Abstract
Characterization of beam energy spread in a space-charge dominated beam is very important to understanding the physics of intense beams. It is believed that coupling between the transverse and longitudinal directions via Coulomb collisions will cause an increase of the beam longitudinal energy spread. At the University of Maryland, experiments have been carried out to study the energy evolution in such intense beams with a high-resolution retarding field energy analyzer. The temporal beam energy profile along the beam pulse has been characterized at the distance of 25 cm from the anode of a gridded thermionic electron gun. The mean energy of the pulsed beams including the head and tail is reported here. The measured rms energy spread is in good agreement with the predictions of the intrabeam scattering theory. As an application of the beam energy measurement, the input impedance between the cathode and the grid due to beam loading can be calculated and the impedance number is found to be a constant in the operation region of the gun.
Highlights
Understanding the physics of the beam energy spread in high-quality intense beams is very important in the applications of advanced particle accelerators for heavyion inertial fusion, high-energy colliders, and freeelectron lasers
In this paper we report an experiment conducted at the University of Maryland Electron Ring (UMER) [7] laboratory to study the energy spread growth in a spacecharge dominated beam
Because of the high resolution of the energy analyzer, we are able to infer the input impedance of the gridded thermionic electron gun based on the beam mean energy measurement
Summary
Understanding the physics of the beam energy spread in high-quality intense beams is very important in the applications of advanced particle accelerators for heavyion inertial fusion, high-energy colliders, and freeelectron lasers. One of the mechanisms causing energy spread growth is the energy transfer from the transverse direction into the longitudinal direction via Coulomb collisions or other collective spacecharge effects This happens in a system with temperature anisotropy, such as a beam accelerated in the longitudinal direction. The first experimental observation of enhanced energy spread growth in a low current electron beam (beam current is on the order of A) was reported by Boersch in 1954 [4] and since this phenomenon has been known as the Boersch effect. This energy spread broadening is believed to be due to Coulomb collisions happening at the beam waist. We introduce a method, as a novel application of the energy analyzer, to measure the beam impedance of the cathode inside the electron gun
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