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This report summarizes the findings of the AF5 Topical Subgroup to Snowmass 2021, which investigated accelerators for rare processes and physics beyond colliders. The report focuses primarily on opportunities for dark sector searches and the need for coordinated development of the Fermilab experimental program for PIP-II and beyond. In addition, a number of other physics opportunities are cataloged and suggestions for synergistic R & D opportunities with various areas of technological development are discussed.

Colliders are essential research tools for particle physics. Numerous future collider proposal were discussed in the course of the US high energy physics community strategic planning exercise Snowmass'21. The Implementation Task Force (ITF) has been established to evaluate the proposed future accelerator projects for performance, technology readiness, schedule, cost, and environmental impact. Corresponding metrics has been developed for uniform comparison of the proposals ranging from Higgs/EW factories to multi-TeV lepton, hadron and ep collider facilities, based on traditional and advanced acceleration technologies. This article describes the metrics and approaches, and presents evaluations of future colliders performed by the ITF.

X-ray free-electron lasers (XFELs) open a new era of X-ray based research by generating extremely intense X-ray flashes. To further improve the spectrum brightness, a self-seeding FEL scheme has been developed and demonstrated experimentally. As the next step, new-generation FELs with high repetition rates are being designed, built and commissioned around the world. A high repetition rate would significantly speed up the scientific research; however, alongside this improvement comes new challenges surrounding thermal management of the self-seeding monochromator. In this paper, a new configuration for self-seeding FELs is proposed, operated under a high repetition rate which can strongly suppress the thermal effects on the monochromator and provides a narrow-bandwidth FEL pulse. Three-dimension time-dependent simulations have been performed to demonstrate this idea. With this proposed configuration, high-repetition-rate XFEL facilities are able to generate narrow-bandwidth X-ray pulses without obvious thermal concern on the monochromators.

One of the key challenges in scientific researches based on free-electron lasers (FELs) is the characterization of the coherence time of the ultra-fast hard x-ray pulse, which fundamentally influences the interaction process between x-rays and materials. Conventional optical methods, based on autocorrelation, are very difficult to realize due to the lack of mirrors. Here, we experimentally demonstrate a novel method which yields a coherence time of 174.7 attoseconds for the 6.92 keV FEL pulses at the Linac Coherent Light Source. In our experiment, a phase shifter is adopted to control the cross-correlation between x-ray and microbunched electrons. This approach provides critical diagnostics for the temporal coherence of x-ray FELs and is universal for general machine parameters; applicable for wide range of photon energy, radiation brightness, repetition rate and FEL pulse duration.

The Snowmass 2021 Implementation Task Force has been established to evaluate the proposed future accelerator projects for performance, technology readiness, schedule, cost, and environmental impact. Corresponding metrics has been developed for uniform comparison of the proposals ranging from Higgs/EW factories to multi-TeV lepton, hadron and ep collider facilities, based on traditional and advanced acceleration technologies. This report documents the metrics and processes, and presents evaluations of future colliders performed by Implementation Task Force., Comment: 68 pages; Submitted to the Proceedings of the US Community Study on the Future of Particle Physics (Snowmass 2021)

In this paper, we present analytic expressions for the coherent synchrotron radiation (CSR)-induced rms energy spread and the resulting emittance dilution due to three common scenarios in electron particle accelerators: An electron bunch transitioning into the steady-state regime while traversing a bending magnet with and without the effects of compression, and an electron bunch coasting into a subsequent drift section. The expressions are used to calculate the CSR-induced rms energy spread and emittance dilution for real bunch compressor systems: the first and second bunch compressors of the LCLS-II CuRF and LCLS-II-HE. The expressions are compared with simulations using elegant for various parameter scans.

Burton Richter had an enormous impact on the field of accelerator science and technology. This paper recounts some of that impact through seven short essays from people who viewed his contributions to aspects of the field with topics ranging from colliding beam rings, linear colliders to Free Electron Lasers.

We present an x-ray regenerative amplifier free-electron laser design capable of producing fully coherent hard x-ray pulses across a broad tuning range at a high steady state repetition rate. The scheme leverages a strong undulator taper and an apertured diamond output-coupling cavity crystal to produce both high peak and average spectral brightness radiation that is 2 to 3 orders of magnitude greater than conventional single-pass self-amplified spontaneous emission free-electron laser amplifiers. Refractive guiding in the postsaturation regime is found to play a key role in passively controlling the stored cavity power. The scheme is explored both analytically and numerically in the context of the Linac Coherent Light Source II High Energy upgrade.

Coherence time is one of the fundamental characteristics of light sources. Methods based on autocorrelation have been widely applied from optical domain to soft X-rays to characterize the radiation coherence time. However, for the hard X-ray regime, due to the lack of proper mirrors, it is extremely difficult to implement such autocorrelation scheme. In this paper, a novel approach for characterizing the coherence time of a hard X-ray free-electron laser (FEL) is proposed and validated numerically. A phase shifter is adopted to control the correlation between X-ray and microbunched electrons. The coherence time of the FEL pulse can be extracted from the cross-correlation. Semi-analytical analysis and three-dimensional time-dependent numerical simulations are presented to elaborate the details. A coherence time of 218.2 attoseconds for 6.92 keV X-ray FEL pulses is obtained in our simulation based on the configuration of Linac Coherent Light Source. This approach provides critical temporal coherence diagnostics for X-ray FELs, and is decoupled from machine parameters, applicable for any photon energy, radiation brightness, repetition rate and FEL pulse duration.

Newtonian fluid dynamics simulations were performed using the Navier–Stokes–Fourier formulations to elucidate the short time-scale (µs and longer) evolution of the density and temperature distributions in an argon-gas-filled attenuator for an X-ray free-electron laser under high-repetition-rate operation. Both hydrodynamic motions of the gas molecules and thermal conductions were included in a finite-volume calculation. It was found that the hydrodynamic wave motions play the primary role in creating a density depression (also known as a filament) by advectively transporting gas particles away from the X-ray laser–gas interaction region, where large pressure and temperature gradients have been built upon the initial energy depositionviaX-ray photoelectric absorption and subsequent thermalization. Concurrent outward heat conduction tends to reduce the pressure in the filament core region, generating a counter gas flow to backfill the filament, but on an initially slower time scale. If the inter-pulse separation is sufficiently short so the filament cannot recover, the depth of the filament progressively increases as the trailing pulses remove additional gas particles. Since the rate of hydrodynamic removal decreases while the rate of heat conduction back flow increases as time elapses, the two competing mechanisms ultimately reach a dynamic balance, establishing a repeating pattern for each pulse cycle. By performing simulations at higher repetition rates but lower per pulse energies while maintaining a constant time-averaged power, the amplitude of the hydrodynamic motion per pulse becomes smaller, and the evolution of the temperature and density distributions approach asymptotically towards, as expected, those calculated for a continuous-wave input of the equivalent power.

Author(s): Hemsing, E; Marcus, G; Fawley, WM; Schoenlein, RW; Coffee, R; Dakovski, G; Hastings, J; Huang, Z; Ratner, D; Raubenheimer, T; Penn, G | Abstract: We present the results from studies of soft X-ray seeding options for the LCLS-II X-ray free electron laser (FEL) at SLAC. The LCLS-II will use superconducting accelerator technology to produce X-ray pulses at up to 1 MHz repetition rate using 4 GeV electron beams. If properly seeded, these pulses will be nearly fully coherent, and highly stable in photon energy, bandwidth, and intensity, thus enabling unique experiments with intense high-resolution soft X-rays. Given the expected electron beam parameters from start to end simulations and predicted FEL performance, our studies reveal echo enabled harmonic generation (EEHG) and soft X-ray self-seeding (SXRSS) as promising and complementary seeding methods. We find that SXRSS has the advantage of simplicity and will deliver 5-35 times higher spectral brightness than EEHG in the 1-2 nm range, but lacks some of the potential for phase-stable multipulse and multicolor FEL operations enabled by external laser seeding with EEHG.

The free-electron laser facilities driven by a superconducting radio-frequency (SRF) linac provide high-repetition-rate electron beam, which makes it feasible to feed multiple undulator lines at the same time. In this paper, we study a method of controlling the energy of multiple electron bunches by off-frequency detuning in the SRF linac. Based on the theoretical analysis, we present the available discrete frequency detunes and the optimal linac energy allocation solutions with given energy overhead and periodic energy pattern. The required off-frequency detune is not larger than half of the beam repetition rate, which can be realized with the frequency tuner in SRF cavities. We adopt the configuration of the high energy upgrade of the Linac Coherent Light Source II (LCLS-II-HE) as an example to discuss the possible schemes in practice to support two undulator lines simultaneously. We also discuss the energy jitter increase due to the off-crest acceleration, the tunable energy range with fixed total SRF linac energy capacity, the beam energy chirp and lattice design for multienergy beam transport.

Plasma wakefield acceleration offers acceleration gradients of several GeV/m, ideal for a next-generation linear collider. The beam optics requirements between plasma cells include injection and extraction of drive beams, matching the main beam beta functions into the next cell, canceling dispersion as well as constraining bunch lengthening and chromaticity. To maintain a high effective acceleration gradient, this must be accomplished in the shortest distance possible. A working example is presented, using novel methods to correct chromaticity, as well as scaling laws for a high energy regime.

Echo-enabled harmonic generation has been used to seed a free-electron laser and has been demonstrated up to the 75th harmonic, producing 32 nm light from a 2,400 nm laser.

The International Particle Accelerator Conference (IPAC) is the main international event for the worldwide accelerator community and industry. It is the venue where researchers present cutting-edge accelerator developments and the latest status of accelerator facilities across the globe. The 9th IPAC, IPAC’18, was held in Vancouver, British Columbia from April 29 to May 4, 2018. With almost 1300 delegates and 90 industrial exhibitors, over 1500 submitted papers, the conference delivered a natural hub for scientific discussion, collaboration meetings and discussions with public program managers and industry and made it a very successful conference. Some conference statistics and scientific high lights will be presented.The International Particle Accelerator Conference (IPAC) is the main international event for the worldwide accelerator community and industry. It is the venue where researchers present cutting-edge accelerator developments and the latest status of accelerator facilities across the globe. The 9th IPAC, IPAC’18, was held in Vancouver, British Columbia from April 29 to May 4, 2018. With almost 1300 delegates and 90 industrial exhibitors, over 1500 submitted papers, the conference delivered a natural hub for scientific discussion, collaboration meetings and discussions with public program managers and industry and made it a very successful conference. Some conference statistics and scientific high lights will be presented.

PAL-XFEL utilizes a three-chicane bunch compression (3-BC) scheme (the very first of its kind in operation) for free-electron laser (FEL) operation. The addition of a third bunch compressor allows for more effective mitigation of coherent synchrotron radiation during bunch compression and an increased flexibility of system configuration. Start-to-end simulations of the effects of radiofrequency jitter on the electron beam performance show that using the 3-BC scheme leads to better performance compared with the two-chicane bunch compression scheme. Together with the high performance of the linac radiofrequency system, it enables reliable operation of PAL-XFEL with unprecedented stability in terms of arrival timing, pointing and intensity; an arrival timing jitter of better than 15 fs, a transverse position jitter of smaller than 10% of the photon beam size, and an FEL intensity jitter of smaller than 5% are consistently achieved.

Many future linear collider designs call for electron and positron beams with normalized rms horizontal and vertical emittances of {gamma}{epsilon}{sub x} = 3{times}10{sup {minus}6} m-rad and {gamma}{epsilon}{sub y} = 3{times}10{sup {minus}8} m-rad; these are a factor of 10 to 100 below those observed in the Stanford Linear Collider. In this dissertation, we examine the feasibility of achieving beams with these very small vertical emittances. We examine the limitations encountered during both the generation and the subsequent acceleration of such low emittance beams. We consider collective limitations, such as wakefields, space charge effects, scattering processes, and ion trapping; and also how intensity limitations, such as anomalous dispersion, betatron coupling, and pulse-to-pulse beam jitter. In general, the minimum emittance in both the generation and the acceleration stages is limited by the transverse misalignments of the accelerator components. We describe a few techniques of correcting the effect of these errors, thereby easing the alignment tolerances by over an order of magnitude. Finally, we also calculate fundamental'' limitations on the minimum vertical emittance; these do not constrain the current designs but may prove important in the future.

A sustained filamentation or density depression phenomenon in an argon gas attenuator servicing a high-repetition femtosecond X-ray free-electron laser has been studied using a finite-difference method applied to the thermal diffusion equation for an ideal gas. A steady-state solution was obtained by assuming continuous-wave input of an equivalent time-averaged beam power and that the pressure of the entire gas volume has reached equilibrium. Both radial and axial temperature/density gradients were found and describable as filamentation or density depression previously reported for a femtosecond optical laser of similar attributes. The effect exhibits complex dependence on the input power, the desired attenuation, and the geometries of the beam and the attenuator. Time-dependent simulations were carried out to further elucidate the evolution of the temperature/density gradients in between pulses, from which the actual attenuation received by any given pulse can be properly calculated.

The superconducting cavities in the CW linacs of LCLS-II will operate at 2 K, where cooling is very expensive. One source of heat is presented by the higher order mode (HOM) power deposited by the beam. Due to the very short bunch length, especially in the L3 region, the LCLS-II beam spectrum extends into the terahertz range. Ceramic absorbers, at 70 K between cryomodules, are meant to absorb much of this power. In this report we perform two kinds of calculations to estimate the effectiveness of the absorbers and the fractional power that remains to be removed at 2 K.

Time-dependent simulations were carried out to study the duty-cycle dependence of the density depression effect in gas attenuators and gas intensity monitors servicing a high repetition rate pulsed Free-electron laser beam. The evolution of the temperature/density gradients in-between the pulses in the entire gas volume, especially during the on-cycle, were obtained to evaluate the performance of any given pulse. It was found that the actual achieved attenuation in the attenuator or the intensity measured by the gas monitor deviates from the asymptotic value expected for a uniformly spaced pulse train after reaching a steady state, becoming progressively more significant as the duty-cycle tends lower.

We analytically examine the bunching factor spectrum of a relativistic electron beam with sinusoidal energy structure that then undergoes an echo-enabled harmonic generation (EEHG) transformation to produce high harmonics. The performance is found to be described primarily by a simple scaling parameter. The dependence of the bunching amplitude on fluctuations of critical parameters is derived analytically, and compared with simulations. Where applicable, EEHG is also compared with high gain harmonic generation (HGHG) and we find that EEHG is generally less sensitive to several types of energy structure. In the presence of intermediate frequency modulations like those produced by the microbunching instability, EEHG has a substantially narrower intrinsic bunching pedestal.

© 2017 authors. Published by the American Physical Society. The shot-noise driven microbunching instability can significantly degrade electron beam quality in x-ray free electron laser light sources. Experiments were carried out at the Linac Coherent Light Source (LCLS) to study this instability. In this paper, we present start-to-end simulations of the shot-noise driven microbunching instability experiment at the LCLS using the real number of electrons. The simulation results reproduce the measurements quite well. A microbunching self-heating mechanism is also illustrated in the simulation, which helps explain the experimental observation.

© 2016 Elsevier B.V. There is a great interest in generating high-power hard X-ray Free Electron Laser (FEL) in the terawatt (TW) level that can enable coherent diffraction imaging of complex molecules like proteins and probe fundamental high-field physics. A feasibility study of producing such X-ray pulses was carried out employing a configuration beginning with a Self-Amplified Spontaneous Emission FEL, followed by a “self-seeding” crystal monochromator generating a fully coherent seed, and finishing with a long tapered undulator where the coherent seed recombines with the electron bunch and is amplified to high power. The undulator tapering profile, the phase advance in the undulator break sections, the quadrupole focusing strength, etc. are parameters to be optimized. A Genetic Algorithm (GA) is adopted for this multi-dimensional optimization. Concrete examples are given for LINAC Coherent Light Source (LCLS) and LCLS-II-type systems. Analytical estimate is also developed to cross check the simulation and optimization results as a quick and complimentary tool.

In a plasma wakefield accelerator (PWFA), using advanced injection techniques such as the density down-ramp injection is a promising approach for generating electron beams with ultralow emittance. In this study, we describe how such a scheme could be realized using the beam from the Linac Coherent Light Source (LCLS) accelerator as a driver. Moreover, we explore the potential applications of such high-brightness e-beams in a facility like LCLS and in novel concepts for compact FELs.

We develop a method to generate a long pulse train of few-cycle coherent radiation by modulating an electron beam with a high power laser. The large energy modulation disperses the beam in a radiating undulator and leads to the production of phase-locked few-cycle coherent radiation pulses. These pulses are produced at a high harmonic of the modulating laser, and are longitudinally separated by the modulating laser wavelength. We discuss an analytical model for this scheme and investigate the temporal and spectral properties of this radiation. This model is compared with numerical simulation results using the unaveraged code Puffin. We examine various harmful effects and how they might be avoided, as well as a possible experimental realization of this scheme.

The recent experimental progress in plasma wakefield accelerator (PWFA) experiments [I. Blumenfeld et al., Nature 445, 741 (15 February 2007)] make it possible to contemplate the application of the PWFA to a future electron positron linear collider. One of the main parameter of such a collider is the luminosity. The luminosity determines the number of occurrence of a given process with a given cross section, per unit time. For example, current designs call for luminosities in the1034 cm−2s−1. To reach such luminosities, particle beams with average powers in the 10 MW range are necessary. In a plasma wakefield accelerator, a plasma works as an energy transformer to convert a relatively high-current, low-energy bunch train to a proportionately lower-current, higher-energy bunch train. This transformation is continued until the desired particle energy is reached to do physics. Microwave powered structures have a demonstrated ability to produce bunch trains with parameters of interest to the PWFA with high efficiency. Such structures operate most efficiently when the bunches are spaced tightly together, e.g. by a few ns. Consequently, the plasma source must be stable and reproducible on the ns time scale. Assuming an energy transfer efficiency from a PWFA drive bunch to the accelerated witness bunch of 35% implies that about 6.5 MW of beam power is left in the plasma. For a beam with an initial energy of 25 GeV, a final energy of 250 GeV and an accelerating gradient of 25 GeV/m, the total plasma length is 9 m, corresponding to an energy deposition of ≈720 kW/m. The energy will be deposited in a very small volume of plasma, typically a radius of about 100 µm for densities of interest. Therefore, while heat evacuation at a large radius may not be a serious issue, thermal shock waves can be expected. These waves will modify the gas and plasma density for the following bunches. The beam and plasma source parameters will be presented, and the main issues with the plasma source discussed.

Beam-based feedback systems play an essential role in the operation of high energy electron-positron linear colliders. Ground motion, vibration and other disturbances can significantly alter the beam trajectory or degrade the beam quality. Feedback systems are required for long term stability and for case and efficiency of operation. Trajectory feedback for the Next Linear Collider (NLC) has been studied extensively to understand both the achievable orbit quality and the time response characteristics. More recently, these studies have been extended to evaluate performance in the presence of ground motion.

This report briefly describes studies performed in the framework of the Collimation Task Force organized to support the work of the second International Linear Collider Technical Review Committee. The post‐linac beam‐collimation systems in the TESLA, JLC/NLC and CLIC linear‐collider designs are compared using the same computer code under the same assumptions. Their performance is quantified in terms of beam‐halo and synchrotron‐radiation collimation efficiency. The performance of the current designs varies across projects, and does not always meet the original design goals. But these comparisons suggest that achieving the required performance in a future linear collider is feasible. Further work of the group is briefly described as well.

Building more compact accelerators to deliver high brightness electron beams for the generation of high flux, highly coherent radiation is a priority for the photon science community. A relatively straightforward reduction in footprint can be achieved by using high-gradient X-band (11.4 GHz) rf technology. To this end, an X-band injector consisting of a 5.5 cell rf gun and a 1-m long linac has been commissioned at SLAC. It delivers an 85 MeV electron beam with peak brightness somewhat better than that achieved in S-band photoinjectors, such as the one developed for the Linac Coherent Light Source (LCLS). The X-band rf gun operates with up to a 200 MV/m peak field on the cathode, and has been used to produce bunches of a few pC to 1.2 nC in charge. Notably, bunch lengths as short as 120 fs rms have been measured for charges of 5 pC (∼3×10^{7} electrons), and normalized transverse emittances as small as 0.22 mm-mrad have been measured for this same charge level. Bunch lengths as short as 400 (250) fs rms have been achieved for electron bunches of 100 (20) pC with transverse normalized emittances of 0.7 (0.35) mm-mrad. We report on the performance and the lessons learned from the operation and optimization of this first generation X-band gun.

This paper presents two-photon photoemission from a copper cathode in an $X$-band photoinjector. We experimentally verified that the electron bunch charge from photoemission out of a copper cathode scales with laser intensity (I) square for 400 nm wavelength photons. We compare this two-photon photoemission process with the single photon process at 266 nm. Despite the high reflectivity ($R$) of the copper surface for 400 nm photons ($R=0.48$) and higher thermal energy of photoelectrons (two-photon at 200 nm) compared to 266 nm photoelectrons, the quantum efficiency of the two-photon photoemission process (400 nm) exceeds the single-photon process (266 nm) when the incident laser intensity is above $300\text{ }\text{ }\mathrm{GW}/{\mathrm{cm}}^{2}$. At the same laser pulse energy ($E$) and other experimental conditions, emitted charge scales inversely with the laser pulse duration. A thermal emittance of 2.7 mm-mrad per mm root mean square (rms) was measured on our cathode which exceeds by sixty percent larger compared to the theoretical predictions, but this discrepancy is similar to previous experimental thermal emittance on copper cathodes with 266 nm photons. The damage of the cathode surface of our first-generation $X$-band gun from both rf breakdowns and laser impacts mostly explains this result. Using a 400 nm laser can substantially simplify the photoinjector system, and make it an alternative solution for compact pulsed electron sources.

Beam instability caused by the electron cloud has been observed in positron and proton storage rings and it is expected to be a limiting factor in the performance of the positron damping ring (DR) of future linear colliders (LC) such as ILC and CLIC [1,2]. To test a series of promising possible electron cloud mitigation techniques as surface coatings and grooves, in the Positron low-energy ring (LER) of the PEP-II accelerator, we have installed several test vacuum chambers including (i) a special chamber to monitor the variation in the secondary electron yield of technical surface materials and coatings under the effect of ion, electron and photon conditioning in situ in the beam line (ii) chambers with grooves [3] in a straight magnetic-free section and (iii) coated chambers in a dedicated newly installed 4-magnet chicane [4] to study mitigations in a magnetic field region. In this paper, we describe the ongoing R&D effort to mitigate the electron cloud effect for the LC damping ring, focusing on the first experimental area and on results of the reduction in the secondary electron yield due to in situ conditioning.

The first experimental observation of magnetic resonances in electron clouds is reported. The resonance was observed as a modulation in cloud intensity for uncoated as well as TiN-coated aluminum surfaces in the positron storage ring of the PEP-II collider at SLAC. Electron clouds frequently arise in accelerators of positively charged particles, and severely impact the machines performance. The TiN coating was found to be an effective remedy, reducing the cloud intensity by three orders of magnitude.

Over the last few years, enormous improvements have been made in the understanding and development of high-brightness compact accelerators. This talk will consider the state-of-the art and describe concepts and fundamental as well as technical challenges of generating EUV and soft X-rays in compact sources. Article not available.

The effect of surface roughness on the secondary electron emission from a sawtooth and isosceles triangular surface as well as a rectangular surface in a magnetic field under electron bombardment is investigated using a Monte-Carlo method. In all cases, some of the secondary electrons emitted from the surface return to the surface within their first few gyrations, resulting in a low effective secondary electron yield. Either sawtooth or isosceles triangle surface can significantly reduce the secondary emission yield in a magnetic field below the multipacting threshold with a weak dependence on the size of the surface and magnetic field. Rectangular surfaces can also reduce SEY below 1.0 but have a large sensitivity to the size of the grooves and the magnetic field. The impedance as well as other implementations of the grooved surfaces are also discussed.

Time-dependent simulation was carried out to study the dynamic response of a gas-based attenuator system designed for the LCLS-II high repetition rate X-ray Free-electron Laser’s, and to further elucidate the impact of the fluctuating energies of proceeding pulses on the actual attenuation factor achieved for the trailing pulses. The filamentation effect in the gas density revealed from an earlier steady-state calculation under a constant Continuous-Wave input power was reproduced with additional ramping behavior and oscillations arising from the onset and the pulsed structure of the beam. More importantly, the actual achieved attenuation for a given pulse was found to vary randomly in response to the fluctuations in the input power.