At the x-ray free-electron laser SwissFEL, at the Paul Scherrer Institute, Switzerland, beam loss monitors are used to determine loss positions along the linear accelerator and protect critical elements such as the undulator magnets from excess radiation. These monitors are integrated into the machine protection system (MPS) allowing beam losses to be limited by dynamically reducing the repetition rate. This paper focuses on the types of loss monitors installed at SwissFEL and their function in protecting the machine.

This paper describes a novel scheme for correcting the second order (chromatic sextupole) magnet lattice and its deployment on a fourth generation multibend achromat electron storage ring. The method is analogous to the well-established linear optics from closed orbits scheme, but uses an off-energy orbit response matrix to characterize the second order optics of a lattice. The matrix is constructed from the difference between two orbit response matrices measured at off-nominal energy, and is approximately linear with chromatic sextupole field strengths. This is utilized in a least squares minimization to find a model which minimizes the difference between the measured and model off-energy orbit response matrix. From this model corrections to the chromatic sextupoles of the machine can be calculated. In effect, for the MAX IV 3 GeV ring, the proposed scheme, NOECO (nonlinear optics from off-energy closed orbits), brings initial sextupole strength variations between achromats in the order of $\pm 8\%$ down to the order of $\pm 1\%$. The chromatic machine functions were symmetrized, the measured chromaticities approached the nominal values, and the lifetime of the beam at delivery conditions, with the same level of emittance coupling, increased to 19 h from 11 h at previously used sextupole settings.

The Compact Linear Collider is one of the two main European options for a collider in a post Large Hadron Collider era. This is a linear $e^{+}{e}^{-}$ collider with three center-of-mass energy stages: 380 GeV, 1.5 TeV, and 3 TeV. The luminosity performance of the first stage at 380 GeV is presented including the impact of static and dynamic imperfections. These calculations are performed with fully realistic tracking simulations from the exit of the damping rings to the interaction point and including beam-beam effects in the collisions. A luminosity of $4.3\times {10}^{34}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$ can be achieved with a perfect collider, which is almost three times the nominal luminosity target of $1.5\times {10}^{34}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$. In simulations with static imperfections, a luminosity of $2.35\times {10}^{34}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$ or greater is achieved by 90% of randomly misaligned colliders. Expressed as a percentage of the nominal luminosity target, this is a surplus of approximately 57%. Including the impact of ground motion, a luminosity surplus of 53% or greater can be expected for 90% of colliders. The average expected luminosity is $2.8\times {10}^{34}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$, which is almost twice the nominal luminosity target.

Coherent transition radiation (CTR) is a widely used approach to characterize the electron beam in accelerator science and technology. Characterization of the CTR with an efficient, versatile and compact setup is in considerable demand for a wide range of applications such as free electron lasers, electron storage rings and other related scientific researches. In this article, we report on spatial-temporal electric field measurements of CTR using the single-shot electro-optic spatial decoding method. A quasi-half-cycle temporal waveform of the THz pulse with peak electric field strength of $\sim 42\mathrm{MV}/\mathrm{m}$ has been measured. A symmetrical field profile across the transverse center and the distorted wave front of CTR THz pulses near the focal plane were demonstrated. Moreover, timing jitters of the ultrashort electron beams are also monitored via this method with tens of femtosecond accuracy. This comprehensive method provides the temporal waveform and the one-dimensional spatial imaging of THz fields simultaneously, showing it has the potential for single-shot diagnostics of ultrashort electron beams.

For the past generation $e^{+}{e}^{-}$ storage ring colliders, we usually used natural bunch length or its impedance lengthened value in beam-beam simulations instead of considering the impedance directly. In the future colliders, such as FCC-ee and CEPC, the beam-beam interaction becomes essentially three dimensional. In order to increase the luminosity, the future accelerators will collide very intense beams of high energy with low emittances and small beta functions at the collision points exploiting the crab waist collision scheme with a large Piwinski angle. For these extreme parameters several new effects become important for the collider performance such as beamstrahlung, coherent X-Z instability, 3D flip-flop so that the longitudinal beam dynamics should be also treated in a self-consistent manner. In this paper we describe the numerical code for the self-consistent 3D beam-beam simulations including beamstrahlung and the longitudinal beam coupling impedance and study interplay of different effects arising in beam-beam collisions of the future colliders.

Electron bunch compressors formed of achromat arcs have a natural advantage over the more commonly used chicane compressors in that linearization of the longitudinal phase space is of the correct sign to compensate for the curvature imprinted by rf acceleration. Here we extend the utility of arc compressors to enable variation of the longitudinal compaction within a fixed footprint. We also show that this variability can be achieved independently order by order in momentum deviation. The technique we employ consists of additional dipoles, leading to the advantageous property that variability can be achieved without incurring significant penalty in terms of chromatic degradation. We show this by comparison to an alternative system where additional quadrupoles are utilized to enable variation of momentum compaction. Each of these alternative approaches are being considered in the context of an upgrade of the MAX IV linac, Sweden, to enable a soft x-ray free-electron laser in addition to its existing functions.

A proposed experiment to demonstrate optical stochastic cooling (OSC) in the Cornell Electron Storage Ring (CESR) based on an arc-bypass design is presented. This arc bypass provides significantly longer optical delay than the dog-leg style chicane, opening up the possibility of a multipass or staged optical amplifier that can achieve the gains required for effective cooling of hadron or heavy ions. Beyond introducing the arc bypass, in this paper we study the stability requirements for the dipoles comprising it and investigate the use of an optical feedback system to relax the dipole and light-path stability tolerances.

High-brightness electron bunches, such as those generated and accelerated in free-electron lasers (FELs), can develop small-scale structure in the longitudinal phase space. This causes variations in the slice energy spread and current profile of the bunch which then undergo amplification, in an effect known as the microbunching instability. By imposing energy spread modulations on the bunch in the low-energy section of an accelerator, using an undulator and a modulated laser pulse in the center of a dispersive chicane, it is possible to manipulate the bunch longitudinal phase space. This allows for the control and study of the instability in unprecedented detail. We report measurements and analysis of such modulated electron bunches in the 2D spectrotemporal domain at the Fermi FEL, for three different bunch compression schemes. We also perform corresponding simulations of these experiments and show that the codes are indeed able to reproduce the measurements across a wide spectral range. This detailed experimental verification of the ability of codes to capture the essential beam dynamics of the microbunching instability will benefit the design and performance of future FELs.

The U.S. electron ion collider will utilize high current electron and ion storage rings with many bunches and large rf systems. Because of the dissimilarity of the two rings, the rf transients created by gaps or variations in the current distributions will be very different in the two rings. These transients cause a shift in the synchronous phase of the beams as a function of rf bucket position, can impact the luminosity through shifts in longitudinal position of the IP, will affect the performance of the rf and LLRF control loops, and may require significant rf power overhead to control. A machine design that uses superconducting crab cavities will also have sensitivity to gap transients and synchronous phase variations along the bunch train with variations in crab cavity voltage seen by each bunch, since the high $Q$ of the crab cavities precludes modulating them to compensate for the time of arrival shifts caused by the gap transients in the main rf systems. All these effects make the problem of managing gap transients crucial to the operation of the EIC. This work presents methods to study the dynamics of the rf and LLRF systems for these heavily beam loaded facilities. An illustrative machine design example is presented and used to investigate the expected magnitudes of the rf gap transients, and exploration of various possible remedies to match the gap transients in the two dissimilar EIC rings. In addition to the study of the power required and gap transients, this work also estimates longitudinal coupled-bunch instabilities due to the baseline cavity fundamental impedance. The work is motivated to emphasize the importance of tools and methods to estimate these effects as part of the early design phase of the Electron-Ion Collider or any high current storage ring design.

We model the effect of ground movement, based on empirical experience, on the transport properties of long neutron guides by ray-tracing simulations. For a simple model, our results reproduce the large losses found by an earlier study, while for a more realistic engineering model of guide mounting, we find the losses to be significantly smaller. A detailed study of the guide for the cold neutron spectrometer BIFROST at the European Spallation Source shows that a realistic loss value is 7.0(5)% for wavelengths of 2.3–4.0 Å, the typical operational wavelength range of the instrument. This amount of loss does not call for mitigation by overillumination as suggested in the previous work. Our work serves to quantify the robustness of the transport properties of long neutron guides, in construction or planning at neutron facilities worldwide.

${\mathrm{Cs}}_x{\mathrm{K}}_y\mathrm{Sb}$ photocathodes grown on GaAs and molybdenum substrates were evaluated using a $-300\mathrm{kV}$ dc high voltage photogun and diagnostic beam line. Photocathodes grown on GaAs substrates, with varying antimony layer thickness (estimated range from $<20\mathrm{nm}$ to $>1\mathrm{um}$), yielded similar thermal emittance per rms laser spot size values ($\sim 0.4\mathrm{mm}\mathrm{mrad}/\mathrm{mm}$) but very different operating lifetime. Similar thermal emittance was obtained for a photocathode grown on a molybdenum substrate but with markedly improved lifetime. For this photocathode, no decay in quantum efficiency was measured at 4.5 mA average current and with peak current 0.55 A at the photocathode.

The typical treatment of time-dependent potentials, such as those used for radio frequency cavities, is to average a potential’s time component through the interval that a reference particle spends in the cavity. Such an approach, using the so-called transit-time factor, uses time as the independent variable in the Hamiltonian. In this paper, we instead propose a fully covariant Hamiltonian to treat a potential’s time component like any other space component. We show how to calculate the dynamics of particles in a pill-box cavity using an explicit symplectic integrator. Finally, we compare the results with the simulator tracewin.

A linear transformer driver (LTD) generator capable of delivering up to 0.9 MA current pulses with 160 ns rise time has been assembled and commissioned at University of California San Diego. The machine is an upgrade of the LTD-III pulser from Sandia National Laboratories, consisting of 40 capacitors and 20 spark gap switches, arranged in a 20-brick configuration. The driver was modified with the addition of a new trigger system, active premagnetization of the inductive cores, a vacuum chamber with multiple diagnostic ports, and a vacuum power feed to couple the driver to plasma loads. The new machine is called compact experimental system for $Z$-pinch and ablation research (CESZAR). The driver has a maximum bipolar charge voltage of $\pm 100\mathrm{kV}$, but for reliability and testing, and to reduce the risk of damage to components, the machine was operated at $\pm 60\mathrm{kV}$, producing 550 kA peak currents with a rise time of 170 ns on a 3.5 nH short circuit. While the peak current is scaled down due to the reduced charge voltage, the pulse shape and circuit parameters are close to the results of the cavity and power feed models but suggest a slightly higher inductance than predicted. The machine was then used to drive wire array $Z$-pinch and gas puff $Z$-pinch experiments as initial dynamic plasma loads. The evolution of the wire array $Z$ pinch is consistent with the general knowledge of this kind of experiment, featuring wire ablation and stagnation of the precursor plasma on axis. The gas puff $Z$ pinches were configured as a single, hollow argon gas shell, in preparation for more structured gas puff targets such as multispecies, multishell implosions. The implosion dynamics agree generally with 1D magnetohydrodynamics simulation results, but large zippering and magneto-Rayleigh-Taylor instabilities are observed. The CESZAR load region was designed to accommodate many load types to be driven by the machine, which makes it a versatile platform for studying $Z$-pinch plasmas. The completion of the CESZAR driver allows plasma experiments on energy coupling from LTD machines to plasma loads, instability mitigation techniques and magnetic field distributions in $Z$ pinches, and interface dynamics in multispecies implosions.

X-ray free electron laser oscillators (XFELOs) are future light sources that produce fully coherent hard x-ray pulses. Based on the low-gain principle, the XFELO traps x-ray pulses in an optical cavity composed of multiple Bragg-reflecting mirrors that have high reflectivity in a bandwidth of about ten meV. The crystal mirrors exposed to intense x-ray beams in the optical cavity are subject to thermal deformations that would shift and distort the Bragg reflectivity curve. Therefore, the stability of the XFELO operation depends on the ability of the mirrors to maintain the Bragg reflection under such thermal load. A new approach was used to analyze the thermal load of the mirrors. The approach utilizes a dedicated Bragg reflection physical process in geant4 to obtain precise absorption information of the XFELO pulses in the crystal. Following transient thermal behavior, including single pulse and multiple pulse inputs, was analyzed by finite element analysis software based on the energy absorption information extracted from the geant4 simulation. It is shown that, for a typical XFELO pulse depositing about ten microjoules energy the over a spot of tens of micrometers in radius, the thermal relaxation time across the thickness is on tens of nanoseconds scale. In this situation, a simplified heat-load model is then developed to integrate the heat load in the XFELO. With the simplified model, the potential impact of the thermal load on the XFELO operation is estimated. When a large amount of heat remains in the crystal, the pulse energy drops significantly and has large oscillations due to negative feedback of the temperature change on the pulse energy.

Space charge is typically one of the performance limitations for the operation of high intensity and high brightness beams in circular accelerators. In the Proton Synchrotron (PS) at CERN, losses are observed for vertical tunes above $Q_y=6.25$, especially for beams with large space charge tune shift. The work presented here shows that this behavior is associated to structure resonances excited by space charge due to the highly symmetric accelerator lattice of the PS, typical for first generation alternating gradient synchrotrons. Experimental studies demonstrate the dependency of the losses on the beam brightness and the harmonic of the resonance, and simulation studies reveal the incoherent nature of the resonance. Furthermore, the calculation of the resonance driving terms generated by the space charge potential shows that the operational working point of the PS is surrounded by multiple space charge driven incoherent resonances. Finally, measurements and simulations on both lattice driven and space charge driven resonances illustrate the different behavior of the beam loss depending on the source of the resonance excitation and on the beam brightness.

Beam intensity, proton fraction, and beam emittance represent the most important characteristics of a beam extracted by a proton source. Beam parameters are strongly related to the parameters of the plasma from which the beam is extracted. The recent improvement of plasma diagnostics devoted to compact ion sources, and, in particular, the development of optical emission spectroscopy, allows an ever more reliable measurement of plasma parameters, also during source operations. In this paper, a zero-dimensional approach able to estimate the beam intensity and the ion fraction from plasma parameters in a proton source is introduced. The model is based on a simplified solution of the balance equation system in a hydrogen plasma achieved by including the electron density, the electron temperature, and the relative neutral abundance ratio $\mathrm{H}/{\mathrm{H}}_2$ within the equations. This approach permits one to reduce the balance equation system to four equations in four unknowns, namely, the ${\mathrm{H}}^{+}$, ${\mathrm{H}}_2^{+}$, and ${\mathrm{H}}_3^{+}$ density and the confinement time. The balance equation system is solved by means of a homemade numerical code. The model has been applied to the beam generated by the Proton Source for the European Spallation Source (PS-ESS), during its commissioning at Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali del Sud in Catania. The extracted current and ion fraction estimated by this approach have been compared with the experimental results obtained by the PS-ESS beam diagnostics. The proton confinement time estimated by this approach has been compared with the results of a theoretical model based on a diffusion model. The comparison demonstrates that the model has achieved a surprising capability of estimation. The paper discusses the main characteristics of the zero-dimensional approach together with its main advantages and limits. The perspectives for further improvement of the model are also discussed.

Operation of particle accelerators and correction of unavoidable magnet or alignment errors critically depend on the assessment of both global and local observables such as tune or resonance driving terms. With most of the observables being a sum of different error sources, careful disentangling is necessary in order to establish an optimal correction and allow for smooth operation. In the LHC, linear coupling has been proven to have a major impact on the beam dynamics and is taken to be one of the main sources of uncertainty when establishing corrections. In this paper an approach to evaluate the change of the Hamiltonian terms with linear coupling is presented. The validity of derived equations is demonstrated on a number of observables and benchmarked against simulations.

A 200 MeV, 10 mA, medium energy high intensity proton accelerator (MEHIPA) is proposed to form the second stage of the Indian accelerator-driven system program. The linac consists of a normal conducting 3 MeV radio frequency quadrupole (RFQ) and a 200 MeV superconducting section consisting of three different families of single spoke resonators (SSR-A, SSR-B, and SSR-C). Here we discuss a detailed physics design of MEHIPA, including electro-magnetic design, multipacting analysis, higher-order mode studies, and beam dynamics. We have designed a compact linac, by careful optimization of cavity $\beta$ for the superconducting section. We also performed detailed optimization studies on the linac lattice and transport line parameters, aimed at minimizing the emittance and beam halo. Reliability analysis and error studies were performed on this optimized linac, including various beam, cavity, and magnet parameters, to estimate the operation tolerances, and the results are reported for a maximum emittance growth of 50%.