A Multi-Cubic-Meter Ioffe Trap for Project 8

Alec Lindman

doi:10.1109/tasc.2020.2974173

Abstract

The Project 8 direct neutrino mass experiment requires a superconducting magnet of unusual scale and complexity. Project 8 will use atomic tritium held in a magnetic bottle of 10 to 100 m 3 to reach well beyond the sensitivity of the presentgeneration experiment. Project 8 is based on a new spectroscopic method, Cyclotron Radiation Emission Spectroscopy (CRES), that requires a highly uniform magnetic field in the fiducial volume at the center of the atom trap. The maximum fluctuation ΔB/B is nominally 10 -7 . To meet these requirements, we propose a modified version of a Ioffe trap to confine the neutral tritium atoms and provide a large fiducial volume for CRES. The key change from the traditional topology is to replace the circular mirror coils that close the trap ends with pleated closures formed from the same conductors as the rest of the trap. This dramatically expands the fiducial volume of the trap. CRES achieves its best sensitivity with a particular particle density; for the highest event rate, all of these particles should be tritium atoms. Cooling is required to remove the atoms' radioactive decay heat and prevent them evaporating from the trap. We leave the magnetic bottle uncapped and maintain an equilibrium atom density by continuously supplying new, cold atoms to replace the warmer ones lost via the loading opening. Pleated trap ends, for certain choices of the number of Ioffe conductors, naturally transition to an extended quadrupole through which we load the trap. For a trapped atom population of ~10 19 atoms at a density of ~10 12 cm -3 , a trap volume of tens of cubic meters is required. Previous magnets have not simultaneously achieved the combination of high field strength, high uniformity, and large volume required for Project 8.

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