Fieldlines starting from a VMEC wout file

[s-sharpe01]

Hi all,

I would like to get a poincare plot out of the code using FIELDLINES. For one of the cases I want to run I have been given an existing VMEC wout file, but no input. I wanted to ask if there was a way to run the FIELDLINES code and get the poincare plot from this wout file? The two tutorial cases in the documentation ( found here: https://princetonuniversity.github.io/STELLOPT/FIELDLINES) use vmec.input files and modify them.

Any advice on how to deal with this would be greatly appreciated

Thanks,
Sophie

[lazersos]

@s-sharpe01 So the FIELDLINES code only reads the EXTCUR array from the &INDATA namelist. In your case you can simply create an input file which matches you wout file. For example if you have wout_example.nc then you'd want your input file to be called input.example. The input file can simply have a blank &INDATA namelist in it. For example

&INDATA
/
&FIELDLINES_INPUT
<Put whatever your parameters are here.>
/

Then when you run FIELDLINES you simply need to pass the -plasma flag so the code knows not to calculate any field beyond the equilibrium.

But can I ask why do you want to make Poincare plots of a VMEC equilibrium instead of just directly plotting the flux surfaces?

[s-sharpe01]

Thank you for your help!

I am trying out different coil optimisation codes and wanted to do a comparison between the plots produced by each code and the VMEC equilibrium, though I suppose plotting the flux surfaces would achieve a similar result

[lazersos]

@s-sharpe01, Ahh then you're much better off plotting the VMEC equilibria. FIELDLINES will not give a machine precision accurate representation of the VMEC magnetic field near the edge of the equilibrium domain. This is becasue we must do an inverse lookup to place the magnetic field from VMEC on the cylindrical R,phi,Z grid of FIELDLINES. This tends to create edge stochasticity inside the VMEC domain which is due to numerics, not reality.