Transverse wakefields generated by intense beams may propagate long distances in a vacuum chamber and dissipate power in different shielded elements such as bellows, vacuum valves, or vacuum pumps. Induced heating in these elements may be high enough to deteriorate vacuum conditions. We have developed a broadband water-cooled bellows absorber to capture and damp these harmful transverse fields without impacting the longitudinal beam impedance. Experimental results at the PEP-II SLAC B-factory demonstrate a high efficiency for this device. This absorber may be useful in super B-factories, the International Linear Collider, the Large Hadron Collider, or synchrotron light sources.

The control of superconducting resonators requires a large amount of rf power due to their narrow bandwidth and the presence of microphonics in the ambience, although only a small part is dissipated inside the resonator to generate the accelerating field. A simple and innovative method has been devised to reduce the effect of microphonics on the quarter-wave resonators. Experiments have been conducted successfully at room temperature and at 4.2 K to demonstrate the effect of the new damping mechanism. A detailed report of the experiments with the superconducting resonators in a test cryostat and an on-line linac cryostat is presented in this paper.

A gap in the vacuum chamber stands between a beam and the outside world, and the theoretical elucidation of the interaction mechanism between the gap and the beam is of great importance to understand the interaction of any device with the beam. In this paper, we will present the formulas for the longitudinal and transverse impedances due to a gap in the beam chamber. In this process, we will derive the complete solutions of electromagnetic fields effective in the entire region, including the inside and the outside of the chamber, in a form that they can be easily numerically evaluated. The newly developed technique can provide new methods of solutions of electromagnetic fields also for a rather broad class of structures such as cavities. The numerical results of impedances are consistent with the ABCI results and their behavior in high frequency agrees well with the prediction of the diffraction theory. Our theory can also accurately reproduce the behavior of the impedance near and above the cutoff frequencies. In addition, our theory is applicable even to the impedances for nonrelativistic beams. We found that the broadband impedance of the small cavitylike structure can be estimated from the gap size and the chamber radius only, regardless of the exact shape of the structure. We also found that the transverse impedance of a gap has a large resonance peak at the frequency where the wavelength is equal to the chamber circumference. This resonance peak appears around 1–2 GHz in most of the cases, and we should be careful to design a ceramic break so that this transverse mode will not leak out to interact with nearby devices.

We recently started testing Chao’s proposed new matrix formalism for describing the spin dynamics due to a single spin resonance. The Chao formalism is probably the first fundamental improvement of the Froissart-Stora equation in that it allows analytic calculations of the beam polarization’s behavior inside a resonance. We tested the Chao formalism using a $1.85\mathrm{GeV}/c$ polarized deuteron beam stored in COSY, by sweeping an rf dipole’s frequency through 200 Hz, while varying the distance from the sweep’s end frequency to an rf-induced spin resonance’s central frequency. Since the Froissart-Stora equation itself can make no prediction inside a resonance, we compared our experimental data with the predictions of the Chao formalism and those of an empirical two-fluid model based on the Froissart-Stora equation. The data strongly favor the Chao formalism.

Coupling of the transverse degrees of freedom modifies analysis of beam coherent motion. A general, simple, and effective rule to do that is derived.

Two partial Siberian snakes were used to avoid all the spin imperfection and vertical intrinsic resonances in the alternating gradient synchrotron (AGS) at Brookhaven National Laboratory. However, the horizontal betatron motion can cause polarization loss resulting from the nonvertical stable spin direction in the presence of two partial snakes. This type of resonance, called a horizontal intrinsic spin resonance, was observed and systematically studied in the AGS. A simplified analytic model and numerical simulation have been developed to compare with experimental data. Properties of the horizontal intrinsic resonance are discussed.

A comprehensive investigation of the effect of Ar, ${\mathrm{H}}_2$, air, and CO on the saturated breakdown field ($E_{\mathrm{sat}}$) of molybdenum, tungsten, and copper has been performed at intensive breakdown conditioning. A significant decrease in $E_{\mathrm{sat}}$ is observed for molybdenum and tungsten when exposed to air. Specifically, at air pressures of $\sim {10}^{-5}\mathrm{mbar}$, the decrease in $E_{\mathrm{sat}}$ is found to be up to 50% and $\sim 30\%$ compared to UHV conditions for molybdenum and tungsten, respectively. In addition, a $\sim 30\%$ decrease is found when molybdenum is conditioned with a $\sim {10}^{-5}\mathrm{mbar}$ CO pressure. Surface analysis measurements and breakdown conditioning in ${\mathrm{O}}_2$ ambience imply that the origin of the decrease in $E_{\mathrm{sat}}$ is closely linked to oxide formation on the electrode surfaces. Ex situ heat treatment of molybdenum is shown to improve the breakdown characteristics drastically.

Here we will derive the general theory of the beam-breakup (BBU) instability in recirculating linear accelerators with coupled beam optics and with polarized higher-order dipole modes. The bunches do not have to be at the same radio-frequency phase during each recirculation turn. This is important for the description of energy recovery linacs (ERLs) where beam currents become very large and coupled optics are used on purpose to increase the threshold current. This theory can be used for the analysis of phase errors of recirculated bunches, and of errors in the optical coupling arrangement. It is shown how the threshold current for a given linac can be computed and a remarkable agreement with tracking data is demonstrated. General formulas are then analyzed for several analytically solvable problems: (a) Why can different higher order modes (HOM) in one cavity couple and why can they then not be considered individually, even when their frequencies are separated by much more than the resonance widths of the HOMs? For the Cornell ERL as an example, it is noted that optimum advantage is taken of coupled optics when the cavities are designed with an $x\mathrm{\text{−}}y$ HOM frequency splitting of above 50 MHz. The simulated threshold current is then far above the design current of this accelerator. To justify that the simulation can represent an actual accelerator, we simulate cavities with 1 to 8 modes and show that using a limited number of modes is reasonable. (b) How does the $x\mathrm{\text{−}}y$ coupling in the particle optics determine when modes can be considered separately? (c) How much of an increase in threshold current can be obtained by coupled optics and why does the threshold current for polarized modes diminish roughly with the square root of the HOMs’ quality factors. Because of this square root scaling, polarized modes with coupled optics increase the threshold current more effectively for cavities that have rather large HOM quality factors, e.g. those without very elaborate HOM absorbers. (d) How does multiple-turn recirculation interfere with the threshold improvements obtained with a coupled optics? Furthermore, the orbit deviations produced by cavity misalignments are also generalized to coupled optics. It is shown that the BBU instability always occurs before the orbit excursion becomes very large.

A comparison of possible undulator designs for the International Linear Collider positron source has resulted in a superconducting bifilar wire design being selected. After a comprehensive paper study and fabrication of the two preeminent designs, the superconducting undulator was chosen instead of the permanent magnet alternative. This was because of its superior performance in terms of magnetic field strength and quality, operational flexibility, risk of radiation damage, ease in achieving the required vacuum, and cost. The superconducting undulator design will now be developed into a complete system design for the full 200 m long magnet that is required.

Preserving the beam brightness and emittance during acceleration for high energy collider operation is a long-standing challenge for synchrotron injectors. Radio-frequency manipulation schemes used to produce bright bunches in these synchrotrons are often responsible for both beam loss and for the emittance dilution; the impact of such effects becomes the main obstacle to improve the collider luminosity. We have developed a scheme for acceleration of intense beam bunches of large longitudinal emittance using two rf systems. The development of such a method primarily came about to improve the overall injection to extraction efficiency of the antiproton beam in the Main Injector used for the proton-antiproton collider operation of the Tevatron at Fermilab. This technique is quite general and can be applied at other facilities. Multiparticle beam dynamics simulations of the scheme have shown that one can eliminate the beam loss and minimize the emittance growth. The scheme has been demonstrated with beam experiments in the Main Injector using the 2.5 and 53 MHz rf systems with harmonic ratio of $1:21$.

A general formalism for treating simultaneously the transverse coupled-bunch and transverse coupled-mode instabilities is presented. In this approach, the equations of motion of a coupled multibunch beam are expanded to yield a system of equations involving correlation moments of the transverse and longitudinal motions. After a proper truncation, the system of equations is closed and can be solved. This approach allows us to formulate within one framework several known instability mechanisms including the single-bunch mode-coupling instability, the coupled-bunch instability, the mode-coupling instability, and the coupled-mode coupled-bunch instability as particular cases.

The baseline Spallation Neutron Source (SNS) accelerator will provide a 1-GeV, 1.4-MW proton beam to a mercury target for the production of neutrons. The main acceleration for the H-beam is provided by 81 superconducting cavities installed in 23 cryomodules operating at 805 MHz. The design of the superconducting linac includes a 2.1-K, 2.5-kW cryogenic plant to maintain the cavities below the helium lambda point for efficient operation at high accelerating gradients. In this paper operating conditions are analyzed rather than the design ones, which still guarantees a high gradient operation without any temperature constraint. From the analysis it appears that the SNS superconducting linac can be operated at temperatures higher than 2.1 K, a fact resulting from both the pulsed nature of the superconducting cavities, the specific configuration of the existing cryogenic plant, and the operating frequency. General conditions are also given regarding the operation of pulsed superconducting cavities resonating at different frequencies.

The linear growth of the two-stream instability for a charged-particle beam that is longitudinally compressing as it propagates through a background plasma (due to an applied velocity tilt) is examined. Detailed, 1D particle-in-cell (PIC) simulations are carried out to examine the growth of the wave packet produced by a small amplitude density perturbation in the background plasma. Recent analytic and numerical work by Startsev and Davidson [Phys. Plasmas 13, 062108 (2006)] predicted reduced linear growth rates, which are indeed observed in the PIC simulations. Here, small-signal asymptotic gain factors are determined in a semianalytic analysis and compared with the simulation results in the appropriate limits. Nonlinear effects in the PIC simulations, including wave breaking and particle trapping, are found to limit the linear growth phase of the instability for both compressing and noncompressing beams.

Driving Term experiments have been performed both at small [PS (Proton Synchrotron) Booster] and large accelerators [SPS (Super Proton Synchrotron)] at CERN. The theory of how to measure driving terms is reviewed. A wealth of SPS experiments is shown together with a successful comparison with model calculations. The PS Booster studies aimed at optimizing the machine performance by measuring and correcting selected driving terms.

We have developed an accelerator architecture that can serve as the basis of the design of petawatt-class $z$-pinch drivers. The architecture has been applied to the design of two $z$-pinch accelerators, each of which can be contained within a 104-m-diameter cylindrical tank. One accelerator is driven by slow ($\sim 1\mu \mathrm{s}$) Marx generators, which are a mature technology but which necessitate significant pulse compression to achieve the short pulses ($\ll 1\mu \mathrm{s}$) required to drive $z$ pinches. The other is powered by linear transformer drivers (LTDs), which are less mature but produce much shorter pulses than conventional Marxes. Consequently, an LTD-driven accelerator promises to be (at a given pinch current and implosion time) more efficient and reliable. The Marx-driven accelerator produces a peak electrical power of 500 TW and includes the following components: (i) 300 Marx generators that comprise a total of $1.8\times {10}^4$ capacitors, store 98 MJ, and erect to 5 MV; (ii) 600 water-dielectric triplate intermediate-store transmission lines, which also serve as pulse-forming lines; (iii) 600 5-MV laser-triggered gas switches; (iv) three monolithic radial-transmission-line impedance transformers, with triplate geometries and exponential impedance profiles; (v) a 6-level 5.5-m-diameter 15-MV vacuum insulator stack; (vi) six magnetically insulated vacuum transmission lines (MITLs); and (vii) a triple-post-hole vacuum convolute that adds the output currents of the six MITLs, and delivers the combined current to a $z$-pinch load. The accelerator delivers an effective peak current of 52 MA to a 10-mm-length $z$ pinch that implodes in 95 ns, and 57 MA to a pinch that implodes in 120 ns. The LTD-driven accelerator includes monolithic radial transformers and a MITL system similar to those described above, but does not include intermediate-store transmission lines, multimegavolt gas switches, or a laser trigger system. Instead, this accelerator is driven by 210 LTD modules that include a total of $1\times {10}^6$ capacitors and $5\times {10}^5$ 200-kV electrically triggered gas switches. The LTD accelerator stores 182 MJ and produces a peak electrical power of 1000 TW. The accelerator delivers an effective peak current of 68 MA to a pinch that implodes in 95 ns, and 75 MA to a pinch that implodes in 120 ns. Conceptually straightforward upgrades to these designs would deliver even higher pinch currents and faster implosions.

Incoherent and coherent synchrotron radiation are issues in most new accelerator projects utilizing bunch compressors to increase the peak current of the electron beam. Here we present a systematic approach to design a bunch compressor chicane suitable for the multi-TeV compact linear collider (CLIC). Based on analytical equations and beam dynamics simulations, we discuss how the influence of incoherent and coherent synchrotron radiation on the transverse beam emittance depends on chicane and electron beam parameters. The optimization of the bunch compressor layout and the electron beam parameters results in two different options which preserve the emittance to well within the specifications.

The well-known Kapchinskij-Vladimirskij (KV) equations are difficult to solve in general, but the problem is simplified for the matched-beam case with sufficient symmetry. We show that the interdependence of the two KV equations is eliminated, so that only one needs to be solved—a great simplification. We present an iterative method of solution which can potentially yield any desired level of accuracy. The lowest level, the well-known smooth approximation, yields simple, explicit results with good accuracy for weak or moderate focusing fields. The next level improves the accuracy for high fields; we previously showed [Part. Accel. 52, 133 (1996)] how to maintain a simple explicit format for the results. That paper used expansion in a small parameter to obtain results of second-level accuracy. The present paper, using straightforward iteration, obtains equations of first, second, and third levels of accuracy. For a periodic lattice with beam matched to lattice, we use the lattice and beam parameters as input and solve for phase advances and envelope functions. We find excellent agreement with numerical solutions over a wide range of beam emittances and intensities.

In this work we consider in general the high-gain SASE x-ray Compton FEL in the quantum regime of amplification arising from the self-consistent set of the Maxwell and relativistic quantum kinetic equations. The analytical solutions are obtained which are valid before the saturation. The quantum seed-shot noise and subsequent SASE power are calculated. A considerable increase in start-up and narrowing of the spectrum of the SASE power for amplifying x-ray frequencies is shown compared with the classical SASE regime.

Pulse length of quasimonoenergetic electrons accelerated by the wakefield generated by 12 TW, 40 fs laser pulses in a gas jet is determined via spectral measurements using a bolometer to detect coherent transition radiation. A quasimonoenergetic electron beam with its mean energy $E=21\mathrm{MeV}$, dispersion $\Delta E=4\mathrm{MeV}$, total charge $q\sim 30\mathrm{pC}$, and the geometrical emittance $0.07\pi \mathrm{mm}\mathrm{mrad}$ is generated with high reproducibility. The averaged duration of only the quasimonoenergetic electron bunches peaked around 20 MeV is $130\pm 30\mathrm{fs}$ (FWHM), while it is $250\pm 70\mathrm{fs}$ (FWHM) for electron bunches with quasimonoenergetic distributions peaked around 4 MeV, at a distance of 180 mm far from the gas jet because of relatively large electron energy spread. Pulse elongation of the electron bunch with the quasimonoenergetic distribution after 180 mm path is $60–220\mathrm{fs}$ (FWHM). Therefore, the initial duration of the electron bunch at the gas-jet rear is expected to be less than 100  fs (FWHM).