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Description
The Reversed Field eXperiment (RFX)-mod2 device, the upgraded version of RFX-mod, is entering its operational phase with significantly enhanced diagnostic capabilities developed within the NEFERTARI (New Equipment for Fusion Experimental Research & Technological Advancements with RFX Infrastructure) project. Work Package 4 (WP-4) focuses on strengthening the main plasma diagnostics to enhance the capability to characterize performance in multiple magnetic configurations, including both Reversed Field Pinch (RFP) and tokamak regimes. The upgrade targets key parameters, including electron (Te) and ion temperature (Ti), through harmonized improvements of Thomson Scattering (TS), Soft X-Ray (SXR), bolometry and Neutral Particle Analyser (NPA).
A major effort has been devoted to the enhancement of the TS system that has been equipped with a second Nd:YAG pulsed laser capable of operating in single-shot or burst mode (0.3–3 kHz). It delivers up to 10 pulses per burst (5 at 0.3 kHz), each with an energy of 4.11 ± 0.04 J, thereby empowering temporal resolution and signal-to-noise ratio. The integration with the existing 100 Hz source required a quasi-coaxial alignment over distances exceeding 15 m. A dedicated campaign ensured that both lasers probe the same scattering volumes along the internal machine diameter. The TS diagnostic has also been upgraded with a modernized acquisition chain based on new digitizers (12-bit at 5 GS/s and 16-bit at 125 MS/s), together with a dedicated manipulator enabling efficient and reliable beam dump maintenance. Furthermore, it provides potential retro-reflection configurations to enhance the scattered signal, particularly beneficial for the tokamak configuration of RFX-mod2, typically operated at low density.
In parallel, the Soft X-Ray (SXR) tomography system has been strengthened through the realization of three new heads operating under ultra-high vacuum. Each includes three photodiode arrays viewing the plasma along defined lines of sight through pinholes and beryllium filters of different thickness. This configuration selects the SXR spectral range while suppressing UV and visible radiation. The geometry of pinholes and detectors has been optimized to maximize plasma coverage, allowing simultaneous measurements of SXR emissivity and Te profiles. Low-noise amplifiers have been procured and tested as well as photodiodes have been comparatively characterized using a calibrated X-ray source exploiting silver anode emission. In addition, a new 128-channel acquisition system operating at 1 MS/s has been designed and assessed.
The bolometric diagnostic has also been upgraded through improvements of the amplification and acquisition hardware, including validation of signal integration within the existing framework.
A Compact Neutral Particle Analyzer (CNPA) was initially proposed to complement the Ti measurements provided by the Diagnostic Neutral Beam Injector improved within WP-5. However, due to the Russian embargo, its procurement was not possible. Therefore, an upgrade of the existing NPA system has been implemented, extending the measurable energy range by a factor of 1.3 (up to 26 keV) and increasing the temporal resolution up to 50 kHz.
Overall, WP-4 has successfully progressed from design and procurement to installation, alignment and commissioning. The upgraded diagnostics will significantly enhance the capacity of RFX-mod2 to measure fundamental parameters, opening new opportunities for advanced studies of plasma phenomena.