Role of template selection in aligning beams in the optical Thomson scattering laser at the National Ignition Facility

Abstract

The National Ignition Facility (NIF) employs 192 laser beams to achieve inertial confinement fusion by irradiating a mm scale fusion target. The optical Thomson scattering (OTS) laser is being deployed to probe the target and understand the target implosion physics. Centroid based approach is one of the common approaches for detecting the position of normal Gaussian beams within the OTS laser for beam alignment. Recently, we reported some results of aligning such a beam in 2021, where a pattern matching technique such as matched filtering was used. However, when we defined the template, it included a very high background noise. The correlated noise resulted in an artificial stability when the template was applied on a set of images taken at the same position in quick succession. However, when applied for alignment on different days, the presence of noise had a lesser effect as it made the noise more uncorrelated. In this paper, we re-evaluate this same dataset as published in 2021. We show that the performance of a matched filtering followed by a weighted centroid can overcome distortions appearing in the beam image and is capable of tracking the pattern motion reliably. This paper aims to explain some of the conclusions reached in the previous work while presenting a better approach.