Characteristics of an arc-seeded microwave plasma torch

The design and operation of a portable microwave plasma torch is presented. An arc plasma torch running at 60 Hz and generated by a torch module is installed on the bottom wall in the narrow section of a tapered S-band rectangular cavity, and is used to seed the microwave discharge at the location where the microwave electric field is at a maximum. This tapered cavity is designed to support the TE₁₀₃ mode. With seeding, only low Q cavity and moderate microwave power (time average power of 700 W) are needed. The microwave-enhanced discharge increases the size, cycle energy, and duty cycle of the torch plasma considerably. This torch can be run without introducing gas flow to stabilize the arc and microwave discharges. Adding gas flow can increase not only the size of the torch plasma, but also its cycle energy which reaches a plateau of about 12 J/per cycle for a gas flow rate exceeding 0.393 l/s. The electron density and excitation temperature, and the composition of torch species are determined by emission spectroscopy. It is shown that, at the bottom of the torch close to the cavity wall, electrons distribute quite uniformly across the core of the torch with density and excitation temperature determined to be about 7×10¹³ cm^-3 and 8000 K, respectively. It is also found that this torch produces an abundance of reactive atomic oxygen.