Tests and calibration of detector strings for the KM3NeT/ARCA neutrino telescope

KM3NeT is a network of underwater Cherenkov neutrino telescopes currently under construction at two sites in the Mediterranean Sea. ARCA, located offshore the Sicilian Coast (Italy), is optimized for the detection of high energy cosmic neutrinos, while ORCA situated off the coast of Toulon (France), is designed for studying atmospheric neutrinos. Both detectors consist of vertical string-like detection units, each comprising 18 optical modules. Each optical module houses 31 3-inch photomultiplier tubes and electronics for control and power supply, an acoustic sensor, tiltmeter-compasses and readout and data transmission. The data are acquired according to a triggerless streaming readout scheme, where the optical modules act as underwater nodes transmitting all data and communicating with the control station on shore via a network of optical fibers. To optimise event direction reconstruction, the optical modules are synchronized to O(∼ 1) ns accuracy using the White Rabbit time synchronization protocol, while their locations underwater are known with accuracy O(∼20) cm at any time using a custom acoustic positioning system. Following the first phase of construction, which relied on a KM3NeT customized version of the White Rabbit protocol, a new network architecture following the standard White Rabbit protocol has been implemented to improve the maintainability and scalability of the system. In this contribution the setup, procedure and protocols adopted to test the ARCA detection units under standard White Rabbit configuration is described. Specifically, the system of instrumentation for optical, acoustic and data readout systems and calibration measurements in the final stage of integration of a detection unit are presented. With this system the detection units are validated under operating conditions prior to their installation at the sea bed.