Update from JLAB

[Jim Stewart <jstewart@jlab.org>]

Hello Everyone

I wanted to say a few words about some calculations Tim has done.

Tim looked in more detail into the design of the o-ring for the tagger 
magnet.
He investigated using a 3/8" inch (9.5mm) o-ring. The manufacturers 
specification
for the sealing deformation of this o-ring is .082 to .108" (2 to 2.7mm) 
for
22% to 28% compression. The force needed to compress a shore-A durometer 
70 o-ring is
27,000 pounds or 12,247 kg. This is roughly the force the inner steel 
slab would
exert on the o-ring just due to its own weight. This means compression 
is no problem.
We just need to remember the wedges in the vacuum chamber gap to support 
the
opening edge. The smallest bending radius of a 3/8" o-ring is about 1" 
which effects the
vacuum chamber o-ring grove. Designing the grove with a reasonable 
amount of material
on both sides of the o-ring and the minimal radius of curvature at the 
ends requires
increasing the dimensions of the sealing surface on the iron and the 
wall thickness of the
vacuum chamber. Tim computed that the side walls should increase from 
15mm to
19mm and the sealing surface at the up and downstream ends of the magnet 
will need to
increase from 15 to 32mm.

Tim then computed the deflection for the pole surface due to the 
magnetic force.
Using the force of 300 metric tons (from Yang's simulation) Tim computed 
that the
bolts in the existing design of the magnet need to be torqued to 70% of 
their yield
point to pre-stress them to match the magnetic load. If this is done 
then the deflection
of the pole is about 0.1mm Yang computed previously a pole deflection of 
about
0.2mm but used much less tension on the bolts. Therefore Tim feels the 
calculations
are compatable. We will probably increase the number of bolts from 1 
bolt every
18" to 1 bolt every 12" to reduce the needed torque per bolt. With a 
total deflection
of 0.1mm due to the magnetic force there is no need to go to a larger 
o-ring. 3/8" is OK.

0.1mm is 1 part in 300 of the total gap. We should estimate what effect 
this could have
on the field uniformity. We may want to have the poles machined so that 
they have a 0.1mm
wedge shape. This way when the magnet is energized, the poles become 
perfectly flat and
parallel.

Many thanks to Tim for crunching the numbers.
Jim