Edge effect studies of a large area cathode for the Electra KrF laser

Summary form only given. Electra is a repetitively pulsed, electron beam pumped krypton fluoride (KrF) laser that will be used to develop the technology required for an inertial fusion energy (IFE) power plant. The electron beams are generated by large area cathodes (2,500 to 10,000 cm/sup 2/). This-paper presents experimental results on the spatial and temporal current distribution at the anode as well as particle-in-cell (PIC) code simulations of the current density along the edge of a large area electron beam. The current is measured with a Faraday cup array at the anode, and the time integrated current density is determined with radiachromatic film. MAGIC PIC code simulations support the authors´ experimental results. The cathode area of the Electra KrF Laser is 27 cm /spl times/ 97 cm, with an A-K gap of 5 cm, producing a 500 kV, 100 kA electron beam with a FWHM duration of 160 ns (100 ns flat top). Using velvet as a reference cathode, the edge current density is roughly twice the average current density. This investigation is essential to the Electra laser program since a beam edge may cut through the vacuum/laser gas window foil during repetitive operation. Simply blocking of the beam edge with an aperture would decrease the overall efficiency. Experiments and simulations showed that recessing the cathode minimizes the electric field at the edge and eliminates the edge effect.