Interferometry

Interferometry measures line-integrated electron density fast and robustly, the fabric's most dependable density diagnostic across multiple chords.

Phase carries density

A probing beam through the plasma accumulates phase proportional to the line-integrated electron density along its chord. Measuring that phase gives a fast, robust density signal that rarely fails, which is why interferometry is a workhorse of the constellation. Multiple chords at different impact parameters constrain the density profile when inverted against the geometry.

From chords to local density

Each chord is a line integral; recovering local density is an inversion that uses the reconstructed flux surfaces. The fabric performs this in the normalization stage, cross-checking the result against localized Thomson density. Agreement between the two independent methods is a core validation check.

import numpy as np
# invert multi-chord line integrals to a flux-surface density profile
def invert_density(chord_integrals, geometry_matrix, reg=1e-3):
    A = geometry_matrix               # maps profile -> chord integrals
    # regularized least squares (Tikhonov) for a smooth profile
    AtA = A.T @ A + reg*np.eye(A.shape[1])
    return np.linalg.solve(AtA, A.T @ chord_integrals)

Fast enough for control

• Continuous and low-latency, interferometry fills the gaps between pulsed Thomson samples.
• Robust against the electromagnetic noise of a 16.84 T machine, since phase is measured optically.
• A fringe-jump fault is caught by validation as an impossible density step and masked.

Both machines

On the breeder, interferometry constrains the density profile feeding the pressure map. On the burner, chords through the central cell and end plugs track the density that the ambipolar potential must confine — end-plug density being one of the burner's central control problems. Chord geometry is versioned per machine.