The Behavior of Thin Dielectrics Under Electron Irradiation
Abstract
An experimental and analytical investigation has been made of the anomalous behavior of an electron beam in the presence of dielectrics. The configuration considered is an internal cavity, with electron beam anode on one end and a Faraday cup on the other. Experiments were performed for an empty cavity (vacuum case) and also with a thin dielectric sheet immediately adjacent to the cylinder wall. Variations include the type of dielectric and the use of a screen at the vacuum-dielectric interface. The experimental evidence is that, while transmitted current is very low for the empty cavity, the presence of dielectric causes considerable enhancement in transmission. This experiment was modelled analytically using MAD2, a 2-D SGEMP code, to determine the cause of this behavior. Low transmission for the vacuum case is shown due initially to beam spreading (coulomb repulsion) and at later stages to extreme space-charge limiting. It is also demonstrated that enhancement is not due to the electromagnetic effect of dielectric charging. Rather, creation of a plasma at the dielectric surface is hypothesized as the probable effect, with plasma electrons providing breakdown of the space-charge barrier, and positive ions providing charge neutralization and pinching of the beam. Detailed calculations using a simple plasma model give good agreement with experimental data. However, no physical mechanism for the plasma creation process is suggested.
Document Details
- Document Type
- Technical Report
- Publication Date
- Mar 01, 1980
- Accession Number
- ADA085583
Entities
People
- A. Greenwald
- B. Goplen
- W. A. Seidler
- W. R. Thomas