Currently proposed energy recovery linac and high average power free electron laser projects require electron beam sources that can generate up to ∼ 1 nC bunch charges with less than 1 mm mrad normalized emittance at high repetition rates (greater than ∼ 1 MHz). Proposed sources are based around either high voltage DC or microwave RF guns, each with its particular set of technological limits and system complications. We propose an approach for a gun fully based on mature RF and mechanical technology that greatly diminishes many of such complications. The concepts for such a source as well as the present RF and mechanical design are described. Simulations that demonstrate the beam quality preservation and transport capability of an injector scheme based on such a gun are also presented.
Clouds of low energy electrons in the vacuum beam pipes of accelerators of positively charged particle beams present a serious limitation for operation at high currents. Furthermore, it is difficult to probe their density over substantial lengths of the beam pipe. We have developed a novel technique to directly measure the electron cloud density via the phase shift induced in a TE wave transmitted over a section of the accelerator and used it to measure the average electron cloud density over a 50 m section in the positron ring of the PEP-II collider at the Stanford Linear Accelerator Center. Low energy background electrons in the beam pipes of high energy accelerators of positively charged beams present a serious challenge to increasing current in these machines. Under the right machine conditions, such as bunch repetition rate, peak current, etc., amplification of the electrons can occur from secondary emission when the electrons strike the beam pipe walls, creating a growth in vacuum pressure along with a number of adverse effects on the circulating beam including severe two-stream instabilities, transverse beam blowup, and heating of cryogenic vacuum chambers. The net result is that the beam intensity is limited and beam quality reduced [1,2]. This effect is important for several future accelerators Electron cloud effects have been primarily observed in a number of high intensity synchrotrons and storage rings [4 -13]. Experimental studies of the electrons have mainly used local detectors (retarding field analyzers) to measure the time dependence, density, and energy spectrum of the electron cloud in a small region near the detector [14 -16]. However, the electron cloud density (ECD) can vary significantly along the beam pipe depending on local beam pipe geometry and surface conditions. Furthermore, the local measurement only detects those electrons that reach the beam pipe walls and can only infer the ECD with the help of computer simulation. Therefore, it is important to develop means of directly measuring the electron clouds over longer sections of the accelerator. Of course, one method of inferring the ring average ECD is from effects on the high energy beam itself, which usually only appear at relatively high beam intensities, however.In this Letter, we present a novel idea [17] and its successful demonstration for measuring the ECD over a much longer section of a storage ring. This idea is based on measuring the time delay (i.e., phase shift) of a microwave signal propagating in the beam pipe due to the change of the index of refraction caused by the electron cloud. In practice, it would be challenging to measure the absolute phase shift of the signal, which is expected to be at most only a few degrees over a hundred meters, in an accelerator environment. Our idea relies instead on measuring the modulation of the phase shift of the microwave signal through the electron plasma by taking advantage of the modulation of the density of the electron cloud from gaps in the fill pattern of the circulating posi...
A condition for improved dynamic aperture for nonlinear, alternating gradient transport systems is derived using Lie transform perturbation theory. The Lie transform perturbation method is used here to perform averaging over fast oscillations by canonically transforming to slowly oscillating variables. This is first demonstrated for a linear sinusoidal focusing system. This method is then employed to average the dynamics over a lattice period for a nonlinear focusing system, provided by the use of higher order poles such as sextupoles and octupoles along with alternate gradient quadrupoles. Unlike the traditional approach, the higher order focusing is not treated as a perturbation. The Lie transform method is particularly advantageous for such a system where the form of the Hamiltonian is complex. This is because the method exploits the property of canonical invariance of Poisson brackets so that the change of variables is accomplished by just replacing the old ones with the new. The analysis shows the existence of a condition in which the system is azimuthally symmetric in the transformed, slowly oscillating frame. Such a symmetry in the time averaged frame renders the system nearly integrable in the laboratory frame. This condition leads to reduced chaos and improved confinement when compared to a system that is not close to integrability. Numerical calculations of single-particle trajectories and phase space projections of the dynamic aperture performed for a lattice with quadrupoles and sextupoles confirm that this is indeed the case.
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.
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