Performance and Capabilities of the Cryogenic Fill-Tube Target Test Facility (CFTF) at the LLEMarch, 2008
A small group of scientists at the Laboratory for Laser Energetics (LLE), who support research at LLE and are devoted to the priorities of the National Ignition Facility (NIF), are currently designing and building a test station to characterize advanced cryogenic targets using deuterium–tritium (DT) fuel. Future direct-drive experiments on the NIF will require that capsules be fueled using fill tubes and be supported along a horizontal orientation. Scientists at LLE are investigating the effect of the fill tube and support structure on fuel-layer uniformity. Advanced cryogenic target designs to be fielded on the NIF introduce perturbations in the spherical isotherms surrounding the capsule and include
- Polar-drive (PD) and Saturn targets
- Foam-shell targets
- Fast-ignition targets
- Capsules in transparent hohlraums
LLE investigators have recently constructed and commissioned two test stations that can fill NIF-scale capsules with deuterium via a fill tube from an integral reservoir. Evaluation of the performance of the fill-tube/reservoir assembly will focus on: (a) the temperature gradient between the reservoir and the capsule necessary to transport the fuel through the fill tube, (b) the corresponding filling times, and (c) the inventory of fuel remaining in the reservoir versus the fuel in the capsule upon solidification. The resulting solid-fuel layer is subsequently brought into balance using infrared laser light injected into a diffuse gold-plated isothermal-layering sphere. The targets are characterized using shadowgraphy—an optical method that reveals nonuniformities in transparent media—and x-ray phase-contrast techniques, including x-ray radiation shielding. This enables investigators to evaluate these two characterization techniques side-by-side on the same fuel layer. The comparison should help determine whether features in the shadowgram are due either to ridges/troughs on the inside surface of the fuel layer or to crystal dislocations/cracks in the layer. Indirect-drive capsules inside transparent hohlraums will also be studied.
The system resides on a pneumatically levitated table and uses a vibration-isolated cryocooler, supported by a floorstand and convectively coupled to the cold finger. Vibration amplitudes of the order of 10 nm are typical with this system. The major components of the test station have been assembled and testing has begun.
Internal cryogenic components are currently being designed and prototyped. A layering laser developed at LLE is scheduled for completion and incorporation into the system by the end of September. The project is on schedule to field and begin layering targets by the end of 2008. During testing of the systems, scientists will discuss the design details of the test stations and review preliminary data obtained regarding the performance and capabilities of the CFTF facility.