Assessment of microwave power flow for reflectometry measurements in tokamak plasmas

Microwave diagnostics, such as reflectometry and electron cyclotron emission, are widely employed in tokamak fusion plasmas, and are also particularly well suited to the burning plasma environment expected in the upcoming ITER device. In existing fusion plasmas, reflectometry has been used to measure electron density profile, density fluctuations, turbulent flow and internal magnetic field strength. The measurement of electron density profile via reflectometry is a particularly high priority in ITER, and so an assessment of electromagnetic wave propagation is important in estimating measurement capability. In ITER, the ratio of plasma size to the required microwave signal wavelength is significantly larger than in current fusion experiments ensuring a more realistic analysis of reflectometry via ray-tracing techniques. The analytical and numerical studies presented in this paper highlight the fact that the group velocity ( or power flow) is strongly dependent on the direction of wave propagation relative to the magnetic field. It is shown that this dependence strongly modifies power flow near the cut-off layer in a manner that embeds the local magnetic field direction in the 'footprint' of the returned power at the launch antenna. It will be shown that this can potentially be utilized to determine the magnetic field pitch angle at the cut-off location. The resultant beam drift and distortion due to the magnetic field also have consequences on the design of reflectometry systems for large, high-field fusion experiments such as ITER