High-Speed Measurements of Target-Support Vibrations Using Line-scan Cameras

The Laboratory for Laser Energetics (LLE) houses the OMEGA 60-beam laser, which is used to carry out inertial confinement fusion (ICF) experiments. The 60 beams delivered by the laser are all focused on a fusion target that is less than a millimeter in diameter. To properly carry out an ICF experiment, the target has to be uniformly irradiated by all 60 of the laser beams. Target vibrations are a critical source of error because they shift the position of the target, disrupting the uniformity of target irradiation. Deviations in target location as little as 20 microns from target chamber center (TCC) can seriously affect the implosion. Line-scan cameras capture images that represent displacement in one dimension as a function of time, and can therefore be used to capture target movement. Displacement data extracted from the images can then be analyzed using a Fast Fourier Transform (FFT) to identify resonant frequencies of the target. This provides valuable information about the target support structure, its responses to various vibration sources, and the possible effects of engineering improvements. Optical measurement techniques are superior to other possible options because they do not directly contact the target. However, camera resolution and image quality limit the camera’s ability to capture data for targets that are resonating at high frequencies with small displacements.