Electrical plants and fast protection systems for the Negative Ion Source NIO1

The NIO1 (Negative Ion Optimization, phase 1) experiment is a versatile test bench operated by Consorzio RFX and INFN in the framework of the activities aimed at the enhancement of negative ion sources for production of large ion beams for plasma heating in nuclear fusion devices and accelerator applications. The nominal beam current of 135mA at -60kV is divided into 9 beamlets, with multi-aperture extraction electrodes. The plasma is continuously sustained by a 2MHz radiofrequency power supply, with a standard matching box. A High Voltage Deck (HVD) is placed inside the lead shielding surrounding NIO1 and is fed by an insulation transformer installed in a nearby box. It contains the radiofrequency generator, the gas control, electronics and power supplies for the ion source. A closed-circuit water cooling system was installed for the whole system, with a branch towards the HVD. The present contribution gives a detailed description of the electrical plants and the fast protection systems for NIO1. High voltage and low voltage circuits are present in NIO1. The description of the former include the extraction and accelerator power supplies and the passive devices devoted to protecting the power supplies themselves against possible breakdowns in the accelerator. Low voltage circuits are dedicated to biasing the plasma grid and the bias plate as well as to producing a static magnetic field (magnetic filter field) and are protected against the effect of accelerator breakdowns. A specific system was developed to coordinate the fast switch-off of the power supplies and to trigger the main control unit in case a breakdown is detected in either high-voltage power supplies. Details on the fast protection circuitry and examples of its operation are presented. This work was performed with partial financial support by EUROfusion.