Radiation Damage Considerations for High-Power Target System
at a Muon Collider and Neutrino Factory

Kirk McDonald
Princeton University
(Feb. 6, 2012)

At a Muon Collider or Neutrino Factory a 4-MW proton beam would
impinge on a free-mercury-jet target that is inside a 20-T
solenoid magnet.  The 20-T magnet is a hybrid with a 5-T copper
insert and a 15-T Nb3Sn outsert.  Nb3Sn must be stabilized with
copper, as a step in the conductor fabrication involves temperatures
close to the melting point of aluminum.  To minimize the magnet 
size, stored energy, and cost, it should be shielded from
radiation damage by the densest practical material, such as
tungsten spheres cooled by He gas.  The magnet should be 
designed for operation subject to maximal radiation damage
consistent with a 10-20 year lifetime, which implies operation
close to the so-called ITER limit of 0.1 mW/g ~ 10 MGy/year.
This dose is 10-100 times larger than that foreseen for operation
of the 5-T Al-stabilized, NbTi magnets of the COMET and mu2e
experiments.