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.
Direct-drive fusion implosion experiments using the 60-beam OMEGA Laser System require ~1% rms uniformity on target. Measurements from laser diagnostics indicate that beam-to-beam power variation has been sufficiently reduced such that the uniformity nearly meets this requirement; however, experimental results suggest otherwise. To better understand this discrepancy, a full-beam-in-tank diagnostic has been developed to characterize the on-shot, full-energy focal spot of a single beam inside the target chamber using a small sample of the beam from the final optic assembly. In this paper, we describe the diagnostic and present the results of commissioning experiments.
The newly developed full-beam-in-tank (FBIT) diagnostic has the capability to characterize multiple beamlines in the target chamber. In addition to measuring multiple beams, we can obtain measurements of the step-by-step changes to achieve smoothing by spectral dispersion (SSD), the SSD kernel, and SSD synchronization. Since other existing diagnostics are all located upstream of the target chamber, this diagnostic can be used to explore a propagating beam through the final optics assembly. In this work, we investigate current discrepancies between laser diagnostics and experimental results by comparing results of on-shot direct measurements using FBIT and the equivalent-target-plane diagnostic.
We present a simple method to measure the spatial coherence of a partially coherent field by analyzing far-field measurements with and without a well-characterized obscuration. From these measurements, the coherence can be estimated for all pairs of points whose centroid is the obstacle's centroid. By scanning the obstacle over the test plane, one can recover the four-dimensional coherence function. In principle, such measurements can be performed without any refractive or diffractive elements, allowing them to be done in higher frequency regimes.