sPHENIX discussion of the superconducting solenoid

[Kin Yip <kinyip AT bnl.gov>]

Hi,

Below is my (ever-continuing) email communication with Pasquale.   You may read and there are probably at least two results:

(1) Pasquale also did a calculation comparing the forces in 3 scenarios:
  (i) BaBar (with barrel and endcap yoke)
  (ii) Only the barrel yoke (no endcap)
  (iii) no yoke at all

His plots are :
https://collab.external.bnl.gov/sites/sPHENIX-Magnet/Shared%20Documents/Documents%20introduced%20or%20sent%20to%20us%20by%20Pasquale%20Fabbricatore/2016-1-28%20Axial%20forces%20in%203%20scenarios.pdf

Pasquale likes to say "integral force" (which is absolutely clear to me) ... But it turns out that the worst case is (ii) which is ~close to the sPHENIX,
in the sense that we have a barrel yoke (HCAL) but the endcap is far away --- like it doesn't exist.  The central B field for (ii) is 1.37 T which is close
to our 1.4 T.  

So, in this aspect, his result kind of confirms what Wuzheng has told us, "no-yoke" is not the worst ... comparatively, the sPHENIX is probably the worst case :-)

(2) He admits that the "no-yoke" case is not as bad as he/they had expected 20 years ago :-) 


Wuzheng told me that the axial forces are mainly due to Br  (as I x Br gives the force in z) and he has plotted the Br(z) and Br(r) using the symmetric sPHENIX
(here he ignores the junction box/chimney).  His B(z) is only half of sPHENIX (as they're symmetric).

His plot is :
https://collab.external.bnl.gov/sites/sPHENIX-Magnet/Shared%20Documents/Dr.%20Meng%20Wuzheng%27s%20calculations/2016-1-28%20radial_field_comp.xlsx

You can see that inside the solenoid, the Br is slightly bigger for the case of symmetric sPHENIX than that without-yoke, and therefore it gives rise to slightly
bigger axial forces.   ( Wuzheng said that people like Achim and Walter can repeat the same result. )

Kin

PS:

Many of Wuzheng's calculations can be found in :
https://collab.external.bnl.gov/sites/sPHENIX-Magnet/Shared%20Documents/Forms/AllItems.aspx?RootFolder=%2Fsites%2FsPHENIX-Magnet%2FShared%20Documents%2FDr.%20Meng%20Wuzheng%27s%20calculations

-------- Forwarded Message -------- Subject: Fwd: Re: FW: Forces on BaBar Coils (with no steel involved) Date: Thu, 28 Jan 2016 14:49:09 +0100 From: fabbric <Pasquale.Fabbricatore AT ge.infn.it> To: Yip, Kin <kinyip AT bnl.gov>

.. I forgot...

If you look at the axial force vs position, you can see that
in the coil center the forces are outwards directed, while at the coil
ends the forces are inward directed. This means that the coil ends are pressed
more than the integral force says. If you look at the TDR
at page 29 this is discussed. Now in TDR the coil was formed by one layer
only , of larger conductor, but the situation is not too different.
In TDR the integral compressive force was calculated to be 3.2 MN.
The force in the coil center was 1.6 MN outwar directed causing a total
compression of 4.8 MN of the coil ends.
This applies also in the present case increaseing the real compressive
force.

Cheers
 Pasquale

-------- Messaggio Inoltrato -------- Oggetto: Re: FW: Forces on BaBar Coils (with no steel involved) Data: Thu, 28 Jan 2016 14:10:40 +0100 Mittente: fabbric <fabbric AT ge.infn.it> A: Yip, Kin <kinyip AT bnl.gov>

Kin,

I confirmed what I said during the meeting of March 26tt
because it was based on the computations and considerations done
20 years ago.

Now I cannot check the computations reported in TDR but I stil have an axi-symmetric
model of Babar coil so I did today some analyses.
Find attached the file with results.

I considered:
1)  Babar Coil with Yoke (End Caps+Barrel)
2) Babar Coil with only barrel yoke (end caps removed)
3) Only coil with no yoke

Al case with the same current.

One can see the axial magnetic force as function of the position
and the integral axial force. This force is in total compressive
both side. The positive values for negative position only means that
the forces are positive directely (the opposit for the other half coil).

Looking at the force integral you can see that the axial compression
for: a)Coil+ Yoke is 3.45 MN; b) Coil + Barrel only is 6 MN;  c) Coil withouth
yoke  is 4.68 MN.

So without the whole yoke the integral axial force is 35% higher. Indeed this increase of
force is really much lower than I expected. This means that you could test the coil without the
iron safely up to 85% of the nominal current. However you have the limitation
due to the fringe field.

If the coil is used with only barrel yoke, the axial force increase of  73%. This is a
much more critical situation.

Kin, consider that my computation is not as  refined as it could be, but the computations done
by you colleagues seems to me even less refined. So I can suggest to re-check the analysis.
Any case I have to recognise that the problem of the increase of axial force is less
critical I believed before.

Best regards

Pasquale

Il 27/01/2016 21:35, Yip, Kin ha scritto:

Hi,

 

We remember that you told us (Mar. 26, 2015) :

(as we wrote a transcript: https://collab.external.bnl.gov/sites/sPHENIX-Magnet/Shared%20Documents/Documents%20introduced%20or%20sent%20to%20us%20by%20Pasquale%20Fabbricatore/2015-3-26%20Meeting%20with%20Pasquale.docx )

“Without an iron tip the radial field at the coil ends increases, causing an increase of the axial force. During the factory test (without yoke) they limited the field to 1/3 of the nominal field. For this coil it is better not to overcome 20 MPa axial pressure on coil.”

 

But we have had our magnet experts do a calculation on the forces/pressure on the coil (by dividing
the coils in 24 sections) without any yoke :
https://collab.external.bnl.gov/sites/sPHENIX-Magnet/Shared%20Documents/Dr.%20Meng%20Wuzheng%27s%20calculations/2016-1-25%20Solenoid%2024-air-coils_forces.pptx

In this case, the central field has decreased to ~1.25 T

In the sPHENIX configuration (with yoke), the magnetic field is 1.4 T and the results on forces/pressure is :
https://collab.external.bnl.gov/sites/sPHENIX-Magnet/Shared%20Documents/Dr.%20Meng%20Wuzheng%27s%20calculations/2015-6-17%20sPhenix_24-coils_forces.pptx

Both calculations use 4596 A of course.

Comparing the 24 sections,  only in a couple places that the forces (without yoke)  are bigger than the sPHENIX forces ... Definitely, this without-yoke configuration doesn't give you the maximum or limit.

So, does this contradict to what you said … We somehow expected that forces should be higher without yoke … but it doesn’t seem so.   

Any comments ?

Kin

 

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