[Halld-pid] B-field in region of TOF PMTs

[Alexander Ostrovidov]

David,

Thanks for looking into this. I just want to remind people
that a steel tube & mu-metal sleeve for Hamamatsu
tubes were measured at JLAB. An outside field of
200 Gauss was reduced to less than 1 Gauss inside. So, 
even  a 50 Gauss field shouldn't be of concern to us 
unless FCAL  steel frame increases it by the orders of 
magnitude.

Sasha

On Thursday 26 August 2010, David Lawrence wrote:
>  Hi Matt,
>
>      That's a good point. Is there a preliminary design for the
> support structure that we could use to try and improve the
> calculation? It would not need to be terribly accurate as we will
> likely need to model it in a few simple shapes in order for the field
> calculation to converge.
>
>  Regards,
>  -David
>
>  On 8/26/10 10:26 AM, Matthew Shepherd wrote:
> Hi all,
>
> Just to add my 2 cents...  I would be weary about any field
> calculation in that region that does not include the steel frame of
> the FCAL.  There will be a lot of steel in the box tubing that makes
> up the core support structure of the FCAL.  The PMTs will be sitting
> very close to this steel, which will likely concentrate the magnetic
> field lines in the region of the PMTs.
>
> Bottom line is I think that ToF design will need a multi-layer
> steel/mu-metal shield (like the FCAL) to cope with potentially high
> fields.  I'm not sure how all of this drives the field spec -- seems
> like a more detailed field calculation coupled with any mechanical
> constraints of the ToF (e.g., maximum shield diameter) is really
> needed to set a spec.
>
> -Matt
>
> On Aug 26, 2010, at 10:14 AM, David Lawrence wrote:
>
>
> Hi All,
>
>     This is mainly directed at the folks at FSU, but since others may
> have an interest and wish to comment, I'm sending this to the
> halld-pid mailing list.
>
>     I'm currently looking at the field specifications for the
> solenoidal magnet. One aspect of this is the fringe field at the
> region of the TOF phototubes. A specification based on the field of
> the LASS magnet was given in GlueX-doc-1542. Elton noticed that this
> differed by nearly a factor of 2 from a map referenced in an earlier
> document, GlueX-doc-912. I made some plots from several of our
> existing field maps in order to compare them. I have placed them on a
> wiki page here:
>
> http://www.jlab.org/Hall-D/software/wiki/index.php/Magnetic_Field_in_
>region_of_TOF
>
> The main issue is simply whether a specification of <50 Gauss in the
> TOF PMT region is sufficient. I'm guessing it is since I believe that
> has been our expectation for some time. However, I would like to make
> sure FSU (and eveyone else) is on board with that before making it an
> official specification for a new magnet design. I suppose the other
> other side of the coin is: can it be higher and if so, how high?
>
> Regards,
> -David
>
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