The High-Luminosity LHC (HL-LHC) is planned to start the operation in 2026 with an instantaneous luminosity of 7.5 x 1034 cm-2s-1. To cope with the event rate higher than that of LHC, the trigger and readout system of ATLAS will be upgraded based on the first-level trigger with a higher rate of 1 MHz and a longer latency of 6 us. The change in the trigger and readout system requires the replacement of the electronics for the Thin Gap Chamber (TGC), which plays a primary role for the first-level muon trigger in the endcap regions. The new design assumes the transfer of all hit data from the frontend to the backend boards, where only selected data are transferred in the current system. The current coincidence-based trigger will be upgraded by a tracking-based trigger to improve the transverse momentum resolution at the first-level trigger.
We have developed the first prototype frontend board of the TGC for the HL-LHC, which has a transceiver to send all hit data of 256 channels with a 16 Gbps bandwidth based on the optical transceivers. The data transfer was tested with pseudorandom numbers, and the bit error rate was less than 8.9 x 10-15. The prototype includes the controller of the threshold voltage for the discriminator, which is also essential for the TGC frontend board. Linear relation between the set and the measured voltages was obtained in a range from -300 mV to 300 mV, which covers the operation range of the current system (from -250 mV to 250 mV). The prototype has been mounted on the spare TGCs for ATLAS, and the data transfer has been demonstrated with the charged particle beam at the CERN SPS beam facility. The single-hit efficiency in the bulk region of the chamber was obtained to be ~99%, which indicates that the data transfer works without problems. The investigation of the radiation tolerance for the elements on the prototype, e.g FPGA, is the next major step. R&D is also ongoing for the backend board. Segment reconstruction with a software-based minimum chi-square method showed a potential of the TGC to provide the segment angle resolution of 3.5 mrad. As a result, the rate for a single muon trigger with a transverse momentum threshold of 15 GeV may be reduced by 30%. The algorithm of the segment reconstruction suited for the implementation in the hardware is under study.