New developments of the ASDEX Upgrade tokamak microwave reflectometer

The ASDEX Upgrade broadband microwave reflectometry system uses frequency modulation of a continuous wave (FM-CW). With a monostatic antenna configuration optimised for eliminating spurious reflections, the system can probe the high and low field sides of the plasma (using both O- and X-modes) covering the density range from 0.3 to 6.76×1019 m−3. A combination of solid state fast tuneable microwave sources (Hyperabrupt Tuned Oscillators, HTO) with active and passive doublers are used. During the recent shutdown period several improvements were made. A new W band (75–110 GHz) channel was installed at the low field side to extend the density range to 15×1019 m−3. A broadband heterodyne detection was designed to improve detection, which is required due to high losses and low power available for this band. A similar detection scheme was implemented to all existing V band channels. Both systems can perform a full band sweep in 10 μs. Also a new Q (35–50 GHz) band O-mode channel with fixed frequency was installed for on-line fast monitoring the L–H transition based on the reduction of the plasma density fluctuations. The control system was upgraded with five new control boards for fixed frequency operation of the reflectometer. The new control software enables the operation in three different modes: broadband, fixed frequency and a mixed mode. A new dedicated data acquisition system came into operation to replace the existing CAMAC/VME configuration, enabling high sampling rate (1 GSPS). Advanced digital signal processing techniques were developed to obtain density profiles. With these new improvements the system is capable of measuring 720 electron density profiles per discharge in the range 0.3 to 15×1019 m−3, each in 20 μs with a separation of ≈40 μs.