Iris 4 of the diffuser (the thickest one) stuck in the closed position towards the end of the last run.

The magnets tripped and when Joe T from Oxford came to examine the diffuser a few days later, that iris was found to operate normally. The diffuser was not, therefore removed (which is non-trivial).

It is possible, therefore, that the sticking was caused by the magnetic field in SSU. All the irises, including iris 4, had, however, been operated previously with the field on.

All the components are nominally non-magnetic although there are some stainless steel components that have a small permeability and will feel a small magnetic force.

The sticking of Iris 4 is therefore mysterious but may have been associated with the magnetic field plus time plus some mechanical wear.

Is it desirable, therefore, to run up SSU before the next run and test the diffuser?

I doubt that a fix could be effected in time if iris 4 is found to stick with the field on, but it could be left open / disabled for the next run.

Alternatively it should be tested at the very start of the next run with the overhead of having to run down / up the magnets if it is found to stick.