Description of and Results from Testing a Serial Powering Chain of Irradiated RD53A Modules

The High-Luminosity LHC (HL-LHC) with its large number of collisions per proton-proton bunch crossing every 25 ns will present a challenging environment for particle detectors. The ATLAS Upgrade Program foresees the complete replacement of the current tracking system with a full-silicon Inner Tracker (ITk) made of a pixel and a strip subdetector. Due to the increased power density in the pixel detector, a serial powering scheme has been chosen for the ITk Pixel Detector. In this scheme, the readout chips of the pixel modules are powered in series; the sensors of several modules on the other hand will be connected in parallel to a common supply line for the depletion voltage. In combination with the ITk grounding requirements and the properties of most commercially available power supplies this architecture results in an effective forward bias on some sensors under certain operating conditions. Although the forward bias will be small, it can still lead to non negligible currents between the sensor backside and the readout chip, in particular for irradiated sensors with large saturation currents. This document presents the findings of a study that was conducted to verify that this behaviour of a serial powering chain does not trigger any damaging effects for neither the sensor nor the readout chips. The results indicate that a potentially costly mitigation strategy is not required.