The LSTB Electron Beam Generator

A 10 cm by 200 cm electron beam generator for driving an excimer laser is described. The system will be used as a Laser Sealing Test Bed at Los Alamos National Laboratories. The system consists of two Marx generators operated in parallel, transfer capacitors, a self-closing output switch, cold cathode electron gun, a forty liter laser cell, magnetic guide field and two electrically triggered diverter switches operated in parallel. The system is capable of operating over a wide range of pulse widths, varying from 400 ns to 2 /spl mu/s. Diode current densities greater than 60 A/cm/sup 2/ have been achieved for the 600 ns case. Energy densities in excess of 200 J/I have been obtained in the laser volume. During operation, two Marx generators are discharged in parallel into a set of energy transfer capacitors. The number of capacitors, and hence the equivalent value of the capacitance, is varied depending upon pulse width and load impedance. An SF/sub 6/ insulated self-breaking output switch transfers the energy from the capacitors to the carbon felt cold cathode. The transfer capacitors provide a fast rise time to ensure good cathode tum-on characteristics. The electrons emitted from the cathode traverse a short drift section before encountering the metal foil vacuum/gas interface. An adjustable magnetic field (up to 2 kgauss) is used to prevent the beam from pinching in the drift section. Peak transmission efficiencies of up to 70% were achieved depending upon the pulse and cathode parameters. Testing was done both into argon, to determine the deposition profile of the beam, and into a vacuum. Results of this testing are presented.