Cryogenics Takes Center Stage for PIP-II at Fermi National Accelerator Laboratory

Construction of the Proton Improvement Plan II (PIP-II) is underway at Fermi National Laboratory (CSA CSM) in Batavia IL. Since the groundbreaking ceremony on March 15, the laboratory team has been preparing for the beam that will contribute to international high energy physics research experiments. The superconducting radio-frequency (SRF)-powered linear accelerator will also power the lab’s flagship project, the Deep Underground Neutrino Experiment (DUNE). It will send trillions of neutrino particles 800 miles through the earth to the four-story high detector that’s part of the Long-Baseline Neutrino Facility (LBNF) currently being built a mile beneath the surface at the Sanford Underground Research Facility in Lead SD. The Mu2e and Muon g-2 experiments, along with DUNE/LBNF, will also rely on PIP-II as the source for beam.

At the start of PIP-II, there is a source of negative hydrogen ions (H-) that are formed into a beam, which is sent down a 700-foot long, superconducting linear accelerator (linac) where it reaches an energy level of 800 MeV, or 800 million electron volts.

Once the H- beam exits the linac it is steered toward the existing booster accelerator, where the ions are stripped of their two electrons leaving only their protons to advance. The beam of protons is then accelerated to 8 GeV, or 8 billion electron volts. From there, the protons will be sent toward various targets; striking them will initiate strings of newly produced particles, some of which will eventually decay into muons. These short-lived particles will be captured by a detector within the MC-1 building at Fermilab.

Other protons exiting the booster will be steered down a different path in the accelerator chain to the existing Main Injector-Recycler complex, a set of rings two miles in circumference. There, the protons will be accelerated to an energy of 120 GeV before they strike a target to produce neutrinos. The neutrinos will travel 800 miles at nearly the speed of light to the Sanford LBNF facility where scientists will study their behavior.

With the completion of Phase 1, the civil engineering preparation, Fermilab is about to begin Phase 2, the civil construction. This phase includes the cryogenic plant building.

Fermilab PIP-II Project Director Dr. Lia Merminga recently spoke with CSA about the project.

She notes that the first step is the construction of the cryogenic building. “It is a crucial building. This building is going to house the cold box, cold compressors and facility utilities used for the entire accelerator. The procurement of the cryo plant is an in-kind contribution from our colleagues at the India Department of Atomic Energy. A bid to design, manufacture and install the cryogenic equipment and components from Air Liquide (CSA CSM) has been accepted by the Indian government,” says Merminga. “The building must be ready for the delivery of this in-kind contribution.” The cryogenic building is scheduled for a review with the US Department of Energy in early 2020, which will mark the beginning of building construction.

Meanwhile, the first cryomodule containing the half wave resonators (HWR) for PIP-II arrived from Argonne National Laboratory (CSA CSM) in Lemont IL in August. “It’s a very exciting time. Our colleagues at Argonne did a fantastic job and delivered an impressive cryomodule; their team has the equivalent of three generations of expertise behind it,” she says. “It was delivered to the Industrial Center Building because we needed to do some alignment work, quality assurance and some small repairs. We’re in the process of doing this now. When we’re done, that cryomodule will be transported to the Cryomodule Test Facility Building where the PIP-II injector test facility is located.”

The PIP-II injector test facility, referred to as PIP2IT, comprises the warm front end of PIP-II and a series of magnets for the beam. It is being prepared to receive the HWR from Argonne. The stands are already put in place and cables have been pulled. The wave guides for the RF distribution system are being installed and the RF controls for the fields inside the resonators of the cryomodule’s half wave resonators are being prepared as well. “There is a tremendous amount of activity at the PIP2IT,” says Merminga.

The HWR cryomodule will move to PIP2IT in the middle of October. “It won’t go to its final location first; it will go a little bit further out where the couplers will be installed in each of the eight cavities,” Merminga clarified. “The couplers are the devices that transfer RF power from the solid-state amplifier power sources, one for each cavity, into the cavity to establish the electromagnetic fields inside each resonator. They are very delicate, so they will be installed last.”

At the same time, the cryogenic system is being tied into the half-kilowatt cryogenic box at PIP2IT for other tests. The cold box operates at 2 K and half a kilowatt. “After the connections are made, we’ll start cooling down the cryomodule for the first time. We’re very excited to see how it performs at 2 K,” notes Merminga. “Once we test the heat load of each cavity and debug any issues, the next step is to turn on RF in the cavities and establish the fields. We’ll test and characterize any noise produced, then we’ll be ready to turn on beam from the gun through the warm front end into the HWR. T¬0 for ‘beam on’ is April 2020.” The beam will go into the cavity in pulses. The cavity’s response to the beam is being measured giving the scientists a chance to compare these measurements with calculations. The beam tests will run for six months.

In parallel, the team at Fermilab is working on the second cryomodule, which consists of single-spoke resonators. This device is being built at Fermilab. It will be installed after the HWR cryomodule sometime in November to be ready for the April 2020 beam test. “We will only have these two of the 23 planned cryomodules in the beam test,” says Merminga. “The other 21 will only be cryogenically and RF, but not beam, tested. The beam test is more of a proof of concept for the two types of cryomodules.”

Genfa Wu is the Fermilab physicist in charge of the SRF and cryogenics systems for PIP-II. The manager for the cryogenic systems is Ben Hansen. Merminga notes, “Both Genfa and Ben are incredibly knowledgeable, very collaborative and level headed. It’s great to work with them.”

With the 60% beam power increase enabled by PIP-II, Merminga is excited about the neutrino research the experiment will make possible. “PIP-II epitomizes the state-of-the-art SRF technology that will lead research in the future. This will enable another 50 years of particle research at Fermilab, which is very exciting. Also, this is the accelerator that will provide the protons that make the neutrinos that will be detected both at Fermilab and Sanford. These experiments will improve our understanding of the universe including why it’s made of matter, rather than antimatter. These are questions that have yet to be answered. To me, this experiment is investigating the most fundamental question: ‘Why are we here?’” ■