Methods of plasma generation and plasma sources

Transcription

1 Methods of plasma generation and plasma sources PlasTEP trainings course and Summer school 2011 Warsaw/Szczecin Indrek Jõgi, University of Tartu Partfinanced by the European Union (European Regional Development Fund

2 Outline of the talk Townsend discharge Glow discharge Arc discharge Corona discharge Dielectric barrier discharge Hollow cathode discharge RadioFrequency discharges Microwave discharges Electron beams 1

3 Glow discharge tube Discharge tube T P (n) Cathode E Anode Simplest case a closed tube filled with an insulating gas There is DC voltage V applied between the electrodes Electric field inside the tube E = V/d When the electric field arises over a certain value, there appears breakdown and the gas becomes conducting Can be self sustaining at several regimes (glow, arc) 2

4 Avalanches Seed electrons cosmic rays or emission from the rough surfaces by electric field Cathode n e x Anode λ i Electrons gain energy in the electric field until they are able to ionize neutrals λ i free path of ionization dn e /dx = n e /λ i Multiplication increases exponentially with the distance n e = n e0 exp(x/λ i ) 3

5 Townsend discharge Townsend ionization coefficient α α = 1/λ i n e = n e0 exp(α x ) ν α = i = v Semiempirically: 1 µ e k i (E/n) E/n α = const V i exp( ) λ Eλ λ ~ p Bp α = Ap exp( ) E function of reduced electric field E/n drift energy ionization energy e 0 λe e 0 V i A and B are properties of gas E λ i Attachment coefficient η η ~ f( E/n ) electronegative gases 4

6 Townsend discharge Secondary electron emission Cathode E Anode Positive ions drift to the cathode where they are recombining but will also extract new electrons The rate at which ions are extracting electrons is given by secondary electron emission coefficient γ i depends on electrode material and ions (typically ) photoionization may also be important γ p The amount of positive ions produced in the gap dis n i = n e0 [exp(α d)1] When attachment has to be taken into account: n i = n e0 [exp((α η) d)1] 5

7 Townsend discharge Paschen law For self sustaining discharge the number of ions produced in gap has to be enough to generate sufficient new electrons at cathode 1 γ n e0 [exp(α d)1]=n e0 α d = ln(1 ) γ V b = E b d Bp 1 Ap exp( ) = ln(1 ) E γ Bpd V b = ln(apd) ln[ln(1 γ 1 )] Scaling with pd V b vacuum insulation V b min high pressure insulation pd 6

8 Townsend discharge Cathode E Anode With increasing voltage (electric field), there is increasing number of multiplication and secondary electron emission increasing current Currents A at small variation of voltage Nonneutral plasma n e , n i cm V Townsend discharge I Light emission increases exponentially close to the anode 7

9 Glow discharge Current increases by several orders of magnitude while voltage remains same Discharge is maintained by positive ions extracting electrons from cathode Most of the voltage falls off close to the cathode where electric field also largest Positive column has a small positive electric field Light intensity strongest in negative glow Weakly ionized i 10 8 to 10 6 and nonequilibrium T e 10 5 K,T i =T 300K n e in the range of cm 3 8

10 Glow discharge V Glow discharge Currents in the range of 10 6 to 10 1 A Resistive ballast for current control I Increasing pressure positive column longer and thinner Increasing electrode distance positive column longer Increasing current increase of cathode glow surface area while current density and voltage remains similar at very low pressure few collisions at very high pressure nonuniformity 9

11 Arc discharge Increasing current will heat the cathode incrasing electron emission Thermionic current e 0 φ j = at 2 exp( ) kt work function of electrode V Arc discharge I The voltage necessary for sustaining the current becomes smaller while currents increase substantially Currents above 1 A High ionization degree 10 3 to 10 1 Plasma closer to equilibrium T e = T i > 10 4 K n e in the range of cm 3 Electrodes have to withstand high temperatures! 10

12 Plasma torch One of the oldest environmentally used plasma Waste gasification at high temperatures K Similar systems can also be used for syngas production and welding Very high currents A Gas flow Usually thermal plasmas with plasma densities up to cm 3 Efficient conversion of electric energy to heat 11

13 Gliding Arc Discharge Highly nonuniform At the shortest gap arc discharge: thermal plasma with high electron density At a certain gap length l cr not able to hold the thermal equilibrium Rapid cooling of gas while electron temperature remains 1 ev This type of plasma retained up to 3l cr Gas flow 10 m/s Most of the power (up to 7580%) dissipated in this regime Various geometries, for example vortex 12

14 Corona discharge Higher electric fields close to highly curved surfaces resulting in increased ionization: highvoltage wires, st. Elmo fires Point to plane Coaxial wire Ionization zone High electric field Ion drift to other electrode Nonuniform distribution of plasma and luminosity Moderately high voltages to prevent arcing 13

15 Corona discharge Properties depend strongly on the polarity of the sharp electrode smaller volume lower electron concentration higher energy electrons Positive corona ionization Negative corona larger volume higher electron concentration lower energy electrons Negative corona more useful for ozone generation Processes which have high activation energy could benefit from positive corona 14

16 Corona discharge The power input in continuous coronas is rather limited due to limited voltages range before evolution of sparks Short pulses allow to increase the maximum voltage Streamer velocity is up to 10 6 m/s Time for streamer development and propagation is ns for 13 cm gaps Voltage pulses shorter than that to prevent spark formations DC corona also used in air ionizers and electrostatic percipitators 15

17 Dielectric barrier discharge Dielectric barrier can also prevent arcing Dielectric barrier Voltage with opposite polarities to allow continuous opperation (500500kHz) Microdischarge radius about 0.1 mm Microdischarge duration 120 ns Microdischarge transfered charge 10 9 C Peak current about 0.1 A Charge density cm 3 Electron energy 110 ev Memory effect 16

18 Dielectric barrier discharge Various configurations for volume discharge Surface barrier discharge Packedbed discharge Coplanar barrier discharge Also useful for surface treatment and aeronautic applications Lower fields High surface area for catalysts Restricted flow 17

19 Hollow cathode discharge pd in the order of Torr cm At low currents ordinary glow discharge Hollow cathode At certain currents negative glow reshapes to virtual anode inside the hollow λ i r pendulum effect Voltage decreases while current increases steeply High electron densities n e cm 3 T e in range of 0.5 ev and above 10 ev r Possible to use arrays of holes without ballast Microhollow cathode Especially at atmospheric pressures 18

20 Radiofrequency discharges Operates in the frequency range of 1100 MHz, typically MHz wavelengths 3300 m larger than the dimensions of reactor planar Capacitively coupled E Inductively coupled coaxial electrodeless B coil coaxial planar spiral electrodeless Suitable at lower pressures ( Pa) and usually used for processing n e in the range of cm 3 and T e in the range of 17 ev 19

21 Radiofrequency discharges planar Capacitively coupled E Sheets and self bias at electrodes high ion energies above 100 ev αmode bulk ionization lower currents, positive IV coaxial γmode secondary emission high currents, partially negative IV electrodeless Different visual appearance n e in the range of cm 3 and T e in the range of 110 ev 20

22 Atmospheric pressure plasma jet Gas flow needle Can be ignited both at khz or RF regimes Plasma bullets with fast speed Stabilized by high gas flow Length of the jet up to few cm Nonthermal n e cm 3, T e 12 ev Used for surface treatment, especially in medical applications 21

23 Radiofrequency discharges Inductively coupled coaxial electrodeless B coil spiral planar Helicon T n e ~ cm 3 at pressures 0.1 Pa n e ~10 12 cm 3 Especially suitable at very low pressures (0.1Pa) Separate control of ion fluxces and energy ICP plasma torches are also reported 22

24 Microwave discharges Energy can be deposited selectively into electrons Ion mass larger and oscillation frequency less than GHz Typical frequency 2.45 GHz wavelength cm f p = 1/2π e 02 n ε 0 m High plasma densities of up to cm 3 High gas temperatures in the range of 10 3 K electron temperatures even higher Can be used in different modes Resonator cavities with standing wave Surfacewave discharges Free expanding torches Capacitive microwave plasmas Electron cyclotron resonance waveguide Nozzles and/or swirls for stabilizing Gas flow resonator Sliding short 23

25 Electron beams Plasma is generated by electron beam from external source Magnetic fields T Large areas in the range of m 2 Good uniformity Energy transfer up to 70 % possible High electron densities can be produced at high pressures Independent control of ion and radical fluxes Special electron accelerators filament accelerator magnetic coil titanium foil Gas flow ebeam 24