Participants in our Summer High School Research Program perform real research, not just summer projects. Furthermore, their work sometimes continues and impacts the Laboratory long after they have returned to their studies. Research projects performed by Bruce Brewington and Daniel Balonek are prime examples of ongoing research that stemmed from a short summer program.
Successful inertial confinement fusion on the National Ignition Facility will require the use of cryogenic targets, in which the deuterium-tritium fuel is frozen as a solid ice layer inside a thin plastic shell (Fig. 1). LLE has acquired considerable experience in making cryogenic deuterium targets, but, prior to Bruce’s work, has not had a method for accurately locating defects. Uniform and defect-free ice layers yield the best experimental performance as defects in the ice prevent compression of the fuel to high densities. Shadowgraphs, two-dimensional images created by passing a beam of light through a target, show defects because the light refracts differently through a defect compared with a smooth ice layer (Fig. 2). However, the location of the defects cannot be determined from a single image since they can be on either side of the target and at any depth. For his project Bruce developed a computer program that analyzes many (typically 50) shadowgraphs taken from different angles around a single target. His software uses a new algorithm to combine the information from the shadowgraphs and create a 3-D model showing the location, size and shape of an ice imperfection. For the target shown in Figure 2, Bruce found that the defect is located on the inner ice surface. Once the defect is located, steps can be taken to find the source of the defect and correct the cause.
As a result of his research, Bruce was one of the 40 finalists nationwide in this year’s Intel Science Talent Search. To the right is a photo of Bruce and his advisor Dr. Stephen Craxton at the Intel competition.
An ideal target (top) irradiated uniformly by overlapping laser beams is compressed to a high density. However, for a target with a defect (bottom, exaggerated), the left side has a smaller mass and accelerates faster toward the center, making high compression impossible.
An ideal target (top) irradiated uniformly by overlapping laser beams is compressed to a high density. However, for a target with a defect (bottom, exaggerated), the left side has a smaller mass and accelerates faster toward the center, making high compression impossible.
Target vibration can degrade the implosion performance of an ICF capsule. Cryogenic targets are particularly sensitive to vibration because the capsules are suspended on spider silk. The goal of the target resonance characterization fixture (TRCF) is to obtain quantitative information that will help isolate and correct design and manufacturing problems associated with these (Fig. 1) delicate target assemblies.
The TRCF evaluates the mechanical integrity of a cryogenic target assembly at room temperature, prior to filling and cooling. A target assembly is excited with a known vibration input, and its vibratory response is measured optically. Initially, this system will be used to measure characteristics of targets in a consistent manner. Subsequent performance of these targets will be evaluated on OMEGA during cryo system testing using high-speed video imaging and during target shots using the cryogenic target characterization diagnostic. Once a correlation is established between performance on OMEGA and the characteristics measured using the TRCF, this system will be used to exclude suboptimal targets prior to the fill process. Other potential uses of this fixture include the evaluation of alternative target assembly methods and materials.
Daniel worked with Dr. Douglas Jacobs-Perkins on the TRCF. Daniel built the optical modules for the TRCF and packaged electronics used for signal conditioning. He also developed procedures for assembly and calibration of this hardware, as well as implementing several design ideas throughout the project.
The vibration power spectral density (PSD) of a cryogenic target is measured using the TRCF at room temperature before filling.
A “mass equivalent” target has a thick shell that mimics the mass of a filled target. As expected, the PSD of this assembly is different due to the increased target mass. Variations in the manufacturing process may also contribute to the different frequency signature.
Target resonance characterization fixture (TRCF)
The TRCF will be used to measure characteristics of targets in a consistent manner. A newly designed target mount uses a spindle with a tapered base. A rare earth magnet holds the target assembly in the spindle. The magnet’s draw-force preloads the joint, resulting in a very stable, repeatable connection.