Plasma position reflectometer in RFX-mod2: system overview, technical issues and bench tests

Plasma position reflectometry (PPR) is a very promising diagnostic technique intrinsically resistant to neutron radiation that aims at estimating the position and shape of the plasma column in Tokamaks. To this extent, a sophisticated PPR system is under design for DEMO, where long pulses and high neutron fluxes are likely to jeopardize the long-term reliability of the internal magnetic sensors. On RFX-mod2 [1,2] (R=2.0 m, a=0.49 m), the upgraded version of the previous RFXmod fusion device, the PPR will consist of four bistatic ultrafast independent reflectometric units working in the K band (18-26 GHz), installed in four different poloidal locations at the same toroidal angle: two on the equatorial plane (High Field Side / Low Field Side) and two at the vertical top/bottom ports. This simplified PPR scheme has been designed in order to test some of the issues related to a plasma position control based on reflectometric measurements: the performance in the high magnetic curvature regions; the integration with the information of the current flowing in the poloidal field coils or coming from other diagnostic systems. To this respect, the RFX-mod2 flexibility in terms of attainable plasma shapes, discussed in this contribution, can offer an almost unique test bench. The integration of the PPR in the RFX-mod2 structure has required the investigation of different technical solutions that will be presented. In particular, the design of the HFS antennae is constrained by the limited physical space: hoghorn reflectors with a complex geometry have been modeled [4] and produced through the additive manufacturing technique and subsequent superficial treatments. The first K band is now being tested on the bench in order to compare the experimental results to the analytical and numerical models and to check the expected performance of the whole system.