Profile of plasma density at the plasma edge
< 1 cm
Microwave low-power signal can be used as a non-perturbing diagnostic of the hot magnetized plasma. Microwave reflectometry utilizes a range of frequencies for which the plasma is reflective, i.e. tens of GHz. Due to a dependency between the frequency and the electron density reflecting the signal, the density profile measurement by the fast frequency sweeping can be realized. For the COMPASS tokamak the frequency range 18 – 90 GHz has been chosen to measure the electron densities of (0.4–10) x 1019 m-3 at the plasma edge and pedestal region. This measurement will complement the measurements by the Thompson scattering system and the Beam Emission Spectroscopy on the Lithium beam system.
The reflectometry system consists of the transmitters, receivers, quasioptical band-combiners and quasioptical antennas. The transmitter emits signal with very fast < 20 µs frequency sweeping. The receiver evaluates the delay of reflected signal so as to localize the position of the reflection. The system will also enable to study the plasma turbulence in so called frequency hopping mode. The highly sophisticated electronics, including the data acquisition and data processing, is developed in IST/CFN Lisbon. The first half of the system for 18 – 40 GHz will be put in operation on COMPASS by the end of 2010.
The whole microwave range 18 – 90 GHz has to be divided to the four frequency bands. Therefore, the system will consist of four transmitters and four receivers. In the band-combiner the four signals are put together and the composite signal is transmitted via the antenna to the plasma. In the receiving path the signals reflected from plasma are decomposited in the same way. The band-combiners and antennas, based on a quasioptical principle, are developed in Institute of Radiophysics and Electronics.
Fig. 1: Reflectometry system – block scheme of electronics for one band.