Fun with magnetic shielding:
Mu-metal shields help screen the experiment from magnetic fields, both varying (such as from local sources of interference) and static (e.g. the earth’s field). Nesting them increases their effectiveness. Mu-metal workings by ‘sucking’ in field lines – they prefer to go through the high permeability material than through air (or free space), so they are diverted around a sensitive sample in the middle.
Magnetic field affects the Josephson Junction by altering its critical current, Ic. The experiment I am performing involves precise measurement of Ic. Ironically, in the middle of all these shields a magnet will be fitted (in the picture this is the smallest item at the right hand side of the line-up). The magnet allows Ic to be changed in a controlled manner when necessary.
The magnet is superconducting so that it can be operated in persistent current mode. This is where the current supply is isolated from the magnet’s superconducting coil using a special switch and the current flowing in the coil then just carries on going round forever. This is good news as it means the field is extremely stable.
At the moment the experiment only has 1 magnetic shield, which is not shown in the picture. Now that I have these new shields, I will nest 3 of them (note I can’t nest them all due to both geometry and space reasons) to improve the experiment. I will assemble the shields inside the dewar (the big blue thing in the background) housing the liquid helium which cools the Josephson junction experiments.