High-resolution millimeter-wave reflectometry for electron density profile measurements in tokamak plasma

A highly reliable density profile reflectometer system, which is relevant for current and future magnetic fusion research devices, has been developed on the DIII-D tokamak. Broadband frequency-modulated (EM) reflectometry has been selected as a benchmark system for this thesis research, while simultaneously assessing the relative merits of alternative techniques. Utilized broadband FM systems are a BWO-based reflectometer and a new solid-state fast-sweep reflectometer. A significant improvement in reflectometer profile measurements has been achieved through the advanced signal analysis utilizing digital complex demodulation (CDM) which improved phase accuracy (${le}pi/10).$ The CDM, which is a software implementation of heterodyne quadrature techniques, has been successfully applied for broadband FM reflectometry for the first time. In processing reflectometer profiles, important radar design parameters such as range resolution, precision and spatial sampling were optimized for reliable measurements. The reflectometer-measured profiles were generally in good agreement with other independently calibrated diagnostics such as Thomson scattering, Langmuir probe and multichannel interferometry. In low density discharges the reflectometer profiles reached almost to the machine center. The measured profiles were also used for physics studies such as the RF wave coupling with plasmas. While the BWO-based system with a moderate frequency sweep rate (400-600 $mu$s) produced reliable profiles with the improved analysis, the time resolution was limited due to tube reset-time ($sim$3 ms) and phase averaging to minimize density fluctuation effects. However, with development of the fast-sweep system, the rf frequency was swept fullband every 100 $mu$S without reset-time. Since fast frequency sweep significantly reduced turbulence effects on measurements, the system has produced very reliable individual profiles, which greatly enhanced the time resolution. The fast-sweep system has resolved fast-changing profiles across L-H transitions and dramatic edge density changes during ELMs with high temporal (${le}100mu$s) and spatial ($sim$1 cm) resolution, which is unprecedented using any other techniques. New information on the edge profile transition time in L-H transitions and ELMs has been obtained from the measurements. The thesis presents details of basic principles, analysis techniques, numerical simulations and implementation of the profile reflectometer system as well as the experimental results obtained in tokamak plasmas.