With a special superconducting cavity, the U.S. Department of Energy’s (DOE) Argonne National Laboratory is providing a boost to the world’s biggest neutrino experiment.
Through a collaboration with DOE’s Fermi National Accelerator Laboratory, Argonne is supplying the first eight of 116 superconducting cavities that will be used to accelerate hydrogen ions to create a stream of neutrinos for the Deep Underground Neutrino Experiment (DUNE). This experiment, which is sited at Fermilab but includes an international collaboration, seeks to understand fundamental properties of neutrinos, ghost-like particles with very little mass that interact only very slightly with normal matter.
The superconducting cavities, grouped into a single cryomodule, will be installed as part of Fermilab’s Proton Improvement Plan-II (PIP-II), a new upgrade to the Fermilab accelerator complex. The installation will likely take place this fall.
“The way the cryomodule works is by starting at low energies,” said Argonne accelerator physicist Zack Conway. “Our cryomodule accelerates particles to higher energies, at which they are fed to different accelerators, where their energies are boosted further, until they hit a target and produce neutrinos.”
The beginning of the process happens upstream of Argonne’s cryomodule, in an ion source that generates hydrogen atoms with an extra electron. Within the cryomodule itself, scientists synchronize the particle beam with electromagnetic waves that provide the acceleration.
This precise timing evokes the way a surfer in the ocean attempts to catch a cresting wave, Conway explained. “Out in the ocean, the waves are moving slowly and gathering speed, and the surfer has to paddle to be able to catch the wave right at its height to get the maximum acceleration,” he said. “If our particle beam arrives at a point on the electromagnetic wave that is too early or too late, it won’t get the proper amount of acceleration.”
Because there are roughly 25 total cryomodules in all, errors in timing — particularly early in the sequence where Argonne’s cavity will be installed — will eventually compound. “The region of the accelerator where our cavities will be deployed is the most sensitive to timing errors,” Conway said. “It’s crucial that we minimize them.”
By using the resonant nature of the cavities to concentrate the electric fields in the regions through which the beam passes, scientists can drive the beam to higher and higher energies until it reaches the target. “It’s important that these cavities are resonant because that property allows them to store energy and build up the strength of the fields to intensities that would otherwise not be possible,” Conway said.
To generate the electric field within the cryomodule, Conway and his colleagues use a series of devices called half-wave resonators. A resonator consists of two concentric niobium cylinders connected at either end and is designed to operate at 162.5 megahertz to generate a wave optimized for particles traveling at around 11 percent the speed of light.
Because the direction and magnitude of the electric field in the middle of the resonator oscillates 162.5 million times per second, getting the beam synchronized to the electric field is very important.
Conway emphasized the importance of the cryomodule staying on beat, as each cryostat must pass the beam precisely so in order to get the beam accelerated all the way to its target energy. “Errors in timing in our cryomodule accumulate as the beam propagates through the accelerator; this makes Fermilab’s job harder when trying to pass the beam onto other accelerators later in the accelerator complex,” Conway said.
The cryomodule itself operates at two degrees Kelvin, or approximately -456 degrees Fahrenheit. It is at these temperatures that the superconducting effect occurs. To keep the cryostat cold, Conway and his colleagues rely on an extremely powerful refrigerator and a very efficient cryomodule design that limits how much the -456 Fahrenheit structure is heated.
“An accumulation of three generations of superconducting radiofrequency physics knowledge and accelerator knowledge has gone into this. We’re taking the sum total of all our experience and putting it into a device to help Fermilab carry out groundbreaking new science.” —Zack Conway, Argonne accelerator physicist.
The development of the half-wave resonator technology and the introduction of superconducting radio frequency accelerating devices fundamentally shifts the possibilities for large-scale accelerators and experiments like DUNE, said Lia Merminga, who heads the PIP-II upgrade. “The creation of these half-wave resonators ushers in a new era for the Fermilab accelerator complex,” she said.
The research represents the culmination of development efforts that have been going on at Argonne since the 1970s. In 1978, superconducting cavities were used to boost the particle beam energy at the Argonne Tandem Linac Accelerator System. “An accumulation of three generations of superconducting radiofrequency physics knowledge and accelerator knowledge has gone into this,” Conway said. “We’re taking the sum total of all our experience and putting it into a device to help Fermilab carry out groundbreaking new science.”
While Conway and his team have optimized the cryostat parameters for the beam that Fermilab will want to generate, he and Merminga indicated that similar radiofrequency accelerator technologies could find use for a number of other projects as well.
“This is a stellar example of how DOE labs work together to execute major projects that involve technological aptitude that no single lab has by itself,” Merminga said. “By leveraging Argonne’s experience in half-wave resonator technology, Fermilab can help make its future a reality and provide the impetus for even more collaboration.”
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