Mixed Refrigerant System Designed to Cool Deep Survey Telescope

When the Large Synoptic Survey Telescope (LSST) first turns to survey the southern sky from its Chilean mountaintop perch in 2022, at least one group of scientists will be focused more on the telescope’s camera than the stunning images it’s expected to render. Those researchers are currently designing and testing a novel cryogenic refrigeration system at SLAC National Accelerator Laboratory that will cool the LSST camera. The 3.2 giga-pixel camera is larger than other CCD (charge-coupled device) telescope cameras, and with that size comes a higher cryogenic heat load. In this case, scientists need to cool the focal plane to -130°C to reduce electronic noise in the CCDs, remove radiative heat loads and process heat from the electronics.

Most CCD telescopes use liquid nitrogen for cooling, either in a boiloff system where dewar reservoirs are refilled daily or in a recirculation system that recondenses vaporized liquid nitrogen (LN2) and sends it back through vacuum insulated flex lines to the camera. A trade study at SLAC concluded that LN2 systems would not be practical. A boiloff system would require nearly 800 liters per night and a heavy recirculating system would have to be designed to move with a telescope that repositions and re-images every 15 seconds.

In 2009, SLAC researchers partnered with MMR Technologies, Inc. (CSA CSM), and began designing a compact Joule-Thomson mixed refrigerant system based on MMR’s commercial patents. Part of the system, the compressors, will reside near the ground on the platform supporting the telescope mount, while the cryostat and its associated utility trunk will be mounted up in the camera. The mixed refrigerants will be compressed and the heat removed at the platform level before being transferred through 150 feet of uninsulated lines up to the cryostat. There, the Joule-Thomson expansion will take place. Temperature in the telescope dome is precisely regulated to ensure image quality, so a counterflow heat exchanger will bring the cold refrigerant back to ambient temperature before it exits the cryostat and reenters the dome to head back to the compressors.

“It’s a complicated system,” says JB Langton, engineering manager for the cryostat subsystem. “Some people view the system and they’re fearful that it’s going to be delicate, that it’s going to be failure-prone. I don’t think so.” Langton joined the SLAC team in 2011 and is responsible for designing the cryostat vacuum chamber and the thermal and support systems for the camera’s focal plane and focal plane electronics.

Layout of LSST Camera Mixed Refrigeration System. Image: SLAC

Layout of LSST Camera Mixed Refrigeration System. Image: SLAC

The cryostat sits at the front of the camera. The focal plane lies just behind L3, the last refractive optic that seals the cryostat’s insulating vacuum. Inside the cryostat is the GRID, the optical bench that supports the focal plane. Six evaporator coils are brazed onto a large copper cryoplate located behind the GRID. The cryostat contains two refrigeration systems. The first consists of six evaporator coils brazed onto a large copper “cryo plate” located behind the GRID. A second “cold plate” composed of two evaporator coils is mounted behind the cryo plate. The cryo system operates at 143K to cool the sensors while the cold system, at 233K, cools the electronics.

Attached to the cryostat is a utility trunk. Inside are the heat exchangers for both refrigeration systems in a separate insulating vacuum. The utility trunk has its own environmental control as it also houses all of the vacuum pumps, power supplies, readout electronics and other electronics necessary to operate other parts of the camera, such as the shutter and filter-changing carousel.

On the ground are the system’s compressors, condensers and oil and phase separators. The latter separates the liquid and vapor components of the refrigerant, removing oil from the stream and returning it to the compressor. Surge tanks, accumulators, filter panels, isolation valves and command, control and monitoring systems for the refrigerators are also on the ground level.

“I think it’s a more sophisticated system than some of the other cooling technology used on these astronomical devices, such as recirculating liquid nitrogen and boiloff nitrogen systems,” says Langton. “[It’s] like comparing the Model-T to a modern car. A modern car is far more sophisticated, but it doesn’t mean it’s less reliable. And it doesn’t mean it’s less maintainable.”

Equally sophisticated are the mixed refrigerants flowing through the system. MMR has developed and tested several blends, the first of which was made from argon, methane, ethane, tetrafluormethane, trifluoromethane and iso-pentane. It cooled well but was flammable. Researchers from both SLAC and MMR are currently testing a non-flammable mixture based on octafluoropropane in a sub-scale system constructed at SLAC. “It’s kind of interesting,” says Rafe Schindler, professor of physics at Stanford and physicist manager for the cryostat subsystem. “In a small system like the commercial products MMR makes it is a lot simpler, but in the LSST system where there are long, very large transfer lines separating the pieces of the system, we have had to adjust the mix for each geometry that we’ve tested.”

When it’s constructed in Chile, the main refrigeration system will use six cryomodules—each with its own compressor, transfer lines, evaporators, etc.—to cool the sensors and electronics. The sub-scale system has one set. Researchers built it in a laboratory with a 70 foot drop to the floor, installing the compressor on the ground and running transfer lines along the wall. The sub-scale camera is mounted on a hexapod, allowing the researchers to mock the telescope’s rotation. It has space for heaters designed to simulate electronic heat loads, a single raft of CCDs, a heat exchanger and an evaporator brazed onto a mock copper cryoplate.

“And so we’ve slowly built up something,” says Schindler, “basically a mock-up of the entire system, and by doing that we’ve been able to uncover places where there are problems that we have to solve so there are no surprises in the real thing.”

One of the first surprises the team encountered was an overabundance of plumbing: The original refrigeration design called for approximately three kilometers. “Within the refrigeration team we just did not appreciate the scale of the task to decontaminate and dry out that plumbing to the level that we need to ensure reliable operation of the refrigerators,” says Langton. “It took a long time with refrigerators that we built as test units and slowly scaled up for it to really dawn on us that it wasn’t really a practical thing to do.” Fixing the problem meant collaborating with the engineers designing the observatory to bring the compressors closer to the camera. Moving the compressors inside the dome to the telescope mount reduced 60 to 70 percent of the total system volume, according to Langton.

Construction of the actual camera is now underway in a clean room at SLAC and is expected to take about 18 months. The refrigerant team has until then to iron out any lingering issues, such as testing transfer line draping or placing capillaries in accessible positions. The team is also testing various evaporator diameters and tweaking the heat exchangers to optimize thermal performance. Most significant, however, is getting the refrigerant mixture adjusted for the system’s final geometry and developing a method to pre-dry the mix from 40 parts per million moisture upon delivery to three to five parts per million within the system.

A final engineering prototype will be constructed and tested at SLAC once the camera is complete. In Chile, the prototype will be used to construct two refrigeration systems, one that cools the camera inside the dome and another for when it’s removed for maintenance. Each system will have a different geometry and thus the team will have to perfect two refrigerant mixes. The team must also develop a process for recovering the refrigerants, separating whatever oil is present and then recharging the system. Original plans called for simply discarding the old refrigerant and recharging with a new mix. “That’s not something we want to do now,” says Langton. “In the grand scheme of things it’s probably not that much money, but it’s still waste. It costs to dispose of it and it costs to replace it and if we can avoid doing that we would prefer not to.”

Engineers can, when necessary, maintain most of the refrigeration system during daylight hours when the telescope is dormant. Some parts, however, can only be accessed every two years during scheduled telescope maintenance. During that period the camera will be removed and placed in a maintenance facility. Engineers could then tweak some camera optics or replace faulty electronic components inside the refrigeration system’s utility trunk, but Langton hopes they won’t ever need to touch the cryostat. “My hope is that once the cryostat is sealed up it is never opened up,” he says. “It’s a very… sophisticated device.”