Erroneous iota near magnetic axis

[kohlerla]

I am attempting to run vmec++ in fixed boundary mode for HSX vacuum configurations. I have developed new python tools to generate the fourier coefficients describing the magnetic axis and the LCFS. When using the input file with these coefficients, the iota peaks erroneously near the magnetic axis. I have attached an example input file (in .txt format to allow it to be attached) that exhibits this behavior and the resulting iota profile. I believe the issue may lie in the fourier coefficients. What types of things could cause an erroneous iota profile.

Thanks for your help!

input_example.txt

[jons-pf]

@kohlerla I ran the input file you provided and noticed that the spectral width <M> is relatively large (~3.5). Even for highly-shaped cases such as W7-X, I would consider values around 1.5 more normal. My gut feeling is that this is the culprit, and it would be consistent with your message around creating new Python tools for making the VMEC input.
I assume you are not taking care of spectral condensation in that context? Could you share these tools, so that we could have a look together how to best approach this?
For context on spectral condensation, please have a look into the Hirshman & Meier (1985) publication, and there is something on this also in "The Numerics of VMEC++" in chapter 4.

[jons-pf]

See also #418

[jurasic-pf]

simsopt has an implementation of spectral condensation hiddenSymmetries/simsopt#576 and you'll find another implementation in our qi stellarator dataset repo https://github.com/proximafusion/constellaration/blob/112b20ae07193910d467d26033fe51022e641b9f/src/constellaration/geometry/spectral_condensation.py#L22

[kohlerla]

@jons-pf @jurasic-pf Thank you for the information. I will implement a spectral condensation scheme and see if that resolves the issue.