[PEPPo] positron capture magnet

[Jay Benesch]

I did a quick and dirty model of a water cooled copper capture solenoid 
30 cm long with 30 cm IR and steel outside 20 cm IR.  In order to get 
1.8 T peak I had to increase the current density to unacceptable levels 
with 10 cm radial extent so that is 24 cm in the attached.  Current 
density 1000 A/cm^2 in coil pack, that in the copper will be larger. 
Steel is 15 cm thick and still well into saturation.  There's a 1 cm gap 
between coil and steel.  Field at [0.0,-50] 1260 G, about a fifth of 
what Andriy showed.  Default "good magnet steel" BH curve used. Field 
will be a few percent lower if 6" cold rolled steel of easily available 
grade is used.
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