Interferometry and Polarimetry Interface

Interferometer chords give fast line-integrated density; polarimetry adds internal field information; the interface unfolds chords into profiles.

Fast density, continuously

Where Thomson scattering is a burst measurement, interferometry runs continuously: laser chords across the plasma measure line-integrated electron density from the phase shift of the probing beam. This is the fast density signal the breeder's fueling loop closes on, feeding the pellet and gas-puff interfaces.

From chords to profile

# line-integrated density per chord -> profile (Abel-like)
for c in chords:
    n_line[c] = dphase[c] / (k_refr * chord_length[c])
# unfold multiple chords into radial n_e(rho)
n_e_profile = invert_chords(n_line, geometry, equilibrium)
# polarimetry: Faraday rotation -> internal B constraint
B_int = faraday_rotation(dpol, n_e_profile)

A single chord gives a line integral; several chords are unfolded (an Abel-type inversion using the equilibrium geometry) into a radial profile. Polarimetry measures Faraday rotation, adding an internal-field constraint that helps pin the current profile the equilibrium reconstruction needs.

Owner and timing

• Owner: L2 for acquisition; L3 for chord inversion; consumed fast by L1 fueling.
• Timing: continuous, fast enough to close the density loop in the MPC cycle.
• Calibration cross-check against Thomson scattering to correct fringe-jump and refraction bias.

Fringe jumps are the classic failure mode; the interface detects and corrects them against the Thomson anchor. Design-and-simulation specification, validated against the twin.