Speaker
Description
In magnetized plasmas, there are many dynamical processes that affect the ion velocity distribution function, both in laboratory and astrophysical environments. Measurements of this quantity can give useful insights for the study of phenomena such as magnetic reconnection, ion heating and acceleration, and turbulence activity. For this purpose, we designed a new diagnostic system that evaluates the ion velocity distribution function at the edge of fusion plasmas. The proposed device, called DIVO (Diagnostic for Ion Velocity Observation), resolves the two components of the ion velocity, parallel and perpendicular to the magnetic field, independently from each other. DIVO will be mounted at the plasma edge in the RFX-mod2 experiment, a Reversed-Field Pinch device, that has been upgraded with a modified magnetic boundary and several improvements on the diagnostic capabilities and will start its operation in 2026. This device offers a direct and local measurement that will enhance our knowledge on the thermal and supra-thermal ion populations at the plasma edge, as RFX-mod was not equipped with any instrument able to locally evaluate the ion distribution functions in velocity space. The working principle of this diagnostic system is based on the force balance between the electric and the Lorentz force, with the aim of getting rid of the Larmor gyration of the ions, which is associated with the perpendicular component of the velocity. Since the balance depends on the ion's perpendicular velocity, a specific externally applied electric field allows for the selection of ions with $v_{\perp}$ within a precise range, while the parallel velocity is evaluated by the ion's impinging position on a matrix of detectors. The instrument is equipped with a series of parallel thin metallic plates that filter the incoming ions. This filtering method is a key element of the device, as it allows to select the particles with the correct gyrophase and $v_{\perp}$ and improve the resolution of the system. Individual particle simulations using Boris algorithm have been performed to optimize the instrument design and performances, assess its expected velocity resolution and operational range, and evaluate the transmission function needed to convert the detected counts into the original distribution function of the ions.
