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Prototype strings

Since line 5 deployment,  a certain number of major events occurred toward the full detector installation.

See the Status and news pages to have an overview of further prototype operation and detector installation.

 

Prototype line architecture (click to enlarge)  

The prototype line was immersed several times in 1998 (test 4) and then, after being equipped with photomultipliers, in December 1999 (test 5). In this latest test the line was successfully connected to the shore through a 40 km electro-optical cable and muon tracks were recorded.

See some pictures of the test 4 line deployment (identical to the test 5 deployment) and some of the very first deployment in ANTARES in 1996.

To gain realistic experience of the issues involved in the deployment, operation and recovery of a full-scale detector, a full-size prototype of an ANTARES detector string has been constructed. It was equipped with a positioning system, slow control network, power distribution, and eight optical modules with their associated readout electronics. Once deployed, the string was linked to a shore station by an electro-optical cable supplying the power to the string as well as enabling the control and readout connection. The string is adequate for a detailed study of the optical background and a measurement of the down-going muon flux.

Mechanical structure

The prototype string is 350 m high, anchored on the sea floor and vertically supported by a buoy. It is composed of two vertical cables spaced 2.3 m apart, supporting both ends of sixteen Optical Module Frames (OMFs). The OMFs are placed every 15 m from a height of 95 m up to 320 m from the sea floor and are constructed from fibre glass to avoid corrosion. Horizontal spacers keep the vertical support cables below the lowest OMF; a spacer is also situated at the middle of every 15 m segment separating the OMFs. Each OMF supports a pair of Optical Modules, separated by 1.6 m from axis to axis, while the central part houses a container for electronics made of corrosion-resistant titanium alloy.

Slow control system

A slow control network is linked to the shore station through an electro-optical cable and employs an architecture whereby each electronics container has a point-to-point connection with the slow control data acquisition system. It permits the control of the power distribution electronics, optical module motherboard, analogue readout electronics and the acoustic positioning system. It also handles the readout and transmission of sensor data, such as the satellites, the acoustic system and the electronic card temperatures or status. This DAQ system is located in the main electronics container at the bottom of the string. The main fibre-optic data link connects this system to the shore station, which receives the slow-control data and provides a user interface for the slow-control system.

Positioning

A string does not provide rigid support for the optical modules. Two independent systems have been incorporated in the prototype string to provide a precise knowledge of the relative position of each OM at any time. The first system is based on a set of tiltmeters and compasses which measure local tilt angles and orientations on the string. The reconstruction of the line shape, as distorted by the water current flow, is obtained from a fit of measurements taken at different points along the line. A successful test of this system was performed during a deployment of the prototype string which was equipped with several sensors with a precision of $0.05^{\circ}$ in tilt and of $0.3^{\circ}$ in direction. A maximum error of $\sim$ 1 m on the reconstructed shape is estimated.

The second system, based on acoustic triangulation, is more precise but requires more complex and expensive electronics. In this system, rangemeters placed on the string send an acoustic signal to a minimum of three transponders fixed to the sea bed. Each transponder replies with its characteristic frequency. A global fit of the measured acoustic paths gives the precise three-dimensional position of the rangemeters, provided that the positions of the transponders and the sound velocity in water are known. The prototype string is equipped with four rangemeters (a hydrophone with its electronics container) communicating with four external autonomous transponders placed on a fixed structure on the sea floor about 200 m away from the string. The measurement of the communication time between one fixed rangemeter and one fixed transponder demonstrates a reproducibility of $\sim 1$ cm in the acoustic path length.

In order to exploit such a system fully, a precise knowledge of the sound velocity in water along the acoustic path is required. This depends strongly on water temperature and also on salinity and depth. The prototype string is thus equipped with sound velocimeters, which measure the local sound velocity with a precision of 5 cm s-1, and with Conductivity Temperature Depth devices (CTDs) to observe the variations of temperature and salinity.

The systems are complementary: a few points of the line can be measured acoustically and other points are obtained by line shape fitting. The tests already performed confirm that the desired precision on relative OM positioning can be achieved.

String deployment

The prototype string was deployed, operated and retrieved several times in Summer 1998, first at 400 m depth and then at 2300 m. It was finally immersed for 6 months in december 1999.

This string was powered by batteries housed in a container at the bottom of the string. The deployment procedure used was different from that used for the mooring lines discussed earlier. In this case, a step-by-step procedure involving two winches on the boat deck was implemented whereby the anchor was immersed first, the string being held securely at 2 points, then each OMF was paid out storey-by-storey. Recovery was performed in a similar way. Deployment and recovery at 2300 m took 18 hours in total. This method allowed the equipment to be deployed in a safe and controlled manner. The exercise also permitted the handling procedures for the electro-optical cable termination attached to the string anchor to be verified. The tiltmeter and compass data were recorded so that the detector string behaviour could be studied during the deployment phase. The fully equipped version of the prototype string was immersed in December 1999.

Successful deployment of the 0.1 km2 array will require strings to be located in well-defined positions with 80 m spacing or less. This was investigated in a multi-string deployment test using the dynamical positioning ship Provence and 400 m test strings instrumented with acoustic beacons.

String positioning precision was investigated by aiming to deploy a second test string 50 m away from the one which had been deployed earlier. Relative positions were measured with the aid of three external acoustic beacons.

The second string was deployed using a winch, with the ship located at the target position. Deployment was halted with the string 200 m above the sea floor, the relative position measured, and the ship moved to adjust as necessary. Using this technique it proved possible to locate the second string to within 10 m of the nominal position.

Deep sea connection and line recovery

See some pictures

In the full-scale detector, each string will be connected to a common point, known as the junction box. A connection scheme from the junction box to a prototype string anchor was tested during a ten-day sea operation at the ANTARES site, using the Nautile submarine and its support vessel the Nadir from IFREMER.

A reel of cable equipped with deep sea connectors on both ends was immersed first. Then the Nautile uncoiled the cable by pulling it and plugged the connector of the cable to its counterpart on the anchor. The connection procedure was successfully performed twice, using a free-flight technique which requires only one of the submarine's arms. This offers the possibility of using IFREMER's second manned submarine, the Cyana, which is equipped with only one arm. The advantage of this is that this submarine is more freely available than the Nautile.

The figure below shows a diagram of the Nautile approaching the anchor, holding the connector and the cable, before plugging it at the end of the arm which can be seen in an upright position at the bottom of the string. Once the connection is made, the Nautile pushes the arm down and plugs it into a fork linked to the anchoring weight. To retrieve the string, acoustic releases are activated from the surface which disconnect the string from the anchoring weight. The buoyancy of the string pulls on the connector which is held back by the fork and is thus unplugged. This system avoids the need for a submarine for string retrieval.

A diagram of the Nautile approaching the anchor, holding the connector and the cable, before plugging it at the end of the arm which can be seen in an upright position at the bottom of the string

The speed of ascent of the string during its trip to the surface is around 1 m s-1. The distribution of weights and buoyancies along the string have to be carefully studied in order to control the relative speed of each storey so that the string does not become entangled when it surfaces.

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Author : Thierry Stolarczyk