We have developed a statistical model for the flashover of a $45°$ vacuum-insulator interface (such as would be found in an accelerator) subject to a pulsed electric field. The model assumes that the initiation of a flashover plasma is a stochastic process, that the characteristic statistical component of the flashover delay time is much greater than the plasma formative time, and that the average rate at which flashovers occur is a power-law function of the instantaneous value of the electric field. Under these conditions, we find that the flashover probability is given by $1-\exp (-E_p^{\beta }{t}_{\mathrm{eff}}C/k^{\beta })$, where $E_p$ is the peak value in time of the spatially averaged electric field $E(t)$, $t_{\mathrm{eff}}\equiv \int [E(t)/E_p{]}^{\beta }dt$ is the effective pulse width, $C$ is the insulator circumference, $k\propto \exp (\lambda /d)$, and $\beta$ and $\lambda$ are constants. We define $E(t)$ as $V(t)/d$, where $V(t)$ is the voltage across the insulator and $d$ is the insulator thickness. Since the model assumes that flashovers occur at random azimuthal locations along the insulator, it does not apply to systems that have a significant defect, i.e., a location contaminated with debris or compromised by an imperfection at which flashovers repeatedly take place, and which prevents a random spatial distribution. The model is consistent with flashover measurements to within 7% for pulse widths between 0.5 ns and $10\mu \mathrm{s}$, and to within a factor of 2 between 0.5 ns and 90 s (a span of over 11 orders of magnitude). For these measurements, $E_p$ ranges from 64 to $651\mathrm{kV}/\mathrm{cm}$, $d$ from 0.50 to 4.32 cm, and $C$ from 4.96 to 95.74 cm. The model is significantly more accurate, and is valid over a wider range of parameters, than the J. C. Martin flashover relation that has been in use since 1971 [J. C. Martin on Pulsed Power, edited by T. H. Martin, A. H. Guenther, and M. Kristiansen (Plenum, New York, 1996)]. We have generalized the statistical model to estimate the total-flashover probability of an insulator stack (i.e., an assembly of insulator-electrode systems connected in series). The expression obtained is consistent with the measured flashover performance of a stack of five 5.72-cm-thick, 1003-cm-circumference insulators operated at 100 and $158\mathrm{kV}/\mathrm{cm}$. The expression predicts that the total-flashover probability is a strong function of the ratio $E_p/k$, and that under certain conditions, the performance improves as the capacitance between the stack grading rings is increased. In addition, the expression suggests that given a fixed stack height, there exists an optimum number of insulator rings that maximizes the voltage at which the stack can be operated. The results presented can be applied to any system (or any set of systems connected in series) subject to random failures, when the characteristic statistical delay time of a failure is much greater than its formative time.

Some high-energy experiments have suffered from bitter experience on synchrotron-radiation background. As a measure, we have developed a new calculation method of synchrotron radiation based on a real beam orbit, aiming at quantitative estimations of synchrotron-radiation backgrounds, and the construction of a possible alarm system for the background. An explanation and a demonstration of our method are given.

Characterization of beam energy spread in a space-charge dominated beam is very important to understanding the physics of intense beams. It is believed that coupling between the transverse and longitudinal directions via Coulomb collisions will cause an increase of the beam longitudinal energy spread. At the University of Maryland, experiments have been carried out to study the energy evolution in such intense beams with a high-resolution retarding field energy analyzer. The temporal beam energy profile along the beam pulse has been characterized at the distance of 25 cm from the anode of a gridded thermionic electron gun. The mean energy of the pulsed beams including the head and tail is reported here. The measured rms energy spread is in good agreement with the predictions of the intrabeam scattering theory. As an application of the beam energy measurement, the input impedance between the cathode and the grid due to beam loading can be calculated and the impedance number is found to be a constant in the operation region of the gun.

Recent experience with $X$-band accelerator structure development has shown the rf input coupler to be the region most prone to rf breakdown and degradation, effectively limiting the operating gradient. A major factor in this appears to be high magnetic fields at the sharp edges of the coupling irises. As a first response to this problem, couplers with rounded and thickened iris horns have been employed and successfully tested at high power. To further reduce fields for higher power flow, conceptually new coupler designs have been developed, in which power is coupled through the broad wall of the feed waveguide, rather than through terminating irises. A “mode-launcher” coupler, which launches the ${\mathrm{TM}}_{01}$ mode in circular waveguide before coupling through a matching cell into the main structure, has been tested with great success. With peak surface fields below those in the body of the structure, this coupler represented a breakthrough in the Next Linear Collider structure program. The design of this coupler and of variations which use beam line space more efficiently are described here. The latter include a coupler in which power passes directly through an iris in the broad wall of the rectangular waveguide into a matching cell, also successfully implemented, and a variation which makes the waveguide itself an accelerating cell. We also discuss in some detail a couple of techniques for matching such couplers to traveling-wave structures using a field solver. The first exploits the cell number independence of a traveling-wave match, and the second optimizes using the fields of an internally driven structure.

A formalism presented in a previous paper for the analysis of cumulative beam breakup with arbitrary time dependence of the beam current [J. R. Delayen, Phys. Rev. ST Accel. Beams 6, 084402 (2003)] is applied to the problem of beam breakup in the presence of random displacements of cavities and focusing elements. A closed-form solution is obtained and is applied to the behavior of a single bunch and to the steady-state and transient behavior of dc beams and beams composed of pointlike bunches.

A microbunching instability driven by longitudinal space charge, coherent synchrotron radiation, and linac wakefields is studied for the linac coherent light source (LCLS) accelerator system. Since the uncorrelated (local) energy spread of electron beams generated from a photocathode rf gun is very small, the microbunching gain may be large enough to significantly amplify rf-gun generated modulations or even shot-noise fluctuations of the electron beam. The uncorrelated energy spread can be increased by an order of magnitude to provide strong Landau damping against the instability without degrading the free-electron laser performance. We study different damping options in the LCLS and discuss an effective laser heater to minimize the impact of the instability on the quality of the electron beam.

A formula is derived for the small-signal gain of a Smith-Purcell free-electron laser. The theory describes the electron beam as a moving plasma dielectric, and assumes that the electron beam interacts with an evanescent mode traveling along the surface of a periodic waveguide with a rectangular profile. The phase velocity of the evanescent wave is synchronous with the electron velocity, but the group velocity is actually negative. The electron beam amplifies the evanescent wave, which does not itself radiate. According to this picture, the radiation observed emanating from the grating is Smith-Purcell radiation enhanced by the bunching of the electrons due to the interaction with the evanescent mode. There will also be radiation from the part of the evanescent mode that is outcoupled from the ends of the grating. This radiation appears at a lower frequency than the Smith-Purcell radiation. The new results explain both the gain and the radiation observed in the experiments of Urata and Walsh, and the cube-root current dependence of the gain inferred by Bakhtyari, Walsh, and Brownell.

The acceleration efficiency of a laser driven linear accelerator is analyzed. The laser power, loss factor, and impedances determine the maximum charge that can be accelerated and the efficiency of that acceleration. The accelerator structure can be incorporated into a laser cavity. The equation for the resultant laser pulse is derived and analyzed. A specific example is presented, and the steady-state laser pulse shapes, acceleration efficiency, and average unloaded gradient are calculated.

The energy loss and gain of a beam in the nonlinear, “blowout” regime of the plasma wakefield accelerator, which features ultrahigh accelerating fields, linear transverse focusing forces, and nonlinear plasma motion, has been asserted, through previous observations in simulations, to scale linearly with beam charge. Additionally, from a recent analysis by Barov et al., it has been concluded that for an infinitesimally short beam, the energy loss is indeed predicted to scale linearly with beam charge for arbitrarily large beam charge. This scaling is predicted to hold despite the onset of a relativistic, nonlinear response by the plasma, when the number of beam particles occupying a cubic plasma skin depth exceeds that of plasma electrons within the same volume. This paper is intended to explore the deviations from linear energy loss using 2D particle-in-cell simulations that arise in the case of experimentally relevant finite length beams. The peak accelerating field in the plasma wave excited behind the finite-length beam is also examined, with the artifact of wave spiking adding to the apparent persistence of linear scaling of the peak field amplitude into the nonlinear regime. At large enough normalized charge, the linear scaling of both decelerating and accelerating fields collapses, with serious consequences for plasma wave excitation efficiency. Using the results of parametric particle-in-cell studies, the implications of these results for observing severe deviations from linear scaling in present and planned experiments are discussed.

Beam blowup due to a half-integer resonance was observed in the HIMAC synchrotron with a nondestructive two-dimensional beam-profile monitor. As the betatron tune approached a half-integer, the vertical beam size became larger by about 13%. The measured rms beam size is in good agreement with a space-charge-included numerical simulation.

In this paper a new type of THz radiation source, based on recirculating an electron beam through a high gradient superconducting radio frequency cavity, and using this beam to drive a standard electromagnetic undulator on the return leg, is discussed. Because the beam is recirculated and not stored, short bunches may be produced that radiate coherently in the undulator, yielding exceptionally high average THz power for relatively low average beam power. Deceleration from the coherent emission, and the detuning it causes, limits the charge-per-bunch possible in such a device.

The generation and transport of planar electron beams is a topic of increasing interest for applications to high-power, high-frequency microwave devices. This paper describes the use of solenoid lenses with elliptical pole apertures to transform a circular high-perveance beam to a planar configuration. The resulting transformation system has a short axial length and is easily fabricated. Furthermore, the approach allows use of a conventional electron gun. We designed lenses and simulated the dynamics of a beam system to drive a 95 GHz, traveling-wave tube experiment at Los Alamos National Laboratory. The three-dimensional self-consistent calculations indicate that a $0.5\mu \mathrm{perv}$ beam can be compressed to a height of less than 0.5 mm with sufficiently low emittance for transport through periodic-permanent-magnet or wiggler arrays.

Induction voltage adders (IVA) and induction accelerators of various types are described and their principles and advantages are discussed. The designs and technologies used in the various subsections and components of high-current IVAs are described. Some features of the pulse power that drives IVAs are discussed. Two representative high-current IVAs are briefly described, and characteristics of other IVAs with different features are mentioned. Some other IVA and induction accelerator variants that could be considered are also suggested.

We present a detailed comparison of the measured characteristics of Thomson backscattered x rays produced at the Picosecond Laser-Electron Interaction for the Dynamic Evaluation of Structures facility at Lawrence Livermore National Laboratory to predicted results from a newly developed, fully three-dimensional time and frequency-domain code. Based on the relativistic differential cross section, this code has the capability to calculate time and space dependent spectra of the x-ray photons produced from linear Thomson scattering for both bandwidth-limited and chirped incident laser pulses. Spectral broadening of the scattered x-ray pulse resulting from the incident laser bandwidth, perpendicular wave vector components in the laser focus, and the transverse and longitudinal phase spaces of the electron beam are included. Electron beam energy, energy spread, and transverse phase space measurements of the electron beam at the interaction point are presented, and the corresponding predicted x-ray characteristics are determined. In addition, time-integrated measurements of the x rays produced from the interaction are presented and shown to agree well with the simulations.

The generation of high intensity, ultrashort x-ray pulses enables exciting new experimental capabilities, such as femtosecond pump-probe experiments used to temporally resolve material structural dynamics on atomic time scales. Thomson backscattering of a high intensity laser pulse with a bright relativistic electron bunch is a promising method for producing such high-brightness x-ray pulses in the 10–100 keV range within a compact facility. While a variety of methods for producing subpicosecond x-ray bursts by Thomson scattering exist, including compression of the electron bunch to subpicosecond bunch lengths and/or colliding a subpicosecond laser pulse in a side-on geometry to minimize the interaction time, a promising alternative approach to achieving this goal while maintaining ultrahigh brightness is the production of a time-correlated (or chirped) x-ray pulse in conjunction with pulse slicing or compression. We present the results of a complete analysis of this process using a recently developed 3D time and frequency-domain code for analyzing the spatial, temporal, and spectral properties an x-ray beam produced by relativistic Thomson scattering. Based on the relativistic differential cross section, this code has the capability to calculate time and space dependent spectra of the x-ray photons produced from linear Thomson scattering for both bandwidth-limited and chirped incident laser pulses. Spectral broadening of the scattered x-ray pulse resulting from the incident laser bandwidth, laser focus, and the transverse and longitudinal phase space of the electron beam were examined. Simulations of chirped x-ray pulse production using both a chirped electron beam and a chirped laser pulse are presented. Required electron beam and laser parameters are summarized by investigating the effects of beam emittance, energy spread, and laser bandwidth on the scattered x-ray spectrum. It is shown that sufficient temporal correlation in the scattered x-ray spectrum to produce sub-100 fs temporal slice resolution can be produced from state-of-the-art, high-brightness electron beams without the need to perform longitudinal compression on the electron bunch.

There has been much recent experimental and theoretical interest in the blowout regime of plasma wakefield acceleration, which features ultrahigh accelerating fields, linear transverse focusing forces, and nonlinear plasma motion. A quantitative understanding of the blowout regime including all these effects has, to this point, been available only through detailed simulations. This paper represents an initial step towards an analytical theory of this regime, in which the mechanism of energy loss in the drive beam is investigated. We find, first from examination of electromagnetic particle-in-cell simulations, and then through analytical investigations, that under short pulse, high-charge conditions, the plasma electrons receive a strong initial push along the direction of beam motion. This nonlinear effect is unanticipated by linear theory, where the return current motion is in the opposite direction. In the limit of short pulses (the $\delta$-function limit), the beam energy loss is shown to be linear in charge even with a nonlinear plasma response dominated by relativistic, electromagnetic effects, despite the fact that the initial plasma electron response changes qualitatively from the familiar electrostatic, nonrelativistic limit.

The particle-core model for a continuous cylindrical beam is used to describe the motion of single particles oscillating in a uniform linear focusing channel. Using a random variation of the focusing forces, the model is deployed as proof of principle for the occurrence of large single particle radii without the presence of initial mismatch of the beam core. Multiparticle simulations of a periodic 3D transport channel are then used to qualify and quantify the effects in a realistic accelerator lattice.

In typical diagnostic applications, intense ion beams are intercepted by a conducting plate associated with devices used to measure beam phase-space projections. This results in the transverse space-charge field near the plate being shorted out, rendering simple envelope models with constant space-charge strength inaccurate. Here we develop corrected envelope models based on analytical calculations to account for this effect on the space-charge term of the envelope equations, thereby removing a systematic source of error in the equations and enabling more accurate comparisons with experiment. For common intense beam parameters, we find that the envelope correction occurs primarily in the envelope angles near the plate and that the effect can be large enough to degrade precision beam matching in periodic transport lattices. Results are verified with 3D self-consistent particle-in-cell simulations based on intense beam experiments associated with driver development for heavy-ion fusion.

In the x-ray, free-electron laser project, the Linac Coherent Light Source (LCLS), a proposal has been made to generate a shorter light pulse by placing a spoiler foil in the middle of a compressor chicane: The foil has a small slot, which selects out the small fraction of particles passing through it (“target particles”) to lase. In this report, using the method of field matching, we obtain longitudinal and transverse impedances and wakefields for several models of the proposed LCLS spoiler foil. We consider the model of a pencil beam and of a cylindrically symmetric, bi-Gaussian beam that is wider than it is long. Third, we generate a Green function that allows us to consider asymmetric beams also. For target particles of the tilted, tri-Gaussian beam that is found at the LCLS spoiler location we obtain approximate analytical formulas and numerical results for wakefield kicks in the three directions. We find that the kicks, after correction using a simple dipole and quadrupole, are all within tolerances.