Evaluation of power absorption for inductively coupled radio-frequency ion source: case study for the ELISE source

The ITER experiment requires Neutral Beam Injectors (NBI) rated for 33 MW for plasma heating and current drive providing one hour pulse operation with a duty cycle of 0.25. The NBI is composed of an ion source capable of producing 40 A of D¯ ions which are accelerated at the energy of 1 MeV by an electrostatic system and finally neutralised. The generated neutral beam is then fed to the plasma in the ITER vacuum vessel. ITER NBI ion source is based on a radiofrequency inductively coupled (RF IC) plasma in which the RF field is produced by a coil in a component called driver which is used to generate and heat the plasma. These sources are generally characterized with high RF power density, low operational pressure of around 0.3 Pa, and operational frequency of 1MHz. They are based on a concept developed at IPP, Garching, Germany, where the most recent test facility in operation is ELISE (Extraction from a Large Ion Source Experiment), half the size of the ITER source. The same type of RF negative ion sources are under development for two projects: SPIDER, the full-size RF negative-ions source and for MITICA, the full-scale prototype of the ITER HNB injector, in the ITER Neutral Beam test Facility in Padova, Italy. This work provides first study and methodology to evaluate the efficiency of the RF power deposition in IC plasma sources as a function of the working frequency and in dependence of other source and plasma parameters. The results obtained are presented for the case of ELISE RF ion source. In particular, based on the available literature, this methodology is coded by considering various relevant mechanism of plasma particle dynamics like stochasting heating, ohmic heating and classical skin depth to derive the plasma equivalent electrical parameters. These formulations are then integrated with a transformer model accounting for the coupling between the RF coil and plasma in the driver. A parametric scan is presented with respect to plasma quantities such as electron density and temperature, geometry and gas pressure in order to analyse the behaviour of both the plasma electrical parameters and the maximum power transfer efficiency as a function of frequency.