Moving a solenoid coil in a strong magnetic field

[_maxim_]

Hi guys,
a solenoid coil 10x4 mm needs to be moved along the vertical axis of a strong magnetic field (up to 14 Tesla) generated by a superconducting magnet. The coil must be contained into a classical probe for NMR spectroscopy, and moved vertically inside that probe.
The main issue I am facing is how to realize such mechanisms to move that coil, housed in a chip of 40x30 km size,in presence of a magnetic field.
Any idea ?
Thanks

 

[Charles Link]

Without ever having done this previously myself, there are two problems that I see. The force required to move it (once inside the chamber) might be minimal, and is given by $F=-\nabla U$ where $U=-\mu \cdot B$, and $\mu=NIA$, where $N$ is the number of turns, $I$ is the current in the solenoid, and $A$ is the cross-sectional area. In a uniform magnetic field, zero force is required to move it, if you maintain the same alignment of the magnetic moment $\mu$ of the solenoid with the magnetic field. What could potentially be much more problematic is a torque $\tau=\mu \times B$ that could result, particularly if the magnetic moment $\mu$ of the solenoid is not aligned (i.e. if it is not parallel) to the magnetic field. A solenoid with magnetic moment anti-parallel to the magnetic field is going to want to try to rotate $180^o$ to align itself with the magnetic field. $\\$ I haven't attempted anything like this previously, but this is the results that arise from magnetostatics that I see as being relevant. $\\$ In addition, bringing a solenoid with current in it into the magnetic field could take some care, because the force $F=-\nabla (\mu \cdot B )$ could be quite large in going from outside to inside the chamber. Inside the chamber, the magnetic field should be much more uniform. $\\$ I assumed here that the solenoid coil has a DC current in it. If it doesn't, then the only problem I see is with the Faraday EMF and currents that could arise from it, if you move the coil too quickly.

 

[_maxim_]

Thank you for having shared your experience.
In NMR, currents in solenoids are in the range of uA o hundreds of uA so the resulting force against the main magnetic field is negligible. Also, the coil will be moved in steps, not continuously, and for any position a different RLC should be connected, in order to set the resonance frequency according with the external magnetic field (which varies almost linearly with the position of the coil respect the main field).
That said, remain the problem: how to mechanically move such solenoid ?

 

[Charles Link]

Thank you for having shared your experience.
In NMR, currents in solenoids are in the range of uA o hundreds of uA so the resulting force against the main magnetic field is negligible. Also, the coil will be moved in steps, not continuously, and for any position a different RLC should be connected, in order to set the resonance frequency according with the external magnetic field (which varies almost linearly with the position of the coil respect the main field).
That said, remain the problem: how to mechanically move such solenoid ?

That's a very good question, but I don't have any immediate answer. Most electro-mechanical motors could/would be affected by the strong magnetic field, and trying to employ them could be quite problematic.

 

[Tom.G]

How about:
Mechanical guides as necessary for path-of-travel definition. (Non-metallic, non-conductive)
If short distance, high resolution movement is needed, a piezo-electric positioner is a possibility.
For longer distance movement, fishing line, or lacing cord, to a remotely mounted, magnetically shielded motor and drum is an option.
-or-
a hydraulic or pneumatic positioner.

Tom

 

[_maxim_]

Interesting ... but we have to consider that everything should fit in few centimeters of space, about 50 mm to be more specific. The maximum vertical distance between the extreme points is in the range ofrom 30-50 cm or so.

 

[_maxim_]

I am in contact with a local supplier for a piezoelectric linear positioner U-523 form PI Physic Instruments.
The main issue I see is the reduced distance covered - few centimeters - while 30-50 cm are needed.
I have no idea how to increase that distance...

 

[Tom.G]

30-50 cm are needed.
I have no idea how to increase that distance...

Physik Instruments is a favorite in optical setups, and has been around long enough that they could probably supply any micro-positioner you can pay for.

1 minute video of operating principle:

 

[_maxim_]

There is also a good piezo motor from Noliac that I am studying now. The question is a mechanical requisite: I do not know the way to avoid loading the piezo motor three too much, since the PAD 7433 is designed for low force systems.
I will address this to mechanical engineer area.
Thanks for now for your ideas.
Great forum