Simulating Positive Ion Distribution in the ProtoDUNE Single Phase Detector

This work covers the validation and implementation of a model to solve for argon ion (Ar+) distribution in the ProtoDUNE-SP liquid argon time projection chamber (LArTPC). ProtoDUNE-SP undergoes a constant flux of cosmic ray muons due to surface operation. The performance of the ProtoDUNE-SP provided valuable insights into single-phase technology, membrane cryostat technology, and calibration of cryogenic instrumentation. Still, challenges in achieving precise measurements persist due to the space charge effect—the distortion of the electric field within the detector due to the accumulation of positive argon ions generated from constant cosmic muon flux. The overarching goal of this thesis is to model the movement and distribution of Ar+ within the detector using computational fluid dynamics software (CFD) Simcenter STAR-CCM+. To achieve this goal, a representative simulation of the detector was created. The simulation incorporates the nominal electric field between the anode plane assemblies and the cathode plane assembly through a linearly varying electric potential maintained along the field cage, and an electrochemical species source term to model the argon ion generation rate in the active volume. The simulation predicted the Ar+ velocities as a sum of both Ar+ drift and liquid argon (LAr) velocity vectors, charge densities, and electric field distortions. These space charge density and electric field maps can be used to inform methods to mitigate offsets in track reconstruction. Through comparison with analytical and simple numerical models from the reported literature, the model is validated. Once validated, this modeling approach is extended to the ProtoDUNE-SP detector. Specifically, a steady-state CFD model of the ProtoDUNE-SP detector is simulated using simplified detector geometry. The LAr velocity generated from this simulation is used as the basis for the transient simulation of argon ion movement and electric field distortions in the detector. This approach showed the charge density, magnitude of the electric field, and combined ion velocities in x, y, and z coordinates throughout the active volume of the detector. This data can be used to inform corrections to the electric field within the detector and account for space charge in areas where the electric field is strongly distorted.