Design of a new reflectometric system for real time plasma position control on the RFX-mod2 device

A diagnostic technique capable of feeding back a reliable real time plasma position estimate alternative to direct magnetic measurements is a crucial topic for position and shape control on future fusion devices. Featuring a high temporal and spatial resolution and being suitable for harsh fusion reactor experimental conditions (long pulses, high neutron fluxes), reflectometry has been proposed as a good candidate for this task. In this respect, some successful tests (limited to the horizontal plasma column control) have been performed on ASDEX-Upgrade [1,2]. In this contribution, we present the main features of a new reflectometric system, specifically designed for real time horizontal and vertical position control studies, to be installed on RFX-mod2 [3] (R=2.0 m, a=0.49 m), the upgraded version of the RFX-mod (R=2.0 m, a=0.46 m). RFX-mod2 is presently under construction and will operate in Reversed Field Pinch (RFP) and tokamak configuration. The new reflectometric system will be composed by four ultrafast K-band units (18-26 GHz) at the same toroidal section: two on the equatorial plane (from inner High Field side and outer Low Field Side respectively) and two at the vertical top/bottom ports. Each unit will be an independent system connected to a bistatic antenna configuration. The reflectometers are designed to operate in X-mode on tokamak plasmas (Bt0=0.5 T; Ip=50-150 kA) in circular and shaped configuration (upper and lower Single Null, Double Null). They will provide the high time resolution edge density profiles needed at first for the development and validation of real time algorithms for basic vertical and horizontal position and to a broader extent, to address the issues linked to the reconstruction of the Last Closed Flux Surface by means of reflectometric data.