Djm Dirk Trienekens

Photoionization capable, extreme and vacuum ultraviolet emission in developing low temperature plasmas in air

Experimental observation of photoionization capable extreme ultraviolet and vacuum ultraviolet emission from nanosecond timescale, developing low temperature plasmas (i.e. streamer discharges) in atmospheric air is presented. Applying short high voltage pulses enabled the observation of the onset of plasma formation exclusively by removing the external excitation before spark development was achieved. Contrary to the common assumption that radiative transitions from the b1∏u (Birge-Hopfield I) and b′1∑+ u (Birge-Hopfield II) singlet states of N2 are the primary contributors to photoionization events, these results indicate that radiative transitions from the c′4 1∑+ u (Carroll-Yoshino) singlet state of N2 are dominant in developing low temperature plasmas in air. In addition to c′4 transitions, photoionization capable transitions from atomic and singly ionized atomic oxygen were also observed. The inclusion of c′4 1∑+ u transitions into a statistical photoionization model coupled with a fluid model enabled streamer growth in the simulation of positive streamers.

Time-discretized extreme and vacuum ultraviolet spectroscopy of spark discharges in air, N2 and O2

In this paper we present time-discretized spectra of spark discharges in air, N2 and O2. In previous work, a system for temporally resolved spectral analysis of extreme ultraviolet (EUV) and vacuum ultraviolet (VUV) emission from spark discharges was presented, along with some initial results. As was noted in this paper, statistical variances and the lacking of an apparatus sensitivity profile limited the usability of the data obtained. We have investigated the cause of these variances and improved the setup to reduce their effect. We also investigated the apparatus sensitivity profile to correct the intensity of measured lines. Newly obtained spectra in dry air, N2 and O2 are presented. Air and N2 show high emission in the vicinity of 100 nm, where direct photoionization of molecular oxygen is possible, in the first 250 ns of the discharge. We conclude this emission originates from nitrogen, which has several intense molecular transitions in this region. This finding is confirmed by our experimental results which show the emission in this region is much lower in oxygen.

Optimizing drive parameters of a nanosecond, repetitively pulsed microdischarge high power 121.6?nm source

Utilizing nanosecond high voltage pulses to drive microdischarges (MDs) at repetition rates in the vicinity of 1 MHz previously enabled increased time-averaged power deposition, peak vacuum ultraviolet (VUV) power yield, as well as time-averaged VUV power yield. Here, various pulse widths (30–250 ns), and pulse repetition rates (100 kHz–5 MHz) are utilized, and the resulting VUV yield is reported. It was observed that the use of a 50 ns pulse width, at a repetition rate of 100 kHz, provided 62 W peak VUV power and 310 mW time-averaged VUV power, with a time-averaged VUV generation efficiency of ~1.1%. Optimization of the driving parameters resulted in 1–2 orders of magnitude increase in peak and time-averaged power when compared to low power, dc-driven MDs.

Stroboscopic images of streamers through air and over dielectric surfaces

We study the propagation of streamer discharges through air and along an epoxy rod with an ICCD camera. We use stroboscopic imaging at frequencies up to 110 MHz to visualize discharge evolution and to calculate velocities. Initial results show that surface streamers along a dielectric surface can be up to twice as fast as streamers through bulk air.

When and why are streamers attracted to dielectric surfaces

Summary form only given. Solid insulation surfaces in gas insulated high voltage (HV) equipment can be advantageous or dangerous with respect to dielectric breakdown by a discharge in the gas insulation, depending on whether the surface blocks the discharge (perpendicular, or dielectric barrier, configuration) or allows the discharge to creep along it (tangential configuration). Although discharge propagation along a surface is an old problem, there is still room for improved understanding of the fundamental physics and for the development of knowledge-based design rules for HV equipment. We thus investigate experimentally the initial (streamer) phase interacting with a dielectric surface. We studied streamers inside a gas-filled vessel using ICCD imaging, both stroboscopically as well as with single-shots. Inside the vessel, HV was applied to a needle located 10-15 cm above a grounded cathode. A dielectric sample was placed in the discharge gap. We varied several experimental parameters, such as pressure, gas composition, relative permittivity, pulse voltage and various geometrical parameters to study their effect on the discharges affinity to prefer the dielectric surface instead of propagating through the bulk gas. Our experimental results provide us with the necessary information to start an in-depth discussion about the important mechanisms governing discharge propagation on surfaces. We show that the local availability of free electrons and the local electric field together determine the behavior of the discharge and explain how several parameters influence this behavior.

Pulsed positive discharges in air at moderate pressures near a dielectric rod

We study pulsed positive discharges in air in a cylindrically symmetric setup with an electrode needle close (about 1 mm) above the top of a dielectric cylindrical rod of 4 mm in diameter mounted at its bottom on a grounded plate electrode. We present ICCD (intensified charge-coupled device) pictures and evaluations of experiments as well as simulations with a fluid discharge model; the simulations use cylindrical symmetry. In the experiments, there is an initial inception cloud phase, where the cylindrical symmetry is maintained, and later a streamer phase, where it is broken spontaneously. At 75-150 mbar, discharges with cylindrical symmetry are not attracted to the dielectric rod, but move away from it. The dielectric rod plays the sole role of an obstacle that shades (in the context of photoionization) a cone-shaped part of the inception cloud; the cone size is determined by the geometry of the setup. The material properties of the dielectric rod, such as its dielectric permittivity and the efficiency of the photon induced secondary electron emission do not have a noticeable effect. This is due to the abundance of photoionization in air, which supplies a positive discharge with free electrons and allows it to propagate along the electric field lines. Using some simple field calculations, we show that field enhancement due to dielectric polarization does not play a significant role in our geometry as long as the discharge maintains its cylindrical symmetry. The field component towards the rod is insufficiently enhanced to cause the discharge to move towards the rod. Any additional electrons produced by the dielectric surface do not influence this discharge morphology. This interpretation is supported by both experiments and simulations. At higher pressures (400-600 mbar) or for larger gaps between the needle and the dielectric rod, the inception cloud reaches its maximal radius within the gap between needle and rod and destabilizes there. In those cases, streamer channels are more likely to turn into a surface streamer. All our experiments and simulations were performed at moderate pressures (75-600 mbar), but we expect that the results will be the same for other pressures assuming that all the lengths scales (including the rod) in the setup are rescaled according to the Townsend scaling of the discharge.