The different reflectometry methods currently used for the determination of electron density profiles of fusion plasmas suffer serious limitations due mainly to plasma fluctuations and to parasitic reflections in the transmission waveguides. An advanced method, based on pulse compression radar techniques, can be applied to overcome these limitations. This paper reports two series of results obtained at JET, which for the first time validate the pulse compression method for reflectometry on thermonuclear plasmas. The first results are for a reflector positioned a short distance beyond the end of a long, complex and oversized waveguide. The same waveguide is used for transmission and reception, inducing many parasitic reflections. The second series of results are from measurements on JET plasmas, using different waveguide configurations. Results both with the reflector and the plasma show that multiple reflections within the transmission line can be eliminated by a correction technique in the signal analysis procedure. A precision of typically 1 cm can be obtained for the measured position of the reflector, or the plasma cutoff layer, with only 20-50 frequency steps. With the fast frequency synthesizers currently available, the complete measurement could be made in 2-5 mu s, thereby freezing plasma fluctuations due to MHD and most of the microturbulence.
Electron density profiles measurements by reflectometry in front of a lower hybrid launcher on WEST
Pulse radar technique for reflectometry on thermonuclear plasmas