Suppression Of Parasitics And Pencil Beams In The High-Gain National Ignition Facility Multi-Pass Preamplifier

The multi-pass amplifier (MPA) is the last subsystem of the NIF1 preamplifier, which feeds the main amplification stages of the NIF beamline. The MPA is based on a flashlamp pumped 5-cm diameter by 48 cm long Nd:glass rod amplifier operated at a single pass small signal gain of 15 to 17. The MPA is an off-axis multi-pass image relayed system, which uses two gain isolating image relaying telescopes and passive polarization switching using a Faraday rotator to output the pulse. We describe the MPA system, techniques used to avoid parasitic oscillation at high gain and suppression of pencil beams. The system is used to generate a well-conditioned 22-joule output from one millijoule input. The output pulse requirements include 22 joules in a square, flat topped beam, and with near field spatial contrast of <5% RMS, square pulse temporal distortion <2.3, and an RMS energy stability of <3%. All of these requirements have been exceeded.The largest impediment to successful operation was overcoming parasitic oscillation. Sources of oscillation could be generally divided into two categories: those due to birefringence, which compromised the polarization contrast of the system; and those due to unwanted reflections fi om optical surfaces. Baffling in the vacuum spatial filters (VSF) helps to control the system sensitivity to unwanted stray reflections from fiat AR coated surfaces. Stress birefringence in the rather large glass volume of the rod (942 cm(3)) and the four vacuum loaded lenses are significant, as each of these elements is double passed between each polarizing beam splitter pass. This lowers the polarization contrast of the system, which can prevent the system from operating at sufficient gain. Careful analysis and layout of the MPA architecture has allowed us to address the challenges posed by a system small signal gain of approximate to 33000 and with an output pulse of as high as 27 joules.