A passive method based on an optimized quasiachromatic condition effectively suppresses emittance variation during the independent tuning of insertion devices (IDs), which is one of the major obstacles for conducting precise experiments in extremely low emittance light source storage rings. This method, simple and easy to apply to any light source storage ring, reduces the emittance variation to potentially less than 1% by carefully optimizing dispersion leakage at the straight sections where the IDs are installed according to an actual condition of the ID independent tuning.

The more than 45-year-old GSI-UNILAC (universal linear accelerator) as well as the heavy ion synchrotron SIS18 will serve as a high-current heavy ion injector for the new Facility for Antiproton and Ion Research (FAIR) synchrotron SIS100. UNILAC will serve together with the SIS18 as FAIR injector for short intense pulse operation; the newly developed heavy ion linac Helmholtz linear accelerator shall deliver cw beams, still required for the comprehensive research program at GSI. In the context of an advanced machine investigation program in combination with the ongoing UNILAC upgrade program, a dedicated beam investigation campaign has been carried out in order to characterize front-to-end high-current medium and heavy ion beam behavior at GSI-UNILAC. A very high beam brilliance at the end of transfer line from UNILAC to SIS18 was achieved recently for medium heavy ion beams in a machine experiment campaign. This is an important step paving the way to fulfill the FAIR heavy ion high-intensity beam requirements. Results of high-current uranium beam measurements applying the novel technique of pulsed hydrogen gas stripping (at $1.4\mathrm{MeV}/\mathrm{u}$) will be presented as well.

For some synchrotron light source beamline applications, a round beam is preferable to a flat one. A conventional method of obtaining a round beam in an electron storage ring is to shift its tune close to a linear difference resonance. The linearly coupled beam dynamics is analyzed with perturbation theories, which have certain limitations. In this paper, we adopt the solution by linear matrices (SLIM) analysis to calculate exact beam sizes to design round beam lattices. The SLIM analysis can deal with a generally linearly coupled accelerator lattice. The effects of various coupling sources on beam emittances and sizes can be studied within a self-consistent frame. Both the on- and off-resonance schemes to obtain round beams are explained with examples. The SLIM formalism for two widely used magnet models: combined-function bending magnets, and planar wigglers and undulators, is also derived.

We present the technical and engineering design of a medium energy (10 MeV) and high average power (1000 kW) electron-beam accelerator intended for irradiation treatment of high-volume industrial and municipal wastewater. The accelerator uses a ${\mathrm{Nb}}_3\mathrm{Sn}$ superconducting radio-frequency (SRF) cavity for producing the high average beam power with $>90\%$ rf to beam efficiency. The design of the accelerator is tailored for industrial settings by adopting the cryocooler conduction-cooling technique for the SRF cavity instead of a conventional liquid helium bath cryosystem. The technical design is supplemented with a detailed analysis of capital and operating cost of the accelerator. The designed accelerator can treat up to 12 million gallons per day of wastewater, requires capital of $\sim \$8\mathrm{M}$ for construction, and has $\sim 13.5¢/\mathrm{ton}/\mathrm{kGy}$ in material processing cost.

The Separator for Capture Reactions (SECAR) is a next-generation recoil separator system at the Facility for Rare Isotope Beams (FRIB) designed for the direct measurement of capture reactions on unstable nuclei in inverse kinematics. To maximize the performance of this system, stringent requirements on the beam alignment to the central beam axis and on the ion-optical settings need to be achieved. These can be difficult to attain through manual tuning by human operators without potentially leaving the system in a suboptimal and irreproducible state. In this work, we present the first development of online Bayesian optimization with a Gaussian process model to tune an ion beam through a nuclear astrophysics recoil separator. We show that this method achieves small incoming angular deviations ($<1\mathrm{mrad}$) in an efficient and reproducible manner that is at least 3 times faster than standard hand-tuning. Additionally, we present a Bayesian method for experimental optimization of the ion optics, and show that it validates the nominal theoretical ion-optical settings of the device, and improves the mass separation by 32% for some beams.

In previous publications, the effect of inhomogeneous beam loading of Landau cavities due to a nonuniform fill pattern in a storage ring has been studied with the aim of predicting the resulting longitudinal profiles of the bunches and their time offsets relative to the main rf [T. Olsson et al., Phys. Rev. Accel. Beams 21, 120701 (2018)]. This work was extended to treat dipolar coupled-bunch modes driven by beam-excited higher-order modes in the rf cavities [F. J. Cullinan et al., Phys. Rev. Accel. Beams 23, 074402 (2020)]. These coupled-bunch modes are of interest because they can become unstable, leading to an increase in the energy spread. The theory has now been extended once again to cover the case of coupled-bunch quadrupole modes where it is the lengths of the bunches that are oscillating, not their centroids. The theory is outlined and its predictions are thoroughly benchmarked against predictions from macroparticle tracking. Observations of the effects of nonuniform fill patterns on coupled-bunch quadrupole instabilities are made and interpreted. Results of measurements at the MAX IV 3 GeV ring, interpreted using theoretical calculations, are then presented to continue the investigation.

Utilizing noninvasive 2D scans with laser wires, we tomographically reconstructed the 4D transverse phase space distribution of a 1-GeV hydrogen ion (i.e., ${\mathrm{H}}^{-}$) beam during operation. The 4D tomography is based on two new advances. First, we extended the formulation of maximum entropy phase space tomography to take 2D projections as input and derived theoretical results relevant to our application. Second, we introduced the method of “perpendicular scans” to obtain cross-plane information from a laser wire emittance scanner. Perpendicular scans are two unconventional 2D measurements performed with perpendicular front and back wires. In contrast, only parallel front and back wires (i.e., “parallel scans”) are utilized in ordinary measurements of the horizontal and vertical 2D phase space distributions. When we applied the technique to the laser emittance station in the high-energy beam transport of the Spallation Neutron Source, experimental results showed that perpendicular scans can provide significant new information to the reconstructed 4D phase space distribution.

A precise determination of the $\beta$ function at different locations of an accelerator is essential to allow accurate optics correction and to ensure high machine performance. The $\beta$ function can only be measured directly at locations where beam position monitors are installed. For other locations, we rely on a $K$-modulation technique. However, this technique presents some limitations resulting in imprecise $\beta$ values when the phase advance between the modulated quadrupole and the observation point is separated by 90°. To mitigate these limitations, we have introduced the same phase advance as an additional constraint in the $K$-modulation algorithm. In this paper, the improvement of the $\beta$ measurement uncertainty is quantified for different optics configurations for both, the LHC and the proton synchrotron (PS) booster. The new algorithm is used to reanalyze measured data during van der Meer scans for luminosity calibration providing significantly more accurate results than obtained previously. Moreover, in the PS booster, this improvement has also reduced the uncertainty of the $\beta$ function at different locations of the machine.

This paper presents the design, fabrication, and low-power rf measurement of an externally powered $X$-band dielectric-loaded accelerating (DLA) structure with a dielectric constant ${ϵ}_{\mathrm{r}}=16.66$ and a loss tangent $\tan \delta =3.43\times {10}^{-5}$. A dielectric matching section for coupling the rf power from a circular waveguide to an $X$-band DLA structure consists of a compact dielectric disk with a width of 2.035 mm and a tilt angle of 60°, resulting in a broadband coupling at a low rf field which has the potential to survive in the high-power environment. Based on simulation studies, a prototype of the DLA structure was fabricated. Results from low-power rf measurements and the comparison with simulated values are presented. A significant discrepancy was found between measurements and simulations. The detailed analysis on the fabrication errors which may cause the discrepancy is also discussed.

The Advanced Photon Source (APS) team is building a fourth-generation storage ring (4GSR), replacing the present double-bend achromat lattice with a multibend achromat system thereby allowing the production of ultrabright x-ray beams. The new lattice enables a 2-order-of-magnitude reduction in horizontal beam emittance and a factor of two increase in beam current. The result is an electron beam of very high energy and power densities. Initial predictions suggest many common ultrahigh-vacuum-compatible materials struck by the full-intensity electron beam will be damaged. Two experimental beam abort studies have been conducted on collimator test pieces in the present APS SR to inform the design of a fully-functional machine protection system for APS 4GSR operations at 200 mA. A comprehensive suite of diagnostics was utilized during the studies. The diagnostics used in these experiments are not new, but employed in different ways to obtain unique data sets. With these sets now in hand, we are developing new numerical tools to guide collimator design using pelegant [M. Borland, elegant: A flexible SDDS-compliant code for accelerator simulation, Technical Report No. LS-287, Advanced Photon Source, 2000; Y. Wang and M. Borland, Implementation and performance of parallelized elegant, in Proceedings of the 2007 Particle Accelerator Conference, http://cern.ch/AccelConf/p07/PAPERS/THPAN095.PDF, pp. 3444–3446], mars [N. V. Mokhov and S. I. Striganov, Fermilab-conf-07/008-ad, AIP Conf. Proc. 896, 50 (2007)], and flash [B. Fryxell et al., flash: An adaptive mesh hydrodynamics code for modeling astrophysical thermonuclear flashes, Astrophys. J. Suppl. Ser. 131, 273 (2000); P. Tzeferacos et al., Laboratory evidence of dynamo amplification of magnetic fields in a turbulent plasma, Nat. Commun. 9, 591 (2018)].

In order to optimize the design of undulators using high-temperature superconductor (HTS) bulks we have developed a method to estimate the critical current density ($J_c$) of each bulk from the overall measured magnetic field of an undulator. The vertical magnetic field was measured along the electron-beam axis in a HTS bulk-based undulator consisting of twenty Gd-Ba-Cu-O (GdBCO) bulks inserted in a 12-T solenoid. The $J_c$ values of the bulks were estimated by an inverse analysis approach in which the magnetic field was calculated by the forward simulation of the shielding currents in each HTS bulk with a given $J_c$. Subsequently the $J_c$ values were iteratively updated using the precalculated response matrix of the undulator magnetic field to $J_c$. We demonstrate that it is possible to determine the $J_c$ of each HTS bulk with sufficient accuracy for practical application within around 10 iterations. The precalculated response matrix, created in advance, enables the inverse analysis to be performed within a practically short time, on the order of several hours. The measurement error, which destroys the uniqueness of the solution, was investigated and the points to be noted for future magnetic field measurements were clarified. The results show that this inverse-analysis method allows the estimation of the $J_c$ of each bulk comprising an HTS bulk undulator.

The advent of x-ray free-electron lasers (FELs) drastically enhanced the capabilities of several analytical techniques, for which the degree of transverse (spatial) coherence of the source is essential. FELs can be operated in self-amplified spontaneous emission (SASE) or seeded configurations, which rely on a qualitatively different initialization of the amplification process leading to light emission. The degree of transverse coherence of SASE and seeded FELs has been characterized in the past, both experimentally and theoretically. However, a direct experimental comparison between the two regimes in similar operating conditions is missing, as well as an accurate study of the sensitivity of transverse coherence to key working parameters. In this paper, we carry out such a comparison, focusing in particular on the evolution of coherence during the light amplification process.

Vector field analysis is presented for a highly overmoded iris-line structure that can act as a medium of transportation for THz radiation. The axisymmetric structure is capable of supporting hybrid modes with desirable features such as low propagation loss, uniformly linear polarization and approximately Gaussian intensity profile across the iris. A specific application that can benefit from these desirable features is the transportation of THz undulator radiation over hundreds of meters to reach the experimental halls at the Linac Coherent Light Source (LCLS) facility at SLAC, Stanford. Such a structure has been modeled before as a boundary-value problem using Vainstein’s complex-impedance boundary condition and assuming infinitely thin screens. Given that physical realizations of such screens must have finite thickness and that the THz wavelength in the 3–15 THz range is expected to be smaller than convenient screen thicknesses in practice, the question of the impact of finite screen thickness on propagation performance becomes rather pressing. To address this question, we present a mode-matching analysis of the structure as an open resonator with finite screen thickness and perturbatively clustered (localized) field expansions, for computational feasibility. The effect of screen thickness is seen to lower the attenuation constant on the iris line, which is dominated by diffraction loss. Ohmic loss due to the finite conductivity of metallic surfaces at the screen edges is found to be small compared to diffraction loss. The propagation loss predictions based on the Vainstein model are compared with the numerical results from mode matching for infinitely thin screens, where the former method is observed to agree with the numerical results better at higher Fresnel numbers (highly overmoded structures). The properties of the dominant mode fields are formally derived from first principles and a recommended approach is discussed for the inclusion of screen-thickness effects into propagation loss estimations.

A continuous system of moment equations is introduced that models the transverse dynamics of a beam of charged particles as it passes through an arbitrary lattice of quadrupoles and solenoids in the presence of self-fields. Then, figures of merit are introduced specifying system characteristics to be optimized. The resulting model is used to optimize the parameters of the lattice elements of a flat to round transformer with self-fields, as could be applied in electron cooling. Results are shown for a case of no self-fields and two cases with self-fields. The optimization is based on a gradient descent algorithm in which the gradient is calculated using adjoint methods that prove to be very computationally efficient. Two figures of merit are studied and compared: one emphasizing radial force balance in the solenoid, the other emphasizing minimization of transverse beam energy in the solenoid.

An eight-cavity, 1.3 GHz, SLAC linac coherent light source II High Energy cryomodule was assembled and tested at Fermilab to verify performance before the start of production. Its cavities were processed with a novel nitrogen doping treatment to improve gradient performance. The cryomodule was tested with a modified protocol to process sporadic quenches, which were observed in Linac Coherent Light Source II production cryomodules and are attributed to multipacting. Dedicated vertical test experiments support the attribution to multipacting. The verification cryomodule achieved an acceleration voltage of 200 MV in continuous wave mode, corresponding to an average accelerating gradient of $24.1\mathrm{MV}/\mathrm{m}$, significantly exceeding the specification of 173 MV. The average ${\mathrm{Q}}_0$ ($3.0\times {10}^{10}$) also exceeded its specification ($2.7\times {10}^{10}$). After processing, no field emission was observed up to the maximum gradient of each cavity. This paper reviews the cryomodule performance and discusses operational issues and mitigations implemented during the several month program.

Phase-space partitioning offers an attractive path for the precise tailoring of complex dynamical systems. In beam physics, the proposed approach involves (i) producing beams with cross-plane correlations to control kinematic-moment invariants known as eigenemittances and (ii) mapping them to invariants of motion associated with given degrees of freedom via a decoupling transformation. Here we report on the direct experimental demonstration of the mapping of eigenemittances to transverse emittances for an electron beam. Measured phase space density confirms the generation of beams with asymmetric transverse emittance ratio $>200$ consistent with the initiated eigenemittance values. In addition, the bunches were produced via photoemission in the space-charge-driven expansion regime ultimately resulting in the generation of uniformly charged three-dimensional ellipsoidal bunches. The ellipsoidal character of the distributions is experimentally confirmed.

The Hamiltonian describing particle motion in an accelerator belongs to a large class of systems, the members of which can be integrated with a new set of high order symplectic integrators. One benefit of these integrators is their strong numerical stability, which results from the inclusion of only forward integration steps, independent of the order of accuracy. Using these integrators, the transfer map of any multipolar accelerator magnet is derived and presented here. From these maps, the Hamiltonian flow in different lattices is simulated and benchmarked against other well established integration schemes in the accelerator community. By comparing quantities such as the linear phase advance and action invariant, the chromaticity, as well as the working point and the tune spread with amplitude, the superiority of the novel symplectic integrators is demonstrated with respect to accuracy and integration cost.

Space-charge effects play an important role in high intensity and high brightness particle accelerators. These effects were generally studied self-consistently by solving the Vlasov-Poisson equations using a particle-in-cell method in the accelerator community. In this paper, we propose an alternative method to simulate the space-charge effects in the accelerator. Instead of solving the Vlasov-Poisson equations, the proposed approach solves the Schrödinger-Poisson equations to study the space-charge effects in accelerators. Using a quantum Schrödinger approach reduces the original problem from six- or four-dimensional phase space down to three or two spatial dimensions. It also provides a possibility to simulate accelerator beam physics on quantum computers by evolving the wave function through quantum gates. Benchmarks of a coasting proton beam through a focusing drift defocusing drift lattice show excellent agreement between the quantum Schrödinger method and the particle-in-cell method.