Transcript Mark_Golkowski_Plasma_Discharge_Phys312

Microwave Plasma Discharge and Its
Applications
Mark Gołkowski
Physics 312
February 1, 2007
Outline





Microwave Plasma Discharge
Hardware
Physics of Microwave Breakdown
Applications
Dry Sterilization Application
2
Microwave Plasma Discharge
 Gas ionization using EM waves: 1 GHz 300 GHz (~2.4 GHz most common)
 Wide range of operation over temperature
and pressure
 mtorr-higher than atmospheric pressure
 Very high to ambient temperature
 Attractive because hardware is cheap and
easy to control
 Safe and wear free: No high voltage
electrodes
3
Magnetron: Microwave Source
 Developed during World War II for RADAR
 Lorentz force from static magnetic field causes
electrons from cathode to execute circular motion
 As electrons sweep past resonant cavities they
excite the resonant modes
 Oscillating E&M fields coupled to antenna (65%
efficiency)
4
Images from HyperPhysics (©C.R. Nave, 2006)
Typical Setup
E-field
Magnetron
Diag.
Cavity
Quartz
Discharge
Tube
(Plasma)
Pressure/Flow
Control
 Wave electric field amplified by resonant cavity (~100 for Q=10000)
 Plasma Discharge takes place at E-field maximum under controlled
flow/pressure in quartz tube
5
Electrons in HF fields
Magnetic field of wave can be neglected, electric field

oscillates as E (t )  E cos(t )  Re{ E exp( jt )}
Equation of motion:
wave E-field
electron velocity


dv

me
 eE(t )  me νv
dt
Using phasors:
jme v  eE  mev
 eE 1
v
me   j
wave term
Electron-neutral
collision frequency
friction term
e
E
cos(t   )
me  2   2
1    
  tan 

6
  
v (t ) 
Kinetic Energy of Electrons
1
1
*
( KE )  me v(t )  v(t )  me v  v
2
4
e2 E 2
( KE ) 
4me ( ν 2   2 )
Average kinetic
energy over 1
HF period
(KE ) not enough for ionization but elastic collisions
allow for the build up of electron energy
Work  Force  Dist  eE(t )  v(t )
PA   eE(t )  v(t )  2 ( KE )
PA is the mean power transfer per electron
7
Increase of Electron Energy
Average kinetic energy increases until excitation or
ionization when the electron loses most of its energy.
Breakdown field function of pressure
MacDonald. Microwave Breakdown in Gases. John Wiley and Sons. New York , 1966
Moisan, Michel, Pelletier, Jacques. Microwave Excited Plasmas. Elsevier. Amsterdam, 1992
8
Applications
 Applications of plasma are in general very
broad
 Because of low-cost, ease of control, wide
temperature/pressure range, microwave
plasmas well suited for enhancing
chemical reactions
 Since electrons are excited/ionized
reaction rates normally associated with
very high temperatures (>10,000 C) are
feasible at ambient temperature
 Examples: Chemical Vapor Deposition
(CVD), filtration/sterilization
9
Dry Sterilization
 Sterilization without use of
chemicals/liquids - no waste product
 Ideal for sterilizing sensitive
equipment, instruments (plastics,
electronics)
 Effective at disinfecting wounds/cuts
 Achievable with microwave plasma
10
Pulsed Plasma Sterilization
Device
 Plasma produced in air at atmospheric pressure
using pulsed microwaves
 Electron temperature ~ 5eV, gas temperature as low
as 30 C
 Plasma is non-thermal, active species:
 free electrons (short lived),
 UV rays,
 strongly oxidizing free radicals (O, O2, OH)
 Neutralizes bacteria, viruses, volatile organic
compounds (VOC)
11
Video
plasma flame
free radicals/active species
virus/bacteria
skin pore
12