Methods of plasma generation and plasma · PDF fileMethods of plasma generation and plasma...
Transcript of Methods of plasma generation and plasma · PDF fileMethods of plasma generation and plasma...
Methods of plasma generation and plasma sources
PlasTEP trainings course and Summer school 2011 Warsaw/Szczecin
Part-financed by the European Union (European Regional Development Fund
Warsaw/Szczecin
Indrek Jõgi, University of Tartu
● Townsend discharge
●Glow discharge
● Arc discharge
● Corona discharge
●Dielectric barrier discharge
Outline of the talk
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●Dielectric barrier discharge
●Hollow cathode discharge
● Radio-Frequency discharges
●Microwave discharges
● Electron beams
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Glow discharge tube
Cathode Anode
+
Discharge tube T P (n)
E–
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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)
Avalanches
Cathode Anode- - ---
-
Seed electrons - cosmic rays or emission from the rough surfaces by electric field
ne
x
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Cathode Anode
– +
Electrons gain energy in the electric field until they are able to ionize neutrals
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Multiplication increases exponentially with the distance
λi - free path of ionization
ne = ne0·exp(x/ λi )
λi
dne /dx = ne/ λi
Townsend discharge
drift energy
αααα = 1/ λi
e0λE
E
-
-
const -V
Townsend ionization coefficient α
αααα = =ννννi
v µµµµe
1E/nki(E/n)
Semiempirically:
function of reduced electric field E/n
ne = ne0exp(αααα x )
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e0λE+
-
λiionization energy e0Vi
αααα =const
λexp( )
Eλ
-Vi
αααα = Ap exp( )E
-BpA and B are properties of gas
λ ~ p
η ~ f( )E/nAttachment coefficient η
- -
electronegative gases
Townsend discharge
Positive ions drift to the cathode where they are recombining but will also extract
new electrons
Secondary electron emission
Cathode Anode
++E
–
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new electrons
γγγγi
The rate at which ions are extracting electrons is given by secondary
electron emission coefficient
depends on electrode material and ions (typically 0.01-0.1)
The amount of positive ions produced in the gap d is ni = ne0[exp(αααα d)-1]
photoionization may also be important γγγγp
ni = ne0[exp((αααα˗η) d)-1]When attachment has to be taken into account:
Townsend discharge
For self sustaining discharge the number of ions produced in gap has to be enough to
generate sufficient new electrons at cathode
Paschen law
γγγγ ne0[exp(αααα d)-1]=ne0 αααα d = ln(1+ )γγγγ
1
-Bp 1
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Ap exp( ) = ln(1 + )E
-Bp
γγγγ
1
Vb = Ebd
-BpdVb = ln(Apd) – ln[ln(1 + γγγγ -1)]
Scaling with pd
Vb
pd
vacuum
insulationhigh pressure
insulation
Vb min
With increasing voltage (electric field), there is increasing number of multiplication
and secondary electron emission – increasing current
Cathode Anode
+E
–
Townsend discharge
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and secondary electron emission – increasing current
Townsend
discharge
V
I
Currents 10-12-10-5 A at small variation of voltage
Light emission increases exponentially close to the anode
Non-neutral plasma ne ∼ 107-108 , ni ∼ 1010 cm-3
10-610-1210-18
Discharge is maintained by positive ions extracting
electrons from cathode
Most of the voltage falls off close to the cathode
Current increases by several orders of magnitude
while voltage remains same
Glow discharge
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Light intensity strongest in negative glow
Most of the voltage falls off close to the cathode
where electric field also largest
Positive column has a small positive electric field
Weakly ionized i ∼ 10-8 to 10-6 and non-equilibrium
Te ∼ 105 K ,Ti=T ∼ 300K
ne in the range of 1010-1012 cm-3
Glow discharge
Increasing pressure — positive column longer and
Currents in the range of 10-6 to 10-1 A
Glow
dischargeV
I
10-610-12 10-4 10-2 100
Resistive ballast for current control
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Increasing electrode distance — positive column
longer
Increasing current — increase of cathode glow
surface area while current density and voltage
remains similar
Increasing pressure — positive column longer and
thinner
at very low pressure few collisions
at very high pressure non-uniformity
Thermionic current
Increasing current will heat the cathode incrasing electron emission
Arc
dischargeV
Ij = aT2 exp( )
kT
-e0φφφφ
work function
of electrode
Arc discharge
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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 Te = Ti > 104 K
Electrodes have to withstand high temperatures!
ne in the range of 1013 cm-3
10-4 10-2 100 102
One of the oldest environmentally used plasma
Similar systems can also be used for syngas production
and welding
Plasma torch
Waste gasification at high temperatures
5000-10000 K
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–Gas flow
Usually thermal plasmas with
plasma densities up to 1017 cm-3
Very high currents 10-1000 A
Efficient conversion of electric energy to heat
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Highly non-uniform
At the shortest gap arc discharge:
thermal plasma with high electron density
At a certain gap length lcr not able to hold the thermal
equilibrium
Gliding Arc Discharge
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Gas flow 10 m/s
Rapid cooling of gas while electron temperature
remains 1 eV
Most of the power (up to 75-80%) dissipated in this regime
This type of plasma retained up to 3lcr
Various geometries, for example vortex
+–
Point to planeCoaxial wire
Higher electric fields close to highly curved surfaces resulting in increased
ionization: high-voltage wires, st. Elmo fires
Ionization zone +--
++ +
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Corona discharge
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Ionization zone
Ion drift to other
electrode
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High electric field+
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Non-uniform distribution of plasma and luminosity
Moderately high voltages to prevent arcing
–
Positive corona
Properties depend strongly on the polarity of the sharp electrode
Negative corona
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++
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++
smaller volumelarger volume
higher electron lower electron
concentration
+ –
Corona discharge
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Negative corona more useful for ozone generation
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higher energy
electrons lower energy
electrons
concentrationconcentration
ionization
Processes which have high activation energy could benefit from positive corona
– +
Short pulses allow to increase the maximum voltage
The power input in continuous coronas is rather limited due to limited voltages
range before evolution of sparks
Streamer velocity is up to 106 m/s
Time for streamer development and propagation is 100-300 ns for 1-3 cm gaps
Corona discharge
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Voltage pulses shorter than that to prevent spark formations
DC corona also used in air ionizers and
electrostatic percipitators
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Dielectric barrier can also prevent arcing
Dielectric
barrier
Voltage with opposite polarities to allow continuous opperation (500-500kHz)
Dielectric barrier discharge
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Microdischarge radius about 0.1 mm
Microdischarge duration 1-20 ns
Microdischarge transfered charge 10-9 C
Charge density 1014-1015 cm-3
Voltage with opposite polarities to allow continuous opperation (500-500kHz)
Memory effect
Peak current about 0.1 A
Electron energy 1-10 eV
Various configurations for volume discharge
Surface barrier discharge
Dielectric barrier discharge
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Surface barrier discharge
Packed-bed discharge
Lower fields
High surface area for catalysts
Restricted flowAlso useful for surface treatment and
aeronautic applications
Co-planar barrier discharge
- pendulum effect
Voltage decreases while current increases steeply
-
+
–
At low currents ordinary glow discharge
pd in the order of Torr·cm
At certain currents negative glow reshapes to
virtual anode inside the hollow
λi ≈≈≈≈ r
Hollow cathode discharge
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Hollow cathode
Voltage decreases while current increases steeply
Microhollow cathode-
–
+–
Especially at atmospheric pressures
r
High electron densities ne 1012 – 1015 cm-3
Te in range of 0.5 eV and above 10 eV
Possible to use arrays of holes without ballast
Operates in the frequency range of 1- 100 MHz, typically 13.56 MHz
Capacitively coupled
E B
Inductively coupledplanar
coaxial “electrodeless”
wavelengths 3-300 m larger than the dimensions of reactor
Radio-frequency discharges
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Suitable at lower pressures (0.1-103 Pa) and usually used for processing
coil
spiral
ne in the range of 109-1015 cm-3 and Te in the range of 1-7 eV
coaxial
planar
“electrodeless”
Capacitively coupled
E
planar
Sheets and self bias at electrodes
α mode
• bulk ionization
high ion energies above 100 eV
• lower currents, positive I-V
Radio-frequency discharges
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ne in the range of 109-1011 cm-3 and Te in the range of 1-10 eV
coaxial
“electrodeless”
γ mode• secondary emission
• high currents, partially negative I-V
Different visual appearance
Plasma “bullets” with fast speed
needle
Can be ignited both at kHz or RF regimes
Gas flow
Atmospheric pressure plasma jet
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Used for surface treatment, especially in medical applications
Length of the jet up to few cm
Stabilized by high gas flow
Non-thermal
ne ∼ 1011-1012 cm-3, Te ∼ 1-2 eV
B
Inductively coupled
coil
spiral
planar
coaxial “electrodeless”
Helicon
0.005-0.03 T
Radio-frequency discharges
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Especially suitable at very low pressures (0.1Pa)
ne ~1012-1013 cm-3 at pressures 0.1 Pa
Separate control of ion fluxces and energy
planar
ne ~1012 cm-3
ICP plasma torches are also reported
Energy can be deposited selectively into electrons
fp = 1/2π e02 n
ε0 mTypical frequency 2.45 GHz
Ion mass larger and oscillation frequency less than GHz
High plasma densities of up to 1013 cm-3
High gas temperatures in the range of 103 K
electron temperatures even higher
wavelength 12.24 cm
Microwave discharges
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Gas flow
waveguide
electron temperatures even higher
Can be used in different modesSliding
shortResonator cavities with standing wave
Capacitive microwave plasmas
Surface-wave discharges
Free expanding torchesresonator
Electron cyclotron resonance
Nozzles and/or swirls for
stabilizing
Plasma is generated by electron beam from external source
Large areas in the range of m2
Good uniformity
Magnetic fields 0.01-0.02 T Special electron accelerators
filament
accelerator
Electron beams
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High electron densities can be produced at
high pressures
Independent control of ion and radical fluxes
Energy transfer up to 70 % possible
magnetic coil
titanium foil
e-beam
Gas flow