Ultrafast frequency sweep heterodyne reflectometer on the Tore Supra tokamak

Abstract

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 ultrafast sweep operations (down to 10 μs) significantly reduce the influence of plasma turbulence on the phase measurements. We also point out the importance of carefully filtering the signal detected in order to keep only information coming from the reflection at the cutoff and get rid of parasitic reflections. It is shown that the phase difference technique does not completely remove fluctuations such as long radial correlation magnetohydrodynamic perturbations. The density profiles determined by the heterodyne reflectometer are in good agreement with the measurements from the other diagnostics of Tore Supra.