Full-wave modeling of arcs within the ITER ICRF antenna for usage in the simulations and design of the RADAR Arc Detection system

The ITER ICRF antenna has been carefully designed to feature electrical fields below tolerable limits (typically, below 2 or 3 kV/mm depending on the location and orientation) when operating at a maximum voltage of 45 kV. In particular, this allows avoiding arcs. However, as for any high-power RF system, arcs can still occur in the ICRF antenna and its power feeding system, during normal operation and especially during the commissioning. Whenever an arc is detected, the RF power shall be immediately tripped (μs timescale) to avoid strong local energy deposition at the location of the arc. Undetected arcs are forbidden. To this aim, several complementary and redundant Arc Detection (AD) systems are foreseen to protect the ITER ICRF antenna. Among these AD systems is the RADAR Arc Detection (or RAAD [2]) which is currently under evaluation for implementation on the ITER ICRF system To provide a first numerical proof of concept of RAAD, full-wave simulations of the ITER ICRF antenna and its power feeding transmission lines have been performed in the radar bandwidth of operation (up to 350 MHz) with the help of CST Studio Suite and ANSYS HFSS commercial codes. In these simulations, the plasma loading has been approximated by a salty water load (with a relative dielectric permittivity 80 and electrical conductivity 1 S/m), while arcs have been modelled with both perfect electric conductor (PEC) cylinders or lumped element shorts. The obtained S-matrices have been then loaded and processed by the RAAD timedomain circuit simulations and signal processing calculations. This paper describes the challenges to simulate the full ITER ICRF antenna with arcs, considering different loading conditions, different materials and different solutions for the arc insertion. It also provides a comparison of the scattering parameters with and without arcs.