Re: Electron at rest - magnetic field?

Sue...:
>Bilge wrote:

>The traditional use of the term 'Faraday shield'
>is a structure which blocks E-plane energy but
>passes H-plane energy so your logic seems correct.

I've never heard the term ``faraday shield'' used and that description
makes no sense. The magnetic amd electric fields in an rf signal are
related by E = cB. If you kill the E field, you kill the B-field and
a reduction in the E field is the more important, since the effects from
B are already down by a factor of c.

There is a such thing as a faraday cup, the purpose of which is to
collect charge at the end of a beam line in order to (1) monitor the beam
current, and (2) normalize different measurements in scattering experiments
to the same number of particles. Often, it's just a plate that is
electrically isolated and has wire running to it. Sometimes, it's an
isolated cylinder which is plugged at the far end with a wire running to
it and electrically isolated. In any case, the choice of material is
dictated by the stopping power and the possible desire to work near it
safely or transport it without a lead box and the protests of the health
physics people. For low energy facilities, one therefore uses aluminum,
for which most of the activation induced by the beam is relatively shrt
lived, i.e., a few hours, tops. For a high energy facility, I would guess
the thickness of aluminum required means aluminum is not an option, in
which case they probably might as well use tungsten or tantalum. Aluminum
is also bad for the vacuum.

The only reason that a magnetic field would be important for a faraday
cup would be if the cup were actually a cup, rather than a plate and the
magnetic field was employed to bend secondary electrons produced at the
end of the cup into the walls of the cylinder rather than have them
escape into the beamline. Allowing the secondary electrons to escape
will produce a false reading for the beam current. In this case, one
certainly doesn't want to shield the magnetic field, so it's entirely
possible or even likely that one would at least make the ground shield
around the cup out of stainless, since stainless is _not_ magnetic.

.

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