Plasma position measurements by O-mode and X-mode reflectometry systems in tokamak plasmas

Plasma Position Reflectometry (PPR) is planned to provide plasma position and shape information for plasma operation in future fusion reactors. Its primary function is to calibrate the drift of the magnetic signals due to the integral nature of magnetic measurement. Here, we attempt to measure plasma position using ordinary mode (O-mode) and extraordinary mode (X-mode) reflectometry systems on two tokamaks. A new physical model based on the phase shift is proposed to deduce the relative movement of the cut-off layer without density inversion. We demonstrate the plasma position measurements by absolute measurement from density profile inversion and relative measurement from phase shift. The combination of X-mode and O-mode reflectometers can minimize the limitations of single polarization reflectometry and further increase the accuracy of plasma position measurement. These results could provide an important technical basis for the further development of a real-time control system based on PPR.

Development of a tunable multi-channel Doppler reflectometer on J-TEXT tokamak

Doppler reflectometer is a powerful diagnostic tool to study the turbulence for tokamak plasmas. It can provide information on the density fluctuation, the poloidal rotation, the radial electric field, its shear, etc. A tunable multi-channel V-band (50-75 GHz) Doppler reflectometer system has been developed on the J-TEXT tokamak for the measurement under various toroidal magnetic fields. A universal serial bus controlled synthesizer is used as a source that can adjust the probing frequency remotely. This Doppler reflectometer can measure the plasma in 0.3 < ρ < 1 . Its radial resolution is <2 cm, and k_⊥ is ∼ 4-12 cm^-1. Based on the Doppler reflectometer, the perpendicular turbulence propagation velocity, the profile of the radial electric field, the geodesic acoustic mode, and some other phenomena have been observed on J-TEXT.