Why to use negative hydrogen in spallation neutron source?

[kiwaho]

Just wondering why to use negative hydrogen H^- ion in spallation neutron source.
I read the literature through http://neutrons2.ornl.gov/facilities/SNS/works.shtml , still not yet get the answer.
If directly use proton, then still easy to accelerate, and no need to strip off the unwanted electrons by stopping carbon foil before entering storage ring.
I think there must be a reason to choose expensive H- instead of economy proton.

 

[mfb]

It makes creating high-intensity pulses easier. CERN has an introduction here and many more references for details.

 

[Vanadium 50]

When you want to move a beam from a linac into a circular machine, you need the injected beam to bend the opposite direction as the circulating beam in order to place the two beams together. If you do this with magnets, that means the linac beam and circulating beam needs to have opposite charge.

 

[mfb]

It does not need to have opposite charge - you can use kicker magnets, as most accelerators for high-energy physics do.
Those magnets are very fast with their change in magnetic fields, so you can inject particles at one field geometry and then quickly go back to the correct field for circular orbits afterwards (I know you know how they work, that explanation is for other readers).