$\gamma$-ray generation by Compton backscattering in an optical lens series with periodic focal points is considered to produce a polarized positron beam for a linear collider. The lens series is composed of 20 unit cells with a length of 210 mm. Each lens has a hole to pass an electron beam with an energy of 5.8 GeV and the generated $\gamma$ rays. It is shown by diffraction analysis that laser beam loss in the series is very small, and the beam size is periodically reduced to $26\mu \mathrm{m}$. Electron beam size is reduced to $34\mu \mathrm{m}$ in a superconducting solenoid with a field of 15 T. To get a required $\gamma$-ray yield of $7\times {10}^{15}\gamma /\mathrm{s}$, only one circularly polarized ${\mathrm{CO}}_2$ laser source with a power of 24 kW is needed.

A new accelerator, LACARA (laser-driven cyclotron autoresonance accelerator), under construction at the Accelerator Test Facility at Brookhaven National Laboratory, is to be powered by a 1 TW ${\mathrm{CO}}_2$ laser beam and a 50 MeV injected electron pulse. LACARA will produce inside a 2 m, 6 T solenoid a 100 MeV gyrating electron bunch, with $\sim 3\%$ energy spread, approximately 1 psec in length with particles advancing in phase at the laser frequency, executing one cycle each 35 fsec. A beamstop with a small off axis channel will transmit a short beam pulse every optical cycle, thereby producing a train of about 30, 3.5 fsec long, 1–3 pC microbunches for each laser pulse. We describe here a novel accelerator, a micron-scale dielectric wake field accelerator driven by a 500 MeV LACARA-type injector that takes the output train of microbunches and transforms them into a near-rectangular cross section having a narrow dimension of $\sim 10\mu \mathrm{m}$ and height of $\sim 150\mu \mathrm{m}$ using a magnetic quadrupole; these bunches may be injected into a planar dielectric-lined waveguide (slightly larger than the bunch) where cumulative buildup of wake fields can lead to an accelerating gradient $>1\mathrm{}\mathrm{GV}/\mathrm{m}$. This proposed vacuum-based wake field structure is physically rigid and capable of microfabrication accuracy, factors important in staging a large number of accelerator modules. Furthermore, the accelerating gradients it promises are comparable with those for plasma accelerators. A LACARA unit for preparing suitable bunches at 500 MeV is described. Physics issues are discussed, including bunch spreading and transport, bunch shaping, coherent diffraction radiation from the aperture, dielectric breakdown, and bunch stability in the rectangular wake field structure.

We consider the dynamical properties of a particle-core model for a uniformly filled triaxial ellipsoid in a periodic lattice of a high intensity linac. The mismatched oscillation modes are analytically computed in the smooth approximation and are compared with the numerical results of a tracking program. The study of the phase space in the mismatched case is performed by the frequency map analysis. In particular, we can analyze the effect of the nonlinear resonances between the envelope modes and the single particle sincrobetatron frequencies. A chaoticity criterion based on the frequency map analysis allows one to compute the stability region around the beam core. An estimate of the transport and its enhancement due to mismatch is provided by tracking orbits at the border of the stability region.

At the Cornell Electron/Positron Storage Ring, a fast luminosity monitor has been implemented. The monitor works by measuring the amplitude of oscillation of selected bunches while the corresponding opposing bunches are shaken. Signal-to-noise ratios of better than 100:1 have been obtained with 1 sec averaging times. A software normalization program ameliorates the unwanted dependence of the signal upon certain vertical lattice parameters such as the vertical tune.

The visible-infrared self-amplified spontaneous emission amplifier (VISA) free electron laser (FEL) is an experimental device designed to show self-amplified spontaneous emission (SASE) to saturation in the near infrared to visible light energy range. It generates a resonant wavelength output from 800–600 nm, so that silicon detectors may be used to characterize the optical properties of the FEL radiation. VISA is designed to show how SASE FEL theory corresponds with experiment in this wavelength range, using an electron beam with emittance close to that planned for the future Linear Coherent Light Source at SLAC. VISA comprises a 4 m pure permanent magnet undulator with four 99 cm segments, each of 55 periods, 18 mm long. The undulator has distributed focusing built into it, to reduce the average beta function of the 70–85 MeV electron beam to about 30 cm. There are four FODO cells per segment. The permanent magnet focusing lattice consists of blocks mounted on either side of the electron beam, in the undulator gap. The most important undulator error parameter for a free electron laser is the trajectory walk-off, or lack of overlap of the photon and electron beams. Using pulsed wire magnet measurements and magnet shimming, we were able to control trajectory walk-off to less than $\pm 50\mu \mathrm{m}$ per field gain length.

A compact proton synchrotron using combined function magnets is proposed to help realize the wider availability of charged particle cancer therapy facilities. This combined function magnet was designed with the help of three-dimensional magnetic field calculations to take account of a realistic fringe and the interference among the magnetic poles. An evaluation scheme for tune values based on particle tracking was developed to improve the magnet design. To verify the magnet design, a model magnet was fabricated and measured. In order to achieve a tune value evaluation from the measured magnetic field, schemes for accurate field mapping and field interpolation were developed. From the tune value evaluation of the measured magnetic field, it was thought that the performance of the model magnet was good enough to construct a synchrotron. In this paper, we report details of the design and the evaluation scheme for the combined function magnet and the results of the field measurements of the model magnet.

Because of a long-range resistive wake, the closed orbit may experience an unstable drift. Unlike the conventional betatron instabilities, this closed orbit instability is not sensitive to the spread of the betatron frequencies. For bunched beams, feedback appears to be the only way to stabilize the closed orbit above threshold. This new instability can be significant for both existing and designed high-intensity rings.

A calculation method of emittance growth of an electron beam due to intrabeam scattering is described. The 3 degrees of freedom are equally treated in the beam rest frame, and the couplings between them are included in a natural way. This formalism is suitable for the calculation of the emittance growth with the beam-envelope method.

The cavities of choice for high-energy synchrotrons and for medium-beta and high-beta accelerating superconducting structures around the world are $\pi$-mode structures. A coupled-circuit analysis, allowing for errors in cell frequencies and cell coupling constants, has been used to determine relative on-axis electric field amplitudes between cells, operating frequencies, end-cell tuning, relative on-axis field phase shifts, and field tilts when operated off resonance. Formulas are given for the above information as well as specific examples showing sensitivities and machining/assembly/control tolerances. A method to determine relative field amplitudes on the basis of the mode spectra was developed.

Cavities for $\pi$-mode operation are used in existing accelerators and are being built for many future systems. One of the steps in cavity construction is tuning individual cells within the cavity to ensure that the absolute values of relative peak on-axis electric field amplitudes in each cell are equal. This process may have multiple iterative steps: measuring the on-axis fields using bead-pull techniques, employing tuning methods to perturb the cells, making cell adjustments, and then repeating the process on the basis of the new results. This paper describes a one-step process for an $N$ cell cavity that provides the necessary adjustment for each cell to achieve equal field amplitude absolute values from one bead-pull measurement. In addition, a method is shown for inferring relative cell fields while a cavity is operational in an accelerator complex.

In a multipurpose cyclotron, it is convenient to use stripping extraction to extract selected positive ions in addition to the usual extraction by stripping of negative ions. The ions are specified as positive or negative according to the sign of their charge during acceleration. After stripping, positive ions travel not only through the pole edge magnetic field gradient, as is the case with negative ions, but can also pass more than once through regions of large sector edge magnetic field gradients. Influence of the magnetic field gradients on the trajectory of a stripped positive ion and on the extracted beam emittance is studied. It is shown that extraction efficiency depends on the position of the point of beam extraction from a cyclotron and on the ratio between ion charge after and before stripping. The position of the point of extraction also strongly affects the design parameters of the stripping extraction system. Extraction quality and design parameters cannot all be simultaneously optimized. The method of extraction system optimization is suggested and applied to the special case of the VINCY cyclotron.

In this paper we present two new techniques for frequency-resolved characterization of longitudinal impedances in storage rings. The first method is based on transient measurements of the growth rates and tune shifts of unstable coupled-bunch modes. In the second approach, estimates of the impedances are obtained from analysis of the steady-state synchronous phases of the bunches for uneven fill patterns. These techniques are applicable to measurements of both fundamental and higher-order mode (HOM) impedances and allow characterization of shunt impedances and quality factors of the HOMs. Methods presented here are complementary to laboratory bench measurements of rf cavities, in that the beam-based measurements directly sense the physical impedance in the installed configuration. Experimental results from the Advanced Light Source and BESSY-II are presented showing the use of these techniques to measure complex impedances.

A simple 1D model is proposed to explore the resonant extraction of intense beams from a synchrotron as performed in the SIS synchrotron in GSI (Darmstadt). The model Hamiltonian consists of a constant focusing, a thin sextupole, and a smooth space charge field. Hyperbolic normal forms are used to estimate the extraction times and the emittance of the extracted beam; the quality of the reconstruction is tested in absence of space charge. The effect of space charge on the dynamical behavior of the beam near the $1/3$ betatron resonance is numerically investigated using the frequency map analysis and qualitatively explained with perturbation theory. A polynomial approximation to the one turn map is obtained by replacing the exact space charge force with a sequence of polynomial kicks, and the resonant normal forms reproduce quite accurately the nonlinear tunes and the fixed points position. At low order an analytical estimate of the area of the stable region is proposed to recover the self-consistency of the model.

Extending the method of Warnock and Ellison for the simulation of the Vlasov-Fokker-Planck equation for bunched beams [in Proceedings of the 2nd ICFA Advanced Accelerator Workshop on the Physics of High Brightness Beams, Los Angeles, CA, 1999 (World Scientific, Singapore, 2000)], we analyze arbitrary radio-frequency potentials and high- $Q$ impedances and study instabilities in stretched bunches. Results for the microwave instability driven by a broadband impedance and instability driven by high- $Q$ rf modes in stretched bunches are compared with measurements of the vacuum ultraviolet ring at the National Synchrotron Light Source. A method for the calculation of response functions is developed, and simulated response functions are compared to experimental results.

We study a method of laser-Compton cooling of electron beams for future linear colliders. Using a Monte Carlo code, we evaluate the effects of the laser-electron interaction for transverse cooling. The optics with and without chromatic correction for the cooling are examined. The laser-Compton cooling for Japan Linear Collider/Next Linear Collider at $E_0=2\mathrm{}\mathrm{GeV}$ is considered.

Superconducting cavities presently used for acceleration of ions in velocity range $\sim 0.01c$ to $0.3c$ (where $c$ is the speed of light) are based on quarter-wave resonators. Currently there are several design proposals in nuclear physics laboratories for application of this type of cavity for acceleration of light and heavy ions. The operating frequencies of the cavities range from $\sim 50$ to 360 MHz to satisfy various specifications. Electrodynamics studies of the field distributions in the beam-cavity interaction area indicate appreciable dipole components of both electric and magnetic fields, especially for higher-frequency cavities. The dipole fields induce beam steering, which is a strong function of rf phase and which couples the longitudinal and transverse motion. This can result in growth in the transverse emittance of the beam. In this paper, we propose two possible methods for the correction of such dynamic beam-steering effects in quarter-wave resonators. We analyze and discuss the correction methods for the particular examples of two quarter-wave resonators operating at 57.5 and 115 MHz designed for the driver linac of the Rare Isotope Accelerator facility.

We recently used a prototype rf dipole magnet to study the spin flipping of a 669 MeV horizontally polarized electron beam stored in the presence of a nearly full Siberian snake in the new MIT-Bates storage ring. We flipped the spin by ramping the rf dipole's frequency through an rf-induced depolarizing resonance. After optimizing the frequency ramp parameters, we used multiple spin flipping to measure a spin-flip efficiency of $94.5\pm 2.5\%$. The spin-flip efficiency was apparently limited by the field strength in the air-core prototype rf dipole magnet. This unexpectedly high efficiency indicates that very efficient spin flipping of the ring's stored polarized electron beam should be possible using the much stronger ferrite spin flipper, which is now being built by the University of Michigan's Spin Physics Center.

We present 2D simulations of both beam-driven and laser-driven plasma wakefield accelerators, using the object-oriented particle-in-cell code XOOPIC, which is time explicit, fully electromagnetic, and capable of running on massively parallel supercomputers. Simulations of laser-driven wakefields with low $\backslash (\sim {10}^{16}\mathrm{W}/{\mathrm{cm}}^2\backslash )$ and high $\backslash (\sim {10}^{18}\mathrm{W}/{\mathrm{cm}}^2\backslash )$ peak intensity laser pulses are conducted in slab geometry, showing agreement with theory and fluid simulations. Simulations of the E-157 beam wakefield experiment at the Stanford Linear Accelerator Center, in which a 30 GeV electron beam passes through 1 m of preionized lithium plasma, are conducted in cylindrical geometry, obtaining good agreement with previous work. We briefly describe some of the more significant modifications to XOOPIC required by this work, and summarize the issues relevant to modeling relativistic electron-neutral collisions in a particle-in-cell code.

Detailed experimental results of staging two laser-driven, relativistic electron accelerators are presented. During the experiment called STELLA (staged electron laser acceleration), an inverse free-electron laser (IFEL) is used to modulate the electron energy, thereby, causing $\sim 3\mathrm{fs}$ microbunches to form separated by the laser wavelength at $10.6\mu \mathrm{m}$ (equivalent to a 35 fs period). A second IFEL accelerates the electrons depending upon the phase of the microbunches entering the second IFEL with respect to the laser beam driving the second IFEL. The data presented includes electron energy spectra as a function of the phase delay and laser power driving the first IFEL. Also shown is a comparison with the computer model, which includes space charge and misalignment effects.