The Motional Stark Effect (MSE) Diagnostic
The motional Stark effect diagnostic (MSE) was invented by Dr. Fred M. Levinton in 1989 and has subsequently become the worldwide touchstone for the measurement of internal magnetic fields in high temperature plasma experiments. Before MSE, plasma equilibria were poorly constrained by external field measurements, which could not fully discern effects of the current distribution inside the plasma. MSE gives a measure of the pitch angle of the magnetic field in the plasma, and this is used along with diagnostics of temperature, density, and external magnetic field to reconstruct the plasma equilibrium.

The concept of MSE is based on the measurement of the polarization state of light emitted from an atomic hydrogen beam as it passes through a magnetized plasma. As the beam moves through the magnetic field, B, at high velocity, v, it experiences in its reference frame an electric field, E = v x B. The beam atoms are excited via collisions with background plasma, and emit visible light which is split into nine spectral lines by the electric field in a phenomemon known as the Stark effect. The polarization of the light is aligned with respect to the magnetic field, so a measurement of the polarization orientation can be used to determine the magnetic field direction in the plasma.

MSE has made important contributions to our understanding of plasma physics in areas such as sawtooth instabilities, transport, and radial electric fields. A selection of important publications follows:

Magnetic Field Pitch Angle Measurements in the PBX-M Tokamak Using the Motional Stark Effect

Stabilization and Onset of Sawteeth in TFTR

Improved Confinement with Reversed Magnetic Shear in TFTR

Radial Electric Field Measurements in Reversed Shear Plasmas