Ultrafast frequency sweep heterodyne reflectometer on the Tore Supra tokamak

A new O-mode dual frequency heterodyne reflectometer has been installed on Tore Supra [Tore Supra Team, Fusion Technol. 29, 417 (1996)] and it has greatly improved the phase determination due to its high dynamic heterodyne detection and ultrafast sweep capabilities (10 μs) provided by its solid state source HTO. This system operates with O-mode electric field polarization in the range of 26–36 GHz and has been designed for density profile measurements. The reflectometer launches two frequencies separated by 320 MHz simultaneously into the plasma. Heterodyne detection improves the dynamics up to 60 dB, and is associated with a sin/cos detection to allow separate analysis of the amplitude and phase of each reflected signal of both probing waves. Therefore, to calculate the density profile, the group delay can be defined in two different ways: (i) from the derivative of the absolute phase of one of the two probing waves or (ii) by calculating the phase difference between the two probing waves. We explain how...