration spectrum.
Abstract: Using a pulse power solenoid, we demonstrate efficient capture of laser accelerated proton beams and the ability to control their large divergence angles and broad energy range. Simulations using measured data for the input parameters give inference into the phase-space and transport efficiencies of the captured proton beams. We conclude with results from a feasibility study of a pulse power compact achromatic gantry concept. Using a scaled target normal sheath acceleration spectrum, we present simulation results of the available spectrum after transport through the gantry.
TL;DR: In this paper, the authors investigated beam dynamics and synchrotron radiation due to the betatron oscillation of the beam in laser-plasma acceleration, characterizing the beam qualities such as energy spread and transverse emittance.
Abstract: Consideration of laser-driven plasma-based electron/positron accelerators with a 2 TeV center-of-mass energy is presented, employing a multistaging scheme consisting of successive multi-GeV laser wakefield accelerators operated at the plasma density range of 10 15 ‐10 18 cm � 3 in the quasilinear regime. A total accelerator length is determined by an operating plasma density and a coupling distance allowed for both laser and beam focusing systems. We investigate beam dynamics and synchrotron radiation due to the betatron oscillation of the beam in laser-plasma acceleration, characterizing the beam qualities such as energy spread and transverse emittance. According to the criteria on the beam qualities for applications and available laser sources, the operating plasma density will be optimized. We note that in the low density operation the required wall-plug power for the laser driver will be much reduced in comparison with the high-density options.
TL;DR: In this paper, the feasibility of a free-electron laser EUV source driven by a multiturn superconducting energy-recovery linac (ERL) was investigated.
Abstract: Optical lithography has been actively used over the past decades to produce more and more dense integrated circuits. To keep with the pace of the miniaturization, light of shorter and shorter wavelength was used with time. The capabilities of the present 193-nm UV photolithography were expanded time after time, but it is now believed that further progress will require deployment of extreme ultraviolet (EUV) lithography based on the use of 13.5-nm radiation. However, presently no light source exists with sufficient average power to enable high-volume manufacturing. We report here the results of a study that shows the feasibility of a free-electron laser EUV source driven by a multiturn superconducting energy-recovery linac (ERL). The proposed $40\ifmmode\times\else\texttimes\fi{}20\text{ }\text{ }{\mathrm{m}}^{2}$ facility, using MW-scale consumption from the power grid, is estimated to provide about 5 kW of average EUV power. We elaborate the self-amplified spontaneous emission (SASE) option, which is presently technically feasible. A regenerative-amplifier option is also discussed. The proposed design is based on a short-period (2--3 cm) undulator. The corresponding electron beam energy is about 0.5--1.0 GeV. The proposed accelerator consists of a photoinjector, a booster, and a multiturn ERL.
TL;DR: In this article, the authors focus on the theory of spectral broadening mechanisms involved during Compton scattering of laser photons from relativistic electron beams and propose a design taking these mechanisms into consideration.
Abstract: The design of narrow-band Compton scattering sources for specific applications using nuclear resonance fluorescence (NRF) is presented. NRF lines are extremely narrow ($\ensuremath{\Delta}E/E\ensuremath{\sim}{10}^{\ensuremath{-}6}$) and require spectrally narrow sources to be excited selectively and efficiently. This paper focuses on the theory of spectral broadening mechanisms involved during Compton scattering of laser photons from relativistic electron beams. It is shown that in addition to the electron beam emittance, energy spread, and the laser parameters, nonlinear processes during the laser-electron interaction can have a detrimental effect on the gamma-ray source bandwidth, including a newly identified weakly nonlinear phase shift accumulated over the effective interaction duration. Finally, a design taking these mechanisms into consideration is outlined.
TL;DR: An analytical model for the spin precession of single beam electrons, and depolarization rates of zero emittance electron beams, is derived in this article, which is in very good agreement with the results from 3D particle-in-cell simulations in the limits of validity of their theory.
Abstract: The acceleration of polarized electron beams in the blowout regime of plasma-based acceleration is explored. An analytical model for the spin precession of single beam electrons, and depolarization rates of zero emittance electron beams, is derived. The role of finite emittance is examined numerically by solving the equations for the spin precession with a spin tracking algorithm. The analytical model is in very good agreement with the results from 3D particle-in-cell simulations in the limits of validity of our theory. Our work shows that the beam depolarization is lower for high-energy accelerator stages, and that under the appropriate conditions, the depolarization associated with the acceleration of 100--500 GeV electrons can be kept below 0.1%--0.2%.
TL;DR: In this article, a simulation of the x-ray free-electron laser (FEL) oscillator is presented that includes the frequency-dependent Bragg crystal reflectivity and transverse diffraction and focusing using the two-dimensional FEL code GINGER.
Abstract: Simulations of the x-ray free-electron laser (FEL) oscillator are presented that include the frequency-dependent Bragg crystal reflectivity and the transverse diffraction and focusing using the two-dimensional FEL code GINGER. A review of the physics of Bragg crystal reflectors and the x-ray FEL oscillator is made, followed by a discussion of its numerical implementation in GINGER. The simulation results for a two-crystal cavity and realistic FEL parameters indicate $\ensuremath{\sim}{10}^{9}$ photons in a nearly Fourier-limited, ps pulse. Compressing the electron beam to 100 A and 100 fs results in comparable x-ray characteristics for relaxed beam emittance, energy spread, and/or undulator parameters, albeit in a larger radiation bandwidth. Finally, preliminary simulation results indicate that the four-crystal FEL cavity can be tuned in energy over a range of a few percent.
TL;DR: In this article, the authors present a scaling law for the "chromatic emittance" of the collector and apply it to the particle energy and angular spectra of the simulation output to find that particle collection by a solenoid magnet satisfies requirements of intensity and beam quality as needed for depth scanning irradiation.
Abstract: Experimental results in laser acceleration of protons and ions and theoretical predictions that the currently achieved energies might be raised by factors 5--10 in the next few years have stimulated research exploring this new technology for oncology as a compact alternative to conventional synchrotron based accelerator technology. The emphasis of this paper is on collection and focusing of the laser produced particles by using simulation data from a specific laser acceleration model. We present a scaling law for the ``chromatic emittance'' of the collector---here assumed as a solenoid lens---and apply it to the particle energy and angular spectra of the simulation output. For a 10 Hz laser system we find that particle collection by a solenoid magnet well satisfies requirements of intensity and beam quality as needed for depth scanning irradiation. This includes a sufficiently large safety margin for intensity, whereas a scheme without collection---by using mere aperture collimation---hardly reaches the needed intensities.
TL;DR: The linac coherent light source (LCLS) is a self-amplified spontaneous emission (SASE) free-electron laser (FEL) operating at fundamental photon energies from 5 to 10 keV as discussed by the authors.
Abstract: The linac coherent light source (LCLS) is a self-amplified spontaneous emission (SASE) free-electron laser (FEL) operating at fundamental photon energies from 05 to 10 keV Characterization of the higher harmonics present in the FEL beam is important to users, for whom harder x rays can either extend the useful operating wavelength range or increase experimental backgrounds We present measurements of the power in both the second and third harmonics, and compare the results to expectations from simulations We also present studies of the transport of harmonics to the users, and the harmonic power as a function of electron beam quality
TL;DR: In this paper, the authors present the experience at Diamond Light Source in the design, implementation, and operation of low momentum compaction factor lattices for the generation of short x-ray pulses and coherent THz radiation.
Abstract: In this paper we present the experience at Diamond Light Source in the design, implementation, and operation of low momentum compaction factor lattices for the generation of short x-ray pulses and coherent THz radiation. The effects of higher-order terms in the expansion of the momentum compaction factor on beam dynamics are reviewed from a theoretical point of view, and the details of both high- and low-emittance solutions at Diamond are discussed. Measurements taken to characterize the lattices under a variety of machine conditions are presented, along with the practical limitations that exist as the momentum compaction factor is made to approach zero.
TL;DR: In this paper, a general method for tailoring the current distribution of relativistic electron bunches is proposed, which relies on a recently proposed method to exchange the longitudinal phase space emittance with one of the transverse emittances.
Abstract: We propose a general method for tailoring the current distribution of relativistic electron bunches. The technique relies on a recently proposed method to exchange the longitudinal phase space emittance with one of the transverse emittances. The method consists of transversely shaping the bunch and then converting its transverse profile into a current profile via a transverse-to-longitudinal phase-space-exchange beamline. We show that it is possible to tailor the current profile to follow, in principle, any desired distributions. We demonstrate, via computer simulations, the application of the method to generate trains of microbunches with tunable spacing and linearly-ramped current profiles. We also briefly explore potential applications of the technique.
TL;DR: In this paper, the influence of betatron coupling on transverse beam emittances is described using the resonance driving terms formalism, and two schemes for automatic compensation of coupling introduced by insertion devices are also presented with results from the first implementation tests.
Abstract: In this paper the influence of betatron coupling on the transverse beam emittances is described using the resonance driving terms formalism. Betatron coupling and vertical dispersion generated by magnetic and installation errors are major sources of vertical emittance. A new scheme for minimizing the latter is presented here, together with results from measurements carried out in 2010 at the ESRF electron storage ring, which provided vertical emittance of about 4.4 pm, a record low for this machine. Two schemes for the automatic compensation of coupling introduced by insertion devices are also presented with results from the first implementation tests. This paper is also an attempt to clarify the various definitions and meanings of vertical emittance in the presence of coupling.
TL;DR: In this article, the positron beam travel through a hollow plasma channel is modeled using laser-induced gas breakdown in hydrogen gas and the results show that hollow channels with plasma densities of order 10−16 and inner channel radii of order 20−20 are possible using currently available terawatt-level lasers.
Abstract: Plasma wakefield acceleration (PWFA) has demonstrated the ability to produce very high gradients to accelerate electrons and positrons. In PWFA, a drive bunch of charged particles passes through a uniform plasma, thereby generating a wakefield that accelerates a witness bunch traveling behind the drive bunch. This process works well for electrons, but much less so for positrons due to the positive charge attracting rather than repealing the plasma electrons, which leads to reduced acceleration gradient, halo formation, and emittance growth. This problem can be alleviated by having the positron beam travel through a hollow plasma channel. Presented are modeling results for producing 10--100 cm long hollow plasma channels suitable for positron PWFA. These channels are created utilizing laser-induced gas breakdown in hydrogen gas. The results show that hollow channels with plasma densities of order ${10}^{16}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}3}$ and inner channel radii of order $20\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ are possible using currently available terawatt-level lasers. At these densities and radii, preliminary positron PWFA modeling indicates that longitudinal electric fields on axis can exceed $3\text{ }\text{ }\mathrm{GV}/\mathrm{m}$.
TL;DR: In this article, higher-order laser modes are analyzed as a method to control focusing forces and improve the electron bunch quality in laser-plasma accelerators, which can be shaped by a superposition of modes.
Abstract: Higher-order laser modes are analyzed as a method to control focusing forces and improve the electron bunch quality in laser-plasma accelerators. In the linear wake regime, the focusing force is proportional to the transverse gradient of the laser intensity, which can be shaped by a superposition of modes. In particular, the transverse wakefield can be arbitrarily small in a region about the axis by adjusting the laser modes. Plasma channel effects, which prohibit the formation of the controlled-focusing region, can be mitigating by introducing a delay between the modes. Modes with parallel polarization produce a beat interference in the laser intensity, which lead to deflecting forces. This can be avoided by using modes with orthogonal polarization, different frequencies, or short pulses that do not overlap. Particle-in-cell simulations are performed of a laser-plasma accelerator in the quasilinear regime driven by high-order modes. Simulations show that, by including the first-order mode, the matched radius of the electron bunch is substantially increased, which for fixed bunch density and emittance implies an increase in the beam charge.
TL;DR: In this paper, a GaAs-based dc high voltage photogun with a drive laser beam positioned away from the electrostatic center of the photocathode is used to reduce photoemission.
Abstract: GaAs-based dc high voltage photoguns used at accelerators with extensive user programs must exhibit long photocathode operating lifetime. Achieving this goal represents a significant challenge for proposed high average current facilities that must operate at tens of milliamperes or more. This paper describes techniques to maintain good vacuum while delivering beam, and techniques that minimize the ill effects of ion bombardment, the dominant mechanism that reduces photocathode yield of a GaAs-based dc high voltage photogun. Experimental results presented here demonstrate enhanced lifetime at high beam currents by: (a) operating with the drive laser beam positioned away from the electrostatic center of the photocathode, (b) limiting the photocathode active area to eliminate photoemission from regions of the photocathode that do not support efficient beam delivery, (c) using a large drive laser beam to distribute ion damage over a larger area, and (d) by applying a relatively low bias voltage to the anode to repel ions created within the downstream beam line. A combination of these techniques provided the best total charge extracted lifetimes in excess of 1000 C at dc beam currents up to 9.5 mA, using green light illumination of bulk GaAs inside a 100 kV photogun.
TL;DR: In this article, the authors applied frequency map analysis (FMA) to study beam-beam effects in a novel crab waist collision approach and found that the ''crab'' focusing of colliding beams results in significant suppression of betatron coupling resonances induced by beam interaction.
Abstract: We applied frequency map analysis (FMA)---a method that is widely used to explore dynamics of Hamiltonian systems---to study beam-beam effects in a novel crab waist collision approach. The ``crab'' focusing of colliding beams results in significant suppression of betatron coupling resonances induced by beam-beam interaction. Application of FMA provides visible information about all working resonances, their widths, and locations in the planes of betatron tunes and betatron amplitudes, so the process of resonances suppression due to the beams crabbing is clearly seen. However, our numerical simulations and further analysis showed that effectiveness of crab waist is considerably restricted in the cases when the colliding beams are not flat. The FMA technique turned out to be very helpful in these studies, as it gave us the key information which would be difficult to obtain in a different way.
Journal Article•
TL;DR: In this paper, a multicharge state inverse Auger effect is proposed to explain the target emission when the conduction band at solid density becomes more atomiclike as energy is transferred from the electrons to the ions.
Abstract: We have studied a solid-to-plasma transition by irradiating Al foils with the FLASH free electron laser at intensities up to 10(16) W/cm(2). Intense XUV self-emission shows spectral features that are consistent with emission from regions of high density, which go beyond single inner-shell photoionization of solids. Characteristic features of intrashell transitions allowed us to identify Auger heating of the electrons in the conduction band occurring immediately after the absorption of the XUV laser energy as the dominant mechanism. A simple model of a multicharge state inverse Auger effect is proposed to explain the target emission when the conduction band at solid density becomes more atomiclike as energy is transferred from the electrons to the ions. This allows one to determine, independent of plasma simulations, the electron temperature and density just after the decay of crystalline order and to characterize the early time evolution.
TL;DR: In this article, an alternative method based on the concept of genetic algorithm was proposed to simultaneously optimize the beam emittance and dynamic aperture for low emittance lattice. And the result shows that the algorithm is particularly useful for the low emittance lattice design, where the beam and the dynamic aperture always conflict with each other.
Abstract: Finding a high quality of lattice that simultaneously meets low beam emittance performance and acceptable dynamic aperture is a challenging task for the storage ring-based light source, especially for the next generation storage ring which is characterized with ultralow beam emittance. This paper presents an alternative method, based on the concept of genetic algorithm, to simultaneously optimize the beam emittance and dynamic aperture for low emittance lattice. Instead of analyzing the nonlinear indicators extracted from the high order nonlinear map, the algorithm can globally optimize the nonlinear performance by the direct dynamic aperture tracking result. So this method is more straightforward and efficient than analyzing the nonlinear driving terms. In order to illustrate this method, the quadrupole and sextupole strengths of a five-bend-achromatic lattice are simultaneously optimized by nondominated sorting genetic algorithm II (NSGA-II). Finally, the optimal linear optics for ultralow emittance lattices with better dynamic aperture are obtained. The result shows that the algorithm is particularly useful for the low emittance lattice design, where the beam emittance and the dynamic aperture always conflict with each other.
TL;DR: In this paper, the authors used magnetic quadrupole lenses with field gradients of 500 T =m to improve the spectral resolution of dipole magnet spectrometers, resulting in measured energy spreads down to 1.0% rms at 190 MeV.
Abstract: The improvement of the energy spread, beam divergence, and pointing fluctuations are some of the main challenges currently facing the field of laser-wakefield acceleration of electrons. We address these issues by manipulating the electron beams after their generation using miniature magnetic quadrupole lenses with field gradients of � 500 T=m. By imaging electron beams the spectral resolution of dipole magnet spectrometers can be significantly increased, resulting in measured energy spreads down to 1.0% rms at 190 MeV. The focusing of different electron energies demonstrates the tunability of the lens system and could be used to filter out off-target energies in order to reduce the energy spread even further. By collimating the beam, the shot-to-shot spatial stability of the beam is improved by a factor of 5 measured at a distance of 1 m from the source. Additionally, by deliberately transversely offsetting a quadrupole lens, the electron beam can be steered in any direction by several mrad. These methods can be implemented while still maintaining the ultrashort bunch duration and low emittance of the beam and, except for undesired electron energies in the energy filter, without any loss of charge. This reliable and compact control of laser-wakefield accelerated electron beams is independent of the accelerator itself, allowing immediate application of currently available beams.
TL;DR: In this article, Niobium surfaces with different process histories are sampled with atomic force microscopy and stylus profilometry and analyzed to trace topography evolution at different scales.
Abstract: Surface topography characterization is a continuing issue for the superconducting radio frequency (SRF) particle accelerator community. Efforts are under way to both improve surface topography and its characterization and analysis using various techniques. In measurement of topography, power spectral density (PSD) is a promising method to quantify typical surface parameters and develop scale-specific interpretations. PSD can also be used to indicate how the process modifies topography at different scales. However, generating an accurate and meaningful topographic PSD of an SRF surface requires careful analysis and optimization. In this report, niobium surfaces with different process histories are sampled with atomic force microscopy and stylus profilometry and analyzed to trace topography evolution at different scales. An optimized PSD analysis protocol to serve SRF needs is presented.
TL;DR: In this paper, the authors used octupoles, decapoles, and longitudinal gradient bending magnets (LGBs) to achieve ultralow emittance and attain sufficient dynamic aperture.
Abstract: Although octupoles, decapoles, and longitudinal gradient bending magnets (LGB) have been studied for many years, they are not usually included in light source lattices. They can, however, be beneficial in order to realize ultralow emittance and attain sufficient dynamic aperture. We present methods for achieving ultralow emittance and discuss optimization of the nonlinear dynamics with multipoles. We demonstrate how control of amplitude-dependent tune shift makes octupoles a powerful tool for dynamic aperture optimization. Control of higher-order chromaticity by octupoles and decapoles is straightforward; however, since this turns out to be not quite as efficient in high-brightness lattices with low arc dispersion, we apply it to a conventional lattice to demonstrate the potential. This paper also illustrates how high-field LGBs can be used to build a compact, bright hard x-ray source. Finally, we demonstrate in detail the application of octupoles as integral components of the MAX IV 3 GeV storage ring lattice.