Longitudinal charge profile measurements or bunch shape measurements are a challenge for temporally short nonrelativistic bunches. The field profile has a larger longitudinal extent compared to the charge profile for such beams. This affects the ability of field sensing devices such as phase pickups or wall current monitors to measure charge distribution. Here we evaluate the feasibility for usage of coherent transition [V. L. Ginzburg and V. N. Tsytovich, Transition Radiation and Transition Scattering (Adam Hilger, New York, 1990)] and diffraction radiation from nonrelativistic beams for single shot bunch shape measurements in the time domain.

Laser-plasma acceleration (LPA) is a compact technique to accelerate electron bunches to highly relativistic energies, making it a promising candidate to power radiation sources for industrial or medical applications. We report on the generation of electron beams from an 80 MeV-level LPA setup based on ionization injection (II) over a duration of 8 hours at a repetition rate of 2.5 Hz, resulting in 72,000 consecutive shots with charge injection and acceleration. Over the full operation time the moving averages of the total beam charge of 14.5 pC and the charge between 70–80 MeV did not drift on a detectable level. The largest source of shot-to-shot jitter was in the beam charge (26% standard deviation), which was most strongly correlated with fluctuations in the plasma density (3.6% standard deviation). Particle-in-cell simulations demonstrate that this was chiefly caused by stronger laser self-focusing in higher density plasmas, which significantly increased the ionized charge along with the emittance of the beam. The nonlinearity of this process imposes tight constraints on the reproducibility of the laser-plasma conditions required for a low jitter II-LPA output if self-focusing plays a role in the laser evolution.

The slow high-efficiency extraction from a ring positron accelerator (SHERPA) project’s aim is to develop an efficient technique to extract a positron beam from one of the accelerator rings composing the $\mathrm{DA}\mathrm{\Phi }\mathrm{NE}$ complex at the Frascati National Laboratory of INFN, setting up a new beam line able to deliver positron spills of O(ms) length, excellent beam energy spread and emittance. The most common approach to slowly extract from a ring is to increase betatron oscillations approaching the third order tune resonance to gradually eject particles from the circulating beam. SHERPA proposes a paradigm change for lepton machines using coherent processes in bent crystals to kick out positrons from the ring, a cheaper and less complex alternative. A description of this innovative nonresonant extraction technique is reported in this manuscript, including its performance preliminary estimation.

A notable impediment in maintaining high electric fields in accelerating structures is the onset of breakdown events. While bulk mechanical properties of the materials are known to significantly affect the breakdown propensity, the underlying mechanisms coupling electric fields to bulk plastic deformation in experimentally relevant thermal and electrical loading conditions remain to be identified at the atomic scale. We present the results of large-scale molecular dynamics simulations (MD) to investigate a possible mode of coupling. Specifically, we consider the activation of Frank-Read sources, which leads to dislocation multiplication, under the combined action of biaxial thermal stresses caused by rf losses and surface tractions induced by electric fields. With the help of a charge-equilibration formalism incorporated in a classical MD model, we show that the creation of surface slipped steps can couple to electric fields in a way that enhances local stresses and facilitates further activation of existing dislocation sources. We quantify the possible enhancement of surface slip under typical microstructural parameters of annealed copper. We show that such a mechanism could potentially promote breakdown precursor formation at very high electric fields, but that its impact is limited at fields typical of the operation of accelerator structures. In this regime, thermal stresses caused by rf losses are expected to be the main drivers of plastic deformation.

The longitudinal-transverse coupling is an important source for space-charge-induced emittance growth in high intensity proton and ion linacs. Different from the equipartitioning method which tries to avoid the longitudinal-transverse coupling, a new design approach has been developed to minimize emittance growth via the low emittance transfer enabled by holding the ratio of longitudinal emittance to transverse emittance in the range of 0.9–1.4. Using a high intensity radio-frequency quadrupole accelerator as an example, a comparison between the new approach and the equipartitioning method has been made. Furthermore, input beams with non-nominal beam intensities and emittances have been applied to the designed accelerator. The simulation results show that the design obtained by following the new approach has a large tolerance for the off-design situations.

Compact dc high-voltage photoelectron guns are able to meet the sophisticated demands of high-current applications such as energy recovery linacs. A main design parameter for such sources is the electric field strength, which depends on the electrode geometry and is limited by the field emission threshold of the electrode material. In order to minimize the maximum field strength for optimal gun operation, isogeometric analysis (IGA) can be used to exploit the axisymmetric geometry and describe its cross section by nonuniform rational B-splines, the control points of which are the parameters to be optimized. This computationally efficient method is capable of describing CAD-generated geometries using open source software (geopdes, nlopt, octave) and it can simplify the step from design to simulation. We will present the mathematical formulation, the software workflow, and the results of an IGA-based shape optimization for a planned high-voltage upgrade of the dc photogun teststand Photo-CATCH at TU Darmstadt. The software builds on a general framework for isogeometric analysis and allows for easy adaptations to other geometries or quantities of interest. Simulations assuming a bias voltage of $-300\mathrm{kV}$ yielded maximum field gradients of $9.06\mathrm{MV}{\mathrm{m}}^{-1}$ on the surface of an inverted insulator electrode and below $3\mathrm{MV}{\mathrm{m}}^{-1}$ on the surface of the photocathode.

The space-charge-induced resonance band just above the betatron phase advance of 90° per cell is of great practical importance. The instability is closely related to the linear collective mode and can thus give rise to severe emittance growth at high beam density. In a circular machine, this type of second-order resonance occurs not only at half-integer tunes but also near quarter-integer tunes, depending on the lattice superperiodicity. Self-consistent numerical simulations are carried out to elucidate the resonance feature above the 90° cell tune. The present results suggest the existence of three different resonance mechanisms working there; namely, the fourth-order incoherent resonance in the beam tail, the second-order and fourth-order coherent resonances in the beam core. It is reconfirmed that no serious emittance growth occurs even if particles deep inside the core satisfy the incoherent resonance condition. The recently proposed stop-band diagram, free from the concept of incoherent tune spread, appears to be consistent with the numerical observations.

The European Spallation Source (ESS), currently under construction in Lund, Sweden, will be a neutron source based on a partly superconducting linac. The ESS linac will be accelerating protons to 2 GeV with a peak current of 62.5 mA and ultimately delivering a 5 MW beam to a rotating tungsten target for neutron production. For a successful tuning and operation of a linac, a beam loss monitoring (BLM) system is required. BLM systems are designed to protect the machine from beam-induced damage and unnecessary activation of the components. This paper focuses on one of the BLM systems to be deployed at the ESS linac, namely the neutron sensitive BLM (nBLM). An overview of the ESS nBLM system design will be presented. In addition to this, results of the tests performed with the full nBLM data acquisition chain will be discussed. These tests represent the first evaluation of the system prototype in a realistic environment. They served as an input to initial study of the procedure for neutron detection algorithm configuration discussed in this contribution as well.

We propose a model to explain power dissipation leading to the formation of hot spots in the inner walls of niobium thin film superconducting rf cavities. The physical mechanism that we explore is due to the constriction of surface electrical current flow at grain interface boundaries. This constriction creates an additional electrical contact resistance which induces localized punctual heat dissipation. The temperature at these spots is derived; and the electrical contact resistance is shown to depend on the magnetic field, on the grain contact size over which dissipation occurs, and on other key parameters, including the effective London penetration depth and the frequency. The surface resistance and the quality factors are determined using our model and are shown to be in excellent agreement with experimental data.

Any proposal for an accelerator facility based upon a multipass energy recovery linac (ERL) must possess a self-consistent match in longitudinal phase space, not just transverse phase space. We therefore present a semianalytic method to determine self-consistent longitudinal matches in any multipass ERL. We apply this method in collider scenarios (embodying an energy spread minimizing match) and FEL scenarios (embodying a compressive match), and discuss the consequences of each. As an example of the utility of the method, we prove that the choice of common or separate recirculation transport determines the feasibility of longitudinal matches in cases where disruption, such as synchrotron radiation loss, exists. We show that any high energy multipass ERL collider based upon common recirculation transport will require special care to produce a self-consistent longitudinal match, but that one based upon separate transport is readily available. Furthermore we show that any high energy multipass ERL FEL driver based upon common recirculation transport requires a larger resultant rf beam load than the one based on separate transport, favoring the separate transport designs.

We report the design, operation, and performance of a high-resolution, low-latency, bunch-by-bunch feedback system for nanobeam stabilization. The system employs novel, ultralow quality-factor cavity beam position monitors (BPMs), a two-stage analog signal down-mixing system, and a digital signal processing and feedback board incorporating a field-programmable gate array. The field-programmable gate array firmware allows for the real-time integration of up to fifteen samples of the BPM waveforms within a measured latency of 232 ns. We show that this real-time sample integration improves significantly the beam position resolution and, consequently, the feedback performance. The best demonstrated real-time beam position resolution was 19 nm, which, as far as we are aware, is the best real-time resolution achieved in any operating BPM system. The feedback was operated in two complementary modes to stabilize the vertical position of the ultrasmall beam produced at the focal point of the ATF2 beamline at KEK. In single-BPM feedback mode, beam stabilization to $50\pm 5\mathrm{nm}$ was demonstrated. In two-BPM feedback mode, beam stabilization to $41\pm 4\mathrm{nm}$ was achieved.

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.