[3dem] Talos Arctica 200 kV Beam Drift Issues

[Stahlberg Henning]

Dear Carolin,

You describe the drift of the beam in tens of nanometers. Is that in total, or per second or per exposure? And you are speaking about beam drift, not image drift, right? 
If you simply widen the beam so that it is a micrometer in diameter, then 10nm drift should not matter....?  Or why is it making life complicated?

If it is beam shift, how do you detect that? Is that from the flucam of the Arctica or from a DED at the bottom? Just to exclude that you have a Moiré effect between a 50Hz disturbance from AC fields and a close camera readout frequency: Does the beam shift look the same if you use different flucam readout speeds?

If it is beam shift, what about beam tilt? And where does that shift, up in the gun or only closer to the sample at the level of the beam deflectors above the sample? 

Sometimes it is easier not to try to make matters better but worse, and that way you at least know where the problem comes from. Any parameter that is causing trouble for you will be at its limiting edge of the tolerance range. If you disturb that more to the slightest amount, you should immediately see the impact. If you instead disturb a parameter that is in a safe zone, then your drift problem should not be affected too much from that. So, you could touch with a warm hand all kinds of water lines or cables or vacuum tubes or warm parts of the column that are exposed to air flow, etc., to find the one parameter that is directly linked to the drift problem.  

Other than that, you could look into correlation with pressure changes in air or water, or temperature changes in these, or vacuum changes, or checking for a bending FEG tip. 

Best, 
Henning.


Henning Stahlberg
Prof., Faculty of Biology and Medicine, UNIL, and
Prof. for Physics, LBEM, IPHYS, SB, EPFL
CH-1015 Lausanne, Switzerland
mailto:henning.stahlberg at epfl.ch
https://urldefense.com/v3/__https://lbem.epfl.ch__;!!Mih3wA!XvRPbD_g_sDfRubiXn09QPG25PhL7liQkTlFZZpSup4x6nDEa01Lu7Y2rZ_cAU4BMQ$  


On 13.04.21, 16:06, "3dem on behalf of Carolin Seuring" <3dem-bounces at ncmir.ucsd.edu on behalf of carolin.seuring at cssb-hamburg.de> wrote:

    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
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