The ATLAS Strip Detector System for the High Luminosity LHC

The ATLAS experiment at the Large Hadron Collider (LHC) is currently preparing for major detector upgrades for the Phase-II of the LHC operation, scheduled to start in 2026. In order to achieve an integrated luminosity up to 4000 fb^-1 within the High Luminosity LHC (HL-LHC) phase, the instantaneous luminosity is expected to reach unprecedented values. To cope with the expected radiation damage and high density of tracks per bunch crossing, a complete replacement of the existing Inner Detector of ATLAS is required.An all-silicon Inner Tracker (ITk) is under development with a pixel detector subsystem surrounded by the strip detector subsystem at larger radii, aiming to provide an increased tracking coverage up to pseudorapidities of four.In this report an overview of the strip detector system design of the ITk is given as well as the results from an extensive prototyping effort are presented. The main focus is on recent testbeam results of the validation of the performance of un-irradiated and irradiated strip detector modules. These are able to reach the specified performance requirements in the HL-LHC environment. The next step of the ITk project is the transition to the production phase to provide a functional detector by 2026.