NIF project now on the path to ignition

"We are definitely on the path to ignition now."

That bold statement by Chris Haynam kicked off the latest in a series of seminars about the National Ignition Campaign (NIC), the program of experiments leading to the ultimate goal of fusion ignition on the National Ignition Facility (NIF).

Haynam is the physicist responsible for modeling and testing the quality and performance of NIF's lasers. Also speaking at the seminar was Alex Hamza, head of NIF's cryogenic target fabrication program.

Haynam described several series of laser commissioning experiments meeting design criteria for energy, pulse shape and timing. In fact, in late February, an expert review committee concluded, "all laser performance completion criteria have been met or exceeded."

Nevertheless, as Haynam said: "It's very exciting to have the laser installed and operational, and like a lot of projects, when they're complete, there's still a lot of work they have to do."

For example, NIF's lasers need to be fine-tuned with regard to 17 adjustable parameters including wavelength, synchronization, spot size and smoothing, among others.

According to Haynam, one of his biggest challenges is maintaining the pristine surfaces of the laser optics.

As the energy in NIF's 192 beamlines increases, so does the possibility that tiny imperfections in the glass will grow. A spot of only 30 microns, formed as residual flaws created during the polishing process, can increase to 300 microns as the laser beams pass through on repeated shots. But Haynam pointed out that even at 300 microns that these damage sites are only the size of the eye of a sewing needle.

New techniques have been perfected to mitigate the effects, including improved fused silica finishing, as well as utilizing beam blockers to selectively prevent the flaws from interacting with the laser beams.

According to Hamza, the target fabrication team has two primary goals: ensuring the performance of the targets meets NIC specifications, and being able to manufacture those targets quickly enough to meet the NIC shot schedule.

The physics requirements for the 10-millimeter long gold container (hohlraum) and the two-millimeter diameter target it holds are rigorous.

Components must be machined to within an accuracy of one micrometer, or one-millionth of a meter. Targets are held in place by polymeric membranes that are 100 nanometers thick, which is just 1/1,000th the width of a human hair.

Furthermore, the fuel inside the target must be a perfectly-formed spherical layer of hydrogen gas, cryogenically cooled to 18.5 degrees Kelvin (-427 degrees Fahrenheit).

Hamza's team has developed a thermal-mechanical package into which the hohlraum assembly can be placed. This allows them to separate the target physics and engineering functions. For example, depending on NIC experimental requirements, the target may need to be constructed with different materials and thickness, or changes may be requested in the length of the hohlraum.

A unique robotic final assembly machine ( see video ) is used to manufacture these targets at a rate of one every two days. A second machine will be available next month. 

The final NIC seminar will be held next Tuesday, May 19, in the Bldg. 123 auditorium. John Edwards will describe the NIC ignition experiments using DT (deuterium-tritium) and THD (tritium hydrogen deuterium) targets.