Effects of Laser Bandwidth in Direct-Drive High-Performance DT-Layered Implosions on the OMEGA Laser.

In direct-drive inertial confinement fusion, the laser bandwidth reduces the laser imprinting seed of hydrodynamic instabilities. The impact of varying bandwidth on the performance of direct-drive DT-layered implosions was studied in targets with different hydrodynamic stability properties. The stability was controlled by changing the shell adiabat from (α_{F}≃5) (more stable) to (α_{F}≃3.5) (less stable). These experiments show that the performance of lower adiabat implosions improves considerably as the bandwidth is raised indicating that further bandwidth increases, beyond the current capabilities of OMEGA, would be greatly beneficial. These results suggest that the future generation of ultra-broadband lasers could enable achieving high convergence and possibly high gains in direct drive ICF.

Impact of the boundary's sharpness on temporal reflection in dispersive media.

We investigate the impact of the finite rise time of a spatiotemporal boundary inside a dispersive medium used for reflection and refraction of optical pulses. We develop a matrix approach in the frequency domain for analyzing such spatiotemporal boundaries and use it to show that the frequency range over which reflection can occur is reduced as the rise time increases. We also show that total internal reflection can occur even for boundaries with long rise times. This feature suggests that spatiotemporal waveguides can be realized through cross-phase modulation even when pump pulses have relatively long rise and fall times.

Inferred UV fluence focal-spot profiles from soft x-ray pinhole-camera measurements on OMEGA.

A method was developed with laser-irradiated Au planar foils to characterize the focal spot of UV laser beams on a target at full energy from soft x-ray emission. A pinhole camera with a back-thinned charge-coupled device detector and filtration with thin Be and Al foil filters provides images of the x-ray emission at photon energies <2 keV. This method requires a careful measurement of the relationship between the applied UV fluence and the x-ray signal, which can be described by a power-law dependence. The measured exponent γ ∼ 2 provides a dynamic range of ∼25 for the inferred UV fluence. UV fluence profiles of selected beams were measured for 100-ps and 1-ns laser pulses and were compared to directly measured profiles from an UV equivalent-target-plane diagnostic. The inferred spot size and super-Gaussian order from the x-ray technique agree within several percent with the values measured with the direct UV measurements.

Power balancing the multibeam OMEGA laser.

Multibeam lasers often require an output beam balance that specifies the degree of simultaneity of the laser output energy, instantaneous power, or instantaneous irradiance (power per unit area). This work describes the general problem of balancing a multibeam laser. Specific techniques used to balance the output power of the 60-beam pulsed OMEGA Laser System are discussed along with a measured reduction of beam-to-beam imbalance. In particular, the square-pulse distortion induced by a simple saturating amplifier operating with its output at some fraction of its saturation fluence is derived, and a method to exchange gain between saturated amplifiers in a single beam that have different saturation fluences to adjust balance is described.

A picosecond beam-timing system for the OMEGA laser.

A timing system is demonstrated for the OMEGA Laser System that guarantees all 60 beams will arrive on target simultaneously with a root mean square variability of 4 ps. The system relies on placing a scattering sphere at the target position to couple the ultraviolet light from each beam into a single photodetector.

What is the Temporal Analog of Reflection and Refraction of Optical Beams?

It is shown numerically and analytically that when an optical pulse approaches a moving temporal boundary across which the refractive index changes, it undergoes a temporal equivalent of reflection and refraction of optical beams at a spatial boundary. The main difference is that the role of angles is played by changes in the frequency. The frequency dependence of the dispersion of the material in which the pulse is propagating plays a fundamental role in determining the frequency shifts experienced by the reflected and refracted pulses. Our analytic expressions for these frequency shifts allow us to find the condition under which an analog of total internal reflection may occur at the temporal boundary.

A single-shot, multiwavelength electro-optic data-acquisition system for inertial confinement fusion applications (invited).

Electro-optic data-acquisition systems encode the output from voltage-history diagnostics onto optical signals. The optical signals can propagate long distances over fiber-optic links without degrading the bandwidth of the encoded signal while protecting the recording electronics from overvoltage damage. The sinusoidal response and tolerance to high-input voltages of the Mach-Zehnder modulator used for the encoding leads to the additional advantage of a high dynamic range and a reduced need for manually swapping attenuators. We have demonstrated a single-shot, electro-optic data-acquisition system with a 600:1 dynamic range. This system provides optical isolation and a bandwidth of 6 GHz. The prototype system uses multiple optical wavelengths to allow for the multiplexing of up to eight signals onto one photodetector.

The use of a generic, patient-based health assessment (SF-36) for evaluation of patients with anterior cruciate ligament injuries.

The MOS 36-item short-form health survey is a generic, patient-based health assessment tool. It has been used to assess functional outcome for many medical conditions, both acute and chronic. The use of this survey in evaluating the effects of treatment of any specific disease or injury allows comparison of treatments across a broad spectrum of disease categories. The purpose of this study was to see if this assessment tool could 1) be used to identify those patients requiring anterior cruciate ligament reconstruction, 2) detect changes in the patients with treatment over time, and 3) correlate with the commonly used knee assessment scales. The short-form health survey could not identify those patients requiring anterior cruciate ligament reconstructive surgery. However, it did show important and significant changes with treatment (surgical and nonsurgical) over time. There was a significant correlation between the short-form health survey and the Lysholm and International Knee Documentation Committee scores during this study. The addition of the MOS 36-item short-form health survey to our traditional knee ligament evaluation tools is encouraged. Its use will permit the orthopaedic community to demonstrate the value of our treatment of anterior cruciate ligament injuries to health care planners and generalist physicians.

Use of the suture anchor in open Bankart reconstruction. A follow-up report.

We previously reported early results of a new technique using a suture anchor to perform a modified Bankart reconstruction. That study included patients from two medical centers and had an average followup of only 1 year. This report includes patients from a single center with followup extended to a mean of 42 months (range, 33 to 61). Between April 1988 and August 1991, 53 patients with recurrent anterior glenohumeral instability underwent modified Bankart reconstruction with the use of a suture anchor. Thirty-two patients met inclusion criteria (identifiable Bankart lesion, open repair with suture anchors, and minimum followup of 2 years); 4 patients were lost to followup. There have been no complications as a result of this technique. Ninety-three percent of the patients in the study had objectively excellent or good results. There were 2 failures with recurrent anterior dislocation. The use of a suture anchor can simplify the Bankart reconstruction. At average followup of 3 years, 26 patients have returned to presurgery activity levels without recurrent dislocation or subluxation. However, careful attention to anchor placement at the junction of articular cartilage and the glenoid neck is necessary to avoid technical failure.

Modification of the Bankart reconstruction with a suture anchor. Report of a new technique.

We assessed the effectiveness of a new suture anchor that has been designed to anchor sutures into a blind, straight hole drilled in bone. The strength of fixation in glenoid bone is 67 N for the No. 0 anchor and suture, and 82 N for the No. 2 device with suture. During 1988 and 1989, 32 patients underwent a modified Bankart reconstruction for recurrent anterior glenohumeral instability at two centers as part of a prospective study of this modified technique. There were no complications as a result of the technique. The four surgeons involved agreed that the suture anchor simplified the procedure. Seventeen patients have been reviewed, with more than 1 year followup. Ninety-four percent had good to excellent results according to the Bankart rating scale. There was one recurrent dislocation in a football player.

Chronic fracture-subluxation of the proximal interphalangeal joint.

Reported methods of treatment of chronic fracture-subluxations of the proximal interphalangeal joint require wide dissection and prolonged immobilization and often result in limited motion. Open reduction with minimal dissection, immobilization for 11 days to allow soft-tissue healing, and the early institution of active flexion exercises protected by an extension block splint was done in four patients. The long-term results demonstrate restoration of a stable joint with satisfactory motion and minimal subjective complaints.