[en] Space-charge modes similar to those observed in recent experiments appear in simulations of nonequilibrium charged particle beams with anisotropy. The modes couple degrees of freedom, causing energy transfer and equipartitioning without halo formation in just a few betatron wavelengths. The rate depends on a single free parameter quantifying the space-charge intensity of the final state. Traditional stability analyses are shown not to apply to high-intensity laboratory beams originating with a large perturbation from equilibrium

[en] In this paper, we discuss the parametric resonance model of halo creation, and compare it with self consistent simulation results. In particular, we employ two different initial distribution functions, and we find agreement with the particle-core model, within the limitations of the latter. Furthermore, using a simple particle tracking algorithm, we are able to follow the trajectories of the halo particles, noting that a large number of them go through the core and re-emerge later.

[en] Multipactor is a vacuum discharge driven by secondary electron emission. Multiple period multipactors have long been known to exist but have been studied less extensively. In a period-n multipactor, electrons undergo multiple impacts in one rf period, with the synchronous phase alternating periodically between multiple values. A novel resonant form is proposed that combines one- and two-surface impacts within a single period, provided the total transit time is an odd number of rf half-periods and the product of secondary yields exceeds unity. For low fD products, the simplest such mode is shown to significantly increase the upper electric field boundary of the multipacting region and lead to overlap of higher-order bands. The results agree nicely with 3-D particle-in-cell code simulations. An alternative, map-based method is introduced for analyzing higher-periodicity multipactor. Practical implications of the findings are discussed, including consequences for multipactor suppression strategies using a dc magnetic field

[en] Multipactor is a vacuum discharge based on secondary electron emission, and can manifest in many resonant and non-resonant modes. Where two or more types of multipactor coexist in the same device, it is found analytically that the one with the highest yield or the lowest order dominates.

[en] This is a numerical study of the effects of a rotational mismatch of a beam at injection. A closely related type of mismatch is the presence of a skew quadrupole at some place in the transporting channel. In both cases, the initial mismatch implies that the emittance of the beam will increase as the beam is transported through an alternating focusing channel. In our simulations, we used the WARP computer code to simulate the University of Maryland Electron Ring (UMER) beam and we have indeed seen a substantial emittance growth (by a factor of 2-3) for mismatches of 10 degrees

[en] With more research and applications demanding high current, high quality electron beams, understanding of longitudinal dynamics of space-charge dominated beams becomes very important. For example, heavy ion beams for High-Energy Density Physics studies need be longitudinally compressed by orders of magnitude to deliver the needed energy on the target, requiring careful control of energy spread. In high-brightness electron injectors for free electron laser applications, fluctuations in the beam waveform must be minimized so as to minimize the effects of coherent synchrotron radiation. At the University of Maryland, we perform experiments on a scaled set-up with low-energy electrons to assess the impact of fluctuations that we deliberately introduce. In this paper, we report experimental observation of the conversion of density modulations into energy modulations. The initial density modulations are generated in the electron gun by varying the operation conditions. Energy perturbation waveforms are measured with a high-resolution energy analyzer. The experimental results are compared with both the linear theory and particle-in-cell code simulations. We discuss the experimental results and the experimental challenges

[en] A model is described for tomographic phase-space mapping and emittance measurement in beams with space-charge. The tomographic results of an experiment with a 10 keV space-charge dominated electron beam are presented. The data are compared against a direct experimental sampling of the phase-space using a pinhole scan. It is found that the accuracy of the tomographic method is within 10% for beams with space-charge intensity less than 90%. With the aid of numerical simulation the observed similarities and differences between the tomography method and pinhole scan are discussed.

[en] When considered as non-neutral plasmas, space-charge dominated charged particle beams have significantly smaller Debye lengths than their beam sizes. Therefore, collective effects due to space-charge forces are very important to the beam dynamics. As an example, density perturbations generated in a space-charge dominated charge particle beam will stimulate longitudinally space-charge waves. To study the modification of transverse beam distributions by longitudinal beam dynamics, experimental studies of low-energy electron beams, with and without longitudinal density perturbations, have been undertaken at the University of Maryland's electron ring and the Long Solenoid Experiment (LSE). We have taken time-resolved beam images on these machines using two different diagnostics: Optical transition radiation, produced from an intercepting aluminized silicon screen, and a fast (<3 ns decay time) phosphor screen. Results from both techniques show that both the transverse size and transverse particle distribution of a space-charge dominated beam are affected by the longitudinal dynamics of the beam. In addition to the fast imaging measurements, longitudinal mean energy profiles of different beams have also been measured at different locations in the LSE system

[en] Intense charged particle beams are nonneutral plasmas as they can support a host of plasma waves and instabilities. The longitudinal physics, for a long beam, can often be reasonably described by a 1-D cold-fluid model with a geometry factor to account for the transverse effects. The plasma physics of such beams has been extensively studied theoretically and computationally for decades, but until recently, the only experimental measurements were carried out on relatively short linacs. This work reviews experimental studies over the past five years on the University of Maryland Electron Ring, investigating longitudinal phenomena over time scales of thousands of plasma periods, illustrating good agreement with simulations.

[en] Tomography based on quadrupole scans has been successfully applied to reconstruct the phase space of intense charged particle beams. This letter develops a tomographic technique based on solenoid scans, which is advantageous for solenoidal systems and injectors. The technique is generalized to the diagnosis of beams that are not axisymmetric, and validated through simulation. Solenoidal tomography is applied experimentally to a system with a nonequilibrium initial distribution, demonstrating its detailed evolution in phase space