Cryogenics remains an integral part of research conducted at several Department of Energy National Labs, from novel cryostats used to cool telescope equipment to the dilution refrigerators driving nanoparticle studies.
SLAC – Menlo Park CA
The SLAC National Accelerator Laboratory, for example, is no stranger to cryogenics. Researchers there are working on two astrophysics and cosmology projects that showcase extreme cold. One is the construction of the 3.2-gigapixel digital camera (the largest of its kind) for the Large Synoptic Survey Telescope (LSST) that will begin surveying the southern sky from a Chilean mountaintop in 2022. It will produce a wide, deep and fast survey of the night sky, cataloging by far the largest number of stars and galaxies ever ob¬served. During a 10-year time frame, LSST will detect tens of billions of objects—the first time a telescope will observe more galaxies than there are people on Earth— and will create movies of the sky with unprecedented details.
SLAC also participates in the operation and data analysis for the current SuperCDMS (Cryogenic Dark Matter Search) Soudan experiment and is ramping up for its leadership role in SuperCDMS Soudan’s successor, SuperCDMS SNOLAB. SuperCDMS SNOLAB is one of three “next-generation” dark matter experiments recently endorsed by the DOE, the National Science Foundation and the Canada Foundation for Innovation. When it turns on in 2018 at the SNOLAB underground science laboratory near Sudbury, Canada, it will be able to see dark matter particles with masses as small as a proton interact with regular matter. SuperCDMS SNOLAB will consist of 42 modular detectors cooled almost to absolute zero (approximately -460°F), bundled together with electronics and wiring into seven self-contained towers.
SLAC’s researchers additionally continue to use its two X-ray user facilities—the Linac Coherent Light Source (LCLS) and the Stanford Synchrotron Radiation Lightsource (SSRL). These facilities store samples and collect data at cryogenic temperatures, making the transport and long-term storage of delicate crystals easier and controlling the state of many samples during experiments.
Both facilities played a role in recent research that combined powerful magnetic pulses with some of the brightest X-rays on the planet to discover a surprising 3-D arrangement of a material’s electrons that appears closely linked to high temperature superconductivity. This unexpected twist marks an important milestone in the 30-year journey to better understand how high temperature super¬conductors conduct electricity with no resistance at temperatures much warmer than conventional metal superconductors but still hundreds of degrees below freezing.
The facilities also provide support to visiting researchers. In a recent study carried out in part using SSRL, scientists from the University of California, San Francisco determined in atomic detail how a potential drug molecule fits into and blocks the channel in cell membranes that Ebola and related “filoviruses” use to infect a victim’s cells. The study marks an important step toward finding a cure for Ebola and other diseases that depend on the channel.
The researchers made crystals containing many copies of the target channel protein and then exposed the crystals to in¬tense X-rays in SSRL and at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory. SSRL’s Beam Line 12-2 was crucial to the successful analysis of these crystals because its bright X-rays are particularly well suited for biomedical diffraction studies and its pixel-array detec¬tor is 1,000 times faster than conventional detectors in logging data.
Fermilab – Batavia IL
Some 2,000 miles away at the Fermi National Accelerator Laboratory (Fermilab) (CSA CSM), engineers at its SRF Department –Technical Division and Cryogenics Department–Accelerator Division are working on another SLAC-hosted project, the LCLS-II upgrade of LCLS.
Current work focuses on a cryogenic distribution system and a prototype cryomodule. “It’s not a prototype in the purest sense,” says Jay Theilacker, department head at Fermilab’s Cryogenic Department. “It has to work, and will be in the machine.”
The LCLS-II cryogenic system and prototype cryomodule are modeled on the European XFEL project at DESY, and the prototype uses cavities from the International Linear Collider (ILC) R&D program. Engineers at Fermilab prepared these cavities for assembly into SRF prototype cryomodules at both Fermilab and the Thomas Jefferson National Accelerator Facility (CSA CSM).
Unlike XFEL and ILC, LCLS-II will be a near-continuous wave X-ray laser (delivering 1,000,000 pulses per second), and will thus have higher heat load, more dynamic losses and increased refrigeration requirements. Fermilab engineers, therefore, chose to mod¬ify the ILC SRF cavities using nitrogen doping and other techniques in order to reduce the cryogenic losses.
The Fermilab team also had to find a way to improve the system’s magnetic shielding. Two layers of shielding have been installed on cavities in the string. A high magnetic field can increase the heat load and deteriorate RF performance, says Theilacker. “You have to shield from stray magnetic fields including the Earth’s. The Earth’s magnetic field is huge compared to what you have to get down to.”
Next up, the Fermilab team will install more instrumentation and cold mass components, including tuners, before placing the string into a vacuum vessel. Work on the prototype is expected to finish in June 2016, with tests beginning thereafter.
JLab – Newport News VA
LCLS-II cryomodules are being com¬missioned at the Thomas Jefferson National Accelerator Facility (JLab) (CSA CSM) as well, but engineers there are also working on the 12 GeV upgrade project. Back on Dec. 14, operators of the Continuous Electron Beam Accelerator Facility (CEBAF) delivered the first batch of 12 GeV electrons (12.065 GeV) to its newest experimental hall complex, Hall D, and since then the newly updated accelerator has delivered beam to three of the lab’s four experimental halls.
The 12 GeV Upgrade project, which is scheduled to be completed in September 2017, was designed to enable the machine to provide 12 GeV electrons, triple its original design and double its maximum operational energy before the upgrade. By increasing the energy of the electrons, scientists are increasing the resolution of the CEBAF microscope for probing ever more deeply into the nucleus of the atom. The $338 mil¬lion upgrade entails adding 10 new acceleration modules and support equipment to CEBAF, as well as construction of a fourth experimental hall, doubling the capacity of the accelerator’s helium refrigeration plant, upgrades to instrumentation in the existing halls and other upgrade components.
“The CEBAF accelerator commissioning and achievement of the design energy required hard work, patience and teamwork,” says Arne Freyberger, Jefferson Lab’s director of accelerator operations. “It’s just fantastic to watch it all come together, and the sense of accomplishment is palpable.”
Once the upgrade is complete, CEBAF will become an unprecedented tool for the study of the basic building blocks of the visible universe. It will be able to deliver 11 GeV electrons into Halls A, B and C (its original experimental areas) and its full-energy, 12 GeV electrons, to the Experimental Hall D complex where scientists hope to produce new particles called hybrid mesons. Hybrid mesons are made of quarks bound together by the strong force, the same building blocks of protons and neutrons, but in hybrid mesons, this force is somewhat modified. It’s hoped that observing these hybrid mesons and revealing their properties will offer a new window into the inner workings of matter.
“This kind of science explores the most fundamental mysteries: Why are we here? Why is it that one particular combination of quarks and forces takes on that material property, while a different combination of quarks and forces makes up the human body?” Lung says. “One particularly compelling question that scientists have had is why do we always find quarks bound together in twos and threes, but never alone? We will have an entirely unique facility designed to answer the question.”
Brookhaven – Upton NY
Some 500 miles up the eastern sea¬board researchers are refreshing yet another cryogenic system, this one at Brookhaven National Laboratory’s Synchrotron Light Source II. Operations initially began on the project in October 2014. NSLS-II is a 3 GeV electron storage ring that produces extremely intense beams of X-ray, ultraviolet and infrared light by circulating electrons around the ring. Researchers use the facility’s experimental stations—called beamlines—to study material properties and functions at nanometer-scale resolution.
To replenish the electron energy loss to synchrotron radiation, Brookhaven Lab’s Radio Frequency (RF) Group installed two 500 MHz superconducting RF niobium cavities in the electron ring.
“If the cavities were not superconducting, 400 kW would be required to provide 4 MV,” says James Rose, head of the RF Systems Group for NSLS-II. “The superconductivity not only reduces energy loss in the cavities but it also pro¬vides for beam stability because we can design the cavities with a large aperture, eliminating higher-order modes that can impede the beam.”
A cryogenic system, designed and installed for Brookhaven Lab RF by Linde Kryotechnik AG, ensures the system’s cavities are immersed in liquid helium. “We maintain a constant volume of 400 l of liquid helium surrounding the cavities,” says William Gash, a cryogenics engineer in Brookhaven’s Utilities Group.
The system’s helium compressors, which compress helium gas to 150 psi, reside in a separate building so that no sound vibrations are picked up by the cavities. The compressed gas is fed into a cold box with three turbines—spinning at 2,600 to 4,300 rps—that liquefy the helium gas as it expands and cools down. A liquid nitrogen system, built by PHPK Technologies (CSA CSM), is used to pre¬cool the helium from a room temperature of 300K down to 77K.
Once liquefied, the helium is stored in a 3,500 l storage dewar at a pressure of 5 psi and distributed via multichannel vacuum transfer lines through two valve boxes directly into RF cavities that operate at a pressure of less than 4 psi.
When the helium is boiled off, the cold gas is returned to the cold box to be used by heat exchangers, then sent to the compressor to start the cycle again.
In the next five years or so, two additional 500 MHz cavities, two 1,500 MHz bunch-lengthening cavities and another valve box will be installed. “This installation will provide the RF power for additional synchrotron light beamlines and extend the lifetime of the electron beam,” says Rose.
ORNL – Oak Ridge TN
Meanwhile, back in the heartland of Tennessee, researchers at Oak Ridge National Laboratory’s Spallation Neutron Source (ORNL SNS) (CSA CSM) recently tested the performance of a nine-foot-tall tritium-compatible cryoviscous compressor (CVC) pump prototype de¬signed for ITER, a burning plasma experiment researchers hope will show the feasibility of fusion-powered commercial power grids.
Robert Duckworth, a researcher with the ORNL plasma technologies and applications group within the fusion materials and nuclear systems division, oversaw the CVC pump testing. “The SNS cryo facility worked great,” Duckworth says. “The SNS staff was very engaged and went above and beyond to assure that we had a positive test experience. The CVC performed well in its first full-scale test and was able to freeze deuterium with helium. We learned much that we can take and use for the final installation of the CVC at ITER.”
Major Tool and Machine fabricated the full-size cryoviscous compressor pump under contract to US ITER. The prototype weighs about 2,500 pounds and is nine feet tall and four feet in diameter. In addition, Meyer Tool & Mfg., Inc. (CSA CSM) fabricated the valve box assembly, instrumentation and controls, while Eden Cryogenics (CSA CSM) provided cryogenic transfer lines.
Ultimately, six cryoviscous compressor pumps will be manufactured and installed on the ITER machine. The primary fusion fuels that will be used on ITER are deuterium and tritium. The CVC pumps will take turns evacuating a mix of deuterium, tritium and helium from the ITER tokamak in order to send the mix on to the tokamak exhaust processing system for fuel recycling. The pumps are “tuned” to freeze out all of the hydrogen species gases so that fuels can be captured and reused.