A simulation of the generation of Smith-Purcell (SP) radiation at microwave frequencies is performed using the two-dimensional particle-in-cell code MAGIC. The simulation supposes that a continuous, thin (but infinitely wide), monoenergetic electron beam passes over a diffraction grating, while a strong axial magnetic field constrains the electrons to essentially one-dimensional motion. The code computes the time-dependent electric and magnetic fields by solving the Maxwell equations using a finite element approach. We find that the passage of the beam excites an evanescent electromagnetic wave in the proximity of the grating, which in turn leads to bunching of the initially continuous electron beam. The frequency and wave number of the bunching are determined, and found to be close to those proposed by Brau and co-workers in recent work. This frequency is below the threshold for SP radiation. However, the bunching is sufficiently strong that higher harmonics are clearly visible in the beam current. These harmonic frequencies correspond to allowed SP radiation, and we see strong emission of such radiation at the appropriate angles in our simulation, again in agreement with Brau’s predictions. We also find that at the ends of the grating, some of the evanescent wave is diffracted away from the surface, and radiation below the threshold occurs. In addition, we observe a second evanescent wave at the same frequency, but with a different wave number. The existence of this wave is also predicted by the theory, although its presence in our simulation is unexpected. Numerical estimates of the growth of the evanescent wave are also in reasonable agreement with the predictions, although the precise form of the dependence of the gain on beam current remains hard to establish.

This paper demonstrates that transverse beam halos can be controlled by combining nonlinear focusing and collimation. The study relies on one-dimensional, constant focusing particle-in-cell (PIC) simulations and a particle-core model. Beams with linear and nonlinear focusing are studied. Calculations with linear focusing confirm previous findings that the extent and density of the halo depend strongly upon the initial mismatch of the beam. Calculations with nonlinear focusing are used to study damping in the beam oscillations caused by the mismatch. Although the nonlinear force damps the beam oscillations, it is accompanied by emittance growth. This process is very rapid and happens within the first 2–3 envelope oscillations. After this, when the halo is collimated using a system of four collimators, further evolution of the beam shows that the halo is not regenerated. The elimination of the beam halo could allow either a smaller physical aperture for the transport system or it could allow a beam of higher current in a system with the same physical aperture. This advantage compensates for the loss of particles due to collimation.

In this paper the results of the experimental measurements at Berlin electron storage ring for synchrotron radiation (BESSY-II) are presented. Grow/damp measurements of the transverse coupled-bunch instabilities were performed using a multibunch feedback system and a synchronized time-domain bunch motion recorder. The results of these experiments are described and explained by the beam-ion instability. Simplified simulations and revised theory of the instability extended to a nonlinear regime are used for the interpretation of the results.

The optical field evolution of an optical klystron free electron laser is analytically described for both low gain and high gain cases. The harmonic optical klystron (HOK) in which the second undulator is resonant on the higher harmonic of the first undulator is analyzed as a harmonic amplifier. The optical field evolution equation of the HOK is derived analytically for both the CHG mode (coherent harmonic generation, the quadratic gain regime) and the HGHG mode (high gain harmonic generation, the exponential gain regime), the effects of energy spread, energy modulation, and dispersion in the whole process are taken into account. The linear theory is given and discussed for the HGHG mode. The analytical formula is given for the CHG mode.

A kinetic model based on the Vlasov equation is used to describe the axial drift compression and transverse focusing of an intense ion charge bunch propagating along the axis of a solenoidal focusing field ${\mathbf{B}}^{\mathrm{sol}}(\mathbf{x})$. The space charge and current of the ion charge bunch are assumed to be completely neutralized by the electrons provided by a dense background plasma. In the absence of self-field forces, the Vlasov equation is solved exactly for general initial distribution function $f_b(\mathbf{x},\mathbf{v},0)$, using the method of characteristics. It is shown that the Vlasov equation possesses a class of exact, dynamically evolving solutions $f_b(W_{\perp },W_z)$, where $W_{\perp }$ and $W_z$ are transverse and longitudinal constants of the motion. Detailed dynamical properties of the charge bunch are calculated during axial compression and transverse focusing for several choices of distribution function $f_b(W_{\perp },W_z)$.

Practical ionization cooling rings could lead to lower cost or improved performance in neutrino factory or muon collider designs. The ring modeled here uses realistic three-dimensional fields. The performance of the ring compares favorably with the linear cooling channel used in the second U.S. Neutrino Factory Study. The normalized 6D emittance of an ideal ring is decreased by a factor of approximately 240, compared with a factor of only 15 for the linear channel. We also examine such real-world effects as windows on the absorbers and rf cavities and leaving empty lattice cells for injection and extraction. For realistic conditions the ring decreases the normalized 6D emittance by a factor of 49.

We numerically study properties of primary dark currents in an $X$-band accelerating structure. For the H60VG3 structure considered for the Next Linear Collider (NLC) we first perform a fairly complete (with some approximations) calculation of dark-current trajectories. These results are used to study properties of the dark current leaving the structure. For example, at accelerating gradient of $65\mathrm{MV}/\mathrm{m}$, considering two very different assumptions about dark-current emission around the irises, we find that the fraction of emitted current leaving the structure to be a consistent $\sim 1\%$. Considering that $\sim 1\mathrm{mA}$ outgoing dark current is seen in measurement, this implies that $\sim 100\mathrm{mA}$ (or 10 pC per period) is emitted within the structure itself. Using the formalism of the Liénard-Wiechert potentials, we then perform a systematic calculation of the transverse kick of dark currents on a primary linac bunch. The result is $\sim 1\mathrm{V}$ kick per mA (or per $0.1\mathrm{pC}$ per period) dark current emitted from an iris. For an entire structure we estimate the total kick on a primary bunch to be $\sim 15\mathrm{V}$. For the NLC linac this translates to a ratio of (final) vertical beam offset to beam size of about 0.2. However, with the assumptions that needed to be made—particularly the number of emitters and their distribution within a structure—the accuracy of this result may be limited to the order of magnitude.

The sudden drop of the beam lifetime and bursts of the background radiation were detected in many machines and associated with microscopic dust particles. We present the model of the dust particle dynamics explaining the long time of the dust events observed in the PEP-II B-factory and BEPC-II machines.

Autocorrelation is applied to analyze sets of finite-sampling data such as the turn-by-turn beam position monitor (BPM) data in an accelerator. This method of data analysis, called the independent component analysis (ICA), is shown to be a powerful beam diagnosis tool for being able to decompose sampled signals into its underlying source signals. We find that the ICA has an advantage over the principle component analysis (PCA) used in the model-independent analysis (MIA) in isolating independent modes. The tolerance of the ICA method to noise in the BPM system is systematically studied. The ICA is applied to analyze the complicated beam motion in a rapid-cycling booster synchrotron at the Fermilab. Difficulties and limitations of the ICA method are also discussed.

We present an improved system for orbit and betatron phase measurement utilizing nonlinear models of beam position monitor (BPM) pickup response. We first describe the calculation of the BPM pickup signals as nonlinear functions of beam position using Greens reciprocity theorem with a two-dimensional formalism. We then describe the incorporation of these calculations into our beam position measurements by inverting the nonlinear functions, giving us beam position as a function of the pickup signals, and how this is also used to improve our calculation of the betatron phase advance. Measurements are presented comparing this system with the linearized pickup response used historically at CESR.

We report the development of a nondestructive technique to measure bunch rms length in the psec range and below, and eventually in the fsec range, by measuring the high-frequency spectrum of wakefield radiation which is caused by the passage of a relativistic electron bunch through a channel surrounded by a dielectric. We demonstrate numerically that the generated spectrum is determined by the rms bunch length, while the specific axial and longitudinal charge distribution is not important. Measurement of the millimeter-wave spectrum will determine the rms bunch length in the psec range. This has been done using a series of calibrated mesh filters and the charge bunches produced by the 50 MeV rf linac system at ATF (Accelerator Test Facility), Brookhaven. We have developed the analysis of the factors crucial for achieving good accuracy in this measurement, and find the experimental data are fully understood by the theory. We point out that this technique also may be used for measuring fsec bunch lengths, using a prepared planar wakefield microstructure.

We recently studied the spin manipulation of $1.85\mathrm{GeV}/c$ vertically polarized deuterons stored in the COSY cooler synchrotron. We adiabatically swept an rf dipole’s frequency through an rf-induced spin resonance and observed its effect on the deuterons’ vector and tensor polarizations. After optimizing the resonance crossing rate and maximizing the rf dipole’s voltage, we measured spin-flip efficiencies of $97\pm 1\%$ and $98.5\pm 0.3\%$ in two separate runs. We also confirmed at higher energy the striking behavior of the spin-1 tensor polarization recently found at IUCF.

We describe a technique for the generation of a solitary attosecond x-ray pulse in a free-electron laser (FEL), via a process of self-amplified spontaneous emission. In this method, electrons experience an energy modulation upon interacting with laser pulses having a duration of a few cycles within single-period wiggler magnets. Two consecutive modulation sections, followed by compression in a dispersive section, are used to obtain a single, subfemtosecond spike in the electron peak current. This region of the electron beam experiences an enhanced growth rate for FEL amplification. After propagation through a long undulator, this current spike emits a $\sim 250\mathrm{\text{attosecond}}$ x-ray pulse whose intensity dominates the x-ray emission from the rest of the electron bunch.

It has previously been shown that the electron beam in a Smith-Purcell free-electron laser interacts with a synchronous evanescent wave. At high electron energy, the group velocity of this wave is positive and the device operates on a convective instability, in the manner of a traveling-wave tube. For operation as an oscillator, the gain must exceed the losses in the external feedback system. At low electron energy, the group velocity of the synchronous evanescent wave is negative and the device operates on an absolute instability, like a backward-wave oscillator, and no external feedback is required. For oscillation to occur, the current must exceed the so-called start current. At an intermediate energy, called the Bragg condition, the group velocity $v_g$ of the evanescent wave vanishes and both the gain and the attenuation due to resistive losses in the grating diverge. It is shown that near the Bragg condition the gain depends on $v_g^{-1/3}$, while the attenuation depends on $v_g^{-1}$. Since the attenuation increases faster than the gain near the Bragg condition, the Smith-Purcell free-electron laser cannot operate at the point of maximum gain. The effects of resistive losses become increasingly important as Smith-Purcell free-electron lasers move to shorter wavelengths.

The neutralized transport experiment (NTX) at the Heavy Ion Fusion Virtual National Laboratory is exploring the performance of neutralized final-focus systems for high perveance heavy ion beams. The final-focus scenario in a heavy ion fusion driver consists of several large aperture quadrupole magnets followed by a drift section in which the beam space charge is neutralized by a plasma. This beam is required to hit a millimeter-sized target spot at the end of the drift section. The objective of the NTX experiments and associated theory and simulations is to study the various physical mechanisms that determine the final spot size (radius $r_s$) at a given distance ($f$) from the end of the last quadrupole. In a fusion driver, $f$ is the standoff distance required to keep the chamber wall and superconducting magnets properly protected. The NTX final quadrupole focusing system produces a converging beam at the entrance to the neutralized drift section where it focuses to a small spot. The final spot is determined by the conditions of the beam entering the quadrupole section, the beam dynamics in the magnetic lattice, and the plasma neutralization dynamics in the drift section. The main issues are the control of emittance growth due to high order fields from magnetic multipoles and image fields. In this paper, we will describe the theoretical and experimental aspects of the beam dynamics in the quadrupole lattice, and how these physical effects influence the final beam size. In particular, we present theoretical and experimental results on the dependence of final spot size on geometric aberrations and perveance.

We present a design of nonscaling fixed field alternating gradient accelerators (FFAG) minimizing the dispersion action function $H$. The design is considered both analytically and via computer modeling. We present the basic principles of a nonscaling FFAG lattice and discuss optimization strategies so that one can accelerate over a broad range of momentum with reasonable apertures. Acceleration schemes for muons are discussed.

The lattice of a laser-electron storage ring with a controlled momentum compaction factor dedicated to the generation of intense x rays under Compton scattering is described. In such a storage ring one can obtain large energy acceptance and keep the long-term stable motion of the electron beam with large energy spread. The intensity of x rays is stable due to the use of electron beam with steady-state parameters for Compton scattering. Parameters of the storage ring, electron beam, and x rays obtained by electron beam dynamics simulation involving both intrabeam and Compton scattering are presented.

The ion-induced desorption experiment, installed in the CERN Heavy-Ion Accelerator LINAC 3, has been used to measure molecular desorption yields for $4.2\mathrm{MeV}/\mathrm{u}$ lead ions impacting under grazing incidence on different accelerator-type vacuum chambers. Desorption yields for ${\mathrm{H}}_2$, $\mathrm{C}{\mathrm{H}}_4$, CO, and $\mathrm{C}{\mathrm{O}}_2$, which are of fundamental interest for future accelerator applications, are reported for different stainless steel surface treatments. In order to study the effect of the surface oxide layer on the gas desorption, gold-, silver-, palladium-, and getter-coated 316 LN stainless steel chambers and similarly prepared samples were tested for desorption at LINAC 3 and analyzed for chemical composition by x-ray photoemission spectroscopy. The large effective desorption yield of $2\times {10}^4\mathrm{\text{molecules}}/\mathrm{P}{\mathrm{b}}^{53+}$ ion, previously measured for uncoated, vacuum fired stainless steel, was reduced after noble-metal coating by up to 2 orders of magnitude. In addition, pressure rise measurements, the effectiveness of beam scrubbing with lead ions, and the consequence of a subsequent venting on the desorption yields of a beam-scrubbed vacuum chamber are described. Practical consequences for the vacuum system of the future Low Energy Ion Ring are discussed.

In this paper, a formula, describing the threshold of regenerative multipass beam breakup (BBU) for a single dipole higher-order mode with arbitrary polarization in a two-pass accelerator with a general-form, $4\times 4$ recirculation matrix, is derived. A new two-dimensional BBU code is also introduced. To illustrate specifics of the BBU in two dimensions, the formula is used to calculate the threshold in several cases including two-dimensional uncoupled optics, reflecting optics, and rotating optics. The analytical results are compared to results of simulations obtained with the new code. A mathematical relation between transfer matrices between cavities of the accelerating structure and recirculation matrices for each cavity, which must be satisfied in order to successfully suppress the BBU by reflection or rotation in several cavities, is presented.

A new method for electron beam conditioning in free electron lasers is proposed. It uses the electron beam interaction with a laser light in two wiggler magnets separated by a strong-focusing channel. The effect of the conditioning is illustrated by the example of a hypothetical single-pass, high-gain free electron laser operating in the self-amplified spontaneous emission mode with the x-ray emission at ${\lambda }_x=1.5\AA$. The proposed conditioner is relatively compact and can be used as a practical add-on device to short x-ray wavelength free electron lasers.