MICO Meeting

Magnetic Field Shielding Issues

11th July 2012

Mike Courthold

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Background toMagnetic Field Shielding Issues

• MICE Cooling Channel magnets were designed without return yokes. Consequently, the fringe fields have the potential to affect much of the MICE Hall and beyond.

• The Magnetic-Shielding Walls were designed to protect the public beyond the MICE Hall, and in the MICE & ISIS Control Rooms, from fields in excess of 5 Gauss. An arbitrary maximum of 5 Gauss was also set for the neighbouring ISIS Linac Hall

• The Magnetic-Shielding Walls do nothing to protect equipment within the MICE Hall, and in fact distort the field locally in ways that create more problems than they solve

• Once the fields in air for all MICE Steps were determined, attention focussed on:• Identifying all magnetically-sensitive components in the Cooling Channel region• Determining for each identified area or item:

• Whether local shielding would be required• The type and amount of shielding required (eg US1010 iron and/or Mumetal)

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Background toMagnetic Field Shielding Issues (cntd)

• The TRD Magnetic-Shielding model was used to predict the magnetic field in air, and the potential shielding requirements in specific areas and to specific equipment

• The model was adapted to analyse the effect of inserting significant ferrous objects into the Cooling Channel region, to predict:

• local distortions of magnetic fields• resultant forces on inserted objects

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Predicted fieldswithout ferrous content

Predicted fieldswith ferrous content

Background toMagnetic Field Shielding Issues (cntd)

• Previously it was assumed that shielding various magnetically-sensitive items would simply be a case of taking the prediction for the field in air, and employing a suitably-thick Mumetal screen.

• However, when items such as air-conditioning units and cryocooler compressors were modelled, it became apparent that the magnetic field was massively enhanced on the surface of such steel cabinets from typically 300 Gauss (30 mT) to 1.7 Tesla

• In hindsight, this effect should have been expected, but was overlooked by those performing the various analysis tasks, and also by various peer reviews, over more than seven years

• The oversight had at least been identified before the arrival of any Cooling Channel magnets, and work began in earnest to quantify the problem and find engineering solutions, ideally for Steps IV to VI, but at least for Step IV.

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Magnetic Field ShieldingNew & Ongoing Activities

• Find engineering solutions for Step IV• 15 compressors involved• Control & monitoring racks for the Cooling Channel magnets• Other electrical racks on the North Mezzanine extension

• Recommend options for moving or replacing magnetically-sensitive components and equipment, rather than shielding

• Consider solutions for Step VI• Obtain addition support and effort by employing a consultant• Understand the full extent of the shielding issues, by:

• Gathering information on all ferrous objects in the MICE Hall• Identifying all magnetically-sensitive items in the MICE Hall, including any

items whose status is unknown• Gathering all available literature from manufacturers & suppliers re

sensitivity/resistance of individual components (eg relays & transformers) and equipment (eg control & monitoring racks, and compressors)

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Step VI: Compressors6

-10000 -5000 0 5000 10000 150000

100200300400500600

Z -location along beamline (mm)

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(Gau

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Step VI: Compressors (cntd)• Looked at the proposed location for the compressors in Step VI along the base of

the south shielding wall.

• On the preceding slide is an image of the model used, which includes two shielding walls, with compressors shielded in stacks two high.

• Initially looked at the fields seen by the compressor when shielded by a double thickness of shielding material – 1mm Mumetal and 5mm of US1010 iron. Assuming that the compressors can be subjected to no more than a 50 Gauss field, only the compressors in the far west positions would survive during Step IV.

• Then steadily increased the shielding levels to find that the amount of shielding that would be required to shield compressors in these locations was a multi-layer comprising 80mm thick US1010 plus 10mm Mumetal. Clearly this is not feasible, and new locations will need to be found in the hall for the compressors in Step VI.

• Suggested locations include directly behind the Beam-Stop, or stacked against the West and North-East walls.

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Step IV: Compressors8

Step IV: Compressors (cntd)• Looking at the beamline for Step IV, the magnets are all located at the upstream

end of the final beamline, so it is possible to use the compressor locations shown on the preceding slide, provided they are shielded with 5mm US1010 iron plus 1mm Mumetal; which is good because work has already been undertaken to install power and water services to these locations.

• Both Vector Fields and Oerlikon-Leybold recommended Magnetic Shields Ltd for shielding solutions.

• Various discussions have since taken place with Steve Locker of Magnetic Shields, who have designed a prototype shielding solution for a single compressor.

• The prototype shield has been ordered, and the plan is to test the shield with a compressor with the AFC magnet in building R9 (once acceptance tests are complete).

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Step IV: Magnet Control Racks Simplified solution shows it is

possible to shield in 5mm US1010 iron and 5mm Mu-metal, if Aluminium racks are used.

In reality, it will be necessary to

provide protected access (labyrinth or air-lock style), to allow access during magnet operation for diagnostics.

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This implies serious modifications to the positions of magnet control racks, their services and the entrance to the trench.

Step IV: Electrical Racks on North Mezzanine

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- 4 racks located on the north mezzanine

- 5mm US1010 iron and 5mm mu-metal, if Aluminium racks are used.

- Estimated weight 3-5 tonnes – requires major modifications to the north mezzanine

Step IV Solutions• In summary, for Step IV it is possible in general to shield items – although finding

solutions for each magnetically sensitive component is time consuming, and none of the options considered so far are trivial to implement.

• Moving items further away from the Cooling Channel magnets is an obvious option, but not feasible in every case, for electrical and/or engineering and/or space reasons

• For the compressors, the options for Step IV are to:• shield them with a combination of 5mm US10101 iron plus 1mm Mumetal, and place them

all in the downstream locations;• Place the compressors against the West and/or North-East walls• Place them behind the Beam-Stop

• For the controls & monitoring racks, the options are to:• Move those that can be moved to safer locations, including:

• Behind the North Magnetic-Shielding Wall (where RF amplifiers will eventually be located for Steps V & VI)

• Next to the West wall• Next to the North-East wall on the upper floor

• Locally shield those racks that cannot be moved – eg the Tracker Cryostat• For the vacuum equipment, the turbo-pumps can be locally shielded (which avoids

issues with long vacuum pipes), suitable gauge heads can be selected and screened, and he backing pumps can be located in a low fringe field

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Step VI Solutions13

Modelling shows that it is not reasonably possible to shield the compressors or electrical racks in their currently proposed locations.

We need to consider:- Compressors moved behind the beam stop,

which would still require shielding, or against the west wall

- Electrical racks either heavily-shielded or moved elsewhere

- What else?

Step VI SolutionsSome thought has been given to Step V1:

• Initially the intention was to find solutions for both Step IV and Step VI, to avoid having to repeat work later. However, initial analysis showed that this would be challenging, for the following reasons:

• For example, up to 80mm US1010 ion and 10mm Mumetal is required to shield a compressor in its current position.

• Also, given the space constraints in the hall, space required for additional shielding will need to be considered.

• For example, it was intended that there would be seven magnet control racks under the north mezzanine, but with shielding there would only be enough space for five.

• Consequently, MICE Project Board decided that the focus should be on finding a Step IV solution

• Consequentially , it will be necessary to find new shielding solutions and/or relocate items for Step VI

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Information GatheringWhat we know we need to consider:

• EMR control rack• Magnet control racks• Vacuum system control racks• Power supplies for conventional magnets• Tracker system• Hydrogen system• Vacuum system• PPS system• Etc...• Often quite small items, but critical to the safe operation of MICE, eg:

• Electro-magnetic actuators• Pnematic/control-valve limit switches (Hall-Effect)• PPS relays• Fire alarms.

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Conclusions16

• Magnetic Field distortion• Previous analysis and reports overlooked massive distortion in magnetic

field profile caused by presence of ferrous objects - including the magnetic shielding itself

• EG: Magnetic field in air in compressor locations ~ 30mT, which becomes 1.7T when ferrous content of cabinet and shielding taken into account.

• What about a partial yoke• Forced to reconsider possibility of partial yoke around Cooling Channel

magnets• Substantial issues re beam physics, forces, weight, and access for services

and maintenance, etc. • Focusing on a Step IV solution

• The plan of action relies on a combination of moving items as much as possible, and then shielding locally, which will still require considerable engineering compromises & effort to achieve, as well as substantial additional analysis