Large-aperture diagnostic system for gain and wavefront measurements on NIF/LMJ amplifiers

Weare in the midst of constructing an amplifier laboratory (Amplab) that will be the physics and engineering proving ground for full sized segmented glass amplifiers ofdesigns that will outfit the National Ignition Facility (NIF) and Laser Megajoule (LMJ) projects. Amplab will demonstrate the cornerstone mechanical, electrical and optical concepts that support the NTF and LMJ amplifier schemes. Here we address the optical diagnostics that will be used to characterize optical performance ofthe amplifiers. We describe, the apparatus that will be used in pulsed measurements ofgain distribution and wave-front distortions. The large aperture diagnostic system or LADS, is now being built through a collaborative effort between CEL-V and LLNL. The LADS will provide measurements ofgain and wave front distortions over the full extracting aperture ofthe NIF and LMJ prototype amplifiers. The LADS will be able to address each of eight apertures via motorized stages and following semi-automated alignment, take data on the aperture ofinterest. The LADS should be operational in mid-'97 at LLNL and will be used to characterize the optical performance ofthe veiy first full scale prototype 4 x 2 NIF and LMJ amplifiers. It will be transported to Bordeaux, France to make similar measurements during activation ofthe first 8-aperture LMJ-like facility (LIL) that is planned to start in the near future. The gain measurement will map the gain distribution ofeach ofeight 40 by 40 cm apertures. Small signal gain of 5 %-per-cm is the nominal operating point (lamps fired at 20% of their explosion energy). It is desired to measure the small signal gain ofthe amplifier with a resolution ofO. 1% (0.005 %per-cm) at the center ofthe aperture, 0.5% at the corners, or better. The amplifier pump distribution is tailored in order to counteract the effects of amplified spontaneous emission that tends to deplete gain in areas near the edges. This diagnostic will be useful in amplifier optimization experiments. Subtle effects of shaping reflector surfaces, the tarnishing of silver or the damage to reflector coatings could be found correlated to the gain data and thus, will be readily monitored. To be useful, the wavefront measurement must resolve features that calculations and Beamlet data show will have fourth order components with peak to valley excursions of about one twentieth wave per amplifier pass (A 1 .053 tm). Even smaller effects are expected from gas stratification/motion inside beam tubes. To chart these subtle wavefront distortions with fidelity it will be necessary to resolve the measured wavefront to A /100. The wavefront and gain measurements will be performed simultaneously. A TwymanGreen interferometer set up will present reference and sample beams to a pulsed, phase shifting interferometer for wavefront analysis. A wavefront map ofthe quiescent state will be acquired 66 milliseconds before the flashlamps fire. In the quiescent state the wavefront data contains the static distortions and will be used as the reference wavefront from which the dynamic distortions will be differentially obtained. Both prompt and delayed maps of wavefront distortions accompanying a shot will be stored. The prompt effects of the firing of the lamps are important at extraction time. Sampling delayed interferograms at say 1 to 5minuteintervals, will be used to monitor the evolution ofwaste heat and to monitor the effects ofcooling flashlamps, edge claddings, convection in the different sections ofthe beam transport. .. etc. The main components for the LADS are the optical relay telescopes, the probe laser, the alignment system, the gain diagnostics cameras and the pulsed, phase shifting interferometer.