Twin E-mode reflectometry for magnetic field measurements in tokamaks

In recent years microwave reflectometry has been more and more used as a successful means for diagnosing fusion plasmas. The method is based on the propagation and reflection of a probing wave in an inhomogeneous plasma. Two modes of operation are possible, depending on the polarization of the injected waves: the Ordinary O or Extraordinary E mode. In the O-mode, the reflection condition (cut-off) depends only on the local plasma density, allowing measurements of the density profile in the swept frequency operation, or of movements of the density layers in the fixed frequency operation. For the E-mode there are two possible reflection layers, at the ''lower'' ωL and ''upper'' ωU cut-off. Each one of the two cut-off conditions depends both on the density and on the magnetic field. Consequently reflectometry in the E-mode is generally considered suitable to give informations on the density profile when the magnetic field is known or viceversa. Here we present the principle and give a preliminary assessment of sensibility and applicability of an alternative method of using reflectometry in the E-mode. (author) 4 refs., 4 figs