ITER Nb3Sn thermal and JxB strain were overwritten

[adrianaghiozzi]

Minor bug where values of thermal strain and JxB strain induced on Nb3Sn for the ITER design were being overwritten with zeros, giving overly optimistic values of critical current density.

Ideally I'd like to find where these strain numbers came from in the first place. I went back in the git blame and saw @orso82 originally added these numbers - any idea what source they came from? They're close but not identical to some thermal strain values found in this paper: https://www.researchgate.net/publication/223365515_Review_of_Nb3Sn_conductors_for_ITER

The JxB strains have been more challenging to track down...

S/O @TimSlendebroek and @ggdose for helping uncover this

[TimSlendebroek]

nice find!

Do you remember what the Liu et all paper was for the j vs B curves you uncovered @adrianaghiozzi ?

[adrianaghiozzi]

@TimSlendebroek Yep, this is the one! https://iopscience.iop.org/article/10.1088/0953-2048/23/2/025002

[orso82]

ping @daveweisberg . The superconducting coils and CS stresses routines came from GASC

[orso82]

what's the rationale for the t_margin ?

[ggdose]

Hey Orso! We caught this T margin because we noticed that in Tim's database LTS magnet had a fairly high Bt with respect to what it is seen in current devices. This DeltaT margin is due to the fact that along the hydraulic circuit of the superfluid helium, the coolant increases its temperature. Even if it enters at 4.2 K along the magnet it will get warmer by 1-1.5 K.
Here is a figure from a paper calculating exactly that:

We found that with this taken into account, we can reproduce current LTS performances.

[ggdose]

full paper as reference: https://iris.polito.it/retrieve/240d659a-666e-4538-af9a-bcbfe85fc608/1-s2.0-S0920379623006130-main.pdf

[TimSlendebroek]

I'd say we merge it!

[adrianaghiozzi]

All that Giacomo said above is a good reason to merge this, and an even better reason is that when adding the t_margin I found that we had been erroneously setting the temperature for ReBCO to 4.2K instead of 20K, so I pushed a fix for that as well!

[orso82]

is there a reason why the margin is not applied to the 20K ReBCo ?

[ggdose]

is there a reason why the margin is not applied to the 20K ReBCo ?

Good question. We could apply it also for REBCO. I speculate that the impact on HTS will be negligible. This is because LTS have a critical temperature very close to the coolant temperature. Therefore, even 1 K difference impacts the performance substantially. REBCO operates at tens of K below the critical temperature. Therefore if the HTS magnet is at 20 or 22 K, the maximum critical density in the conductor is not impacted much. This was not true for a LTS operating at 5.2-5.7 K instead of 4.2 K.

[TimSlendebroek]

I did some testing with the higher temp of rebco and the magnets are limited by stresses more so than jcrit for HTS so tmargin with that in mind will not be very impactful there

[bclyons12]

@TimSlendebroek @adrianaghiozzi This change is causing the ARC and CAT tests to fail in FUSE