Recent studies have demonstrated the effectiveness of feedback techniques in suppressing the bursting behavior caused by microbunching instabilities, leading to the stabilization of terahertz (THz) coherent synchrotron radiation (CSR) emission in storage rings. However, the use of feedback loops on the accelerator voltage without proper caution can lead to destabilization of the bunch position and result in beam losses, which limits the optimal feedback parameters. In this paper, we propose a solution to this limitation by introducing a second feedback loop on the phase of the accelerating voltage. The effectiveness of this strategy has been tested at the SOLEIL storage ring, representing a significant step forward in mitigating microbunching instability at high current charges.

A record accelerating rate was achieved earlier in standing wave (SW) SRF cavities when their shape was optimized for a lower peak surface magnetic field sacrificing the peak surface electric field. In view of new materials with higher limiting magnetic fields, expected for SRF cavities, in the first line of the ${\mathrm{Nb}}_3\mathrm{Sn}$, the approach to optimization of cavity shape should be revised. A method of equidistant optimization, offered earlier for traveling wave (TW) cavities is applied to SW cavities. It is shown here that without limitation by magnetic field, the maximal accelerating rate is defined not only by limitations of the electric field but to a significant degree by the cavity shape. For example, for a cavity with the aperture radius of $R_a=35\mathrm{mm}$, the minimal ratio of the peak surface electric field to the accelerating rate is about $E_{\mathrm{pk}}/E_{\mathrm{acc}}=1.54$. So, with the maximal surface field experimentally achieved $E_{\mathrm{pk}}\sim 125\mathrm{MV}/\mathrm{m}$, the maximal achievable accelerating rate is about $80\mathrm{MeV}/\mathrm{m}$ even if there are no restrictions by the magnetic field. Optimized cavity shapes with and without limitations by a magnetic field are presented. Another opportunity—optimization for a low magnetic field, is opening for the same material, ${\mathrm{Nb}}_3\mathrm{Sn}$, with the purpose of having a high quality factor and increased accelerating rate that can be used for industrial linacs with cryocooler-based cooling scheme.

Following the spectacular success of the third-generation light source over the past decades, a few new-generation light sources based on the multibend achromat (MBA) scheme have come into operation. Other such facilities are also under construction while existing ones are being upgraded. Likewise, the Swiss Light Source, which has been in operation for more than 20 years at the Paul Scherrer Institute, is to be upgraded with the present storage ring being replaced by an MBA low-emittance ring. A natural emittance of 158 pm at a beam energy of 2.7 GeV is achieved for a storage ring of comparatively short circumference, 288 m, providing a gross straight section of 83.6 m. The objectives of the upgrade were met under tight constraints by the application of two novel concepts: the introduction of both a longitudinal gradient and a reverse bend into the unit cell and pushing the superperiodicity to the number of arcs, effectively with “pseudosymmetry.” A detailed account of the MBA design and its expected performance is presented and the experience gained is highlighted with the aim to facilitate next-generation light source lattice design.

The LHC heavy-ion program with ${{}^{208}\mathrm{Pb}}^{82+}$ beams will benefit from a significant increase of the beam intensity when entering its high-luminosity era in Run 3 (2023). The stored energy is expected to surpass 20 MJ per beam. The LHC is equipped with a betatron collimation system, which intercepts the transverse beam halo and protects sensitive equipment such as superconducting magnets against beam losses. However, nuclear fragmentation and electromagnetic dissociation of ${{}^{208}\mathrm{Pb}}^{82+}$ ions in collimators generates a flux of secondary fragments, which are lost in downstream dispersion suppressor and arc cells. These secondary ions may pose a performance limitation in upcoming runs since they can induce magnet quenches. In order to mitigate this risk, an alternative collimation technique, relying on bent crystals as primary collimators, will be used in forthcoming heavy-ion runs. In this paper, we study the power deposition in superconducting magnets by means of tracking and fluka shower simulations, comparing the standard collimation system against the crystal-based one. In order to quantify the predictive ability of the simulation model, we present absolute benchmarks against beam loss monitor measurements from the 2018 ${{}^{208}\mathrm{Pb}}^{82+}$ run at $6.37Z\mathrm{TeV}$. The benchmarks cover several hundred meters of beamline, from the primary collimators to the first arc cells. Based on these studies, we provide a detailed analysis of ion fragmentation and leakage to the cold magnets and quantify the expected quench margin in future ${{}^{208}\mathrm{Pb}}^{82+}$ runs.

In the context of wave propagation in solids caused by particle-matter interactions, the composite structure of copper diamond is believed to have a significant impact on the material’s response. This limits the accuracy of isotropic homogeneous elastic and elastic-plastic models used in earlier studies modeling the material’s behavior under such conditions. This study aims to investigate the mesoscopic behavior of the copper diamond and discusses the advantages and limitations of modeling the internal composite structure of the material. The material response of CuCD was modeled in a 2D finite element simulation, considering internal wave propagation as a result of external impact and an internal thermal shock. Various homogeneous models were considered and compared with a mesoscopic model. The homogeneous models tested were found to be able to capture wave propagation effects in the material, and the inclusion of a hardening model allowed their performance to approach that of the mesoscale model considered, which is significantly more computationally demanding.

A relatively simple method of a guess of multipactor appearance on a cavity cell equator was described earlier in Phys. Rev. Accel. Beams 19, 102002 (2016). For this guess, evaluation of electric field components in two definite points near the equator is sufficient, it gives the value of a so-called geometrical parameter $p$ of the cell. If this parameter is big enough, for niobium superconducting cavities, it was found as $p>0.28÷0.3$, conditions for multipactor exist. However, this method works, as described, only for the inner cells of a multicell cavity or for single-cell cavities. An explanation is in the breakdown of symmetry of the electric field near the equator when the end cell consists of asymmetric half-cells and has a regular inner cell from one side and a beam pipe from another side. Multipactor in such cells shifts from the equator point. This paper is an addendum to the earlier presented paper, here it is shown how to find a correct value of the geometrical parameter $p$ taking the asymmetry into account.

The amplitude dependent tune shifts from either space charge or beam-beam interactions are calculated analytically with the inclusion of synchrotron oscillations and multiple interactions around the ring. Simpler formulas are derived under limits of bunches longer than the transverse sizes, equal and unequal transverse sizes, etc. This is used to derive semianalytical forms for the density distribution of the tune shifts. The tune spread and the density distribution are needed to understand beam decoherence or Landau damping with either interaction. The tune footprints due to space charge in IOTA are simulated using pyorbit and found to be in good agreement with the theoretical predictions.

$C$-band technology is emerging as an exciting innovative approach to the creation of compact new accelerators. Besides the possibility to sustain higher gradients and higher repetition rate operation at normal conducting temperature, it also allows one to increase the machine performance in terms of beam brightness. We propose the design study of a normal conducting, high gradient $C$-band injector aiming at the production of high brightness electron beams—up to $2.6\times {10}^3\mathrm{TA}/{\mathrm{m}}^2$—at high repetition rate—up to 1 KHz—as desired to enhance the capabilities of modern radiation sources. This paper reports on beam dynamics studies that guided the injector design looking for a good compromise between the machine compactness and performances. For this purpose, a new standing wave $C$-band gun consisting of 2.6 cells has been designed to enhance the final beam brightness together with its own mode launcher to ensure the needed peak field at the cathode—up to $180\mathrm{MV}/\mathrm{m}$. Several working points in terms of peak current, transverse emittance and brightness are explored, showing the flexibility allowed by the proposed photoinjector. The integration of the gun mode launcher, that relies on a four-port configuration, in the injector design and its effect on the beam quality are also reported.

Polarization switching of synchrotron radiation has wide application prospects in various areas, requiring a high switching frequency between horizontally linear polarization (HLP) and vertically linear polarization (VLP) or between left-hand circular polarization (LCP) and right-hand circular polarization (RCP). To make it possible for realizing a fast polarization switching, in this paper we simplify and incorporate the previous schemes based on segmentally helical undulators connected by phase shifters. Adjacent undulators are set by the same or opposite polarization state to generate circularly or linearly polarized radiation. Theoretical analysis and simulations for radiation characteristics are reported in details. By combining the polarization adjustment of each helical undulator with the phase shifters, both the switching for HLP/VLP and LCP/RCP can be realized in the same undulator scheme. The on-axis heat load can always be effectively suppressed. Benefiting from the extremely low emittance and taking into account the space occupied by phase shifters, the proposed scheme shows a significant advantage in preserving the photon flux with a high polarization degree in a diffraction limited storage ring compared with a typical third generation storage ring. In addition, we also study the influence of beam deviation at each undulator segment, the segment number limited by the practical length of straight section and the issues related to user experiments.

Beam stability is conventionally described by means of the stability diagram, which is a threshold line in a complex plane of a particular intensity parameter, the coherent tune shift; the diagram separates stable and unstable states. The incoherent tune spread of the beam particles normally causes the transverse stability diagram to be an asymmetric bell-shaped curve above the real axis; for a given optical nonlinearity, the diagram is determined by the beam distribution function only. Recently, such a diagram was measured by means of an antidamper, see Antipov et al., Phys. Rev. Lett. 126, 164801 (2021). Such measurements open the door to a new method of beam diagnostics, finding the beam phase space density as a solution of an inverse stability problem. The main idea, methods of its implementation, possible obstacles and optimizations are discussed.

We measured the surface resistance of titanium, amorphous carbon, and nonevaporable getter (NEG) coatings on copper samples, representative of surface treatments of beam-facing components in the High-Luminosity Large Hadron Collider at CERN. Measurements were done at room temperature as well as at liquid nitrogen temperature (77 K) by means of a novel large dielectric resonator operating at 3.4 GHz. The impact of a 100 to 400 nm titanium layer or a 50-nm amorphous carbon layer was negligible both at room temperature and down to 77 K in comparison to copper, while a thick (1140 nm) NEG layer showed a measurable increase in the surface resistance, being more significant at low temperature. The dielectric resonator proved to be a useful tool to characterize the surface resistance of flat samples with different surface treatments and sizes up to 10 cm for accelerator applications. Furthermore, its construction and operation are much simpler compared to other test devices currently in use, and it can provide accurate experimental data for the evaluation of the beam coupling impedance.

Particle-in-cell simulations are among the most essential tools for the modeling and optimization of laser-plasma accelerators, since they reproduce the physics from first principles. However, the high computational cost associated with them can severely limit the scope of parameter and design optimization studies. Here, we show that a multitask Bayesian optimization algorithm can be used to mitigate the need for such high-fidelity simulations by incorporating information from inexpensive evaluations of reduced physical models. In a proof-of-principle study, where a high-fidelity optimization with fbpic is assisted by reduced-model simulations with wake-t, the algorithm demonstrates an order-of-magnitude speedup. This opens a path for the cost-effective optimization of laser-plasma accelerators in large parameter spaces, an important step toward fulfilling the high beam quality requirements of future applications.

One of the fundamental challenges for future leptonic colliders and neutrino factories as well as for high-sensitivity studies of lepton universality is to design and construct new high-intensity sources of muons and positrons. The next-generation sources should increase the intensity of the presently operating ones by at least 3 orders of magnitude and include an important option of producing longitudinally polarized leptons. The main effort to achieve this goal has been focused, so far, on the proton-beam-driven muon sources and electron-beam-driven positron sources. In this paper, we present exploratory studies of an alternative scheme which is based on high-intensity megawatt-class photon beams. Such beams could be delivered in the future by the Gamma Factory (GF) project. One of the GF multiple goals is to increase the energy range (by more than one order of magnitude) and the intensity (by more than 6 orders of magnitude) of presently operating photon sources. Such a leap can be achieved by extending the present hadron-collider modus operandi of the LHC with a new GF-operation-mode, allowing the collisions of beams with laser pulses. The exploratory studies presented in this paper demonstrate that more than ${10}^{13}$ muons of both signs and more than ${10}^{16}$ electrons/positrons per second can be produced by a GF source.

Dynamic aperture (DA) is an important nonlinear property of a storage ring lattice, which has a dominant effect on beam injection efficiency and beam lifetime. Generally, minimizing both resonance driving terms (RDTs) and amplitude dependent tune shifts is an essential condition for enlarging the DA. In this paper, we study the correlation between the fluctuation of RDTs along the longitudinal position and the DA area with double-bend and multibend achromat lattices. The fluctuation of one RDT is quantitatively represented by the average RDT at the positions of nonlinear magnets. It is found that minimizing the RDT fluctuations is more effective than minimizing the commonly-used one-turn RDTs in enlarging the DA, and thus can serve as a very powerful indicator in the DA optimization. It is also found that minimizing low-order RDT fluctuations can also reduce both higher-order one-turn RDTs and higher-order RDT fluctuations, which also indicates that even higher-order RDTs can also be controlled due to the reduction of higher-order RDT fluctuations. This could be the underlying physics behind the strong correlation between minimizing the RDT fluctuations and enlarging the DA. And this also suggests that controlling lower-order RDT fluctuations in the DA optimization can avoid the need for calculating higher-order RDTs to a large extent, which are not only more computationally complicated but also more numerous. Besides, the effectiveness of controlling the RDT fluctuations in enlarging the DA confirms that the local cancellation of nonlinear effects used in some diffraction-limited storage ring lattices is more effective than the global cancellation.

The Future Circular Collider (FCC) $e^{+}{e}^{-}$ injector complex needs to produce and transport high-intensity $e^{+}$ and $e^{-}$ beams at a fast repetition rate for topping up at collision energy. Two options are considered for a preaccelerator ring, to be used for an intermediate accumulation and acceleration before the bunches are transferred to the high-energy booster. One option is a slightly modified Super Proton Synchrotron (SPS), and the other is designing a new ring. We explore the needs and parameters of the existing SPS as prebooster ring and also develop a conceptual design of an alternative accelerator ring. After establishing the basic parameters, we describe the optics design and layout and discuss the single-particle linear and nonlinear dynamics optimization, including magnetic and alignment errors. In addition, we present synchrotron radiation power studies and analytical estimates of various collective effects for both prebooster ring design options, including space charge, intrabeam scattering, longitudinal microwave instability, transverse mode coupling instability, ion effects, electron cloud, and coherent synchrotron radiation.

Imposing a density modulation on an electron beam may improve the brightness of a Thomson source by orders of magnitude via superradiant emission. In this paper, we analytically and numerically analyze electron beam modulation via the ponderomotive force due to the copropagating beat wave formed by two laser pulses at different frequencies. We show that energy modulation favorably scales with electron beam energy but is limited by the interaction length imposed by the finite waist of the laser pulses. Additionally, the effect of initial emittance and energy spread on the quality of microbunching is studied. Finally, we propose a superradiant extreme ultraviolet Thomson source based on ponderomotive bunching.

Pulsed power generators create high-energy-density conditions by rapidly delivering an immense pulse of electrical current to a compact imploding load. Accurately measuring the shape and amplitude of this load current pulse is essential to understanding the behavior of all pulsed power experiments. At the Z Pulsed Power Facility, the closest-in load current measurements are provided by velocimetry techniques such as VISAR (velocity interferometer system for any reflector) and PDV (photonic Doppler velocimetry). Here, fiber-coupled interferometers measure the velocity history of an exploding metallic flyer plate that is embedded in the vertical walls of the current return can. The flyer plate is driven outward by the magnetic pressure from the load current such that magnetohydrodynamic modeling can be used to determine the load current waveform from the measured velocity history. In this paper, we present the first load current velocimetry measurements to be made from the horizontal top flyer plate that carries current radially inward from the return can to the load. These spatially resolved measurements, which span $R=5–9\mathrm{mm}$, are enabled by a transformative new velocimetry diagnostic—a line-imaging velocity interferometer called Z Line VISAR (ZLV)—whose optical performance overcomes the measurement challenges presented by the steep velocity gradients encountered on the top flyer plate. To validate ZLV’s capabilities, a 14-MA, 100-ns experiment was conducted to losslessly couple current up the return can and radially inward across the top flyer plate. Comparisons between the ZLV data obtained from this experiment and two-dimensional magnetohydrodynamic simulations driven with the current measured on the return can indicate that the current delivery across the top flyer plate is indeed lossless to within the few-percent uncertainty of the ZLV data. Given that the current coupling is lossless, the experimental results are used to demonstrate that one-dimensional current unfold techniques can be applied to generate a radially resolved load current map from the ZLV velocity data. This analysis provides a template for how to use the ZLV diagnostic to determine the efficacy of current delivery in future experiments where losses may occur in close proximity to the load.

This paper presents a quantitative description of field emission in superconducting rf cavities. The goal is to fully reconstruct the physics involved in the phenomenon, from electron current generation at the emitter to the radiation pattern measured by the x-ray detector. The field emission process in the cavity was reconstructed using CST Particle Studio, and then the generation of x-ray radiation and its propagation toward the detectors were modeled analytically. The entire model was then applied to the specific case of the European Spallation Source medium beta superconducting cavities, and the theoretical predictions were cross-checked with experimental data collected during cavity vertical tests.

Applications of laser-plasma accelerators (LPA) require independent control of electron beam parameters. However, due to the complex coupling of the many variables governing the laser-plasma interaction, precisely tuning these parameters based on simple scalings is often impossible or at least suboptimal. Here, we apply multiobjective Bayesian optimization to derive optimal tuning curves for LPAs, both in simulations and experiments. For electron energies between 150 and 250 MeV, we demonstrate tuning of the charge over a range of nearly 100 pC, while preserving optimal beam loading conditions with energy spreads below 5%. The derived tuning curves can explain the sometimes counterintuitive interplay between laser and plasma control variables that is necessary to find the best trade-off between competing beam properties.

A novel method of measuring an electron beam’s energy spectrum and divergence using Compton-scattered x rays created by colliding a laser with an electron beam has been developed and tested using the compact laser-Compton x-ray source at LLNL. The method only requires an x-ray imaging device and a filter material whose $K$-edge energy matches that of the Compton-scattered x rays. $K$-edge filtering of energy-angle correlated Compton x rays causes large variations in intensity over the viewing angle. These intensity variations contain information about the electron beam’s properties. By using simulation tools and adjusting the beam parameters to match the shape of the acquired image, the electron beam can be characterized. As a demonstration of this technique, a $75\text{−}\mathrm{\mu }\mathrm{m}$ Sn foil was used to filter 30-keV Compton x rays created from Compton scattering of a $532\text{−}\mathrm{\mu }\mathrm{m}$ laser beam by a 30-MeV electron beam. The measured parameters were mean energy $E=28.51\pm 0.06\mathrm{M}\mathrm{eV}$, energy spread ${\sigma }_E<0.3\%$, and beam divergence ${\sigma }_{\theta }=1.8\pm 0.1\mathrm{m}\mathrm{rad}$.