The Design, Fabrication, and Assembly of Material Equation-of-State Targets for the National Ignition Facility

摘要

The unique capabilities available at the National Ignition Facility (NIF) allow users to recreate the pressure-temperature conditions within the cores of giant planets for materials such as Carbon and Iron. The Equation-of-State (EOS) of these materials can be measured using ramp compression experiments. These experiments require use of high-fidelity targets as shown in Figure 1 and sophisticated diagnostics to acquire the ramp compression data.When shot in the NIF, the high-fidelity EOS targets convert laser energy from the NIF into x-ray energy via gold hohlraums (essentially, thin walled cylinders with partial openings at each end). The X-ray energy then strikes the backside of an ablative material (the ablator) to create a shock that will be transmitted to the material to be studied. As the shock transfers through the material of interest the velocity of the front-side of the material is measured via a diagnostic that is part of the NIF (see Figure 2). The EOS can be determined from the velocity measurements of the free surface. Therefore, the two critical components of an EOS target are the hohlraum and the experimental package which will typically contain the ablator, shielding, and the material to be studied (e.g., carbon or iron).In order to minimize negative effects on the velocity measurements caused by the deposition of large amounts of laser energy into the target, the target hohlraums are filled with 0.2 atm of gas; the gas slows down the formation of an undesirable shock. To contain the gas, the hohlraums must be sealed. The hohlraums are filled with gas while installed in the NIF target chamber (a vacuum environment) prior to shot time.Not all of the laser energy directed at the target is desirable. A large portion of the laser energy is undesirable unconverted light that can damage the experiment and possibly damage the laser facility via reflections off of the target. To mitigate this concern, extensive shielding is incorporated into the target to protect the target and scatter the undesirable laser energy away from critical hardware.The target is held in the center of the target chamber via a 5-meter long target positioning arm. To simplify mounting of the target to the positioning arm, a universal kinematic mount is included as part of the overall target; it is attached to the hohlraum via a carbon rod. To ensure that the target is properly located in the target chamber, additional features (target positioning features - in this case, alignment fibers) are added to the target to allow the facility users to visually verify via multiple cameras that the target is positioned properly.Designing and making the components just described and assembling these components into a functional EOS target will be the focus of this paper. Three of the significant challenges include making the experimental packages with stepped features out of carbon and iron, measuring the thickness of the stepped features, and assembling the fragile target components into usable targets.