Los Alamos National Laboratory has assembled an array of experimental and theoretical tools to optimize amplifier design for future single-pulse KrF lasers. The next opportunity to exercise these tools is with the design of the second-generation NIKE system under construction at the Naval Research Laboratory with the collaboration of Los Alamos National Laboratory. Major issues include laser physics (energy extraction in large modules with amplified spontaneous emission) and diode performance and efficiency. Low cost is increasingly important for larger future KrF single-pulse systems (low cost and high efficiency is important for larger repetitively pulsed applications such as electric power production). In this article, we present our approach to amplifier scaling and discuss the more important design considerations for large single-pulse KrF amplifiers. We point out where improvements in the fundamental database for KrF amplifiers could lead to increased confidence in performance predictions for large amplifiers and address the currently unresolved issues of anomalous absorption near line center and the possibility of diode instabilities for lowimpedance designs. Los Alamos has applied these amplifier design tools to the conceptual design of a 100-kJ Laser Target Test Facility and a 3-MJ Laboratory Microfusion Facility.
IntroductionThe electrooptic Kerr effect is the interaction of an applied electric field with a birefringent fluid medium resulting in a change in the refractive index of the fluid.It has found a variety of applications in the measurement of the applied voltage and electric field by analyzing the polarization rotation of light shined through the fluid.Potentials of kilovolts to megavolts, and risetimes as fast as nanoseconds have been measured. A Kerr measurement system has the usual advantages of a photonic sensor: non -perturbing, immunity to EMI, and high bandwidth.An unusual advantage of the Kerr effect is derived from the quadratic dependence of the polarization phase shift on the applied electric field, such that the precision of a highvoltage measurement is typically "0.1 %. The accuracy, limited by the uncertainty of the Kerr constant of the fluid, is about 2 %. This chapter presents the theory of the Kerr effect; a historical overview of its applications; the results of a high-bandwidth, multi -megavolt voltage measurement in the harsh environment of a pulsed power accelerator; a discussion of the accuracy and precision of Kerr voltage measurements; a comparison with conventional voltage monitors; and consideration of further possible developments.The high-bandwidth, non -intrusive, and reliability aspects of Kerr measurements will be emphasized in this chapter, in as much as those advantages depend on the photonic nature of the measurement. A more general and thorough discussion of Kerr measu ment systems can be found in an excellent review article by Hebner et al.( 1). TheoryA simplified schematic of a device to measure voltage or electric field using the Kerr effect is shown in Figure 1. A high voltage, V(t), is applied between two conductors producing a strong electric field, É(t), in a Kerr -active fluid. The randomly oriented dipole moments of the fluid become partially aligned to the applied field producing a difference in the indices of ref çtion of the fluid in directions parallel and perpendicular to E, such that'where X is the wavelength of the light and B is the Kerr coefficient of the fluid. Conducto (2)Applied Voltage VA 111 err Fluid A mug %;:a H rx:. Light BeamPolarizer WWWWWWWWWWWWWWW Analyzer Fig. 1
The AURORA KrF laser at Los Alamos became operational in August 1989. AURORA is the first integrated system for demonstrating the capability of a KrF laser to perform target physics experiments for inertial confinement fusion (ICF) and is currently configured as a 5-kJ, 5-ns, 96-beam device. Both laser physics and ICF target physics experiments have been performed over the last year. Of the four major amplifiers in the AURORA laser system, one performed better than expected, one performed about as expected, and two performed below expectations. The causes of the variability in the amplifier performance are now well enough understood that this information can be used to improve the detailed design of the NIKE laser currently under construction at the Naval Research Laboratory. High-dynamic-range pulse shapes have been propagated with minimal distortion through the AURORA amplifier chain, verifying theoretical predictions. Target physics experiments have been performed with intensities greater than 100 TW/cm 2 , pulse lengths ranging from 2-7 ns, and sp<">t-size diameters from 500-1100 /xm. The analysis of this first-generation kjclass KrF laser target physics facility identified the strengths and weaknesses of KrF lasers for ICF applications. Detailed measurements of amplifier performance led to a better understanding of issues for KrF laser-fusion systems, and design studies for future KrF lasers for ICF applications incorporate improvements based in part on AURORA experience.
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