Re: How can you physically collapse magnetic flux lines?

    I am not sure what you mean by your question.  If I assume that your 
word "collapse" means "compress" I can answer from personal experience.  
In the summer of 1964 I worked at Los Alamos in a group doing explosive 
compression of magnetic fields.  The idea was to make ultra high magnetic 
fields.  A typical experiment involved a tungsten cylinder about 5 inches 
in diameter and a foot long.  The cylinder was placed in a Helmholtz coil 
powered by a room size capacitor bank.  Around the tungsten cylinder was 
placed a large doughnut of very high energy explosive which weighed 
perhaps 100 pounds!  It had about 100 detonators arranged on the outside 
of the doughnut.
     At the end of the countdown the capacitor bank discharged pushing a 
very high current into the Helmholtz coil.  This made a field of about 
20,000 Oe.  The rise time of the field was slow enough that it could 
diffuse through the tungsten cylinder.  At the peak field the explosive 
was detonated which compressed the cylinder so quickly that the field 
lines could not diffuse out.  The hole in the cylinder got smaller until 
the pressure of the field equaled the pressure from the explosive.  The 
result was a field of over 15,000,000 Oe which was a record that stood 
for over 20 years!    
     The experimentors (including me) were in a concrete and steel bunker 
only 25 feet or so from the explosion.  It was quite an experience.  I 
have a lot of sympathy for the folks in Iraq who have to worry about 
similar explosions without the bunker.