Apparatus for treating a gas in a conduit of a substrate processing system are provided. In some embodiments, an apparatus for treating a gas in a conduit of a substrate processing system includes: a dielectric tube to be coupled to a conduit of a substrate processing system to allow a flow of gases through the dielectric tube, wherein the dielectric tube has a conical sidewall; and an RF coil wound about an outer surface of the conical sidewall of the dielectric tube, the RF coil having a first end to provide an RF input to the RF coil, the first end of the RF coil disposed proximate a first end of the dielectric tube and a second end disposed proximate a second end of the dielectric tube. In some embodiments, the RF coil is hollow and includes coolant fittings to couple the hollow RF coil to a coolant supply.