III. Applied Plasma Physics: theory, simulation, experiments

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III. Applied Plasma Physics theory, simulation,
experiments

• 8. The Plasma Laboratory     
• 8.1 Plasma Generation
•       8.2 Plasma Confinement
•      8.3 Plasma Diagnostic
• 8.4 Plasma Heating
• 9. Nuclear Fusion
•       9.1 Inertial Confinement
•       9.2 Magnetic Confinement
• 9.3 Electrostatic Confinement
• 10. Industrial Plasmas
• 10. 1 Plasma Processing
• 10. 2 Plasma Torches
• 11. Plasma Propulsion
• 11.1 Electric Plasma Propulsion
• 11.2 The VASIMR Experiment
• 12. Space and Astrophysical Plasmas
• 12.1 The Ionosphere
• 12.2 The Stars
• 12.3 Space Plasmas

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Part III
BACK

• Applied Plasma Physics theory, simulation,
  experiments

3
III. Applied Plasma Physics theory, simulation,
experiments

• 8. The Plasma Laboratory
• 9. Nuclear Fusion
• 10. Industrial Plasmas
• 11. Plasma Propulsion
• 12. Space and Astrophysical Plasmas

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8. The Plasma Laboratory
BACK

• 8.1 Plasma Generation
• 8.2 Plasma Confinement
• 8.3 Plasma Heating
• 8.4 Plasma Diagnostics

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8.1 Plasma Generation

• 8.1.1 Plasma Sources Generalities
• 8.1.2 RF Plasma Sources
• 8.1.3 RF Magnetized Plasma Sources
• 8.1.4 DC Plasma Sources

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8.1.1 Plasma Sources Generalities

• To produce a plasma, electron separation from
  atoms or molecules in the gas state (ionization)
  is required.
• Ionization takes place when an atom or a molecule
  gains enough energy from an external source or
  through collisions
• Common kinds of plasma sources gaseous,
  metallic, and laser-based plasma sources

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Plasma Sources Generalities (II)

• Many plasma sources, such as direct current (DC),
  RF-Glow Discharge, and electron cyclotron
  resonance (ECR) sources are operated at
  low-pressure since the breakdown electric field
  is smaller and the current is more controllable.
• These plasma sources can generate large area
  uniform plasma with a well controlled electron
  density.
• Some other plasmas such as corona discharge and
  arc plasma are generated at atmospheric pressure

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Plasma Sources Generalities (III)

• In general, plasma is generated by applying a
  potential through the gas
• Breakdown potential depends on the pressure and
  discharge gap width.

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Plasma Sources Generalities (IV)
Relationship between the breakdown potential of
air and pressure
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Plasma Sources Generalities (V)

• The relationship between the current and voltage
  in a low-pressure gas discharge shows four
  regions
• (1) dark or Townsend discharge prior to
  spark ignition,
• (2) normal glow
• (3) abnormal glow
• (4) arc discharge where the plasma becomes
  highly conductive

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Plasma Sources Generalities (VI)
Relationship between the current and voltage in
a low-pressure gas discharge
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Plasma Sources Generalities (VII)

• In atmospheric pressure the discharge has a
  simpler characteristic
• The discharge can be divided into two regions
• (1) corona discharge where the discharge current
  is very small, and
• (2) arc discharge where the gas becomes highly
  conductive and a rapid drop in voltage with
  increasing current occurs.
• The arc discharge mode is widely used in welding,
  cutting, and plasma spraying.

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8.1.2 RF Plasma Sources

• Radio frequency (RF) glow discharge plasma source
  (RFGD) produce a large volume of stable plasma
• RF electric potential, coupled with the plasma
  through antennas, produces ionization
• The RF discharges are classified into two types
  according to the method of coupling the RF power
  with the load capacitive or inductive coupling
• Capacitive RF discharges were the etching
  industry standard for years, and are slowly being
  replaced by inductively coupled plasmas.
• Both modes can use internal or external electrodes

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RF Plasma Sources (II)
Currentvoltage Characteristic of a RF Glow
Discharge plasma source.
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RF Plasma Sources (III)

• Inductive coupling RF current is passed through
  a conducting coil separated from the plasma
  chamber by an insulating window
• A current is induced in the plasma much in the
  same way as in an air core transformer
• With external electrodes a discharge tube that is
  made of glass (quartz or borosilicate glass) is
  used
• External electrodes eliminate the plasma
  interaction on the electrode materials
• RF at 13.56 MHz is a industry-standard for in
  RFGDs

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RF Plasma Sources (IV)
Vacuum Chamber
Typical low-pressure inductively coupled plasma
(ICP) source
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RF Plasma Sources (V)

• RFGD typical parameters
• The pressure during discharge is between 10-3 and
  100 Torr (1 atm760 torr1.013 10 3 Pa)
• The electron density in low-pressure (10-3 to 1
  Torr) varies from 1015 to1017 m-3,
• the electron density in medium pressure (1100
  Torr) can reach 1018 m-3
• Electron temperature is several eV and the ion
  temperature is a fraction of eV

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RF Plasma Sources (VI)

• Microwave cavity discharges. A resonant
  microwave cavity is placed around a glass or
  quartz tube
• The resulting standing wave excites a plasma
  through the glass
• This technique is commonly used for creating a
  downstream oxygen plasma for plasma photoresist
  stripping in semiconductor manufacturing

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8.1.3 RF Magnetized Plasma Sources

• Electron cyclotron resonance (ECR) plasma source
  generate high-density plasmas at low-pressure.
• Microwave power is introduced into the plasma
  chamber through a quartz window
• Magnetic coils are arranged around the periphery
  of the chamber to achieve the ECR conditions

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RF Magnetized Plasma Sources (II)
Schematic of an ECR plasma source
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RF Magnetized Plasma Sources (III)

• When a magnetic field is applied to the plasma,
  the electrons begin to rotate in a helical orbit
  about the magnetic field lines.
• Plasma resonance can be obtained by adjusting the
  magnetic field to match the cyclotron frequency
  of the electrons with the RF (microwave)
  frequency
• In general, an ECR plasma source is operated at a
  pressure between 10-5 and 10-3 Torr.
• The electron density can reach 1018 m-3

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RF Magnetized Plasma Sources (IV)

• The mean ion charge state is high because of the
  high collision frequency between the electrons
  and ions.
• Plasma distribution is not very uniform
• Electron temperature is larger than that of the
  ion temperature

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8.1.4 DC Plasma Sources

• Simplest type of plasma discharge DC voltage
  across two electrodes inside of a plasma chamber
• The gas is ionized and maintained in the plasma
  state
• A negative bias is applied to one electrode
  (cathode) and the grounded chamber walls is used
  as the positive electrode (anode)
• DC discharges are typically used in metal
  sputtering and for ion implantation into
  mechanical materials for surface treatment.

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DC Plasma Sources (II) - Corona

• Corona discharge plasma source appears as a
  luminous glow localized in space around a pointed
  tip in a highly non-uniform electric field
• If the characteristic size of the anode is
  comparable to that of the cathode, a voltage
  between the wires produces a spark instead of a
  corona discharge.
• Typical voltage applied to the anode exceeds
  several kVs and the magnitude of the discharge
  current varies from 10-10 to 10-10 A.

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DC Plasma Sources (III) - Corona
Schematic of a corona discharge plasma source
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DC Plasma Sources (IV) - Corona

• In the plasma near the tip electrode the plasma
  density rapidly decreases with distance from
  about 1019 to 1015 m-3.
• The electron temperature within the plasma
  averages about 5 eV, and the ion temperature is
  very low.
• In the region outside the discharge, the electron
  density is much lower and near 1012 m-3

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DC Plasma Sources (V) - Torch

• Plasma spray torch a typical configuration
  includes a nozzle-shaped anode serving also as a
  constrictor.
• Stick-type cathode with a conical tip
• Outer grounded and water-cooled shield as an
  anode may be used
• In order to sustain a plasma torch, the discharge
  current and power density are very high
• The plasma develops between the cathode and anode
  where an electrically conducting gas at T gt 8000
  K and 105 Pa

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DC Plasma Sources (VI) - Torch


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Schematic of a plasma spray torch
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DC Plasma Sources (VII) - Torch

• The electron density in the plasma torch ranges
  from 1022 to 1025 m-3,
• Electron temperature varies from 7.0 to 9.0 eV,
  but the ion temperature is an order of magnitude
  lower at 0.30.9 eV
• High plasma flow velocity (nearly 103 m/s)
• The high-temperature in the plasma causes the
  melting of almost all solid particles
• Atmospheric plasma torches are widely used in
  plasma spraying

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DC Plasma Sources (VIII) - Vacuum Arc

• In general, a vacuum arc plasma source is
  composed of two parts plasma production unit and
  macro-particle filter
• When a high voltage pulse (several kVs) is
  applied to the trigger electrode, an arc
  discharge between the cathode and anode is
  ignited.
• The arc discharge current is concentrated at the
  cathode surface
• Formation of non-stationary locations of
  extremely high current density, in the order of
  1012 A/m2 (cathodic spots)

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DC Plasma Sources (IX) - Vacuum Arc
Schematic of a vacuum arc plasma source
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DC Plasma Sources (XI) - Vacuum Arc
Schematic of a vacuum arc plasma source
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DC Plasma Sources (XII) - Vacuum Arc

• The high current density is associated with the
  extremely high local power density (on the order
  of 1013 W/m)
• Conditions for the localized transformation from
  the solid cathode material to fully ionized
  plasma
• The plasma produced at the cathode spots expands
  rapidly into the vacuum ambient
• Typical ion flow ion velocities are in the range
  of 104 m/s corresponding to approximately20 eV
  for light elements and 200 eV for heavy elements.

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DC Plasma Sources (XIII) - Vacuum Arc

• Curved macro-particle filters are used to
  mitigate macro-particle (impurities)
  contamination from the cathodic arc plasma stream
• Neutral macro-particles are not be affected by
  the magnetic field and impact the outer surface
  of the curved duct
• Cathodic arc process produces of a large quantity
  of ions of the cathode materials
• Almost every electrical conductive material can
  be made into a cathode.

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DC Plasma Sources (XIII) - Vacuum Arc

• The electron density in the cathode spot plasma
  can reach 1026 m3
• Expanding plasma produces a highly ionized jet
  with ion charge states typically between 1 and 3
• High plasma flux velocity prevents diffusion
  effects
• Plasma flux is not very uniform in both the axial
  and radial directions

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8.1.5 Laser Plasma Source

• Plasma generated by the interaction of
  high-density laser pulses with a solid target
• Low laser fluence laser beam passes nearly
  unattenuated through the vapor produced by the
  leading edge of the pulse
• Evaporation occurs from the liquid metal
• High laser fluence the vapor temperature is high
  enough to cause appreciable atomic excitation and
  ionization.
• Vapor begins to absorb the incident laser
  radiation leading to vapor breakdown and plasma
  formation

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Laser Plasma Source (II)
Schematic of a laser plasma source
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Laser Plasma Source (IV)
Plasma in a laser source
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Laser Plasma Source (V)

• Typical laser intensity in the order of 1012 to
  1014 W/m2.
• Metallic and composite targets that coupled with
  gas feeding can generate gaseous plasma and
  compensate for gas loss during evaporation.
• Max. electron density 1024 to 1026 m-3
• Electron/Ion temperature in the range of 15 eV