Microwave Reflectometry Based On Amplitude Modulation

Reflectometry is a technique, based on the radar principle, with a response mainly sensitive to the plasma regions where the microwaves are reflected. The reflectometry technique used in plasma devices measures the phase of a wave reflected by a plasma cut-off layer at a given electron density. It has become a promising experimental technique for the measurement of the electron density profile in plasma devices, due to its good spatial and temporal resolution, together with the moderate access requirements and its ability to perform profile inversion along a single line of sight. Since its first implementation on a plasma device for electron density profile measurement, the high cut-off X-mode polarization has convinced many scientists to use this technique on most of the main devices, such as ASDEX, DIII-D, Gamma 10, TJII, TFTR, W7AS etc, and it is planned for edge and scrape of layer profile determination on ITER.HT-7 tokamak is a circular cross-section limiter superconducting tokamak with a 1.22m major radius and 0.27m minor radius. Many experiments such as IBW heating and LHW heating have been conducted in predominantly deuterium target with a toroidal magnetic field B-T up to 2.0Tesla, plasma current I-p up to 200kA and central line averaged electron density (n) over bar (e) (0) = (0.5 - 6.0) x 10(19) m(-3). In order to obtain density profiles in these experiments, a microwave reflectometry based on amplitude modulation, is being developed on HT-7 tokamak.The reflectometry was designated according to HT-7 specifications for profile measurements over a rather large range. This system will operate with the extraordinary mode in the range of 48-72 GHz. The cut-off frequency as a function of the radius r for different central densities is shown in fig. 1.A hyper-abrupt varactor-tuned oscillator (HTO), which routinely provides fast sweep capabilities, in the range of 12-18GHz is adopted in combination with active multipliers, which can generate the required frequency of microwave source. Emission and reception are performed with two separate cone antennae. Both antennae were located in the equatorial plane close each other and inside the vacuum vessel. The microwave amplitude modulation frequency is 200 MHz. Both the probing signal and reference signal are mixed with a 198 MHz local oscillator. The phase comparison is accomplished at 2 MHz by I/Q phase detector. As the amplitude of the reflection is of great importance to determine the starting point, the AD8302 chip was used due to its excellent features. Results of testing of this system will be reported in this article.This work was supported by the National Natural Science Foundation of China under Grant No 10335060.