Re: Pole contour

[Jim Stewart <jstewart@jlab.org>]

Hello Yang

With the preparation of the collaboration meeting I did not get a chance 
to reply to your mail. Here is my long overdue reply.
"If the tagger magnet will only operate at 1.5 T, there is no need to 
use a Rogowski type profile.-Yang"
You made plots of the Mu for the pole but you had a rather unfortunate 
scale on the plots. I would like to ask you to set the scale so that the 
maximum mu is 1000. I think then one would see much better the 
improvement in the mu within the iron. I believe that a higher mu would 
make the magnet more immune to fluctuations in chemical composition as 
you go along the length. However in the absence of a study I do not know 
how much an effect this could have. In general I feel the higher the mu 
the more robust the design of the magnet. This is a very weak argument 
in the absence of any studies.

I modeled the magnetic field in ANSYS for the original Glasgow magnet. I 
have included horizontal sweeps in the y (vertical) component of the 
field in the two attached ps files. In Tg_1.5_hsweep1.ps you will find 
zoomed in plots of B_y in the pole region. Please excuse the lack of 
axises and definition of the coordinates. The lower right plot is the 
field in the horizontal plane. x=0 is the center of the pole and the 
vertical axis is the B_y field in T. The lower left plot is the field 
5mm above the horizontal plane. Here you already see a small increase in 
the field at the corners. In the upper right plot you are 10mm above the 
horizontal plane and the effect is larger. In the upper left plot you 
are at 15mm which is along the pole surface and see the spike in field 
up to ~1.8T. I believe this effect will be weaker with the Rogowski 
contour. Could you please check this. If the out of plane variations are 
smaller then the alignment errors will have a smaller impact on the 
integrated B.dl. This would be a clear positive advantage of the more 
complicated contour even at 1.5T and even on the one surface. I was 
going to look at this but I would need a step file and it looks like we 
are going to have to go back and forth a bit before yours and Jame's 
model match.

"I should point out that if we don't use Rokowski profile at both short 
edges, I am sure the magent effective length and the field uniformity 
will be affected at high field region, this will not affect the tagger 
resolution, but it is big enough to effect the dispersion and the tagger 
energy calibration." -yang

Here I believe you are trying to argue that by putting the Rokowski 
profile on all the surfaces you would improve the performance at higher 
fields at negligible cost. I am trying to think of what studies you 
could do to demonstrate this without costing you much work You have both 
the 25mm-45degree contour design and the 4 surface Rokowski contour 
modeled and meshed. If you take both models and again crank up the 
current then you can compare them at 1.8T. You can see how large the 
uniform field region is in both cases. You can also then measure the 
change in effective field length in the two cases. Now you will have 
real numbers to back up your argument for a 4 surface Rokowski design. 
(If you want to get really fancy here you can also bolt a 200mm thick 
plate on the back (-x face) of the magnet to reduce the saturation in 
the return yoke. However you would then need to re-mesh and may have 
problems with too many nodes. If JLAB goes to 18GeV then the extra plate 
would be a simple upgrade if we foresee it in advance. )

"For the edge near the return yoke, because the width of the pole is 
minimized for the Rowkiski profile, if we want to use other profile,  
the effective pole width must be increased, because the good field 
region will be smaller. That means we will not save a lot of materials, 
but we can save some machine cost if we use other profile.  There will 
be also a small effect on the energy calibration. "-Yang

I don't understand this. What I understood from you before is that for 
the same uniform field width the Rowkiski profile increased the pole 
width by ~1cm.

"So I insist we should use the Rogoeski profile all around the pole, the 
reason is that it's effect on the cost is small and its effect on the 
tagger performance is positive.We have already saved some money from 
other places, for example, the strongback support, and we only need one 
NMR detector instead of two. "-Yang

The price of steel has increased by a factor of 2 since the last quotes 
so what ever we do we will be over budget.
Any change  we make has to be justified against the cost. I also do not 
want to reduce the number of NMR probes. We need a minimum of 2 probes 
so we have spares. I would then rather have it installed an read out 
than sitting on a shelf. With 2 probes you can also stabilize on one 
probe and measure the variations with the other. This gives a cross 
check on any hysteresis or variations in mu with temperature.

In general I am in favor of the 4 surface Rokowski contour but if asked 
during the review why we need it I need backup.

Hope this is clear.
Let me know what you think.
Jim

Guangliang Yang wrote:
> Dear Jim,
>
>    If the tagger magnet will only operate at 1.5 T, there is no need 
> to use a Rogowski type profile.  The reason that we want to use a 
> Rogowski type profile is that if we want to run the magnet at higher 
> magnetic field for example 1.8 T. If we can measure the field map for 
> every point we want to run, there will be no need to use a Rogowski 
> type profile. I should point out that if we don't use Rokowski profile 
> at both short edges, I am sure the magent effective length and the 
> field uniformity will be affected at high field region, this will not 
> affect the tagger resolution, but it is big enough to effect the 
> dispersion and the tagger energy calibration.
>   For the edge near the return yoke, because the width of the pole is 
> minimized for the Rowkiski profile, if we want to use other profile,  
> the effective pole width must be increased, because the good field 
> region will be smaller. That means we will not save a lot of 
> materials, but we can save some machine cost if we use other profile.  
> There will be also a small effect on the energy calibration.
>  So I insist we should use the Rogoeski profile all around the pole, 
> the reason is that it's effect on the cost is small and its effect on 
> the tagger performance is positive.We have already saved some money 
> from other places, for example, the strongback support, and we only 
> need one NMR detector instead of two.
>
>
> Cheers
>
> Yang
>
>
>
>
>
> ----- Original Message ----- From: "Jim Stewart" <jstewart@jlab.org>
> To: "tagger list" <halld-tagger@jlab.org>
> Sent: Friday, September 12, 2008 12:37 AM
> Subject: Pole contour
>
>
>> Hello Yang
>>
>> We loaded the step file you sent us and have a few questions. Tim 
>> will make a drawing which will make
>> clear what we want to verify but there is one point I want to raise 
>> early.
>>
>> In the last meeting Dan pointed out that we really have no need for 
>> the rowgowsky contour on the up and downstream
>> faces of the magnet nor the long inside face.
>>
>> If we keep the sealing surface as it is now and put the 25mm 45% 
>> contour on those three surfaces we could save then 5-10K
>> according to Tims estimate.
>>
>> Any thoughts on this proposal? Can you think of any reason why it 
>> would not be a good idea?
>>
>> Jim
>>
>>
>>
>

Tg_1.5T_hsweep2.ps

Tg_1.5T_hsweep1.ps