1. MAE /NCSU
Unipolar Injection of Charge into Quiescent Gaseous Dielectrics Alexei V. Saveliev Department of Mechanical and Aerospace Engineering North Carolina State University Presented at workshop on “ Electrostatic Atomization of Electrically Insulating Fluids: Principles and Applications” University of Southampton, March 2, 2009

2. Outline 1. Introduction 2. Breakdown of Gaseous Dielectrics
Plasma and ionized gases
2. Breakdown of Gaseous Dielectrics
Uniform field – Paschen’s law
Non-uniform field breakdown
Positive and negative coronas
Corona charge injection devices
Pulsed streamer corona discharges
3. Discharge initiation in Supercritical Fluids
SCFs as a cluster fluid
Generation of SCF plasma in carbon dioxide
Potential applications of SCF plasmas
4. Breakdown and charging on liquid-gas interfaces

3. Plasmas and Ionized Gases
Plasma is an ionized gas consisting of charged and neutral particles and exhibiting collective behavior (Langmuir, 1929)
Plasma may carry a current
Plasma is often considered to be electrically neutral
Plasma is an ionised gas
Ionized gas is not always plasma

4. E-field Breakdown Thermal Plasma Non-Thermal Electrical Discharges:
Te = Ti = Tg
Non-Thermal Plasma
Te >> Ti>Tg
Non-Thermal Electrical Discharges:
Electrons are accelerated in the electric field gaining energy sufficient for ionization of neutral molecules
Secondary electrons are produced in collisions forming electron avalanche propagating in the interelectrode gap
Development of electron avalanche
1 eV  K Ionization energy  10 eV
E/N parameter me << mi,mn e >> i Ar
E-field
Ar + e  e + e + Ar+

5. Kinetics of Ions and Electrons
Elementary processes:
Ionization e + Ar  2e + Ar+
Electron attachment e + O2  O2
Electrode processes - ion impact - thermionic emission - field emission
Fowler – Nordheim, 1928

6. E-field Breakdown – Uniform Field
Townsend criterion d
Multiplication of charges during avalanche propagation

7. Paschen’s Law
Breakdown voltage and breakdown reduced e-field between two parallel electrodes

8. Breakdown in Non-uniform Field
Modified Townsend criterion
Peek’s formula
Ecr –critical e-field in kV/cm; r – radius of inner sphere in cm;  - relative air density

9. Corona Discharge Corona configurations
A – wire-in-cylinder; B – sphere-in-sphere; C – point-and-plane; D – parabola-to-plane; E – wire-to-plane E)

10. Applications of Corona Discharge
Charging and discharging of solids and fluids
Manufacture of ozone
Electrostatic precipitators
Pollution control (VOCs, SOx, NOx)
Processing of material surfaces
Photocopy
Ionic wind devices
Gas ionizers for mass spectrometers and DMA

11. DC Corona Current
Typical voltage-current characteristics for negative point-to-plane corona
Solution for wire in cylinder corona
Vcr – ignition voltage

12. Streamer Propagation and Branching
Streamer Model:
Positive corona:
Streamer propagation modes:
Ionization in the negative corona is due to multiplication of avalanches. Continuity of electric current from the cathode into the plasma is provided by the secondary emission from the cathode (mostly induced by ion impact). Ignition of the negative corona actually has the same mechanism as the Townsend breakdown generalized taking into account non-uniformity and possible electron attachment.
Ionization in the positive corona cannot be provided by the cathode, since the electric field there is low. Here the ionization processes are related to the formation of cathode directed streamers. Ignition criterion can be described using the criteria of cathode directed streamer formation. Generalization of the Meek criterion is a good approximation in this case.
Veldhuizen and Rutgers, J. Phys. D. Appl. Phys. 35, 2002

13. Streamer Propagation and Branching
Morphology, length , and velocity of streamers are extremely important for discharge structure and ion generation efficiency
Ionization in the negative corona is due to multiplication of avalanches. Continuity of electric current from the cathode into the plasma is provided by the secondary emission from the cathode (mostly induced by ion impact). Ignition of the negative corona actually has the same mechanism as the Townsend breakdown generalized taking into account non-uniformity and possible electron attachment.
Ionization in the positive corona cannot be provided by the cathode, since the electric field there is low. Here the ionization processes are related to the formation of cathode directed streamers. Ignition criterion can be described using the criteria of cathode directed streamer formation. Generalization of the Meek criterion is a good approximation in this case.
(a)
(b)
Fast intensified CCD imaging of the fast transient plasma generation with 5 ns gating: (a) pulsed corona discharge in air, (b) pulsed corona discharge in CO2.

14. Positive Corona
Existence regions for positive sphere-to-plane corona forms in atmospheric air, r =10 mm

15. Negative Corona
Existence regions for negative sphere-to-plane corona forms in atmospheric air

16. DC Corona Chargers Cathode
The need for more efficient methods to charge gas and aerosol streams has resulted in a renewed interest in corona discharge ionizers. + + + + + + +
Anode + + + + + + + +
Space charge buildup in the positive wire-to-cylinder corona. E
Ionization region r

17. DC Corona Diffusion Chargers
Several types of such chargers have been proposed in recent years, most of them based on the original design of Hewitt (1957)
Biskos et al., Journal of Electrostatics 63(2005)
Distribution of ion concentration at various pressures

18. DC Corona Diffusion Chargers
Sharp-point electrode ionizer
Alguacil and Alonso, Aerosol Science 37 (2006)
Total ion number concentration at ionizer outlet

19. Pulsed Streamer Corona Discharge
Pulsed corona discharge can be readily generated at atmospheric pressure
Voltage is applied to the sharp electrode as a series of fast rising pulses:
100 ns duration
10 ns rise time
20 kV peak voltage
Electron energies up to 20 eV (~230,000 K )
Generates and efficiently separates the charges

20. Pulsed Electrostatic Precipitators
The first use of corona to remove particles from an aerosol was by Hohlfeld in 1824
Electrostatic precipitators were first used in industry to remove of sulfuric acid mist from exhaust fumes in the beginning of the 20th century
Modern controls minimize sparking and prevent arcing by applying pulsed voltage and thus avoiding damage to the components.

21. Pulsed Corona Discharge for VOC Removal
10-10
102
10-6
10-4
10-2 1
10-8
Electrons
Radicals
By-Products
Time, s
High energy
electrons
Radical-molecule
Ion-molecule
Clustering
Heterogeneous
Excitation
Ionization
Dissociation
Reactions O N
N4+
N2+ OH
CH3 H
H2O
NO2 CO
CO2 O3

22. Elevated Pressure Plasmas
Breakdown in Supercritical Fluids
The area of non-thermal plasmas is expanding to elevated pressure gases, liquids and microplasmas
For a given voltage V
Te = f(E/n)=F(V/nd)
n - neutral density d - characteristic system size
Liquid Plasmas
SCF plasma
1 atm
Elevated Pressure Plasmas
Microplasma
Nanoplasma
Traditional area
1 m

23. SCF Plasma - Problem Statement and Motivation
The generation of non-thermal plasma in supercritical fluid (SCF) media is interesting both from fundamental and applied viewpoints. Sustaining plasmas in SCF media bridges the gap between gaseous and liquid processing since SCFs have properties that are intermediate between the gas and liquid states
Applications of supercritical plasmas:
- pollutant removal
- bacterial deactivation
- plasma polymerization
- material synthesis

24. Supercritical Carbon Dioxide
Critical point : 304 K, 73 bar
Environmentally friendly “green” solvent
The properties of supercritical fluids are intermediate to corresponding gas and liquid states
SCF combines heterogeneous chemistry with efficient mass transfer realized by the low viscosity, high diffusivity and zero surface tension

25. Clusters in Supercritical CO2
SCF is often referred as a cluster fluid
Clusters are effectively formed near the critical point
Clusters affect reaction and transport properties of SCF
Clusters are crucially important for plasma breakdown mechanism

26. SCF Plasma Reactor Technical Approach
Non-thermal plasma is initiated in supercritical carbon dioxide at pressures above 75 bar using high-voltage pulses of nanosecond duration
The pulses are applied to the system of microelectrodes arranged in point-to-plane and wire-to-plane geometries
The discharge and plasma parameters are studied using optical and electrical diagnostics

27. Microbreakdown in Supercritical CO2
d = 80 m
IEEE Trans. Plasma Sci., 33: 850, 2005
Ito and Terashima, Appl. Phys. Lett., 80: , 2002.

28. Electron Kinetics in Supercritical CO2
High
energy
electrons
(CO 2 ) n CO +
Recombination
+ e
Attachment O - 1
Ionization
Dissociation
Ionization
Recombination
Attachment
Dissociation

29. Wire-in-cylinder Corona Discharge
Discharge inception voltages in gaseous and supercritical CO2. The original Paschen’s data are shown for reference.
IEEE Trans. Plasma Sci., 33: 850-3, 2005

30. Wire-in-cylinder Corona Discharge
The ionization potential for solid CO2 clusters is well known. It can be estimated that the ionization potential of the supercritical fluid near the critical point is 10 % lower than in the gaseous phase. Then for electron temperature of 1 eV, the required breakdown voltage at supercritical state can be estimated from comparison of ionization rates suggesting value 3.5 times lower than that in the corresponding gaseous phase.
Dependence of critical E/p on pd in gaseous air and CO2 for uniform and non-uniform fields.
IEEE Trans. Plasma Sci. , 34: 2467, 2006

31. Point-to-plane Corona Discharge
Atomic Force Microscopy
Electrode assembly
Electrode gap

32. Periodic Pulsed Corona Discharge
Distance 200 m
Needle tip 5 m
T= 60 C
P= 83 bar
Voltage 13 kV
Pulsed corona discharge is generated with frequency of 1 Hz. When the duration between pulses is long enough transition from corona to spark discharge is observed. The video is made with normal video camera. Further ICCD diagnostics of the streamer propagation will be conducted.

33. Sliding Liquid-Jet Discharge
The high voltage nanosecond pulses are applied to the micro-jet
Overcharged micro-jet generates corona discharge with sharp liquid electrode
Application for material processing and sensors
Plasma as an ignition source for engines and combustors using liquid fuels
Taylor Cone: cone-shaped pendant drop that emits a fine jet from its tip when is exposed to a strong electrical field
Electric forces exceed surface tension forces
Fluid accelerates into apex region
Micro and nano jet brakes into charged droplets creating “electrospray”
100 m

34. Experimental Setup
Surface discharge operation

35. Experimental Setup

36. Discharge Propagation - Electrolyte Jet
a) b) c) d) e)
Discharge propagation modes. The lower electrode is anode, R = 840 kΩ, L = 18 mm, δ = 2 mm; (a) – low current degenerated glow discharge at U0 =10 kV, (b) – conversion of the glow discharge to the single sliding surface discharge, (c, d) – developed surface discharge at U0 =23 kV; (e) - degenerated arc discharge at U0 =30 kV

37. Discharge Modes – Distilled Water
Voltage-current characteristics of the discharge for distilled water: L = 18 mm, Q = 0.3 ml/s, d = 0.4 mm, С = 0.47 nF, δ = 1 (1) , 2 (2), 3 (3), 4 mm (4). I – the area of a continued degenerated glow discharge; II – pulsed surface discharge.

38. Discharge Regimes - Electrolyte Jet
Voltage-current characteristics at С = 0.73 nF, L = 20 mm, d = 0.5 mm, Q = 0.5 mL/s.
Ring electrode of 4 mm diameter; σ = 6·10-2 (1), 7·10-2 (2), 2·10-1 (3), 1.2 S/m (4);
I – the area of a continued degenerated glow discharge;
II –the pulse surface discharge, III – continued degenerated arc discharge.

39. Jet Atomization
Water jet atomization at Q = 0.5 mL/s, U = 2 (a), 10 kV (b). Appearing the surface
discharge at U = 13 kV (c), I = 8 mA and its development at U = 16 kV, I = 16 mA (d).

40. Summary
Unipolar injection of charge into quiescent gaseous dielectrics is well established scientific area.
Understanding of ionization and charge separation processes is developed
Corona discharges are widely applied for charging of fluids and solid surfaces
Pulsed discharges represent modern trend in plasma generation for various charge injection devices
Discharges in supercritical fluids, liquids, at material interfaces and discharges in micro and nanoscale are current areas of research interest
THANK YOU!

41. Energy Systems Laboratory 2003
Acknowledgements
Collaborators:
Dr. Lawrence A. Kennedy Dr. Alexander A. Fridman
Dr. Vladimir Shmelev Dr. Evgenyia Lock
Dr. Ozlem Yardimci
Dr. Wilson Merchan-Merchan
Emiliano Giacchetti
Mario Sobacchi
Research support:
NSF grants # , #
DOE grant FWP 49885
CRDF grant # RUC2-2824
Argonne National Laboratory Texaco, Inc.
Air Liquide, Inc.
Innovative Energy Solutions, Inc.
Energy Systems Laboratory