Vacuum chambers of particle accelerators are typically equipped with radio-frequency couplers. The couplers are employed to excite modes for particle acceleration, to extract the energy of higher-order modes, or for diagnostic purposes. From a network theory perspective, these couplers represent terminal ports by which means the structure can exchange energy with its exterior. Usually, these ports are terminated with fixed impedances corresponding to the characteristic impedances of the coaxial lines attached to them. In this paper, we investigate the influence of the termination conditions of vacuum chambers on beam coupling impedances. For this purpose, we introduce a novel approach that allows us to determine beam coupling impedances for arbitrary port terminations. A full-wave Maxwell solver is employed to determine a generalized scattering matrix of the vacuum chamber and its couplers terminated with prespecified reference impedances. Often, these impedances are chosen to be the characteristic line impedances of the waveguides so that coupler ports are free of reflection. Using the generalized scattering matrix, the beam coupling impedances can be readily determined by means of a computationally inexpensive postprocessing step that takes into account arbitrary impedance loads at the coupler ports. Thus, the influence of various port terminations on the beam coupling impedances can be conveniently examined. This is relevant to improve older structures that were designed when no sophisticated design tools were available or to improve the operation of existing structures for a purpose they were initially not designed for. Using the proposed approach, we investigate the 33-cell 200 MHz traveling-wave accelerating structures of the SPS at CERN. It is shown that port termination conditions do have an important influence on the beam coupling impedance and, therefore, must be taken into account in beam stability considerations.

A novel approach to detect anomalies in superconducting radio-frequency (rf) cavities is presented, based on the parity space method with the goal to detect quenches and distinguish them from other anomalies. The model-based parity space method relies on analytical redundancy and generates a residual signal computed from measurable rf waveforms. The residual is a sensitive indicator of deviation from the model and provides different signatures for different types of anomalies. This new method not only helps with detecting faults but also provides a catalog of unique signatures, based on the detected fault. The method was experimentally verified at the European X-ray Free Electron Laser (EuXFEL). Various types of anomalies incorrectly detected as quenches by the current quench detection system are analyzed using this new approach.

The operation of RHIC collider rings in polarized proton runs includes helical snakes, which allow for preserving polarization during acceleration to store energies. The RHIC lattice also includes spin rotators, operated when nonvertical polarization or corrections to the orientation of polarization at the interaction points are required. Utilization of OPERA field maps of snakes and rotators has been systematized in the past decade, in order to assess in detail the effects of these spin devices on beam polarization, and their perturbative effects on beam optics. The method is also used in ongoing studies regarding the future Electron Ion Collider, to permit increasing average store polarization to at least 70% at 275 GeV and the acceleration of polarized helion with low polarization losses. This paper reviews various applications and outcomes of these field map methods. It is thereby also a review of studies undertaken as part of beam polarization research activities at RHIC in recent years.

We compute a spectrum of parametric x-ray radiation inside a crystal from a bunch of electrons, which is periodically modulated in density. We consider that the bunch of electrons is exiting from an x-ray free-electron laser (XFEL) channel. We demonstrate that in the case of a resonance between the frequency of parametric x-ray radiation and a frequency of modulation of an electron bunch, the sequence of strong quasimonochromatic x-ray pulses is formed—superradiant parametric x-ray emission (SPXE) with frequencies that are multiples of the modulation frequency. The number of photons in the pulse of SPXE in the case of extremely asymmetric diffraction is comparable with the photon number in the pulse of an XFEL. Moreover, the SPXE is directed under the large angle to the electron velocity and every harmonic in the spectrum is emitted under its own angle.

Accelerators produce too many signals for a small operations team to monitor in real time. In addition, many of these signals are only interpretable by subject matter experts with years of experience. As a result, changes in accelerator performance can require time-intensive consultations with experts to identify the underlying problem. Herein, we focus on a particular anomaly detection task for radio-frequency (rf) stations at the SLAC Linac Coherent Light Source (LCLS). The existing rf station diagnostics are bandwidth limited, resulting in slow, unreliable signals. As a result, anomaly detection is currently a manual process. We propose a beam-based method, identifying changes in the accelerator status using shot-to-shot data from the beam position monitoring system; by comparing the beam-based anomalies to data from rf stations, we identify the source of the change. We find that our proposed method can be fully automated while identifying more events with fewer false positives than the rf station diagnostics alone. Our automated fault identification system has been used to create a new dataset for investigating the interaction between the rf stations and accelerator performance.

High-power particle accelerators are complex machines with thousands of pieces of equipment that are frequently running at the cutting edge of technology. In order to improve the day-to-day operations and maximize the delivery of the science, new analytical techniques are being explored for anomaly detection, classification, and prognostications. As such, we describe the application of an uncertainty aware Machine Learning method using the Siamese neural network model to predict upcoming errant beam pulses using the data from a single monitoring device. By predicting the upcoming failure, we can stop the accelerator before damage occurs. We describe the accelerator operation, related Machine Learning research, the prediction performance required to abort the beam while maintaining operations, the monitoring device and its data, and the uncertainty aware Siamese method and its results. These results show that the researched method can be applied to improve accelerator operations.

We describe a novel technique, based on image compression and machine learning, for transverse phase space tomography in two degrees of freedom in an accelerator beamline. The technique has been used in the CLARA accelerator test facility at Daresbury Laboratory: results from the machine learning method are compared with those from a conventional tomography algorithm (algebraic reconstruction) and applied to the same data. The use of machine learning allows reconstruction of the 4D phase space distribution of the beam to be carried out much more rapidly than using conventional tomography algorithms and also enables the use of image compression to reduce significantly the size of the data sets involved in the analysis. Results from the machine learning technique are at least as good as those from the algebraic reconstruction tomography in characterizing the beam behavior, in terms of the variation of the beam size in response to variation of the quadrupole strengths.

The eighth-order space-charge-induced resonance $8{\nu }_y=171$ was identified as the source of beam losses and suppressed in the main ring synchrotron of the Japan Proton Accelerator Research Complex. Identification and interpretation were performed using investigations of the lost particles, Poincaré map simulations, and analytical calculations. In addition, the dependencies of the simulated beam loss on the vertical phase advance and resonance potential ${\widehat{U}}_{0,8,171}$ were very similar. Besides validating our assumption, this relationship indicates that the vertical phase advance can be exploited for controlling the beam loss. We, therefore, developed new beam optics by changing the vertical phase advance and successfully reduced the resonance width by a factor of 4. In an experimental study, the new optics reduced the beam loss during the beam injection period by 20% from that of the present optics.

We present the design, fabrication, and cold testing of a THz-driven field emission electron gun. The gun is designed as a two-cell standing-wave structure with a copper tip inserted halfway into the first cell that serves as the field emission source. It is powered by a 110 GHz gyrotron and is expected to produce 51 fC, 360 keV electron bunches with an input power of 500 kW. Several gun structures were fabricated using a high precision diamond turned mandrel and copper electroforming. The frequencies of the cavity resonances were mechanically tuned using azimuthal compression.

To reduce the size of a rotating gantry for a particle therapy system, it is desirable to reduce the size of the scanning system by substituting separated scanning magnets with a combined-function scanning magnet. A two-dimensionally designed scanning magnet with distributed winding coils (DW-SCM) is proposed. The DW-SCM can generate a rotating dipole magnetic field using a single power supply. The two-dimensional static magnetic field distributions of the DW-SCM were calculated using poisson superfish code and compared with those of an octupole scanning magnet (CW-SCM) and cos-theta-type scanning magnet ($\mathrm{Cos}\theta$-SCM) with the same bore diameter of 98 mm. The calculated two-dimensional magnetic field showed that the DW-SCM had the highest magnetic field strength of the three types of scanning magnets, and compared with the CW-SCM, the DW-SCM was 129% higher and the $\cos \theta$-SCM was 72% higher. The good field region (radius) of field homogeneity within $\pm 0.5\%$ of the DW-SCM had a symmetrical shape centered on a current phase of 30°, and the minimum value was 24 mm. Also compared with the CW-SCM, the minimum good field region was 11% higher for the DW-SCM and 65% higher for the $\cos \theta$-SCM. The results indicate that the proposed scanning magnet could generate a high-strength rotating dipole magnetic field, and it might be possible to reduce the size of a scanning system by substituting separated scanning magnets with the DW-SCM.

A compact $X$-band two-stage rf pulse compression system has been successfully designed, fabricated, and tested at Tsinghua $X$-band high-power test stand. The pulse compression system consists of a correction cavity chain, a first-stage, and a second-stage storage cavity. The correction cavity chain adopts a new design whose transmission loss and length are reduced by half compared with the old one. A detuning device is applied to the second-stage storage cavity so that the system can work in one-stage or alternatively two-stage compression mode. In the one-stage compression mode, a 150-ns, 70-MW flattop output, with a standard deviation of 1.5% in amplitude and 1° in phase, was generated with a gain factor of 3. In the two-stage compression mode, a first two-stage pulse compression experiment with correction cavities in the $X$ band was performed. A peak power of 320 MW was achieved with a gain factor of 9.7 and full-width at half-maximum pulse durations of 53 ns.

Accelerator performance often deteriorates with time during a long period of operation due to secular changes in the machine components or the surrounding environment. In many cases, some tuning knobs are effective in compensating the performance drifts and optimization methods can be used to find the ideal machine setting. However, such intervention usually cannot be done without interrupting user operation as the optimization algorithms can substantially impact the machine’s performance. We propose an optimization algorithm, Safe Robust Conjugate Direction Search, which can perform accelerator tuning while keeping the machine performance within a designated safe envelope. The algorithm builds probability models of the objective function using Lipschitz continuity of the function as well as characteristics of the drifts and applies to the selection of trial solutions to ensure the machine operates safely during tuning. The algorithm can run during normal user operation constantly, or periodically, to compensate for the performance drifts. Simulation and online tests have been done to validate the performance of the algorithm.

Highly overmoded iris-line structures carry desirable features for the efficient transportation of THz radiation over long distances. Previous studies have analyzed the iris line, modeled approximately as a long open-resonator structure with thin screens, using methods such as Vainstein’s impedance boundary condition or perturbative mode matching. The aim in those methods was to seek the eigensolution that represents the dominant (least lossy) propagation mode in a long iris-line structure at a steady state. In this paper, a forward-traveling wave analysis is presented, wherein a short THz pulse paraxially traverses the oversized structure cell by cell, including the transient regime. The iris line’s periodic discontinuities are analyzed in terms of forward-wave orthogonal mode decompositions, which are then used to build a “forward-scatter” model (matrix) for each cell. The presented approach predicts a diffraction loss behavior that agrees with that of the eigensolution found by the previous analytical methods for a long waveguide at the steady state but adds the ability to analyze waveguides of finite lengths, showing transients as they evolve and settle to the dominant mode down the line, and offers numerical implementation speeds that are typically an order-of-magnitude faster than those previously reported for the mode-matching method. This approach may also be easily extended to analyze iris lines with mechanical imperfections (e.g., misalignments or aperiodic defects), allowing for investigations of performance sensitivity against manufacturing tolerances.

We present the design of a cylindrical corrugated waveguide for use in the A-STAR accelerator under development at Argonne National Laboratory. A-STAR is a high gradient, high bunch repetition rate collinear wakefield accelerator that uses a 1-mm inner radius corrugated waveguide to produce a $90\mathrm{MV}{\mathrm{m}}^{-1}$, 180-GHz accelerating field when driven by a 10-nC drive bunch. To select a corrugation geometry for A-STAR, we analyze three types of corrugation profiles in the overmoded regime with $a/\lambda$ ranging from 0.53 to 0.67, where $a$ is the minor radius of the corrugated waveguide and $\lambda$ is the free-space wavelength. We find that the corrugation geometry that optimizes the accelerator performance is a rounded profile with vertical sidewalls and a corrugation period $p\ll a$. Trade-offs between the peak surface fields and thermal loading are presented along with calculations of pulse heating and steady-state power dissipation. In addition to the ${\mathrm{TM}}_{01}$ accelerating mode, properties of the ${\mathrm{HEM}}_{11}$ mode and contributions from higher order modes are discussed.

The use of high-field superconducting magnets has furthered the development of medical diagnosis, fusion research, accelerators, and particle physics. High-temperature superconductors enable magnets more powerful than those possible with Nb-Ti (superconducting transition temperature $T_c$ of 9.2 K) and ${\mathrm{Nb}}_3\mathrm{Sn}$ ($T_c$ of 18.4 K) conductors due to their very high critical field $B_{c2}$ of greater than 100 T near 4.2 K. However, the development of high-field accelerator magnets using high-temperature superconductors is still at its early stage. We report the construction of the world’s first high-temperature superconducting ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_x$ (Bi-2212 with $T_c$ of $\sim 82\mathrm{K}$) accelerator dipole magnet. The magnet is based on a canted-cosine-theta design with Bi-2212 Rutherford cables. A high critical current was achieved by an overpressure processing heat treatment. The magnet was constructed from a nine-strand Rutherford cable made from industrial 0.8 mm wires. At 4.2 K, it reached a quench current of 3600 A and a dipole field of 1.64 T in a bore of 31 mm. The magnet did not exhibit the undesirable quench training common in Nb-Ti and ${\mathrm{Nb}}_3\mathrm{Sn}$ accelerator magnets. It quenched a dozen times without degradation. The magnet exhibited low magnetic field hysteresis ($<0.1\%$) as measured by a cryogenic Hall sensor. It was fast cycled to 1.47 T at $0.54\mathrm{T}/\mathrm{s}$ without quenches. This work validates the canted-cosine-theta Bi-2212 dipole magnet design, illustrates the fabrication scheme, and establishes an initial performance benchmark.

Complex three-dimensional field maps can be efficiently denoised and compressed in data volume by high order single value decomposition. Extension of the method to inter/extrapolation and determination of basis function representation is reported here. Inter/extrapolation of denoised and compressed maps is applied to spatial coordinates and especially to the parameters defining the geometry of devices creating such maps. It significantly increases the efficiency of the provision of compact and noise-free maps combined with high resolution. The method is illustrated by creating corresponding maps for rf cells of radio-frequency quadrupoles.

SwissFEL has a unique capability, among the normal conducting linac-based light sources, to simultaneously serve two separate undulator lines (Aramis and Athos) up to the machine repetition rate of 100 Hz using the double bunch operation mode. It increases twice the experiments throughput of the facility with modest additional investment. Two electron bunches spaced 28 ns apart are extracted from the cathode by two laser pulses with individually controlled repetition rates. The bunches are accelerated up to about 3 GeV in the main linac using the same rf macropulse. After separation, one bunch serves the Athos soft x-ray beamline and the other is further accelerated to serve the hard x-ray beamline – Aramis. A fast and high-stability beam kicker separates the two bunches without disturbing the electron beam and consequently the x-ray lasing. The timing and control system sets hybrid machine modes utilizing independent operation of the two undulator lines with individually programmed repetition rates. Beam diagnostics and feedback systems have to operate with two closely spaced bunches where the two beams share the same machine path. The low-level rf system manipulates the rf amplitude and phase within a fraction of the rf macropulse to provide decoupling of the acceleration parameters of the first and the second bunch. This manuscript presents measurements that show that the bunch separation does not degrade FEL lasing stability.

At the Photon Factory storage ring (PF ring) of the High Energy Accelerator Research Organization (KEK), a pulsed sextupole magnet (PSM) has been used for top-up injection for the first time. Following its successful operation at the KEK, this new injection scheme was expanded to one with a nonlinear kicker in BESSY II and was then applied at UVSOR and Aichi SR in Japan. Also, the PSM system in the PF ring was improved by introducing a rectangular shape for the magnet’s bore to strengthen the magnetic field. However, this upgraded PSM was not implemented for user operation because it induced a horizontal oscillation of the stored beam with a maximum amplitude of 5.0 mm. This oscillation was assumed to be caused by eddy currents originating from the uniform inner coating of the ceramic chamber. We, therefore, developed a modified ceramic chamber with a new patterned coating designed to suppress the eddy current–induced magnetic field. The pattern shape was optimized to suppress this magnetic field and, simultaneously, to reduce the resistive wall impedance. A fine-line pattern-coating process, originally developed for a ceramic chamber with an integrated pulsed magnet, was modified for application with the magnet’s chamber of the sextupole. The maximum horizontal oscillation amplitude was suppressed by a factor of 16 by using the new PSM chamber without any issues related to beam instability. Details of the historical background of the upgrading of the PSM system, an investigation of the cause of the abnormal oscillation problem, the development of the new process for coating the ceramic chamber, and the results of the beam-response measurements for the new ceramic chamber installed in the PF ring are reported in this article.