Speaker
Description
The Neutral Beam Test Facility in Padova, Italy, is the site where the development of the Neutral Beam Injector (NBI) for the ITER tokamak is being carried out. In this context, the Megavolt ITER Injector and Concept Advancement (MITICA), the full-scale prototype of the NBI, is currently in the installation and commissioning phase. The system aims at producing neutrals with energies up to 1 MeV to be injected into the plasma, through the acceleration and subsequent neutralization of a negative ion beam. The accelerator is based on a multi-grid, multi-aperture system, where high-voltage holding in vacuum over long distances represents one of the most critical challenges for reliable operation. In-vacuum discharges and breakdowns can interrupt operations and potentially damage the system. These phenomena are investigated at the High Voltage Padova Test Facility (HVPTF), which consists of a cylindrical vacuum vessel equipped with two electrodes that can sustain voltage differences up to 800 kV DC. In HVPTF, several electrode configurations have been tested, including needle-to-plane, plane-to-plane, and sphere-to-plane geometries, in order to study the dependence of discharge behavior on the electric field topology. During operation, current and voltage are monitored for the two independent power supplies, together with the pressure inside the chamber, while multiple lines of sight allow complementary diagnostics. The dynamics of the discharges have been studied using cameras operating in the visible, UV, and IR spectral ranges. Particular attention has been devoted to X-ray emission from the vacuum chamber, generated by bremsstrahlung processes associated with energetic electrons interacting with the electrodes and the chamber walls during discharge events. The emitted radiation spans a broad energy range, from a few keV up to approximately 800 keV. Initial measurements were performed with inorganic scintillators; however, the extremely high X-ray fluxes involved, exceeding 10^6 photons/cm2⋅s, severely limit the applicability of scintillator or solid-state detectors due to saturation effects, pile-up, and loss of linearity. In this regime, the use of Micro-Pattern Gaseous detectors such as a GEM becomes essentially mandatory. Their intrinsically low detection efficiency to high energy x-rays, combined with a very high rate capability, allows them to operate reliably under such intense flux conditions, avoiding saturation while preserving temporal resolution. This makes GEM detectors uniquely suited for studying the fast time dynamics of X-ray emission associated with discharge events in HVPTF. Different GEM-based detectors have been developed and tested on the HVPTF facility, demonstrating their capability to significantly improve both the temporal and spatial characterization of the X-ray emission. Their implementation has enabled a more detailed investigation of the discharge dynamics, providing enhanced resolution in tracking the evolution of the radiation in both time and space. A strong correlation has been observed between the current signals from the power supplies and the X-ray signals measured with GEM detectors, indicating that X-ray diagnostics can provide valuable insight into the physics of discharges. This opens the possibility of using such measurements for the characterization of breakdown phenomena and for the development of mitigation and prevention strategies.
| Name of the speaker | Gabriele Croci |
|---|---|
| Eligible for the Georges Charpak Young Scientist Award. | no |