The design of this
torches is very similar the one of the metallic torches. The
main difference is the propagation mode of the electromagnetic
waves, as there is no guide-to-coaxial line transition. In the
semi-metallic torches, the plasma gas flows through a quartz
tube (which is transparent to microwave radiations). Instead of
burning out on top of the nozzle, the discharge is initiated in the
zone where the quartz tube intersects the rectangular wave
guide [65] (see Fig. 25).