(Color online) Top figure: a schematic drawing of the plasma needle. Bottom figure: a picture taken of the tip of the needle in operation. The needle itself is a sharpened metal wire with a diameter of . The perspex tube around it has an inner diameter of and serves to direct the flow of helium along the needle. The needle is isolated for a large part in order to prevent a discharge from forming within the tube. Refer to Ref. 17 for more details concerning the device.
(Color online) A schematic diagram of the numerical grid and the circuit driving the discharge. The discharge region was bounded on the right by a metal plane and on the left and in radial direction by a dielectric. The needle itself was located on the axis of the system. The metal plane was connected to earth and the needle to a rf power supply via a capacitor.
The voltage, conduction current and total (i.e., conduction plus displacement) current as a function of time during steady-state operation of the needle for the standard conditions ( and ). The horizontal line at indicates the bias voltage that offsets the needle potential with respect to earth.
The electron-density profiles, averaged over the rf cycle, for a needle at from a plane, driven at . The contour lines show the density at a logarithmic scale, the different levels of gray show the density at a linear scale. Note that not the entire calculation domain is shown here.
The axial, time-averaged densities of the various species as a function of the distance to the needle tip. Most species existed only close to the needle and got rapidly converted in ions and electrons. These then diffused outward and recombined further away from the needle.
The time-dependent reaction rates at a distance of from the needle tip. The numbers near the curves correspond with the numbers in Table II. Refer to Fig. 3 for the phase of the applied voltage as a function of time.
The axial electron density and the potential at four different phases during the rf cycle. The bold continuous line in the top part of the graph shows the time-averaged ion density.
The dissipated power as a function of the applied voltage for a distance of between needle and plane.
The dissipated power as a function of the distance between needle and plane for an applied voltage of .
The axial electron density and the potential at four different phases during the rf cycle for the situation in which the current is not restricted by a bias voltage across a capacitor. The bold continuous line in the top part of the graph shows the time-averaged ion density. Compare with Fig. 7 for the biased situation.
The list of species for which the balance equation was solved, including references to the literature sources of the various coefficients.
The set of reactions treated in the model, with the references to the literature from which the reaction-rate coefficients were obtained.
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