Experimental study of nanosecond pulsed discharges and DC-driven helium plasma jets
Creators: name: Wang, XingXing
Publisher: name: Purdue University Graduate School
Year: 2021
Subjects: Aerodynamics (excl. hypersonic aerodynamics)
Plasma physics; fusion plasmas; electrical discharges

description: Atmospheric pressure plasma has been found valuable for numerous kinds of applications in multiple fields. The capabilities of it being operable under atmospheric pressure/room temperature, efficiently producing active gas species, and quickly depositing energy into the gas volume have earned its irreplaceable role in the field of aerodynamics, combustion, food, medicine, material processing, and nanotechnology. This work focuses on the diagnostics of two specific kinds of atmospheric pressure plasmas: nanosecond pulsed discharges with pin-to-pin configuration, and DC-driven helium plasma jet, with an emphasis on the application of microwave Rayleigh scattering for the electron number (density) measurement. Firstly, a large set of diagnostic tool infrastructure was built. It includes development of diagnostic techniques based on the traditional methods that expand their default capabilities - for instance, OES measurement assisted with HV probing pulse. In addition, the accuracy of the electron number density measurement by MRS was further improved by the simultaneous measurement of gas density by LRS. Secondly, the study of the nanosecond high-voltage discharges with pin-to-pin configuration was conducted, in which multiple discharge parameters were measured, including discharge voltage, current, temperatures (gas/rotational/vibrational), gas density, electron number/number. Multiple discharge conditions were also studied, such as different pulse energy, gap distance between electrodes, and pulsing frequency. To start with, two discharge regimes (spark and corona) were identified by different discharge properties and visual presentations. It was further demonstrated that discharges are operating within spark regime at smaller gap distances (&lt;6 mm) and corona at larger distances (&gt;8mm). Within the spark regime under single pulsed condition, a higher electron number density, current, temperature was achieved at smaller gap distances, with a maximum value of 7.5´10<sup>15</sup> cm<sup>-3</sup>, 22 A, and 4000 K (10 μs after discharge) at a gap distance of 2 mm, respectively. Lower electron number density/current/temperature was observed at 5mm gap by lowering the pulse energy with additional current-limiting resistors. Electrons after discharge decayed with a characteristic decay time of 150-200 ns governed by a mix of dissociative recombination and three-body attachment. Gas properties including temperature and density recovered to ambient level within approximately 1 ms. In addition, higher temperature, lower gas density, and lower electron number density were measured at higher repetition frequency where: above 5000 K (minimum gas temperature), 2.0´10<sup>18</sup> cm<sup>-3</sup>, 1.0´10<sup>15</sup> cm<sup>-3</sup> were measured at a repetition frequency of 100 kHz, respectively. The decay rate of electrons was also lowered at higher repetition frequency due to a lower gas density and electron temperature. A substantially lower pulse energy, gas heating, and electron production was achieved within the corona regime. Thirdly, a new type of atmospheric pressure plasma jet was proposed in this work driven by DC high-voltage where the supplied DC voltage was held constant during operation. The discharges however were firing repetitively in the kHz range in auto-oscillatory mode. Each discharge pulse is associated with a streamer discharge event with a current amplitude ~ 1mA and a duration ~ 5 μs. The electron number density of the resulting plasma volume was measured to be on the order of 10<sup>11</sup> cm<sup>-3 </sup>by microwave Rayleigh scattering. The gas temperature of the plasma jet was also measured to be less than 400 K with optical emission spectroscopy. The governing breakdown mechanism of this DC-driven plasma jet, namely, flashing corona, was further investigated. It has been shown that an enclosure of dielectric material could increase the pulsing frequency by 10 times and enhance the intensity of each discharge so that the peak current increases from 14 to 35 mA. Furthermore, the applicability of such discharge for sterilization was assessed by quantifying the energy cost of the production of ozone and NOx, which were 150 eV and 1950 eV per molecule, respectively.