Comparison of Different Reflectometry Techniques

Abstract

Reflectometry applied to the measurement of density profiles on fusion plasmas has been subject to many recent developments. For ITER, reflectometry has the great advantage of having only waveguides nearby the vessel, while the windows and the electronics can be at a distance. Different methods have been experimented: linear frequency sweep (broadband or narrowband), dual frequency differential phase, amplitude modulation, pulsed radar, which can be extended to ultra-short pulse, pulse compression, and noise correlation radar. Up to a certain degree, they all encounter the main difficulties limiting the performances, namely the plasma fluctuations and the quality of the transmission lines. Based on actual applications, the implementation of the different techniques is presented, with an analysis of the technological requirements as well as their respective limitations and merits. Expected technical evolution for these techniques, or development of more recent and alternative techniques, is also discussed. Keeping in mind that a whole reflectometry system is based on a method of measurement, but also on the available technique and on the required performances, a comparison is made in prospect of their application in the challenging environment which will be imposed by ITER.