A multi-pass rebuild of the HELEN high power laser at AWE

Summary form only given. Taking advantage of the availability of substantial numbers of Nd:glass disk amplifiers as a result of the closing of the Nova laser at Lawrence Livermore National Laboratory (LLNL) in California, the three main arms of HELEN consist of four 20.8 cm aperture each passed four times. Off-axis propagation is employed to separate the four passes at the pinhole plane of the main transport spatial filter. This allows injection to occur at small aperture close to the pinhole plane, and four passes of the amplifiers an also achieved by diverting the beam at a similar point into a small aperture reverser. The output lenses of the east and west spatial filters expand the beam to a diameter of 29 cm before conversion to the second harmonic at 527 nm wavelength by Type II KH/sub 2/PO/sub 4/ crystals. The backlighter beam is expanded to the smaller size of 18 cm before also being converted to the second harmonic. In this paper we report on the commissioning of the HELEN laser in multi-pass mode, giving details of all aspects of system performance and making comparison with the performance of the previous HELEN configuration.