The KM3NeT4RR project in Bologna

KM3NeT4RR is a project for the Kilometre Cube Neutrino Telescope (KM3NeT), which is a large European research infrastructure composed of two underwater large-scale neutrino telescopes, ARCA and ORCA, located in the Mediterranean Sea off-shore Portopalo di Capo Passero (Sicily, Italy) and Toulon (Provence, France) respectively. The telescopes are mainly designed for studying cosmic neutrinos and neutrino properties, but the observatory infrastructure also offers opportunity for geological and marine sciences research, providing instrumentation connections for long-term, high-bandwidth and continuous data collection. The KM3NeT4RR project, led by the Italian Istituto Nazionale di Fisica Nucleare, has started at the end of 2022 and during the following 30 months it includes a set of activities aimed at extending the ARCA seafloor network and the Italian on-shore facilities, building and operating an additional significant number of detection elements, thus significantly advancing the final completion of the infrastructure. Within this project, the KM3NeT laboratories present at the INFN-Bologna section are going to be expanded and a new setup will be created that will serve for both the detectors calibration studies and for deep-sea environmental research.


Introduction
The ARCA and ORCA underwater neutrino detectors [1] consist of large-volume three-dimensional arrays of Digital Optical Modules (DOMs) [2] hosting photomultipliers tubes (PMTs) and are placed on the Mediterranean seabed at a depth of ∼3500 m offshore Portopalo di Capo Passero (Sicily, Italy) for ARCA and of ∼2450 m offshore Toulon (Provence, France) for ORCA.DOMs are positioned along and sustained by vertically aligned Detection Units (DUs), each hosting 18 DOMs.Currently, ARCA and ORCA are equipped with 28 DUs and 18 DUs respectively, and in their final configurations they will have 230 DUs and 115 DUs.Each DOM contains 31 3-inch PMTs, calibration and positioning instrumentation including a light emitter and readout electronic boards named Central Logic Boards (CLBs).The CLB is responsible for the data acquisition from DOM PMTs and for the data transfer to the shore-stations where they are processed.Each DU is equipped with a top buoy to provide vertical support and is anchored to the seafloor with a heavy anchor frame which also incorporates the DU Base Module and calibration devices such as hydrophones, lasers, acoustic beacons.The DU Base Module hosts the electronics for powering the DU and a CLB for instrument control and data readout.DUs are connected to submarine electrical and optical fiber distribution systems, called Junction-Boxes (JBs) in ARCA and Nodes in ORCA, which distribute power and data from the main electro-optical cables to the DUs and vice-versa.JBs are also endowed with hydrophones, lasers and acoustic beacons.While both ARCA and ORCA feature the same detection elements, they have distinct layouts designed to suit their respective scientific objectives; in particular, ARCA is designed to study cosmic neutrinos from the TeV to PeV energy scale and their possible astrophysical sources while ORCA is optimised for the detection of atmospheric neutrinos at GeV energy scale for oscillation studies.The PMT array detects Cherenkov light emitted when relativistic charged particles traverse the detector volume.The recorded data is then utilised to determine the direction and energy of the incoming neutrino, which is responsible for generating these particles.To achieve an angular resolution for reconstructed tracks smaller than 1 • , detector active elements must be synchronised with nanosecond accuracy [3], and their exact location determined with an accuracy < 20 cm [4].The former is obtained by means of the CERN White Rabbit time synchronization protocol [5] implemented with White Rabbit switches on-shore and with a dedicated White Rabbit core on the -1 -FPGA of the CLBs off-shore, and the latter via an acoustic positioning system based on the acoustic emitters and receivers located on the seafloor network and on DOMs.

KM3NeT Data Acquisition System
For the first 31 ARCA DUs (and the Calibration Unit, see section 3) and 48 ORCA DUs, the detector Data Acquisition (DAQ) network follows an asymmetrical topology, referred to as the "Broadcast" scenario.In the "Broadcast" scenario, the shore-station communicates with the off-shore detector with a single downlink channel, whereas every CLB exploits one direct uplink to transmit data to the shore-station resources [6].To cope with the asymmetric design of the network, the firmware of the White Rabbit switches used in the shore-station and of the CLB has been customised, diverting from the standard.In fact, only the DU Base Module CLBs are in a true, even though not pure standard, Master-Slave relation with the White Rabbit switches on shore, while the DOM CLBs ignore the incoming synchronization White Rabbit stream and do not perform the packets exchange of the synchronization protocol, sending back to shore only the rest of the data streams.
On the shore-station, the incoming optical and acoustic data streams are routed to the so called Trigger and DataAcquisition System, a multi-stage processing chain realised with C++ software applications [7].In this processing chain, specialized processes gather and filter incoming optical raw data by applying various causality-trigger algorithms to identify candidate tracks of neutrino-induced muons or shower-like events.Acoustic data instead are used for the reconstruction of the time of arrival of signals emitted by the beacons, recognized by their different sound-waveforms.The times of arrival are then analyzed for the offline positioning of the detector elements.Finally, another process is responsible for writing the incoming stream of selected data into ROOT files on a local storage system.In addition, two copies of each file are secured in two different remote repositories: CNAF-INFN, in Italy, and CC-Lyon, in France.The coordination of all Trigger and DataAcquisition System processes with the detector is facilitated by a central software component known as the Control Unit.This DAQ system is designed to scale with the increasing number of deployed DUs in order to keep the pace with the increasing raw data throughput from off-shore and the complexity of the filtering algorithms.This is achieved by increasing the number of due TriDAS processes, distributing in an automatized way their replicas on servers in the shore-station when additional resource allocation is needed.The collaboration has developed a monitoring system, consisting of a collection of Python-based web applications showing the activity status of the several detector components (DOMs and Base Modules), plots of the trigger algorithms efficiency, the CLBs synchronisation status and plots for monitoring the quality of the data taking.The very same framework is also exploited in the DAQ stations of the integration and test sites of the collaboration.
Due to the limitation on the available number of fibers inside the submarine electro-optical cables which connect the shore-stations with the off-shore infrastructures, the "Broadcast" network topology is not scalable to the design goal of the ARCA and ORCA detector configurations.The limit will be reached at 31 DUs in ARCA and 48 DUs in ORCA.Then, a different network implementation will take over referred to as "full White Rabbit".In this scenario the DU Base Module has been re-designed in order to include two customized versions of White Rabbit switches (only the backplane differs from the standard one) that act as aggregation layers for the fibers coming from the DOMs of the DU.From each of the switches, a single 1Gbps uplink transmits DU data to shore.The -2 -abundance of ports on the switches also allows cold redundancies of DOM connections that can be exploited in case of failures.With this kind of network, each DU is going to have only two fiber uplinks instead of the 19 (for the Base Module CLB and the DOMs CLBs), allowing a reduction of a factor ten in the number of fibers inside electro-optical cables.
Moreover, this implied a change of paradigm concerning the time synchronization of DOMs: the presence of the two White Rabbit switches inside the DU Base Module allows the DU CLBs to become pure standard White Rabbit Slave units, with a new dedicated firmware and a simplified time-calibration procedure.On-shore, the new White Rabbit switch fabric will also use standard firmware, thus benefitting from the support by the international White Rabbit community.The first ARCA DUs implementing this design are currently under production and are going to be deployed in 2024.Eventually, data from deployed "Broadcast" DUs and "full White Rabbit" DUs will be merged and combined in the TriDAS processes, contributing together to the detector data taking.

KM3NeT Calibration Units
The accuracy of the positioning system can be improved by characterizing sound propagation in the detector medium and measuring the current speed and direction along the water column.To do so, KM3NeT will deploy dedicated systems called Calibration Units [8,9].The Calibration Unit is composed of two structures, the Calibration Base and the Instrumentation Unit, connected by a an electro-optical inter-link cable (see figure 1).The Calibration Base will be connected to a Junction Box/Node for power and communication with the shore-station.Its anchor frame hosts an acoustic beacon and a hydrophone (and a laser beacon in the ORCA Calibration Base, -3 -used for in-situ time calibration), contributing to the acoustic positioning system.They are connected to a Base Module which contains the electronics for managing the power and communication both with the shore-station and with the Instrumentation Unit.The Base Module is a titanium pressure vessel identical to a DU Base Module and hosts a CLB board for communication with the shore-station and with Instrumentation Unit sensors, and a power supply board to convert and deliver the high-voltage power provided by the Junction Box/Node to the CLB and to instrumentation.

16th Topical Seminar on Innovative Particle and Radiatio
The Instrumentation Unit is composed of a recoverable anchor frame and a 750m-long inductive line, kept vertical by a top buoy, that provides support for the oceanographic sensors and acts as transmission medium for the inductive transducers used to communicate with the sensors.The frame hosts a pressure gauge and an inductive modem module to allow communication between the CLB inside the Base Module on the Calibration Base and the sensors on the inductive line.Sensors that are not native inductive are connected to serial inductive modems.The sensors acquire data regarding sea water properties, and are therefore placed on the full length of the line in order to characterize the medium at the depth of the detector elements.Instrumentation Units host sound velocimeters, Doppler current sensors, and Conductivity-Temperature-Depth recorders which perform indirect measurements of the sound velocity in water.
The ORCA Calibration Unit is foreseen to be deployed at the end of 2023, while the ARCA Calibration Unit in 2024.

The KM3NeT4RR project in Bologna
KM3NeT4RR [10] is a project led by the Italian Istituto Nazionale di Fisica Nucleare (INFN) and funded by the NextGenerationEU investments, aimed at upgrading and expanding the ARCA sub-sea infrastructure and the collaboration integration and test sites in Italy.KM3NeT4RR has started on 01/12/2022 and will last 30 months: its final goal is the deployment of 55 DUs by the time the project is completed, making it possible, together with other already funded DU production and deployment projects, to instrument about 2/3 of the ARCA full size (230 DUS).The INFN-Bologna section, which has been playing a crucial role in the KM3NeT collaboration hosting a Base Module integration site and a DAQ test site, is participating to the project and has begun the expansion of its laboratories and the creation of a new one.A review of the existing infrastructure together with a description of the upgrades funded by K3NeT4RR is given in the following sections.

Bologna Common Infrastructure laboratory
INFN-Bologna hosts the Bologna Common Infrastructure (BCI) laboratory, a unique facility in the entire KM3NeT collaboration which puts in place all the data processing key-points of the complete implementation of the full KM3NeT DAQ chain, summarized in section 2, within a controlled environment.It integrates all main aspects from control and monitoring, to the time synchronization, to the networking, and to the online software for data processing.It is used to develop and test DAQ-related hardware, firmware and software components before their installation either on the shore-station servers or the deployment on the submarine infrastructure.With KM3NeT4RR, the facility is going to be upgraded in order to be able to cope with the increased needs of the collaboration and to fit the new DAQ architecture of the "full White Rabbit" scenario.
At present, the BCI comprises two full DU-like test benches (see figure 2), with 19 CLBs on each test bench: 18 running with DOM firmware and 1 running with the Base Module firmware.The CLBs -4 - are connected to the necessary front-end switching fabric and their data is sent to servers running TriDAS processes, recreating a setup which is compliant to the ARCA shore-station one.Dedicated customized boards plugged onto the CLBs, named "OctoPAES" [11], emulate optical signals for the PMTs and acoustic signals.In particular, one test bench implements the "Broadcast" network configuration, while the other implements the "full White Rabbit" upgrade, featuring prototype CLBs of the latest version and prototype "Wet" White Rabbit switches.The "Broadcast" test bench also includes a Base Module power board, which is used to power the Base Module CLB, a hydrophone and an acoustic beacon as in a deployed ARCA DU Base Module.
With KM3NeT4RR the BCI is going to be expanded and moved to a bigger, renovated room and an additional "full White Rabbit" DU-like test bench will be built with the most recent CLBs v5 and the ultimate "Wet" White-Rabbit switches featuring the most recent backplane version.Including both new standard and White Rabbit switch fabrics on the "shore-side", this test-bench will become an exact replica of the future DUs that are going to be deployed from the next sea-campaign.Also, required laboratory equipment (scopes, generators, optical and acoustic analyser) has been procured as data analysis tools.

Bologna Base Module integration laboratory
The INFN-Bologna has been hosting the first Base Module integration site of KM3NeT and has a fundamental role in this task having designed several boards and actively participated in the testing phase of the prototype modules.For the ARCA "full White Rabbit" DUs production, the laboratory, like other Base Module integration sites, has recently started the integration of its first new Base Module, that features a completely renewed design.To cope with the presence of the two White Rabbit switches, the new Base Module has a different form-factor and features a different power -5 -board and connections, thus requiring a new integration and test procedure with respect to previous "Broadcast" Base Modules: with KM3NeT4RR the laboratory has been renovated and adapted to work with the new switches and boards, and has been equipped with the necessary tools to perform the optical fibers splicing as well as accurate optical, electrical and thermal measurements that guarantee the correct functioning of the system.

Bologna Laboratory for User-ports
The ARCA detector infrastructure includes the so-called "user ports": dedicated ports of the Junction Boxes which serve as physical connection for power and data transmission to structures such as the Calibration Unit, allowing the interface between oceanographic devices and the KM3NeT DAQ system.A new laboratory, called Bologna Laboratory for User-ports (BLU), is going to be set up in the same room where the new BCI will be located.This will be used for the design and the implementation of interfaces to oceanographic and environmental sensors to be mounted on KM3NeT Calibration Units, and for the integration of their data acquisition in the KM3NeT DAQ system.The sensors can be both the same ones used in the KM3NeT Calibration Units, and different ones provided by external users who intend to exploit KM3NeT infrastructure for deep-sea environmental studies.Moreover, BLU allows to provide technical support to those external users from both the (Italian) deep sea science and industry communities to develop their capability to work in deep sea.
The BLU test bench will simulate an ARCA Calibration Unit; the mock Calibration Base consists of a bare Base Module power board and a CLB; the mock Instrumentation Unit includes: • Seabird [12] Inductive Modem Module; • Seabird Inductive Cable Coupler; • Seabird SBE-44 Underwater Inductive Modem; • Seabird Underwater Inductive Modem Module; • Valeport [13] mini-SVS sound velocimeter; • Seabird SBE37-IMP MicroCat Conductivity and Temperature recorder; • Idronaut [14] Conductivity-Temperature-Depth recorder • AAndera [15] ZPulse Doppler Current Sensor; • 50 m copper test cable, but thanks to the flexibility of inductive technology, the line can host up to tens of any other kind of sensors using either serial or inductive communication that can be provided by external users.The CLB will be connected to BCI networking and computing infrastructure, in order to integrate sensor data with the DAQ system, and develop the control of the acquisition by the Control Unit and the storage on KM3NeT servers.The BLU laboratory will allow for an even stronger synergy with the EMSO consortium [16], which will also have access to oceanographic data recorded by Calibration Unit sensors.

Figure 1 .
Figure 1.KM3NeT Calibration Unit components: the Calibration Base (on the right) and the Instrumentation Unit (on the left).The ORCA Calibration Base also hosts a Laser beacon not shown here.

Figure 2 .
Figure 2. (a): the BCI test bench for the "Broadcast" network implementation.(b): the BCI test bench for the "full White Rabbit" network implementation.Below the first floor of CLB boards a splitter (the white box with optical fibers connections) is used to implement the redundacies on the two "Wet" White Rabbit switches which can be seen below it, recognizable by their red custom backplanes developed by the collaboration.