Microwave-plasma Interactions In Photoionized, Tmae-seeded Helium Gas Mixtures

We report on experimental measurements of the microwave attenuation in highly collisional plasmas created by the photoionization of tetrakis-dimethylamino-ethylene (TMAE) mixed in atmospheric pressure helium contained in a conical frustum, thin-film-mylar vessel. Separate experiments, examining microwave interactions from two different photoionization sources, were performed. The sources were placed near the bottom baseplate consisting of 60-cm diameter, 360-cm focal length aluminum reflector. Best results for peak microwave attenuation were achieved with a windowless, two-dimensional spark array dissipating total energies as high as 135 Joules in 10 μs. This light source is expected to generate substantial energy in the vacuum ultraviolet (VUV), peaking at 80 nm, thus efficiently ionizing TMAE (ionization potential= 5.4 eV). Lower ionization levels, although lasting for significantly longer durations, were achieved with modified commercial, high-energy flashtubes dissipating as much as 2400 Joules.