We investigated the feasibility on the application of a superconducting radio frequency (SRF) niobium cavity to an accelerator-based neutron source for boron neutron capture therapy (BNCT). Neutron source is the key component of BNCT and adopting rf-linac based neutron source realizes a medical care system sufficient to be compact, which can be installed in a hospital and to generate intensive neutron yields that the BNCT requires. However, it is still desirable to improve the efficiency of input power on neutron yields and the achievable accelerate field gradient. SRF accelerator technology potentially allows us to enhance the performance because of its prominent lower ohmic loss and higher sustainable accelerating fields. This paper presents a first feasibility study on the application of a SRF niobium cavity to an accelerator-based neutron source for BNCT, assuming that a superconducting radio frequency quadrupole (SC-RFQ) composed of pure bulk niobium at 4.2 K accelerates the proton/deuteron beams to a beryllium or lithium target for the neutron production of BNCT via ${}^7\mathrm{Li}(\mathrm{p},\mathrm{n}){}^7\mathrm{Be}$, ${}^9\mathrm{Be}(\mathrm{p},\mathrm{n}){}^9\mathrm{B}$, or ${}^9\mathrm{Be}(\mathrm{d},\mathrm{n}){}^{10}\mathrm{B}$. The following beam parameters were used: beam energy of 2.5 MeV (for Li target)/5 MeV (for Be target), ion source current (50 keV, CW 30 mA), normalized beam emittance of 0.02 cm mrad, and resonance frequency of 325 MHz (for proton)/162.5 MHz (for deuteron). Based on these conditions, we evaluated the feasibility on the following three criteria: comparison of the cooling capacity of the refrigerator to the amount of heat, power consumption of AC, and size of the BNCT system. First, we evaluated the amount of heat generated in a cryomodule by adding the ohmic loss of SC-RFQ $Q_{\mathrm{rf}}$, beam losses in SC-RFQ $Q_{\mathrm{b}}$, heat penetration into the cryomodule $Q_{\mathrm{ext}}$, and beam losses of molecular ion beams and poor quality beams emitted from the ion-source at/near the RFQ entrance $Q_{\mathrm{emit}}$. In this study, we typically regarded $Q_{\mathrm{ext}}$ as 20 W at 4.2 K and considered a new low-energy beam transport system that can suppress $Q_{\mathrm{emit}}$ to 0. In addition, $Q_{\mathrm{rf}}$ and $Q_{\mathrm{b}}$ were numerically evaluated by beam simulation and electromagnetic calculation. The obtained results revealed that the sum of the heat amounts could be sufficiently suppressed below the typical cooling capacity of a commercially available helium refrigerator. Second, we compared the ac power consumption of BNCT between a conventional and SC-BNCT systems, which indicated that the BNCT system adopting the SRF cavity effectively reduced the ac power consumption of SC-BNCT by almost $1/4$ times. Third, the length of the SC-RFQ could be shortened by adjusting the peak surface E-field, as compared to conventional existing RFQs such as J-PARC, SNS, and IFMIF. Eventually, this study demonstrated that the application of the SRF cavity for the rf-linac-based neutron source of BNCT is feasible, and thus provides a foundation for the future development of design for next-generation BNCT systems.

Understanding beam and spin dynamics are fundamental in accelerator-based experiments concerning polarized beams. And there has been growing interest in having more precise estimations of the beam and spin dynamics variables, as more high precision particle physics experiments in the intensity frontier appear. This paper provides analytical estimations of some of the important variables such as beam transverse chromaticities and corrections to the spin precession frequency in the simplest type of particle accelerator: a circular magnetic storage ring with weak vertical focusing. We attempt to precisely obtain the next order approximations from the small betatron oscillations or the momentum dispersion, verified by high precision spin tracking simulation. We also discuss a potential way to suppress the corrections to the spin precession frequency, which relevant experiments may find beneficial.

Recent advances in superconducting rf (SRF) technology allow for increasingly more reliable and higher power hadron linacs. They hold enormous potential for economic, scientific, medical, environmental, and national security fields. Moving this technology from one of a kind laboratory demonstration devices to industrial scale applications imposes strict requirements on the linac performance in terms of its stability, reliability, and availability. The Spallation Neutron Source (SNS) is currently the highest average power superconducting proton linac in the world that has been routinely operated since 2006. We present statistical data on the SNS SRF linac reliability obtained from its practical operational experience during 2020. We analyze the frequency and duration of SRF cavity trips and identify their causes. These data will show that SRF cavity trips are the most common source of single-point failure within the linac. In an industrial environment, where redundancy is a necessity to prevent interruption of facility operation, the linac beam can be quickly restored by redistributing the failed cavity function to downstream energy reserve SRF cavities. We present a practical demonstration of this approach. We intentionally turn off one of the cavities and recover the beam by readjusting the phases of downstream cavities to maintain the same linac beam energy output. We discuss limitations of this approach at SNS and how they can be overcome.

CERN’s Super Proton Synchrotron (SPS) accelerates protons to $450\mathrm{GeV}/c$ and transfers them into the Large Hadron Collider (LHC). It is currently one of the limiting factors in increasing the beam intensity and thus the luminosity of the LHC. As part of the LHC injectors upgrade project, the SPS 200 MHz rf system has been modified during the CERN long shutdown 2 (January 2019–April 2021), resulting in a new layout containing two additional cavities. The goal is to improve longitudinal stability required for the planned doubling of the beam intensity for the high luminosity LHC. In parallel with the upgrade of the high-power rf, a new low-level rf (LLRF) system has been designed, including a new cavity field regulation system. This work presents a model of the beam-rf interaction which includes a detailed representation of the LLRF controlling the cavity. This model is used to determine the optimal LLRF design for maximum loop stability and beam loading compensation. Finally, the performance of the upgraded LLRF is estimated.

Significant progress has been made at Argonne National Laboratory in the development of a compact wakefield accelerator based on a cylindrical corrugated waveguide with a 2-mm ID and fine corrugations on the wall. The fabrication process of 10-cm-long corrugated waveguide structures has been established and a high quality of the final product has been confirmed by precision metrology. Several samples have been tested using the electron beam at Brookhaven National Laboratory’s Accelerator Test Facility. The frequency and group velocity of the fundamental monopole ${\mathrm{TM}}_{01}$ mode of sub-terahertz Čerenkov radiation produced by a 1.5-ps-long electron bunch propagating in the corrugated waveguide, as well as the frequencies of the dipole ${\mathrm{HEM}}_{11}$ and the quadrupole ${\mathrm{HEM}}_{21}$ modes have been measured and found to be in good agreement with the design values calculated using CST Microwave Studio. The energy modulation of a 5-ps-long electron bunch caused by the self-induced wakefield has also been measured and found to be in good agreement with the calculated values.

Exact numerical solutions for the single particle equations of motion have revealed conditions for highly nonlinear rapid acceleration near cyclotron resonance for electrons injected into a ${\mathrm{TE}}_{111}$-rotating-mode cylindrical microwave cavity immersed in a uniform axial magnetic field. We dub this the eCRA interaction. Magnitudes of acceleration energy in eCRA are shown to exceed to a large degree the limits for the related cyclotron autoresonance acceleration (CARA) interaction, wherein autoresonance acceleration is sustained for traveling rotating ${\mathrm{TE}}_{11}$-mode waves in a cylindrical waveguide. As with CARA, all injected electrons in an idealized eCRA enjoy equal energy gain without bunching. Injection of high currents that involve heavy beam loading allow acceleration in eCRA to multi-MeV levels for beams with average powers of hundreds of kW and rf-to-beam power efficiencies that exceed 80%. It is shown, to cite one example, that an effective acceleration gradient of over $90\mathrm{MV}/\mathrm{m}$ can be sustained with a maximum cavity surface field of only $40\mathrm{MV}/\mathrm{m}$, when producing a 4.5 MeV, 300 kW average power electron beam, with an rf-to-beam efficiency of about 86%. In that example, the cavity operates at 2.856 GHz, and the cavity’s average surface heating rate is $100\mathrm{W}/{\mathrm{cm}}^2$. Other examples are given for beams with over one MW levels of average power and energies up to about 20 MeV. This paper’s goal is only to elucidate and give examples of the basic mechanism for the strongly nonlinear acceleration that is predicted to occur in eCRA, rather than to present a particular engineered design. Still, the predicted parameters for an idealized eCRA suggest that practical realizations could emerge to satisfy a range of needs for efficient, compact accelerators for industrial, commercial, and national security applications.

The standing wave TESLA niobium-based superconducting radio frequency structure is limited to an accelerating gradient of about $50\mathrm{MV}/\mathrm{m}$ by the critical rf magnetic field. To break through this barrier, we explore the option of niobium-based traveling wave (TW) structures. Optimization of TW structures was done considering experimentally known limiting electric and magnetic fields. It is shown that a TW structure can have an accelerating gradient above $70\mathrm{MeV}/\mathrm{m}$ that is about 1.5 times higher than contemporary standing wave structures with the same critical magnetic field. The other benefit of TW structures shown is $R/Q$ about 2 times higher than the TESLA structure that reduces the dynamic heat load by a factor of 2. A method is proposed how to make TW structures multipactor-free. Some design proposals are offered to facilitate fabrication. Further increase of the real-estate gradient (equivalent to $80\mathrm{MV}/\mathrm{m}$ active gradient) is also possible by increasing the length of the accelerating structure because of higher group velocity and cell-to-cell coupling. Realization of this work opens paths to International Linear Collider energy upgrades beyond 1 to 3 TeV in competition with CLIC. The paper will discuss corresponding opportunities and challenges.

This paper presents results of simulations of synchrotron radiation from electron beams affected by instabilities in position and angle at the source point. The goal of this work is to develop and test instrumentation for studies of the effects of the electron beam’s vibrations and drifts on beamline performance at synchrotron light sources. To perform these simulations, numerical methods were developed and implemented in the Python version of the Synchrotron Radiation Workshop (srw) code. The simulation results were compared against analytical formulas and experimental results obtained from an x-ray pinhole-camera diagnostics beamline at NSLS-II. We compared the measured data with the prediction by our code for amplitudes of orbit oscillation from $6–60\mu \mathrm{m}$ and frequencies between 1 and 40 Hz, also taking into account the effects of misalignment of the pinhole. The simulation results are in good agreement with both analytical estimates and experimental data.

In our previous study of bunch lengthening for the planned Hefei Advanced Light Facility (HALF) storage ring, a periodic transient beam loading effect was found in the presence of passive harmonic cavities for some cases, which can dramatically affect the bunch lengthening as well as the beam stability [T. L. He et al., Bunch lengthening of the HALF storage ring in the presence of passive harmonic cavities, in Proceedings of the IPAC2021, Campinas, SP (JACoW, Campinas, SP, Brazil, 2021), pp. 2082–2085, TUPAB265]. In this paper, we continue to use HALF as an example, via both tracking simulation and semianalytical calculation, to comprehensively study the characteristics of this periodic transient effect, analyze its dependence on some relevant factors and investigate the impact of fill pattern on its periodicity. It is found that there exits a threshold current for this periodic transient effect, which can be increased by the reduction of $R/Q$ of harmonic cavity (HC) and the increase of main cavity (MC) voltage, HC detuning, momentum compaction, and energy spread, as well as the inclusion of short-range wake. In addition, the periodicity of bunch centroid oscillation can be affected by the damping time and MC beam loading, and completely interrupted due to the nonuniform fill pattern, but this is not helpful for improvement of bunch lengthening.

Typically, in self-Amplified spontaneous emission free electron laser (SASE FEL) based short-pulse schemes, pulse duration is limited by FEL coherence time. For hard x-ray FELs, coherence time is in a few hundred attosecond range while for XUV and soft x-ray FELs it is in the femtosecond regime. In this paper the modification of so-called chirp-taper scheme is developed that allows to overcome the coherence time barrier. Numerical simulations for XUV and soft x-ray FEL user facility FLASH demonstrate that one can generate a few hundred attosecond long pulses in the wavelength range 2–10 nm with peak power reaching hundreds of megawatts. With several thousand pulses per second this can be a unique source for attosecond science.

For a precise determination of the rf properties of superconducting materials, a calorimetric measurement is carried out with the aid of a so-called quadrupole resonator (QPR). This procedure is affected by certain systematic measurement errors with various sources of uncertainties. In this paper, to reduce the impact of geometrical uncertainties on the measurement bias, the modified steepest descent method is used for the multiobjective shape optimization of a QPR in terms of an expectation measure. Thereby, variations of geometrical parameters are modeled by the polynomial chaos expansion technique. Then, the resulting Maxwell’s eigenvalue problem with random input data is solved using the polynomial chaos-based stochastic collocation method. Furthermore, to assess the contribution of the particular geometrical parameters, the variance-based sensitivity analysis is proposed. This allows for modifying the steepest descent algorithm, which results in reducing the computational load needed to find optimal solutions. Finally, optimization results in the form of an efficient approximation of the Pareto front for a three-dimensional model of the QPR are shown.

Short bunch proton beams are of great significance for the applications of white neutron beams and muon beams. The accelerator complex of the China Spallation Neutron Source (CSNS) was designed to support the applications mainly based on neutron scattering techniques where the proton pulse length is not very sensitive. Some theoretical and experimental studies have been performed to see if one can extract a short-bunch proton beam by bunch rotation from the rapid cycling synchrotron (RCS) at CSNS. The experimental results at RCS have evidently displayed the bunch lengthening and rotation process, which demonstrates the effectiveness of this method even with a very short available time for the rf gymnastic processes and a high-intensity beam. With a beam power of 50 kW and normal longitudinal emittance at the injection, the proton beam with a bunch length of about 53% with respect to the one in the normal operation mode was obtained and transported to the spallation target. With a reduced longitudinal emittance at injection and the beam power of 30 kW, the shortest extraction bunch length obtained is about 26% of the one in the normal operation mode. Different machine settings have also been tested to show the impact of the desynchronization between the rf and magnetic fields, the influence of the nonadiabatic rise time, and the adiabatic decay time of the rf voltage on the extraction bunch length. The experimental results are well consistent with the theoretical and simulated ones. It is interesting to observe that space charge has a beneficial effect on the bunch lengthening which will result in a shorter bunch at the extraction with the later bunch rotation. The controlled desynchronization method between the rf and magnetic fields in an RCS was also proven successful.

The coupled-bunch instability for arbitrary multibunch configurations is investigated in its generality. The theoretical framework adopted is based on the general eigenvalue analysis based on the known formulas of the complex frequency shifts for the uniform multibunch configuration case. Closed formulas are derived for special cases. For the configuration consisting of a uniform filling pattern with a gap of missing bunches, the theoretical results are found to be in good agreement with measurements of the transverse coupled-bunch instability driven by the resistive wall impedance performed at the NSLS-II storage ring.

Recent studies have shown a novel coherent head-tail instability induced by beam-beam interaction with a large Piwinski angle. The instability has become an important issue during the designs of CEPC and FCC-ee. Simulations have further revealed that the longitudinal impedance has a strong impact on the beam stability, squeezing the horizontal stable tune area seriously. The cross-wake force has been introduced to represent beam-beam interaction. A mode coupling theory based on the localized wake (impedance) force has been developed to explain the instability. However, the theory did not consider the effects of longitudinal impedance. In this paper, we develop a new transverse mode coupling analysis method that could be used to study the beam-beam instability with and without longitudinal impedance. The result shows that the distortion of longitudinal phase space trajectory and the incoherent synchrotron tune shift induced by longitudinal impedance would reduce the stable tune area.

Laser-triggered ionization injection is a promising way of generating controllable high-quality electrons in plasma-based acceleration. We show that ionization injection of electrons into a fully nonlinear plasma wave wake using a laser pulse comprising of one or more Laguerre-Gaussian modes with combinations of spin and orbital angular momentum can generate exotic three-dimensional spatial distributions of high-quality relativistic electrons. The phase dependent residual momenta and initial positions of the ionized electrons are encoded into their final phase space distributions, leading to complex spatiotemporal structures. The structures are formed as a result of the transverse (betatron) and longitudinal (phase slippage and energy gain) dynamics of the electrons in the wake immediately after the electrons are injected. Theoretical analysis and three-dimensional simulations verify this mapping process leads to the generation of these complex topological beams. These beams may trigger novel beam-plasma interactions as well as produce coherent radiation with orbital angular momentum when sent through a resonant undulator.

We present design and realization of an ultrabroadband optical spectrometer capable of measuring the spectral intensity of multioctave-spanning light sources on a single-pulse basis with a dynamic range of up to eight orders of magnitude. The instrument is optimized for the characterization of the temporal structure of femtosecond long electron bunches by analyzing the emitted coherent transition radiation spectra. The spectrometer operates within the spectral range of 250 nm to $11.35\mu \mathrm{m}$, corresponding to 5.5 optical octaves. This is achieved by dividing the signal beam into three spectral groups, each analyzed by a dedicated spectrometer and detector unit. The complete instrument was characterized with regard to wavelength, relative spectral sensitivity, and absolute photometric sensitivity, always accounting for the light polarization and comparing different calibration methods. Finally, the capability of the spectrometer is demonstrated with a coherent transition radiation measurement of a laser wakefield accelerated electron bunch, enabling to determine temporal pulse structures at unprecedented resolution.

The electron gun of the REXEBIS charge breeder at the REX/HIE-ISOLDE facility at CERN has been upgraded from a standard magneto-immersed type to a gun using a nonadiabatic magnetic element. The results from the cathode emission and electron beam propagation tests are presented, as well as the charge breeding efficiency for the new design. Complete mass-scans of the extracted beam have been performed from which the level of cathode-originating contaminations could be established, as well as partial pressures of the most abundant residual gases in the ion trapping region. Furthermore, optimal breeding times for a broad range of elements and charge states, either introduced as a gas or externally injected as singly charged ions into the trapping region, are given for different electron currents. From these values, effective electron current densities have been derived. Finally, the axial ion energy distributions of various elements and charge states were also measured, and the derived ion temperatures were correlated with the ion and electron beam overlap factors.

When a relativistic beam moves along a curved trajectory it loses energy on coherent radiation. The effect of this radiation on the beam dynamics is described by the so call coherent synchrotron radiation (CSR) longitudinal wake field. Among different models of the CSR wake, the simplest is the one-dimensional (1D) one where the beam is treated as a line-charge. The self-field of the beam on a curved trajectory also creates a transverse component of the force. Unfortunately, there is a confusion in the literature as to whether a 1D model can be worked out for the transverse force inside the beam. In this paper, we show how such a 1D model can be consistently derived for a general curvilinear beam orbit if one formulates the equations of motion for the beam particles in Hamiltonian form and uses a renormalized transverse force. Unfortunately, this scheme cannot be applied to the classical problem of relativistic beam passing through a single bending magnet, because the scalar potential in the initial conditions for the Hamiltonian variables cannot be defined in the 1D model. In this case, one should use the 3D transverse force, for which we derive analytical expressions. For the steady state, our calculations show an excellent agreement with computer simulations. We also calculate the transient effects for this force at the entrance to and the exit from the bend. Our results provide a new way to accommodate the transverse force into the existing and new simulation codes which is important for many applications of the high-current, small-emittance relativistic beams.

We demonstrate experimentally that hydrodynamic optical-field-ionized (HOFI) plasma channels can be generated at kHz-scale pulse repetition rates, in a static gas cell and for an extended period. Using a pump-probe arrangement, we show via transverse interferometry that the properties of two HOFI channels generated 1 ms apart are essentially the same. We demonstrate that HOFI channels can be generated at a mean repetition rate of 0.4 kHz for a period of 6.5 h without degradation of the channel properties, and we determine the fluctuations in the key optical parameters of the channels in this period. Our results suggest that HOFI and conditioned HOFI channels are well suited for future high-repetition rate, multi-GeV plasma accelerator stages.