Re: possible pair spectrometer magnets (fwd)

[Jim Stewart <jstewart@jlab.org>]

Hello Hrachya

I do not understand this. The vacuum chamber will need to be longer to
insure that 12 GeV electrons don't go down the beam pipe and to give enough 
space for shielding. The window cut out for the 9GeV particles also has to 
be far enough from the beam line to have mechanical strength. These 
criteria are directly affected by the Int[B*dL]. On the low energy side 
the particles never hit the yoke on one side so as long as the particles 
do not curl up inside the field you can still detect them.

I suggest that you look at what the vacuum chamber would look like for the
20x42 and then compare how the design would be with the 24-8-72. The 72 
inch long pole gives plenty of Int[B*dL] and the 24" pole would still give 
a uniform field over about 500mm even if we put plates in the gap.


At some time we may need to exchange sketches.

Jim


On Wed, 4 Jun 2008, Hrachya.Hakobyan wrote:

> Hello Jim,
>
> The C-shape magnet mentioned has a BL=2T*m. This ls slightly shorter than
> expected(2.6T*m) for the wide range of momentum analysed  from 3 to
> 8GeV/c at maximal field setting as I showd in the draft of PS proposal.
> But decrease of the gap, currently 20cm, down  to f.e. 5cm , will
> slightly increase the mean field and its homogeneity. If there is no
> other choice, this magnet may  be used with reduced acceptance of momentum
> analysis and the measurements of the photon spectrum in the range of
> 6-12GeV (within assumption  of reasonable energy resolution)  will be
> realized with  more than one magnetic field setting,so
> not simultaneously. That is acceptable in the presence of the beam
> monitors for the absolute integral flux measurements.
>
> Hrachya
>
> On Tue, 3 Jun 2008, Jim Stewart wrote:
>
>>
>> Hello Dan
>>
>> How serious a concern do you think this is? If we reduce the gap to 40mm
>> or so, there should not be that much bulging of the field. The pole shape
>> is however rounded at the ends so the magnet will need mapped.  I think we
>> definitely want to use the pair spectrometer to calibrate the tagger
>> absolute energy so we need a good energy sum measurement.
>>
>> Jim
>>
>>
>> On Tue, 3 Jun 2008, Daniel Sober wrote:
>>
>>> Dear Jim et al.,
>>> The one disadvantage of the C-magnet is that it would require a more
>>> detailed field map to precisely identify symmetric pairs for an energy
>>> calibration measurement -- in case that is on anyone's agenda.
>>> Dan Sober
>>>
>>> Jim Stewart wrote:
>>>> Dear tagger
>>>>
>>>> While at BNL I went looking for possible magnets for the pair
>>>> spectrometer. I found several which look like good candidates.
>>>>
>>>> The 20x42 looks to be very interesting. It is a C-Magnet with the
>>>> following characteristics:
>>>>  pole width  508mm
>>>>  pole gap    203mm
>>>>  pole length 1067mm
>>>>  Max field   1.85T
>>>>
>>>> The gap is too large but it would be easy to add plates to the poles to
>>>> reduce this. A C-Magnet has the advantage that on one side of the magnet
>>>> we can measure very low momentum particles. Installing the vacuum chamber
>>>> can also be done without taking the magnet apart.
>>>>
>>>>
>>>> The 24-8-72 is an H-magnet with a field clamp and relatively wide poles.
>>>>  pole width  609mm
>>>>  pole gap    203mm
>>>>  pole length 1829mm
>>>>  Max field   1.85T
>>>> To reduce the gap here you would need to add tapered plates. This could
>>>> be done but requires a simulation.
>>>>
>>>> The 18D36 is also an H-magnet. It has a somewhat narrower pole and is more
>>>> compact.  (18" wide pole and 36 inches long)
>>>>
>>>> They may also have power supplies for the magnets above. We would need to
>>>> install new controls in any supply we got.
>>>>
>>>> I feel that the 20x42 is a rather good match to our needs.
>>>>
>>>> Please let me know what you think!
>>>>
>>>> Jim
>>>>
>>>> ------------------------------------------------------------------------
>>>>
>>>>
>>>> ------------------------------------------------------------------------
>>>>
>>>>
>>>> ------------------------------------------------------------------------
>>>>
>>>
>>> --
>>> /Daniel Sober
>>> Professor and Chair
>>> Physics Department
>>> The Catholic University of America
>>> Washington, DC 20064
>>> Phone: (202) 319-5856, -5315
>>> E-mail: sober@cua.edu/
>>>
>>
>
>