Photodetachment of H- at the GBAR Experiment

The GBAR experiment aims to measure the gravitational acceleration on antimatter, $\bar{g}.$ A crucial step to realizing this goal is the production of $\overline{\text{H}}^+$ atoms through the reaction $\overline{\text{H}} + \text{Ps} \rightarrow \overline{\text{H}}^+ + e^-,$ which does not have a precisely known cross-section. To measure this cross-section, GBAR seeks to measure the cross-section for the charge-conjugated reaction. GBAR is provided a source of H$^-$ ions from ELENA, so it is first necessary to photodetach these ions in-line before they interact with positronium. A neutral beam of H atoms will also be useful for beam diagnostics and alignment. The aim of my project was to experimentally realize the photodetachment of H$^-$ in the GBAR experiment. A theoretical model for the photodetachment was developed, and simulations of the photodetachment process were run to better understand how the photodetachment efficiency depends on the pulsed laser beam energy. Experimentally realizing the photodetachment of H$^-$ proceeded in three steps. First, the laser beam was aligned from the laser room to the reaction chamber. Then, a beam-block assembly was printed and assembled to absorb and detect the beam at the exit port of the reaction chamber. Lastly, the trigger timing for the pulsed laser flashlamp and Q-switch was tuned so that the laser beam arrives in the reaction chamber on the incoming H$^-$ ions. We observed the successful photodetachment of H$^-$ ions in the reaction chamber via an MCP detector located downstream in the beamline, achieving the goal set out at the start of the project.