Search Results (19 total)

We propose and analyze a regenerative-amplifier free-electron laser (FEL) to produce fully coherent, hard x-ray pulses. The method makes use of narrow-bandwidth Bragg crystals to form an x-ray feedback loop around a relatively short undulator. Self-amplified spontaneous emission (SASE) from the leading electron bunch in a bunch train is spectrally filtered by the Bragg reflectors and is brought back to the beginning of the undulator to interact repeatedly with subsequent bunches in the bunch train. The FEL interaction with these short bunches regeneratively amplifies the radiation intensity and broadens its spectrum, allowing for effective transmission of the x rays outside the crystal bandwidth. The spectral brightness of these x-ray pulses is about 2 to 3 orders of magnitude higher than that from a single-pass SASE FEL.

A Bragg waveguide consisting of multiple dielectric layers with alternating index of refraction becomes an excellent option to form electron accelerating structure powered by high power laser sources. It provides confinement of a synchronous speed-of-light mode with extremely low loss. However, laser field cannot be coupled into the structure collinearly with the electron beam. There are three requirements in designing input coupler for a Bragg electron accelerator: side coupling, selective mode excitation, and high coupling efficiency. We present a side-coupling scheme using a distributed grating-assisted coupler to inject the laser power into the waveguide. Side coupling is achieved by a grating with a period on the order of an optical wavelength. The phase matching condition results in resonance coupling thus providing selective mode excitation capability. The coupling efficiency is limited by profile matching between the outgoing beam and the incoming beam, which has normally a Gaussian profile. We demonstrate a nonuniform distributed grating structure generating an outgoing beam with a Gaussian profile, therefore, increasing the coupling efficiency.

We are concerned with coherent longitudinal motion in a storage ring, especially with situations in which coherent synchrotron radiation (CSR) can influence stability of the beam. The collective force from CSR is usually described by an impedance or a wake function in such a way that the force depends only on the charge distribution at the present time. This description is exact only for a rigid bunch, since causality demands that the force depend on the prior history of the bunch. We show how to treat a deforming bunch by applying the “complete impedance” Z(n,ω), a function of wave number and frequency. We derive this impedance and study its analytic properties for a special model: radiation from circular orbits shielded by parallel plates representing the metallic vacuum chamber. We analyze the corresponding collective force, obtaining the usual formula as a first approximation, plus easily computed corrections that depend on present and prior values of the time derivative of the charge density. In related papers we have applied these results in numerical simulations of instabilities induced by CSR.

We examine the effect of the collective force due to coherent synchrotron radiation (CSR) in an electron storage ring with small bending radius. In a computation based on time-domain integration of the nonlinear Vlasov equation, we find the threshold current for a longitudinal microwave instability induced by CSR alone. The model accounts for suppression of radiation at long wavelengths due to shielding by the vacuum chamber. In a calculation just above threshold, small ripples in the charge distribution build up over a fraction of a synchrotron period, but then die out to yield a relatively smooth but altered distribution with eventual oscillations in bunch length. The instability evolves from small noise on an initial smooth bunch of rms length much greater than the shielding cutoff.

The delay line distribution system is an alternative to conventional pulse compression, which enhances the peak power of rf sources while matching the long pulse of those sources to the shorter filling time of accelerator structures. We present an implementation of this scheme that combines pairs of parallel delay lines of the system into single lines. The power of several sources is combined into a single waveguide delay line using a multimode launcher. The output mode of the launcher is determined by the phase coding of the input signals. The combined power is extracted from the delay line using mode-selective extractors, each of which extracts a single mode. Hence, the phase coding of the sources controls the output port of the combined power. The power is then fed to the local accelerator structures. We present a detailed design of such a system, including several implementation methods for the launchers, extractors, and ancillary high power rf components. The system is designed so that it can handle the 600 MW peak power required by the Next Linear Collider design while maintaining high efficiency.

In this Letter we calculate the effects of a linearly varying focusing field on radiation damping and quantum excitation in an electron storage ring using a quantum mechanical perturbation approach. This model predicts correctly the limits of pure bending and pure focusing. We find that quantum excitation can be exponentially suppressed by the focusing field when the radiation formation length is comparable to the transverse-oscillation wavelength. This new result on quantum excitation may have interesting applications in the generation of ultralow emittance beams.

A compact laser-electron storage ring (LESR) is proposed for electron beam cooling or x-ray generation. The LESR uses an intense laser pulse stored in a high-finesse resonator to interact repetitively with a circulating electron beam in the energy range from a few MeV to a few hundred MeV. The rapid damping caused by laser-electron interaction counterbalances the intrabeam scattering effect, thus allowing electron beams with relatively low energy to be cooled or stabilized in the storage ring to very low transverse emittances. Intense x rays are produced simultaneously from Compton backscattering and can be used for x-ray lithography and other applications.

We describe a method for determining the stability of a system consisting of several highly relativistic bunches of charged particles circulating in a storage ring. The particles interact with magnets designed to guide the beam as well as with electromagnetic fields induced by the particles themselves. Previous calculations considered multibunch modes with one type of internal motion; our method includes coupling between these modes. We also include effects of feedback systems designed to correct these dipole motions. We include an example from a real storage ring design.

Research is under way at SLAC to develop accelerator structure for a next generation linear collider. A full-scale prototype X-band structure has been built in which the dipole mode frequencies were detuned to suppress the long-range transverse wake field by about 2 orders of magnitude. To verify that the detuning works as expected, a facility, the Accelerator Structure Setup, was constructed in the SLAC Linear Collider to measure the long-range wake field. This Letter presents the results from the measurement of the prototype X-band structure with this facility.

We show that the radiation damping rate of the transverse action of a particle in a straight, continuous focusing system is independent of the particle energy, and that no quantum excitation is induced. This absolute damping effect leads to the existence of a transverse ground state to which the particle inevitably decays and yields the minimum beam emittance that one can ever attain, γε_min  =  /2mc, limited only by the uncertainty principle. Because of adiabatic invariance, the particle can be accelerated along the focusing channel in its ground state without any radiation energy loss.

We explore an algorithm for the construction of symplectic maps to describe nonlinear particle motion in circular accelerators. We emphasize maps for motion over one or a few full turns, which may provide an economical way of studying long-term stability in large machines such as the Superconducting Super Collider (SSC). The map is defined implicitly by a mixed-variable generating function, represented as a Fourier series in betatron angle variables, with coefficients given as B-spline functions of action variables and the total energy. Despite the implicit definition, iteration of the map proves to be a fast process. The method is illustrated with a realistic model of the SSC. We report extensive tests of accuracy and iteration time in various regions of phase space, and demonstrate the results by using single-turn maps to follow trajectories symplectically for 10⁷ turns on a workstation computer. The same method may be used to construct the Poincaré map of Hamiltonian systems in other fields of physics.

We describe a nonperturbative numerical technique for solving the Hamilton-Jacobi equation of a nonlinear Hamiltonian system. We find the time-periodic solutions that yield accurate approximations to invariant tori. The method is suited to the case in which the perturbation to the underlying integrable system has a periodic and not necessarily smooth dependence on the time. This case is important in accelerator theory, where the perturbation is a periodic step function in time. The Hamilton-Jacobi equation is approximated by its finite-dimensional Fourier projection with respect to angle variables, then solved by Newton’s method. To avoid Fourier analysis in time, which is not appropriate in the presence of step functions, we enforce time periodicity of solutions by a shooting algorithm. The method is tested in soluble models, and finally applied to a nonintegrable example, the transverse oscillations of a particle beam in a storage ring, in two degrees of freedom. In view of the time dependence of the Hamiltonian, this is a case with ‘‘21/2 degrees of freedom,’’ in which phenomena like Arnol’d diffusion can occur.

In this paper we present methods for studying and controlling transverse coupled-bunch instabilities. Our primary motivation is the study of damping rings for next-generation linear colliders in which many bunches are damped in the same ring simultaneously; however, the methods presented are also applicable to other situations. The theory developed here treats the motion of the bunch centroids, since the coherent dipole modes of coupled-bunch oscillation are expected to be strongly dominant. A formalism to obtain the n normal modes of oscillation of n bunches is developed. The imaginary part of the frequency of each normal mode determines its stability. However, not only the long-term stability of each oscillation mode, but also the transient behavior of the bunches just after injection, must be considered in damping rings. Two methods of studying the transient behavior are presented: (1) A Laplace-transform method, using the eigenmodes and corresponding eigen-frequencies found by the normal-modes formalism, and (2) computer tracking, using a localized-kick approximation. Examples are given for damping-ring designs appropriate to a linear collider of about 0.5-1.0 TeV center-of-mass energy.

By constructing action variables that are very nearly invariant in a region Ω of phase space, and by examining their residual variation, we set long-term bounds on any orbit starting in an open subregion of Ω. A new and generally applicable method for constructing the required high-precision invariants is applied. The technique is illustrated for transverse oscillations in a circular accelerator, a case with 21/2 degrees of freedom and strong nonlinearity.

In this paper, we study multibunch beam breakup, with emphasis on theoretical methods applicable to the design of a linear collider with a center-of-mass energy near 1 TeV. One way to significantly improve the luminosity and energy transfer efficiency of such a collider is to accelerate a train of bunches rather than just a single bunch each time the linac accelerating structure is filled with a pulse of rf energy. For the required bunch charges and intensities, the transverse instability due to the wake fields produced in the accelerating structure is very severe unless measures are taken to control it. Therefore, we examine the effects of several methods of reducing this instability: (1) use of damped acceleration cavities, (2) placing the bunches near nodes of the transverse wake fields produced by preceding bunches, (3) introducing a spread (over different cells of the accelerating structure) of the individual mode frequencies in the transverse wake field, and (4) varying the strength of the transverse focusing from bunch to bunch, in such a way as to partially cancel the effects of the wake fields from preceding bunches. We present examples illustrating the effectiveness of these cures, using realistic linear-collider design parameters.

We have used relativistic klystron technology to extract 290 MW of peak power at 11.4 GHz from an induction linac beam, and to power a short 11.4-GHz high-gradient accelerator. We have measured rf phase stability, field emission, and the momentum spectrum of an accelerated electron beam. An average accelerating gradient of 84 MV/m has been achieved with 80 MW of relativistic klystron power.