Dante Upgrade Sheds ‘Light’ on Hohlraum Temperatures

A recent upgrade of NIF’s workhorse Dante x-ray diagnostic is helping researchers gain a better understanding of the temperatures produced in NIF hohlraums. When heated by NIF’s laser beams, the hohlraum can reach temperatures in excess of three million degrees and glow beyond white-hot, producing x rays which can be used for a wide variety of physics experiments. Accurately measuring how bright the hohlraum glows is critical to improving researchers’ understanding of nearly every NIF experiment.

The Dante upgrade involved the installation of new state-of-the-art, high-speed digital oscilloscopes that replaced aging recording hardware from the era of underground nuclear testing—adding flexibility, improving measurement accuracy, and reducing maintenance costs. In addition to modernizing the recording system, other enhancements enable more efficient operations and automated shot setup. The upgrades have significantly reduced the workload for shot operations staff and helped to reduce costs and increase the NIF shot rate.

The two Dante diagnostics, named after the 14th century Italian poet whose “Divine Comedy” describes a journey through the fiery inferno of Hell, looks deep into the inferno of the hohlraum (see “Dante: Measuring NIF’s Inferno”). Each Dante consists of 18 absolutely calibrated detectors, each of which is filtered using reflections from mirrors and transmission through different elements in order to measure only a certain energy range (or “color”) of x-rays. Combined, these signals provide a time history of the x-ray spectrum and temperature produced inside the hohlraum.

Signals from each x-ray detector are transported on long cables from the Target Bay to the recording instruments on the diagnostic mezzanines. The thick concrete walls of the Target Bay, along with shielded cabinets, protect the delicate instruments from neutrons and gamma rays produced by the imploding target.

Not all potential hazards can be eliminated through shielding, however; if an x-ray filter breaks, for example, large voltages could travel down the normal signal path and damage the instruments. To mitigate the potential for damage, special protection circuits that work like a very fast fuse, developed by NIF’s collaborators at the Laser Megajoule facility in France, were implemented in the upgraded design.

Ultimately the analog signals from the x-ray detectors are converted into digital format by the new oscilloscopes, which were developed by Keysight Technologies in Santa Clara, California. Each “’scope” is capable of recording four separate signals at 10 billion samples per second with a vertical resolution of about one in 1,000 (10 bits). Unlike their predecessors, the sensitivity of each channel can be remotely adjusted to match the expected signal level.

To accommodate the large range of hohlraum temperatures that NIF can produce, attenuation of the x-ray detector signal is an important element of the diagnostic. Remotely controllable attenuators have been introduced to streamline operation of the upgraded Dante; attenuation is now controlled directly by shot setup software. Previously, target diagnostic operators would have to manually insert individual attenuators into each of the 18 signal paths depending on shot expectations—a tedious and time-consuming task which had to be performed before nearly every NIF shot.

The former analog vacuum-tube oscilloscopes were at the end of their life cycle and were difficult and costly to maintain; replacements using similar technology would have cost about $1.6 million and still would have relied on analog technology. Going digital—incorporating time-saving enhancements while increasing reliability and signal recording quality—significantly reduced the cost. “Dante has gone from being one of the most expensive diagnostics to one of the more cost-effective diagnostics we run today on NIF,” said Perry Bell, program manager for Target Diagnostics.

Controlling the new hardware and incorporating the new functionality into the NIF Integrated Computer Control System (ICCS) was a large part of the project. Software engineers from across NIF’s software team redesigned the automated setup calculations for the 18 channels, rewrote shot participation rules, and developed new control and analysis code without affecting ongoing shots.

The upgrade was accomplished during a few weeks this summer; the old hardware was removed, the new hardware was installed, the new software was released, and Dante 1 entered the digital world—saving money, helping to increase the shot rate, and improving the quality of the scientific data being recorded.

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