VMEX

Rename note: vmec_jax is now vmex; the deprecated import vmec_jax compatibility shim still ships with VMEX 0.5.

VMEX is a JAX implementation of VMEC for stellarator and tokamak ideal-MHD equilibria. It reads standard VMEC input files, solves fixed- and free-boundary problems, writes standard wout_*.nc files, and provides exact implicit derivatives of converged fixed-boundary equilibria for optimization.

Install

pip install vmex
vmex --doctor
vmex --test

Python 3.10+ is supported. VMEX installs CPU JAX, SciPy, plotting, NetCDF, and booz_xform_jax; install an accelerator-enabled JAX wheel separately using the JAX installation guide. Optional integrations are vmex[optimizers] for JAXopt/Optax, vmex[neoclassical] for NEO_JAX effective ripple, vmex[freeb] for differentiable virtual casing, vmex[coils] for ESSOS, and vmex[turbulence] for GKX.

An editable source install remains connected to its checkout, so pip install -e . only needs to be repeated when packaging metadata or dependencies change—not after each git fetch or checkout.

Solve and inspect an equilibrium

import vmex as vj

inp = vj.VmecInput.from_file("input.circular_tokamak")
result = vj.solve_multigrid(inp, verbose=True)
wout = vj.wout_from_state(inp=inp, state=result.state,
                           fsqr=result.fsqr, fsqz=result.fsqz, fsql=result.fsql,
                           niter=result.iterations, converged=result.converged)
vj.write_wout("wout_circular_tokamak.nc", wout)
vj.plot_wout("wout_circular_tokamak.nc", "figures")

The CLI provides the same workflow:

vmex input.circular_tokamak
vmex --plot wout_circular_tokamak.nc
vmex input.nearby --restart wout_circular_tokamak.nc

VMEX uses the input file's NS_ARRAY, FTOL_ARRAY, and NITER_ARRAY. verbose=True prints the VMEC iteration table; typed errors distinguish invalid inputs, Jacobian failures, non-convergence, and numerical failures.

Magnetic field and derivatives

Converged equilibria evaluate the field inside the LCFS, including spatial derivatives and exact VJPs in the originating optimization problem's degrees of freedom:

import jax.numpy as jnp

final_equilibrium = problem.equilibrium_from_x(result.x)
final_equilibrium.set_points_xyz([[x, y, z]])

B = final_equilibrium.B()
absB = final_equilibrium.absB()
gradB = final_equilibrium.gradB()
gradgradB = final_equilibrium.gradgradB()
gradgradgradB = final_equilibrium.gradgradgradB()

dBdx = final_equilibrium.B_vjp(jnp.ones_like(B))
dgradBdx = final_equilibrium.gradB_vjp(jnp.ones_like(gradB))
d2Bdx = final_equilibrium.gradgradB_vjp(jnp.ones_like(gradgradB))
d3Bdx = final_equilibrium.gradgradgradB_vjp(
    jnp.ones_like(gradgradgradB))

All field components and spatial derivative axes above are Cartesian. Each VJP returns one entry per problem.dof_names, including selected boundary and current-profile variables. Use set_points_flux([[s, theta, phi]]) instead to place interior points in VMEC flux coordinates; returned vectors and tensors remain Cartesian, and parameter VJPs hold those mapped Cartesian points fixed. The poloidal coordinate degenerates at s=0, but the physical field does not: VMEX applies the regular spectral axis limit, so B and its first three Cartesian spatial derivatives can be queried on the magnetic axis. VmecExtender covers points outside the plasma by adding the plasma-current contribution from virtual_casing_jax to a supplied coil or MGRID field. Virtual casing alone is not the total exterior field.

Effective ripple is an optional in-memory diagnostic—no boozmn file is needed. examples/epsilon_effective.py computes and plots the conventional NEO transport quantity $\epsilon_{\rm eff}^{3/2}$; --plot adds the same bounded-resolution radial trend to the pressure panel when NEO_JAX is installed.

field = vj.VmecExtender.from_file(
    "wout_example.nc", external_field=coils.B, nphi=32, ntheta=32
)
field.set_points([[1.8, 0.0, 0.0]])

B = field.B()              # (n, 3), Cartesian
modB = field.absB()        # (n,)
gradB = field.gradB()      # (n, B_i, x_j)
d2B = field.gradgradB()
d3B = field.gradgradgradB()
grad_modB = field.GradAbsB()

Install vmex[freeb] for the finite-beta path. Points must be outside the last closed flux surface, away from the source surface and external currents. MGRID queries must also remain inside the tabulated R-Z domain. The resulting vacuum region can contain islands or stochastic field lines; VMEX does not assume nested surfaces there.

equilibrium.exterior_field() builds the plasma contribution from the live VMEX spectral state, rather than a materialized wout, so JAX derivatives with respect to the equilibrium boundary are retained for single-stage objectives. Run examples/vmex_get_B_gradB.py for the finite-beta interior API and examples/free_boundary_essos_coils.py for the released ESSOS 0.16 coil interface. Exterior coil VJPs and field-line tracing need ESSOS branch rj/vmex-optimization-interfaces: pip install "essos @ git+https://github.com/uwplasma/ESSOS.git@rj/vmex-optimization-interfaces".

The common CLI operations are:

See the CLI reference for resolution, device, convergence, coil, plotting, and Boozer options.

Hot restart

Pass a previous state or wout to initialize a nearby run. VMEX adapts the boundary and skips completed multigrid rungs when possible.

base = vj.solve_multigrid(inp)
nearby = vj.solve_multigrid(changed_input, initial_state=base.state)
from_file = vj.solve_multigrid(changed_input, restart_from="wout_base.nc")

The CLI equivalent is vmex input.changed --restart wout_base.nc; a deck may instead set RESTART_WOUT. Optimization trial solves hot-restart automatically. See the restart guide for grid changes and validation rules.

Optimizer-neutral problems

Objective tuples use (function, target, weight), with weight multiplying the squared cost by default; a one-dimensional weight applies different penalties to profile rows, such as a stronger edge penalty. The resulting problem works directly with SciPy, JAXopt, Optax, or a user optimizer.

from dataclasses import replace
import jax.numpy as jnp
import numpy as np
from scipy.optimize import least_squares

from vmex import optimize as opt
from vmex.core.omnigenity import QIResidual

max_mode = 5
mpol = max(max_mode + 2, 5)
inp = replace(inp, delt=0.5).change_resolution(
    mpol=mpol, ntor=mpol, ntheta=2 * mpol + 6, nzeta=2 * mpol + 4)
qi = QIResidual(np.linspace(0.1, 1.0, 6))

def iota_floor(equilibrium_state, solver_context):
    return jnp.maximum(
        0.33 - jnp.abs(opt.mean_iota(equilibrium_state, solver_context)), 0.0)

problem = opt.VmecProblem.from_tuples(inp, [
    (qi, 0.0, 1.0),
    (opt.aspect_ratio, 5.0, 0.005),
    (iota_floor, 0.0, 10.0),
], max_mode=max_mode, use_ess=True)

result = least_squares(problem.residual, problem.x0,
    jac=problem.residual_jac, x_scale=problem.scales, max_nfev=50, verbose=2)
optimized_input = problem.input_from_x(result.x)
optimized_equilibrium = problem.equilibrium_from_x(result.x)

VMEX implicitly differentiates the converged equilibrium by default. For a residual vector, auto checks each block-response column against the linearized VMEC equations. If any column fails, it recomputes the Jacobian with the reverse adjoint. Cost weights, hot restarts, and one-column batches are defaults.

problem.value_and_grad and problem.jax_value_and_grad expose the same scalar contract. problem.evaluate(x) reports solve effort, failed trials, derivative fallbacks, fsq, fsq_ratio, and whether the implicit derivative was certified. The runnable examples show SciPy least squares, BFGS/L-BFGS-B, JAXopt, Optax Adam, QI/QS objectives, high-accuracy final solves, input/wout output, and plotting.

Joint boundary/coil and coil-only free-boundary scripts are previews for the same ESSOS branch.

QA, QH, QP, and QI examples

The scripts in examples/optimization/ optimize QA (NFP=2), QH (NFP=4), QP (NFP=2), and QI (NFP=2) from simple seeds; each writes an optimized input, WOUT, and standard plots. Run QA_optimization.py, QH_optimization.py, QP_optimization.py, or QI_optimization.py, then python examples/plot_optimized_families.py to reproduce the composites below. Each column shows four toroidal cuts separated by π/(2 NFP), the 3-D LCFS colored by |B|, and LCFS |B| in Boozer coordinates.

examples/optimization/stellarator_asymmetry/ contains matching vacuum and finite-beta examples with LASYM=True; each visibly seeds and optimizes the additional RBS and ZBC boundary families.

Validated QI inputs spanning NFP=1–4 are bundled in examples/data/; the same plotting script reads them directly.

Finite beta, free boundary, and mirrors

examples/free_boundary_essos_coils.py holds the Landreman–Paul QA coil currents fixed while increasing beta and re-solving the NESTOR free boundary. The magnetic-axis displacement is the expected Shafranov shift.

VMEX also solves open-ended mirrors. examples/mirror/mirror_fixed_boundary_nonaxisymmetric.py compares an axisymmetric mirror with a non-axisymmetric rotating ellipse; examples/mirror/mirror_free_boundary_beta_scan.py continues an ESSOS-coil free boundary from 0% to 80% central beta. The latter plots the solved on-axis field against the MHD paraxial scaling B/Bvac = sqrt(1-beta) implied by p + B²/(2 μ0) = Bvac²/(2 μ0). The 0–10% lane is supported; higher-beta points remain clearly marked as extended validation pending refined-grid promotion.

Equilibrium and kinetic diagnostics

vmex --plot wout_X.nc produces cross-sections, profiles, a full-resolution 3-D LCFS, and the compact summaries below. They combine Mercier DMerc, Glasser DR, and $V''(s)$ on zero-aligned axes; add a 3-D LCFS; and show the second adiabatic invariant in the Velasco polar coordinates $x=s\cos\alpha$, $y=s\sin\alpha$. A separate stability figure decomposes DMerc and shows the frozen-geometry response to a pressure ramp; finite-pressure points must be re-solved for certification. Boozer $|B|$ appears automatically, while --booz only saves a reusable boozmn_*.nc file.

This finite-pressure NFP=3 QI example reaches $\langle\beta\rangle=2.38%$.

The vacuum QA example has pres=0 and DWell=0 exactly: VMEX adds no pressure floor. DMerc can retain shear, current, and geodesic terms; for a current-free vacuum it reduces to the shear term and $D_R=0$, so these curves are not a finite-beta pressure margin.

QA_optimization_bootstrap.py, QH_optimization_bootstrap.py and QI_optimization_bootstrap.py first fit a bootstrap-consistent seed, then optimize the boundary and a stage-refined current spline together against Redl, Mercier, and resistive-interchange targets. The QI variant uses helicity_n=0, since a quasi-isodynamic field carries no helical symmetry for the Redl isomorphism to shift; Redl is a fit to quasisymmetric calculations, so there it is an analytic estimate rather than a converged kinetic answer. Their controls are explained in the objective reference; published-equilibrium and SFINCS comparisons live in benchmarks/. Each script also writes a direct Redl-versus-equilibrium bootstrap-current overlay. In the vacuum QA example, setting TRIAL_BETA enables differentiable frozen-geometry pressure proxies for DMerc and DR; a finite-pressure re-solve remains the stability certificate.

Physics and interoperability

VMEX includes VMEC pressure/current/iota profiles, multigrid continuation, NESTOR free boundary, mgrid and direct coil fields, Boozer transforms, QI/QS and maximum-J objectives, Mercier and ballooning diagnostics, bootstrap-current objectives, dimensional scaling, mirror equilibria, and standard wout/mout output. The capability reference states the validation level and limitations of each path.

VMEX outputs are intended for existing VMEC workflows: wout_*.nc files load in SIMSOPT, booz_xform, and other downstream tools. VMEC2000 compatibility and deliberate differences are documented in the compatibility reference.

Solver feature comparison

This matrix was checked on 2026-08-11 against current STELLOPT/VMEC2000 and VMEC++ sources. ✅ denotes a public path, ⚠️ a documented limitation, and ❌ no public path; the linked VMEX capability contract defines the validation scope.

Convergence parity and implementation size

On the bundled NFP=4 QH case at ns=51, VMEX follows VMEC2000 and VMEC++ through the full force-residual trace (fresh local run: VMEX d7347c9, VMEC2000 512375c, VMEC++ 0.5.3). Reproduce it with python benchmarks/make_readme_figures.py --only convergence; the benchmark discovers local solver installations or accepts VMEX_XVMEC2000 and VMEX_VMECPP_PY.

The following cloc 2.11 snapshot counts implementation code and comments, excluding tests, generated code, and third-party sources. VMEX counts vmex/core (the toroidal solver); VMEC2000 counts VMEC2000/Sources but not shared STELLOPT libraries; VMEC++ counts src/vmecpp C++/headers/Python. These scopes make the comparison reproducible, not a claim of identical feature breadth.

VMEX reduces duplication by expressing spectral operators as vectorized JAX array programs and using the same equations for CPU, accelerators, and automatic differentiation. It also deliberately omits some legacy modes, so the smaller codebase reflects both architecture and narrower compatibility surface.

Performance and parallelism

JAX compilation is paid once per array structure and reused from a machine-local cache. Warm runs are the relevant measure for continuation, parameter scans, and optimization.

Independent solves use vj.parallel.solve_ensemble(inputs, workers=None). A single equilibrium already uses XLA's internal threading; ensemble workers are therefore bounded by both the number of cases and the CPUs made available by the host scheduler. Explicit workers=1 gives a reproducible serial baseline, and GPU/device placement can be selected with device=.

benchmarks/optimization.py profiles QI, QA, QH, QP, scalar objectives, SciPy/JAX contract agreement, finite differences, optimizer choices, and the max_fsq_ratio policy.

Documentation and development

The documentation is organized as tutorials, task-focused how-to guides, API/reference pages, and numerical explanations. Start with:

• first equilibrium
• first gradient
• first optimization
• optimization reference
• objectives reference
• parallel and HPC usage

For development:

git clone https://github.com/uwplasma/vmex
cd vmex
pip install -e ".[dev]"
pytest -q -m "not full and not weekly"
python -m ruff check vmex tests examples benchmarks

See contributing and the test manifest. Release notes are on GitHub. VMEX uses the MIT license.

Roadmap

• Promote the experimental boundary-Schur free-boundary adjoint after reducing its remaining local-force/NESTOR cold compile and GPU memory costs, then promote coil-only free-boundary single-stage optimization.
• Promote rotating-ellipse stellarator–mirror hybrids from extended validation with refinement, independent force checks, and practical optimization examples.
• Broaden trapped-particle-fraction benchmarks against near-axis theory across QA/QH/QP/QI, retaining the physically nonzero on-axis QI trapped fraction.
• Complete the VMEX-state-to-NEO differentiable lane for effective ripple, validate its forward sensitivities against finite differences and STELLOPT NEO, then add Gamma_c and the associated trapped-particle diagnostics.