Input file reference

vmex accepts two input formats, auto-detected by vmex.core.input.VmecInput.from_file():

• the classic VMEC2000 &INDATA Fortran namelist (input.<case>), and

• VMEC++-style JSON (.json suffix or leading {) with identical key names.

Both round-trip: VmecInput.to_json writes a VMEC++-schema JSON deck that parses back to the same input. All VMEC2000 defaults and readin.f normalizations are applied on construction (see the vmex.core.input docstring for the exact rules).

INDATA variables

Symmetry and resolution

Variable	Default	Meaning
LASYM	F	non-stellarator-symmetric mode (enables the rbs/zbc and *mns/*mnc partners)
NFP	1	number of field periods
MPOL	6	poloidal modes m = 0 .. mpol-1
NTOR	0	toroidal modes n = -ntor .. ntor
NTHETA / NZETA	0	angular grid points (0 selects the VMEC default)

Multigrid ladder and stepping

Variable	Default	Meaning
NS_ARRAY	[31]	radial surfaces per multigrid stage
FTOL_ARRAY	[1e-10]	force tolerance per stage (converged when fsqr, fsqz, fsql are all below it)
NITER_ARRAY (or NITER)	[100]	iteration cap per stage
DELT	1.0	initial time step
TCON0	1.0	constraint-force multiplier (spectral condensation)
APHI	[1, 0, ...]	radial-flux remap polynomial
PHIEDGE	1.0	total enclosed toroidal flux [Wb]
NSTEP	10	iterations between progress prints

Pressure profile

Variable	Default	Meaning
PMASS_TYPE	power_series	one of power_series, two_power, gauss_trunc, cubic_spline, akima_spline, line_segment, pedestal, … (see vmex.core.profiles)
AM	zeros	profile coefficients (dense, indices 0..20)
AM_AUX_S / AM_AUX_F	—	spline knots in s / values for tabulated profile types
PRES_SCALE	1.0	pressure scale factor [Pa]
GAMMA	0.0	adiabatic index (JSON alias: adiabatic_index)
SPRES_PED	1.0	pressure pedestal location in s

Current and iota profiles

Variable	Default	Meaning
NCURR	0	0: prescribed iota (AI), 1: prescribed toroidal current (AC)
PCURR_TYPE	power_series	current profile type; *_i forms prescribe \(I(s)\), *_ip forms prescribe \(I'(s)\)
AC / AC_AUX_S / AC_AUX_F	zeros / —	current profile coefficients / spline knots and values
CURTOR	0.0	total toroidal current [A]
PIOTA_TYPE	power_series	iota profile type
AI / AI_AUX_S / AI_AUX_F	zeros / —	iota profile coefficients / spline knots and values
BLOAT	1.0	profile-argument expansion factor

Axis and boundary shape

Variable	Default	Meaning
RAXIS_CC / ZAXIS_CS	zeros	axis initial guess, cos/sin coefficients for n = 0 .. ntor
RAXIS_CS / ZAXIS_CC	zeros	asymmetric axis partners (LASYM = T)
RBC(n,m) / ZBS(n,m)	zeros	boundary Fourier coefficients of \(R\cos / Z\sin(m\theta - n\,\mathrm{NFP}\,\zeta)\)
RBS(n,m) / ZBC(n,m)	zeros	asymmetric boundary partners (LASYM = T)

Free boundary

Variable	Default	Meaning
LFREEB	T	free-boundary mode; forced F when MGRID_FILE = 'NONE'
MGRID_FILE	'NONE'	MAKEGRID vacuum-field file, or 'DIRECT_COILS' for direct Biot-Savart coil fields (with vmex --coils)
EXTCUR	—	external coil-group currents [A]
NVACSKIP	1	vacuum-solve cadence (<= 0 falls back to NFP)
MFILTER_FBDY / NFILTER_FBDY	-1	boundary spectral filtering
PRECON_TYPE / PREC2D_THRESHOLD	NONE / 1e-30	2D preconditioner selection (accepted for compatibility)

VMEC++-style JSON

The JSON schema follows vmecpp.VmecInput verbatim: the keys are the lower-case INDATA names (lasym, nfp, mpol, ntor, ntheta, nzeta, ns_array, ftol_array, niter_array, delt, tcon0, aphi, phiedge, nstep, pmass_type, am, am_aux_s, am_aux_f, pres_scale, adiabatic_index, spres_ped, ncurr, pcurr_type, ac, ac_aux_s, ac_aux_f, curtor, piota_type, ai, ai_aux_s, ai_aux_f, bloat, raxis_c, zaxis_s, raxis_s, zaxis_c, rbc, zbs, rbs, zbc, lfreeb, mgrid_file, extcur, nvacskip, ...).

Boundary coefficients are sparse lists of {"n": int, "m": int, "value": float} objects; axis arrays are dense of length ntor + 1:

{
  "lasym": false,
  "nfp": 5,
  "mpol": 5,
  "ntor": 4,
  "ns_array": [31],
  "ftol_array": [1e-12],
  "niter_array": [2000],
  "phiedge": 0.5,
  "raxis_c": [1.0, 0.1, 0.0, 0.0, 0.0],
  "zaxis_s": [0.0, 0.1, 0.0, 0.0, 0.0],
  "rbc": [
    {"n": 0, "m": 0, "value": 1.0},
    {"n": 0, "m": 1, "value": 0.3}
  ],
  "zbs": [
    {"n": 0, "m": 1, "value": 0.3}
  ]
}

Extensions beyond VMEC++ (ignored by VMEC++ but accepted here): the spline and pedestal profile types listed above, with the same key names as INDATA.