Interferometry for Density Control
How laser interferometry provides the fast, reliable density measurement that closes the density-control loop.
Measuring density with light
A plasma changes the phase of a laser beam passing through it by an amount proportional to the electron density integrated along the path. An interferometer compares the plasma beam against a reference beam and reads that phase shift, yielding the line-integrated density along each chord - fast, non-perturbing, and every shot.
Why it suits control
Density control needs a measurement that is fast (kilohertz or faster), reliable, and requires no fragile calibration each shot. Interferometry meets this: the phase shift is a clean, direct function of density, updated continuously. It is the standard real-time density signal feeding gas and pellet fueling loops.
Line-integrated versus profile
A single chord gives the density integrated along its path, not the local density. Multiple chords at different heights can be inverted to a profile, but even the line-averaged value from one central chord is enough to regulate overall density. For profile control, several chords or a complementary diagnostic like reflectometry are combined.
Practical issues
- Fringe jumps: rapid density changes can skip interference fringes and corrupt the count, so robust fringe-tracking is essential
- Mechanical vibration: shifts the reference path and mimics density change, mitigated by a second wavelength
- Refraction: strong density gradients can bend the beam off the detector
- Access: chords must have a clear line of sight through the plasma
In the control loop
The interferometer feeds the state estimator, which combines it with the reconstructed geometry to report density and Greenwald fraction. The density controller then trims gas and pellet fueling to track the reference. A dropped or corrupted interferometer chord is a serious event, so its health is monitored as a control-critical signal.