Cancelling systems in adjacent rooms

  • By spicerconsulting
  • 09 Apr, 2015

Can we put magnetic field cancelling systems in adjacent rooms?

 There are two main concerns:
1. Will the two systems interact with each other and cause instability?
2. Will the field from one system cause the field in the adjacent room to be non-uniform?
 

1. Will the two systems interact with each other and cause instability?
Two Spicer Consulting cancelling systems will work fine in adjacent rooms. This is because the amount of crosstalk is typically less than 12%. Each cancelling system can cancel any field made by the other one at its sensor position. Therefore this concern is not usually a problem.

There is one proviso – do not place a cable from one system on the other side of the same wall as the cable of another system. These would couple strongly to each other causing instability in both systems. If fitting wall-mounted cables in adjacent rooms, put at least one of the cables on the far wall away from the other room.
 

2. Will the field from one system cause the field in the adjacent room to be non-uniform?

Yes it will to some extent. This could be a problem if the non-uniformity in the field exceeds the specification of a large high resolution TEM in one of the rooms. It is not a problem with small columns such as SEMs and Dual Beam machines. The size of the non-uniformity depends on the size and position of the cables and the size of the field that is being cancelled.

As an example we have two adjacent rooms of size W=5m, D=5m and H=3m. We want to put a TEM in the centre of each room. We fit double loop room cables in the first room, centred on the room and we calculate the field that this installation makes in the centre of the second room. Let’s say the second room is in the X direction from the first room. Then the Z cables make a non-uniform X field in the second room. The field over the length of the TEM column is about ±4% of the field in the centre of the first room, approximately zero at 1.5m and with opposite sign at the top of the room compared to the bottom. If the Z field to be cancelled is 500 nT pk-pk, then a non-uniform X field of 20 nT pk-pk will be created at the column in the second room. An X field in the first room makes a ±2% non-uniform field in the Z field of the second room. Other smaller non-uniformities are also created.
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