[3dem] Talos Arctica 200 kV Beam Drift Issues

[Carolin Seuring]

Hi 3DEM community,

We are looking for support in tracking down a beam drift issue on our Arctica Talos 200kV FEG system.

The problem: For the better part of the past 6 months, and mostly during the day, the beam drifts over a few hours in one direction, and then randomly backwards - never getting back to its original position (but close). The drift is in the tenth of nanometers and makes data acquisition impossible during that time. The beam is stabilising often in the late evenings. Magnetic Field compensation is always on. The facility also houses 2 Krioses in separate rooms adjacent and further away from the Arctica room, but zero issues there. 

Parameters we have checked so far:
- room temperature stability -> humidity controlled environment with airconditioning, 21-21.5°C
- operator in the room vs operator on external remote controls -> using remote access from a separate room seems to cause less issues
- air movement in the room -> seems negligible, only slow air movements towards air exchange vents
- dewar filling time -> beam seems to drift less after dewar filling
- whether centrifuges and instrumentation one floor up above the microscope are on —> results not conclusive
- tested efficiency of magnetic field compensation a two points in the room —> ok
- we used a carton paper to cover the 2m^2 Arctica top (thanks to Wim, who posted that somewhere) 
- we changed the C2 aperture set, that seemed to helped a bit a few months back

Beam drift is now still on about once a day at fairly random times! So annoying…

Has anyone encountered anything like this or has any other suggestions or parameters we should consider?

Thanks so much for your help!
Carolin