Fermilab’s Proton Improvement Plan II, or PIP-II, will enable the world’s most intense neutrino beam and help scientists search for rare particle physics processes. These investigations will require intense beams of protons, which will produce gushers of other neutrinos that scientists can then study in greater detail.
The raw material for experiments at PIP-II is protons, lots of them, which are used to generate other types of particles for multiple experiments.
Protons are first emitted from a source and formed into a beam. The proton beam then speeds down a 250-meter superconducting linear accelerator, or linac, to an energy of 800 million electronvolts (or 800 megaelectronvolts, MeV). The PIP-II linac is situated on the infield of the (decommissioned) Tevatron accelerator on the Fermilab site. This siting takes advantage of existing cryogenic, electrical and water infrastructure.
Once it exits the 800-MeV linac, the proton beam is steered towards the existing Booster accelerator, where it is accelerated to 8 billion electronvolts (or gigaelectronvolts, GeV).
Some of the protons exiting the Booster will head directly toward a variety of targets, striking them. These will initiate strings of newly produced particles, of which some fraction eventually decay into muons. The muons will be captured within the MC-1 Building, right on the Fermilab site. There they will enter a detector, where scientists can make measurements of this short-lived particle.
The other protons exiting the Booster will take a different path, continuing down the accelerator chain. They will be transferred and accelerated within the existing Main Injector-Recycler complex — a set of 3.3-kilometer-circumference rings that will produce a beam of protons at an energy of 120 billion electronvolts (or 120 gigaelectronvolts, GeV). These protons then strike a target, eventually producing neutrinos. The neutrinos will then fly through the Earth at nearly light speed. Under the PIP-II scheme, they will be directed to the Long-Baseline Neutrino Facility experimental area, planned to be built at Homestake, South Dakota, 1,300 kilometers away. The LBNF detectors will help researchers better understand the behavior of neutrinos, which are notoriously difficult to observe because of their flighty nature.
Read the PIP-II white paper.
Superconducting radio-frequency technology
At the heart of the PIP-II accelerator is a technology that provides for a highly efficient way to accelerate particle beams. Superconducting radio-frequency (SRF) cavities make it possible to accelerate intense proton beams to higher energies in relatively short distances.
PIP-II SRF cavities come in a number of shapes and sizes, but the engineering principle of particle acceleration is the same for all of them.
Cavities are highly polished, perfectly shaped niobium structures whose task is to generate electric fields that propel the particle beam forward. As a superconducting metal, niobium can generate these electric fields without creating wasted heat, as would be generated if one were to use a normal-conducting metal such as copper. So long as the niobium’s temperature is kept to a few kelvin —a few degrees above absolute zero – it can accelerate particles with supreme efficiency.
A string comprising several of these cavities nestles in a vessel called a cryomodule, which bathes them in liquid helium and keeps them at the ultracold temperature that is key to their operation and efficiency.
Cavities are constructed from one or more cells, compartments that enclose one cycle of an oscillating electric field. Cells can be strung together to form cavities. Their number and shape depend on acceleration requirements.
The electric field runs down the center of the single-cell and multicell cavities. It oscillates between positive and negative, swelling to a peak and sinking to a valley within the space of a single cell; it is as if each cell rapidly switches between a positive and negative charge.
The cycles are timed to kick charged particles riding the wave from cell to cell. Each time a positively charged proton enters a cell, the cell’s charge changes to negative, which attracts the proton. As the proton leaves the cell, the cell’s charge changes to positive and pushes the proton forward. Traversing the next cell, the proton is propelled in the same fashion. This process continues until the particle has shot all the way through the accelerator.