Upgrade of the RFX microwave frequency modulated reflectometer to ultrafast sweeping rate

The swept homodyne reflectometer installed on the RFX experiment is composed of five units which cover the frequency range 26.5-110 GHz. This diagnostic system is based on solid state IMPATT sources and can be operated in the O or X mode. During the first operation, at a sweep rate of 0.1 GHz/ mu s, the typical density fluctuations present in the reversed field pinch configuration usually prevented correct profile reconstruction, which was possible only in some special cases. To increase the sweep speed and overcome this limit, a new IMPATT driver has been built and IF amplifiers, control, and acquisition systems have been modified accordingly. The new configuration has been extensively tested in the 34-38 GHz range with a modulation rate exceeding 4 GHz/mu s on many different plasma conditions. Measurements on plasma showed that both the phase and reflected power level can be correctly recovered from the IF signals. These results indicated the criteria necessary to extend the fast sweep capability to the full 26.5-110 GHz range. In this article we present how the fast sweep capability is now being extended to the whole reflectometric diagnostic and the first successful plasma measurements obtained using a test reflectometric system. (C) 1999 American Institute of Physics. [S0034-6748(99)51301-4].