Transverse coupling impedance measurements of the Spallation Neutron Source (SNS) beam extraction system were performed and the results are here reported. The SNS beam extraction system is composed from 14 subsystems, each of which consists of a vertical kicker magnet plus a pulse forming network (PFN). Impedance bench measurements were performed on one large and one small aperture magnet, stand-alone as well as assembled with the first-article production PFN. The impedance measuring methods to cover the interesting frequency range from below 1 to 100 MHz are described in considerable detail. The upper frequency range is properly covered by the conventional twin-wire method but it had to be supplemented at the low-frequency end by a direct input impedance measurement at the magnet busbar. Required modifications of the PFN to maintain the impedance budget are discussed. The total impedance estimate was finally obtained by quadratic scaling with vertical aperture from the two tested kicker subsystems.

During heavy ion operation, large pressure rises, up to a few orders of magnitude, were observed at CERN, GSI, and BNL. The dynamic pressure rises were triggered by lost beam ions that impacted onto the vacuum chamber walls and desorbed about ${10}^4$ to ${10}^7$ molecules per ion. The deterioration of the dynamic vacuum conditions can enhance charge-exchange beam losses and can lead to beam instabilities or even to beam abortion triggered by vacuum interlocks. Consequently, a dedicated measurement of heavy-ion induced molecular desorption in the $\mathrm{G}\mathrm{e}\mathrm{V}/\mathrm{u}$ energy range is important for Large Hadron Collider (LHC) ion operation. In 2003, a desorption experiment was installed at the Super Proton Synchrotron to measure the beam-loss induced pressure rise of potential LHC collimator materials. Samples of bare graphite, sputter coated (Cu, TiZrV) graphite, and 316 LN (low carbon with nitrogen) stainless steel were irradiated under grazing angle with $158\mathrm{G}\mathrm{e}\mathrm{V}/\mathrm{u}$ indium ions. After a description of the new experimental setup, the results of the pressure rise measurements are presented, and the derived desorption yields are compared with data from other experiments.

In recent high luminosity colliders, the finite crossing angle scheme becomes popular to gain the luminosity with multibunch or long bunch operation. Success of the KEKB factory showed that the finite crossing angle scheme has no problem achieving beam-beam parameters up to 0.05. We have studied the beam-beam interactions with and without crossing angle toward higher luminosity. We discuss how the crossing angle affects the beam-beam parameter and luminosity in the present KEKB using computer simulations. The simulations showed that crab cavities, which realize the head-on collision effectively, can be expected to double the luminosity.

We compare simulations to experiments measuring electron yield from heavy ions striking metal surfaces, in particular at grazing incidence, for 1.0 MeV potassium and 182.0 MeV gold ions striking stainless steel as measured in two recent experiments. We find the electron yield is proportional to within 3% for the potassium experiments and 13% for the gold experiments to the simulated energy deposited by the ions in a thin ($\approx 20\AA$) layer at the target surface. We discuss how nonequilibrium stopping and ion wake fields may account for the larger disagreement with the gold experiments. An analytic estimate based on specular reflection predicts the peak in simulated and measured electron yield as a function of angle to within a few percent.

In the currently envisioned configurations for heavy ion fusion, it is necessary to longitudinally compress the beam bunches by a large factor after the acceleration phase. Because the space-charge force increases as the beam is compressed, the beam size in the transverse direction will increase in a periodic quadrupole lattice. If an active control of the beam size is desired, a larger focusing force is needed to confine the beam in the transverse direction, and a nonperiodic quadrupole lattice along the beam path is necessary. In this paper, we describe the design of such a focusing lattice using the transverse envelope equations. A drift compression and final focus lattice should focus the entire beam pulse onto the same focal spot on the target. This is difficult with a fixed lattice, because different slices of the beam may have different perveance and emittance. Four time-dependent magnets are introduced in the upstream of drift compression to focus the entire pulse onto the same focal spot. Drift compression and final focusing schemes are developed for a typical heavy ion fusion driver and for the integrated beam experiment being designed by the Heavy Ion Fusion Virtual National Laboratory.

The force between two moving point charges, because of its inverse square law singularity, cannot be applied directly in the numerical simulation of bunch dynamics; radiative effects make this especially true for short bunches being deflected by magnets. This paper describes a formalism circumventing this restriction in which the basic ingredient is the total force on a point charge comoving with a longitudinally aligned, uniformly charged string. Bunch evolution can then be treated using direct particle-to-particle, intrabeam scattering, with no need for an intermediate, particle-in-cell, step. Electric and magnetic fields do not appear individually in the theory. Since the basic formulas are both exact (in paraxial approximation) and fully relativistic, they are applicable to beams of all particle types and all energies. But the theory is expected to be especially useful for calculating the emittance growth of the ultrashort electron bunches of current interest for energy recovery linacs and free-electron lasers. The theory subsumes coherent synchrotron radiation and centrifugal space charge force. Renormalized, on-axis, longitudinal field components are in excellent agreement with values from Saldin et al. [DESY Report No. DESY-TESLA-FEL-96-14, 1995; Nucl. Instrum. Methods Phys. Res., Sect. A 417, 158 (1998).]

For several processes at colliding beams, macroscopically large impact parameters give an essential contribution to the standard cross section. These impact parameters may be much larger than the transverse sizes of the colliding bunches. In that case, the standard calculations have to be essentially modified. The corresponding formulas for such a beam-size effect were given 20 years ago without taking into account correlations of particle coordinates in the beams. In the present paper we derive formulas necessary to take into account quantitatively the effect of particle correlations in the spectrum of bremsstrahlung as well as in pair production. Our results are quite different from those obtained in recent papers [V. N. Baier and V. M. Katkov, Phys. Rev. D 66, 053009 (2002); V. N. Baier and V. M. Katkov, hep-ph/0305304.]. We point out the origin of this difference.

A simple scheme for field enhancement in synchrotron radiation sources such as undulators and wigglers is proposed, which is based on the fundamental nature of the superconducting loop where the magnetic flux is preserved. A superconductor ring placed to enclose the magnetic pole works as a kind of permanent magnet. The magnetization is performed by electromagnetic induction brought by the opening movement of the magnetic gap. Since neither additional external power supplies nor current leads are necessary, high-temperature bulk superconductors can easily be implemented in this scheme. Calculations to check the effectiveness of the new concept show that the critical current density of the superconductor is crucial to the performance of the synchrotron radiation sources based on this concept. Experiments were performed to verify the principle of the proposed scheme, which gave promising results to strongly support it.

A simple analytical formula is described for bunch lengthening by a linear horizontal betatron motion in an electron storage ring. An example of calculation shows that this lengthening is larger than the intrinsic bunch shortening limit for most dispersive sections, which strongly limits the wavelength region of coherent radiation.

Presented are details of the staged electron laser acceleration (STELLA) experiment, which demonstrated high-trapping efficiency and narrow energy spread in a staged laser-driven accelerator. Trapping efficiencies of up to 80% and energy spreads down to 0.36% ($1\sigma$) were demonstrated. The experiment validated an approach that may be suitable for the basic design of a laser-driven accelerator system. In this approach, a laser-driven modulator together with a chicane creates a train of microbunches spaced apart by the laser wavelength. These microbunches are sent into a second laser-driven accelerator designed to efficiently trap the microbunches in the ponderomotive potential well of the laser electric field while maintaining a narrow energy spread. The STELLA scientific apparatus and procedures are described in detail. In-depth comparisons between the data and model are given including the predicted energy spectrum, energy-phase plot, and microbunch length profile. Data and model comparisons as a function of the phase delay between the microbunches and the accelerating wave are presented. The model is exercised to reveal how the high-trapping efficiency process evolves during the acceleration process.

External fields are necessary for emittance compensation and beam focusing in rf photoelectron guns. For rf guns with superconducting cavities two fields have been discussed up to now. The first field is a specially designed radial component of the rf field immediately after the cathode and the second is a static magnetic field downstream of the superconducting cavity. In this paper we discuss a third possibility. Inside the cavity the magnetic rf field of a TE mode focuses the electron beam and prevents the increase of the transverse emittance. The results depend only weakly on the phase of the TE mode. For a bunch charge of 1 nC, an emittance of 0.7 mm mrad has been obtained with a surface field strength of the magnetic field below the quench limit.

The electron cloud instability (ECI) in the Beijing Electron Positron Collider (BEPC) was studied systematically with experiments and simulations in recent years. Several electron cloud detectors have been installed into the BEPC storage ring to measure the electron yield. The effects of solenoid, the clearing electrode, the chromaticity, and the octupole on ECI have been investigated in BEPC experimentally. To evaluate the ECI in BEPCII, the upgrade project of BEPC, a computer code has been developed to simulate the electron density in the antechamber under different conditions, as well as the progress of the single bunch and coupled-bunch instabilities caused by the electron cloud. The experimental and simulation results, as well as the activities, are reported in this paper.

We have studied the various conditions and limitations for achieving compact fixed-field alternating-gradient (FFAG) accelerators to be widely used in heavy-ion cancer therapy. For the case of a normal-conducting FFAG accelerator, our linear calculation indicates 12-cell radial sectors with a field index of 10.5 as a suitable configuration. We found that its ring circumference can be as small as 70 m and that triple-cascade rings are needed to accelerate a carbon beam from $40\mathrm{k}\mathrm{e}\mathrm{V}/\mathrm{u}$ to $400\mathrm{M}\mathrm{e}\mathrm{V}/\mathrm{u}$. In this paper, we report a systematic analysis based on a linear optical model, a comparison of various types of FFAG, and a design example with some technical concerns. An important result is that viable radial-sector designs are possible with circumference factor $C$ significantly lower than the value 4.45 previously quoted.

Gas desorption and electron emission coefficients were measured for 1 MeV potassium ions incident on stainless steel at grazing angles (between 80° and 88° from normal incidence) using a new gas-electron source diagnostic (GESD). Issues addressed in design and commissioning of the GESD include effects from backscattering of ions at the surface, space-charge limited emission current, and reproducibility of desorption measurements. We find that electron emission coefficients ${\gamma }_e$ scale as $1/\cos (\theta )$ up to angles of 86°, where ${\gamma }_e=90$. Nearer grazing incidence, ${\gamma }_e$ is reduced below the $1/\cos (\theta )$ scaling by nuclear scattering of ions through large angles, reaching ${\gamma }_e=135$ at 88°. Electrons were emitted with a measured temperature of $\sim 30\mathrm{e}\mathrm{V}$. Gas desorption coefficients ${\gamma }_0$ were much larger, of order ${\gamma }_0={10}^4$. They also varied with angle, but much more slowly than $1/\cos (\theta )$. From this we conclude that the desorption was not entirely from adsorbed layers of gas on the surface. Two mitigation techniques were investigated: rough surfaces reduced electron emission by a factor of 10 and gas desorption by a factor of 2; a mild bake to $\sim 220°$ had no effect on electron emission, but decreased gas desorption by 15% near grazing incidence. We propose that gas desorption is due to electronic sputtering.

The potential use of two planar superconducting elliptical undulators—a vertically wound racetrack coil structure and a staggered array structure—to generate a circularly polarized hard x-ray source was investigated. The magnetic poles and wires of the up and down magnet arrays were rotated in alternating directions on the horizontal plane, an elliptical field is generated to provide circularly polarized light in the electron-storage ring and the energy-recovery linac accelerator. Rapid switching between right- and left-circularly polarized radiations is performed using two undulators with oppositely rotated wires and poles. Given a periodic length of 15 mm and a gap of 5 mm, the magnetic-flux densities in the elliptical undulator are $B_z=1.2\mathrm{T}$ ($B_x=0.6\mathrm{T}$) and $B_z=0.35\mathrm{T}$ ($B_x=0.15\mathrm{T}$) in the planar vertically wound racetrack coil and the staggered structure with poles rotated by $35°$ and $25°$, respectively. In maximizing the merit of the flux and the width of the effective field region in the two superconducting elliptical undulators, the trade-off rotation angles of the coils and poles are $20°$ and $5°$, for vertically wound racetrack coil and staggered undulators, respectively.

The ultrahigh-vacuum electrical breakdown characteristics of copper, molybdenum, and tungsten have been explored in a setup based on a capacitor discharge. Upon repeated sparking, tungsten and molybdenum showed improvement of the maximum applicable field before breakdown (conditioning) in contrast to copper, which experienced alternate improvement and degrading. After conditioning, tungsten withstood the highest applied field followed by molybdenum and copper. This behavior was correlated with that of the field enhancement factor $\beta$ extracted from measurements of the field emission current. These results are compared with the tests performed on 30 GHz test accelerating structures for the future Compact Linear Collider.

We derive the space-charge limit for a train of periodic finite-size charged bunches which are slightly displaced from the axis of a circular, perfectly conducting pipe. The general calculation assumes that the bunches, while periodic, may have arbitrary radial and longitudinal density distributions with respect to the beam axis. We present results for a few specific distributions, as well as the limits of unbunched and single-bunch beams. Finally, we show how the bunched beam space-charge limit along with the radial rms envelope equation for a bunched beam is applied to predict operational limits of periodic permanent magnet focusing klystrons, such as the SLAC 11.4 GHz 75 MW XP-3 device.

Grazing collisions with the stainless steel beam pipes of gold ions, the so-called “halo scraping,” result in large secondary electron emission and surface molecular desorption yields in the Relativistic Heavy Ion Collider. Here we estimate electron emission yields as function of incidence angle, we show that desorption rates will follow a similar angular dependence at small angles, and we propose a simple approach to mitigate these effects.

We present experimental data from the Los Alamos Proton Storage Ring (PSR) showing long-lived linac microbunch structure during beam storage with no rf bunching. Analysis of the experimental data and particle-in-cell simulations of the experiments indicate that space charge, coupled with energy spread effects, is responsible for the sustained microbunch structure. The simulated longitudinal phase space of the beam reveals a well-defined separatrix in the phase space between linac microbunches, with particles executing unbounded motion outside of the separatrix. We show that the longitudinal phase space of the beam was near steady state during the PSR experiments, such that the separatrix persisted for long periods of time. Our simulations indicate that the steady state is very sensitive to the experimental conditions. Finally, we solve the steady-state problem in an analytic, self-consistent fashion for a set of periodic longitudinal space-charge potentials.