UC Davis PDG: MIR on TEXTOR
A prototype 1-D Microwave Imaging Reflectometer (MIR) system has been constructed and installed on the TEXTOR tokamak, in collobaration with the Princeton Plasma Physics Laboratory (PPPL) and the FOM Institute for Plasma Physics 'Rijnhuizen'. In this apparatus, a portion (~10 cm) of the poloidal cross section is illuminated by a large probing beam, and a wide aperture optical system is used for collecting the reflected waves and for forming an image of the plasma cutoff layer onto a linear array of 16 detectors.
Shown to the right is an artist's sketch of the combined ECE Imaging and MIR system on the TEXTOR tokamak in Juelich, Germany. Both systems share a pair of large diameter reflecting optics (mirrors), using a gridded mesh beamsplitter to separate the MIR (at 88 GHz) and ECEI (at 95-125 GHz) signals. Previous MIR implementations which utilized multiple high density polyethelene (HDPE) lenses had significant standing wave problems arising from multiple reflections of the lens surfaces shared by the probing and reflecting beams, leading to the development of the current reflective optics design.
The primary difference between the configuration of the MIR and ECEI subsystems is that the MIR system has the added complication of the probing beam. In order to maximize the robustness of the collection optics to details of the plasma profile, the probing beam is focused at the center of the radius of curvature of the cutoff surface, so that the incident wavefront matches the shape of the reflecting layer. The focal point is, of course, different in the poloidal and toroidal directions. MIR receiver optics, on the other hand, image the plasma cutoff layer onto the detector array, thereby reducing much of the interference effects seen by non-imaged reflectometer systems. A beam splitter separates the transmitted beam path from the detection path and final detector optics, and allows the systems to be independently focused for maximum flexibility.
A photograph of the system, installed on TEXTOR, is shown to the right.
Additional MIR systems are planned for the KSTAR and NSTX plasma devices. For further technical details on these systems, please examine the following links:MIR on DIII-D
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