interferometry is the diagnostic technique that measures a plasma's density from the phase shift a beam of light or microwaves accumulates while passing through it.
The word joins interference — the way overlapping waves reinforce or cancel — with the -metry suffix of measurement, and outside 📝fusion it names a broad family of precision techniques used everywhere from astronomy to gravitational-wave detection. The fusion application rests on a simple fact: the refractive index of a 📝plasma depends on its electron density. An interferometer splits a laser or microwave beam, sends one arm through the plasma and the other around it, and recombines the two; the fringe shift between them is directly proportional to the density integrated along the beam's path.
That line-integrated character is the near-neighbor distinction against 📝Thomson scattering: an interferometer cannot say where along its chord the density sits, only how much plasma the beam traversed in total, while Thomson scattering resolves the value point by point. What interferometry gives up in locality it repays in robustness — it needs no high-power pulsed laser, runs continuously through a discharge, and delivers its answer fast enough to feed real-time fueling control, which is why nearly every magnetic confinement device carries one. Machines typically field several chords at once, and multi-chord arrays can be inverted to reconstruct an approximate density profile.