Speaker
Description
In RFX-mod, several plasma measurements would have benefited from improved accuracy, particularly magnetic field diagnostics, while key core measurements such as ion temperature, impurity density, and impurity flow were not available. These quantities are of high relevance for the investigation of QSH and SHAx plasma states. For this reason, the RFX-mod2 upgrade includes the implementation of Charge eXchange Recombination Spectroscopy (CXRS) and Motional Stark Effect (MSE) diagnostics, whose development status is presented in this work.
The operation of these diagnostics requires a high-quality neutral beam, motivating the optimization of the Diagnostic Neutral Beam Injector (DNBI) transmission line in order to maximize the equivalent neutral current entering the RFX-mod2 vacuum vessel. Steady-state simulations identified beam reionization losses as being primarily localized in the connecting duct between the DNBI and RFX-mod2, which was consequently redesigned to minimize such losses.
In addition, to ensure economically sustainable operation of the DNBI, the original open-cycle cryopumps (2 × 24 kL/s) were replaced with closed-cycle units (2 × 10 kL/s), eliminating helium consumption but significantly reducing the available pumping speed. This change required a revalidation of the vacuum system and was explicitly taken into account during the duct redesign, including an assessment of the impact of the new equilibrium pressure on beam reionization losses.
Simulation results were compared with experimental data from previous RFX-mod campaigns, where DNBI transmission losses were observed to be significantly higher than expected. The analysis indicates that the measured losses exceed steady-state reionization predictions by approximately one order of magnitude, ruling out RFX-mod neutral gas filling pressure as the primary source of reionization of the observed past anomalies.