Introducing hairpin probe for electron density measurement in a KAMABOKO-III negative ion source

摘要

Neutral beam injection based on a negative ion source is one of the most promising candidates for heating and current drive in future fusion reactors. The physical and chemical processes involved in the filter field region of an ion sources are especially important to investigate, as this is the region where the negative ions are mainly generated and extracted. The externally applied magnetic/filter field around 0.012T cools the high-energy electrons in order to avoid collisional destruction of the negative ions. The electron density (ne) measurements using conventional diagnostic techniques such as Langmuir Probe (LP) and Optical Emission Spectroscopy becomes complicated in presence of negative ions as well as magnetic field and gives a qualitative result. For accurate det ermination of ne in such complex plasma, the floating resonance hairpin probe¹ (HP) is applied in KAMABOKO-III negative ion source, at the MANTIS test bed in CEA Cadarache, France. The technique is based on measuring the plasma permittivity using a U-shaped microwave resonating structure whose relative shift of characteristics frequency in plasma from that in vacuum directly gives the ne without rely on any other plasma parameter. However, in the presence of magnetic field the plasma density becomes anisotropic and thus the HP gives an effective ne measurement within its spatial resolution. The measured ne is compared with the positive ion density measured by planar LP. The ne is studied as a function operating source parameters such as arc power, pressure, applied magnetic field and the plasma grid bias exclusively in the filter field region. The effect of probe orientation to the B-field on the resonant properties is also studied under given plasma conditions. The performance of the HP shows that the technique is more reliable, sensitive and accurate diagnostic tool for such ion source.