RADIUS MEASUREMENTS FROM TIME-RESOLVED IMAGES OF A RELATIVISTIC ELECTRONBEAM

Abstract

Abstract : Images of the cross-section of an intense relativistic electron beam are obtained using a fast framing camera called a Gated Optical Imager (GOI). Cherenkov radiation generated by the beam passing through a thin quartz plate is viewed by the GOI, which provides 2-ns resolution of the beam current density profile at three times during the beam pulse on each shot. These images have been characterized quantitatively to provide measurements of the electron beam radius and position, without assuming a particular form of the profile. Using a direct, pixel-by-pixel approach rather than an iterative technique to evaluate the image, the beam centroid and various measures of the radius are found, including for example finding all the contours and choosing one, such as the one that contains half the beam current. The algorithms that have been developed will be described, as well as the physical significance of the results, in terms of both a general profile and the electron beam in the experiment. These techniques may be applicable to quantitative measurements of the profile of any beam, such as a laser beam or an ion beam, and to the analysis of any two-dimensional image, such as the cross-section of a plasma. These algorithms have been implemented for an experiment that studies beam propagation through the atmosphere. Intense relativistic electron beams propagating through gas are subject to the resistive hose instability. The GOI is being used to study various beam conditioning techniques to reduce the effect of the hose instability and extend the propagation distance of the beam. The 5-MeV, 25-kA, 40-ns beam from SuperIBEX is injected into a 1-atm gas cell. Preliminary experimental results will be presented.