Email from Dave:

From mack@jlab.org Fri Sep 17 11:44:50 2004
Date: Wed, 15 Sep 2004 16:26:45 -0400 (EDT)
From: Dave Mack 
To: Mark Pitt 
Cc: Roger Carlini , finn@physics.wm.edu,
     'Neven Simicevic' ,
     'Jim Birchall' ,
     'Juliette Mammei' , 'Klaus Grimm' ,
     armd@jlab.org, 'Allena Opper' ,
     'Greg Smith' , 'Norman Morgan' ,
     'Mike Finn' , 'Yongguang Liang' ,
     'Shelley Page' ,
     'Tony Forest' , richard.t.jones@uconn.edu
Subject: Re: Qweak- Collimator Working Group

    [ The following text is in the "X-UNKNOWN" character set. ]
    [ Your display is set for the "ISO-8859-1" character set.  ]
    [ Some characters may be displayed incorrectly. ]


	Mark et al,

	Yes, it's fair to reopen the discussion since the Chairman wasn't
present. 

	The notes I sent out the other day suggest that the defining edge
of Jim's collimator was far enough from the target center already (~140
cm). Further is better (all other things being equal), but longitudinal
alignment tolerances do NOT appear to be a decision driver.

	Minitorus ripple is several orders of magnitude smaller than beam
motion at the line frequency even when fast feedback is on. (See my
attached notes.) I don't see this as a decision driver.

	I suspect a collimator in either location would give us an
experiment. So what are the REAL trade-offs? 

1. Defining collimator after the minitorus:

	bad news: We need a 1% field map and as-yet-unspecified alignment
tolerances to avoid screwing up our Q^2 determination. This is annoying,
not fatal. 

	good news: We presumably get a crisper definition of the angular
acceptance, therefore a tighter Q^2 distribution without long tails, and
better separation of elastic and inelastic electrons. We await proof of
this from simulations. (At the last meeting we remembered that we hadn't
rechecked the elastic/inelastic separations in aeons. At least I think
that is a true statement.)


2. Defining collimator before the minitorus

	bad news: Generally longer tails on the Q^2 distribution.
Annoying, not fatal. But inferior separation of elastics and inelastics
could be serious. We await those results.

	good news: clean Q^2 determination without having to know any
magnetic fields.


	regards,

	Dave


Dave's attachment:


ripple_note.txt