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Chilling Out

Engineers install, test cryogenic system for HETDEX detectors

VIRUS is catching cold.

The detectors in the suite of 156 spectrographs for HETDEX will be chilled to roughly minus-148 degrees Fahrenheit (-100 C) with liquid nitrogen. Engineers have installed the plumbing to support that temperature and are testing the system prior to the installation of the VIRUS spectrographs on the Hobby-Eberly Telescope (HET) later this year. “We’re trying to understand the characteristics of the system,” says project engineer Richard Savage. “So far, it’s behaving as expected.”

The liquid nitrogen tank (right) and its plumbing after installation outside the Hobby-Eberly Telescope
The intricate plumbing at the base of the storage tank A crane lifts the liquid nitrogen tank for installation Technicians lower the tank into position A tanker truck will deliver about 6,000 gallons of liquid nitrogen every week The storage tank is delivered to the top of Mount Fowlkes The racks that will hold the VIRUS units in place on the Hobby-Eberly Telescope

Each VIRUS unit contains a CCD similar to those in digital cameras to capture the faint light of galaxies that are billions of light-years away. The heat of the cameras and their related electronics creates “noise” that can fog the view. Cooling the CCDs eliminates the fog, allowing the cameras to obtain sensitive spectra of HETDEX’s one million target galaxies.

Reaching the super-cold temperatures needed to achieve that goal requires a sophisticated plumbing system. It begins with a three-story, 11,000-gallon tank for storing the liquid nitrogen, which was installed outside the HET dome in October 2014. When the system is operational, a tanker truck from Praxair, Inc., a vendor near Dallas, will deliver 6,000 gallons every week.

Some of the nitrogen in the tank will vaporize, creating pressure (about 34 psi) that will force the liquid nitrogen through pipes into the HET dome, where it will fill two 60-gallon phase-separator tanks atop the telescope structure.

The liquid nitrogen will then flow “down hill” out of these tanks through a system of pipes that deliver liquid nitrogen to the VIRUS units, which will be paired in 78 containers. The liquid nitrogen won’t enter the spectrographs, though. Instead, it will make contact with a copper support structure, which then will cool the CCDs. The CCDs are housed in vacuum cryostats to eliminate heat losses to the outside.

In all, the system contains about 1,000 feet of vacuum-jacketed pipeline. “It’s a very special pipe -- it’s consists of a pipe within a pipe,” says Savage. “The liquid nitrogen flows through a stainless steel pipe that’s inside a larger pipe. The space between them is a vacuum, which provides insulation.” Nitrogen gas generated by the cooling process will exit the dome through similar pipe.

The system underwent initial testing in February after all the piping was installed. The warm storage tank was chilled to allow it to handle the super-cold (–321 F/–196 C) liquid nitrogen, then was filled over a period of a few hours. Initial checks verified that there were no leaks in the system and that the piping was maintaining the proper vacuum. “We would have seen any leaks pretty easily because we would have had condensation on the pipe,” says Savage.

The next set of tests verified that the system was achieving and maintaining the proper temperatures. “There are 80 places to hook up, and we tested eight of them,” says Savage. “Now, we’re measuring how much liquid nitrogen we’ll need. We’re also testing heat losses to see how well they are matching our estimates, and so far we’re close.”

Engineers also are testing the control system, which maintains the proper temperatures and pressures throughout the system. If it detects any problems it sets off an alarm in the HET control room and places automated phone calls to the operations staff. The system also includes a series of oxygen monitors around the telescope to make sure there are no leaks. If the oxygen drops below pre-determined levels, then the system alerts the staff of a possible problem.

“We’re determining what the proper set points are and tuning the system,” Savage notes. “We’ll tune it the best we can without having VIRUS on the telescope. With VIRUS, we’ll get a lot more heat, so the operational parameters will change a bit.”

Once the spectrographs are installed, it should take a day or so to fully cool the entire system. After that, the spectrographs will be kept cold around the clock, keeping them at the ready for their many nights of observing distant galaxies.