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

• Ron Burgundypants
In summary, the conversation discussed using a solenoid to measure the magnetocaloric effect of dysprosium and the challenges of doing so. The effect is strongest at the Neel temperature of 180K, which is achieved using a solenoid in a cryostat. However, the presence of a thick copper heat exchanger surrounding the solenoid was found to reduce the self inductance of the coil by about 20%, likely due to eddy currents. The conversation also mentioned the use of graphs to estimate the reduction of inductance in conductive boxes and the opposite effect of a ferrite slug in the coil. The speaker requested a diagram of the system for better understanding.
Ron Burgundypants
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.

Tak.

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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.

## 1. What is altered inductance around a solenoid in a cryostat?

The altered inductance refers to a change in the ability of a solenoid to store energy in the form of a magnetic field when placed in a cryostat. This change is caused by the presence of eddy currents, which are induced currents that flow in a conductive material in response to a changing magnetic field.

## 2. How are eddy currents related to altered inductance?

Eddy currents are responsible for the altered inductance around a solenoid in a cryostat because they create opposing magnetic fields that counteract the original magnetic field of the solenoid. This results in a decrease in the overall inductance of the solenoid.

## 3. What factors can influence altered inductance around a solenoid in a cryostat?

The amount of altered inductance depends on the strength and frequency of the applied magnetic field, as well as the material and geometry of the solenoid and cryostat. The temperature and electrical conductivity of the materials also play a role in the amount of altered inductance.

## 4. What are the effects of altered inductance on the performance of a solenoid in a cryostat?

The presence of altered inductance can cause a decrease in the efficiency and accuracy of the solenoid in a cryostat. It can also lead to increased power consumption and heating of the solenoid, as well as potential damage to the cryostat if the eddy currents are too strong.

## 5. How can altered inductance be mitigated in a solenoid in a cryostat?

There are several methods that can be used to reduce the effects of altered inductance in a solenoid in a cryostat. These include using materials with lower electrical conductivity, designing the solenoid and cryostat to minimize eddy current flow, and applying shielding or insulating materials to the solenoid.

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