Fig. 1: Overview of the cryogenic storage ring (CSR), the DR process, and the effect of internal cooling of D₂H⁺.

a Schematic of the CSR with the electron cooler, interaction zone, and neutral particle detector locations noted. The CSR features a nested vacuum structure, with a large outer cryostat and inner ultra-high vacuum chambers that are cooled by liquid helium to temperatures below 6 K. b Schematic of the recombination event, showing the molecular break-up and detection of the neutral fragments. c Upper panel: Stick diagram of the estimated initial population distribution of the rovibrational states of D₂H⁺. Assuming a temperature of 3000 K, the population is dispersed over hundreds of individual states. Lower panel: Simulated populations for D₂H⁺ after storage of 1000 s inside the cryogenic vacuum of the CSR, with a combined  ~80% of the population ending up in the two ground states of the respective nuclear spin manifolds (0₀₀ for ortho-D₂H⁺, and 1₀₁ for para-D₂H⁺).