Simulations on the Role of the Resonance of the Probing Wave on Reflectometry Measurements in Fluctuating Plasmas

In fusion plasmas for energy, turbulence is clearly associated to the anomalous transport of energy and particles. Diagnostics using electromagnetic waves is usually used to measure the plasma parameters and the turbulence characteristics. One of the diagnostics is reflectometry, which is based on the radar principle and can be used to reach these different goals: density profile, turbulence characterization, plasma positioning, etc. During the plasma probing, fast phase variations called phase jumps have been measured, and part of them can be explained by a local enhancement of the probing field. Using an analytical model to compute the probing electric field amplification, a new expression for the phase variations has been written, taking into account the amplification of the probing electric field. This formula exhibits a good agreement with the full-wave computations. A possible improvement of the reflectometer sensitivity can be done using the local enhancement of the probing electric field induced by Bragg resonant density perturbations, which build resonant cavities. The computed cases show that it is possible to improve the reflectometry measurements when the local enhancement of the probing electric field exists. To illustrate the possible improvements, simulations of the radial wavenumber spectrum reconstruction have been done, and they confirm the possibility to increase the sensitivity and the spatial resolution for a given range of wavenumber.