Use of electrical measurements for non-invasive estimation of plasma electron density in the inductively coupled SPIDER ion source

SPIDER (source for the production of ions of deuterium extracted from RF plasma) is the full-scale prototype of the ITER neutral beam injector ion source. The plasma heating takes place inside eight drivers via inductive coupling, through the radiofrequency (RF) coil. To achieve optimum conditions during source operation it is necessary to know the plasma behavior under various operational conditions. One of the essential parameters is the plasma electron density. It is possible to experimentally measure this parameter and, currently, different methods are being explored, such as the estimations coming from a Langmuir probe and optical emission spectroscopy. However, these methods are either available for temporary measurements or require dedicated analyses with large uncertainties. In this regard, alternative, reliable, and fast diagnostic tools will be beneficial for the estimation of the order of magnitude of electron density inside the driver. Two models for the estimation of electron density are recalled, discussed, and further modified in this work: one is based on the classical power balance equation, and the other is recently described in the literature and relies on classical plasma conductivity. Both models use equivalent electrical parameters of the driver derived from available electrical measurements at the output of the RF generator and through the knowledge of the matching network and the transmission line length. This work explicitly focuses on the application of these models to estimate the plasma electron density in a single driver of SPIDER. Furthermore, the estimations are compared with the first experimental results obtained from temporary electrostatic probe measurements and are found to be in good agreement in terms of magnitude and trends.