Revisiting The Propagation And Focussing Of A High Intensity Electron Beam In A Low-Pressure Gas Cell

Intense relativistic electron beam propagation has been subject to extensive studies in many laboratories throughout the world, in particular during the 70's and 80's. CESAR is a 700 kV, $2 \Omega$ generator facility at CESTA which is used to study fast hydrodynamic response of materials. Typically, this device delivers a 350 kA, 70 ns FWHM electron beam at 700 keV from a graphite cathode. The electron beam exits the diode through a thin aluminized mylar anode and it is transported and focussed in a gas-cell filled with dry air. The pressure in the cell varies from 0.1 to 10 mbar, and an external focusing coil produces a magnetic field up to 3 T. In addition to the regular electrical diagnostics, such as B-dot probes and Faraday cups for current measurements along with capacitive dividers for diode voltage estimations, we have installed an optical diagnostic line in order to analyze the beam behavior along the propagation axis. This diagnostic consists of a fused silica target which intercepts the beam at some given location inside the gas cell. When the beam impinges on the silica target, visible Cerenkov light is produced and some part of this light can be collected both by a gated camera and a streak camera. In this paper, we report on the progress of this experiment where the issues of beam homogeneity, both in time and space, are addressed.