The goal of this study was to improve the electrical efficiency of a metallic torch for ICP plasma at medium frequency. We focused our study on two characteristic parameters of the system: the Joule losses of the cold cage and the shield effect that reduces the magnetic field available inside the torch.
We first tried to optimise the classical design of the torch using a 3D electromagnetic computation. We have pointed out the main parameters that are involved: the numbers of sectors and the width of the slits. Contrary to the state of art of the building of metallic cages, the high division ends up being prejudicial to the performance, while the increase of the width of the slits is always beneficial. Some experiments have confirmed these results.
We then designed a new low losses cage to reduce considerably the Joule losses in the sectors. This was achieved by cutting the current loop with an insulating material in order to maintain water cooling. We reduced the electrical losses by a factor of 2.5 at 100 kHz, thereby increasing the electrical efficiency from 65% to 87%.