A compact rf cavity was constructed that simultaneously resonates at many harmonic modes when excited by a repetitive bunched electron beam passing through its bore. The excitation of these modes provides a Fourier description of the temporal characteristics of the bunch train. The cavity was used to noninvasively characterize electron bunches produced from thin and thick GaAs photocathodes inside a DC high voltage photogun illuminated with 37 ps (full width half maximum, FWHM) laser pulses at repetition rates near 1500 MHz, at average beam current from 5 to $500\mu \mathrm{A}$, and at beam energy from 75 to 195 keV. The cavity bunch-length monitor could detect electron bunches as short as 57 ps (FWHM) when connected directly to a sampling oscilloscope, and could clearly distinguish bunches with varying degrees of space-charge induced growth and with different tail signatures. Efforts are under way to detect shorter bunches by designing cavities with increased bandwidth. This demonstration lends credibility to the idea that these cavities could also be used for other applications, including bunching and shaping, when driven with external rf.

New measurements are reported for the time dependence of the vertical polarization of a $0.97\mathrm{GeV}/\mathrm{c}$ deuteron beam circulating in a storage ring and perturbed by an rf solenoid. The storage ring is the cooler synchrotron (COSY) located at the Forschungszentrum Jülich. The beam polarization was measured continuously using a 1.5 cm thick carbon target located at the edge of the circulating deuteron beam and the scintillators of the EDDA detector. An rf solenoid mounted on the ring was used to generate fields at and near the frequency of the $1-G\gamma$ spin resonance. Measurements were made of the vertical beam polarization as a function of time with the operation of the rf solenoid in either fixed or continuously variable frequency mode. Using rf-solenoid strengths as large as $2.66\times {10}^{-5}\mathrm{revolutions}/\mathrm{turn}$, slow oscillations ($\sim 1\mathrm{Hz}$) were observed in the vertical beam polarization. When the circulating beam was continuously electron cooled, these oscillations completely reversed the polarization and showed no sign of diminishing in amplitude. But for the uncooled beam, the oscillation amplitude was damped to nearly zero within a few seconds. A simple spin-tracking model without the details of the COSY ring lattice was successful in reproducing these oscillations and demonstrating the sensitivity of the damping to the magnitude of the synchrotron motion of the beam particles. The model demonstrates that the characteristic features of measurements made in the presence of large synchrotron oscillations are distinct from the features of such measurements when made off resonance. These data were collected in preparation for a study of the spin coherence time, a beam property that needs to become long to enable a search for an electric dipole moment using a storage ring.

The purpose of this addendum is to clarify some of the information presented in the original paper.

Knowing the number and structure of the free electron laser’s (FEL) growing modes is of fundamental importance to its theoretical description and its applications. The one-dimensional FEL dispersion relation, including the space-charge effects, is well established in the FEL theory, but questions remain about the number of the growing solutions (modes). In this paper, we provide the definite answers to the latter question and identify the amplification bandwidth of these modes.

Nonlinear simulations are devoted to the comparative study of the equilibrium electrons’ motion and stability in the combination of a three-dimensional helical wiggler and a positive or reversed guide magnetic field, where effects of the self-field, off-axis guiding center and adiabatic magnetic field are included. It is shown that a reversed guide magnetic field configuration brings the electron motion much smaller Larmor radius and transverse displacement span than a positive guide magnetic field does, and consequently, provides better transportation quality of the electron beam. Although the electron motion far from the resonance and antiresonance is stable in both the positive and the reversed guide magnetic field configurations, fluctuation induced by a reversed guide magnetic field is smaller than that by a positive guide magnetic field, especially in the adiabatic region of the wiggler. In a certain parameters domain, the self-field of the electron beam is advantageous to confine the electrons’ motion at antiresonance.

Self-consistent particle-in-cell simulations are performed to verify earlier theoretical predictions of adiabatic thermal beams in a periodic solenoidal magnetic focusing field [K. R. Samokhvalova, J. Zhou, and C. Chen, Phys. Plasmas 14, 103102 (2007); J. Zhou, K. R. Samokhvalova, and C. Chen, Phys. Plasmas 15, 023102 (2008)]. In particular, results are obtained for adiabatic thermal beams that do not rotate in the Larmor frame. For such beams, the theoretical predictions of the rms beam envelope, the conservations of the rms thermal emittances, the adiabatic equation of state, and the Debye length are verified in the simulations. Furthermore, the adiabatic thermal beam is found be stable in the parameter regime where the simulations are performed.

The computation of tunes and matched beam distributions are essential steps in the analysis of circular accelerators. If certain symmetries—like midplane symmetry—are present, then it is possible to treat the betatron motion in the horizontal, the vertical plane, and (under certain circumstances) the longitudinal motion separately using the well-known Courant-Snyder theory, or to apply transformations that have been described previously as, for instance, the method of Teng and Edwards. In a preceding paper, it has been shown that this method requires a modification for the treatment of isochronous cyclotrons with non-negligible space charge forces. Unfortunately, the modification was numerically not as stable as desired and it was still unclear, if the extension would work for all conceivable cases. Hence, a systematic derivation of a more general treatment seemed advisable. In a second paper, the author suggested the use of real Dirac matrices as basic tools for coupled linear optics and gave a straightforward recipe to decouple positive definite Hamiltonians with imaginary eigenvalues. In this article this method is generalized and simplified in order to formulate a straightforward method to decouple Hamiltonian matrices with eigenvalues on the real and the imaginary axis. The decoupling of symplectic matrices which are exponentials of such Hamiltonian matrices can be deduced from this in a few steps. It is shown that this algebraic decoupling is closely related to a geometric “decoupling” by the orthogonalization of the vectors $\overrightarrow{E}$, $\overrightarrow{B}$, and $\overrightarrow{P}$, which were introduced with the so-called “electromechanical equivalence.” A mathematical analysis of the problem can be traced down to the task of finding a structure-preserving block diagonalization of symplectic or Hamiltonian matrices. Structure preservation means in this context that the (sequence of) transformations must be symplectic and hence canonical. When used iteratively, the decoupling algorithm can also be applied to $n$-dimensional systems and requires $\mathcal{O}(n^2)$ iterations to converge to a given precision.

Low-energetic ion and antimatter beams are very attractive for a number of fundamental studies. The diagnostics of such beams, however, is a challenge due to low currents down to only a few thousands of particles per second and significant fraction of energy loss in matter at keV beam energies. A modular set of particle detectors has been developed to suit the particular beam diagnostic needs of the ultralow-energy storage ring (USR) at the future facility for low-energy antiproton and ion research, accommodating very low beam intensities at energies down to 20 keV. The detectors include beam-profile monitors based on scintillating screens and secondary electron emission, sensitive Faraday cups for absolute intensity measurements, and capacitive pickups for beam position monitoring. In this paper, the design of all detectors is presented in detail and results from beam measurements are shown. The resolution limits of all detectors are described and options for further improvement summarized. Whilst initially developed for the USR, the instrumentation described in this paper is also well suited for use in other low-intensity, low-energy accelerators, storage rings, and beam lines.

Development of a nondestructive, efficient electric-charge-stripping method is a key requirement for next-generation high-intensity heavy-ion accelerators such as the RIKEN Radioactive-Isotope Beam Factory. A charge stripper employing a low-$Z$ gas is an important candidate applicable to high-intensity uranium beams for replacing carbon-foil strippers. In this study, a high-beam-transmission charge-stripping system employing helium gas for ${}^{238}\mathrm{U}$ beams injected at $10.8\mathrm{MeV}/\mathrm{u}$ was developed and demonstrated for the first time. The charge-state evolution measured using helium in a thickness range of $0.24–1.83\mathrm{mg}/{\mathrm{cm}}^2$ is compared with theoretical predictions. Energy attenuation and energy spread due to the helium stripper are also investigated.

The rapid cycling synchrotron (RCS) of Japan Proton Accelerator Research Complex (J-PARC) and the accumulator ring (AR) of Spallation Neutron Source (SNS) can be used as high-power pulsed neutron sources. In both cases, the injection region becomes one of the highest activation areas in the ring. However, residual dose distributions have revealed that the highest activation points in the J-PARC RCS and the SNS AR are different in detail. The dose of the charge exchange chamber in the SNS is more than 100 times larger than that of the RCS though the ratio of beam power is less than 10. We investigated the reason of this difference by Geant4 and MARS, and the calculation results indicated that the difference was due to the dependence of the neutron and pion production rate on the injection energy.

Estimating emittance growth is important for evaluation of performance of linear accelerators, especially where stable and low emittance beam is required, such as linear colliders. Usually, estimation of emittance growth is performed using tracking simulations, including the Monte Carlo method, which tend to take a long time. We have developed a much faster and simpler method of quantitative estimation. Formulas of orbit and emittance due to random misalignment without corrections were reported in our previous paper [K. Kubo,Phys. Rev. ST Accel. Beams 14, 014401 (2011)]. Here, formulas of emittance growth after a simple orbit correction are reported. This method is valid for very long linacs with many components, where statistical treatment is justified. It is shown that the results from the method agree well with tracking simulations for the International Linear Collider main linac.

The high transition temperature and low surface resistance of ${\mathrm{MgB}}_2$ attracts interest in its potential application in superconducting radio frequency accelerating cavities. However, compared to traditional Nb cavities, the viability of ${\mathrm{MgB}}_2$ at high rf fields is still open to question. Our approach is to study the nonlinear electrodynamics of the material under localized rf magnetic fields. Because of the presence of the small superconducting gap in the $\pi$ band, the nonlinear response of ${\mathrm{MgB}}_2$ at low temperature is potentially complicated compared to a single-gap s-wave superconductor such as Nb. Understanding the mechanisms of nonlinearity coming from the two-band structure of ${\mathrm{MgB}}_2$, as well as extrinsic sources of nonlinearity, is an urgent requirement. A localized and strong rf magnetic field, created by a magnetic write head, is integrated into our nonlinear-Meissner-effect scanning microwave microscope [T. Tai et al., IEEE Trans. Appl. Supercond. 21, 2615 (2011)]. ${\mathrm{MgB}}_2$ films with thickness 50 nm, fabricated by a hybrid physical-chemical vapor deposition technique on dielectric substrates, are measured at a fixed location and show a strongly temperature-dependent third harmonic response. We propose that several possible mechanisms are responsible for this nonlinear response.

Choke-mode damped structures are being studied as an alternative design to waveguide damped structures for the main linac of the Compact Linear Collider (CLIC). Choke-mode structures have the potential for lower pulsed temperature rise and simpler and less expensive fabrication. An equivalent circuit model based on transmission line theory for higher-order-mode damping is presented. Using this model, a new choke geometry is proposed and the wakefield performance is verified using Gdfidl. This structure has a comparable wakefield damping effect to the baseline design which uses waveguide damping. A prototype structure with the same iris dimensions and accelerating gradient as the nominal CLIC design, but with the new choke geometry, has been designed for high-power tests.

Short-lived, $\sim 10\mathrm{ps}$, deep plasma channels, with their lengths of $\sim 1\mathrm{mm}$ and diameters of $\sim 20\mu \mathrm{m}$, are observed and characterized in Ar gas jets irradiated by moderate intensity, $\sim {10}^{15–16}\mathrm{W}/{\mathrm{cm}}^2$, laser pulses with a duration from subpicosecond to several picoseconds. The channels, upon 2D particle-in-cell simulations including ionization, fit well in the guiding of high intensity femtosecond laser pulses and, therefore, in laser wakefield acceleration with a controllable electron self-injection.

A metallic pipe with wall corrugations is of special interest in light of recent proposals to use such a pipe for the generation of terahertz radiation and for energy dechirping of electron bunches in free electron lasers. In this paper we calculate the surface impedance of a corrugated metal wall and show that it can be reduced to that of a thin layer with dielectric constant $ϵ$ and magnetic permeability $\mu$. We develop a technique for the calculation of these constants, given the geometrical parameters of the corrugations. We then calculate, for the specific case of a round metallic pipe with small corrugations, the frequency and strength of the resonant mode excited by a relativistic beam. Our analytical results are compared with numerical simulations, and are shown to agree well. They are also shown to be more accurate when compared to the earlier used analytical model.

Very short-range wakefields in strongly tapered disk-loaded waveguide structures are investigated in the regime where the so-called “catch-up” distance is much larger than the length of the tapered structure. It is shown that in this case the very short-range wakefields are not being an average over all irises of the tapered structure; instead they are dominated by the irises with the smallest aperture radius. A new analytical model for short-range wakefield calculation in this regime is proposed. The results of the analytical model and numerical simulations are compared for the case of X-band accelerating structures.

Niobium is an important technological superconductor used to make radio frequency cavities for particle accelerators. Using laser confocal microscopy we have directly investigated hydride precipitates formation in cavity-grade niobium at 77 and 140 K. We have found that large hydrides were usually formed after chemical or mechanical treatments, which are known to lead to a strong degradation of the quality factor known as $Q$ disease. From our experiments we can conclude that hydrides causing $Q$ disease are islands with a characteristic thickness of $\gtrsim 100\mathrm{nm}$ and in-plane dimensions $1–10\mu \mathrm{m}$. Our results show that mechanical polishing uploads a lot of hydrogen into bulk niobium while electropolishing leads to a mild contamination. Vacuum treatments at $600–800°\mathrm{C}$ are demonstrated to preclude large hydride formation in line with the absence of $Q$ disease in similarly treated cavities.

We report the first observation of indirect resonantly enhanced transverse laser cooling of a bunched, stored ion beam of 40 keV ${}^{24}\mathrm{Mg}^{+}$. The longitudinal velocity distribution and the horizontal beam size of the laser-cooled ${}^{24}\mathrm{Mg}^{+}$ ion beam with 280 nm laser light were simultaneously observed by the use of standard fluorescence-based techniques. Keeping the operation point at (2.068, 1.105), the synchrotron tune (${\nu }_s$) was changed from 0.0376 to 0.1299. A strong decrease in the cooled horizontal beam size and a corresponding increase in the equilibrium cooled momentum spread were observed at the expected synchrobetatron resonance coupling condition of ${\nu }_s=0.068$, which is the first observation of a resonantly induced coupling for enhanced indirect transverse laser cooling of an ion beam, and is an important step towards creating a crystalline ground state of the beam.