Theoretical and numerical studies of the transport in vacuum of multi-nC, multi-MeV electron beams are performed using several methods, including envelope models, a novel semianalytic approach using ellipsoidal shell decomposition, a modified electrostatic particle-in-cell method, and a point-to-point interaction model. The effects of space-charge forces on the longitudinal and transverse bunch properties are evaluated for various bunch lengths, energies, energy spreads, and charges. An evaluation of the various methods for studying space-charge effects in large energy spread, high charge beams is summarized. Examples are given for beam distributions typical of those generated by plasma-based accelerators. It is found that, for the highly correlated beams produced in the self-modulated regime, the high energy portion of the beam can gain significant energy while propagating in vacuum due to space-charge effects.

The pulse line ion accelerator concept was motivated by the desire for an inexpensive way to accelerate intense short pulse heavy ion beams to regimes of interest for studies of high energy density physics and warm dense matter. A pulse power driver applied at one end of a helical pulse line creates a traveling wave pulse that accelerates and axially confines the heavy ion beam pulse. Acceleration scenarios with constant parameter helical lines are described which result in output energies of a single stage much larger than the several hundred kilovolt peak voltages on the line, with a goal of $3–5\mathrm{MeV}/\mathrm{\text{meter}}$ acceleration gradients. The concept might be described crudely as an “air core” induction linac where the pulse-forming network is integrated into the beam line so the accelerating voltage pulse can move along with the ions to get voltage multiplication.

Light at 1560 nm from a gain-switched fiber-coupled diode laser and ErYb-doped fiber amplifier was frequency doubled to obtain over 2 W average power at 780 nm with $\sim 40\mathrm{ps}$ pulses and pulse repetition rate of 499 MHz. This light was used to drive the 100 kV DC high voltage GaAs photoemission gun at the Continuous Electron Beam Accelerator Facility at Jefferson Laboratory to produce a high average current beam ($100\mu \mathrm{A}$) of highly spin-polarized electrons ($>80\%$). This new drive-laser system represents a significant advance over laser systems used previously, providing significantly higher power and enhanced reliability.

A theory of transverse instability of a bunch in a rectangular potential well is developed. A series of equations adequately describing the instability is derived and solved both analytically and numerically. The dependence of the instability growth rate and threshold on bunch factor is investigated for various beam coupling impedances. The theory is applied to the Fermilab recycler ring.

Multipactoring phenomena in a radio frequency (rf) input coupler, which could cause serious problems during accelerator operation, are described based on electrodynamics with the superposition state of the input and reflected waves in the coupler. Since the reflection coefficient varies widely with the beam loading in the case of high beam current storage rings, multipactoring zones should be represented in the two-dimensional space of the input rf power and the reflection coefficient. Based on this map, we have performed a simulation study for normal-conducting rf accelerating cavities and developed a method to operate an rf cavity with multipactoring problems so as to minimize the influence of multipactoring.

A new iterative method is developed to numerically calculate the periodic, matched beam envelope solution of the coupled Kapchinskij-Vladimirskij equations describing the transverse edge trajectory of a beam in a periodic, linear focusing lattice of arbitrary complexity. Implementation of the method is straightforward. It is highly convergent and can be applied to all usual parametrizations of the matched envelope solutions. The method is applicable to all classes of linear focusing lattices without skew couplings, and also applies to all physically achievable system parameters—including cases where the matched beam envelope is strongly unstable. Example applications are presented for periodic solenoidal and quadrupole focusing lattices. Convergence properties are summarized over a wide range of system parameters.

One scheme for harmonic generation employs free electron lasers (FELs) with two undulators: the first uses a seed laser to modulate the energy of the electron beam; following a dispersive element which acts to bunch the beam, the second undulator radiates at a higher harmonic. These processes are currently evaluated using extensive calculations or simulation codes which can be slow to evaluate and difficult to set up. We describe a simple algorithm to predict the output of a harmonic generation beam line in the low-gain FEL regime, based on trial functions for the output radiation. Full three-dimensional effects are included. This method has been implemented as a Mathematica® package, named CAMPANILE, which runs rapidly and can be generalized to include effects such as asymmetric beams and misalignments. This method is compared with simulation results using the FEL code GENESIS, both for single stages of harmonic generation and for the LUX project, a design concept for an ultrafast x-ray facility, where multiple stages upshift the input laser frequency by factors of up to 200.

The problems of spin-polarized free-electron beam interaction with electromagnetic wave at electron-spin resonance conditions in a magnetic field and of superradiant spin-flip radiative emission are analyzed in the framework of a comprehensive classical model. The spontaneous emission of spin-flip radiation from electron beams is very weak. We show that the detectivity of electron spin resonant spin-flip and combined spin-flip/cyclotron-resonance-emission radiation can be substantially enhanced by operating with ultrashort spin-polarized electron beam bunches under conditions of superradiant (coherent) emission. The proposed radiative spin-state modulation and the spin-flip radiative emission schemes can be used for control and noninvasive diagnostics of polarized electron/positron beams. Such schemes are of relevance in important scattering experiments off nucleons in nuclear physics and off magnetic targets in condensed matter physics.

A simulation of the generation of Smith-Purcell (SP) radiation at terahertz frequencies has been performed using the two-dimensional particle-in-cell code MAGIC. The simulation supposes that a thin (but infinitely wide) monoenergetic electron beam passes over a diffraction grating. We simulate two configurations, one similar to the Dartmouth SP free-electron laser, with a low-energy continuous beam (we use an axial magnetic field to constrain the electrons to essentially one-dimensional motion). The other is similar to the recent MIT experiment that uses a prebunched 15 MeV beam.

Electron clouds limit the performance of many major accelerators and storage rings. Significant quantities of electrons result when halo ions are lost to beam tubes, generating gas which can be ionized and ion-induced electrons that can multiply and accumulate, causing degradation or loss of the ion beam. In order to understand the physical mechanisms of ion-induced electron production, experiments studied the impact of 50 to 400 keV ${\mathrm{K}}^{+}$ ions on stainless steel surfaces near grazing incidence, using the 500 kV ion source test stand (STS-500) at LLNL. The experimental electron yield scales with the electronic component ($d{E}_e/dx$) of the stopping power and its angular dependence does not follow $1/\cos (\theta )$. A theoretical model is developed, using TRIM code to evaluate $d{E}_e/dx$ at several depths in the target, to estimate the electron yield, which is compared with the experimental results. The experiment extends the range of energy from previous works and the model reproduces the angular dependence and magnitude of the electron yield.

We describe a model of damage in rf cavities and show how this damage can limit cavity operation. We first present a review of mechanisms that may or may not affect the ultimate fields that can be obtained in rf cavities, assuming that mechanical stress explains the triggers of rf breakdown events. We present a method of quantifying the surface damage caused by breakdown events in terms of the spectrum of field enhancement factors, $\beta$, for asperities on the surface. We then model an equilibrium that can develop between damage and conditioning effects, and show how this equilibrium can determine cavity performance and show experimental evidence for this mechanism. We define three functions that quantify damage, and explain how the parameters that determine performance can be factored out and measured. We then show how this model can quantitatively explain the dependence of cavity performance on material, frequency, pulse length, gas, power supply, and other factors. The examples given in this paper are derived from a variety of incomplete data sets, so we outline an experimental program that should improve these predictions, provide mechanisms for comparing data from different facilities, and fill in many gaps in the existing data.

A barrier bucket with very small separation between the rf barriers (relative to the barrier widths) or even zero separation has its synchrotron tune decreasing rather slowly from a large value towards the boundary of the bucket. As a result, a large area at the bucket edges can become unstable under the modulation of rf voltage and/or rf phase. In addition, chaotic regions may form near the bucket center and extend outward under increasing modulation. Application is made to those barrier buckets once used in the process of momentum mining at the Fermilab Recycler Ring.

In this paper we present a comprehensive analysis of the self-consistent, collective behavior associated with the space-charge-driven (Montague) coupling resonance near $2Q_x-2Q_y=0$, including the effect of linear coupling. Based on analytical work and particle-in-cell simulation in the 2D coasting beam limit, we derive scaling laws for stop-band widths and growth rates, which may be applied to circular machines as well as to linear accelerators. For slow crossing of the stop bands, we find a strong directional dependence. In the case of crossing from below—assuming that the rising tune pertains to the direction, in which the initial emittance is the larger one—the emittance exchange is a smooth and fully reversible process. For crossing from above, we encounter a discontinuous behavior, which disappears largely, if an external linear coupling is applied.

We have observed the interference of optical diffraction radiation (ODR) and optical transition radiation (OTR) produced by the interaction of a relativistic electron beam with a micromesh foil and a mirror. The production of forward directed ODR from electrons passing through the holes and wires of the mesh and their separate interactions with backward OTR from the mirror are analyzed with the help of a simulation code. By careful choice of the micromesh properties, mesh-mirror spacing, observation wavelength, and filter band pass, the interference of the ODR produced from the unperturbed electrons passing through the open spaces of the mesh and OTR from the mirror are observable above a broad incoherent background from interaction of the heavily scattered electrons passing through the mesh wires. These interferences (ODTRI) are sensitive to the beam divergence and can be used to directly diagnose this parameter. We compare experimental divergence values obtained using ODTRI, conventional OTRI, for the case when front foil scattering is negligible, and computed values obtained from transport code calculations and multiple screen beam size measurements. We obtain good agreement in all cases.

We determine the low frequency transverse impedance of axially symmetric tapered structures. Higher-order perturbation theory is used to improve previous estimates due to Yokoya and Stupakov. For linear tapers, accurate numerical results are obtained using the ABCI electromagnetic simulation code. Based upon insight gained from the perturbation calculations, we introduce a simple parametrization that provides an excellent fit to all of our ABCI data—including cases with gradual and steep tapers as well as large and small change in cross section.

The temperature distribution and heat flow in the planar optical Bragg acceleration structure, fed by a train of high-power laser pulses, are analyzed. Dynamic analysis of a high-repetition rate train of pulses indicates that the stationary solution is an excellent approximation for the regime of interest. Analytic expressions for the temperature and heat distributions across the acceleration structure are developed. Assuming an accelerating gradient of $1\mathrm{GV}/\mathrm{m}$ and a loss factor similar to that existing in communication optical fibers $1\mathrm{dB}/\mathrm{km}$ $(\tan \delta \sim {10}^{-11})$, the temperature increase is less than 1 K and the heat flow is of the order of $1\mathrm{W}/{\mathrm{cm}}^2$, which is 3 orders of magnitude lower than the known technological limit for heat dissipation. Obviously, using materials with a significantly higher loss tangent may lead to unacceptable temperatures and temperature gradients as well as confinement difficulties and phase mismatch.

The beam coupling impedances of small discontinuities of an accelerator vacuum chamber have been calculated [e.g., Kurennoy, Gluckstern, and Stupakov, Phys. Rev. E 52, 4354 (1995)] for ultrarelativistic beams using the Bethe diffraction theory. Here we extend the results to an arbitrary beam velocity. The vacuum chamber is assumed to have an arbitrary, but uniform along the beam path, cross section. The longitudinal and transverse coupling impedances are derived in terms of series over cross-section eigenfunctions, while the discontinuity shape enters via its polarizabilities. Simple explicit formulas for two important particular cases—circular and rectangular chamber cross sections—are presented. The impedance dependence on the beam velocity exhibits some unusual features: for example, the reactive impedance, which dominates in the ultrarelativistic limit, can vanish at a certain beam velocity, or its magnitude can exceed the ultrarelativistic value many times. In addition, we demonstrate that the same technique, the field expansion into a series of cross-section eigenfunctions, is convenient for calculating the space-charge impedance of uniform beam pipes with arbitrary cross section.

The single-particle dynamics in a time-dependent focusing field is examined. The existence of the Courant-Snyder invariant, a fundamental concept in accelerator physics, is fundamentally a result of the corresponding symmetry admitted by the harmonic oscillator equation with linear time-dependent frequency. It is demonstrated that the Lie algebra of the symmetry group for the oscillator equation with time-dependent frequency is eight dimensional, and is composed of four independent subalgebras. A detailed analysis of the admitted symmetries reveals a deeper connection between the nonlinear envelope equation and the oscillator equation. A general theorem regarding the symmetries and invariants of the envelope equation, which includes the existence of the Courant-Snyder invariant as a special case, is demonstrated. As an application to accelerator physics, the symmetries of the envelope equation enable a fast numerical algorithm for finding matched solutions without using the conventional iterative Newton’s method, where the envelope equation needs to be numerically integrated once for every iteration, and the Jacobi matrix needs to be calculated for the envelope perturbation.

Heavy ion drivers for heavy ion fusion and high energy density physics applications use space-charge-dominated ion beams which must undergo longitudinal bunch compression in order to meet the requisite beam intensities desired at the target. The Neutralized Drift Compression Experiment-1A (NDCX-1A) at Lawrence Berkeley National Laboratory is used to determine the effective limits of neutralized drift compression, which occurs due to an imposed longitudinal velocity tilt on the drifting beam and subsequent neutralization of the beam’s space charge with background plasma. The accurate and temporally resolved measurement of the ion beam’s current and pulse length, which has been longitudinally compressed to a few nanoseconds duration at its focal plane, is a critical diagnostic. This paper describes the design and experimental results for a fast and accurate ion beam probe, which reliably measures the absolute beam current in the presence of high density plasma at the focal plane as a function of time. A particle-in-cell code has been used to model the propagation of the intense ion beam and to design the diagnostic probe.

We have developed a two-macroparticle bunch to explore the longitudinal beam dynamics through various components of the Fermilab/NICADD photoinjector. Such a two-macroparticle bunch is generated by splitting the ultraviolet pulse from the photocathode drive laser. The presented method allows the exploration of radio-frequency-induced compression in the 1.625 cell radio frequency gun and the booster cavity. It also allows a direct measurement of the momentum compaction of the magnetic bunch compressor. The measurements are compared with analytical and numerical models.