Development of X-Ray collimators to identify sources of radiation in devices insulated by large vacuum gaps

The paper describes the development of a diagnostic system to localize the sources of X-ray in a High Voltage Direct Current (HVDC) device insulated by large vacuum gaps. Electrons extracted from cathodic surfaces by Field Emission (FE) processes are accelerated to energies below 1MeV producing bremsstrahlung radiation from the impact areas on anodic surfaces. This radiation is one of the main experimental evidences observable during the high voltage conditioning in systems insulated by large vacuum gaps with voltages beyond $50 \div 100$ kV. The technological challenge associated to the achievement of a stable accelerating voltage in MITICA, the prototype of the Neutral Beam Injector for ITER, will require diagnostic systems able to monitor the evolution of the high voltage conditioning of the electrostatic accelerator by detectors located outside the beam source vessel. The measurement of the spatial distribution of the X-ray sources could be essential to understand the weak points in term of voltage holding for the MITICA electrostatic accelerator. The adoption of a X-ray collimator embedding a Cerium doped Lutetium-based scintillation crystal has been proposed to cope this target. A prototype of a Single Pixel Collimator with variable geometry shield by lead has been manufactured and tested at the High Voltage Padova Test Facility (HVPTF). High voltage tests with acceleration voltages up to 780kVdc have been carried out to improve the collimator design.