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Jeff
USA
2 Posts |
 Posted - May 27 2015 : 22:10:16
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Hello,
I am an undergrad student who just started working with FastHenry. So far I have read the user manual and the sparse bits of help out there.
I have been asked to find the loop inductance of a SQUID. So far my results have been off by about 7 pH so I wanted to test whether what I was writing was actually finding the answer I thought it was.
There is a well-known formula from Ketchen and Jaycox that says for a square washer with no gnd plane and infinitely large outer diameter
L = 1.25 u0 d
Where d is the inner diameter and u0 = 4 pi / 10 pH/um. My inner_d is 10um so I get L~15pH. However when I run a simulation of this with the outer_d>>inner_d to get an "infinite" plane my answer is about 1nH.
I am wondering if this has something to do with how I am setting up my models. I am using fastmodel so I basically just place nodes and create segments that correspond to the shape I am trying to make. To get the loop inductance I do a .external which runs through the entire shape I created.
Thank you for any help you can provide.
Here is a copy of the code I am using.
*"Infinite" Outer_d square washer*
.units um
.default z=0 w=1990 h=.16 nwinc=10 nhinc=10 lambda=0.085
*Two Large Plates
N1 x=-2000 y=-1000
N2 x=2000 y=-1000
N3 x=2000 y=1000
N4 x=-2000 y=1000
*Tiny Plates Connecting Two Larger Ones*
N5 x=1000 y=-5
N6 x=1000 y=5
N7 x=-1000 y=5
N8 x=-1000 y=-5
*w=1990 should leave 10 um diameter square hole in the center*
E1 N1 N2
E2 N3 N4
E3 N5 N6
E4 N7 N8
*Connecting three of the four contacts between
*the large and small plates*
.equiv N2 N5
.equiv N6 N3
.equiv N4 N7
*I think would give the loop inductance*
.external N1 N8
.freq fmin=1 fmax=1 ndec=1
.end |
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Jeff
USA
2 Posts |
Posted - May 27 2015 : 22:19:53
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| Sorry. I had to write this twice because my internet crashed. I forgot to mention I am working with the superconducting version of FastHenry. |
Edited by - Jeff on May 27 2015 22:20:34 |
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Enrico
550 Posts |
Posted - May 29 2015 : 20:11:49
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I am not an expert of the FastHenry version supporting superconductors, as this has been modified by Steve Whitleley from Whitleley Research Inc.
However, your geometric definition is wrong, if you are looking to simulate a loop. There are two main issues:
1. sides are in one case too long and in the other too short. The design of the loop should be symmetric. Note that perpendicular segments can (and should) overlap, as they model current in orthogonal directions. You can check the User's Guide on this point, in particular when it discusses ground planes.
1.bis (consequence of 1) Connections of nodes as you made them assume that the current 'jumps'. This should not be the case.
2. The number of filaments is most probably insufficient, expecially if you expect a high field gradient. Note also that you should use odd values for the 'nhinc' and 'nwinc' values.
In general, if your goal is to model a very large flat inductor, you may want to use a ground plane with a hole. Or, you should use either more filaments or more segments at least.
Best Regards,
Enrico
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Finding inductance through a hole.
