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Description
The RFX experiment has been in operation since 1991. A first upgrade, leading to the RFX-mod configuration, was completed in 2004. This upgrade enabled significant improvements in the understanding of fundamental plasma physics and the development of technological solutions, particularly in the field of active MHD control. These systems proved successful in mitigating the effects of plasma modes and in investigating the properties of the Reversed Field Pinch (RFP) configuration up to a plasma current of 2 MA, the highest value ever achieved in an RFP experiment.
RFX-mod operated for more than ten years and, in 2016, entered a new upgrade phase aimed at overcoming the limitations encountered during its operation. This upgrade led to the development of RFX-mod2. The main upgrades include a new vacuum vessel, a new boundary structure surrounding the plasma, and a redesigned layout of magnetic sensors. These modifications are expected to improve the plasma start-up phase, while the slight increase in plasma radius and the reduction of the distance between the plasma and the stabilizing wall are expected to enhance equilibrium control, reduce error fields, decrease the amplitude of tearing modes, and improve plasma transport.
Thanks to funding obtained within the framework of the NEFERTARI project under the Italian National Recovery and Resilience Plan (PNRR), a major revamping of key RFX-mod2 technological systems has been carried out.
In particular, the acquisition of an absolute laser tracker, a high-precision 3D metrology system, enabled the accurate assembly of the new machine boundary of RFX-mod2. This includes the installation and alignment of the graphite tiles composing the first wall, the conductive stabilizing shell with its fixing rings, and the vacuum vessel. The same system will also be employed for the installation and alignment of the new diagnostics developed within the NEFERTARI project.
Additional major activities comprised the refurbishment of the vacuum pumping system and of its control system, which now also integrates the gas injection control [1]. Furthermore, the boronization system has been upgraded in order to comply with the updated ATEX regulations.
Particular attention has been devoted to first-wall conditioning techniques, namely Glow Discharge Cleaning (GDC) [2] and Pulse Discharge Cleaning (PDC).
For GDC, a new system of movable electrodes has been designed, manufactured, and tested. This system will complement the existing mobile electrode system, which has also been modified with respect to the original design and will operate only in continuous current without the RF power. In addition, a new control system for the GDC plant has been developed and tested.
Due to the presence of gaps in all conducting structures (the copper shell and the vacuum vessel), high-temperature baking with induced currents will not be possible in RFX-mod2. Therefore, wall conditioning will rely on Pulse Discharge Cleaning. The PDC system, originally built in 2000, had not been modernized or commissioned and has therefore required significant upgrades.
[1] M. Zausa et al., Upgrade and Optimization of the Vacuum System for RFX-mod2 within the NEFERTARI Project, in this book of abstract
[2] M. Fadone et al., Glow Discharge cleaning efficiency analyses in view of RFX-mod2 operation, in this book of abstract