Liquid Deuterium Thermosyphon for an Ultracold Neutron Source

The TUCAN (TRIUMF Ultracold Advanced Neutron) EDM experiment seeks to measure the neutron electric dipole moment (EDM) with an uncertainty $\delta d_n=10^{-27}~e$cm. A new spallation-driven He-II ultracold neutron (UCN) source is developed at TRIUMF so that the goal statistical uncertainty can be reached. In the final layer of neutron moderation prior to UCN production, a liquid deuterium (LD$_2$) volume surrounds the He-II to efficiently moderate hotter spallation neutrons to the desired CN energies. The LD$_2$ moderator experiences a heat load of 60~W for the design proton beam current of 40~$\mu$A, and is cooled to 20~K using a distant cryocooler at higher elevation. This poster describes studies of the engineering design and performance of a natural circulation system (thermosyphon) used to provide cooling to the LD$_2$ volume near the hot spallation target. The thermosyphon features no moving parts and single-phase (liquid) operation. A key discovery made through these studies is that the thermosyphon will continue to flow despite the duty cycle from proton beam pulsing at minute-long timescales.