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

[Richard (Rick) Karlquist]

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