RE: Notes Meeting Wed Jan 28 / tagger

["James Kellie" <j.kellie@physics.gla.ac.uk>]

Dear Elton,

Thankyou for providing an accurate summary of the proposed design for an
external tagger vacuum system which was discussed yesterday. Please treat
the design as a possible option, which should be considered alongside other
options before a final design can be determined.

Indeed in an e-mail from Yang, he mentioned another possibility suggested by
Paul Brindza, which should definitely be considered.

The suggestion - as I understand it - is to take a horizontal slice from the
bottom of the top pole shoe and the top of the bottom pole shoe and
construct a single welded vacuum chamber which would incorporate the two
slices of pole shoe. This would make the vacuum box a reasonable weight, the
box would be inserted between the pole shoes, could be taken out of the
magnet if there are vacuum leaks without taking the magnet apart, and could
be manufactured and tested independently of the magnet. We would have to
make sure the box was sufficiently strong to withstand the atmosphere
forces. When the box is inside the magnet, extra strengthening would be
available by screwing the top and bottom lids to the field clamps. Outside
the magnet, removable spacers could be inserted between the slices of pole
shoes for vacuum testing. There would probably be some distortion of the box
between the pole shoes, the effect of which on the field uniformity will
need to be investigated.

An alternative to welding, is to use O-ring seals between the vacuum box and
a lip on the poleshoe slices. I think this is probably OK, since pressure
could be applied to the seals by having tightening screws, outside the
vacuum seals, which would go through the - reasonably thin - pole slices and
be threaded into the walls of the vacuum chamber. I shall send a sketch to
clarify what I mean. This is safer that using similar seals between the
whole poleshoes and the vacuum box since the O-ring sealing then depends on
very accurate machining of the whole structure of the spectrometer, which is
not affected  when the magnetic forces are applied. At 1.8T this force is
400 tonnes.

Cheers,

Jim.






-----Original Message-----
From: Elton Smith [mailto:elton@jlab.org]
Sent: 28 January 2004 19:30
To: halld-jlab@jlab.org
Cc: Richard Jones; sober@cua.edu; crannell@cua.edu; Jim Kellie
Subject: Notes Meeting Wed Jan 28 / tagger



Hall D Meeting Wed Jan 28 / tagger
----------------------------------

Guangliang Yang arrived over the weekend, but due to the snow storm and
the lab closing, meetings with Ravi and Paul could not be scheduled till
Tuesday. Guangliang met with Ravi and Paul yesterday and reviewed the
drawings that Jim produced and sent around for comments.

This is a new concept based on the magnet being completely inside a large
tube which is evacuated. The tube itself is approximately 7 m long, the
tube is 2 m in diameter. The vacuum would be connected directly to the
accelerator through various apertures. The suggested vacuum for this
region is 10^-4 torr (Hall B is at about 10^-5). The pipe would consist of
approximately 270 degrees of round pipe. The remaining section would
consist of two flat sides: one to support the magnet and the other
parallel to the focal plane detectors.

The magnet is 90 tons (6m x 1.5m)

Questions raised included the cost of pumps and speed with which they can
pump down the system. Beneath the large tube would be attached a smaller
but longer (11 m total) vacuum chamber with an exit window which would be
close to the tagging focal plane.

Access to the water and electrical connections on the magnet would be
provided through the upstream flange which is a cap to the tube. However
it would require breaking vacuum for any maintenance and reestablinshing
adequate vacuum afterwards. This activity must be able to be accomplished
on a reasonable time scale.

Access to the magnet and coils would be by detaching the 270 degree round
side which is under vacuum through a o-ring seal. Presumably this would
happen very infrequently and in the current civil design perhaps require
removal of the roof on the tagger building.

This design raises many questions especially related to breaking and
reestablishing vacuum, and maintaining vacuum at the required level. First
the the level (10^-4 torr) should be reviewed as it may drive many other
decisions. Also, maintaining a vacuum region which is in contact with
coils may be an issue as they may trap gas and/or outgas. Finally the cost
of pumps of the necessary size to establish vacuum in a relatively short
time needs to be determined.

Ravi is putting some of these concepts into electronic drawings which can
be used for further discussion. He will continue working with Guangliang
till tomorrow when we will review where we are and see if there are any
more issues which can be addressed during this visit. These considerations
can serve as a basis for further discussions by the collaboration.

Cheers, Elton.


--
Elton Smith
Jefferson Lab
elton@jlab.org
(757) 269-7625