Inductively-coupled plasma volume where there is a maximum interaction between the electromagnetic field and the plasma gas (mostly argon). This is the plasma’s hottest area and has a toroidal configuration.
Source:
PAC, 2021, 93, 647. 'Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)' on page 728 (https://doi.org/10.1515/pac-2019-0203)