Electrostatic Design of the MITICA Intermediate Electrostatic Shield

Two heating Neutral Beam Injectors (NBIs), required for plasma heating and current drive, are foreseen for ITER operation. Each beam will be generated by a 40A current of Deuterium negative ions, accelerated up to the specific energy of 1MeV and then neutralized, delivering to the plasma a power up to 16.5 MW each. The beam source (BS) will be constituted by an RF-driven negative ion source at -1 MV potential and by an electrostatic accelerator (consisting of 5 stages at intermediate potentials). All components will be installed in a vacuum vessel, together with a high-capacity cryo-pumping system that controls the background gas pressure.In order to validate the ITER NBI design and address all the outstanding issues related to these demanding requirements, a full-scale prototype called MITICA (Megavolt ITER Injector & Concept Advancement) is under construction in Padova at Consorzio RFX. Voltage insulation in vacuum and/or very low-pressure gas on a single gap is indeed one of the expected issues that MITICA will have to deal with. An effective solution for increasing the voltage holding capability of the system consists in the use of an additional intermediate electrostatic shield, biased at an intermediate potential, placed between the ion source and the vacuum vessel. In this paper, the electrostatic design of the shield is presented, by considering the voltage holding capability. A novel 3D version of a numerical tool, called Voltage Holding Prediction Model (VHPM), is applied to the additional intermediate shield design to assess the expected voltage holding capability of the experiment in high vacuum.