Heat dissipation tests on the titanium Junction Box of theANTARES experiment

M.Ameri1, M.Anghinofi1, P.Cocconi1, S.Cuneo1, R.Papaleo2, F.Parodi1, G.Raia2 and A.Rottura1

I.N.F.N.- Sezione di Genova, Via Dodecaneso 33, GENOVA I-16146

I.N.F.N.- Laboratori Nazionali del Sud, Via S.Sofia 44, CATANIA

Abstract

The Junction Box is the part of the ANTARES project where the main electro optical cable from the shore is splitted to the 13 stings of the detector. The different active components housed inside will produce a total amount of heat greater than 1 Kwatt; it is therefore important to guarantee a good heat dissipation in order to avoid damages to the instrumentation due to local overheating. This report describes the test performed in our laboratory using the titanium box equipped withdifferent heaters and merged on a water bath at constant temperatureto simulate thereal conditions. The result have defined the optimal configuration in order to guarantee a safe margin with respecttothe maximum temperature allowed by the instrumentation; simulations performedwith the FEA code show good agreement with the measured temperatures.

1. Introduction

The aim of theinternational ANTARES collaboration is the realization of an undersea detector to measure high energy neutrinos. The detector, ready to take data starting from the year 2003, will be located on the bottom of in the Mediterranean sea at2500 mdepth , close to the town of Toulon in 

France. When completed, the effective area of the detector will be 0.1 Km2 , sufficient to measure fluxes of cosmic neutrinos.The configuration of the detector is shown in fig.1: 13 flexible strings support different optical modules (OM) to form anarray ofa total of 1000 PMTs to sense the Cherenkov light produced by the muons on the sea water.Starting from the bottom each string includes:

-a socket with the electro optical connection to anchor the string in the sea bed (BSS).

-100 m of electro mechanical cable.

Fig.1: The ANTARES undersea detector

-30 segments each composed by a frame to supportthe three PMTs , a local control module (LCM) to house the read out electronics and 12 m of electro mechanical cable.

-a top buoy to keep the string in the vertical position. 

Each string is then connected by an electro optical cableto a junction box (JB) which ensure the link to the shore station via a 40 Km long cable containing a 5 K Volt line and 24 opticalfibers.The JB is a component extremely important: it represents, in fact,the ‘ single point failure’ of the apparatus and a

Fig. 2: The Junction Box and the frame

possible malfunctioning would compromise the result of all the experiment. The JB, represented in fig. 2, is composed by a container and a frame for support and handling. The design and the realization of the JB has been performed by our group using two titanium hemispheres available from the Moscow University collaborators. In particular, the volume inside the two hemispheres was found to be largely insufficient to house the required instrumentation and cable connections (penetrators): the final design includes therefore a specially machinedtitanium spacer, the two hemispheres, a small box for the plug-in of the main cable and a large number of seals; a detailed description of the mechanical executive design can be found in ref ().

Table1
 
designation
quantity
total dissipation (W)
main transformer
1
1000
clock+ROR module
1
10
output breaker
7
50
output breaker/switch
16
50
low voltage supply
1
50
JB control module
1
50
timed relay
2
2
short mode relay
1
2

The JB will contain different items including a transformer to lower the 5 K Volt AC from the shore down to 1 K Volt AC and 25 Volt DC, the splitter to supplythe 13 strings with the possibility to switch off a single line, different optical connections for data transmission and multiplexing, an intelligent unit to drive the slow controls and to generate a fast trigger to the DAQ. For the purpose of this note, themain issue is the total amount of power dissipated by these components listed in table 1: 900 mWatts by the transformer and 250 m Watts from the rest.

The JB , in fact, is expected to continuously operatefor a nominal 10 year period and any local overheating of the electronics should be carefully avoided.We have therefore decided to perform some measurements on the heat transmission using the JB in the configurationdescribed in section 2; the results of these tests confirm that the total power listed in table 1 can be safely dissipated provided that thegeometrical configuration where the main transformer is located in the top region of the JB is chosen. The successful comparison to the simulations using a xxx program are finally discussed in section 3.

2. The experimental apparatus and the measurements.

The JB in its final configuration willbe mounted by October 2000 since the titanium spacer needs a dedicated time to be produced and machined while the inner components are expected to come a bit later. We therefore decided to start our test for heat dissipation using the JB in its simplest configurationwhere the two available hemispheres are separated by a stainless disk to generate two regions: region A to house the transformer and region B for the electronics. The general layout of the apparatus which has been mounted in our laboratory in Genova is shown in fig 3 and includes:

Fig. 3: The experimental apparatus for the test

-a box of 30x30x50 cmmade by a 0.3 cm thick copper sheet with dimension very close tothe real transformer. Six flat heater resistors ofdimension 2x20x0.1 cm (mod xxxx)are located in the inner part of each side of the box; each resistor is supplied with 250 V AC to give a power of 150 mWatts: the total amount of heat released in the box is therefore 900 mWatts as in the 40 Kvolt amp transformer. The box is fixed 5 cm. from the disk by a support; this clearance is necessary to insulatethe transformer from metallic components surrounding it.Both the inner part of the box and the volume of region are filled with xxx liters ofmineral oil ( sigla)using an external tube connected to one oh the three holes already present in the top of thehemisphere.The remainingtwo holes are used for exhaust and to drive the supply to the heaters;

- a stainless (tipo) disk of 80 cm diameter and 1.5 cm thickness to separate the two regions. The disk is sandwiched between the 2.5cm large flangespresent in thehemispheres; only the flange of region A has a slot carved on it: to ensure sealing we used an o-ringwhile a glue xxxx was used for region B;

-two cylindrical heaters 1cm diameter, 20 cm length located in region B. The heaters are located in the central region with a support fixed in the disk and produce 250 mWatt to simulate the heat from the electronics of table 1.Region B is filled with air at atmospheric pressure; however a tube connects this region with a pumping system located outside which can be used to make a low vacuum before filling with another gas.

In order to be able to monitor the temperature at different locations, 8 platinum resistors are located in the system as shown in fig 3 :3 on the oil bath(PT6,PT8,PT4), 1 on the surface(PT7) and 1 on the inner part of the copper box(PT5), 2 on the air of region B(PT2, PT3) and 1 in the stainless disk (PT1).The sensors (PT100 mod) are read out by a xxxx which scans up to 8 different channels.

In this configuration, the system was located in the middle of a big cylindrical tank containing 1500 liters of tab water maintained at constant temperature in the range 14 +/- 1.5 degree Celsius by a chiller mod xxxx.The same temperaturewas measured in the sea water at 2500 m depth in the location where the JB will be positioned: we are therefore confident to realistically simulate the real conditionsof the experiment.

When all the parts of the system had reached the temperature of the water, we switched on the power and recorded the 8 temperatures in 5-30 minutes intervals up to the equilibrium value. This procedure was repeated in three different conditions:

(a)the transformer is in the upward region ; region B is filled with air at 1 atm

(b)the transformer is in the downward region; region B is filled with air at 1 atm

(c)the transformer is in the downward region; region B is filled with helium at 1 atm

The temperature Tn of the nth sensor was fitted as a function of the time t from the initial temperature T0=14 degree, using the following expression:

Tn=Te-(Te-T0)*exp(-t/A) (1)

where Te and A are free parameters representing, respectively, the final temperature and the half time need to reach it.The values of Te and An for the (a), (b) and (c) conditions are listed in table 2

Table 2
 
sensors
PT1
PT2
PT3
PT4
PT5
PT6
PT7
PT8
Te (a)
36
30
30
39
58
28
49
34
A(a)
18
12
10
70
88
90
30
92
Te (b)
55
42
42
17
70
57
54
42
A(b)
70
12
11
74
73
41
100
Te (c)
50
35
32
17
73
60
55
44
A(c)
125
8
18
100
96
53
145

The fit is generally goodas shown in the example of fig 4.

These results clearly indicate that in condition (a) where the transformer is in the upper region of the JBthe heat exchange with respect to the water is optimized and the temperature of both the oil and air areminimized.Indeed we tested also conditions (b) since this geometry wouldavoid possible oil leakage from the transformer to the electronics in the downward region. However, the values obtainedin (b) are too close to the limit allowed by the instrumentation: xx deg for the transformer, yy for the optic fiber in region B and zz for the switches.An improvement is obtained in condition (c) where the helium is used to improve the thermal exchange with the titanium shell.

Of course the two heaters in region B are only a crude approximation of the real electronic configuration; in particular, being our empty volume much higher than the real conditions, it might be possible that our results for conditions (b) and (c) are optimistic;moreover it is not know the effect ofthe use of helium gas in electronic devices for a long period .All these considerations 

Fig. 4: The fit of the temperature in the copper box in condition (a) as a function of time

support our decision to choose configuration (a) for the JB; of course a lot of attention will be paid for the design and choice of the feed through to be located in the separation disk to allow the transmission of the voltage and anode cable connected to the transformer.

3. The simulations