Norman A. Kurnit

Phase locking two beams by means of seeded Brillouin scattering

We have demonstrated that the phases of the stimulated Brillouin scattering reflections of two separately focused pump beams can be locked to a common phase by a backward seed of pump laser light.

KrF amplifier design issues and application to inertial confinement fusion system design

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.

High-Damage-Threshold Pinhole for Glass Fusion Laser Applications

We are investigating methods to fabricate high-damage- threshold spatial-filter pinholes that might not be susceptible to plasma closure for relatively high energies and long pulses. These are based on the observation that grazing-incidence reflection from glass can withstand in excess of 5kJ/cm2 without plasma formation. The high damage threshold result from both the cos(theta) spreading of the energy across the surface and the reflection of large fraction of the energy from the surface, thereby greatly reducing the field strength within the medium.

Demonstration of an optical mixing technique to drive Kinetic Electrostatic Electron Nonlinear waves in laser produced plasmas

A nitrogen gas Raman cell system has been constructed to shift a 70 J 527 nm laser beam to 600 nm with 20 J of energy. The 600 nm probe and a 200J, 527 nm pump beam were optically mixed in a laser produced (gas jet) plasma. The beating of the two laser beams formed a ponderomotive force that can drive Kinetic Electrostatic Electron Nonlinear (KEEN) waves discovered in Vlasov-Poisson simulations by Afeyan et al [1,2]. KEEN waves were detected in these experiments where traditional plasma theory would declare there to be a spectral gap (ie no linear waves possible). The detection was done using Thomson scattering with probe wavelengths of both 351 nm and 263.5 nm.

Cyclic plasma shearing interferometry for temporal characterization of a laser-produced plasma

A cyclic shearing interferometer has been employed to characterize a laser-produced plasma with 180 ps resolution. Counterpropagation maintains an equal path length for the probe and reference beams, and the shear is provided solely by the plasma, which appears within the circuit after the reference beam has passed the laser focal spot. The background is virtually fringe free because of the overlapping of the reference and probe beams so that analysis is simplified. The plasma, which is formed by a line focus, is seen to expand in a cylindrical manner away from the line focus with an exponential density profile. In addition, the interferometer shows evidence of a bow shock when an interaction beam is introduced into the plasma parallel to the direction of the line focus.

Improved KrF laser design for the Laboratory Microfusion Facility

A conceptual design of the KrF laser-driven Laboratory Microfusion Facility (LMF) has been completed. LASNEX calculations predict an indirect-drive target yield of 400 MJ from the 3-MJ, 480-beam driver system. Nine final amplifiers with individual output energy of 412 kJ are used. The 480 beams are transmitted through helium to reduce losses and are delivered to target through a series of buildings designed for radiation safety. The total cost of the KrF laser-driven LMF is estimated by an independent cost assessment to be

High-power microwave frequency modulated KrF laser

921 million in 1992 dollars.<<ETX>>

New Developments in Optical Phase Conjugation

We report on the development of a 260 mJ 248 nm KrF laser with 98.7% depth of modulation at 1.3 GHz and 77% depth of modulation at 2.6 GHz using a 1.3 GHz microwave source driving a Pockels cell. Subsequent single-pass amplification of the modulated KrF source to the 1/4-joule level is shown to be straight forward with no noticeable degradation of the modulation depth.

Measurement of nonlinear index by a relay-imaged top-hat Z-scan technique

Optical phase conjugation has ceased to be regarded as merely a scientifically interesting laboratory phenomenon. It is now being invoked as a tool to deal with various laser problems that do not yield easily to more conventional solutions. Thus, in addition to the continuing research on the fundamental mechanisms in various methods of phase conjugation, activities at the Los Alamos National Laboratory have also addressed development issues of more practical relevance. For a tutorial introduction to the concepts of optical phase conjugation we direct the reader to references [1] and [2], whereas more details on various subtopics can also be found in [2].