Design of e-gun for large KrF amplifiers

The design of very large single-aperture laser amplifier for an angular multiplexed laser fusion system requires advances in excimer laser e-gun technology beyond existing designs. Scaling considerations dictate the use of multiple e-guns to pump a single laser; in the present case the authors will discuss the scaling and design features of one of the ten e-guns being developed to pump the Los Alamos Polaris Power Amplifier Module. Multiple e-guns minimize the diode self-magnetic field, lowering the size of the imposed guide magnetic field, and reducing the diode impendance collapse. Multiple guns also result in lowered current rise times, reduce the development cost of the technology at the prototype stage, and, of course, limit the cost due to operation failures in the e-gun. The present design utilizes the expanding electron flow diode to provide uniform electron flow into the gas from a high-current density cold cathode (approx. =50 A/cm2). Laminated iron and an imposed dipole field are utilized for B-field shaping. The applied B field lines trace from the anode, terminate on the cathode, and are then conducted through the shank to beyond the bushing. This feature not only provides for fully expanded electron flow from cathode to anode, but it also allows for self-magnetic field insulation of the shank and bushing, thus minimizing voltage standoff distances, inductance, and rise time. A single large aspect ratio racetrack-shaped bushing on each e-gun is provided with robust grading to limit field concentration at the ends