Re: What happens when an EMP field intersects an EM field?

I will try to answer your questions in reverse order.  So, at the end of
your message, you ask if it is possible to use an intense electromagnetic
field to shield out an electromagnetic pulse (EMP), and what the
limitations of that shielding might be.

The short answer is that you can't generally use an EM field to stop an
EMP; that is, generally the fields will simply pass through each other
without interacting.  I say "generally" because, in certain very special
materials, intense electromagnetic waves (also known as light) will
interact, up to the point of stopping one of the waves from propagating. 
The field of "nonlinear optics" is devoted to studying these types of
interactions, and finding applications for the materials that have these
nonlinear properties.  Before you get your hopes up, however, I should
point out that the samples used in these studies are typically very small,
and that the lasers used to generate the intense fields are pulsed -- in
other words, making an EMP 'shield' that provides continuous protection is
a long way off from what exists in the laboratory right now.  *IF* it is
possible at all.

As for 'EMP guns' and pacemakers, you are correct that the usual EMP
shielding (a metal box or cage, also known as a "Faraday cage") is not
practical for the driver or passengers of a car.  The EMP that fries the
car's ignition system would also fry the pacemaker, leaving the owner
vulnerable to whatever condition(s) led to the installation of the
pacemaker in the first place.  However, the dirty secret is that the car's
ignition system can be shielded fairly easily by someone who is willing to
spend a little time on it.  In the lab where I work, we regularly shield
microvolt- or millivolt-scale signals from kilovolt-scale pulses and waves,
and the techniques we use are fairly standard and well-known.

Besides, the police couldn't really use an EMP gun if their own cars were
vulnerable to the same EMP.  So there will need to be shielded cars from
the first day that the EMP guns are used.  And, if the police can get
shielded cars and EMP guns, then so can the criminals.  My guess is that
this proposal will go nowhere, for the simple reason that the EMP guns
would be expensive, heavy, dangerous to bystanders, and (in the end)
useless for stopping criminals.

I hope that this answers your questions.  If not, please submit a
followon question to MadSci.