The compression of relativistic electron beams resulting from partial space charge neutralization by thermal ions is simulated to obtain self-consistent solutions. The numerical modeling is based on a finite difference approach, using under-relaxation to assure convergence in solving this nonlinear problem. The results show a nonuniform fraction of neutralization, increasing as a function of radius. Neutralization on axis is higher for colder compensating ions and for lower electron energy. In general, the temperature of the ions turns out to be higher than that of the electrons. With respect to the non-neutralized, not-thermally-dispersed beam, higher compression factors result at higher beam energies. The analytic solutions, known as the Bennett pinch, are well matched at corresponding settings of the parameters.

When charged particles pass through a metal pipe, they are accompanied by an image current on the metal surface. With intense short bunches passing near or even into the metal surface, the peak image current density can be very high. This current may result in substantial temperature rise on the surface, especially in high peak current, multibunch operation. In this paper, we derive an explicit formula for the surface temperature rise due to this previously unrecognized pulsed heating effect and show that this effect dominates the proposed linear coherent light source collimator spoiler and wire scanner heating. Without proper account, it can result in component and instrument failures. The result also applies to optical transition radiation screens, profile screens, wire scanners, exit windows, and targets, which the beam crosses.

Bunch length compression of a single proton bunch is studied by manipulations of the rf voltage and phase. Analytical expressions for the compression ratio, defined as the ratio of the initial to the final bunch lengths, are derived for a linear model. Results obtained from numerical simulations are compared with experimental results. The ultimate maximum compression ratios are found to be $\sqrt{2}/\sqrt{3}{\sigma }_{\phi ,i}$ for the phase shift method and $\sqrt{8}/\sqrt{3\pi }{\sigma }_{\phi ,i}$ for the voltage manipulation method, where ${\sigma }_{\phi ,i}$ is the rms rf-phase spread of the initial beam bunch at the maximum rf voltage. Effects of space charge force are also studied.

$7.6\times {10}^6$ x-ray photons per 3.5 ps pulse are detected within a 1.8–2.3 Å spectral window during a proof-of-principle laser synchrotron source experiment. A 600 MW ${\mathrm{CO}}_2$ laser interacted in a head-on collision with a 60 MeV, 140 A, 3.5 ps electron beam. Both beams were focused to a $\sigma =32\mathrm{}\mu \mathrm{m}$ spot. Our next plan is to demonstrate ${10}^{10}$ x-ray photons per pulse using a ${\mathrm{CO}}_2$ laser of $\sim 1\mathrm{TW}$ peak power.

In conventional low energy electron coolers, the electron beam is immersed in a continuous solenoid, which provides a calm and tightly focused beam in a cooling section. While suitable for low energies, the continuity of the accompanying magnetic field is hardly realizable at relativistic energies. We consider the possibility of using an extended solenoid in the gun and the cooling section only, applying lumped focusing for the rest of the electron transport line.

The conceptional design of the proposed linear electron-positron collider TESLA is based on 9-cell 1.3 GHz superconducting niobium cavities with an accelerating gradient of $E_{\mathrm{acc}}\ge 25\mathrm{MV}/\mathrm{m}$ at a quality factor $Q_0\ge 5\times {10}^9$. The design goal for the cavities of the TESLA Test Facility (TTF) linac was set to the more moderate value of $E_{\mathrm{acc}}\ge 15\mathrm{MV}/\mathrm{m}$. In a first series of 27 industrially produced TTF cavities the average gradient at $Q_0=5\mathrm{}\times {10}^9$ was measured to be $20.1\pm 6.2\mathrm{MV}/\mathrm{m}$, excluding a few cavities suffering from serious fabrication or material defects. In the second production of 24 TTF cavities, additional quality control measures were introduced, in particular, an eddy-current scan to eliminate niobium sheets with foreign material inclusions and stringent prescriptions for carrying out the electron-beam welds. The average gradient of these cavities at $Q_0=5\mathrm{}\times {10}^9$ amounts to $25.0\pm 3.2\mathrm{MV}/\mathrm{m}$ with the exception of one cavity suffering from a weld defect. Hence only a moderate improvement in production and preparation techniques will be needed to meet the ambitious TESLA goal with an adequate safety margin. In this paper we present a detailed description of the design, fabrication, and preparation of the TESLA Test Facility cavities and their associated components and report on cavity performance in test cryostats and with electron beam in the TTF linac. The ongoing research and development towards higher gradients is briefly addressed.

We present an experimental and theoretical study of the effect of desynchronism modulation on short pulse free-electron laser (FEL) oscillators. We find that the output power and the micropulse length of the FEL beam oscillate periodically at the modulation frequency and that the minimum micropulse length during the cycle can be significantly shorter than that which can be obtained without modulation. For example, when the desynchronism of our FEL is modulated at 40 kHz, the minimum measured micropulse length is 300 fs. Without modulation the minimum is about 700 fs. We show that when the desynchronism is modulated, the FEL can operate for part of the cycle in the normally inaccessible portion of the output power curve where the FEL gain is less than the cavity losses. It is even possible for the FEL to operate periodically in the region of negative desynchronism where gain, as normally defined, does not exist.

The transverse dynamics of space-charge dominated beams are investigated both analytically and computationally, in order to understand the mechanisms for emittance oscillations and growth due to nonlinear space-charge fields. This work explores the role of space-charge dominated equilibrium and its relationship to phase space wave breaking, which is responsible for the irreversible emittance growth in these systems. The physics of both coasting and accelerating beams are examined in order to illuminate the most effective approaches to beam handling during the emittance compensation process as well as during subsequent beam transport. These results are discussed within the context of recent ultrahigh brightness rf photoinjector designs.

Non-Gaussian beam tails producing low beam lifetimes and background to the experimental detectors can be a serious performance limitation in colliding beam facilities. We describe simulations and measurements of non-Gaussian beam tails performed on the $e^{+}{e}^{-}$ collider LEP that revealed the importance of inelastic particle scattering as launching processes of particles to large amplitudes.

The conventional method of spin flipping a stored polarized beam is based on slowly crossing an rf induced depolarizing resonance. This paper discusses a different approach where the polarization reversal is achieved by trapping the beam polarization into a stable spin-flipping motion on top of rf induced resonance at a half-revolution frequency.

Measurement of the betatron phase in the vertical and horizontal planes as well as the transverse horizontal-vertical coupling is a standard procedure now used at the Cornell Electron/Positron Storage Ring. The measurement is made by shaking the beam and observing the phase of oscillation at detectors located around the ring. The measurements allow quadrupolar errors to be corrected.

This paper summarizes the low-loss design for the Spallation Neutron Source accumulator ring [“Spallation Neutron Source Design Manual” (unpublished)]. A hybrid lattice consisting of FODO arcs and doublet straights provides optimum matching and flexibility for injection and collimation. For this lattice, optimization focuses on six design goals: a space-charge tune shift low enough (below 0.15) to avoid strong resonances, adequate transverse and momentum acceptance for efficient beam collimation, injection optimized for desired target beam shape and minimal halo development, compensation of magnet field errors, control of impedance and instability, and prevention against accidental system malfunction. With an expected collimation efficiency of more than 90%, the uncontrolled fractional beam loss is expected to be at the ${10}^{-4\mathrm{}}$ level.

It is shown that energy conservation in the longitudinal envelope equation can be used to derive analytic expressions to model fast bunch compression and the effect of space charge in terms of a dimensionless Coulomb parameter $\Sigma$ $(\propto 1/\eta$, with $\eta$ the slip factor). For small $|\eta |$ (below transition, hence $\Sigma \gg 1$), the rf voltage required is nearly independent of $\eta$ and dominated by space charge repulsion. The extra voltage generates the coherent momentum spread $\delta \propto 1/\sqrt{|\eta |}$ required to compensate the increasing space charge force gradient during compression. This sets a clear limit to the useful approach to transition. An $\eta$ jump scheme is discussed to minimize this effect. Particle-in-cell computer simulation confirms the validity of our results also for more realistic beam distributions. Noticeable tails in momentum space due to the nonlinear space charge force are found for Gaussian line density bunches and $\Sigma \gg 1$.

Collective processes in intense charged particle beams described self-consistently by the Vlasov-Maxwell equations are studied using a 3D multispecies nonlinear perturbative particle simulation method. The newly developed beam equilibrium, stability, and transport (BEST) code is used to simulate the nonlinear stability properties of intense beam propagation, surface eigenmodes in a high-intensity beam, and the electron-proton ($e-p$) two-stream instability observed in the Proton Storage Ring (PSR) experiment. Detailed simulations in a parameter regime characteristic of the PSR experiment show that the dipole-mode two-stream instability is stabilized by a modest spread (about 0.1%) in axial momentum of the beam particles.

The use of ${\mathrm{TE}}_{12}$ in circular waveguide with smooth walls was suggested for low-loss transport of rf signals in multimoded systems [S. G. Tantawi et al., in Advanced Accelerator Concepts: Eighth Workshop, edited by Wes Lawson, AIP Conf. Proc. No. 472 (AIP, New York, 1999), pp. 967–974]. Such systems use the same waveguide to transport different signals over different modes. In this report we detail a series of experiments designed to measure the characteristics of this mode. We also describe the different techniques used to generate it and receive it. The experiments were done at $X$ band around a frequency of 11.424 GHz, the frequency of choice for future linear colliders at $X$ band [The NLC Design Group, Report No. LBNL-PUB-5424, SLAC Report No. 474, Report No. UCRL-ID 124161, 1996; The JLC Design Group, KEK-REPORT-97-1, 1997]. The transportation medium is 55 m of highly overmoded circular waveguide. The design of the joining flanges is also presented.

The dynamics of particles in laser pulse-driven wakefields over multistages in a collider is studied. A map of phase space dynamics over a stage of wakefield acceleration induced by a laser pulse (or electron beam) is derived. The entire system of a collider is generated with a product of multiple maps of wakefields, drifts, magnets, etc. This systems map may include offsets of various elements of the accelerator, representing noise and errors arising from the operation of such a complex device. We find that an unmitigated strong focusing of the wakefield coupled with the alignment errors of the position (or laser beam aiming) of each wakefield stage and the unavoidable dispersion in individual particle betatron frequencies leads to a phase space mixing and causes a transverse emittance degradation. The rate of the emittance increase is proportional to the number of stages, the energy of the particles, the betatron frequency, the square of the misalignment amplitude, and the square of the betatron phase shift over a single stage. The accelerator with a weakened focus in a channel can, therefore, largely suppress the emittance degradation due to errors.

We have used 2D cylindrically symmetric particle-in-cell simulations to investigate the dynamics of a high energy electron beam propagating through an underdense plasma. The simulation parameters are relevant to a recent plasma wakefield experiment conducted at the Stanford Linear Accelerator Center [R. Assmann et al., Stanford Linear Accelerator Center Proposal, 1997]. We model the dynamic development of the beam and wakefield excitation over meters of propagation length. To most clearly illustrate the dynamics of both the beam and the wakefield, a video of the simulation data is presented. The main observation is that the beam dynamics, i.e., its betatron motion in the resulting ion channel, agree well with the theoretical predictions while the plasma wake remains almost invariant over the entire propagation distance. The video illustrates subtle details regarding the interplay between the beam dynamics and wakefield generation. The results presented here complement results published separately [S. Lee et al., Phys. Rev. E 61, 7014 (2000)].

By modulating the rf voltage at near twice the synchrotron frequency, the longitudinal bunch shape can be modulated. This method can be used to shorten bunches. We show experimentally that the bunch shape can be modulated while preserving the longitudinal emittance when the rf voltage modulation is turned on adiabatically. Experimental measurements will be presented along with theoretical predictions.

In this paper we present a study of beam halo based on a three-dimensional particle-core model of an ellipsoidal bunched beam in a constant focusing channel including the effects of nonlinear rf focusing. For an initially mismatched beam, three linear envelope modes—a high frequency mode, a low frequency mode, and a quadrupole mode—are identified for an azimuthally symmetric bunched beam. The high frequency mode has three components all in phase; the low frequency mode has the transverse components in phase and the longitudinal component 180° out of phase; the quadrupole mode has no longitudinal component, and the two transverse components in the mode are 180° out of phase. We also study the case of an ellipsoidal bunched beam without azimuthal symmetry and find that the high frequency mode and the low frequency mode are still present but the quadrupole mode is replaced by a new mode with transverse components 180° out of phase and a nonzero longitudinal component. Previous studies, which generally addressed the situation where the longitudinal-to-transverse focusing strength is roughly 0.6 or less, conclude that the oscillation of the high frequency mode is predominantly transverse, and that of the low frequency mode is predominantly longitudinal. In this paper we present a systematic study of the features of the modes as a function of the longitudinal-to-transverse focusing strength ratio. We find that, when the ratio is greater than unity, the high frequency mode may contain a significant longitudinal component. Thus, excitation of the high frequency mode in this situation can be responsible for the formation of longitudinal beam halo. Furthermore, while previous studies have observed halo amplitudes roughly 2–3 times the matched beam edge, for the present parameters we observe much larger amplitudes (5 times or more). This is due to the fact that the longitudinal-to-transverse focusing ratio used here is greater than that of previous studies. The finding of large transverse halo amplitude can have significant impact on the design of high-intensity ion accelerators where the longitudinal-to-transverse focusing ratio is slightly greater than unity in some parts of the linac.