Dual-polarization single-chord plasma interferometry in stellarators/torsatrons

The well known ordinary wave based interferometry is of frequent use for plasma diagnostics in fusion facilities. With the probing direction being perpendicular to the magnetic field of the facility, the ordinary wave phase shift is proportional to the average density along the probing path at frequencies much higher than the plasma frequency of electrons. In the case of extraordinary waves, the phase shift depends on both the plasma density and the confining magnetic field. For the facilities of stellarator/torsatron types, the magnetic field is known; therefore, the measurement of the extraordinary wave phase shift can also provide information on the plasma density profile. The plasma density measurements with the use of dual-polarization interferometers were carried out on the Uragan-3M and Uragan-2M torsatrons. For correct interpretation of the experimental results, numerical simulation was performed to describe the probing wave propagation through the plasma in the magnetic field of variable direction. The transmitting-to-receiving horn scattering matrix has been obtained in a wide range of plasma parameters. In the course of the interpretation of measurements, restrictions on the use of the Wentzel–Kramers–Brillouin approximation were determined. This has made it possible to develop the optimized layout of the dual-polarization interferometer for measuring the plasma density in stellarators/torsatrons. The here proposed upgrade of the interferometers on Uragan-2M will enable one to obtain additional information on the temporal evolution of the plasma density.