Plasma Dynamical Processes and Turbulence Studies using Advanced Microwave Diagnostics
Phased array antenna / beam steering
Background: Doppler reflectometry has been developed as a direct radial electric field Er measurement technique. Here, a microwave beam approaches the plasma cutoff layer (boundary of non-propagation) at an angle and is partially back-scattered from turbulent fluctuations. The intensity of the back-scattered beam gives information on the turbulent fluctuation amplitude at a specific wavenumber k_perp while the Doppler frequency shift gives the plasma flow velocity and thus the radial electric field.
The technique has the benefit of high spatial (radial) and temporal (sub-microsecond) resolution. However, antennas with fixed tilting require careful optimization of the probing beam pattern, the antennae position and alignment in both the poloidal and toroidal directions. Fixed alignment also means that only one preselected k value is used which restricts the information and may not be optimal for all plasma configurations. This can be overcome using a steerable antenna to adjust the probing angle, for example via an adjustable front-end mirror. However, mechanical moving mirrors or antennas in-vessel can be problematic and undesirable in W7-X, ITER and future fusion devices. The proposal here is to develop non-mechanical steerable antenna options.
Approach: The standard radar approach to beam steering is via phased arrays - which use mono-mode waveguides for the feeds and phase shifter. Two sub-projects have been defined with different approaches to creating the controlled phase shifting:
Sub-project: Frequency keyed phase shifting
The first approach, which avoids any mechanical movement in the phase shifter, is a frequency-controlled phased array. Here, the length of the individual delay lines in the mono-mode section are optimized such that the phasing and thus the beam steering is obtained by frequency shifting. The challenge here is to develop an antenna which operates at several centre frequencies, while a small (negligible) variation of the centre frequency would lead to a strong variation of the radiation angle.
Project lead by IGVP in collaboration with TUM: Stefan Wolf (PhD) Dr. Walter Kasparek Dr. Burkhard Plaum
Sub-project: Active phase shifting
The appropriate phasing can also be set by squeezing the individual waveguides in the H-plane using a pressure element (such as a single gas pressure or piezo drive) creating a monotonic variation of the phase difference between adjacent waveguides. The phase-shifter is followed by one-dimensional up-tapers to obtain an antenna array with high directionality in both planes.
Project lead by TUM in collaboration with IGVP: Christoph Eisner Prof. Thomas Eibert Dr. Uwe Siart