Niobium is a technologically important material, which is typically used for superconducting radio-frequency applications. Superconductive cavities made of niobium require contamination-free and smooth surfaces to ensure the best performance of a particle accelerator. Interior surfaces of niobium cavities are usually obtained by centrifugal barrel polishing, buffered chemical polishing, and electropolishing or a combination of these methods. However, a standard inspection of the inner cavity surface after the treatment still shows the presence of sharp features limiting cavity performance. Laser polishing is a potential alternative or can be used as an additional final step for a more efficient surface treatment toward higher electric field gradients. In the present study, a chemically polished (110)-oriented single crystal niobium surface was processed with a focused pulsed laser with a pulse duration of 3.5 ns and a repetition rate of 10 Hz. The laser fluence and the number of pulses were varied in the range from 0.68 up to $4.27\mathrm{J}/{\mathrm{cm}}^2$ and from 20 up to 200, respectively. The magnitude of laser-induced surface structures and boiling traces was systematically studied by means of scanning electron microscopy, atomic force microscopy, and optical profilometry. Finally, the local field emission behavior was investigated and correlated with the observed surface modifications. Typical current-field characteristics and a field enhancement statistic of laser-processed areas are presented in the study. The surface processing with a rather low laser fluence of $0.68\mathrm{J}/{\mathrm{cm}}^2$ yielded high onset electric fields of $650–940\mathrm{MV}/\mathrm{m}$ with field enhancement factors below 10. The processing with a higher laser fluence and/or a higher number of pulses resulted in boiledlike structures emitting at $70–190\mathrm{MV}/\mathrm{m}$.

Advanced targets based on thin films of graphene oxide covered by metallic layers have been irradiated at high laser intensity ($\sim {10}^{19}\mathrm{W}/{\mathrm{cm}}^2$) with 40 fs laser pulses to investigate the forward ion acceleration in the target normal sheath acceleration regime. A time-of-flight technique was employed with silicon-carbide detectors and ion collectors as fast on-line plasma diagnostics. At the optimized conditions of the laser focus position with respect to the target surface was measured the maximum proton energy using Au metallic films. A maximum proton energy of 2.85 MeV was measured using the Au metallization of 200 nm. The presence of graphene oxide facilitates the electron crossing of the foil minimizing the electron scattering and increasing the electric field driving the ion acceleration. The effect of plasma electron density control using the graphene oxide is presented and discussed.

The development of fast detection methods for comprehensive monitoring of electron bunches is a prerequisite to gain comprehensive control over the synchrotron emission in storage rings with their MHz repetition rate. Here, we present a proof-of-principle experiment with a detailed description of our implementation to detect the longitudinal electron bunch profiles via single-shot, near-field electro-optical sampling at the Karlsruhe Research Accelerator (KARA). Our experiment is equipped with an ultrafast line array camera providing a high-throughput MHz data stream. We characterize statistical properties of the obtained data set and give a detailed description for the data processing as well as for the calculation of the charge density profiles, which were measured in the short-bunch operation mode of KARA. Finally, we discuss properties of the bunch profile dynamics on a coarse-grained level for the example of the well-known synchrotron oscillation.

The collision of ultrashort high-density $e^{-}$ or $e^{-}$ and $e^{+}$ beams at 10s of GeV, to be available at the FACET II and in laser wakefield accelerator experiments, can produce highly collimated $\gamma$ rays (few GeVs) with peak brilliance of $10^{27}\mathrm{ph}/\mathrm{s}{\mathrm{mm}}^2{\mathrm{mrad}}^{2}0.1\%\mathrm{BW}$ and up to $10^5$ nonlinear Breit-Wheeler pairs. We provide analytical estimates of the photon source properties and of the yield of secondary pairs, finding excellent agreement with full-scale 3D self-consistent particle-in-cell simulations that include quantum electrodynamics effects. Our results show that beam-beam collisions can be exploited as secondary sources of $\gamma$ rays and provide an alternative to beam-laser setups to probe quantum electrodynamics effects at the Schwinger limit.

The process of diffracted x-ray transition radiation by a high-energy “half-bare” electron in a crystalline target is considered in Bragg geometry. The electron is supposed to appear in the mentioned state, in which the field around it is significantly suppressed comparing to the equilibrium Coulomb one, in the result of preliminary penetration through an upstream amorphous target situated on some distance from the crystal. It is shown that the process of the electron’s field regeneration after its exit from the upstream target dramatically modifies both the diffracted transition radiation angular distribution and its total yield making them dependent on the distance between the targets. Analytical expressions for these quantities, as well as their simple approximations, are derived. Almost arbitrary crystal orientation is considered. Special attention is drawn to the case of backward radiation geometry when the emitted radiation has to penetrate through the upstream target on its way to the detector.

The Large Hadron Collider (LHC) at CERN is a 7 TeV proton synchrotron, with a design stored energy of 362 MJ per beam. The high-luminosity (HL-LHC) upgrade will increase this to 675 MJ per beam. In order to protect the superconducting magnets and other sensitive equipment from quenches and damage due to beam loss, a multilevel collimation system is needed. Detailed simulations are required to understand where particles scattered by the collimators are lost around the ring in a range of machine configurations. merlin++ is a simulation framework that has been extended to include detailed scattering physics, in order to predict local particle loss rates around the LHC ring. We compare merlin++ simulations of losses during the squeeze (the dynamic reduction of the $\beta$ function at the interaction points before the beams are put into collision) with loss maps recorded during beam squeezes for run 1 and 2 configurations. The squeeze is particularly important, as both collimator positions and quadrupole magnet currents are changed. We can then predict, using merlin++, the expected losses for the HL-LHC to ensure adequate protection of the machine.

We report the measurement of the internal dark current in a 17 GHz, high gradient accelerator cavity and its comparison with theory. The cavities were fabricated from copper and had a sidewall that was either uncoated or coated with diamondlike carbon or TiN. The dark current was monitored by a downstream detector and by detectors behind two small slits made in the cavity sidewall. With an increasing gradient, the downstream current increased monotonically, as expected for field emission. The variation of the internal, side dark current was not monotonic but showed the onset of peaks at gradients near 45 and $65\mathrm{MV}/\mathrm{m}$. These were identified as the $N=2$ and $N=1$ single point multipactor resonances. The total internal dark current was estimated at $\sim 15–30\mathrm{A}$. The magnitude of the internal dark current and its dependence on the gradient were in good agreement with simulations using the cst code as well as an in-house code. Processing to a higher gradient, $\sim 90\mathrm{MV}/\mathrm{m}$, eliminated the $N=2$ mode, but the $N=1$ mode persisted. The coated sidewall cavities showed the same multipactor resonances as the uncoated structure. However, at the highest gradient achieved in testing, the coated structures showed a modest reduction in the internal dark current.

A fast chopper system has been developed and tested for single bunch selection with the radio-frequency quadrupole (RFQ) accelerating element of the Soreq Applied Research Accelerator Facility (SARAF). The fast chopper consists of a high voltage (HV) deflector just before the RFQ, providing both positive and negative HV deflections and fast HV switching between polarities to enable momentary transmission of a single prebunch to the RFQ. Presently, the system enables single bunch selection for protons and deuterons at a repetition rate as determined by the user of up to 200 kHz, with bunch transmission of up to 50%, and with neighboring bunch contamination of less than 15%. Single bunch selection provides SARAF with fast neutron time-of-flight (TOF) capabilities. Measurements performed with liquid scintillation detectors show clear gamma and neutrons peaks, with TOF resolution of about 1 nanosecond FWHM. Beam dynamics simulations suggest possibilities for further improvements of the fast chopper and single bunch selection characteristics, with a significant lowering or elimination of the neighboring bunches, enhanced TOF resolution, and increased repetition rate to above 200 kHz. Fast neutron TOF capabilities, especially at phase II of SARAF, will provide exceptional opportunities for neutron induced reaction measurements for nuclear technology and fundamental research.

In order to achieve high intensity beams for Fermilab’s neutrino program, slip stacking is used in the Recycler in which bunches overlap and slip through other bunches multiple times. As the bunch intensity is increased in the future, space charge effects during the overlap periods become more detrimental. In order to investigate the size of these space charge effects, the beam simulation program Synergia has been extended to allow copropagation of bunch trains at different momenta.

A new scheme for a laser-driven proton accelerator based on a sharply tailored near-critical-density plasma target is proposed. The designed plasma profile allows for the laser channeling of the dense plasma, which triggers a two-stage acceleration of protons—first accelerated by the laser acting as a snowplow in plasma, and then by the collisionless shock launched from the sharp density downramp. Thanks to laser channeling in the near-critical plasma, the formed shock is radially small and collimated. This allows it to generate a significant space-charge field, which acts as a monochromator, defocusing the lower energy protons while the highest ones remain collimated. Our theoretical and numerical analysis demonstrates production of high-energy proton beams with few tens of percent energy spread, few degrees divergence angle and charge up to few nC. With a PW-class ultrashort laser this scheme predicts the generation of such high quality proton beams with energies up to several hundreds of MeV.

We present the design and test results of an induction cavity, which is a full-scale prototype cell of a prospective six-stage induction voltage adder. This induction cell runs in positive polarity and the output transmission line operates in the vacuum insulation state. The design methods and configurations of the azimuthal transmission line, insulation stack, magnetic core, cathode plate, and output transmission line in the induction cell are given in detail. The induction cavity is driven by one pulsed power system consisting of a tesla transformer, intermediate storage capacitor, laser triggered gas switch, pulse forming line, self-breaking oil switch, and a water transmission line. A maximum voltage about 1.05 MV could be achieved across one $7.0\mathrm{\Omega }$ water resistor load. A full circuit model including the primary energy storage section and pulse forming section is proposed. The circuit model also includes the two-dimensional circuit model of the induction cavity [Phys. Rev. Accel. Beams 20, 020401 (2017)]. The voltage and current waveforms across the feed port, azimuthal transmission line, insulation stack, and resistor load obtained during the experiment are compared with the circuit simulation results. It demonstrates that they can agree with each other very well.

We report on the radiation studies performed at the CLEAR facility of CERN in the sub-THz range, exploiting picosecond ultrarelativistic electron bunches for the production of coherent radiation. The coherent radiation, produced by different mechanisms (in particular coherent transition radiation), has been fully characterized using different techniques and detectors. The main aim has been that to setup a new beam-based source of radiation in the mm-waves for external users, individuating the performances and the limitations. Moreover the coherent radiation has been used for longitudinal diagnostics, providing reliable bunch length values consistent with other diagnostics. Transverse shaping of the radiation source has been also demonstrated via control of the size and divergence of the electron bunch at the source plane. The mechanism yielding the highest peak-power has been the Cherenkov-Diffraction Radiation, providing $\sim 0.1\mathrm{MW}$. The current performances of the CLEAR THz source seem to be more suitable for high-average-power than high-peak-power applications. Future plans and strategies will look toward the realization of a high-peak-power source, providing $\sim 10–100\mathrm{MW}\mathrm{THz}$ pulses.

The improvement of the performance of superconducting rf cavities has recently motivated a considerable research effort in order to elucidate the effect of trapped magnetic flux on the surface resistance $R_s$. In this paper, we show that, by introducing a nonlinear pinning force in the Gittleman-Rosenblum equations for the rf power dissipation due to a trapped magnetic flux in a superconductor, we can empirically describe the linear dependence on the rf field amplitude $B_{rf0}$ of the additional surface resistance $R_{fl}$. We also show that the proportionality between the rf-field-dependent and -independent terms $R_{fl}^1$ and $R_{fl}^0$ and the frequency dependence of $R_{fl}^1$ follow naturally from this approach.

We developed a novel method for element detection utilizing an x-ray source based on parametric x-ray radiation (PXR), an accelerator-based light source at the Laboratory for Electron Beam Research and Application (LEBRA), Nihon University. The method is a type of $K$-edge subtraction (KES) imaging that uses the drastic change of x-ray absorption around the $K$-shell absorption edge of the target element. Using the properties of PXR, simultaneous KES imaging is possible, and can easily be applied to 3-dimensional (3D) computed tomography (CT). We demonstrated the feasibility of simultaneous KES-CT in our previous work. In this study, we investigate the quantitative performance of simultaneous KES imaging for a sample containing the element strontium (Sr) for which the $K$-edge energy is 16.1 keV. Results of a simultaneous KES-CT experiment employing the LEBRA-PXR source confirm that the imaging method can provide a 3D distribution of the element with a value proportional to the Sr concentration. Concerning sensitivity to element concentration, at least in the region of 0.5%-concentration, the sample was successfully distinguished from the region without Sr in the 3D tomographic image obtained using the element-imaging technique.

SuperKEKB is an electron-positron collider with asymmetric energies located at the High Energy Accelerator Research Organization (KEK) in Tsukuba, Japan. After more than five years of upgrading work on KEKB, phase-1 commissioning commenced in February 2016 and ended in June of the same year. Following a 20-month shutdown for the installation of a particle detector, BELLE II, phase-2 commissioning commenced in March 2018 and ended in July of the same year. This paper describes one major issue faced by SuperKEKB: the electron cloud effect (ECE) in the positron ring, which was observed during phase-1 commissioning. In the high-beam-current region, electron clouds, i.e., the source of the ECE, existed in the beam pipes at drift spaces of the ring, which had antechambers and a titanium nitride (TiN) film coating as countermeasures against the ECE. Permanent magnets and solenoids used to generate magnetic fields in the beam direction were attached to the beam pipes as additional countermeasures before the next commissioning phase commenced. Consequently, during phase-2 commissioning, experiments showed that the threshold of the current linear density for exciting the ECE increased by a factor of at least 2 compared to that during phase-1 commissioning. While the countermeasures were strengthened, the effectiveness of the antechambers and TiN film coating of the real beam pipe was reevaluated. Through various simulations and dedicated experiments during phase-2 commissioning, the antechambers were found to be less effective than anticipated with regard to reducing the number of photoelectrons in the beam channel. In contrast, the TiN film coating had a low secondary electron yield, as expected.

Magnetic fields used to control particle beams in accelerators are usually controlled by regulating the electrical current of the power converters. In order to minimize lifetime degradation and ultimately luminosity loss in circular colliders, current-noise is a highly critical figure of merit of power converters, in particular for magnets located in areas with high beta-function, like the High-Luminosity Large Hadron Collider (HL-LHC) insertions. However, what is directly acting upon the beam is the magnetic field and not the current of the power converter, which undergoes several frequency-dependent transformations until the desired magnetic field, seen by the beam, is obtained. Beam screens are very rarely considered when assessing or specifying the noise figure of merit, but their magnetic frequency response is such that they realize relatively effective low pass filtering of the magnetic field produced by the system magnet-power converter. This work aims at filling this gap by quantifying the expected impact of different beam screen layouts for the most relevant HL-LHC insertion magnets. A well-defined postprocessing technique is used to derive the frequency response of the different multipoles from multiphysics finite element method (FEM) simulation results. In addition, a well-approximated analytical formula for the low-frequency range of multi-layered beam screens is presented.