Plasmas

Plasmas

1. What is a plasma?

   • Quasi-neutral
   • Thermodynamic equilibrium
     • Thermal plasmas
     • Non-thermal plasmas
   • Ionized gas
     • Weakly ionized: ionization degree 10^-7 - 10^-4
     • Completely ionized
   • Practical applications
     • Electron temperatures 1-20 eV
     • Electron densities 10^6 - 10^18 /cm3

2. Plasma physics

   • Formation mechanisms
     • Low pxd (< 200 Torr.cm)
       • Dark discharge
         • Self-sustained Townsend discharge (m > 1)
         • Townsend avalanches
           • 1st Townsend coefficient
             • Ionization rate per unit length
             • Increases with E/p
           • 2nd Townsend coefficient
             • Cathode emission due to impact of photons
             • Cathode emission due to positive ions
             • Gas ionization by positive ions
         • Free electrons available
           • Cosmic rays ionization
           • Cathode exposure to UV
       • Glow discharge
         • Space charge accumulation
           • High E zone near cathode (cathode layer)
             • Higher ionization and photon emission
           • Low E zone near anode (positive column)
       • Electric arc
         • Lower potential
         • Fueled by cathode thermionic emissions
     • High pxd (> 300-500 Torr.cm)
       • Spatial inhomogeneity
         • Micro discharges
           • Electrons dissipate in 40 ns, but ions take some micro seconds
           • initiated by an avalanche and developed until electron current termination
           • Micro discharge remnant: remaining excitation
             • Facilitates a new microdischarge in the same spot (memory effect)
           • Quasi repulsion between microdischarges
         • Corona discharges
           • Negative (around cathode)
           • Positive (around anode)
       • Ionization waves
         • Streamer propagation due to space charge
           • Dipole with characteristic length and radius around 1/alpha and charge N_e in the order of exp(alpha x)
           • Much faster than the time required for ions to cross the gap
             • High velocities due to strong field at the tip of the streamer (which acts as anode), leading to high electron drift
             • Growth rate of around 10^8 cm/s
           • Streamer channel around 0.01-0.1 cm, maximum size of avalanche head
           • Plasma density around 10^12-10^13/cm3, corresponds to the maximum density in the primary avalanche
           • Meek criterium (alpha x d > 20)
         • Negative x positive streamers
           • Negative streamers form when gap and overvoltage are large
           • Positive streamers form when the avalanche reaches the anode
         • Two streamer models
           • Quasi-self-sustained streamer
           • Ideally conducting streamer channel
       • Arc prevention
         • Electric field strong only near to a small-curvature electrode
         • Pulsating power supplies
         • Dielectric barrier

3. Plasma chemistry

   • Neutral particles
     • Vibrational excitation
     • Rotational excitation
     • Electronic excitation
     • Dissociation by electron impact
     • Relaxation
       • Vibrational-translational (VT)
       • Vibrational-vibrational (VV)
       • Rotational-rotational (RR)
       • Rotational-translational (RT)
       • Excited states relaxation
         • Collisions
         • Relaxation to vibrational/rotational excitation
   • Collision processes
     • Elastic
       • Kinetic energy constant
     • Inelastic
       • Kinetic energy transformed into internal energy
     • Superelastic
       • Internal energy transformed into kinetic energy
   • Ionization processes
     • Direct ionization by electron impact
       • Energy required calculated by Thomson formula
     • Stepwise ionization by electron impact
     • Ionization by collision of heavy particles
     • Photo-ionization
     • Surface ionization
     • Associative ionization
   • Electronic emission
     • Thermionic emmission
     • Electric field emmission
     • Secondary electron emmission
     • Photo/thermal ionization of aerosols
   • Electron energy distribution
     • Low E/n: rotational excitation and elastic collisions (<<1 eV)
     • Intermediate E/n: vibrational excitation (around 1eV)
     • High E/n: electronic excitation and ionization
     • Strongly dependent on gas composition
   • Ion reactions
     • Positive ions reactions
       • Dissociative electron-ion recombination
       • Three-body electron-ion recombination
       • Radiative electron-ion recombination
       • Ion molecular reactions
       • Ion-atomic charge transfer
     • Negative ions reactions
       • Dissociative electron attachment/associative detachment
       • Three-body electron attachment
     • Ion recombination
       • Two-body
       • Three-body

4. High voltage nanosecond pulsed discharges

   • Advantages
     • Produce high energetic electrons instead of heating the gas
     • Prevent glow-to-arc transition
   • Fast rising time and short duration