We present a method to actively outcouple radiation from an x-ray regenerative amplifier. The method uses an undulator within a cavity formed by the diamond mirror and is driven by a high repetition rate linac to build up a high power, narrow bandwidth seed within the cavity. Shaping the electron beam via laser heater modulation on a subsequent pass increases the bandwidth of the x-ray free-electron laser pulse beyond the bandwidth of the diamond mirrors, transmitting the radiation from the cavity. A fraction of the power from the shaped pulse is returned to the cavity, reducing the subsequent number of passes needed to rebuild the seed and increasing the duty factor. The cavity seeding may be used to produce short pulses that are significantly more stable than those produced via self-amplified spontaneous emission (SASE) or enhanced SASE.

In this paper, we present the effects of linear transverse-longitudinal coupling on beam size at the interaction point (IP) of a collider with a local crab crossing scheme, when time-dependent transverse deflection (crab kicks) and dispersive orbit intertwine near IP. The analytic propagation formula and the closed orbit form of the crab dispersion and momentum dispersion are derived. The nonzero momentum dispersion at crab cavities and the nonideal phase from crab cavities to IP are detailed with the derived propagation formula to predict the beam size distortion at IP with or without the beam-beam interaction. The linear results are compared with nonlinear simulation using the weak-strong beam-beam code.

The usage of optical transition radiation (OTR) for profile monitoring of relativistic electron beams is well known. This paper presents the case for beam diagnostic application of OTR for nonrelativistic ion beams. In addition to expected linearly polarized transition radiation in the plane of observation, a significant component of observed radiation is unpolarized. The unpolarized contribution increases as the target is tilted toward the grazing angles with its intensity, an order of magnitude higher than the polarized radiation. This unpolarized radiation is shown to have the characteristics of OTR and is qualitatively explained as the OTR generated on a rough target surface. This increase in observed radiation can be used advantageously for transverse profile measurements. Further systematic effects such as the dependence of light yield on beam current, comparison of the measured transverse profiles with secondary electron emission based grid, target heating, and prompt nature of observed radiation are reported.

We have developed a new chromatic compensation scheme for the electron storage ring with two low-beta interaction regions in the Electron-Ion Collider. The hybrid scheme consists of the modular chromatic matching of periodic systems and beamlines. The first-order chromatically matched solutions are linearly parameterized with the local linear chromaticities that control the higher order chromatic beatings. The parameterization enables an efficient optimization of dynamic aperture. As a result, we successfully achieve the 1% design criterion for the momentum aperture in the ring.

Understanding and controlling of nonlinear beam dynamics are essential to the performance of storage ring light sources. In order to measure and correct limitations due to nonlinear beam dynamics, an effective method is needed to excite the beam oscillation to large amplitudes. Autoresonance is a method that enables the control of the amplitude of a driven nonlinear oscillator by sweeping the frequency of the driver. In this study, autoresonant excitation of beam transverse oscillations in storage rings is studied. The threshold of the drive amplitude for autoresonance is theoretically obtained, for cases with or without damping. The results are in good agreement with simulations for a simple storage ring model, as well as for models of actual storage rings. Application of the theory to experimental data on the SPEAR3 storage ring also validates the results.

In an electron storage ring, the emittances are determined by the beam energy and the linear lattice; the effects of lattice nonlinearity are usually negligible. In this study, we show a case where the vertical emittance has a strong quadratic dependence on the vertical chromaticity, a form of lattice nonlinearity. In this case, a crab cavity is present in the lattice, which couples the longitudinal and vertical motions of the beam. A theoretic analysis is conducted to derive the emittance dependence on chromaticity, using the normal form approach to decouple the longitudinal and vertical motion. Particle tracking simulations are performed to verify the analytic results and a good agreement is found. The dependence of equilibrium emittances on nonlinear lattice properties as demonstrated in this paper may provide new possibilities for design options and machine control.

This article presents a reliability analysis method and a series of case studies of different architectures for interlock systems of large and complex research infrastructures. Interlock systems play a crucial role in the protection of different types of machines, including present and future particle accelerators and fusion experiments. These infrastructures require multibillion Euros investments and accidents could cause irreparable damage. Protection systems are needed to prevent damage from an unintended release of large amounts of stored energy or power. Interlock systems define the signal exchange between the sensors that detect non-nominal conditions and the actuators that bring the machine into a safe state through a protection shutdown. The design of machine protection systems in general and interlock systems, in particular, is caught between the desired machine safety and machine availability, and the requirements vary between different infrastructures. For some infrastructures, interlock systems must be designed to strictly avoid unintentional shutdowns, as these can have a significant impact on the lifetime of vital equipment or their primary operational purpose. For others, unintentional shutdowns due to a failure in the interlock system are acceptable as long as their number is small compared to protection shutdowns caused by failures of other equipment, in order to maximize their scientific output. The case studies presented in the article compare different interlock architectures based on the probability of specific failure scenarios occurring.

Measuring the electron source size is essential for determining the emittance of synchrotron radiation. Pinhole imaging is the most common technique which measures the source profile in all transverse directions. The grating interferometry technique measures the coherence function of the x-ray beam, which indirectly determines the source size. The newly developed phase-space beam position and size monitor (ps-BPM) system provide information on electron source properties: position, angle, size, and divergence. These three techniques were used to determine the source size at a bend magnet beamline at the Canadian Light Source. In this work, we compare experimental results from these measurements at the same time, which is used as a cross-calibration procedure for each method.

Tuning machine parameters of particle accelerators is a repetitive and time-consuming task that is challenging to automate. While many off-the-shelf optimization algorithms are available, in practice their use is limited because most methods do not account for safety-critical constraints in each iteration, such as loss signals or step-size limitations. One notable exception is safe Bayesian optimization, which is a data-driven tuning approach for global optimization with noisy feedback. We propose and evaluate a step-size limited variant of safe Bayesian optimization on two research facilities of the PSI: (a) the SwissFEL and (b) HIPA. We report promising experimental results on both machines, tuning up to 16 parameters subject to 224 constraints.

Parametric x-ray radiation (PXR) is the quantum process analog to Laue diffraction where a pseudophoton is scattered out of a crystalline structure as a radiation photon. In this work, PXR emitted by electron beams generated in a compact laser-plasma accelerator and interacting afterward with a Si 220 crystal is observed in single-shot operations. The combination of laser wakefield acceleration and PXR is an efficient and table-top way to obtain monochromatic, pulsed, ultrashort, and stable emission of high-energy photons: to our knowledge such a complete set of performance parameters is new. Unlike other relevant radiation mechanisms, such as betatron radiation or incoherent bremsstrahlung, PXR is insensitive to the beam energy spread. With a long crystal, we demonstrate the stability of the PXR bandwidth toward shot-to-shot variations of the beam energy and divergence.

This study presents a general analysis of how the transverse rotational symmetry of a beam imposes equality constraints among transverse beam moments. Efficient analytic methods are developed to derive symmetry-imposed constraints among $k$th order moments in beams with continuous or $n$-fold rotational symmetry for arbitrary $k$ and $n$. The formalism also enables one to construct symmetry arguments based on how discrete and continuous rotational symmetries manifest themselves differently in terms of beam moments. Three case studies on beams with continuous, twofold, and threefold rotational symmetries are conducted. We prove that, regardless of their triangulation, beams with threefold rotational symmetry (e.g., from electron cyclotron resonance ion source) always have the same rms properties as beams with cylindrical symmetry. These counterintuitive results derived purely from symmetry considerations have clarified beam dynamics at the Facility for Rare Isotope Beams.

Experiments were performed at the Fermilab Accelerator Science and Technology facility to elucidate the effects of long-range wakefields in TESLA-type superconducting rf cavities. In particular, we investigated the higher-order modes (HOMs) generated in the eight, nine-cell cavities of a cryomodule (CM) due to off-axis steering with correctors located $\sim 4\mathrm{m}$ upstream of the CM. We have observed correlated submacropulse centroid slews of a few hundred microns and centroid oscillations at $\sim 240\mathrm{kHz}$ in the rf beam-position-monitor data after the CM. The entrance energy into the CM was 25 MeV, and the exit energy was 100 MeV with $125\mathrm{pC}/\mathrm{bunch}$ and $400\mathrm{pC}/\mathrm{bunch}$ in 50-bunch pulse trains. These experimental results were evaluated for machine learning training aspects which will be used to inform the commissioning plan for the Linac Coherent Light Source-II injector CM. With an entrance beam energy of $<1\mathrm{MeV}$ in this case (or for $\sim 6\mathrm{MeV}$ at the European X-ray Free-electron Laser injector), the HOM-effect mitigation would be particularly critical.

The generation of highly coherent radiation in a high repetition rate free electron laser driven by a superconducting linear accelerator has become a topic of growing interest. External seeding schemes like high-gain harmonic generation (HGHG) and echo-enabled harmonic generation are proven to be able to generate coherent radiation in the extreme ultraviolet and x-ray range. However, the repetition rate of current laser systems with sufficient power to modulate the electron beam is limited to the kilohertz range. Recently, to achieve seeding at a high repetition rate, an optical resonator scheme has been introduced to recirculate the radiation in the modulator to seed the electron bunches. In this paper, two harmonic optical klystron resonator configurations either starting with a seed laser or starting from shot noise are studied. With the harmonic optical klystron as the seeding source, the efficiency of harmonic radiation generation at a comparatively high harmonic can be significantly enhanced when compared with a standard single-stage HGHG. Simulation results show that highly coherent, stable, and high repetition rate pulses in the water window range could be generated by the proposed seeding schemes. Some practical considerations including beam energy chirp effects and the power density effects on mirrors are discussed.

An experimental study of the afterglow mode was performed with a third generation electron cyclotron resonance ion source, SECRAL-II (Superconducting ECR ion source with Advanced design in Lanzhou No. II), under double frequency heating. The experimental results show that intense pulsed beams of highly charged ions (e.g., $266\mathrm{e}\mu \mathrm{A}$ of ${{}^{129}\mathrm{Xe}}^{34+}$ and $169\mathrm{e}\mu \mathrm{A}$ of ${{}^{129}\mathrm{Xe}}^{38+}$) could be produced at high frequency ($24+18\mathrm{GHz}$) and high power ($\sim 8\mathrm{kW}$), even compared with the beam intensity records of SECRAL-II obtained in continuous wave (cw) mode at higher microwave frequency ($28+18\mathrm{GHz}$) and higher power ($\sim 10\mathrm{kW}$), the gain factor is also up to $\sim 3$. Meanwhile, it is found that the afterglow decay time in our study is much longer than that obtained with the second generation ECR ion sources typically operating at 10–18 GHz, and the corresponding peak duration is greater than 2 ms. This study provides a viable solution for heavy ion synchrotron accelerator complex such as High Intensity Heavy Ion Accelerator Facility project that requires intense pulsed beams of highly charged ions with long peak duration.

The evolution of two intense pulsed electron beams for an x-ray flash radiography application is characterized in a coupling of experimental and simulation studies. For this purpose, experiments were performed at Commissariat à l’Energie Atomique et aux Energies Alternatives with a new dual-pulse inductive voltage adder named “Mi2.” This experimental setup has been designed and built to produce two intense pulsed electron beams of 2 kA, 700 keV, and 80 ns full width at half maximum from a velvet cold cathode. The delay between the pulses ranges from tens of nanoseconds to a few microseconds. This study quantifies the effect of the plasma dynamics on the beam current of the second pulse, which represents an important parameter from the beam transport point of view. A multidimensional particle-in-cell simulation model is used to provide a numerical analysis in order to determine the influence of velvet plasma dynamics on the second pulse. Analysis of the experimental results indicates that the motion of the electron extraction zone within the expanding plasma remains lower than 1 mm for $3\mu \mathrm{s}$ delay, inducing a minor impact on the beam current extracted from the velvet cathode. Based on the developed model, this plasma expansion induces an increase of 5% for the current extracted from the second pulse with a delay of $1\mu \mathrm{s}$ for two pulses having the same voltage, which is consistent with linear induction accelerator injector parameters for multiple pulse use.

Brookhaven National Laboratory (BNL) and Thomas Jefferson National Accelerator Facility are collaborating on the design and construction of the next Electron Ion Collider (EIC) to be built at BNL. The EIC is a unique high-energy, high-luminosity, polarized electron-proton/ion collider. The EIC accelerator complex needs about 10 new rf and SRF systems with frequencies spanning 24 MHz to 1773 MHz, requiring more than 60 new high-power fundamental power couplers (FPCs). These couplers will operate in either pulsed mode or continuous wave mode with peak traveling wave power ranging from 10 s to 380 kW. Here we present our design for a 1-MW broadband rf window suitable for EIC rf and SRF systems with operating frequencies up to 591 MHz. This design takes advantage of the numerous synergies between the various rf and SRF systems to make it broadly applicable. The rf window design criteria are based on the requirement for the 591-MHz electron storage ring (ESR) SRF cavities, as it will operate at both the highest traveling wave power and the highest peak power over the EIC $\mathrm{rf}/\mathrm{SRF}$ complex. The results presented will detail the FPC power requirement, rf window choices, design criteria, and multiphysics performance in the most critical application, the ESR SRF cavity, and how the rf window design applies to other EIC $\mathrm{rf}/\mathrm{SRF}$ systems.

There has been a renewed interest in applying multiobjective (MO) optimization methods to a number of problems in the physical sciences, including to rf structure design. The results of these optimizations generate large datasets, which makes visualizing the data and selecting individual solutions difficult. Using the generated results, Pareto fronts can be found giving the trade-off between different objectives, allowing one to utilize this key information in design decisions. Although various visualization techniques exist, it can be difficult to know which technique is appropriate and how to apply them successfully to the problem at hand. First, we present the setup and execution of MO optimizations of one standing wave and one traveling wave accelerating cavity, including constraint handling and an algorithm comparison. In order to understand the generated Pareto frontiers, we discuss several visualization techniques, applying them to the problem, and give the benefits and drawbacks of each. We found that the best techniques involve clustering the resulting data first to narrow down the possible choices and then using multidimensional visualization methods such as parallel coordinate plots and decision maps to view the clustered results and select individual solutions. Finally, we give some examples of the application of these methods and the cavities selected based on arbitrary design requirements.

The study of collective beam instabilities has long been an important and active research topic in particle accelerators. The collective effects are typically divided into two categories: the short-range single-bunch instabilities and the long-range multibunch or multiturn instabilities. Robinson instability can be the mostly encountered long-range multiturn instability. We recently explored a collective multiturn instability driven by the short-range coherent undulator radiation in the laser cavity modulator of a steady-state microbunching (SSMB) storage ring. In this paper, we formulate and compare such an instability with the Robinson instability. Considering the radiation slippage effect and the finite duration, the functional form of the radiation wakes is essentially different from that of the Robinson case. We also relate a special case of the newly explored instability to the high-gain free-electron laser process and find that a similar cubic dispersion equation can be obtained. The discussions and comparisons presented in this paper may shed light on the underlying physical mechanisms of the collective beam dynamics in the laser cavity modulator of an SSMB storage ring.