RE: Comments on new Tagger magnet vacuum tank

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

Dear Elton,

Here are comments on some of the points raised in your e-mail below.

1. The water and power to the magnet coils is by means of hollow copper
tubes. There will be a total of 16 or 20 tubes (depending on the field gap
which may have to be different for different designs), and I think they
should all come from the coils to outside the vacuum using individual feed
throughs. The feed throughs should all be located in a single plate which
would have an O-ring seal to the bottom of the base plate. The feed through
plate could be made from a stiff electrically insulating material. The water
and electrical manifolds would then be outside the vacuum, and the only way
a water leak could occur in vacuum is if one of the copper tubes fractured.
This whould be extremely unlikely since  the cooling water pressure between
inlet and outlet is only 1 bar for a 20 degree centigrade rise in cooling
water temperature.

2. I think the monitoring of the magnet can all be done externally, so there
is no need to have any vacuum feed throughs.ie the cooling water flow rate
and temperature rise, and the resistance of each colling circuit which
monitors the temperature of each cooling circuit, can all be measured
externally.

3.  I have been consulting one of my colleagues, who is in the Gravitational
Waves group which works at pressures of 10 to(-8) mbar,  about outgassing
problems from the magnet surfaces which could result in a poor vacuum. If
the iron surfaces are cleaned, heated with heating tape and then a polyamide
coating, which is very tough, is applied before the surfaces can absorb
water vapour, pressures of 10 to(-6) should be possible. As a lubricant
between metal surfaces we could use vacuum grease. Anyway, we should be very
careful to treat all the surfaces that will be in vacuum if it is necessary.

4. Would it be possible to install a fast acting valve near the vacuum tank,
so that if there is a vacuum leak in the tank, the accelerator vacuum would
be protected.

There are many pros and cons to discuss.

Yang, who arrived safely back in Glasgow, tells me that Ravi is already
making CAD drawings which I am looking forward to looking at. Yang also
implied that how the magnet should be assembled and installed, and
deassembled if necessary, is a bit vague. I'm also uncertain, since I'm not
sure what lifting equipment will be available and what the access to the
magnet will be.

I hope to have sketches of the seals for a vacuum system using pole slices
and O-ring seals which could be inserted between the pole shoes ready soon.
I shall send them across as jpg files.

Regards,

Jim.






-----Original Message-----
From: owner-halld-jlab@jlab.org [mailto:owner-halld-jlab@jlab.org]On
Behalf Of Elton Smith
Sent: 29 January 2004 22:24
To: halld-jlab@jlab.org
Subject: Comments on new Tagger magnet vacuum tank



Comments/thoughts on the new tagger vacuum tank design
------------------------------------------------------

I do not have much experience with the tagger designs, so some of these
off-the-top-of-my-head comments are probably obvious to those that have
been thinking about this. I also spoke briefly with Dave Kashy about the
tagger design with external vacuum chamber to get his thoughts on this.
Here are some observations.

Turbo pumps could be about $30K, not excessive compared to magnet cost of
about $1.5 M. Vacuum is 10^-5, compared to 10^-5 in beam line and 10^-6
around target.  Time for initial pumpdown is hours, 10-12 hours to steady
state vacuum.

- All cooling water (LCW), and power for magnet must be in vacuum,
requiring corresponding feedthroughs.
- All instrumentation on magnet would need feedthroughs into vacuum
system.
- Any water leaks would contaminate the vacuum space.
--> Maintenance issues need to be addressed

Current Hall B design uses the rigidity of the magnet itself to support
the vacuum chamber. The vacuum chamber tank design requires external
strengthening.

Pressure on vacuum chamber is approximately for 1m x 7m = 40in x
280in=11,000 in2 x 14.2 lb/in2 = 0.3 x10^6 lbs = 150 tons, or comparable
to weight of magnet.

Volume evacuated is large compared to what is actually required for
physics.

We discussed several of these issues with Ravi, Guangliang, and Elliott.
Ravi has entered many of the hand drawings into the computer and will make
drawings available on the web as pdf files for discussion.



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