Cool journey to the center of the Earth

Patience and complexity are the hallmarks of fundamental scientific research. It takes time to do what we do at the Department of Energy (DOE) Office of Science.

Case in point: Technical staff at the DOE’s Fermi National Accelerator Laboratory have built a prototype of a superconducting cryomodule for the Proton Improvement Plan II (PIP-II) project.

Four of these 39-foot-long vessels, which weigh an astonishing 27,500 pounds each, will be responsible for accelerating hydrogen ions to more than 80% of the speed of light. Ultimately, the cryomodules will comprise the last section of the new linear accelerator, or linac, that will drive Fermilab’s accelerator complex.

Physicists like to accelerate particles to higher and higher energies. The higher the energy, the more finely penetrating and discriminating a particle probe can be. That increased precision allows scientists to study the tiniest of structures.

There are many benefits of faster and faster accelerators. To name a few: destroying cancer cells; revealing the structure of proteins and viruses; creating vaccines and new drugs; and advancing our knowledge of the origins of our universe.

For the PIP-II linac, each superconducting cryomodule vessel will contain a chain of devices called “cavities” at its core. These cavities look like oversized soda cans stacked end-to-end. They’re made of pure niobium, a superconducting material. Electricity flows through the superconducting material with no energy loss when the niobium is kept well below the average temperature of outer space.

Note the snippet “cryo” in the word cryomodule, meaning involving or producing cold. Especially extreme cold. In order to reach superconducting state, the cavities need to be kept at super-cold temperatures, hovering around absolute zero.

To keep things cool, the team fills the inside of the vessel with liquid helium. The vessel has many layers of insulation to protect the cavities from outside temperatures that are too warm.

Once the prototype is functioning properly, four of the modules will be assembled to build out the last section of Fermilab’s new linear accelerator. 

Here’s how the journey will unfold. The superconducting cryomodules will power beams of hydrogen anions, which are hydrogen atoms made up of one proton and two electrons, instead of the usual one proton and one electron.

The beams will reach a final energy of 800 million electronvolts, or MeV, before they exit the accelerator.

From there, the beam will transfer to the upgraded Booster and Main Injector accelerators. There it will gain more energy before being turned into neutrinos.  

The machine will then send these neutrinos on a 1,300-kilometer journey (800 miles) through Earth to the Deep Underground Neutrino Experiment (DUNE) at the Long Baseline Neutrino Facility in Lead, South Dakota.

The team is now making sure that all the preparations have paid off as the modules are tested at Fermilab’s Cryomodule Test Facility. This will reveal how well the modules function after practice shipments between Fermilab and the United Kingdom.

The final modules will be built by PIP-II’s partners around the world. Three will come together at Daresbury Laboratory, run by the Science and Technology Facilities Council of United Kingdom Research and Innovation, and shipped to Fermilab.  

The fourth will be assembled at Fermilab using components provided by the Raja Rammana Centre for Advanced Technology of India’s Department of Atomic Energy.

International partners from India, Italy, France, Poland and the United Kingdom are contributing to many aspects of the PIP-II project.

All of this work is done as part of the PIP-II project, an essential enhancement to the Fermilab accelerator complex. PIP-II will provide neutrinos for DUNE scientists to study. In parallel, the high-power proton beams delivered by the PIP-II accelerator will enable muon-based experiments to search for new particles and forces at unprecedented levels of precision. The diverse physics program is powering new discoveries for decades to come.

This research was funded by the Department of Energy Office of Science and international partners.