Motional Stark Effect Spectroscopy

Local magnetic field measurements by Motional Stark Splitting

The neutral beam atoms themselves emit light that can be measured separately, as the Doppler effect at the velocity of beam atoms causes a distinct shift in the wavelength of their light. Moreover this radiated light has specific features due to the fact that the beam atoms cross very rapidly through a strong magnetic field – its characteristic spectral lines are split and polarised. By measuring these features one can determine the direction of magnetic field lines even inside the hot plasma. The technique, called Motional Stark Effect Spectroscopy, is quite challenging but vital as the plasma confinement depends so much on the exact topology of the magnetic field, and the topology depends on electric currents in the plasma…

Toroidal Alvén Eigenmodes

Simulation of one part of the new TAE antenna structure

The Toroidal Alfvén Eigenmode (or TAE) diagnostic helps making detailed measurements of magnetic field line oscillations, known as Alfvén waves . It consists of two sets of four antennas. Some of the TAE antennas emit electromagnetic waves to actively modify the Alfvén waves, while the others will passively observe the response of TAE. With this diagnostic, JET is the fusion facility best equipped to study and interpret interaction between Alfvén waves and alpha particles, e.g. Helium nuclei produced in fusion reactions. From computer simulations it is believed that the interaction of Alfvén waves and alpha particles plays a significant role in the confinement of the alpha particles and in the overall stability of plasmas in future fusion reactors.