Measuring plasma electron density by detecting the phase shift imparted to a probing laser or microwave beam
Interferometry exploits the fact that a plasma has a refractive index slightly less than unity for electromagnetic waves above the plasma frequency. When a coherent beam traverses the plasma, it accumulates a phase shift relative to a reference beam traveling through vacuum. This phase shift is directly proportional to the line-integrated electron density along the beam path. By comparing the two beams in an interferometer—typically a Mach–Zehnder or Michelson configuration—the electron density integral ∫ne dl can be extracted with high temporal resolution.1
The choice of probing wavelength involves a trade-off. Far-infrared lasers (e.g., CO2 at 10.6 μm) provide high sensitivity but are subject to mechanical vibration noise and refraction bending at steep density gradients. Microwave interferometers operating at millimeter wavelengths are more robust against vibration but have a lower density resolution and can encounter cutoff if the plasma density approaches the critical density for the chosen frequency.2
Multi-chord interferometry arrays pass several parallel beams through the plasma cross-section at different vertical or radial positions. Abel inversion or more sophisticated tomographic algorithms then reconstruct the local density profile from the set of line-integrated measurements. On ITER, a toroidal interferometer-polarimeter system will provide real-time density measurements essential for plasma position and density feedback control.3
When the probing beam propagates parallel to the magnetic field, the Faraday effect rotates its polarization plane by an amount proportional to ∫neB∥ dl. Combining interferometric phase shift with Faraday rotation—a technique called interferometry-polarimetry—allows simultaneous measurement of electron density and the parallel magnetic field component, providing constraints for equilibrium reconstruction.4
Interferometry has been a workhorse diagnostic since the earliest tokamak experiments in the 1960s and remains the primary means of real-time density control on every major confinement device.