Transport Phenomena in Plasma

620 Pages·2007·9.6 MB·English

by  A. Fridman, Y.I. Cho, George A. Greene and Avram Bar-Cohen (Eds.)

Transport Phenomena
in Plasma
Advances in Heat Transfer
Volume Editors/Serial Editors
A. Fridman and Y.I. Cho
Department of Mechanical Engineering and Mechanics
Drexel University
Philadelphia, Pennsylvania
Coordinating Technical Editor
George A. Greene
Energy Sciences and Technology
Brookhaven National Laboratory
Upton, New York
Serial Editor
Avram Bar-Cohen
Department of Mechanical Engineering
University of Maryland
College Park, Marland
Volume 40
Founding Editors
y
Thomas F. Irvine, Jr. State University of New York at Stony Brook, Stony Brook, NY
y
James P. Hartnett University of Illinois at Chicago, Chicago, IL
Amsterdam Boston London New York Oxford Paris
San Diego San Francisco Singapore Sydney Tokyo ACADEMIC
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PREFACE
For more than 40 years, Advances in Heat Transfer has filled the infor-
mation gap between journals and university-level textbooks. The series
presents review articles on topics of current interest, starting from widely
understood principles and bringing the reader to the forefront of the topic
being addressed. The favorable response of the international scientific and
engineering community to the 40 volumes published to date is an indication
of the success of our authors in fulfilling this purpose.
In recent years, the editors have published topical volumes dedicated to
specific fields of endeavor. Examples of such volumes are Volume 22 (Bio-
engineering Heat Transfer), Volume 28 (Transport Phenomena in Materials
Processing) and Volume 29 (Heat Transfer in Nuclear Reactor Safety). The
editors have continued this practice of topical volumes with the publication
of Volume 40, which is dedicated to Heat Transfer in Plasma Physics.
The editors would like to express their appreciation to the contributing
authors of Volume 40, who have maintained the high standards associated
with Advances in Heat Transfer. Finally, the editors would like to acknowl-
edge the efforts of the staff at Academic Press and Elsevier, who have
maintained the attractive presentation of the volumes over the years.
xiii
CONTENTS
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii
Non-Thermal Atmospheric Pressure Plasma
A. FRIDMAN, A. GUTSOL, Y.I. CHO
I. Non-Thermal Plasma Stabilization at High Pressures . . . . . . . . 1
II. Townsend and Spark Breakdown Mechanisms . . . . . . . . . . . . 4
A. The Townsend Mechanism of Electric Breakdown of Gases . . . . . . . . . . . . 4
B. The Critical Electric Field of Townsend Breakdown . . . . . . . . . . . . . . . . . 6
C. The Townsend Breakdown Mechanism in Large Gaps. . . . . . . . . . . . . . . . 7
D. The Spark Breakdown Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
E. Electron Avalanches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
F. The Streamers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
G. The Meek Criterion of Streamer Formation . . . . . . . . . . . . . . . . . . . . . . . 13
H. The Streamer Breakdown Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
I. The Leader Breakdown Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
III. The Corona Discharge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
A. Overview of the Corona Discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
B. Negative and Positive Coronas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
C. Ignition Criterion for Corona in Air . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
D. Active Corona Volume . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
E. Influence of Space Charge on Electric Field in Corona. . . . . . . . . . . . . . . . 21
F. Current-Voltage Characteristics of a Corona Discharge . . . . . . . . . . . . . . . 22
G. Power Released in a Continuous Corona Discharge . . . . . . . . . . . . . . . . . 23
IV. Pulsed Corona Discharge. . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
A. Overview of Pulsed Corona Discharges . . . . . . . . . . . . . . . . . . . . . . . . . . 24
B. Corona Ignition Delay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
C. Flashing Corona . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
D. Trichel Pulses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
E. Pulsed Corona Discharges Sustained by Nano-Second Pulse Power Supplies 27
F. Configurations of Pulsed Corona Discharges. . . . . . . . . . . . . . . . . . . . . . . 28
V. Dielectric-Barrier Discharge . . . . . . . . . . . . . . . . . . . . . . . . . . 30
A. Overview of Dielectric Barrier Discharges. . . . . . . . . . . . . . . . . . . . . . . . . 30
B. Properties of Dielectric Barrier Discharges . . . . . . . . . . . . . . . . . . . . . . . . 31
C. Phenomena of Microdischarge Interaction: Pattern Formation . . . . . . . . . . 35
v
vi CONTENTS
D. Surface Discharges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
E. The Packed-Bed Corona Discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
F. Atmospheric Pressure Glow DBD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
G. Ferroelectric Discharges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
VI. Spark Discharges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
A. Development of a Spark Channel, a Back Wave of Strong Electric Field and
Ionization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
B. Laser Directed Spark Discharges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
VII. Atmospheric Pressure Glows . . . . . . . . . . . . . . . . . . . . . . . . . 52
A. Resistive Barrier Discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
B. One Atmosphere Uniform Glow Discharge Plasma . . . . . . . . . . . . . . . . . . 55
C. Electronically Stabilized APG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
D. Atmospheric Pressure Plasma Jet. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
E. Role of Noble Gases in Atmospheric Glows . . . . . . . . . . . . . . . . . . . . . . . 72
VIII.Microplasmas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
A. Micro Glow Discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
B. Micro DBDs for Plasma TV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
C. Micro Hollow Cathode Discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
D. Other Microdischarges and Microdischarge Arrays . . . . . . . . . . . . . . . . . . 89
IX. Gliding Discharges (GD) and Fast Flow Discharges. . . . . . . . . 96
X. Plasma Discharges in Water. . . . . . . . . . . . . . . . . . . . . . . . . . 104
A. Needs for Plasma Water Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
B. Conventional Methods for Drinking Water Treatment . . . . . . . . . . . . . . . . 105
C. Water Treatment Using Plasma Discharge . . . . . . . . . . . . . . . . . . . . . . . . 106
D. Production of Electrical Discharges in Water . . . . . . . . . . . . . . . . . . . . . . 109
E. Previous Studies on the Plasma Water Treatment . . . . . . . . . . . . . . . . . . . 111
F. Mechanism of Plasma Discharges in Water. . . . . . . . . . . . . . . . . . . . . . . . 116
G. Process of the Electrical Breakdown in Water. . . . . . . . . . . . . . . . . . . . . . 120
H. New Developments in Plasma Water Treatment at Drexel Plasma Institute . 124
XI. Final Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
Heat Transfer in Plasma Spray Coating Processes
J. MOSTAGHIMI, S. CHANDRA
I. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
II. Plasma Spray Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
A. Direct Current (DC) Plasma Gun . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
B. Radio-Frequency Inductively Coupled Plasma (RF-ICP) . . . . . . . . . . . . . . 148
C. Wire-Arc Spraying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
III. Droplet Impact, Spread and Solidification. . . . . . . . . . . . . . . . 150
A. Axi-Symmetric Impact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
B. Splashing and Break-Up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
CONTENTS vii
IV. Mathematical Model of Impact . . . . . . . . . . . . . . . . . . . . . . . 156
A. Fluid Flow and Free Surface Reconstruction . . . . . . . . . . . . . . . . . . . . . . 156
B. Heat Transfer and Solidification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
C. Thermal Contact Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162
D. Effect of Solidification on Fluid Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . 162
E. Numerical Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163
F. Simulation of Splat Formation in Thermal Spray . . . . . . . . . . . . . . . . . . . 163
G. Effect of Roughness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
V. Laboratory Experiments on Droplet Impact . . . . . . . . . . . . . . 173
A. Large Droplets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173
B. Small Droplets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175
C. Transition Temperature Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
D. Effect of Substrate Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183
VI. Thermal Spray Splats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185
A. Wire-Arc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185
B. Plasma Particle Impact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189
VII. Simulating Coating Formation . . . . . . . . . . . . . . . . . . . . . . . 196
A. Direct Coating Model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
B. Stochastic Coating Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202
Plasma Spraying: From Plasma Generation to Coating Structure
P. FAUCHAIS, G. MONTAVON
I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205
II. Plasma Spray Torches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207
A. General Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207
B. Plasma Jet Characterization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211
C. Direct Current Stick-Type Cathode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212
D. Velocity and Temperature Distributions . . . . . . . . . . . . . . . . . . . . . . . . . . 223
E. Soft Vacuum or Controlled Atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . 230
F. Other d.c. Torches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231
III. RF Plasma Spray Torches . . . . . . . . . . . . . . . . . . . . . . . . . . . 236
A. Conventional Torches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236
B. Supersonic Torches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239
IV. Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239
A. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239
B. General Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240
C. RF Plasma Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240
D. d.c. Plasmas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241
E. In-Flight Particles Interaction with the Plasma Jet . . . . . . . . . . . . . . . . . . . 246
F. Corrections Specific to Plasmas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251
G. Chemical Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257
viii CONTENTS
H. In-flight Particle Measurement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262
I. Ensemble of Particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267
V. Coating Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279
A. General Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279
B. Characteristic Times . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281
C. Diagnostics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282
D. Models and Results on Smooth Substrates Normal to Impact
Direction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286
E. Transition Temperature when Preheating the Substrate . . . . . . . . . . . . . . . 294
F. Models and Measurements on Rough Orthogonal Substrates . . . . . . . . . . . 303
G. Impacts on Inclined Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305
H. Splashing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307
I. Parameters Controlling the Particle Flattening . . . . . . . . . . . . . . . . . . . . . . 311
J. Adhesion of Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315
K. Splat Layering and Coating Construction. . . . . . . . . . . . . . . . . . . . . . . . . 320
L. Coating Architecture. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321
VI. Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325
Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329
Heat Transfer Processes and Modeling of Arc Discharges
E. PFENDER, J. HEBERLEIN
I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345
II. General Features of Thermal Arcs . . . . . . . . . . . . . . . . . . . . . 347
A. Relatively High Current Densities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 348
B. Low Cathode Fall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 348
C. High Luminosity of the Column . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 348
III. Thermodynamic and Transport Properties Relevant to Thermal
Arcs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355
A. Equilibrium Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355
B. Non-equilibrium Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391
IV. Modeling of Thermal Arcs . . . . . . . . . . . . . . . . . . . . . . . . . . . 412
A. Simple Models Based on LTE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 412
B. Models for Non-LTE Arcs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 420
V. Heat Transfer Processes in Thermal Arcs . . . . . . . . . . . . . . . . 428
A. General Considerations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 428
B. Anode Heat Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 431
C. Cathode Heat Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 439
VI. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 444
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 446
CONTENTS ix
Heat and Mass Transfer in Plasma Jets
S.V. DRESVIN, J. AMOUROUX
I. The General Concepts of Convective Heat Transfer . . . . . . . . . 451
A. What is a Convective Heat Transfer? The Newton’s Formula. . . . . . . . . . . 451
B. The Energy Conservation Law at the Solid Wall Interface . . . . . . . . . . . . . 453
C. Similarity Criteria (Numbers): Reynolds and Nusselt’s Numbers. . . . . . . . . 454
D. On the Boundary Layer and Similarity Theory . . . . . . . . . . . . . . . . . . . . . 457
E. Boundary Layer Thickness Evaluation and the First Possibility of Expressing
the Heat Transfer Coefficient with Flow Parameters . . . . . . . . . . . . . . . . . 459
F. The Full Energy of The Oncoming Flow: The Stanton’s Number . . . . . . . . 461
G. The Prandtl and Peklet Numbers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 463
H. The Equations of the Laminar Boundary Layer . . . . . . . . . . . . . . . . . . . . 465
I. Estimation of the Thermal Boundary Layer Thickness . . . . . . . . . . . . . . . . 469
J. The Approximate Expression for the Convective Heat Transfer Coefficient
as Function of Medium and Flow Parameters . . . . . . . . . . . . . . . . . . . . . . 470
K. The Exact Calculation of the Heat Transfer Coefficient a (Laminar Thermal
Boundary Layer at the Plane Plate) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 472
L. Heat Transfer Formulas for Sphere, Cylinder and Plate . . . . . . . . . . . . . . . 476
II. The Convective Heat Transfer in Plasma. . . . . . . . . . . . . . . . . 482
A. The Key Concepts and Its Considerations . . . . . . . . . . . . . . . . . . . . . . . . 482
B. Experimental Studies of Heat Transfer in Plasma . . . . . . . . . . . . . . . . . . . 492
C. Comparison and Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 511
Author Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 523
Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 543
ARTICLE IN PRESS
ADVANCES IN HEAT TRANSFER VOL. 40
Non-Thermal Atmospheric Pressure Plasma
A. FRIDMAN, A. GUTSOL and Y.I. CHO
Department of Mechanical Engineering and Mechanics, Drexel University, Philadelphia,
PA 19104
I. Non-Thermal Plasma Stabilization at High Pressures
Plasma-chemical and plasma-processing systems are traditionally divided
into two major categories: thermal and non-thermal ones [1]. Thermal
plasma of arcs or radio-frequency (RF) discharges is associated with Joule
heating and thermal ionization that enables to deliver high power (to over
50MW per unit) at high operating pressures. However, low excitation
selectivity, very high gas temperature, quenching requirements and electrode
problems result in limited energy efficiency and applicability of thermal
plasma sources. Non-thermal plasma is usually very far from thermo-
dynamic equilibrium: while temperature of electrons reaches 1–3 eV and
provides intensive ionization, gas as whole remains cold. Non-thermal
plasma offers high selectivity and energy efficiency of plasma-chemical
reactions; it is able to operate effectively at low temperatures, in contact
with fragile and delicate materials and does not require any quenching. Thus
it is the non-thermal plasma, which this chapter is to be focused on.
Electric energy of plasma sources is initially absorbed by electrons, and
then transferred from the electrons to the neutral gas. If the rate of energy
transfer from the plasma electrons to the neutral gas is significant, but
cooling of the gas is not effective, then the plasma becomes thermal. If the
rate of energy transfer from the plasma electrons to the neutral gas is limi-
ted, and/or cooling of the gas is fast and effective, then the electron temper-
ature significantly exceeds that of neutrals (TecT0) and the plasma becomes
non-thermal and strongly non-equilibrium. Most of the conventional non-
thermal plasma discharges are organized at low pressures, where the neutral
gas cooling by the walls is much faster. Such low-pressure non-thermal
plasma discharges can be represented by traditional glow, inductively (ICP)
and capacitively (CCP) coupled RF discharges, and are widely used in
modern electronics and reviewed particularly in Ref. [1,2].
Advances in Heat Transfer 1 Copyright r 2007 Elsevier Inc.
Volume 40 ISSN 0065-2717 All rights reserved
DOI: 10.1016/S0065-2717(07)40001-6
ARTICLE IN PRESS
2 A. FRIDMAN ET AL.
Organization of the non-thermal, strongly non-equilibrium plasma at
atmospheric pressure is much more challenging. Energy transferred from the
plasma electrons to neutral gas tends at high pressures to be transferred to
heat through different fast channels of the thermal (ionization-overheating)
instability. Increase of temperature significantly accelerates the process, and
cooling mechanisms at high pressures are limited. Nevertheless, several
approaches have been developed to overcome the problems and organize the
strongly non-equilibrium plasma at atmospheric pressure. Between those,
we can point out the following major approaches:
Low discharge power. If the discharge power is sufficiently low, it
obviously limits overheating and gas temperature, while electron
temperature should be anyway on the level of 1–3 eV to provide
effective ionization. Such situation takes place, in particular, in the
well-known stationary corona discharges. The approach is not very
much attractive: there is no overheating, but there is no intensive
plasma as well.
Short pulse discharges. If the duration of pulses is short enough, over-
heating can be avoided even locally. The discharges can generate high
concentration of active plasma species and initiate multiple plasma-
chemical processes, while gas temperature remains very low. Good
example of the approach is a pulsed corona discharge, which becomes
today more and more attractive for many exciting applications.
Dielectric barrier discharge (DBD). While the pulse duration is con-
trolled electronically in the short pulse discharges, the DBD pulses are
controlled naturally by dielectric barriers even when the conventional
AC voltage is applied. Simplicity of the DBD has made this dis-
charges probably the most widely used today. The important problem
of the DBDs is their space non-uniformity related to streamer mech-
anisms of the generation of the discharges.
Helium discharges. Uniform discharges can be organized at atmos-
pheric pressure in helium without overheating due to its high thermal
conductivity and possibility to ionize the gas at relatively low voltages
and powers. In the case of DBD, only small additions of electron-
egative gases to helium are permitted without disturbing DBD uni-
formity. In the case of RF discharges, admixtures of molecular and
electronegative gases can be significant. Applications of the dis-
charges are obviously limited by usage of helium.
Fast-flow discharges. The non-thermal, strongly non-equilibrium dis-
charges can be stabilized at atmospheric pressure in fast gas flows.
Intensive convective cooling is able to stabilize even very powerful
discharges without significant overheating. Flow organization and