[time-nuts] "Shaking" of magnetic shields in atomic clocks

[Joseph Gwinn]

On Wed, 02 Sep 2020 17:49:47 -0400, time-nuts-request at lists.febo.com 
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> Message: 7
> Date: Wed, 2 Sep 2020 14:31:04 -0700
> From: "Richard (Rick) Karlquist" <richard at karlquist.com>
> To: Discussion of precise time and frequency measurement
> 	<time-nuts at lists.febo.com>
> Subject: [time-nuts] "Shaking" of magnetic shields in atomic clocks
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> 
> In the NIST paper available at the URL below:
> 
> 
<http://pdfs.semanticscholar.org/47ac/742de238c0ece5e91ff7d12c515b9173eb60.pdf>
> 
> At the beginning of page 2 (4th line) the paper
> states:
> 
> "Note that the shield permeability is a nonlinear function of the 
> magnetization and increases to a maximum value of umax =400,000 at
> higher applied fields. ?Shaking? the shields by continuously
> applying an alternating magnetic field is a way to take advantage of umax."
> 
> Another paper "The effect of shaking on magnetic shields"
> has this abstract:
> 
> "The increase of the shielding factor due to shaking was measured in a 
> scale model for a magnetically shielded room. The increase was found to 
> be 7 dB for a single-layer square cylinder biased by the Earth's 
> magnetic field. The shielding factor of a large-volume three-layer 
> Mumetal ? room was estimated to increase by a factor of 30, thus 
> confirming the feasibility of shaking in magnetic shields. The shaking 
> parameters, amplitude, and frequency are not critical according to the 
> experiments. Winding the shaking coils along the edges of the cubic 
> shield leads to minimum disturbances inside the cube, and the winding 
> can also be applied to demagnetize the shield by an alternating field of 
> 25 A/m, 50 Hz. The relative incremental permeability of Mumetal was 
> studied as a function of the shaking and biasing fields. The 
> permeability was found to increase considerably by shaking and by 
> decreasing the biasing field. With zero biasing and with shaking field 
> of H s = 5 A/m root mean square (rms), 50 Hz, the permeability reached 
> its maximum value of 89 000, which is sevenfold the value without shaking."
> 
> In all my work on atomic standards, I never heard of this.  Has
> anyone else heard of this?  I don't understand how a large
> AC mag field can be applied to the shield without getting inside
> and messing up the atoms by means of Zeeman effect.
> 
> BTW, the NIST paper has a nice exact formula for a spherical
> magnetic shield (eqn 3 on page 1).  Good reading, as usual from
> NIST.
> 
> Rick Karlquist
> N6RK
> 

I have not read the paper yet, but this reminds me of the use of AC 
bias to linearize the magnetic coating of recording tape.  That has the 
property of causing linear behavior right through zero average field.

Joe Gwinn