UC Davis PDG: Introduction and Background of Microwave Imaging Reflectometry

 
 
 
 

             Fig. 1 Characteristic frequencies of TEXTOR-94.

 

Microwave reflectometry is a radar technique used to infer the electron density characteristics by probing the density-dependent cutoff layer in magnetized plasma. Referring to Fig.1, an electromagnetic wave propagating through magnetized plasma is reflected from a cutoff surface where the EM wave frequency is equal to the local plasma frequency or the R or L circular frequency depending upon polarization.

In standard electron density fluctuation measurements with microwave reflectometry, the probing wave is launched and received on the equatorial plane using a pair of small antennas. The measurement is essentially a point measurement, and does not provide direct information on the spatial structure of density fluctuations.  A significant improvement in the capability of this technique is the method of correlation reflectometry, where the radial structure of plasma fluctuations is inferred from waves reflected from closely spaced cutoff layers.
The normal 1-D geometrical optics approximation unfortunately breaks down in the case of multidimensional turbulent fluctuations, which is precisely the case of interest for magnetic fusion plasma diagnostics where there are large radial and poloidal variations. The need for MIR has been well documented through detailed reflectometry studies by E. Mazzucato of the Princeton Plasma Physics Laboratory who is one of our collaborators in this research. These studies have revealed the crucial need for reflectometric imaging (together with the failure of standard fluctuation reflectometry). The difference between standard 1-D reflectometry and 2-D is readily apparent from Fig.2 below, where it is seen that with 1-D fluctuations, (a) the reflection layer will move back and forth in the radial direction, resulting in the phase changes in the reflected wave; while with 2-D fluctuations, (b) the backward field propagating along different directions results in a complicated interference pattern at the detector plane, so that both the amplitude and phase of the reflected wave will be disturbed by the fluctuations perpendicular to the probing beam, leading to a breakdown of the simple relationship between phase fluctuations and density fluctuations.
 

             Fig.2 Comparison of 1-D (a) and 2-D (b) reflectometry.

In standard reflectometry, the reflected waves from a "corrugated" cutoff layer interfere at the detector, causing both amplitude and phase modulation. Hence, the measured phase no longer directly corresponds to the density fluctuation. Fortunately, the use of imaging optics can solve this problem.
 

For further technical details, and Microwave Imaging Reflectometry measurements and results from TEXTOR, please examine the following links:

Microwave Imaging Reflectometry Set up in the lab

The features of the Mivrowave Imaging Reflectometry diagnostics derive from the use of wideband, low cost monolithic and hybrid Schottky diode mixer arrays. Follow the links below to learn more about both the technology and the techniques employed in MIR.

Imaging array design and fabrication
 
 

Comments to: Jian Wang