Colder than Freezing: The Cryogenics of FRIB

The world in which we live gets really weird when it approaches the extreme limits on the fringes of physics. The smallest particles known to exist become entangled, instantly reacting to changes in the other despite being light years apart, a phenomenon Einstein described as “spooky.” At the Large Hadron Collider, lead ions are smashed together creating a quark-gluon plasma at trillions of degrees creating a chaotic soup of unbound particles similar to the first moments of the universe.

And then you have the physics of the ultra-cold.

Matter in these systems has lost so much energy that everything down to electrons almost completely stops. This also creates strange phenomena, such as pendulums speeding up instead of slowing down and superconductivity, an attribute very important to particle physics.

ReA3 cryomodule

A cryomodule in production for ReA3, a linear accelerator using similar technology to that of FRIB.

When certain metals are cooled below their critical temperature – 4.6 Kelvin or -451.1 Fahrenheit for the NSCL’s niobium-titanium magnets – they lose all resistance to electricity. They do not lose energy because of electrons being impeded while trying to move through the wire. Not only does this mean that closed systems theoretically can run forever after a single charging, it means more electricity can fit into much smaller wires.

The discipline of keeping these metals at these superconducting temperatures in order to take advantage of these characteristics is called cryogenics.

“Cryogenics is the science and engineering associated with phenomena that occur below -243 Fahrenheit and the technology enables a wide spectrum of scientific discovery,” says John Weisend, FRIB’s Cryogenics Group Leader. “These projects range from the very large to the quite small and they cover a wide range of temperatures and disciplines.”

However, Weisend is mainly concerned only with its application to his current project, the Facility for Rare Isotope Beams (FRIB). Cryogenics allow scientists to make superconducting magnets that create powerful magnetic fields in a compact space, which are used to steer and focus beams of rare isotopes. Superconducting radio frequency (SRF) cavities are charged with up to two million volts of resonating electricity that accelerate nuclei as they pass through. These magnets and accelerators aren’t very big, but if it weren’t for superconductivity, their size would be unmanageable.

With roughly 350 of these SRF cavities planned and more than 100 superconducting magnets, FRIB will require a lot of cooling power. This is why the lab recruited one of the best cryogenic engineers in the world to augment its already strong cryogenics group . Weisend has plenty of experience with cold temperatures, having worked with the cryogenic systems at the Stanford Linear Accelerator Center, the Deutsches Elektronen-Synchrotron (DESY) in Hamburg, Germany and the Centre d’Etudes Nucleaires in Grenoble, France.

FRIB cut section

A cut-away section of the planned linear accelerator and cryogenics system for FRIB.

“The challenges for the FRIB cryogenics system are challenges in terms of its size,” says Weisend. “It’s not beyond the realm of technology, so we’re not pushing the state-of-the-art. But it will be a big system, even compared to existing laboratories here in the States.”

While FRIB’s cryogenics system won’t be the largest, it certainly will be impressive. Roughly 17,000 liters of liquid helium at temperatures below -456 Fahrenheit and weighing about 7,500 pounds will be shipped into the cryogenics plant. Once there, it will be distributed through a complex system of pipes capable of cooling down and warming up each individual cryomodule containing several SRF cavities and at least one superconducting magnet. The distribution system itself will be kept insulated by thermal shielding at -360 Fahrenheit, helping to screen the liquid helium from the heat of the outside world.

But the liquid helium will absorb heat from FRIB’s machines, causing it to heat up. As the liquid helium turns into a gas, it will be transported back to the cryogenics plant in a closed-loop system, where it can be cooled back into a liquid through a 15 kilowatt cryogenics plant and reused. Since helium is a limited resource and its cost is increasing, it is important to conserve it and reuse as much as possible.

FRIB cut section

A mock-up of what a FRIB cryomodule will look like.

What’s more, due to the construction and maintenance of this large facility, John will be able to launch a new cryogenics research program at Michigan State University, which will join the Massachusetts Institute of Technology, the University of Wisconsin-Madison and Florida State University as one of cryogenic’s very few major graduate training programs. This will be a great opportunity for a few exceptional students, as the somewhat obscure field is expanding.

And it isn’t just accelerators like FRIB that will put these future cryogenic engineers to work. Weisend points out several projects that use liquid nitrogen and liquid helium systems including particle detectors at the Large Hadron Collider in Geneva, Switzerland, searches for dark matter at the underground laboratory in Ontario, Canada called SNOLAB, and a prototype muon ionization cooling device in the Rutherford Appleton Laboratory in Didcot, U.K.

“We’re looking for people to come to NSCL and I can easily think of three or four different laboratories and institutions that also are looking for trained cryogenic people at the moment,” says Weisend. “So the demand is beginning to outstrip the supply of people that we’re generating. And the amount of work that’s available will probably increase as more and more things use this technology, particularly in the area of science.”