Optical And Electrical Investigations Into Cathode Ignition And Diode Closure

A combination of optical and electrical diagnostics has been fielded on the Electron Beam Test Facility to study ignition and closure in large area electron beam diodes. A four-channel framing camera is used to observe the formation of microplasmas on the surface of the cathode and the subsequent movement of these plasmas toward the anode. Additionally, a perveance model is used to extract information about this plasma from voltage and current profiles. Results from the two diagnostics are compared. Closure velocity measurements are presented showing little dependance on applied magnetic field for both velvet and carbon felt emitters. We also report the first observation of the screening effect in large area cold cathode diodes.