Re: Hall D / Tagger magnet orientation (fwd)

[Elton Smith <elton@jlab.org>]

---------- Forwarded message ----------
Date: Thu, 29 Jun 2000 18:53:00 -0400
From: Richard T. Jones <Richard.T.Jones@uconn.edu>
Reply-To: jonesrt@uconnvm.uconn.edu
To: Jay Benesch <benesch@JLAB.ORG>
Cc: Elton Smith <elton@JLAB.ORG>, douglas@JLAB.ORG, harwood@JLAB.ORG,
     lebedev@JLAB.ORG, andrew@JLAB.ORG
Subject: Re: Hall D / Tagger magnet orientation

Dear Jay,

Thank you for looking into the question of rotating the beam ellipse going
into the Hall D radiator.  The reason we asked for this is that we want to
minimize the spot size in the direction PERPENDICULAR to the bend.  I was
concerned that rotating the magnet without rotating the beam at the same
time would degrade our ability to tag the coherent part of the bremsstrahlung
beam.  Your  assumption that we would want to minimize the spot size in the
bend plane is perfectly reasonable, assuming that we want to optimize the
spectrometer resolution.  However this is not what we want, for two reasons.



1) Our design criterion of 0.1% r.m.s. energy resolution in the tagger is
not rigid -- the figure of merit does not change very much if the resolution
degrades to 0.15%.  We will continue to use 0.1% as our goal, but keep in mind
that if under some senario it comes out as 0.12% no one is going to get worried.
Please note that this statement only applies to THIS parameter, and is Not a
general statement implying that other parameter specifications for the Hall D
beam line are soft.

2) According to our optics calculation for the Hall D tagging spectrometer,
a bend-plane spot size of 1.7mm rms and beam energy spread of dp/p=0.0008
gives us a tagged-photon energy resolution of 0.10% for the design setup,
with roughly equal contributions from beam energy spread and spot size.
Further optimization of the beam spot size in the bending plane is not
useful to us.

3) We would very much like to have a beam spot size in the perpendicular
plane as small as possible.  Right now the size is 0.7mm.  We would benefit
from getting it as small as possible.  Vertical collimation of the recoil
electron in the spectrometer focal plane would allow us to increase our
tagging efficiency, which limits our ability to efficiently tag at high rates.
Therefore, we would like to optimize the beam spot size, vertical axis in
the situation where the bend plane is horizontal.  I think our present design
is close to optimal in that parameter, isn't it?



I hope that I have been able to clarify this, because I have been asked by
several others why we don't want improve the energy resolution by rotating
the spot or the magnet.  That is not desirable; the beam spot is the way it
is intentionally.

>From your note it sounds like it is a big deal to rotate the ellipse, compared
to the alternative ways we might use to bend the beam slightly downward into
a near-grade beam dump.  The reason that this idea came up was to save the cost
involved in piling up shielding around a beam dump located 1.5m above grade.
An obvious alternative would be to keep the present optics design, tag in the
horizontal plane, and then put a vertical dipole after the tagger that gives
the beam a downward 3-degree kick into a dump near grade level.  What would
you recommend, given our limited real-estate?

Richard Jones