Electromagnetic design and performance of conduction cooled superconducting magnet for spoke resonator cryomodule for Proton Improvement Plan (PIP)-II

The Proton Improvement Plan (PIP)-II project is part of Fermilab’s upgrade of its proton accelerator complex, to provide a powerful, high-intensity proton beam to the laboratory’s upcoming research program. The project includes an 800 MeV superconducting (SC) linear accelerator (linac), with five flavours of cavities and cryomodules. The medium energy section of the linac contains two types of superconducting Single Spoke Resonator (SSR) RF cavities (SSR1 and SSR2), which are interleaved with strong solenoid focusing lenses. A unified design of the solenoid has been developed, with one solenoid design satisfying both SSR1 and SSR2 requirements. The integral focusing strength requirement of 4.5 T²m with a full width half maximum (FWHM) of 180 mm indicates the peak field strength ∼ 6.8 T in the magnet aperture, necessitating a superconducting design within the limits of NbTi as magnet wire strand. These are complex combined units that include one focusing solenoid with bucking coils to minimize fringe fields and four corrector coils each, with independent current leads to produce dipole and quadrupole fields. To simplify the current lead design and reduce complexity, the project opted conduction cooling for these magnets, thus requiring a redesign compared to previous prototype bath cooled units. Existing designs for high energy accelerators adopt bath cooled design of the solenoid focusing lenses for medium energy cryomodules. The present design explores a unique and technically superior solution for the cryomodule operation by decoupling the magnet and cavity cooling to certain extent. Reliability in cryomodule operations shall be studied after integration of magnets in the Linac beamline. Here we discuss the design requirements, challenges, electromagnetic design, superconducting wire selection and the results from magnetic measurements of the first pre-series units.