Experimental Studies of the Mechanisms for Reflectometer Spectrum Formation for Normal and Oblique Probing

The dynamics of spectra formation was studied experimentally using radar stroboscopic time-of-flight measurements. Both conventional fluctuation reflectometry spectra (obtained with probing normal to the cutoff surface) and Doppler reflectometry spectra were studied. Two mechanisms of spectrum formation were distinguished even at low time resolution that did not allow capturing the spectra formed after single radiation pass to the cutoff surface and back. The first mechanism that manifests itself for the normal probing and works for the unshifted component of the Doppler reflectometry spectrum is typical for the nonlocal formation of the fluctuation reflectometry spectra in small machines. This can be associated with the radiation propagation along the lengthy trajectory between the cutoff surface and the chamber wall, as well as with the small-angle scattering that occurs during this propagation along the trajectory. The second mechanism, which manifests itself during the formation of the bulk of the Doppler reflectometry spectrum (which is dominant in power and is frequency shifted), occurs without a time delay and most likely corresponds to the classical single-pass backscattering near the cutoff surface.