I Altered inductance around a solenoid in a cryostat (from Eddy currents?)

I'm trying to use a solenoid to measure the magnetocaloric effect of dysprosium. The effect is highest at the Neel temp which is down near 180K. I have a solenoid in a cryostat to get the correct temperature. The cryostat's heat exchanger is made of a thick copper cylinder and I think it is retarding the inductance of the coil.

The heat exchanger totally surrounds the solenoid and seems to be reducing the self inductance of the coil by about 20%. I measured inductance with and without the copper core several times and its a consistent drop.

I think the drop is due to eddy currents, but I'm not totally convinced. Does it seem reasonable? It also seems like a very high retardation. Having no idea about the magnitude of eddy currents I'm not sure if there is more to this problem.

The coil is about 30mm long and has roughly 3mm thick of 0.3mm diameter copper wire wrapped around it. Then there's an air gap and then the copper heat exchanger which is around 10mm thick and surrounds the entire coil.

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Science Advisor
When a solenoid inductor is operated in a conductive box, the current sheet in the coil is reflected back to the inductor as an inverted current, which cancels part of the inductance. There were graphs used to estimate the reduction of inductance based on inductor and box dimensions. If the walls are good conductors there will be a strong reflection with very little power loss.

Where a conductive slug is enclosed by a coil, the effective area of the coil is reduced so the inductance is also reduced. That has the opposite effect to a ferrite slug in the coil.

You have reported your observations. What would you like to change?
Please post a diagram with the detail and dimensions of your system.

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