Showing papers in "Physical review accelerators and beams in 2018"
TL;DR: In this article, the authors used machine learning methods to predict the longitudinal phase space (LPS) distribution of particle accelerators using only nondestructive linac and e-beam measurements as inputs.
Abstract: We report on the application of machine learning (ML) methods for predicting the longitudinal phase space (LPS) distribution of particle accelerators. Our approach consists of training a ML-based virtual diagnostic to predict the LPS using only nondestructive linac and e-beam measurements as inputs. We validate this approach with a simulation study for the FACET-II linac and with an experimental demonstration conducted at LCLS. At LCLS, the e-beam LPS images are obtained with a transverse deflecting cavity and used as training data for our ML model. In both the FACET-II and LCLS cases we find good agreement between the predicted and simulated/measured LPS profiles, an important step towards showing the feasibility of implementing such a virtual diagnostic on particle accelerators in the future.
87 citations
TL;DR: In this paper, the effect of cavity material on rf breakdowns with short $X$-band standing wave accelerating structures was investigated, and it was shown that harder materials can reach larger accelerating gradients for the same breakdown rate.
Abstract: Vacuum radio-frequency (rf) breakdown is one of the major factors that limit operating accelerating gradients in rf particle accelerators. The occurrence of rf breakdowns was shown to be probabilistic, and can be characterized by a breakdown rate. Experiments with hard copper cavities showed that harder materials can reach larger accelerating gradients for the same breakdown rate. We study the effect of cavity material on rf breakdowns with short $X$-band standing wave accelerating structures. Here we report results from tests of a structure at cryogenic temperatures. At gradients greater than $150\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ we observed a degradation in the intrinsic cavity quality factor, ${Q}_{0}$. This decrease in ${Q}_{0}$ is consistent with rf power being absorbed by field emission currents, and is accounted for in the determination of accelerating gradients. The structure was conditioned up to an accelerating gradient of $250\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ at 45 K with $1{0}^{8}$ rf pulses and a breakdown rate of $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}/\mathrm{pulse}/\mathrm{m}$. For this breakdown rate, the cryogenic structure has the largest reported accelerating gradient. This improved performance over room temperatures structures supports the hypothesis that breakdown rate can be reduced by immobilizing crystal defects and decreasing thermally induced stresses.
63 citations
TL;DR: The genetic algorithm has been enhanced and applied to the multi-objective optimization problem presented by the dynamic aperture of the National Synchrotron Light Source II (NSLS-II) Storage Ring and the quality of the population increases and produces more competitive descendants accelerating the evolution process significantly.
Abstract: With the aid of machine learning techniques, the genetic algorithm has been enhanced and applied to the multi-objective optimization problem presented by the dynamic aperture of the National Synchrotron Light Source II (NSLS-II) Storage Ring. During the evolution processes employed by the genetic algorithm, the population is classified into different clusters in the search space. The clusters with top average fitness are given ``elite'' status. Intervention on the population is implemented by repopulating some potentially competitive candidates based on the experience learned from the accumulated data. These candidates replace randomly selected candidates among the original data pool. The average fitness of the population is therefore improved while diversity is not lost. Maintaining diversity ensures that the optimization is global rather than local. The quality of the population increases and produces more competitive descendants accelerating the evolution process significantly. When identifying the distribution of optimal candidates, they appear to be located in isolated islands within the search space. Some of these optimal candidates have been experimentally confirmed at the NSLS-II storage ring. The machine learning techniques that exploit the genetic algorithm can also be used in other population-based optimization problems such as particle swarm algorithm.
58 citations
TL;DR: In this paper, the authors presented systematic beam measurements with varying rf amplitudes and phases of the CH cavity, as well as phase space measurements for heavy ion beams with different mass to charge ratio.
Abstract: Very compact accelerating-focusing structures, as well as short focusing periods, high accelerating gradients and short drift spaces are strongly required for superconducting (sc) accelerator sections operating at low and medium energies for continuous wave (cw) heavy ion beams. To keep the GSI-super heavy element (SHE) program competitive on a high level and even beyond, a standalone sc cw linac (Helmholtz linear accelerator) in combination with the GSI high charge state injector (HLI), upgraded for cw operation, is envisaged. Recently the first linac section (financed by Helmholtz Institute Mainz (HIM) and GSI) as a demonstration of the capability of 217 MHz multigap crossbar H-mode structures (CH) has been commissioned and extensively tested with heavy ion beam from the HLI. The demonstrator setup reached acceleration of heavy ions up to the design beam energy. The required acceleration gain was achieved with heavy ion beams even above the design mass to charge ratio at high beam intensity and full beam transmission. This paper presents systematic beam measurements with varying rf amplitudes and phases of the CH cavity, as well as phase space measurements for heavy ion beams with different mass to charge ratio. The worldwide first and successful beam test with a superconducting multigap CH cavity is a milestone of the R work of HIM and GSI in collaboration with IAP in preparation of the HELIAC project and other cw-ion beam applications.
41 citations
37 citations
TL;DR: In this article, a parametric study of longitudinal and transverse instabilities caused by the resistive wall is performed for the case of the Future Circular Collider lepton machine, by taking into account also the effects of coating, used to fight the electron cloud build up.
Abstract: The Future Circular Collider study, which aims at designing post-LHC particle accelerator options, is entering in the final stage, which foresees a conceptual design report containing the basic requirements for a hadron and a lepton collider, as well as options for an electron-proton machine. Due to the high beam intensities of these accelerators, collective effects have to be carefully analyzed. Among them, the finite conductivity of the beam vacuum chamber represents a major source of impedance for the electron-positron collider. By using numerical and analytical tools, a parametric study of longitudinal and transverse instabilities caused by the resistive wall is performed in this paper for the case of the Future Circular Collider lepton machine, by taking into account also the effects of coating, used to fight the electron cloud build up. It will be proved that under certain assumptions the coupling impedance of a two layer system does not depend on the conductivity of the coating and this property represents an important characteristic for the choice of the material itself. The results and findings of this study have an impact on the machine design in several aspects. In particular the quite low threshold of single bunch instabilities with respect to the nominal beam current and the not negligible power losses due to the resistive wall are shown, together with the necessity of a new feedback system to counteract the fast transverse coupled bunch instability. The importance of a round vacuum chamber to avoid the quadrupolar tune shift is also discussed. Finally the crucial importance of the beam pipe material coating and thickness choice for the above results is underlined.
35 citations
TL;DR: In this paper, an analytic derivation of the expression describing the scattered radiation linewidth was presented and a model was proposed to describe the effects of horizontal and vertical emittance on the properties of the scattering radiation.
Abstract: Rising interest in inverse Compton sources has increased the need for efficient models that properly quantify the behavior of scattered radiation given a set of interaction parameters. The current state-of-the-art simulations rely on Monte Carlo--based methods, which, while properly expressing scattering behavior in high-probability regions of the produced spectra, may not correctly simulate such behavior in low-probability regions (e.g. tails of spectra). Moreover, sampling may take an inordinate amount of time for the desired accuracy to be achieved. In this paper, we present an analytic derivation of the expression describing the scattered radiation linewidth and propose a model to describe the effects of horizontal and vertical emittance on the properties of the scattered radiation. We also present an improved version of the code initially reported in Krafft et al. [Phys. Rev. Accel. Beams 19, 121302 (2016)], that can perform the same simulations as those present in cain and give accurate results in low-probability regions by integrating over the emissions of the electrons. Finally, we use these codes to carry out simulations that closely verify the behavior predicted by the analytically derived scaling law.
34 citations
TL;DR: In this article, collisionless shock acceleration of protons and ions has been achieved by the interaction of a near-critical density plasma with a secondary laser, which enables the production of beams with peaked spectra with energies of 10% -20% with up to $3/ifmmode\times\else\texttimes\fi{}{10}^{9}$ particles within these narrow spectral features.
Abstract: Collisionless shock acceleration of protons and ${\mathrm{C}}^{6+}$ ions has been achieved by the interaction of a ${10}^{20}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$, $1\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ laser with a near-critical density plasma. Ablation of the initially solid density target by a secondary laser allows for systematic control of the plasma profile. This enables the production of beams with peaked spectra with energies of $10--18\text{ }\text{ }\mathrm{MeV}/\mathrm{amu}$ and energy spreads of 10%--20% with up to $3\ifmmode\times\else\texttimes\fi{}{10}^{9}$ particles within these narrow spectral features. The narrow energy spread and similar velocity of ion species with different charge-to-mass ratios are consistent with acceleration by the moving potential of a shock wave. Particle-in-cell simulations show shock accelerated beams of protons and ${\mathrm{C}}^{6+}$ ions with energy distributions consistent with the experiments. Simulations further indicate the plasma profile determines the trade-off between the beam charge and energy and that with additional target optimization narrow energy spread beams exceeding $100\text{ }\text{ }\mathrm{MeV}/\mathrm{amu}$ can be produced using the same laser conditions.
34 citations
TL;DR: In this article, a stable relativistic electron source with a high charge per pulse up to 24 ǫ pC/shot was demonstrated using very tight focusing of the laser pulse in conjunction with microscale supersonic gas jets.
Abstract: We report on electron wakefield acceleration in the resonant bubble regime with few-millijoule near-single-cycle laser pulses at a kilohertz repetition rate. Using very tight focusing of the laser pulse in conjunction with microscale supersonic gas jets, we demonstrate a stable relativistic electron source with a high charge per pulse up to 24 pC/shot. The corresponding average current is 24 nA, making this kilohertz electron source useful for various applications.
32 citations
31 citations
TL;DR: In this article, a quasi-optical approach and direct particle-in-cell simulations were used to study the dynamics of relativistic SWO with 2D periodical corrugated structures of cylindrical geometry.
Abstract: Based on a quasioptical approach and direct particle-in-cell simulations, we study dynamics of oversized relativistic surface-wave oscillators (SWOs) of the Cherenkov type with 2D periodical corrugated structures of cylindrical geometry. Such corrugation allows significant rarefication of the spectrum of modes with different azimuthal indices. As a result, selective excitation of a mode with a given azimuthal index is possible. Azimuthal index of the generated mode depends on the voltage rise time. For short (nanosecond scale) rise time, generation of an azimuthally symmetric mode can be realized. For longer (hundreds nanoseconds to microseconds) rise time, the modes with high azimuthal indexes would be excited. These conclusions are supported by the experiments where Ka-band SWOs with 2D corrugated structures were realized based on the $300\text{ }\text{ }\mathrm{keV}/100\text{ }\text{ }\mathrm{A}/4\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$ thermionic accelerator SATURN. For an oversize factor of 16, stable narrow-band generation with output power of 1.5--2 MW was obtained at the frequency of 32.5 GHz corresponding to the mode with an azimuthal index of $m=3$. The project of Ka-band subgigawatt power SWOs operating at the azimuthally symmetric mode based on $500\text{ }\text{ }\mathrm{keV}/4\text{ }\text{ }\mathrm{kA}/20\text{ }\text{ }\mathrm{ns}$ high current explosive-emission accelerator SINUS-6 is under development.
TL;DR: In this paper, the authors investigated whether explosive electron emission may be the initiating mechanism of vacuum breakdown in the accelerating structures of TeV linear electron-positron colliders (Compact Linear Collider).
Abstract: It has been investigated whether explosive electron emission may be the initiating mechanism of vacuum breakdown in the accelerating structures of TeV linear electron-positron colliders (Compact Linear Collider). The physical processes involved in a dc vacuum breakdown have been considered, and the relationship between the voltage applied to the diode and the time delay to breakdown has been found. Based on the results obtained, the development of a vacuum breakdown in an rf electric field has been analyzed and the main parameters responsible for the initiation of explosive electron emission have been estimated. The formation of craters on the cathode surface during explosive electron emission has been numerically simulated, and the simulation results are discussed.
TL;DR: In this article, the authors investigated beam loading and emittance preservation for a high-charge electron beam being accelerated in quasilinear plasma wakefields driven by a short proton beam.
Abstract: We investigate beam loading and emittance preservation for a high-charge electron beam being accelerated in quasilinear plasma wakefields driven by a short proton beam. The structure of the studied wakefields are similar to those of a long, modulated proton beam, such as the AWAKE proton driver. We show that by properly choosing the electron beam parameters and exploiting two well known effects, beam loading of the wakefield and full blow out of plasma electrons by the accelerated beam, the electron beam can gain large amounts of energy with a narrow final energy spread (%-level) and without significant emittance growth.
TL;DR: In this article, a single cell superconducting radiofrequency cavity was treated with low temperature baking in a nitrogen environment and a significant increase in quality factor was observed when the cavity was heat treated in the temperature range of $120--160\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ with a nitrogen partial pressure of $ensuremath{\sim}25\text
Abstract: We report the rf performance of a single cell superconducting radiofrequency cavity after low temperature baking in a nitrogen environment. A significant increase in quality factor has been observed when the cavity was heat treated in the temperature range of $120--160\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ with a nitrogen partial pressure of $\ensuremath{\sim}25\text{ }\text{ }\mathrm{m}\text{ }\mathrm{Torr}$. This increase in quality factor as well as the $Q$-rise phenomenon (anti-$Q$-slope) is similar to those previously obtained with high temperature nitrogen doping as well as titanium doping. In this study, a cavity ${\mathrm{N}}_{2}$-treated at $120\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ and at $140\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ showed no degradation in accelerating gradient, however the accelerating gradient was reduced by $\ensuremath{\sim}25%$ with a $160\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ ${\mathrm{N}}_{2}$ treatment, compared to the baseline tests after electropolishing. Sample coupons treated in the same conditions as the cavity were analyzed by scanning electron microscope, x-ray photoelectron spectroscopy and secondary ion mass spectroscopy revealed a complex surface composition of ${\mathrm{Nb}}_{2}{\mathrm{O}}_{5}$, NbO and ${\mathrm{NbN}}_{(1\ensuremath{-}x)}{\mathrm{O}}_{x}$ within the rf penetration depth. Furthermore, magnetization measurements showed no significant change on bulk superconducting properties.
TL;DR: In this article, it was shown that dc quadrupole field can correct for both types of couplings and exactly cancel their emittance growths, but the degree of cancellation of the rf skew quadrupoles emittance is limited by the electron bunch length.
Abstract: Weak, rotated magnetic and radio frequency quadrupole fields in electron guns and injectors can couple the beam's horizontal with vertical motion, introduce correlations between otherwise orthogonal transverse momenta, and reduce the beam brightness. This paper discusses two important sources of coupled transverse dynamics common to most electron injectors. The first is quadrupole focusing followed by beam rotation in a solenoid, and the second coupling comes from a skewed high-power rf coupler or cavity port which has a rotated rf quadrupole field. It is shown that a dc quadrupole field can correct for both types of couplings and exactly cancel their emittance growths. The degree of cancellation of the rf skew quadrupole emittance is limited by the electron bunch length. Analytic expressions are derived and compared with emittance simulations and measurements.
TL;DR: In this paper, the effect of linear coupling on transverse beam stability was carried out, and a new mechanism that could incite transverse instabilities by causing a loss of Landau damping has been found.
Abstract: In the past, transverse coherent instabilities have been observed at the Hadron-Electron Ring Accelerator (HERA) proton ring that were instigated by the presence of linear coupling. Linear coupling can also potentially explain some transverse instabilities that were observed in the Large Hadron Collider (LHC) in both run I and run II, however a detailed description of the destabilizing mechanism of linear coupling was not known at the time. A study into the effect of linear coupling on transverse beam stability was carried out, and a new mechanism that could incite transverse instabilities by causing a loss of Landau damping has been found. The study includes time domain simulations with PyHEADTAIL and frequency domain computations based on analytical approaches, and was then verified by measurements with a single proton bunch in the LHC.
TL;DR: J.J. Leckbee, T. Wisher, M. L. Savage, W. R. Woodworth, and N. T. Stoltzfus are among those who have contributed to the development of Yazzie Sandia National Laboratories' high-resolution 3D image recognition system.
Abstract: J. D. Douglass, B. T. Hutsel, J. J. Leckbee, T. D. Mulville, B. S. Stoltzfus, M. L. Wisher, M. E. Savage, W. A. Stygar, E.W. Breden, J. D. Calhoun, M. E. Cuneo, D. J. De Smet, R. J. Focia, R. J. Hohlfelder, D. M. Jaramillo, O. M. Johns, M. C. Jones, A. C. Lombrozo, D. J. Lucero, J. K. Moore, J. L. Porter, S. D. Radovich, S. A. Romero, M. E. Sceiford, M. A. Sullivan, C. A. Walker, J. R. Woodworth, and N. T. Yazzie Sandia National Laboratories, Albuquerque, New Mexico 87185, USA
TL;DR: In this article, the authors describe the process of fabricating entire accelerator structures simplifies production and can be advantageous in a wide variety of applications. But, they do not discuss the cost of constructing accelerator structures.
Abstract: Fabricating entire accelerator structures simplifies production and can be advantageous in a wide variety of applications.
TL;DR: In this article, measurements of the spatially dependent variations in height and surface potential can be used to compute the electron beam mean transverse energy (MTE), one of the key determining factors in evaluation of brightness.
Abstract: The performance of free electron laser x-ray light sources, and systems for ultrafast electron diffraction and ultrafast electron microscopy, is limited by the brightness of the electron sources used. The intrinsic emittance, or equivalently, the mean transverse energy (MTE) of electrons emitted from the photocathode determines the maximum possible brightness in such systems. With ongoing improvements in photocathode design and synthesis, we are now at a point where the physical and chemical surface roughness of the cathode can become a limiting factor. Here we show how measurements of the spatially dependent variations in height and surface potential can be used to compute the electron beam mean transverse energy (MTE), one of the key determining factors in evaluation of brightness.
TL;DR: In this article, the resistive wall impedance associated with a layer of nonevaporable getter (NEG) coating on the vacuum chamber required for electron cloud mitigation was investigated by means of numerical simulations.
Abstract: The Future Circular Collider FCC-ee is a study toward a high luminosity electron-positron collider with a centre-of-mass energy from 91 GeV to 365 GeV. Due to the beam parameters and pipe dimensions, collective effects and electron cloud can be very critical aspects for the machine and can represent the main limitations to its performance. An estimation of the electron cloud build up in the main machine components and an impedance model are required to analyze the induced instabilities and to find solutions for their mitigation. Special attention has been given to the resistive wall impedance associated with a layer of nonevaporable getter (NEG) coating on the vacuum chamber required for electron cloud mitigation. The studies presented in this paper will show that minimizing the thickness of this coating layer is mandatory to increase the single bunch instability thresholds in the proposed lepton collider at 45.6 GeV. For this reason, NEG thin films with thicknesses below 250 nm have been investigated by means of numerical simulations to minimize the resistive wall impedance. In parallel, an extensive measurement campaign was performed at CERN to characterize these thin films, with the purpose of finding the minimum effective thickness satisfying vacuum and electron cloud requirements.
TL;DR: In this article, a new scheme to produce very low emittance muon beams using a positron beam of about 45 GeV interacting on electrons on target is presented, where the beam is stored in a low-emittance ring with a thin target, that is directly inserted in the ring chamber to produce muons.
Abstract: A new scheme to produce very low emittance muon beams using a positron beam of about 45 GeV interacting on electrons on target is presented. One of the innovative topics to be investigated is the behaviour of the positron beam stored in a low emittance ring with a thin target, that is directly inserted in the ring chamber to produce muons. Muons can be immediately collected at the exit of the target and transported to two ${\ensuremath{\mu}}^{+}$ and ${\ensuremath{\mu}}^{\ensuremath{-}}$ accumulator rings and then accelerated and injected in muon collider rings. We focus in this paper on the simulation of the ${\mathrm{e}}^{+}$ beam interacting with the target, the effect of the target on the 6-D phase space and the optimization of the ${\mathrm{e}}^{+}$ ring design to maximize the energy acceptance. We will investigate the performance of this scheme, ring plus target system, comparing different multi-turn simulations. The source is considered for use in a multi-TeV collider in [F. Zimmermann, in Strategy for Future Extreme Beam Facilities, Accelerator Technology Institute of Electronic Systems, Vol. 44 (Warsaw University of Technology, 2017).].
TL;DR: In this article, the results from a diffraction radiation beam size monitor are presented on the Cornell Electron Storage Ring using a 1 mA (1.6 × 10 particles per bunch) single bunch electron beam at 2.1 GeVenergy.
Abstract: In recent years, there has been an increasing demand for noninvasive beam size monitoring on particle accelerators. Ideally, these monitors should be cost effective and require little or no maintenance. These monitors should also be suitable for both linear and circular machines. Here, the experimental setup is described in detail, and the results from a diffraction radiation beam size monitor are presented. This monitor has been tested on the Cornell Electron Storage Ring using a 1 mA (1.6 × 10 particles per bunch) single bunch electron beam at 2.1 GeVenergy. Images of the target surface and the angular distribution of the emitted diffraction radiation were acquired at wavelengths of 400 and 600 nm. These measurements are compared to two analytical models.
TL;DR: In this article, the beam breakup instability of the drive bunch in the structure-based collinear wakefield accel- erator is considered and a stabilizing method is proposed.
Abstract: The beam breakup instability of the drive bunch in the structure-based collinear wakefield accel- erator is considered and a stabilizing method is proposed. The method includes using the specially designed beam focusing channel, applying the energy chirp along the electron bunch, and keeping energy chirp constant during the drive bunch deceleration. A stability condition is derived that defines the limit on the accelerating field for the witness bunch.
TL;DR: In this article, a single-shot diagnostic technique for the 4D average and core phase space densities of low-charge, high-brightness electron beams, based on the analysis of shadow point-projection images of metal grids, is presented.
Abstract: This paper details the development of a single-shot diagnostic technique for the 4D average and core phase space densities of low-charge, high-brightness electron beams, based on the analysis of shadow point-projection images of metal grids. This technique is similar to the standard pepper pot method, although it allows much greater transmission of the beam and therefore is more suitable for low-charge electron beams. Transverse coupling terms are included in the analysis, allowing the complete 4D transverse beam matrix to be reconstructed. The 4D beam phase space information is extremely important for the characterization of nonround beams. An analysis of the resolution limits and experimental benchmarking of the technique with pepper-pot emittance measurements are presented.
TL;DR: In this article, a method based on muon spin rotation (mu SR) is developed to probe these parameters on samples, which combines measurements from two different spectrometers, one being specifically built for these studies and samples of different geometries.
Abstract: The performance of superconducting radiofrequency (SRF) cavities used for particle accelerators depends on two characteristic material parameters: field of first flux entry H-entry and pinning strength. The former sets the limit for the maximum achievable accelerating gradient, while the latter determines how efficiently flux can be expelled related to the maximum achievable quality factor. In this paper, a method based on muon spin rotation (mu SR) is developed to probe these parameters on samples. It combines measurements from two different spectrometers, one being specifically built for these studies and samples of different geometries. It is found that annealing at 1400 degrees C virtually eliminates all pinning. Such an annealed substrate is ideally suited to measure H-entry of layered superconductors, which might enable accelerating gradients beyond bulk niobium technology.
TL;DR: In this article, the authors present an experimental study of the rf losses, manifested as degradation of the measured cavity quality factor (Q) in a copper cavity operated at cryogenic temperatures and high gradients.
Abstract: The development of high brightness electron sources can enable an increase in performance and reduction in size of extreme X-ray sources such as free electron lasers (FELs). A promising path to high brightness is through larger electric fields in radio-frequency (rf) photoinjectors. Recent experiments with 11.4 GHz copper accelerating cavities at cryogenic temperatures have demonstrated $500\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ surface electric fields with low rf breakdown rates. However, when the surface electric fields are larger than $300\text{ }\text{ }\mathrm{MV}/\mathrm{m}$, the measured cavity quality factor, ${Q}_{0}$, decreases during the input rf pulse by up to 30%, recovering before the next rf pulse. In this paper, we present an experimental study of the rf losses, manifested as degradation of ${Q}_{0}$, in a copper cavity operated at cryogenic temperatures and high gradients. The experimental conditions range from temperatures of 10--77 K and rf pulse lengths of 100--800 ns, using surface electric fields up to $400\text{ }\text{ }\mathrm{MV}/\mathrm{m}$. We developed a model for the change in ${Q}_{0}$ using measured field emission currents and rf signals. We find that the ${Q}_{0}$ degradation is consistent with the rf power being absorbed by strong field emission currents accelerated inside the cavity.
TL;DR: The new optimization algorithm was modified from the popular Nelder-Mead simplex method to make it noise aware and noise resistant and showed that the algorithm is robust for optimization problems with complex functional dependence, high cross-coupling between parameters, and high noise.
Abstract: A new optimization algorithm is introduced for online optimization applications. The algorithm was modified from the popular Nelder-Mead simplex method to make it noise aware and noise resistant. Simulation with an analytic function is used to demonstrate its performance. The algorithm has been successfully tested in experiments, which showed that the algorithm is robust for optimization problems with complex functional dependence, high cross-coupling between parameters, and high noise. Advantages of the new algorithm include high efficiency and that it does not require prior knowledge of the parameter space such as an initial conjugate direction set.
TL;DR: In this paper, an x-ray Hanbury Brown-Twiss interferometry was used to diagnose a femtosecond electron bunch (e-bunch) profile at SPring-8 Angstrom Compact free-electron LAser (SACLA).
Abstract: An x-ray Hanbury Brown-Twiss interferometry to diagnose a temporal profile of a femtosecond electron bunch (e-bunch) is presented We show that intensity interference of spontaneous x-ray radiation from the e-bunch reflects the e-bunch profile Based on this relationship, a temporal profile of the 81-GeV e-bunch at SPring-8 Angstrom Compact free-electron LAser (SACLA) that generates x-ray free-electron laser (XFEL) light is characterized through the intensity interference measurement Combining this e-bunch profile with a numerical simulation, the XFEL pulse duration generated by the e-bunch is estimated to be less than 10 fs
TL;DR: In this article, an adiabatically-tapered dielectric-lined waveguide was proposed to accelerate nonrelativistic charged particles to synchronous acceleration.
Abstract: We present a general concept to accelerate nonrelativistic charged particles. Our concept employs an adiabatically-tapered dielectric-lined waveguide which supports accelerating phase velocities for synchronous acceleration. We propose an ansatz for the transient field equations, show it satisfies Maxwell's equations under an adiabatic approximation and find excellent agreement with a finite-difference time-domain computer simulation. The fields were implemented into the particle-tracking program astra and we present beam dynamics results for an accelerating field with a 1-mm-wavelength and peak electric field of $100\text{ }\text{ }\mathrm{MV}/\mathrm{m}$. Numerical simulations indicate that a $\ensuremath{\sim}200$-keV electron beam can be accelerated to an energy of $\ensuremath{\sim}10\text{ }\text{ }\mathrm{MeV}$ over $\ensuremath{\sim}10\text{ }\text{ }\mathrm{cm}$ with parameters of interest to a wide range of applications including, e.g., future advanced accelerators, and ultra-fast electron diffraction.