MICE Spectrometer Solenoid Review

Some inputs for the in person meeting

to be discussed at the

November 13, 2009 phone meeting

P.Fabbricatore on behalf the review board

In the last month the review committee has analyses many documents (drawings, photos, log data) related to the design, construction and tests of the two MICE spectrometer solenoid. Though a lot of information is available, something is still needed in view of the in-person review at Berkeley.

A crucial information is coming from the measured data. At the Nov.2 phone meeting we had the ones related to the test of magnet 2. These data do not include the temperatures measured by Cernox (insensitive to the magnetic field), but only the PT100 and diode sensors. We have found some inconsistencies as shown later. For the review these problems should be fixed and the data for magnet 1 test should be available.

The central point is related to the whole thermal balance and our aim is to identify a clear coherent scenario for the effective heat loads. This means that the measured temperature distribution shall be coherent with heat flows and boil-off of cryogens (LN2 and LHe for magnet 2, LHe for magnet 1).

Two main problems were found: 1) Limited thermal conductance of the Cu plate causing problems to the current leads; 2) High thermal impedance of the connection between Cu-plate and shield. However at present time the quantitative analysis related to these problems is not completely self-consistent as explained later. During the review we shall try to find a satisfactory quantitative explanation.

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Shield

Shield _below_cryocoolersShield_flange_leftShield_flange_rigth

Te

mp

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ture

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Progressive measurement

Shield temperatures measured on magnet 2

The graph shows the temperature measured in 3 locations of the shield: just below the cryo-coolers and on the flanges at the sides. This graph is clearly wrong unless the temperature of the shield close to cryo-cooler (TPR02) is exchanged with one of the temperature at side (TPR07) . If it is the case we can assume that the temperature of the point closest to cryo-cooler is 105 K in steady state conditions

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all_temp

Shield_flange_left

CRYO_1

CRYO_3

CUplate_CL

Te

mp

era

ture

(K

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Progressive measurement

Temperature distribution on the Cu plate

From these measurements we can assume an average temperature of the Cu plate of 70 K (We are interested to the heat flow from the shield)

Heat flow between Cu plate and shield

Tom Bradshaw has proposed to use a simple but effective spreadsheet for analyzing the thermal impedance between Cu plate and shield. Imposing 70 K as the temperature of the Cu plate, 105 K as temperature of the shield (see previous slides) and considering the materials and geometries one can find that the measured temperature are compatible with a heat flow of 23 W for each connection, so 69 W total. Question: where 69 W are coming from?

Spectrometer Solenoid 70 K - Shield connection

a b x-sect Area Length K # KA/l Res

Copper plate 70.0

Weld 25 5 1.25E-04 5 200 10 50.000 0.020 70.5

Transition (Cu-Al) 25 25 6.25E-04 10 375 10 234.375 0.004 70.6

Weld 25 5 1.25E-04 5 100 10 25.000 0.040 71.5

Aluminium tube (6061) 628.3 6 3.77E-03 400 113 1 1.065 0.939 93.1

Aluminium links (pure 1100) 19 6.4 1.22E-04 200 300 12 2.189 0.457 103.6

Al Weld to shield 15 5 7.50E-05 10 150 10 11.250 0.089 105.7

Shield 105.7

Total Res 1.549K/W

Power 23W

dT 35.627K

Type of heat load Heat Load @60K (W) Heat Load @4.2 K (W)

Radiation 30 ( if we have 2 W/m2)

0.15

Conduction – Cold mass support 2.4 0.24

Conduction – Shield supports 1.2 -

Conduction – Neck tubes 1.9 0.06

Conduction – Cooler sleeve tubes 7.5 0.75

Conduction- Instrum. wires 1.0 0.05

Current leads (upper) With/Without current

90/45 -

HTS leads with current - 0.56

Joule heating in joints - 0.40

TOTAL LOADS 134/89 2.21

Let’s re-analyze the heat load chart

In red the heat low to the shield. With respect last time we increased of a factor 2 the radiation losses (2W/m2). However also with this value the heat flow from shield to Cu plate is half we need for justifying the measured DT.

Three possibilities are in front of us:

1)The heat loads to the shield are much higher (that is not impossible)

2)The thermal conductance of some components shown in the spreadsheet is

higher than supposed (possible guilty is here the Cu/Al transitions placed on the

top of the AL 6061 tube)

3)A combination of the two above effects

It is recommended that during the in person review these aspects will be

analyzed having in hand more information as the one coming from the

measurement on magnet 1.

Temperature distribution on the Cu plate

A 2D thermal model of the Cu plate was considered with following assumptions:1)Cooling power 50W x 3 =140 W2)Heat load 75W3)Heat load due to current lead 65 W

This model accounts for the measured distribution (with no current) but: 1) The cooling power should be a bit less than nominal (50 W per cryo-cooler); 2) The heat load is mainly due to a high load from shield; 3) The heat load from CL ishall be higher than calculated (65W vs 46 W).

For the in person meeting, it would be nice to have more thoughts on these aspects, with the aim to correct this scheme as better as possible. The connection of a 4th cryo-cooler should be discussed in this frame quantifying the thermal connection (geometry, material)

It is still unclear the role of the LN2 reservoir used in magnet 2. It was said

that people were expecting that at a certain moment the LN2 would have

frozen, but this did not happen and they observed a continuous boil-off of

LN2. From the drawing it appears that the reservoir is not only thermally

connected to the shield but also to the I stage of the cryo-coolers. Now, down

to 77 K at the I stage, the LN2 helps, but as the temperature of the I stage

goes below 77K, part of the cooling power is dedicated to coldown and freeze

the LN2.

Measured data on magnet 1 can help the understanding (looking at the

differences with magnet 2). This point shall be discussed during the review.

What we can say at present time?

It seems that at the basis of the problems found with the two magnets there are three causes:

1)The thermal load to the Al shield is quite high (may be up to 70W – 75 W)

2)The cooling of the shield is limited due to a high thermal impedance between copper plate and shield (specially the 6061 tube is quite unsuitable for a good thermal connection)

3)The current leads causes a concentrated heat dissipation, which the copper plate is not able to efficiently drain away