Helium may be the second most abundant element in the universe, but on Earth it is a finite, non-renewable resource. Helium is too light to be trapped in the lower layers of Earth's atmosphere, and its relatively low reactivity makes it extremely difficult to capture. Liquid helium is a key component in refrigeration equipment used to study quantum systems and image atoms, as well as in high-powered magnets used in MRI scanners and particle accelerators. But if not carefully contained, helium can travel to the furthest reaches of the atmosphere, or even out into space if it boils off.
Xiaowei LiThe University of California, San Diego chemist says helium's scarcity has become increasingly apparent in the more than 10 years he's worked with instruments that require ultracold conditions: During that time, he's seen the price of liquid helium rise from about $2 a liter to $20.
So Lee decided to take the problem into his own hands and build a system in his lab to condense and recycle the helium. At the APS March meeting In Minneapolis, Liya Bi, a graduate student working with Lee, explained how they did it: Cobbling together air compressors, brewing equipment, and hardware-store parts, the UCSD team built a relatively inexpensive system that can recover 92 percent of the liquid helium the lab uses, without generating the vibrations that would shake up sensitive equipment.
Helium is produced by the radioactive decay of elements underground and is often mined in the same places as natural gas. But humanity is consuming helium faster than it can be replenished by natural processes, Li says. Geopolitical and economic turmoil threatens scientists' ability to maintain a steady, affordable supply. A provision in a 2013 law allows the United States, the world's largest producer of this vital element, to: Sell ​​the country's helium stockpileThe company plans to sell its main oil and gas development sites, oil and gas pipelines and wells, to a private company later this year. It is unclear what impact the sale will have on researchers.
Liquid helium, whose cryogenic boiling point is only a few degrees above absolute zero, is essential for cooling Lee's scanning tunneling microscope (STM). The STM uses an extremely sharp tip to image and manipulate single atoms on a surface. Because the atoms must be stationary, extremely low temperatures are necessary to operate. “Thermal perturbations will ruin the measurement,” Lee says. His lab uses the STM to study the quantum states of single atoms, which is key to understanding the underlying causes of chemical reactions. “To study quantum systems, you need the surroundings to be as cold as possible,” he says.
Over the past decade, companies have developed systems to conserve liquid helium. But those systems aren't suitable for a lab like Li's because they cost more than $10 million and are aimed at systems that use large amounts of liquid helium (designed to liquefy about 100 liters per day), so they're typically shared among multiple labs or medical imaging suites. Li's lab doesn't have the pipes to connect to one of those systems, because the university hospital is just across the highway from his lab. Other options would vibrate too much and wouldn't be able to share space with the sensitive STM.
“Helium is becoming increasingly expensive and difficult to manage.” Paul WeissWeiss, the UCLA chemist, also uses scanning tunneling microscopes, and says helium is a major expense in labs that run them: These instruments are very sensitive to vibrations, Weiss says, which often makes it difficult or inefficient to implement helium-recovery systems.
Li's group got their helium recovery system to work by isolating the noisy parts of the helium storage system from the STM. Helium evaporated from the STM's cryostat is piped to a separate room, where the vibrations of a noisy air compressor are not an issue. The gas is condensed with cooling coils (normally used to cool beer) that Li purchased from a brewer, and stored in a metal container called a Dewar. The most expensive component was the condenser, which cost about $70,000. Li says the entire system cost about $100,000 to build, and he expects to recoup the cost in helium savings within two years. Li's group describes their design as follows: Submit a preprint to arXiv January of this year.
Though Lee was trained in chemistry, he says this kind of DIY work isn't too far-fetched for him. He likes building new equipment, and a large part of his work is spent building new imaging technology. At the March conference, Lee said he was “a very creative person.” Hyperdimensional scanning tunneling microscopecan provide high-resolution information about the spatial and temporal motion of electrons in response to a laser pulse. He said this work was inspired by his doctoral supervisor at the University of California, Irvine, Wilson Ho, who would often tell him that if what he needed didn't exist, he should try to make it himself.
Weiss calls Lee's helium recycler a “clever integrated solution,” noting that the UCSD team is able to collect high-quality STM data even with the system turned on, proving that even small labs can work with this sensitive equipment.
Li says the helium-recovery design should be feasible for other small labs housing sensitive instruments, and the team is applying for funding from the National Science Foundation to further develop the system.