vmec_jax.radial

Radial operators for vmec_jax.

VMEC uses a normalized radial-like coordinate s in [0, 1].

We need radial derivatives of the Fourier coefficients that are stored on an (ns, K) grid. We deliberately start with a simple and robust finite-difference operator that is:

• fast on a laptop

• differentiable end-to-end (JAX-friendly)

• easy to replace later (e.g. Chebyshev / global spectral radial basis)

The operator below is 2nd-order centered in the interior and 1st-order one-sided at both ends.