Ion cyclotron wall conditioning (ICWC) is being developed for ITER as a baseline conditioning technique in which the ion cyclotron heating and current drive system will be employed to produce and sustain the current-less conditioning plasma. The TOMAS project (TOroidal MAgnetized System, operated at the FZ-Juelich, Germany) proposes to explore several key aspects of ICWC. This project stands on two pillars featuring plasma and material studies: (a) plasma-induced material modification and optimization of the wall conditioning efficiency via exposure of probes made of real PFC and the use of tracers; (b) detailed research on ICWC plasma production and optimisation to benchmark codes.
The ICRF system requirements to fulfill the above aims are: (a) ability to couple op to 6 kW of RF power to low density and low temperature plasma (1011/cm3, 3-10 eV) (b) ability to initiate plasma in broad frequency range (15 to 45MHz) for plasma production studies.
For this purpose we have designed an ICRF system made of a single strap antenna within a metallic box, connected to a feeding port and a pre-matching system. We discuss the design work of the antenna system with the help of the commercial electromagnetic software CST Microwave Studio. The simulation results for a given geometry provide input impedance matrices for the two-port system. These matrices are afterwards inserted into various circuit models to assess the accessibility of the required frequency range.
The sensitivity of the matching system to uncertainties on plasma loading and capacitance values is notably addressed. With a choice of three variable capacitors we show that the system becomes resilient to such uncertainties. We also demonstrate that the system can cope as well with the high reflected power levels during the short breakdown phase of the RF discharge.