[3dem] DC EMI cancellation

[Stahlberg Henning]

Hi,
We had strong EM fields from a nearby metro on one side of the building and a DC cable running hundreds of amperes DC current close to the intended EM site on the other side of the building. DC field variations at the intended EM location were stronger than 4000nT, while you wish to keep that below 50nT.   DC fields decay linearly with the distance from the source cable, while AC fields decay with distance squared from the source cable. So, in a certain distance, DC fields are the remaining problem.
Fields can be shielded passively with annealed mu-metal, which is very expensive and in addition requires to install a room in a room, which swallows 20cm from each wall. After that, fields inside the mu-metal cage are homogeneous and easier to shield passively. But the price is very high.
Fields can also be shielded actively with Helmholtz coils in X, Y and Z, providing a counter-field that neutralizes the field at least for the one location in the room where the system's field sensor is. That works well for a TEM without energy filter, if the field sensor of that active cancellation system is close to the sample stage, where the sensitivity is the highest. If in addition you also need a low field in a post-column filter of that TEM, then an active two-sensor / two-coil-pair system might be an option, or you need to go for the much more expensive passive shield solution. The active cancellation system compensates fields from 0Hz up to 1000Hz for one location in the room. If fields are inhomogeneous, e.g., from a nearby cable, then active cancellation would correct at one spot in the room but make matters worse everywhere else in the room. 

Working closely with the excellent Systron AG in Zurich, Switzerland, we decided to move the instruments as far away from the source as possible (about 12 m in this case), and then install only the active cancellation systems MR-3 by Stefan Mayer in each EM room. This was 40x cheaper than passive cancellation and didn't consume any space. It worked well, all 300kV instruments reach 1.2Å or better on apoferritin, also the one with postcolumn filter, even though the latter will benefit from the better location into which we will move the instruments soon.

Best wishes, 
Henning.

Henning Stahlberg
Laboratory of Biological Electron Microscopy
Institute of Physics, School of Basic Sciences, EPFL, and
Dep. of Fund. Microbiology, Faculty of Biology and Medicine, UNIL, 
Cubotron, BSP421, 1015 Lausanne, Switzerland
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From: 3dem <3dem-bounces at ncmir.ucsd.edu> on behalf of Andrzej Żak <andrzej.zak at pwr.edu.pl>
Date: Thursday, 6 July 2023 at 08:32
To: Hannah Turner <hannaht at scripps.edu>
Cc: 3dem <3dem at ncmir.ucsd.edu>
Subject: Re: [3dem] DC EMI cancellation

Dear Hannah,  
I had such situation near the road, probably city start using other/more trams. The main reasons of DC are trams, trains, metros, elevators, etc. If pre measurement was taken during holidays, during the normal work day could be different (and getting worse in time, if anyone around mount some fancy equipment or you have more buildings and stuff around). As far as I know field cancelation like Spicer is the only way to go for DC. And nowadays it's good practice to mount it for any new TEM, just in case for the future. 
Best wishes 
Andrzej 
Andrzej Żak, PhD Eng
Wroclaw University of Science and Technology,
tel. (+48)883314999
e-mail: mailto:andrzej.zak at pwr.edu.pl

śr., 5 lip 2023, 22:06 użytkownik Hannah Turner <mailto:hannaht at scripps.edu> napisał:
Dear Colleagues, 
 
Has anyone had to correct for DC EMI after a site survey? Are there ways to reduce or block occasional near DC peaks other than installing an entire cancellation sytem? 
 
Thank you,
Hannah L. Turner
Lab Manager, Andrew Ward Lab
Scripps Research Institute, La Jolla CA, USA
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