sPHENIX discussion of the superconducting solenoid

[Kin Yip <kinyip AT bnl.gov>]

Hi,

I've re-read the following 1999 IEEE paper and used my brain a bit more and seem to understand much more than my previous cursory reading :

https://collab.external.bnl.gov/sites/sPHENIX-Magnet/Shared%20Documents/BaBar%20Documents/IEEE%20TRANSACTIONS%20ON%20APPLIED%20SUPERCONDUCTIVITY,%20VOL.%209,%20NO.%202,%20JUNE%201999.pdf

{ There have been a few versions with similar titles but this one has the section about the "magnetic forces" at the end. }

The info. probably traced back to the Acceptance/Testing in 1998  which can be found under the directory
"Volume 5 DESIGN AND QUALITY CONTROL REPORT/5.1 GENERAL AND MAIN COMPONENTS", the file
Volume 5.1 Design and Quality Control Report GENERAL MAIN COMPONENTS.pdf 
or
Searchable_Volume 5.1_OCR.pdf (which Roberto made to make it to searchable)

on pages 33-34.

But it's better explained in the above IEEE paper.  On the 2nd-4th line on the first paragraph under "VII. Magnetic Forces" at the bottom-right corner of the 4th page,    it said : " In order to have an offset force in one direction only (no inversion during the ramp up), the coil was positioned with 30 mm ..."

It said specifically “during the ramp up”.  So, in principle, without this 3 cm offset, the net axial force on the entire magnet coil will change signs at certain current.  Of course, it happens at "ramp down" or/and as Pasquale likes to say （fast) discharge, => when the current changes.

In the paper, they told us that with the 3 cm offset, they originally hoped to get a max. force at about 2500 A and then fall to a few Metric tons at full current (which one can read from Fig. 8 in this IEEE paper).  But from the measured result, the maximum turned out to be at ~3800 A and didn't fall off as much as they had hoped at full current.   They think the result was consistent with a ~3.2 cm offset --- and one can find such a simulation curve on p.14 of the above-mentioned PDF file (Acceptance/Testing）in our BaBar server.

Therefore, my point is that this offset question is just due to change of direction of the net/total axial force at different currents.   { Wuzheng said that it's because of different saturation of the yoke ... }

Wuzheng has created a 2-D model of BaBar in Opera (which doesn't have any asymmetric offset such that the center of the return yoke is the same as the center of the coil) a couple weeks ago (when LeReC couldn't provide him info. to work). { Wuzheng only found approximate dimensions and measured by ruler etc.  He thinks it's not a perfect BaBar model as he finds 1.49 T at 4596 A :-)  But I think it's good enough for my purpose. }  Under Wuzheng's instructions/tutorials, I or we have just run "Opera" and calculated the axial forces on the entire magnet coil at full current and also 10% of the full current => the sign of the force clearly changes.   I was thinking of doing it 0.1, 0.2 ..... 1.

The 2D model finished very quickly ... like a few minutes.   I'm doing the same thing for the 3-D sPHENIX model (also without
offset).   My previous experience was that it'd take > 5 hours to finish for one calculation.... I'll try to see whether the sign changes for the sPHENIX Model as sPHENIX endcaps are far away and the return yoke doesn't have much saturation.

I understand/learn from Wuzheng that he's way of modelling (using preprocessor) is a bit time-consuming when you created the model.  But the payoff is that it runs faster in Opera than those created by "Modeler" in Opera (a method that maybe Walter used by reading engineering files).

Kin