FELIX: readout upgrade for the ATLAS Trigger DAQ system in HL-LHC

The High Luminosity LHC (HL-LHC) will start to operate in 2029 (Run 4) with an instantaneous luminosities five times higher than the LHC nominal value, where the ATLAS experiment will be operated in an increasingly harsh collision environment. This has motivated a series of upgrades, to be installed during the long experimental shutdown period from 2026 to 2029. One key change among these is the upgrade of the Front-End Link eXchange (FELIX) system, which was developed for the third major LHC data taking period (Run 3) from 2022 to 2025 to improve the capacity and the flexibility of the detector readout system for selected ATLAS systems. After the HL-LHC upgrade, all ATLAS systems will be read out via the FELIX system, with sub-detector specific processing taking place in a common software processing framework. One major upgrade challenge for the system in Run 4 is to support the trigger rate increase from 100 kHz in Run 3 to 1 MHz in Run 4, along with an overall increase in event size. This presentation will cover the FELIX design for Run 4, as well as the results of a series of performance tests carried out with Run 3 hardware at Run 4 rates, which will inform the next stages of development.