Microwave polarimetric setup for plasma density measurement in compact Ion sources

In this contribution, an innovative K-band (18-26.5 GHz) microwave polarimetric system is presented. The experimental setup for the detection of the magnetoplasma-induced Faraday rotation [1] is based on a precision rotating polarimetric system and on waveguide Orthomode Transducers (OMTs). In presence of a known longitudinal magnetic field component, (0.1 T in our case), the system is able to provide reliable line-integrated electron density measurements, in the range 1017-1019m-3. It is worth noting that any non-intrusive plasma electron density measurement in compact microwave-based Ion Sources is a challenge, especially due to the small size of the plasma chamber with respect to the required probing wavelength. The approach considered here overcomes many drawbacks: an analysis method has been developed on purpose in order to detect the polarization plane excluding the effects of the cavity resonator when a wire grid polarizer is placed inside the cavity itself. The measurement strategy and data analysis allow being sensitive even in the extreme unfavourable conditions λp≈Lp≈Lc, being λp, Lpand Lcthe probing signal wavelength, the plasma dimension and the plasma chamber length respectively. The developed noninvasive diagnostic method will represent a powerful tool for probing in a non-intrusive way very compact devices, such as plasma-based Electron Cyclotron Resonance Ion Sources and, more generally, compact magnetic traps containing microwave-generated plasmas. Finally, our measurements can be directly compared (and crosschecked) with the already developed Frequency-Modulated Continuous-Wave (FMCW) interferometer setup [2].