UC Davis PDG: Microwave Imaging Reflectometry (MIR)
An electromagnetic wave propagating through a plasma can be reflected from plasma cutoff layers in the plasma interior. Microwave reflectometry is a radar technique used to infer the electron density characteristics by probing the density-dependent cutoff layer in plasmas. Since its first use for the investigation of low frequency microturbulence in tokamak plasmas, microwave reflectometry has become widely used for density profile and fluctuation measurements. In the presence of density fluctuations, the reflected electromagnetic wave spectrum is broadened with a strong weighting by those fluctuations in the vicinity of the cutoff layer.
In standard electron density fluctuation measurements with microwave reflectometry, the probing wave is launched and received 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.
Microwave reflectometry has been extensively used in tokamak plasmas for the detection of turbulence, due to its relatively simple implementation and its high sensitivity to small perturbations of electron density. Despite its widespread and long-standing use, however, the interpretation of reflectometry data from fluctuations remains an outstanding issue. For 1-D turbulence (see Fig. 1(a)), the plasma permittivity is stratified. The reflection layer will move back and forth in the radial direction, resulting in only phase changes in the reflected wave. In the presence of two dimensional turbulent fluctuations, the interpretation of reflectometry becomes considerably more complex. But this is precisely the case of interest for tokamak plasma, which exhibit both toroidal and poloidal fluctuations. The difficulty arises from the fact that when the plasma permittivity fluctuates perpendicularly to the direction of propagation of the probing wave, the spectral components of the reflected field propagate in different directions. This can result in a complicated interference pattern on the detector plane (see Fig. 1(b)), from which it is very difficult to extract any useful information about the plasma fluctuations since the detector sees a complicated amplitude and phase modulated signal rather than simply phase fluctuations.
Fig.1 Comparison of (a) 1-D and (2) 2-D reflectometry
Microwave Imaging Reflectometry (MIR), which was conceived by Dr. Mazzucato of the Princeton Plasma Physics Laboratory, is a technique in which large-aperture optics at the plasma edge are used to collect as much of the scattered wavefront as possible and optically focus an image of the cutoff layer onto an array of detectors, thus restoring the integrity of the phase measurement. In MIR, a broad area of the plasma cutoff layer is illuminated by a millimeter-wave source as illustrated in Fig. 2. Focusing optics transform the output of the illuminating source into an extended beam whose curved wavefront is designed to roughly match the poloidal/toroidal shape of the plasma cutoff surface. This wavefront curvature matching projects a nearly constant phase front onto the fluctuating layer, while ensuring that a sufficient fraction of the incident electromagnetic power is coupled to the MIR receiver for detection. The reflected radiation passes back through the same imaging optics as the illuminating beam, with additional optics utilized to image the reflecting layer onto the detector array. A beam splitter separates the transmitted beam path from the detection path and final detector optics.
Fig.2 Schematic layout of the MIR diagnostic.
The first 1-D MIR system was installed on the TEXTOR tokamak and decommissioned in 2006. Recently in 2012, the TEXTOR MIR detector array was incorporated into a proof-of-principle MIR system on KSTAR, with a planned upgrade in coming years. MIR systems are currently under development for the DIII-D and EAST tokamak devices. For further technical details on these systems, please examine the following links:
The Microwave Imaging Reflectometry diagnostic derives much from the use of wideband, low cost Schottky diode mixer arrays. Follow the link below to learn more about this innovative technology.
Comments to: Calvin Domier