[BTeam] non-linearity

[Jay Benesch]

Thinking about the 2R problem after a discussion with Yves and the way 
the differential orbits go into phase in the first pass, the beam pipe 
through the trapezoidal dipoles came to mind.  After 3D modeling in 2008 
I recommended a 57 mm hole with a centered 50 mm OD, 4 mm wall carbon 
steel beam pipe to reduce field and remove gradients from the beam 
running through the return steel.  Tommy Hiatt increased the return 
steel thickness from that Robin specified and I modeled, did 2D models 
in three Z locations, presented those results to someone, and it was 
done.  I have not modeled the installed magnets, the XT and XS, with 
their stainless beam pipes.

It seems to me that replacing the stainless beam pipes running through 
the return steel in the four corners of the main machine with carbon 
steel can only have benefits.  Whether 42 mm ID suffices in the 
transport recombiner I haven't checked, but suspect not.  Even a direct 
replacement of stainless with carbon steel tube would help reduce 
gradients there.