Study of electron transport across the magnetic filter of NIO1 Negative ion Source

In the framework of the accompanying activities in support to the ITER NBI test facility a relatively compact radiofrequency (RF) ion source, named NIO1 (Negative Ion Optimization, phase 1) was developed in Padua, Italy, in collaboration between Consorzio RFX and INFN. Negative hydrogen ions are formed in a cold, inductively coupled plasma with a 2MHz, 2.5 kW external antenna. A cooling of the electron energy distribution is necessary to increase the survival probability of negative ions in the proximity of the extraction area. This goal is accomplished by means of a transversal magnetic field, confining the high energy electrons better than the colder electrons. In NIO1, this filter field can cover different topologies, exploiting different set of magnets and high current paths. In this contribution we study the property of the plasma in the vicinity of the extraction region for different B field configrations. For this experiment the source was operated in pure volume conditions, in hydrogen and oxygen plasmas. The experimental data, measrued by spettroscopic means is interpreted also with the support of finite element analyses simulations of the magnetic field and a dedicated numerical model for the electron transport across it, including Coulomb and gas collisions.