Challenging magnets

Mike Lamm, head of the Magnet Systems Department and Mu2e Level 2 Manager for Solenoids, wrote this week’s column.

Mike Lamm

For the past 18 months, the Technical Division magnet program has been working on the development of several complex magnets for Mu2e (pronounced mew-2-e), one of the flagship experiments of Fermilab’s Intensity Frontier program. A few weeks ago, we achieved an important milestone when our detailed, conceptual design for the Mu2e magnets passed a three-day Director’s Technical Design Review of the entire project.

The Mu2e experiment will provide a strong test for beyond the Standard Model theories. Mu2e will look for the predicted but not-yet-observed direct conversion of a muon into an electron, a process known as charged lepton flavor violation. We know that all quarks can change flavor, such as a charm quark turning into an up quark, and we have recently learned that leptons without charge can change flavor too, such as a muon neutrino transforming into an electron neutrino. Hence we suspect that charged leptons such as muons might be able to likewise change flavor by directly converting into an electron. If they do, it will be a very rare process, and its discovery will require a special beamline and particle detector.

The Mu2e experiment will smash an intense beam of protons from Fermilab’s Booster accelerator into a gold target to produce lots of low-energy muons. A magnet known as the production solenoid will slow and collect these particles (see graphic). A transport solenoid will guide the muons through the S-shaped chicane that weeds out unwanted particles. Then the muons will be captured in an aluminum target. If and when a muon converts to an electron within the target, an electron detector within a detector solenoid will identify the emerging electron.

The production solenoid and detector solenoid resemble the superconducting solenoid magnets currently used in Tevatron and LHC experiments, but with additional requirements. The production solenoid must achieve 5 Tesla, or 100,000 times the earth’s magnetic field–the highest central magnetic field of any solenoid in particle physics. Its coils will experience 170 tons of force during operation, or the weight of four fully loaded 18-wheeler trucks. The detector solenoid will be comparable in diameter to the massive ATLAS central solenoid at the LHC, but will be longer, with a total length of more than 11 meters. It will store about the same amount of energy as the ATLAS solenoid, but will feature a more uniform magnetic field.

The transport solenoid will be like nothing else ever built. Because of its complex S-shape its superconducting coils will experience strong forces and torques that will pull in opposite directions when the adjacent coils are forced to power down during an operational hiccup known as a quench. This made its design very challenging.

With the detailed, conceptual design of the Mu2e magnets approved and almost complete, we are moving one step closer to building this experiment, and one step closer to a better understanding of our universe.