The possibility of using a radio frequency undulator field to accelerate a high intensity ion beam in a linac is discussed. Such an accelerator can be realized using the periodical interdigital H-type resonator structure. The accelerating force is produced by an electric field which is a combination of two or more spatial harmonics, none of them being synchronous with the ion beam. The value of this force is proportional to the squared charge. The equations of motion in Hamiltonian form are derived by means of smooth approximation. The analysis of the 3D effective potential function allows finding the conditions of the beam focusing and acceleration. Two ways to increase ion beam intensity are considered: (i) to enlarge beam cross section; (ii) to neutralize the beam space charge by accelerating ions with opposite charge signs within the same bunch. The basic results are confirmed by a numerical simulation.

The electro-optic (EO) effect is a powerful diagnostic tool for determining the time profile of ultrashort relativistic electron bunches. When a relativistic bunch passes within a few mm of an electro-optic crystal, its transient electric field is equivalent to a half-cycle THz pulse passing through the crystal. The induced birefringence can be detected with polarized femtosecond laser pulses. A simulation code has been written in order to understand the faithfulness and the limitations of electron bunch shape reconstruction by EO sampling. The THz pulse and the laser pulse are propagated as wave packets through the EO crystal. Alternatively, the response function method is applied. Using experimental data on the material properties of zinc telluride (ZnTe) and gallium phosphide (GaP), the effects of velocity mismatch, pulse shape distortion, and signal broadening are explicitly taken into account. The simulations show that the most severe limitation on the time resolution is given by the transverse-optical (TO) lattice oscillation in the EO crystal. The lowest TO frequency is 5.3 THz in ZnTe and 11 THz in GaP. Only the Fourier components below the TO resonance are usable for the bunch shape reconstruction. This implies that the shortest rms bunch length which can be resolved with moderate distortion amounts to $\sigma \approx 90\mathrm{fs}$ in ZnTe and $\sigma \approx 50\mathrm{fs}$ in GaP. The influence of the crystal thickness on the amplitude and width of the EO signal is studied. The optimum thickness is in the range from 100 to $300\mu \mathrm{m}$ for ZnTe and from 50 to $100\mu \mathrm{m}$ for GaP.

Maintaining stable phasing in a linear accelerator is crucial for maintaining optimal performance. If phasing is incorrect, the beam will in general have an energy error and increased energy spread. While an energy error can be readily detected and corrected using position readings from beam position monitors at dispersion locations, this method is not useful for correcting energy spread in a system with many possible phase errors. While energy spread can be corrected by looking at beam size at a dispersive location, this typically involves a beam-intercepting diagnostic and is not compatible with top-up operation. Uncorrected energy spread results in poor capture efficiency in downstream accelerators, such as the Advanced Photon Source (APS) particle accumulator ring or booster synchrotron. To address this issue, APS has implemented beam-to-rf phase detectors in the linac, along with software for automatic correction of phase errors. We discuss the design, implementation, and performance of these detectors, as well as their use in feedback to automatically correct linac phase errors during top-up operation.

The linac coherent light source (LCLS), an x-ray free-electron laser project presently under construction at SLAC, uses a 2.856 GHz rf photocathode gun with a copper cathode for its electron source. While the copper cathode is performing well for the LCLS project, a cathode material with higher quantum efficiency would reduce the drive laser requirements and allow a greater range of operating conditions. Therefore a robust $\mathrm{CsBr}/\mathrm{Cu}$ photocathode with greater than 50 times the quantum yield at 257 nm relative to the present LCLS copper cathode has been investigated. Preliminary experiments using a dedicated electron source development test stand at SLAC/SSRL are encouraging and are presented in this paper.

In this paper, we present a new lattice design for the International Linear Collider (ILC) positron damping ring. The same lattice could be used also for the electron damping ring. This lattice is based on modified FODO arc cells, and has a freely tunable momentum compaction factor from $2\times {10}^{-4}$ to $6\times {10}^{-4}$. In comparison with the baseline design which uses the theoretical minimum emittance arc cells, the advantages of this design are: freely tunable momentum compaction factor; smaller number of quadrupole and sextupole magnets; larger dynamic aperture; simplified layout and lower cost. The design principles and analysis techniques used in this paper to change the momentum compaction factor by a large amount while keeping a very low emittance and large dynamic aperture are very important for the ILC damping ring; at the same time it could be very useful for any high bunch density storage ring.

Laser triggered, megavolt, megampere gas switches are frequently utilized to synchronize multiple pulsed power driver modules for inertial-confinement fusion, isentropic compression, and radiation physics experiments. The device developed to synchronize the 36 modules of the refurbished accelerator at Sandia National Laboratories is a 5.4 MV, 700 kA, sulfur-hexafluoride () filled, laser triggered gas switch. At this operating level, switch jitter is 5 ns, the prefire rate is less than 0.1%, the average optic lifetime is greater than 200 shots, and the flashover rate is less than 1%. Over 1000 shots on a single-module test facility were conducted while iterating several potential design improvements, including utilizing low-erosion electrode material, varying pressure, and modifying internal switch geometry all while keeping the basic switch architecture and footprint constant. Results of this development effort are presented herein.

A dynamics simulation model is used to estimate limits of performance of the positron-electron project (PEP-II). The simulation captures the dynamics and technical limitations of the low level radio frequency (LLRF) system, the high-power rf components, and the low-order mode coupled-bunch longitudinal beam dynamics. Simulation results showing the effect of nonlinearities on the LLRF loops, and studies of the effectiveness of technical component upgrades are reported, as well as a comparison of these results with PEP-II measurements. These studies have led to the estimation of limits and determining factors in the maximum stored current that the low energy ring/high energy ring (LER/HER) can achieve, based on system stability for different rf station configurations and upgrades. In particular, the feasibility of the PEP-II plans to achieve the final goal in luminosity, which required an increase of the beam currents to 4 A for LER and 2.2 A for HER, is studied. These currents are challenging in part because they would push the longitudinal low-order beam mode stability to the limit, and the klystron forward power past a level of satisfactory margin. An acceptable margin is defined in this paper, which in turn determines the corresponding klystron forward power limitation.

We have developed a high-resolution cavity-beam position monitor (BPM) to be used at the focal point of the ATF2, which is a test beam line that is now being built to demonstrate stable orbit control at $\sim \mathrm{\text{nanometer}}$ resolution. The design of the cavity structure was optimized for the Accelerator Test Facility (ATF) beam in various ways. For example, the cavity has a rectangular shape in order to isolate two dipole modes in orthogonal directions, and a relatively thin gap that is less sensitive to trajectory inclination. A two stage homodyne mixer with highly sensitive electronics and phase-sensitive detection was also developed. Two BPM blocks, each containing two cavity BPMs, were installed in the existing ATF beam line using a rigid support frame. After testing the basic characteristics, we measured the resolution using three BPMs. The system demonstrated 8.7 nm position resolution over a dynamic range of $5\mu \mathrm{m}$.

A three-dimensional analysis of radiation generation in a free-electron laser (FEL) is performed in the small signal regime. The analysis includes beam conditioning, harmonic generation, flat beams, and a new scaling of the FEL equations using the six-dimensional beam brightness. The six-dimensional beam brightness is an invariant under Liouvillian flow; therefore, any nondissipative manipulation of the phase space, performed, for example, in order to optimize FEL performance, must conserve this brightness. This scaling is more natural than the commonly used scaling with the one-dimensional growth rate. The brightness-scaled equations allow for the succinct characterization of the optimal FEL performance under various additional constraints. The analysis allows for the simple evaluation of gain enhancement schemes based on beam phase space manipulations such as emittance exchange and conditioning. An example comparing the gain in the first and third harmonics of round or flat and conditioned or unconditioned beams is presented.

This paper presents a detailed study of the deflection phenomena of a $400\mathrm{GeV}/c$ proton beam impinging on a new generation of bent silicon crystals; the tests have been performed at the CERN Super Proton Synchrotron H8 beam line. Channeling and volume reflection angles are measured with an extremely precise goniometer and with high resolution silicon microstrip detectors. Volume reflection has been observed and measured for the first time at this energy, with a single-pass efficiency as large as 98%, in good agreement with the simulation results. This efficiency makes volume reflection a possible candidate for collimation with bent crystals at the CERN Large Hadron Collider.

Different applications of TE modes in accelerator physics are discussed. In this discussion, the Fourier transform of the squared axial component of magnetic field, $B_z{}^2(z)$, plays an important role. If it turns out to be zero, the rf field-particle energy transfer is negligible and the focal length of the TE mode lens is phase independent. Such rf lens focuses continuous beams just as a solenoid. In order to compensate spherical aberrations and emittance growth caused by field nonlinearities of a focusing solenoid, the rf focusing is used also even if the beam space charge is taken into account. If the transform has its maximum, excitations of TE mode resonances and electron beam self-focusing are possible. Furthermore, the rf field of the TE mode can be used to expand the radial acceptance of a FEL for the THz region.

Longitudinal bunching factors in excess of 70 of a 300-keV, 27-mA ${\mathrm{K}}^{+}$ ion beam have been demonstrated in the neutralized drift compression experiment [P. K. Roy et al., Phys. Rev. Lett. 95, 234801 (2005)] in rough agreement with particle-in-cell source-to-target simulations. A key aspect of these experiments is that a preformed plasma provides charge neutralization of the ion beam in the last one meter drift region where the beam perveance becomes large. The simulations utilize the measured ion source temperature, diode voltage, and induction-bunching-module voltage waveforms in order to determine the initial beam longitudinal phase space which is critical to accurate modeling of the longitudinal compression. To enable simultaneous longitudinal and transverse compression, numerical simulations were used in the design of the solenoidal focusing system that compensated for the impact of the applied velocity tilt on the transverse phase space of the beam. Complete source-to-target simulations, that include detailed modeling of the diode, magnetic transport, induction bunching module, and plasma neutralized transport, were critical to understanding the interplay between the various accelerator components in the experiment. Here, we compare simulation results with the experiment and discuss the contributions to longitudinal and transverse emittance that limit the final compression.

Vacuum-post-hole convolutes are used in pulsed high-power generators to join several magnetically insulated transmission lines (MITL) in parallel. Such convolutes add the output currents of the MITLs, and deliver the combined current to a single MITL that, in turn, delivers the current to a load. Magnetic insulation of electron flow, established upstream of the convolute region, is lost at the convolute due to symmetry breaking and the formation of magnetic nulls, resulting in some current losses. At very high-power operating levels and long pulse durations, the expansion of electrode plasmas into the MITL of such devices is considered likely. This work examines the evolution and dynamics of cathode plasmas in the double-post-hole convolutes used on the $Z$ accelerator [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)]. Three-dimensional particle-in-cell (PIC) simulations that model the entire radial extent of the $Z$ accelerator convolute—from the parallel-plate transmission-line power feeds to the $z$-pinch load region—are used to determine electron losses in the convolute. The results of the simulations demonstrate that significant current losses (1.5 MA out of a total system current of 18.5 MA), which are comparable to the losses observed experimentally, could be caused by the expansion of cathode plasmas in the convolute regions.

We report the production of optically spaced attosecond electron microbunches produced by the inverse free-electron-laser (IFEL) process. The IFEL is driven by a Ti:sapphire laser synchronized with the electron beam. The IFEL is followed by a magnetic chicane that converts the energy modulation into the longitudinal microbunch structure. The microbunch train is characterized by observing coherent optical transition radiation (COTR) at multiple harmonics of the bunching. Experimental results are compared with 1D analytic theory showing good agreement. Estimates of the bunching factors are given and correspond to a microbunch length of 410 attosec FWHM. The formation of stable attosecond electron pulse trains marks an important step towards direct laser acceleration.

The large-bore magnets in the injection system of a high-intensity proton synchrotron are placed close to each other as compared to their bore sizes. In such cases, the magnetic cores interfere with the fringe fields of adjacent magnets. Moreover, the incoming beam passes through several kinds of time-varying fields along the injection line and the nonlinear field region of the ring quadrupole magnet. The beam behavior under these conditions is analyzed by using the Runge-Kutta method. Although the interference of the magnetic fields does not result in emittance growth, it produces a closed-orbit distortion. Field fluctuations of the time-varying field cause an emittance growth and the nonlinear field deforms the beam profile in phase space of the incoming beam. These effects lead to the modification of the injection scheme. This study focuses on the particle tracking with magnetic field interferences during the early stage of beam commissioning, when the space charge force is not important.

I point out that the claim of an “unexpected” reduction of the resonance width in spin-flip studies reported in the paper by Krisch et al. [Phys. Rev. ST Accel. Beams 10, 071001 (2007)] is invalid, because the authors employed a theoretical expression for the resonance width which did not account for all of the contributions to the spin flip induced by a radial field rf dipole spin flipper.

Accelerator science and technology have evolved as accelerators became larger and important to a broad range of science. Physical Review Special Topics - Accelerators and Beams was established to serve the accelerator community as a timely, widely circulated, international journal covering the full breadth of accelerators and beams. The history of the journal and the innovations associated with it are reviewed.

The repeated passage of a coasting ion beam of a storage ring through a thin target induces a shift in the revolution frequency due to the energy loss in the target. Since the frequency shift is proportional to the beam-target overlap, its measurement offers the possibility of determining the target thickness and hence the corresponding luminosity in an experiment. This effect has been investigated with an internal proton beam of energy 2.65 GeV at the COSY-Jülich accelerator using the ANKE spectrometer and a hydrogen cluster-jet target. Possible sources of error, especially those arising from the influence of residual gas in the ring, were carefully studied, resulting in an accuracy of better than 5%. The luminosity determined in this way was used, in conjunction with measurements in the ANKE forward detector, to determine the cross section for elastic proton-proton scattering. The result is compared to published data as well as to the predictions of a phase shift solution. The practicability and the limitations of the energy-loss method are discussed.

The vertical-beam emittance in an electron storage ring is mainly determined by two factors: the linear betatron coupling and the spurious vertical dispersion generated by magnet errors. We find that the contribution of spurious vertical dispersion is larger than that generated by the linear betatron coupling. Using the independent component analysis (ICA) method, we develop stop band corrections to reduce the vertical emittance. We demonstrate our method by making ICA and correction to a quadruple-bend achromatic low emittance lattice. Six families of skew quadrupoles can effectively minimize both the vertical dispersion and the linear betatron coupling.

The Compact Linear Collider (CLIC) main linac is sensitive to dynamic imperfections such as element jitter, injected beam jitter, and ground motion. These effects cause emittance growth that, in case of ground motion, has to be counteracted by a trajectory feedback system. The feedback system itself will, due to jitter effects and imperfect beam position monitors (BPMs), indirectly cause emittance growth. Fast and accurate simulations of both the direct and indirect effects are desirable, but due to the many elements of the CLIC main linac, simulations may become very time consuming. In this paper, an efficient way of simulating linear (or nearly linear) dynamic effects is described. The method is also shown to facilitate the analytic determination of emittance growth caused by the different dynamic imperfections while using a trajectory feedback system. Emittance growth expressions are derived for quadrupole, accelerating structure, and beam jitter, for ground motion, and for noise in the feedback BPMs. Finally, it is shown how the method can be used to design a feedback system that is optimized for the optics of the machine and the ground motion spectrum of the particular site. This feedback system gives an emittance growth rate that is approximately 10 times lower than that of traditional trajectory feedbacks. The robustness of the optimized feedback system is studied for a number of additional imperfections, e.g., dipole corrector imperfections and faulty knowledge about the machine optics, with promising results.