A GEM-based high-rate x-rays diagnostic for flux measurement during high voltage conditioning in vacuum insulated systems

The MITICA experiment at Consorzio RFX Padova, within the framework of the European Magnetic Confinement Fusion effort, is a full-scale prototype of the Neutral Beam Injector (NBI) heating system envisioned for ITER, which will be required to produce 1MeV beams of deuterium with an ion current of 40 A for up to 1 hour. These specifications far exceed any current NBI devices with a compact electrostatic accelerator to operate at 1MV being double the voltage reached up to now. To achieve these conditions the entire beam source and accelerator of MITICA are insulated by a vacuum instead of the more commonly used insulating gas. This is to avoid radiation induced conductivity in the gas by radiation produced from fusion reactions. However, this serves to be an overall more complex system and vacuum insulation has its own substantial hurdles regards stability and voltage holding. The High Voltage Padova Test Facility (HVPTF) is a concurrent R&D project focused on understanding the physical processes behind breakdown and micro-discharges in high-voltage insulation, as well as developing new diagnostics, models, and operational modes to address these challenges and aid in the stable operation of MITICA. The highresolution charac-terization of the X-ray energy spectrum during the high voltage conditioning of a multi electrode vacuum insulated system such as HVPTF could be vital to understand the fundamentals behind voltage breakdowns and which stage is causing the micro discharge onset. This technique seems particularly promising for measuring small dark currents (< 10?A), which are generally difficult to be measured with standard techniques. The X-rays produced during HV experiments are mainly due to bremsstrahlung interaction between the electrons escaping from the HV electrodes (field emission effect) and the background gas or the chamber walls. The X-rays spectrum extends from low energy ( ? keV) up to several hundreds of keV depending on the maximum potential difference applied to the electrodes (up to 800 kV). During these events high photon flux (>106 photons/cm2s) is produced. This paper shows the development and preliminary results of a newly designed X-Ray diagnostic based on Gas Electron Multiplier (XR-GEM). This detector is able to stand very high rate (> MHz) in single photon counting mode and can cover the energy range from 3 - 50 keV. The XR GEM detector is equipped with anodic pads (256 pads 6x6 mm2) readout with a new data acquisition system called GEMINI, which gives the possibility to obtain a counting rate of several MHz and, thus, sub-ms time resolution together with mm spatial resolution.