1. Preliminary work on microplasmas and APGDs p.m.bryant@liv.ac.uk Paul Bryant Department of Electrical Engineering and Electronics, Brownlow Hill, University of Liverpool, L69 3GJ.

2. Recent technological and scientific advances in atmospheric pressure discharges have opened up an exciting area of research with opportunities for novel applications. These can be broadly classified into: Large area atmospheric pressure discharges AC and RF excited barrier and barrier free discharges e.g. DBDs (Dielectric barrier discharges) and bare electrode discharges. (Textiles treatment) Packed bead reactors (de-toxification of car gas exhausts NOx etc) Corona and Arc discharges (Textile treatment, environmental detoxification of gas plumes etc) and … Micron - sized discharges (microplasmas) PDPs Plasma display panels (micro DBDs) Si based microplasma arrays (displays, UV sources) Plasma needles (dentistry) Micro hollow cathodes Introduction

3. 1)Si-based Microplasmas (with J. G. Eden and S. J. Park, University of Illinois, USA) 2500 (50 x 50)  plasma pixel array based on pSi. Each pixel is an inverted pyramid 50  m square by 30  m deep. AC pSi Polyimide 3  m Ni Si 3 N 4 1.8  m 1.7  m Plasma ball Distance between pixels approx. 50  m.

4. Experimental set- up < 1 kV pk-pk, < 20 mA 5, 10, 15 and 20 kHz AC Pwr supply Vacuum chamber and rotary pump IdId VdVd Active strain gauge Ar Ne SRS Delay Gen. Computer Osc.ICCD camera Plasma on! Micro- Langmuir probe Filters: 810nm, 750nm

5. Preliminary Results AC Polyimide capacitor ZsZs ZaZa <<  plasma array IdId ipip IV independent of pressure, gas etc !  plasma is a parasitic load with I d >> i p C 10kHz = 0.46 nF I p per pixel approx 1.20mA / 2500 = 0.48  A!  plasma current peaks (green) increase with V d and move towards voltage peak. C 5Khz 0.57nF C = 0.4 nF (measured independently)

6. Imaging results: (with Dr Greg Clarke) 700Torr Ne

7. 7 Matching unit 2) APGD Atmospheric Pressure Glow Discharge Dielectric Barrier Discharge d = 1 – 2 mm AC Filamentary plasma Homogeneous plasma unstable ! Glass, quartz rf DBD MHz 13.56 MHz Stable! With  and  modes like in low pressure rf CCP discharges AC Barrier free discharge 10 -100 kHz Flow: 5 slm He 50Hz, kHz Ar, He, Ne, O 2

8. Collaborative Project with In high pressure plasmas excimers (excited molecular complexes) are readily formed via three body collisions. The resulting excimer Rg 2 * is unstable and rapidly disintegrates (within a few ns) typically emitting photons via spontaneous emission. Table 1: Rare gas excimer peak radiation wavelength in nm He 2 * Ne 2 * Ar 2 * Kr 2 * Xe 2 * 7483126146172 Glass Stack TiO 2 ZnO 2 Ag ZnO 2 ZnSnO x 18-20 nm 5 nm 10-12 nm 3-5 nm 40 nm Figure 2. Schematic of glass sample stack structure UV wavelengths good for photon-induced bond-breaking. E.g. Modifying properties of glass stacks. Need large area, stable & homogeneous plasma source. Rg* + Rg + Rg  Rg 2 * + Rg.

9. Experimental set-up 1-2 mm Current probe High voltage probe HH audio amp. Audio Osc. Oscillo. Figure 1. Schematic of the existing APGD source set-up. Gas in 2800 : 54 Active strain gauge He Ne Ar Vacuum chamber & Rotary pump 1 st stage: Ignite a barrier-free discharge (simpler!) 2 nd stage: rfDBD – need RF power feedthrough, ceramic plates < 5KV, < 2A Barrier free arrangement with 2 cm diam. Stainless steel electrodes.

10. Preliminary results: At ignition (approx 1 kV p-p ) observe a discharge column: Stable Ne, Ar glow High voltage, low current No electrode damage Ne 700 Torr 10kHz Not an arc ! In rfDBD by decreasing V a discharge column should fill electrode gap with uniform plasma glow … But in our case it extinguishes! Possible reason: Gas flow – reduces breakdown voltage, easier to ignite volume

11. Summary Microplasma array:  plasma array ignited only at several points during the driving voltage cycle, and seem to be related to the  plasma current spikes and power maximums. Various other studies are underway (i.e. Paschen curves, micro-Langmuir probe, filtered ICCDs, material treatment) to shed further light on plasmas ignition process. APGD: Upon ignition a stable discharge column (filamentary discharge?) appears. However, unlike in rfDBDs and other barrier free discharges, reducing the applied voltage does not lead to a uniform glow filling the inter-electrode space. Reason might be due to absence of flow of approx 5slm! Is there room for dust in microplasmas / APGDs? Yes! See “Observation of individual particles and Coulombic solids in a microdischarge” John, P.C., et al IEEE Trans. Plasma Science, 27, 199. APGDs? Reactive gases could produce dust disrupting processes, coating of dust particles, dust crystals at high pressure  new phenomena!