Recent results of the National Ignition Facility Beamlet demonstration project

B.M. Van Wonterghem, J.A. Caird, C.E. Barker,J.H. Campbell, J.R. Murray, D.R. Speck.University of CaliforniaLawrence Livermore National LaboratoryP0 Box 808, L-490Livermore, CA 94551-9900(510) 423-9494 1 FAX (510) 422-7748ABSTRACTThe activation of a full scale single beam prototype of a multipass amplifier cavity based fusion class laser has beencompleted. A 35 x 35 cm2 beam is amplified during four passes through an 1 1 slab long amplifier in a cavity, and isswitched out using a full aperture Pockels cell and polarizer. Further amplification is achieved in a five slab long boosteramplifier, before being frequency tripled by a Type I! Type II frequency converter. We present initial performance resultsof this laser system, called Beamlet. At ico, energies up to 17.3 kJ have been generated in a a 10 ns pulse, and frequencytripled beams up to 8.3 U in a 3 ns pulse.Keywords: inertial confinement fusion, neodymium glass lasers1. INTRODUCTIONThe United States Department of Energy has proposed building a National Ignition Facility for Inertial Confmement Fu-sion. The laser driver ofthis system wiliproduce 1.8 MJ in a 3.5 ns long shaped frequency tripledpulse to ignite Hohlraumtargets and study propagation of thermonuclear burn. The conceptual design of this facility is discussed elsewhere in theseproceedings'. The LMJ (Laser Megajoules) is a similarly sized system, being developed by the CEA (France) and also usesmultipass amplifiers, although using a different switch geometry.The architecture of this proposed neodymium glass laser driver is based upon an optical cavity where the beam makesmultiple passes through multi-segment amplifiers. This allows the construction of compact and cost effective laser bundlearrays, since only one beam size is propagated. Key components for this design are the multisegment amplifier, and a fullaperture Pockels cell and polarizer to switch the beam out of the cavity. A small pointing offset of the beams in the cavityseparates the foci of individual passes in the spatial filter that is used to image the injection beam onto the cavity mirrors.This separation allows the insertion of a small mirror near the focal plane to inject the preamplifier pulse in the cavity.The Beamlet project integrated full scale 2x2 multisegment amplifiers and a 36 x 36 cm2 clear aperture Pockels cell in afunctional laser system. Its activation started in 1992 and was completed in September of 1994. In addition to the maincavity, Beamlet also demonstrated new optical pulse generation technology using waveguide intensity modulators2,fiberoptic pulse transport to high gain multipass preamplifiers, flexible spatial beam shaping and wavefront correction bydeformable mirrors3.Figure 1 shows a lay-out of major components of the system. The beam is generated in the master oscillator room, wherewaveguide modulators apply temporal shaping to the pulse. This 200 pJ pulse is transported though a 60 m long fiber to ahigh gain ring regenerative amplifier, that brings the energy up to the nil level. At this point, the round beam is transformedinto a square beam with smooth edges to minimize edge diffraction ripples during propagation. The pulse is furtheramplified by a four pass rod amplifier up to the Joule level before being injected in near the focal plane of the cavity spatialfilter. The beam is amplified by four passes through an 1 1 slab long amplifier assembly in the cavity, and by a single passthrough a five slab booster section. A second spatial filter, identical to the cavity spatial filter, images the beam onto a