Aug 21 2019
The U.S. Department of Energy’s (DOE) Argonne National Laboratory has developed an exclusive superconducting cavity, further advancing the world’s biggest neutrino experiment.
Argonne engineer Bill Jansma works on a cryomodule that Argonne has delivered to Fermilab as part of Fermilab’s Proton Improvement Plan-II (PIP-II) accelerator project. The cryomodule will allow PIP-II to provide accelerated protons that will generate neutrinos for Fermilab’s Deep Underground Neutrino Experiment. (Image credit: Argonne National Laboratory)
Argonne has collaborated with DOE’s Fermi National Accelerator Laboratory and is providing the first eight of 116 superconducting cavities that will be employed for the acceleration of hydrogen ions to produce a neutrino stream for the Deep Underground Neutrino Experiment (DUNE).
Although this experiment is carried out at Fermilab, it involves international collaboration and seeks to gain insights into the basic properties of neutrinos—ghost-like particles that have very little mass and interact with normal matter only very slightly.
The superconducting cavities will be combined to form a single cryomodule and installed as part of Fermilab’s Proton Improvement Plan-II (PIP-II), a new upgrade to the Fermilab accelerator complex. The installation will possibly be done this fall.
The way the cryomodule works is by starting at low energies. 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.
Zack Conway, Accelerator Physicist, Argonne National Laboratory
The process commences upstream of the cryomodule at Argonne, in an ion source that produces hydrogen atoms with an additional electron. Inside the cryomodule, researchers synchronize the particle beam using electromagnetic waves that induce the acceleration.
This accurate timing elicits the way a surfer in the ocean tries to get hold of a cresting wave, stated Conway. “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 added. “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.”
Since there are approximately 25 cryomodules in total, flaws in timing—specifically early in the sequence in which Argonne’s cavity will be installed—will compound finally. “The region of the accelerator where our cavities will be deployed is the most sensitive to timing errors,” stated Conway. “It’s crucial that we minimize them.”
Researchers can use the cavities’ resonant nature to concentrate the electric fields in the areas through which the beam travels and thrust the beam to increasingly 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,” stated Conway.
Conway and his team used a range of devices known as half-wave resonators to produce the electric field inside the cryomodule. A resonator includes two concentric niobium cylinders attached to either end and is engineered to function at 162.5 MHz to produce a wave optimized for particles moving at about 11% the speed of light.
Since the magnitude and direction of the electric field at the center of the resonator oscillates at a rate of 162.5 million times per second, it is highly crucial to make the beam synchronized to the electric field.
Conway reiterated the significance of the cryomodule to stay on beat since it is necessary for each cryostat to pass the beam accurately 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,” stated Conway.
The cryomodule itself works at a temperature of 2 K, or about −456 °F. The superconducting effect emerges exactly at these temperatures. Conway and his group maintain the cryostat cold by relying on an extremely robust refrigerator and a highly effective cryomodule design that restricts the extent to which the −456 °F structure is heated.
According to Lia Merminga, who leads the PIP-II upgrade, the creation of the half-wave resonator technology and the launch of superconducting radio-frequency accelerating devices essentially shifts the possibilities for large-scale accelerators and experiments such as DUNE. “The creation of these half-wave resonators ushers in a new era for the Fermilab accelerator complex,” she noted.
The study is the culmination of development work ongoing at Argonne from the 1970s. In 1978, superconducting cavities were employed to increase 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. 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, Accelerator Physicist, Argonne National Laboratory
Although Conway and his colleagues have optimized the cryostat parameters for the beam that Fermilab will intend to produce, he and Merminga suggested that analogous radiofrequency accelerator technologies could be applied for several other projects.
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. 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.
Lia Merminga, Head of The Proton Improvement Plan-II, Fermilab