This paper introduces a novel bead-falling measurement method for the precise and efficient mapping of electromagnetic fields within radio-frequency (rf) cavities, which are crucial components in the design of accelerators. The traditional bead-pull method, while effective, involves mechanical complexities and is prone to errors from wire perturbations. The innovative method reported here leverages the simplicity and accuracy of free-falling beads to mitigate these issues. This technique eliminates the need for a wire-pulley system, thereby simplifying the experimental setup and reducing potential mechanical errors. We detail the development and operational principles of this new method, including the design of a compact, portable measurement device that integrates a bead/droplet release system and a bead detection system linked to a vector network analyzer. The device has been tested with a three-gap buncher cavity and a scaled Alvarez-type cavity, demonstrating its ability to perform rapid, reliable field measurements under various challenging conditions, including low signal-to-noise ratios and environmental vibrations. The results confirm the method’s superiority in precision and operational efficiency, potentially setting a new standard for rf cavity diagnostics and tuning.

The generation of electron bunches on the attosecond timescale is important for a multitude of accelerator-based applications. Here, we report on a tomographic measurement of the (pre)linearized longitudinal phase space of a low charge 3 MeV electron bunch generated with the 1.6 cell Pegasus photoinjector for the generation of attosecond bunches. The nonlinear correlations in the longitudinal phase space induced by space charge at the photocathode, radiofrequency field curvature of the gun, and vacuum dispersion are compensated using a compact X-band linearizer. Then, the initial and compensated phase of the picosecond electron bunch is precisely reconstructed by neural network assisted tomographic reconstruction from momentum spectra at varying buncher linac phase. Finally, we combine the measured phase space shape with particle tracking simulations to show that electron bunches as short as 941 as develop downstream the beamline.

In the main ring (MR) of the Japan Proton Accelerator Research Complex, one of the primary sources of beam loss is the betatron resonance. A factor determining the strength of betatron resonances is the symmetry of optics, which relies on the quadrupole field errors in the ring. In the beam tuning of the MR, correcting quadrupole field errors plays a significant role in reducing beam loss. This paper introduces and examines a precise method to evaluate time-varying quadrupole field errors at individual quadrupole magnets around the ring through closed-orbit measurements in the MR. By applying this method, we experimentally determined substantial quadrupole field errors due to eddy currents on the vacuum ducts during acceleration. The methods developed in this paper are employed in the beam tuning for the user operations of the MR.

The beam response to an external periodic excitation delivers relevant information about the optics, tune distribution, and stability of a circulating beam in a storage ring. In this contribution, the horizontal beam response to the excitation (transfer function) under conditions typical for slow extraction is presented for a coasting beam. The resulting spectrum exhibits a splitting behavior. The single particle dynamics is discussed and an interpretation based on the simulation results is presented.

In this paper, we use one-dimensional measurements to infer the four-dimensional phase space density of an accumulated proton beam in the Spallation Neutron Source (SNS) accelerator. The reconstruction was performed by maximizing the distribution’s entropy subject to the measurement constraints and thus represents the most conservative inference from the data. The reconstructed distribution reproduces the measured profiles down to the noise level, and simulations indicate that the problem is reasonably well constrained. Similar measurements could serve as benchmarks for beam dynamics simulations in the SNS or hadron accelerators.

Plasma cleaning is extensively employed in superconducting radio-frequency (SRF) accelerators to mitigate field emissions induced by hydrocarbon contaminants. This study explores the plasma physical characteristics and modulation techniques in a commonly utilized 1.3 GHz 9-cell tesla-shaped SRF cavity through the integration of optical emission spectroscopy and a plasma fluid model. Investigations of argon and neon plasma demonstrate that argon generates a higher electron density due to its lower ionization energy, while neon plasma exhibits higher electron temperature and potential that promotes the decomposition of oxygen and the rate of surface chemical reactions. Furthermore, the research reveals that the electron density, temperature, and electric potential of the neon plasma within the cavity exhibit a centrally symmetric distribution, with maximum values of $1\times {10}^{15}{\mathrm{m}}^{-3}$, 0.53 eV, and 24.5 V, respectively. Subsequently, the effects of power, gas pressure, and frequency on the physical properties of the plasma are systematically analyzed. Among these, the 640 nm spectral line was selected as a qualitative indicator of glow intensity and plasma state changes within the cavity due to its strongest peak intensity. Findings indicate that increasing power and gas pressure enhance electron density and 640 nm spectral line intensity while reducing electron temperature. Additionally, comparative analysis of plasma properties in cells 1-9 reveals that most cells exhibit higher electron temperature and electron density at frequency II. (Frequency I serves as the transfer mode for plasma transfer, while frequency II acts as the stable mode for plasma stabilization.) Consequently, plasma cleaning of the 1.3 GHz 9-cell cavity using neon gas at this frequency is anticipated to yield superior cleaning outcomes. These findings enhance our understanding and control of plasma behavior within the SRF cavity and provide valuable insights for optimizing plasma cleaning processes.

For double-ring colliders, it is a vital task to make the two beams meeting each other properly at the interaction point. BEPCII is a double-ring collider that operates in the decay mode, as the beam currents decrease over time, the beam orbits need to be continuously adjusted to maintain the optimum collision conditions. Originally, this task was carried out manually, with operators adjusting three offset control knobs ($x,y,y^{\prime }$) according to the luminosity. There is a crucial necessity to implement advanced automated control approaches. However, the feedback methods are ineffective due to the constraints imposed by machine characteristics. Nevertheless, the optimization methods are also limited by the slow response speed of BEPCII and have intrinsic limitations. Reinforcement learning (RL), which learns from past experiences, provides a new approach for handling such problems. In this paper, we implemented two automated control methods: the dither method and the deep $Q$-network (DQN) RL method. The dither method is a numerical optimization method used to provide more online data for DQN training. With the help of the historical data from the dither method, we successfully trained a DQN agent to control the offset knobs to optimize the luminosity. The DQN method exhibited significantly better performance than the dither method, achieving faster optimization speeds and yielding higher integral luminosity. Furthermore, the DQN method has been integrated into the daily operations of BEPCII, achieving long-term stable deployment and effectively substituting manual labor.

Typical parameters of electron bunches available now in undulator x-ray FELs enable generation of induced radiation in crystal-based x-ray volume free electron laser (XVFEL). An important peculiarity of an undulator x-ray FEL is electron bunch spatial modulation matching the radiation wavelength. Therefore, x-ray FEL can be considered as a modulator of electron bunch density. As a result, a facility combining an undulator, which produces a modulated beam, and a crystal, on which the modulated beam is incident, can be considered to be similar to a klystron, which is a well-known source for microwaves emission. The use of a premodulated beam enables to generate and observe superradiation and induced radiation in a crystal-based XVFEL. The number of quanta radiated in a crystal-based XVFEL corresponds (or even exceeds) the number of quanta produced by the XFEL in the same frequency range.

Contaminations on the active surface of high-power microwave resonators, such as radio-frequency accelerator cavities, can greatly enlarge electron field emission and reduce their performance. In this work, we demonstrate the use of ozone to clean the surface of niobium superconducting radio-frequency cavities resulting in reduced field emission and enhanced accelerating gradient before onset of field emission. We report results from processing four cavities.

We have conducted 0D, 1D, 2D, and 3D simulations of an idealized coaxial impedance-matched Marx generator (IMG) [Phys. Rev. Accel. Beams 20, 040402 (2017)]. The 0D calculations were conducted with a four-element circuit model; the 1D, 2D, and 3D calculations were conducted with highly resolved, fully electromagnetic representations. The IMG consists of 30 stages distributed axially and connected electrically in series. Each stage is powered by two bricks separated by 180° and connected electrically in parallel. Each brick comprises two opposite-polarity capacitors in series with a single switch. The bricks drive an internal impedance-matched coaxial transmission line terminated by a resistive load. The simulations neglect effects due to the switch-triggering circuit, the capacitor-charging circuit, external conducting boundaries, and reactive components of the load. We find dimensionality does not significantly affect the electrical power delivered by the IMG to its load: peak load powers estimated by the 0D, 1D, 2D, and 3D simulations agree to within 1%. The 3D calculations demonstrate that electromagnetic power radiated by the bricks, and axial gaps between stages, reduces the peak load power by less than $\sim 1\%$. Each simulation assumes the load impedance is 34% above that at which the load power is maximized. Operating an IMG with such an overmatched load offers several advantages while decreasing the peak load power by only 2%. The 0D, 1D, 2D, and 3D models outlined herein could be adapted to assess computationally competing IMG designs, and conduct a variety of numerical IMG experiments, before an IMG is constructed.

ISAC-TRIUMF operates targets under proton irradiation in the high-power regime of 50 kW ($100\mathrm{\mu }\mathrm{A}$, 500 MeV) to produce radioactive isotope beams using the isotope separation on-line technique. The targets irradiated at ISAC have a total stopping power significantly lower than 500 MeV resulting in a residual high-power proton beam downstream of the target. The coupling of a supplementary target in a downstream position allows for additional scientific output. Irradiation conditions have been analyzed with the fluka code and relevant achievable research within operational limitations explored. Prominently, the study of material degradation under high radiation fields is of increasing interest due to the steady demand for high-performance materials for new-generation nuclear applications and beam-intercepting components at high-power particle accelerator facilities. Successive irradiations at ISAC yield relevant radiation damage of bulk material samples which can then be characterized using already existing facilities at TRIUMF and partner facilities. Additionally, a variety of isotopes produced by transmutation with research applicability can be harvested from these irradiations. With the purpose of unlocking this complementary research at ISAC-TRIUMF, a secondary target, attached to the primary ISAC target system, has been designed and commissioned.

GaAs photocathodes in dc high-voltage photoguns are highly susceptible to ion back-bombardment, which reduces the photocathode quantum efficiency and limits the useful operating lifetime for producing polarized electron beams. This paper demonstrates that applying a small positive bias to the photogun anode can significantly suppress ion back-bombardment and increase charge lifetime. This technique was studied extensively using the Continuous Electron Beam Accelerator Facility photogun, where highly polarized electron beams created using a strained-superlattice $\mathrm{GaAs}/\mathrm{GaAsP}$ photocathode were used and charge lifetimes improved by almost a factor of 2. A new simulation code ionator was developed to model ion production and tracking in order to better understand and explain the factors that led to the performance improvement. Results of the experiments and simulations are discussed in detail.

In this paper, we present detailed simulations with asymmetric initial beam settings in the context of the proposed Future Circular Collider $e^{+}{e}^{-}$ (FCC-ee) using the xsuite framework. We compare simulated equilibrium bunch sizes and luminosities against an already existing analytical model, which shows remarkably good agreement for realistic small perturbations. We investigate the longitudinal top-up injection, the currently preferred injection scheme for the FCC-ee, using self-consistent simulations featuring beam-beam collisions with beamstrahlung and the injection process for the first time. We present and assess the sensitivity and required precision of the nominal beam parameters in a potential real-life operation by providing first estimates of the tolerances in the initial asymmetry of several machine parameters, with respect to the 3D flip-flop mechanism, obtained from parameter scan simulations.

Several analytical formulas have been developed to calculate the Touschek lifetime of beam in electron storage rings under different assumptions. Among them, Piwinski’s formula is the most general and precise for unpolarized beams, widely applied in the mainstream accelerator simulation software. Lee’s formula is conventionally adopted to evaluate the influence of beam polarization on the Touschek lifetime, but it is less general than Piwinski’s formula when applied to unpolarized beams. In this paper, a new formula for the Touschek lifetime of ultrarelativistic polarized beams is derived in the theoretical framework of Piwinski. Its applications to several lattices of electron storage rings reveal that the influence of beam polarization on the Touschek lifetime is weaker than that estimated with Lee’s formula especially when the vertical emittance is large.

Harmonic up-conversion schemes represent the most successful approach in externally seeded free-electron lasers (FELs) for obtaining radiation with laserlike properties at short wavelengths. They are, however, limited in the shortest achievable wavelengths, as the harmonic conversion efficiency decreases with increasing harmonic number, even for echo-enabled harmonic generation (EEHG). In the following, we introduce a novel externally seeded cascaded FEL scheme based on two EEHG stages, capable of obtaining unprecedented bunching fraction that is independent on the final FEL wavelength. The second stage can be operated both in conventional harmonic up-conversion but also in down-conversion, i.e., going toward (slightly) longer wavelengths. Higher bunching enables emission of laserlike radiation in the soft x-rays with large output power in a compact scheme. Its performance at 4 nm and shorter is investigated, demonstrating the ability to achieve multi-GW level, fully coherent pulses with laserlike properties and paving the way for externally seeded radiation at 1 nm and beyond.

While plasma-based accelerators have the potential to positively impact a broad range of research topics, a route to application will only be possible through improved understanding of their stability. We present experimental results of a laser wakefield accelerator in the nonlinear regime in a helium gas jet target with a density transition produced by a razor blade in the flow. Modifications to the target setup are correlated with variations in the plasma density profile diagnosed via interferometry and the shot-to-shot variations of the density profile for nominally equal conditions are characterized. Through an in-depth sensitivity study using particle-in-cell simulations, the effects of changes in the plasma density profile on the accelerated electron beams are investigated. The results suggest that blade motion is more detrimental to stability than gas pressure fluctuations, and that early focusing of the laser may reduce the deleterious effects of such density fluctuations.

Laser surface structuring has proven to be an effective technique for achieving a copper surface with secondary electron yield (SEY) values close to or below unity. However, the attributes that minimize SEY, such as moderately deep grooves and redeposited nanoparticles, may lead to undesirable consequences, including increased radio frequency surface resistance. This investigation systematically examined data about different cleaning procedures designed to eliminate redeposited adsorbed particulates. Various analysis techniques were used iteratively after each consecutive cleaning step, providing insights into the evolving surface characteristics. The collected experimental results identified distinct impacts of microgrooves, groove orientation, and associated particulates on secondary electron yield and surface resistance. Exposing the crests while retaining high particulate coverage in the grooves leads to reduced SEY values and surface resistance, suggesting that the tips of the grooves exert a more significant influence on surface current density than the groove depth. At the same time, nanoparticles in the grooves have a more significant impact on SEY values than the exposed tips at the surface.

The observation of transverse Schottky spectra provides insight into intrabunch particle dynamics and serves in numerous facilities to derive important beam and machine parameters, such as the betatron tune, first-order chromaticity, and transverse emittance. The original theory confirming the validity of these observations has been, however, developed by taking strong assumptions on the particle motion in both the longitudinal and transverse planes. In this work, we loosen these assumptions, by proving that the relationships linking the spectra with transverse emittance and betatron tune remain universally unchanged regardless of the shape of the rf waveform and chromaticity up to any order, while for the first-order chromaticity, we derive compact formulas for error estimates. The derived theory is supported by macroparticle simulations and applied to typical beam and machine parameters in the Large Hadron Collider.

Double-aperture young interferometry is widely used in accelerators to provide a one-dimensional beam measurement. We improve this technique by combining and further developing techniques of nonredundant, two-dimensional, aperture masking, and self-calibration from astronomy. Using visible synchrotron radiation, tests at the ALBA synchrotron show that this method provides an accurate two-dimensional beam transverse characterization, even from a single 1 ms interferogram. The nonredundancy of the aperture mask in the technique enables it to be resistant to spatial phase fluctuations that might be introduced by vibration of optical components or in the laboratory atmosphere.

In the case of a single-digit picosecond bunch length, synchrotron light sources produce intense coherent radiation up to the THz range. The reduction of the bunch length by lowering the momentum compaction factor (low-$\alpha$) gives rise to the microbunching instability, which is on the one hand a crucial roadblock in the x-ray range during to the resulting effective bunch lengthening, but on the other hand also an opportunity for the generation of intense THz radiation if it can be controlled appropriately. In the KIT storage ring KARA (Karlsruhe Research Accelerator), two parallel plates with periodic rectangular corrugations are planned to be installed as a proof of principle in an electron storage ring. These plates create an additional longitudinal impedance based on their geometry, which can affect the beam dynamics. The resulting impedance manipulation will be used to study and control the longitudinal electron beam dynamics and the emitted coherent synchrotron radiation. This paper presents the results of systematic studies in simulation of the impact of additional corrugated plate impedances on the longitudinal beam dynamics using the example of the KARA storage ring. If the periodicity of the wake function of the corrugated plates matches the size of the substructures in the longitudinal bunch profile, the instability threshold can be effectively manipulated. This extends intense THz radiation to different beam current regimes.