MITICA BS Mockup Pumping System

The high-Q operation of the ITER tokamak will require two Neutral Beam Injectors (NBIs) for plasma heating and current drive. Each beam will be generated by a 40 A current of Deuterium negative ions, accelerated up to the specific energy of 1 MeV and then neutralized. The power delivered to the plasma by each NBI shall reach 16 MW with duration up to 1 h. The beam source will be constituted by an RF-driven negative ion source at -1 MV potential, by a Multi-Aperture, Multi-Grid (MAMuG) electrostatic accelerator (consisting of 5 stages at intermediate potentials), and a gas-box neutralizer at ground potential. All components will be installed in a vacuum vessel (also at ground potential), together with a high-capacity cryo-pumping system which controls of 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 which MITICA will have to deal with. For this reason, before the installation of the MITICA Beam source, an experimental program for anticipating the full-scale voltage holding tests is studied and proposed, using the real MITICA vacuum vessel (on-site) together with a beam source mock-up without and with an intermediate electrostatic shield, in order to get a first voltage holding assessment of the expected MITICA voltage holding capability, to evaluate the effect of the intermediate shield and to assess the effect of the background pressure on the voltage holding capability of the system.