title: Robust measurement of group delay in the presence of density fluctuations by means of ultrafast swept reflectometry.
abstract: It is well known that reflectometric measurements can be seriously disturbed by density fluctuations. In this paper the basic mechanisms that critically affect the measurements are outlined with the help of a simplified two-dimensional model which simultaneously mimics the dynamic of the plasma turbulence and the frequency sweep. It turns out that an adequate modulation rate can easily overcome the adverse effect of wave scattering produced by short wavelength fluctuations even when measurements taken at a single frequency would fail. Nevertheless the Doppler effect caused by long wavelength fluctuations can produce a substantial shift in the group delay, which accounts for nonmonotonous density profile reconstructions. In turn this effect can be corrected using a very fast ascending and descending frequency modulation, similarly to what is done in FM radars. The technical feasibility of such a system is demonstrated using the Ka band reflectometer of RFX, where a triangular modulation of 2.5 MHz, with a bidirectional modulation rate up to 4 GHz/200 ns, has been obtained.
authors.affiliation: Consorzio RFX, Associazione Euratom, ENEA, Corso Stati Uniti 4, I-35127 Padova, Italy.
authors.forename: Roberto
authors.initials: R
authors.lastname: Cavazzana
authors.forename: Maurizio
authors.initials: M
authors.lastname: Moresco