To improve RHIC luminosity for heavy ion beam energies below $10\mathrm{GeV}/\text{nucleon}$, the Low Energy RHIC electron Cooler (LEReC) is currently under commissioning at BNL. The linac of LEReC is designed to deliver a 1.6–2.6 MeV electron beam, with rms $dp/p$ less than $5\times {10}^{-4}$. A 704 MHz superconducting radio frequency booster cavity in this linac provides up to 2.2 MeV accelerating voltage. With such a low energy and very demanding energy spread requirement, control of higher order modes (HOMs) in the cavities becomes critical and needs to be carefully evaluated to ensure minimum impact on the beam. In this paper, we report the multiphysics design of the HOM damper for this cavity to meet the energy spread requirement, as well as experimental results of the cavity with and without the HOM damper.

An experimental campaign was recently completed at the U.S. Naval Research Laboratory’s Mercury pulsed-power facility, where the feasibility of using a 5 MV inductive voltage adder (IVA) as a pulsed photoneutron source was studied. In these experiments, a large-area bremsstrahlung diode was fielded on the Mercury accelerator, producing an intense, pulsed x-ray beam, which generated photoneutrons when striking an appropriate target. This paper reports on simulations that were performed to study the production of the electron beam in the diode, and the generation and transport of the x-ray beam. Comparison is made between the numerically predicted beam properties and results obtained during the experimental campaign. Various models of electron and ion emission from the electrodes in the generator were simulated, and the effect of model parameter choices on the dose predictions is described.

Measurements of transverse profiles using ionization profile monitors (IPMs) for high brightness beams are affected by the electromagnetic field of the beam. This interaction may cause a distortion of the measured profile shape despite strong external magnetic field applied to impose limits on the transverse movement of electrons. The mechanisms leading to this distortion are discussed in detail. The distortion itself is described by means of analytic calculations for simplified beam distributions and a full simulation model for realistic distributions. Simple relation for minimum magnetic field scaling with beam parameters for avoiding profile distortions is presented. Further, application of machine learning algorithms to the problem of reconstructing the actual beam profile from distorted measured profile is presented. The obtained results show good agreement for tests on simulation data. The performance of these algorithms indicate that they could be very useful for operations of IPMs on high brightness beams or IPMs with weak magnetic field.

The coherent process of particle deflection by aligned atomic strings and planes of oriented crystals is accompanied by incoherent scattering by atomic cores. While the coherent particle deflection, described by the axial or planar averaged potential, becomes more and more classical at high energies, the incoherent scattering of relativistic particles remains essentially quantum. Though the incoherent scattering of relativistic particles in crystals reminds the scattering by atoms of an amorphous medium at a large momentum transfer, at small ones the incoherent scattering process in crystals is modified by the influence of the inhomogeneity of the atom distribution in the plane normal to the crystal axis or plane. We present a theory of incoherent scattering of high energy particles in oriented crystals, which takes into consideration both its quantum nature and the impact of the atom distribution inhomogeneity. The axial case is considered as a more general example. The way to incorporate the revealed quantum features into classical trajectory simulations is also outlined.

Operation of superconducting radio frequency (SRF) cavities with high loaded quality factors is becoming increasingly preferred for applications which involve low beam loading including energy recovery linacs (ERL). Vibration induced microphonics detuning poses a major operational bottleneck in these low bandwidth systems, adversely affecting field stability. Besides passive measures of mitigating the vibration sources, modern SRF cavities are also attached to fast tuners incorporating piezoelectric actuators. We demonstrate the narrow band active noise control algorithm for realizing active resonance control and propose a modification based on the least mean square approach to adaptively tune the control parameters and study its stability and performance. We discuss our experience of using passive mitigation techniques while commissioning the main linac cryomodule of the Cornell-BNL ERL test accelerator and report a net reduction in peak detuning by more than a factor of 2 in its unstiffened cavities. Finally, we demonstrate stable performance of our resonance control system with consistent reduction of peak microphonics detuning by almost a factor of 2 on multiple cavities.

An amplitude dependent closest tune approach (ADeCTA) was first observed in the LHC in 2012. This potentially harmful effect can be generated by linear coupling in conjunction with octupoles. In this article we demonstrate experimentally and in simulations a configuration of the octupoles to suppress this recently observed effect. Furthermore, ADeCTA is investigated for the first time with driven oscillations. We show analytically that skew quadrupoles and normal octupoles do not generate ADeCTA for forced oscillations in the same way as for free oscillations. This adds an additional constraint to measure it since the forced oscillations, generated by an ac dipole, are the preferred option for optics measurements in the LHC. The analytical result is confirmed by both measurements and simulations.

In this paper, we extend the Mattis-Bardeen theory to obtain the surface impedance of a two-band superconductor, and apply it to magnesium diboride (${\mathrm{MgB}}_2$) for radiofrequency (rf) superconductivity applications. The numerical results for ${\mathrm{MgB}}_2$ are in good agreement with the previously published experimental results. The surface impedance properties are clearly dominated by the smaller gap, significantly limiting utility in the 10-20 K regime that might otherwise have been attractive.

At the Istituto Nazionale di Fisica Nucleare–Laboratori Nazionali del Sud (INFN-LNS), the commissioning of the high intensity proton source for the European Spallation Source (PS-ESS) has been recently completed. Optical emission spectroscopy measurements carried out with PS-ESS permitted, in a noninvasive way, the evaluation of the electron density, temperature, and relative abundances of the neutral components of a hydrogen plasma. This approach is helpful in finding the optimal source parameters exploiting protons or $H_2^{+}$ beams production.

Particle storage rings are a rich application domain for online optimization algorithms. The Cornell Electron Storage Ring (CESR) has hundreds of independently powered magnets, making it a high-dimensional test-problem for algorithmic tuning. We investigate algorithms that restrict the search space to a small number of linear combinations of parameters (“knobs”) which contain most of the effect on our chosen objective (the vertical emittance), thus enabling efficient tuning. We report experimental tests at CESR that use dimension-reduction techniques to transform an 81-dimensional space to an 8-dimensional one which may be efficiently minimized using one-dimensional parameter scans. We also report an experimental test of a multiobjective genetic algorithm using these knobs that results in emittance improvements comparable to state-of-the-art algorithms, but with increased control over orbit errors.

In this paper we propose a double chirp-taper method to produce two subfemtosecond x-ray free-electron lasers (XFELs) using electron beams with sinusoidal energy modulation. The taper of the undulator is optimized to control the power profile of each XFEL pulse to obtain a single-spike power shape. We present start-to-end numerical simulations of this method to demonstrate the generation of two XFEL pulses with $\sim 0.4\mathrm{fs}$ full width at half maximum, with a time separation tunable from 0 to tens of fs and energy separation limited by the available undulator tuning range. The output of this method provides a powerful tool for XFEL pump-probe experiments at the attosecond timescale.

Today’s superconducting radio-frequency (srf) cavities approach quality factors up to $10^{11}$. With the decrease in overall dissipation the loss contributions previously considered minor gain in importance. One contribution is trapped magnetic flux. Superconducting rf cavities have to be cooled down to their cryogenic operating temperature. During the process, high temperature differences can occur in the system driving thermoelectric currents in the cavity wall and the surrounding liquid helium tank. The associated magnetic field can get trapped in the superconductor during its phase transition from the normal conducting into the superconducting (sc) state and significantly increase dissipation during later operation. The existence of this effect has been proven in general. The study presented here now adds experimental data and simulations on the amplitude and distribution of the trapped magnetic flux after the sc phase transition in the complete nine-cell cavity geometry. Furthermore, the dynamic behavior in the thermoelectrically generated magnetic field during the phase transition is evaluated.

We develop a general method for assessing the evolution of the energy spectrum of relativistic electrons that undergo small quantum losses, such as the ionization losses when the electrons pass through matter and the radiation losses in periodic fields. These processes are characterized by a small magnitude of the recoil quantum as compared with the particle’s initial energy. We convey the statistical consideration of the radiating electrons and demonstrate that, for a small average number of recoils, the electron energy spectrum can be described as a composition of consecutive convolutions of the recoil spectrum with itself, weighted with a Poisson distribution. In this stage, the electron’s spectrum reveals some individual characteristics of the recoil spectrum. Later, the spectrum loses individuality and allows for an approximate description in terms of statistical parameters. This consideration reveals that the width of the electron’s spectrum is increasing with the number of recoils according to a power law, with the power index being inverse to the stability parameter, which gradually increases with the number of recoils from one to two. The increase of the spectrum width limits the ability of the beam to generate coherent radiation in the hard x-ray and gamma-ray region.

An electron beam experiences chromatic emittance growth in a plasma-based accelerator if it is not matched to the focusing force in the plasma wake. A ramped plasma density profile at the entrance and exit of the plasma source can control the focusing of the beam into and out of the plasma accelerator, limiting emittance growth. Here, we present a comprehensive, analytic theory to describe the transverse beam dynamics and emittance growth in a nearly arbitrary plasma ramp profile. For a given incoming beam, this theory can be used to determine the length of the ramp required to correctly focus the electron beam, the optimal location of the beam’s vacuum focus, and the chromatic emittance growth in the ramp. In addition, the theory can be used to determine the effect that errors in the beam focusing and plasma profile have on the emittance of the beam. We illustrate two example ramps to demonstrate the theory: one that provides very fast focusing for beam matching, and one that is robust to errors in the plasma density profile.

The transverse mode coupling instability of a bunched beam is investigated in the paper at different forms of the bunches, with space charge included. An equation of transverse motion of the bunch in the parabolic potential well of synchrotron oscillations is derived and analyzed. The bunch of constant density (flat bunch) is examined in detail to make a comparison with the square well model. It is shown that both models result in very close instability thresholds of the flat bunch. Then different form bunches are investigated in the parabolic potential well. It is shown that decrease of the bunch RMS length leads to the growth of its threshold, that is the flat bunch model gives only a minimal estimation of the threshold. The results are treated in terms of Landau damping due to a spread of the space charge tune shift.

This paper considers how the finite dimensions of a photonic crystal placed inside a resonator or waveguide affect the law of electron beam instability. The dispersion equations describing e-beam instability in the finite photonic crystal placed inside the resonator or waveguide (a bounded photonic crystal) are obtained. Application of photonic crystals for development of THz sources at electron beam current densities available at modern accelerators is discussed.

Accelerator-based terahertz (THz) radiation has been expected to realize a high-power broadband source. Employing a low-emittance and short-bunch electron beam at a high repetition rate, a scheme of coherent diffraction-radiation in an optical cavity layout is proposed. The scheme’s stimulated radiation process between bunches can greatly enhance the efficiency of the radiation emission. We performed an experiment with a superconducting linac constructed as an energy recovery linac (ERL) test facility. The electron beam passes through small holes in the cavity mirrors without being destroyed. A sharp THz resonance signal, which indicates broadband stimulated radiation correlated with beam deceleration, was observed while scanning the round-trip length of the cavity. This observation proves the efficient beam-to-radiation energy conversion due to the stimulated radiation process.

Free-electron lasers (FEL) and synchrotron sources of high brilliance x-rays have proven to be of tremendous value in basic and applied research. Inverse-Compton sources (ICS) can achieve brilliance matching the requirements of many applications pioneered at those FEL and synchrotron facilities—including phase contrast imaging, macromolecular x-ray crystallography, and x-ray microscopy—but with size, cost, and complexity compatible with a small laboratory. The free-electron laser inverse-Compton interaction compact x-ray source at the University of Hawaii at Manoa is a unique approach to an ICS which employs an FEL as the laser source. We have measured a total average flux of $3.0\times {10}^5\mathrm{photons}/\mathrm{second}$ with an average brilliance of $2.0\times {10}^7\mathrm{photons}/\mathrm{s}{\mathrm{mm}}^2{\mathrm{mrad}}^2$ 0.1% of bandwidth (BW) with a peak energy of 10.9 keV from the source. While these results are modest in comparison to the standards set by other IC sources, upgrades to the system have the potential to increase the total average flux to $9.2\times {10}^{11}\mathrm{photons}/\mathrm{second}$ with an average brilliance of $1.9\times {10}^{12}\mathrm{photons}/\mathrm{s}{\mathrm{mm}}^2{\mathrm{mrad}}^2$ 0.1% BW: comparing more favorably to other sources. We discuss the scientific program, the progress made in design and development, and the achievements of the source to date. We also outline future upgrades and integration needed to yield an enabling source for emerging high brilliance x-ray applications.

A hybrid seven-bend-achromat lattice that features very strong focusing elements and a relatively small vacuum chamber has been proposed for the Advanced Photon Source (APS) upgrade. The design lattice parameters during commissioning will need to be quickly achieved in order to minimize the interruption of user operation. This paper describes start-to-end simulation of the machine commissioning beginning from first-turn trajectory correction, progressing to orbit correction, and culminating in lattice correction and coupling adjustment. The automated commissioning procedure shows that the rapid commissioning of the ultralow emittance lattice is possible. It also enables a more rigorous statistical evaluation of expected performance of proposed lattices.

We describe the use of a genetic algorithm to apply active feedback to a laser wakefield accelerator at a higher power (10 TW) and a lower repetition rate (5 Hz) than previous work. The temporal shape of the drive laser pulse was adjusted automatically to optimize the properties of the electron beam. By changing the software configuration, different properties could be improved. This included the total accelerated charge per bunch, which was doubled, and the average electron energy, which was increased from 22 to 27 MeV. Using experimental measurements directly to provide feedback allows the system to work even when the underlying acceleration mechanisms are not fully understood, and, in fact, studying the optimized pulse shape might reveal new insights into the physical processes responsible. Our work suggests that this technique, which has already been applied with low-power lasers, can be extended to work with petawatt-class laser systems.