### Dr. Park Corrects An Error in Navy Paper.

Posted:

**Sat Aug 23, 2014 4:39 pm**Hello,

I am doing a write up on the Navy paper.

I am analyzing the magnetic field, using simple equations and 22,000 amp*turns. Here is a summary of what I did:

Huh. The field calculated and the one listed in the paper are off by 26%! Wow. So I emailed Dr. Park about this. Here is what I got back:

I am doing a write up on the Navy paper.

I am analyzing the magnetic field, using simple equations and 22,000 amp*turns. Here is a summary of what I did:

Huh. The field calculated and the one listed in the paper are off by 26%! Wow. So I emailed Dr. Park about this. Here is what I got back:

Please see the e-mail below. It looks like there was a mistake on the Amp. turns value. Thank you for pointing out and we will correct it. Since the B-field values are the meaningful quantity in the analysis, I expect the physics results stand as written.

Sincerely,

Jaeyoung

---------- Forwarded message ----------

Date: Mon, Aug 18, 2014 at 8:21 PM

Subject: Re: Question about B-Field analysis

There was an error in the paper. The amp-turns in the appendix were incorrect, but the field values used throughout the paper were correct.

I don't know where 22 kA-turns came from. It popped into the paper somewhere between "v4" and "v6", March 4-11. It replaced an earlier and more wildly incorrect value. Shot 15640 had magnet current 256 A, and at 160 turns per magnet that's 40960 A-turns.

The field at the face of a solitary coil with that current would be 0.375 T. The other five coils reduce the field 27%, so the final field is 0.27 T. (The repeating 27 is a coincidence.)

I checked these numbers against an online magnet calculator (http://www.netdenizen.com/emagnet/offax ... ulator.htm) and also confirmed with Excel (using elliptic integrals tabulated in Abramowitz and Stegun for the 4 coils that aren't on the same axis as the calculation point).

In response to this gentleman, let him know the amp-turns were in error, but the field values are correct. You could also let him know that the other five coils reduce the face value by 27% at the magnet plane (not at the true max field point, but very close).