Converter for VMEC inputs (INDATA namelist) to JSON.

Default values for the input parameters are set in read_indata_namelist() in vmec_input.f.

Prior to reading the namelist, the extcur array is filled with a large constant (cbig in vparams). If extcur entries are specified in the namelist, they overwrite the unreasonably large values in extcur, which is checked for when determining the size of the extcur array to export to the JSON file. This way, the mgrid file does not need to be read to determine the size of the extcur array. In order to implement this logic, I needed to slightly adjust vmec_input.f. The modified version is included in this repository at src/vmec_input.f

This behavior can be disabled by specifying --truncate_extcur as a separate command line argument. Then, only the part of the extcur array from the first element to the last non-zero element is exported to JSON. This is more similar to the original VMEC behavior, but you must take care yourself to check if you specified all required coil currents.

A subfolder can be specified, which will be put in front of the mgrid_file variable. Specify this on the command line as --mgrid_folder some_subfolder and, for a mgrid_file = 'mgrid_w7x.nc' entry in the INDATA namelist, the following will be printed to the JSON output: "mgrid_file":"some_subfolder/mgrid_w7x.nc". Note that a slash (/) is hardcoded for now (sorry Windows users).

Furthermore, after reading the namelist, a few fixups are done there as well:

• If all entries in niter_array stayed at their default values of -1, they are all set to the value of niter (which defaults to 100).
• raxis --> raxis_cc and zaxis --> zaxis_cs for all non-zero entries in raxis,zaxis
• lfreeb is initialized to true in read_indata_namelist(), but re-set to false if mgrid_file remains at the default value NONE.
• bloat is set to 1 if it was set to 0, which is invalid.
• if running in constrained-iota mode (ncurr .ne. 1), bloat is checked to equal 1
• mpol and ntor are set to their absolute values; negative counts of Fourier harmonics make no sense.
• the absolute value of tcon0 is taken and it is coerced to a maximum value of 1
• The entries in ns_array are checked to be monotonically increasing
• if nvacskip was not set, it is initialized to nfp

For further details, refer to the actual code (linked below).

Further documentation of the input parameters can be found below or in the following places:

• STELLOPT Wiki: VMEC Input Namelist (v8.47)
• V3FIT: VMEC Equilibrium
• vmec-internals
• Rotating Ellipse Equilibrium Calculation

This utility is based on the input file reading routines in VMEC, in particular:

• LIBSTELL/.../Modules/vmec_input.f
• PARVMEC/.../TimeStep/vmec.f
• PARVMEC/.../TimeStep/runvmec.f
• PARVMEC/.../Input_Output/readin.f
• PARVMEC/.../Input_Output/read_indata.f

This repository uses two Git submodules (LIBSTELL and json-fortran) which need to be properly cloned for the build process to work.

Here is how you get this repo including the submodules:

git clone --recursive https://github.com/jonathanschilling/indata2json.git

and here is how you get the submodules if you accidentially cloned the repo without the submodules using the command in the first line here:

git clone https://github.com/jonathanschilling/indata2json.git
cd indata2json
git submodule update --init --recursive

This is a totally standard Makefile setup. Just type

make

and you should be good to go. The executable then is indata2json and it can be called as follows:

./indata2json demo_inputs/input.w7x_ref_167_12_12

which in turn creates a file w7x_ref_167_12_12.json in the current working directory.

The following lists the input parameters for VMEC which are extracted from the INDATA namelist and exported to the json file.

lasym flag to indicate non-stellarator-symmetry

nfp number of toroidal field periods (=1 for Tokamak)

mpol number of poloidal Fourier harmonics; m = 0, 1, ..., (mpol-1)

ntor number of toroidal Fourier harmonics; n = -ntor, -ntor+1, ..., -1, 0, 1, ..., ntor-1, ntor

ntheta number of poloidal grid points; is rounded to next smaller even number, if odd

nzeta number of toroidal grid points; must match nzeta of mgrid file if using free-boundary

ns_array [numGrids] number of flux surfaces per multigrid step

ftol_array [numGrids] requested force tolerance for convergence per multigrid step

niter_array [numGrids] maximum number of iterations per multigrid step

delt initial value for artificial time step in iterative solver

tcon0 constraint force scaling factor for ns --> 0

aphi radial flux zoning profile coefficients

lforbal =T, use non-variational forces to ensure = 0; =F, use variational form of forces, ~ 0

nstep printout interval for convergence information

phiedge total enclosed toroidal magnetic flux in Vs == Wb

pmass_type Specifies parameterization type of pmass function 'power_series' - p(s)=Sum[ am(j) s ** j] - Default 'gauss_trunc' - p(s)=am(0) (exp(-(s/am(1)) ** 2) - exp(-(1/am(1)) ** 2)) others - see function pmass

am mass or pressure (gamma=0) expansion coefficients (series in s) in MKS units [NWT/M**2] Interpretation changes with pmass_type

am_aux_s Auxiliary array for mass profile. Used for splines, s values

am_aux_f Auxiliary array for mass profile. Used for splines, function values

pres_scale factor used to scale pressure profile (default value = 1) useful so user can fix profile and change beta without having to change all AM coefficients separately

gamma value of compressibility index (gamma=0 => pressure prescribed)

spres_ped location of pressure pedestal in s

ncurr 0: constrained-iota; 1: constrained-current

piota_type Specifies parameterization type of piota function 'power_series' - p(s)=Sum(am(j) s**j) - Default others - see function piota

ai expansion coefficients for iota (power series in s) used when ncurr=0 Interpretation changes with piota_type

ai_aux_s Auxiliary array for iota profile. Used for splines, s values

ai_aux_f Auxiliary array for iota profile. Used for splines, function values

pcurr_type Specifies parameterization type of pcurr function 'power_series' - I'(s)=Sum[ ac(j) s ** j] - Default 'gauss_trunc' - I'(s)=ac(0) (exp(-(s/ac(1)) ** 2) - exp(-(1/ac(1)) ** 2)) others - see function pcurr

ac expansion coefficients for the normalized (pcurr(s=1) = 1) radial derivative of the flux-averaged toroidal current density (power series in s) used when ncurr=1 Interpretation changes with pcurr_type

ac_aux_s Auxiliary array for current profile. Used for splines, s values

ac_aux_f Auxiliary array for current profile. Used for splines, function values

curtor value of toroidal current in A. Used if ncurr = 1 to specify current profile.

bloat bloating factor (for constrained toroidal current)

lfreeb =T, run in free boundary mode if mgrid_file exists

mgrid_file full path for vacuum Green's function data

extcur array of currents in each external current group. Used to multiply Green''s function for fields and loops read in from MGRID file. Should use real current units (A).

nvacskip number of iteration steps between accurate calculation of vacuum response; use fast interpolation scheme in between

raxis_cc magnetic axis coefficients for R ~ cos(n*v); stellarator-symmetric

zaxis_cs magnetic axis coefficients for Z ~ sin(n*v); stellarator-symmetric

raxis_cs magnetic axis coefficients for R ~ sin(n*v); non-stellarator-symmetric

zaxis_cc magnetic axis coefficients for Z ~ cos(n*v); non-stellarator-symmetric

rbc boundary coefficients of COS(mtheta-nzeta) for R [m]; stellarator-symmetric

zbs boundary coefficients of SIN(mtheta-nzeta) for Z [m]; stellarator-symmetric

rbs boundary coefficients of SIN(mtheta-nzeta) for R [m]; non-stellarator-symmetric

zbc boundary coefficients of COS(mtheta-nzeta) for Z [m]; non-stellarator-symmetric

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