In this work, we develop a machine learning (ML) model with aleatoric uncertainty for the low energy beam transport (LEBT) region of the LANSCE linear accelerator in which we model the transport of a space-charge-dominated 750 keV proton beam through a lattice of 22 quadrupole magnets. Our ML model is developed based on data generated by a Kapchinsky–Vladimirsky (KV) envelope model of beam transport. We show that a recurrent neural network can be used as a dynamical surrogate model for fast prediction of the LEBT beam envelope. Furthermore, we endow the model with the prediction of aleatoric uncertainty and compare three different approaches. We demonstrate that the ML-based uncertainty quantification models are well calibrated and produce good estimates of the regions where the model is less certain about its predictions. This ML framework is a necessary step in the development of a real-time virtual diagnostic tool with uncertainty quantification that can be integrated into more complex downstream tasks (e.g., adaptive control or learning flexible control policies via reinforcement learning) for improved efficiency in beam operations. In future work, we plan to expand on this preliminary study by considering more realistic envelope models that include longitudinal momentum spread and dispersive effects in bending magnets, as well as particle tracking codes with 3D space charge (such as hpsim and gpt).

The radio-frequency quadrupole (RFQ) linac is often used as the first radio-frequency (rf) structure for both bunching and preaccelerating proton or heavy ion beams directly behind an ion source or a low-energy beam transport section. Typically, RFQ linacs operate at frequencies below 450 MHz and beam velocity ranges from $\beta =0.01–0.08$. Taking the MYRRHA project as an example, a 704.4 MHz continuous-wave (cw) RFQ linac has been recently proposed to realize an efficient fourfold frequency jump at $\beta \approx 0.2$. Due to its high frequency, small dimensions, and cw operation mode, careful design studies for the RFQ cavity structure including the water-cooling channels have been further performed, integrating the idea of using additive manufacturing technology for future construction. Systematic rf and thermal simulations and analysis demonstrated the reliability of the 704.4 MHz RFQ linac at cw operation and the frequency tuning ability via water-speed adjustments. This investigation shows that it could be a highly promising solution to perform an efficient frequency jump with a high-frequency RFQ accelerator at $\beta \approx 0.2$ for large-scale hadron linacs, which could potentially shorten the low- and medium-$\beta$ parts of the whole facility.

A high-intensity muon source driven by a continuous-wave superconducting linac holds the potential to significantly advance the intensity frontier of muon sources. Alongside advancements in accelerator technologies, breakthroughs in muon production target and collection schemes are essential. After a brief introduction to the development of the accelerator-driven system superconducting linac, a novel muon production target is proposed, utilizing a free-surface liquid lithium jet capable of handling the heat power generated by a proton beam with an energy of 600 MeV and a current of 5 mA. It is predicted by our simulation studies that the lithium target is more efficient in surface muon production compared to the rotating graphite target. The parameter space of the front end consisting of a lithium target and a large-aperture capture solenoid is explored, from the perspective of production efficiency, capture efficiency, and characteristics of the surface muon beam.

This paper provides an overview of the worldwide first commissioning of a gantry beamline with a rotator at the MedAustron synchrotron-based proton/ion cancer therapy facility in Wiener Neustadt, Austria. The gantry beamline consists of the high energy beam transfer (HEBT) line and the gantry beam transport system. It transports the beam from the synchrotron to the gantry-room isocenter. The HEBT transports the beam from the synchrotron to the gantry entrance, which is the coupling point between the HEBT and the gantry. The rotator is one of the HEBT modules, thus it is an integral part of the gantry beamline. The MedAustron rotator is the worldwide first rotator system used to match slowly extracted asymmetric beams from the synchrotron to the rotating gantry. In this paper, main attention is paid to ion-optical and beam-alignment aspects of the beamline commissioning. A novel orbit-correction and beam-alignment technique has been developed specifically for the beamline with the rotator. While the theoretical concept of the rotator has existed for almost two decades, the MedAustron rotator is the first hardware implementation of this concept all over the world. The presented overview of the beamline commissioning includes a description of the principal technical solutions and main results of the first beam-transport measurements. Since the measured beam size and beam position agree well with theoretical predictions, one can conclude that the proof-of-concept of the rotator-matching has been successfully accomplished.

Taking advantage of a higher field than a conventional permanent magnet undulator, superconducting undulators (SCUs) have been rapidly developed in recent years for applications at synchrotron radiation sources and free-electron lasers. The Institute of High Energy Physics, Chinese Academy of Sciences, has developed a 1.5-m long planar SCU with a short period of 15 mm. This SCU has been commissioned and optimized in the horizontal position. In contrast to the vertical position, where the undulator coils are immersed in liquid helium for superconductivity, the horizontal test requires the assembly of the complete device, including the cryostat, current conductor, cryocooler, vacuum chamber, and undulator coils. This test reflects the operation status and measures the SCU’s characteristics under working conditions. Although experiences from the vertical test provided good guidance for horizontal optimization, special care needs to be taken for effects such as gravity. After improving field uniformity through iterative measurements, the rms phase error of the SCU is reduced to 7.3° at a current of 400 A. Additionally, the functions of the end correction coils to mitigate field integral errors have been verified. This paper also demonstrates and analyzes other phenomena and interesting observations during the commissioning process. It discusses important issues that impact field quality and measurement accuracy and suggests further improvement measures for an even better field.

Cesium telluride photocathodes are known to offer high quantum efficiencies under UV illumination combined with good lifetimes compared to other semiconductor photocathodes, making them very popular electron sources for particle accelerator applications. The development of photocathode preparation, characterization, and related expertise at a single accelerator laboratory can be challenging, expensive, and time consuming. Recognizing this, we explored the use of a custom-designed ultrahigh vacuum suitcase for transportation of CERN-made (Switzerland) cesium telluride photocathodes to Daresbury Laboratory (UK) for characterization. We report the synthesis and characterization of a batch of four cesium telluride photocathodes corresponding to our second attempt of transport, following design and process improvements through lessons learned from our first attempt. The photocathode characterization involved, where possible, measurements of the surface elemental composition using x-ray photoelectron spectroscopy (XPS), surface roughness with an in-vacuum scanning tunneling microscope (STM), and quantum efficiency (QE) measurements. Transverse energy distribution curves were obtained over a wide range of illumination wavelengths using the transverse energy spread spectrometer (TESS) at room- and cryogenic temperatures, and the values for mean transverse energy (MTE) were extracted. The photocathodes exhibited distinct thicknesses ranging from $\sim 50$ to $\sim 120\mathrm{nm}$ and significant MTE beyond the photoemission threshold which is attributed to the presence of ${\mathrm{Cs}}_x\mathrm{O}$ and Cs phases, as confirmed by XPS analysis. The photocathode that exhibited no carbon or oxygen contamination was measured to have the highest QE of 2.9% at a wavelength of 265 nm at the end of the performance characterization process. The results presented herein offer an insight into the achievements possible through international collaborations by successfully utilizing long-distance transportation of photocathodes by land under ultrahigh vacuum conditions.

The High Intensity Proton Accelerator facility (HIPA) delivers a 590 MeV cw (50.6 MHz) proton beam with up to 1.4 MW beam power (2.4 mA) to spallation and meson production targets serving particle physics experiments and material research. The main accelerator is the ring cyclotron, an isochronous proton machine accelerating an injected 72 MeV beam to a final 590 MeV. A few meters downstream of the ring cyclotron, an electrostatic beam splitter was installed in the 1980s and originally designed to peel off from a $200\mathrm{\mu }\mathrm{A}$ beam up to $20\mathrm{\mu }\mathrm{A}$ (12 kW beam power). Future initiatives will also make use of the splitter. Specifically, as part of the Isotope and Muon Production using Advanced Cyclotron and Target technologies (IMPACT) upgrade project, Targeted Alpha Tumour Therapy and Other Oncological Solutions (TATTOOS), an online isotope separation facility will allow to produce promising radionuclides for diagnosis and therapy of cancer in quantities sufficient for clinical studies. The TATTOOS facility includes a dedicated beamline intended to operate at a beam intensity of $100\mathrm{\mu }\mathrm{A}$ (60 kW beam power), requiring continuous splitting of the high-power main beam via the splitter. As a step forward toward reaching the desired beam intensity, a beam study was carried out to test the viability of the existing splitter for TATTOOS. The results of this study show that a record of $90\mathrm{\mu }\mathrm{A}$ (53 kW beam power) was peeled off a horizontally and vertically enlarged beam by the splitter. The successful beam strategy employed during the study as well as the results of several key measurements are presented in this paper, with particular emphasis on diagnostic measurements. Additionally, to support the measurements, a computational model of the splitter has been implemented using Monte Carlo simulation tools, including realistic geometry, electrostatic fields, beam optics, and power deposition calculations. Overall, the results of this paper show that through the combination of beam measurements and simulations, the existing splitter can be used to reach the $100\text{−}\mathrm{\mu }\mathrm{A}$ beam intensity requirement for TATTOOS.

Vertical fixed-field accelerators (vFFAs) feature a magnetic field that increases exponentially in the vertical direction, resulting in vertically stacked nonplanar orbits. Their magnetic field is highly nonlinear, and their solenoid and quadrupolar components induce strongly coupled optics. The detailed study of their beam dynamics must account for the transverse motion linear and nonlinear coupling. Specifically, the study of linear beam dynamics requires adequate coupling parametrizations, and the study of nonlinear beam dynamics, including the characterization of the dynamic aperture (DA), must be performed in the full 4D phase space. The zgoubi ray-tracing code is ideally suited to study the vFFA transverse dynamics, as it can perform step-by-step particle tracking in vFFA complex geometry and magnetic fields. This paper provides an in-depth study of the vFFA prototype ring designed under the ISIS-II proton driver prototype project to accelerate proton beams from 3 to 12  MeV. The magnets of this vFFA ring exhibit slow magnetic field falloffs, resulting in a significant influence of the neighboring cells on the optical lattice parameters. The determination of stable orbits and tunes requires superimposing 3D magnetic field maps in zgoubi to account for the neighboring cell residual fields. The study of nonlinear beam dynamics revealed the appearance of fourth-order stability islands. A complete characterization of the DA in the 4D phase space was conducted to give a measure of the stability domain and characterize the performance and limitations of this lattice. This study paves the way for further validation studies with experimental data and field maps.

The Accelerator Research Experiment at Sinbad (ARES) linac at DESY aims to generate and characterize ultrashort electron bunches (fs to sub-fs duration) with high momentum and arrival time stability for the purpose of applications related to accelerator research and development, e.g., development of advanced and compact diagnostics and accelerating structures, test of new accelerator components, medical applications studies, machine learning, etc. During its commissioning phase, the bunch duration characterization of the electron bunches generated at ARES has been performed with an rf-phasing technique relying on momentum spectra measurements, using only common accelerator elements (rf accelerating structures and magnetic spectrometers). The sensitivity of the method allowed highlighting different response times for Mo and ${\mathrm{Cs}}_2\mathrm{Te}$ cathodes. The measured electron bunch duration in a wide range of machine parameters shows excellent agreement overall with the simulation predictions, thus demonstrating a very good understanding of the ARES operation on the bunch duration aspect. The importance of a precise in situ experimental determination of the phase velocity of the first traveling wave accelerating structure after the electron source, for which we propose a simple new beam-based method precise down to a variation of one part per ten thousand respective to the speed of light in vacuum, is emphasized for this purpose. A minimum bunch duration of 20 fs rms, resolution-limited by the space charge forces, is reported. This is, to the best of our knowledge, around 4 times shorter than what has been previously experimentally demonstrated based on rf-phasing techniques with a single rf structure. The present study constitutes a strong basis for future time characterization down to the sub-fs level at ARES, using dedicated X-band transverse deflecting structures.

In this work, the process of scattering of high-energy positively charged particles in the field of atomic planes of an ultrashort silicon crystal was studied. In the parabolic potential approximation of atomic planes, analytical expressions are found for the dependence of the coordinates and velocities of particles in a crystal on time and initial conditions. The relationship between the particle incidence angle on the crystal and its deflection angle has also been determined. It is shown that, under certain conditions, a beam can be split by an ultrashort crystal into two beams diverging at an angle equal to twice the angle of planar channeling.

A beam vacuum window is a thin interface of separation between a volume under vacuum and a volume at a higher pressure traversed by particle beams. Their application is not only limited to particle accelerators, where they are typically installed inside the beamline to separate vacuum sectors, but also extends to other fields of nuclear research and to high-power hadron beam applications, such as spallation neutron sources and accelerator-driven systems. The main issue concerning the beam window technology resides intrinsically in the dual role of these components: the thickness of the window is supposed to be as thin as possible to allow the passage of the particles through matter with minimal interaction but, at the same time, enough resistant to maintain the required differential pressure between the two environments.

With their high quantum efficiency (QE) and excellent photoemissive properties, cesium telluride (Cs-Te) photocathodes are the current workhorse in the high average current electron accelerators around the globe. Their ability to generate high brightness and high charge electron beams has opened the doorway to numerous applications, including fundamental particle physics research, radiation therapy, high-energy physics experiments, and high repetition rate free-electron lasers (FELs). Their long-term performance is critical. In this work, we analyze the systematic production and rejuvenation of photocathodes via thermal evaporation and deposition techniques (both sequential and co-deposition), showing excellent values of QE exceeding 20% using deep ultraviolet illumination ($\lambda =262–266\mathrm{nm}$). To extend the photocathode lifetime, the rejuvenation process is performed via multilayer thin-film co-deposition, demonstrating the feasibility of reliably recovering the initial QE even after air exposure. We evaluated their final performance in both high gradient dc and rf gun setups and obtained consistent results of QE and lifetimes. The technique can be used in situ at the gun level, which significantly extends the sustained accelerator operation without major interventions. These approaches significantly enhance the robustness and performance of Cs-Te photoinjectors and represent a significant advancement for reliable high average current electron accelerators and FELs.

A conduction-cooled cryogenic permanent magnet undulator, CU15, with a period length of 15 mm and 133 periods has been successfully developed. This undulator has been operated at the Taiwan Photon Source (TPS) storage ring at a beam current of 500 mA for more than 3 years. In CU15, two cryocoolers are used to cool the PrFeB magnets to 80 K. The effective magnetic field measures 1.33 Tesla for a magnet gap of 4.2 mm, maintaining an rms phase error below 2.6°. The bunch length of the electron beam in TPS is as short as 16 psec, leading to a serious issue with high beam-induced heating in CU15. This report addresses the above issues and provides detailed adaptations of CU15 to operate at a beam current of 500 mA.

This paper reports the design, fabrication, and results of the high-gradient conditioning and testing for a two-cell, $\pi$-mode, standing wave normal-conducting prototype booster cavity for the proposed 3 GeV proton linac upgrade at Los Alamos Neutron Science Center. Increasing the energy of proton beam from the existing 800 MeV to 3 GeV will improve resolution of the proton radiography by up to 10 times. The proposed energy boost can be achieved with a compact normal-conducting high-gradient radio-frequency (rf) linac section. The C-band section of the booster linac was designed with optimized-shaped copper accelerator cavities with distributed rf coupling. A short two-cell test prototype structure was designed for the frequency of 5.712 GHz, fabricated, and tested at the C-band Engineering Research Test Facility in New Mexico (CERF-NM) at Los Alamos National Laboratory. The maximum klystron power coupled into the test structure was 8.3 MW with $1\mathrm{\mu }\mathrm{s}$ pulse length and 100 Hz repetition rate. The breakdown probabilities were recorded as functions of the accelerating gradient and peak surface fields. Operation of the test cavity at accelerating gradients of up to $100\mathrm{MV}/\mathrm{m}$ was demonstrated.

Based on the technique of the discrete one-turn transfer maps, the problem of linear coupling between horizontal and vertical betatron oscillations in an accelerator has been treated exactly and entirely in explicit form. The stability region in the fractional part of the horizontal and the vertical betatron tune space as a function of the linear coupling strength has been obtained, and the increment/decrement of the horizontal and the vertical betatron oscillations in the case of the linear sum resonance has been shown to be approximately equal to the half of the coupling strength. The normal form parameterization of the one-turn linear map with horizontal-to-vertical coupling has been developed in detail in the spirit of Edwards and Teng formalism. The motion in the normal mode in the new normal form coordinates is decoupled by implying that two independent Courant-Snyder invariants exist, which have been found explicitly.

The Low Energy Ion Ring (LEIR) at CERN is the first synchrotron in the Large Hadron Collider ions injector chain. The performance of LEIR is mainly determined by the number of charges extracted from the machine and transferred to the downstream chain of accelerators. Ions are delivered by the linear accelerator Linac 3. In the intensity accumulation phase, the machine operates with coasting beams: each injected beam is cooled, reducing its transverse dimensions and momentum spread, and brought into a stacking momentum position to allow subsequent injections. In this context, the evolution of the beam parameters for an injected beam under the effects of electron cooling, impedance, intrabeam scattering (IBS), and space charge is of interest in order to optimize the machine working point with respect to the accumulated intensity and to contribute to the understanding of the interplay of the different collective effects. This work describes the advancement in modeling coasting beam dynamics for LEIR accounting for the interplay of electron cooling, impedance, IBS, and space charge. Each effect is presented and progressively included to compute the simulated equilibrium longitudinal Schottky spectrum, which is found in good agreement with the measured one.

The China initiative Accelerator-Driven subcritical System (CiADS) project aims to design and build the world’s first accelerator-driven subcritical system demonstration facility. The facility will attain 2.5 MW thermal power in 2026 to realize accelerator-target-reactor coupling experiments. As the first approved accelerator-driven subcritical system facility, the CiADS driver linac will adopt cutting-edge accelerator technologies to achieve the challenging goals of high beam power and high reliability. The accelerator will operate in cw mode to produce a 500 MeV, 5 mA proton beam. To meet the extremely high demands on beam availability, the linac design lays strong emphasis on beam loss control and increased fault tolerance via a novel fault-compensation method. The accelerator complex consists of a room temperature front-end section, a main linac that employs superconducting accelerator technologies, and a high-energy beam transport line. The beam-loss-oriented beam dynamics design of the driver linac complex is presented in the paper.

Accelerator-based x-ray free-electron lasers (XFELs) are the latest addition to the revolutionary tools of discovery for the 21st century. The two major components of an XFEL are an accelerator-produced electron beam and a magnetic undulator, which tend to be kilometer-scale long and expensive. A proof-of-principle demonstration of free-electron lasing at 27 nm using beams from compact laser wakefield accelerators was shown recently by using a magnetic undulator. However, scaling these concepts to x-ray wavelengths is far from straightforward as the requirements on the beam quality and jitters become much more stringent. Here, we present an ultracompact scheme to produce tens of attosecond x-ray pulses with several GW peak power utilizing a novel aspect of the FEL instability using a highly chirped, prebunched, and ultrabright tens of MeV electron beam from a plasma-based accelerator interacting with an optical undulator. The FEL resonant relation between the prebunched period and the energy selects resonant electrons automatically from the highly chirped beam which leads to a stable generation of attosecond x-ray pulses. Furthermore, two-color attosecond pulses with subfemtosecond separation can be produced by adjusting the energy distribution of the electron beam so that multiple FEL resonances occur at different locations within the beam. Such a tunable coherent attosecond x-ray sources may open up a new area of attosecond science enabled by x-ray attosecond pump/probe techniques.