NIF’s millimeter-sized targets are complicated, precision assemblies that require a complex interplay among target designers, materials scientists, and precision engineers and technicians.
NIF’s targets are designed and fabricated to meet precise specifications for density, concentricity, and surface smoothness. The extreme temperatures and pressures targets encounter during experiments make the results highly susceptible to any imperfections in fabrication. As a result, target components must be machined to an accuracy of 1 micron, or 1 millionth of a meter. Some joints can be no larger than 100 nanometers (1/1000th the width of a human hair).
To aid in the extremely delicate manufacturing, NIF researchers developed a precision robotic target assembly machine. Workers regularly inspect the target materials and components using nondestructive characterization methods to ensure that target specifications are met and that all components are free of defects.
Different target designs
Many different target designs exist. Some are designed for stockpile stewardship purposes, others for high-energy-density science, and still others are dedicated to the pursuit of achieving ignition. Specialized ignition-related targets provide information on shock timing, capsule implosion shape, implosion velocity, and the extent of mixing of the colder fuel with the fuel core “hot spot.”
Targets designed to achieve ignition feature four essential components: a capsule with fill tube, deuterium and tritium (D-T) fuel, a cylinder the size of a pencil eraser known as a “hohlraum” enclosing the capsule, and thermal control hardware. Livermore scientists have pioneered procedures to precisely form the frozen layer of D-T fuel inside the fuel capsule.
The fuel capsule is made from light elements that perform well as “rocket fuels” when ablated by the x rays created in the hohlraum. To achieve ignition, the capsule needs to have a precise spherical shape with surfaces smooth to the nanometer (billionth of a meter) level. Filling the capsule with D-T gas requires a small fill hole less than 5 microns across.
Fuel capsule surface polished to perfection
Surface conditions for the assembled fuel capsules have stringent specifications. The NIF team developed a proprietary polishing technique that can remove or reduce bumps to less than 150 nanometers in height. Because surface debris can interfere with the uniformity of capsule heating and compression, dust particles greater than 5 microns in diameter on the capsule wall must be eliminated. Meeting this standard requires assembly in a Class 100 cleanroom.
The capsule is fitted inside a hohlraum cylinder measuring 9 millimeters high by 5 millimeters in diameter. The hohlraum is made from a high-Z (high-atomic-number) material such as gold and uranium.
Once assembly is complete, the target is integrated with the cryogenic target positioning system and placed at target chamber center. Target position is maintained to within 2 microns, and temperature is held in the range of 18 to 20 kelvins (-427 to -424 degrees Fahrenheit) with a stability of 1/1,000th of a kelvin.