Langmuir probe characterization of the NIO1 ion source plasma

In view of the future experiments on the large ion sources used for the neutral beam injection system of ITER and DEMO reactor, a small scale negative ions source NIO1 (negative ion optimization, phase 1) is operated at Consorzio RFX since 2014. At this stage H- are mainly formed by volume processes, while the use of cesium vapor to enhance the surface production is foreseen in second stage. The plasma is sustained by a 2.5 kW RF generator, inductively coupled via a seven turn coil surrounding the plasma chamber using 2 MHz frequency. The production and survival of negative ions strongly depends on the plasma properties in the vicinity of the apertures from which particles are extracted and a beam is formed. In order to characterize these properties against the variable pressures, input power and magnetic field strength in the source a dedicated campaign with a movable Langmuir probe immersed in the plasma was carried out. In this paper the installation of such probe on NIO1 is described, and the data acquired in configuration of the magnetic field are then presented and discussed.