A welding-type device and gas system for use therewith are disclosed. The amplifier may include a multi-level H-bridge, which may include pairs of transistor switches coupled to a first, second, and third supply potential. Irregularities on the surface of the test piece result in a progressive diffusion of the circulating beam about the path. This allows the generation of a stable thermal plasma when a reaction gas containing an organic halide and water vapor is supplied into the outer tube through the annular gap with a microwave transmitted from the rectangular waveguide into the cavity.