Non-thermal plasma jets in open air are composed of ionization waves commonly known as 'plasma bullets' propagating at high velocities. We present in this paper an experimental study of plasma bullets produced in a dielectric barrier discharge linear-field reactor fed with helium and driven by microsecond high-voltage pulses. Two discharges were produced between electrodes for every pulse (at the rising and falling edge), but only one bullet was generated. Fast intensified charge coupled device camera imaging showed that bullet velocity and diameter increase with applied voltage. Spatially resolved optical emission spectroscopy measurements provided evidence of the hollow structure of the jet and its contraction. It was shown that the pulse amplitude significantly enhances electron energy and production of active species. The plasma bullet appeared to behave like a surface discharge in the tube, and like a positive streamer in air. A kinetics mechanism based on electron impact, Penning effect and charge transfer reactions is proposed to explain the propagation of the ionization front and temporal behavior of the radiative species.

Using a one-way coupled model of neutral gas flow and plasma dynamics we report a mechanism to explain the ring-shaped emission pattern that has been observed experimentally in plasma bullets at atmospheric pressure. We solve a fluid model with the local field approximation in one-dimensional cylindrical coordinates, corresponding to a cross-section of a plasma bullet. Pulselike uniform electric field is assumed to be applied perpendicular to the simulation domain. Time and spatially resolved spectroscopic measurements support the simulation results.

In most expanding-plasma thrusters, ion acceleration occurs due to the formation of ambipolar-type electric fields; a process that depends strongly on the electron dynamics of the discharge. The electron properties also determine the heat flux leaving the thruster as well as the maximum ion energy, which are important parameters for the evaluation of thruster performance. Here we perform an experimental and theoretical investigation with both magnetized, and unmagnetized, low-pressure thrusters to explicitly determine the relationship between the ion energy, E i , and the electron temperature, T e0 . With no magnetic field a relatively constant value ofis found for xenon, while when a magnetic nozzle is present, E T / i e0 is between about 4-5. These values are shown to be a function of both the magnetic field strength, as well as the electron energy distribution function, which changes significantly depending on the mass flow rate (and hence neutral gas pressure) used in the thruster. The relationship between the ion energy and electron temperature allows estimates to be made for polytropic indices of use in a number of fluid models, as well as estimates of the upper limits to the performance of these types of systems, which for xenon and argon result in maximum specific impulses of about 2500 s and 4500 s respectively.

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Direct thrust measurements of an Electron Cyclotron

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Increasing concerns over atmospheric pollution has motivated research into technologies able to remove volatile organic compounds (VOC’s) from gas streams. The aim of this paper is to understand the chemical and physical mechanisms implied in the decomposition of VOC’s in a filamentary nonthermal plasma discharge. Experiments have been carried out on three pollutants (propane, propene, and isopropyl alcohol) in dry air at atmospheric pressure using a wire to cylinder corona discharge generated by a homemade nanosecond rise time high voltage pulse generator. The resulting plasma efficiently destructs propane, propene, or isopropyl alcohol at a concentration of 500ppm with low specific input energies (less than 500J∕L), but the poor oxidation rate leads to the formation of numerous by-products (acetone, formaldehyde, formic acid, and methyl nitrate) whose concentration can reach some hundreds of ppm. We also investigated the effect of pulse parameters on VOC removal efficiency. Neither pulse peak value nor rise time (in the range of 4–12ns) appears to have a significant influence on the VOC decomposition rates. Therefore, we believe that the way the energy is deposited in the plasma does not modify the density of active species (radicals, ions) in the streamers. The production of energetic electrons is not enhanced by the external applied field, and the only effective parameter may be the local field in the streamer head, which is almost the same (around 500Td) whatever the voltage (above the inception value).

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