Gas discharges. For low temperature plasmas:electric fields are important (stationary (DC) or alternating current (AC)) E - -

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1 Gas discharges For low temperature plasmas:electric fields are important (stationary (DC) or alternating current (AC)) example. DC-glow discharge: h Kathode _ E Anode + a primary electron leaves the cathode, e.g. via a photon it gains energy due to an electric field more electrons are generated due to collisions

2 DC-glow discharge Ions hit the cathode: continuous generation of electrons via secondary electron emission: Model for the existence and ignition of a glow discharge (Townsend): number of ionisation collisions/distance for electrons [1/m] number of ionisation collisions/distance for ions [1/m] number of electrons generated by secondary emission (ions that hit the cathode)

3 DC glow discharge: ignition condition A primary electron (generated at the cathode) generates other electrons: e e 0 N 1 d dx simplified: d 1 e The generated ions hit the cathode and generate new electrons, avalanche-like amplification of the primary electron: cathode 1.(start) avalanche N 1 1 N 1 1 e 2.avalanche d d d N 2 e 1 N 2 anode d e 1 e nth avalanche d N n e 1 ( n1) Ignition for : ( e d 1) 1

4 DC-glow discharge:avalanche current Ignition for: ( e d 1) 1 current time Typical runtime for ions typically 10 s

5 Probability for ionisation Stoß Ionisation towards the anode E Electron flux reduces due to inelastic collisions: de dx ( x) e e e Stoß (0) e x Stoß e ( ion ) e (0) e ion Stoß e (0) e E ion / ee Stoß p ~ e( ion ) e (0) e E ion / ee Stoß e c 1 p E e c 1 pd U

6 DC-glow discharge: Paschen curve Ansatz for ionisation probability.: B p exp Apd U Now insert in ignition condition: ( e d 1) 1 d ln1 1 Bpd exp Apd U ln1 1 Apd U 1 ln ln1 ln Bpd

7 DC-glow discharge: Paschen curve Apd U results in critical igniton voltage: ln ln1 ln Bpd U Z A ln( B p d 1 p d ) ln ln (11/ ) for high voltages: field emission for high pressures:arcing (see below) B pd eln(11/)

8 DC-glow discharge:close at the ignition condition Total current (anode current): amplified cathode current I A I K e d Cathode current: external current (e.g photo emission) + secondary electron current(under stationary conditions) I K d I0 I I0 I K ( e 1) Close to the ignition condition: divergence of the current Ik I 0 e d 1 (e d 1)

9 standard glow discharge a)small current density, small amplification cathode (0) anode (+U ) d =d virtual anode b) medium current, density medium amplification Ionen Elektronen j tot d c)large current density, large amplification Ionen d E ~ U/d Elektronen

10 standard glow discharge (rel.e.) E=U/d E=U/d has a maximum with increasing virtual anode Vorrücken der virtuellen Anode E/p (rel.e.) instabil! stabil! d dx ( 0 d e 1) ( e 1) E=U/d (rel.e.)

11 Light emission

12 U/I characteristics of a glow discharge U (V) 500 ignition 400 Glow discharge arc discharge not-ignited dark discharge subnormal normal anomale I (A)

13 Das Bild kann zurzeit nicht angezeigt werden. transition glow- arc discharge U arc subnormal (unstable) glow discharge virtuelle Anode I anomal glow discharge normal glow discharge

14 the arc discharge without arc spot with arc spot

15 arc discharge: plasma welding

16 flashover At high pressures: about 1000 times faster than glow discharge breakdown (10 ns instead 10 s) Extremely high electric field: photons are produced and re-absorbed extrem hohes E -Feld durch Raumladung U Paschenkurve typ. > 0,5 bar Photonen-Emission und Absorption aufgrund hoher Gasdichte Glimmentladung Funkendurchschlag (Streamer) p (d) -

17 Applications: fluorescent lamp ( neon tube) (low pressure arc discharge) Hg-resonance line at 254 nm Visible light cathode Luminescent substance anode negative glow light positive column

18 neon tube: spectrum problem: no continuous spectrum mix different fluorescent substances Colour temperatures:

19 fluorescent lamp : energy saving lamps lm/w (light bulb: 15 lm/w) Licht Wärme

20 Application:high pressure lamp direct generation of light Na lamp (highest efficiency factor: 150 lm/w)

21 application: plasma screen