Design and Operation of the SpinQuest Polarized Target System

The 120 GeV proton beam from the Fermilab Main Injector strikes the transversely polarized solid ammonia (NH₃) target, which is maintained at 1K under a 5T magnetic field, to investigate the transverse spin structure of the nucleon. The target material is loaded into an oval-shaped, 8 cm-long target cell, which is the longest target cell used in a 1 K polarized target system, enabling the highest instantaneous luminosity achieved on a solid polarized target. The superconducting split-pair magnet generates a 5T homogeneous magnetic field surrounding the target material. It is configured with helium bath pumping capability, which enables it to withstand the thermal loads induced by the high-intensity proton beam and increase the potential operating maximum intensity before reaching the superconducting magnet critical temperature. A helium-4 evaporation refrigerator provides the necessary cooling power during both the dynamic nuclear polarization (DNP) process and the beam–target interaction. A high-capacity roots pump stack, with a pumping speed of 17,000 m³/h is connected to the refrigerator and delivers approximately 5W of cooling power at 1.1K, making it one of the highest-performing helium evaporation refrigerators. The Extended Interaction Oscillator (EIO) generates microwaves at approximately 140 GHz, which are guided through a series of waveguides and directed toward the target material to enhance the polarization during the DNP process. The helium vapor pressure and the NMR signal area from thermal equilibrium measurements are analyzed to extract the offline calibration constant, which is then used to evaluate the updated polarization of the target material during production runs. In this talk, an overview of the polarized target setup and an outline of the polarization data analysis methodology will be presented.