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

126.07.2011

●Dielectric barrier discharge

●Hollow cathode discharge

● Radio-Frequency discharges

●Microwave discharges

● Electron beams

Summer School, Warsaw

Glow discharge tube

Cathode Anode

+

Discharge tube T P (n)

E–

226.07.2011Summer School, Warsaw

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


Cathode Anode

– +

Electrons gain energy in the electric field until they are able to ionize neutrals

--

-

-

-

-

-

-

- -

--

-- -

-

- -

--

-- -

-

-

-

-

--

-

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 )


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

–


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


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


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


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


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


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

1126.07.2011

+

–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

Summer School, Warsaw

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


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 +--

++ +

+

+

+

+

Corona discharge


Ionization zone

Ion drift to other

electrode

-

High electric field+

+

+

+

+

-

-

-

-

-

-

--

-+

++

+

++

++

+

+

+

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

--

+

++

-

--

+

++

smaller volumelarger volume

higher electron lower electron

concentration

+ –

Corona discharge


Negative corona more useful for ozone generation

-

-

-

-

+

++

+

++

+

-

-

-+

++

-

--

-

-+

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


Voltage pulses shorter than that to prevent spark formations

DC corona also used in air ionizers and

electrostatic percipitators

-

-

-

-

- -

-

-

-

+

–

-

-

-

-

- -

--

Dielectric barrier can also prevent arcing

Dielectric

barrier

Voltage with opposite polarities to allow continuous opperation (500-500kHz)

Dielectric barrier discharge


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


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


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


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



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


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



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


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


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

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