Estimation of Plasma Electron Density Inside the Radio Frequency Inductively Coupled Driver of Spider

SPIDER radio-frequency (RF) inductively coupled ion source is a full-size prototype of ITER Heating Neutral Beam Injector ion source, equipped with 100 keV accelerator system for the particles. It is in operation since June 2018 in the premises of Neutral Beam Test Facility located in Padova, Italy. The ion source includes a plasma source where plasma is generated and heated by 8 RF drivers operating with Hydrogen/Deuterium at a gas pressure of ∼ 0.3 Pa and maximum RF power of 100 kW per driver at 1 MHz frequency. There are 4 RF circuits present in SPIDER, each comprises of a RF oscillator and a RF load. The RF load is defined by a transmission line: a capacitor-based matching network and two driver coils connected in series.