Development of a novel fast frequency modulation scheme for the Joint European Torus multichannel reflectometer

A novel frequency modulation scheme has been developed for the multichannel reflectometer used to measure density profiles and density fluctuations on the Joint European Torus (JET) tokamak. This reflectometer normally uses slow frequency sweeping (6 ms), combined with fixed-frequency operation, to measure the group delay, as well as plasma fluctuations, at ten different microwave frequencies. The novel scheme uses continuous frequency modulation on a time scale much shorter (1/65 μs) than that of plasma fluctuations, the main aim being to make the group delay measurement more robust against plasma fluctuations. This article discusses the theoretical background of the scheme, gives a detailed description of the system, and presents results from plasma measurements. Although the implementation used suffered from an excessive power-dependent group delay distortion (resulting in a positional error of 10 cm), the main objectives have been achieved.