The KM3NeT observatory hosts two undersea neutrino telescopes, ARCA and ORCA, located at two abyssal sites of the Mediterranean sea. The detectors consist of a 3D array of optical modules, each housing 31 3-inch photomultiplier tubes to detect the Cherenkov light emitted by charged particles produced in neutrino interactions in water. Although still under construction, both detectors are already in operation and use the same data acquisition model in compliance with a triggerless-streaming readout approach. In this architecture all the data collected by the optical modules are transmitted to shore, where online processes running on dedicated resources filter and record the relevant data for physics analyses and calibration procedures. To accomplish the target scientific goals, stringent constraints on the precision of the position and timing of the modules are set. In particular the clock distribution must provide a nanosecond synchronisation of the modules which are tens of kilometers away from the on-shore clock references and occupy a large volume that, in the case of ARCA, will reach the cubic kilometer scale. This requirement is met by exploiting the White Rabbit technology. During the initial phase of construction of KM3NeT, the data acquisition system was based on a custom White Rabbit implementation that deviates significantly from the standard design. This architecture concerns the first part (Phase 1) of ARCA and it will be used for the complete construction of ORCA. Recently the submarine architecture of the ARCA telescope was significantly revised to accomplish a mandatory optimisation necessary to scale it to the cubic kilometer size. In this new scenario, which is referred to as the Phase 2 of ARCA construction, it was possible to revise also the implementation of the White Rabbit technology for the experiment, aiming at a standard design. In this contribution we review the evolution of the ARCA data acquisition system from Phase 1 to Phase 2, focussing on the new design, its implementation with the new detector components that were installed and are operational since the fall of 2024, and the integration of the Phase 1 and Phase 2 sector.
Gualandi et al. (Wed,) studied this question.