[PEPPo] PEPPo current monitoring

[Joseph Grames]

Dear PEPPo Collaborator:

There have been a few questions regarding our diagnostics to monitor 
electron beam current for PEPPo.

1. There is a beam current monitor cavity ~1 meter upstream of the 5 MeV 
dipole named as IBC0L02 on layouts.  This is a non-invasive device with 
a threshold of about 40nA which is limited by the existing downconverter 
system.  I have a request in the queue for a new dedicated reciver that 
can in principle aim toward <1 nA but is not guaranteed for the Fall.

2. There is a Faraday cup about ~3 meter downstream of the 5 MeV dipole, 
I believe appears as IFY0L02 on layouts.  It is normally used to 
calibrate the BCM at current >1 uA, but can similarly be used by us to 
calibrate the raw signal we may record from BCM above.  I measured a 
threshold of 400-500pA however using a picoammeter with a resolution of 
200pA so it is questionable which limit I was hitting.  We have another 
series of picoammeter with resolution of 0.2pA which we can configure 
for another test in the Fall.

In brief, the current monitoring we seek 20-100 pA will require more 
testing/work.  A new dedicated BCM receiver may be the option to push.  
As both Riad and Larry pointed out there are clearly ways to reduce the 
current to the pA's level.  However, measuring this is not presently 
within our reach.  So, control of HC systematics should be a part of the 
program, i.e., demonstrating with the use of flipping the optical 
waveplate or analyzing magnet that we can flip the sign of the measured 
experimental asymmetry.