Design of an X-mode fast-scanning reflectometry for edge density profile measurement on HT-7 tokamak

An amplitude modulation reflectometry system with fast-scanning microwave source is designed for edge density profile measurement on the HT-7 superconducting tokamak (R-0 = 1.22 m, a = 0.28 m, B-t: 1.8-2.3 T, plasma pulses of 1-20 s). The reflectometer launches microwave power from a very fast-scanning backward wave oscillator in 53-78 GHz to probe the plasma and receive the reflection by the density cutoff layer. It is an extraordinary mode reflectometer, which can measure the edge density profiles in normalized radius r/a from scrape-off layer up to 0.5 when B-t = 2.2 T, N (e0) = 3 x 10(19) m(-3) in typical HT-7 operation parameters. The microwave amplitude modulation frequency is 200 MHz. A 2 MHz reference signal and the probing signal are obtained by mixing a 198 MHz local oscillator power with the demodulated microwave signals. Phase comparison is then accomplished at 2 MHz by I/Q phase detector. The scanning time over the full band of 53-78 GHz is about 0.5 ms. The reflectometry will fill in the space left by the five-chord interferometer, which measures in the core plasma region and the Langmuir probes, which work in the scrape-off layer; especially the reflectometry is used to study the lower hybrid wave coupling into plasmas for the current drive and the radio frequency wave coupling problems for heating on HT-7. The reflectometry is also considered to be useful on the future HT-7U superconducting tokamak. (C) 2003 American Institute of Physics.