1. Microwave Plasma Discharge and Its Applications
Mark Gołkowski
Physics 312
February 1, 2007

2. Outline
Microwave Plasma Discharge
Hardware
Physics of Microwave Breakdown
Applications
Dry Sterilization Application

3. Microwave Plasma Discharge
Gas ionization using EM waves: 1 GHz 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

4. 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)
Images from HyperPhysics (©C.R. Nave, 2006)

5. Typical Setup
E-field
Diag.
Magnetron
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

6. Electrons in HF fields
Magnetic field of wave can be neglected, electric field oscillates as
Equation of motion:
wave E-field
electron velocity
Electron-neutral
collision frequency
friction term
Using phasors:
wave term

7. Kinetic Energy of Electrons
Average kinetic energy over 1 HF period
not enough for ionization but elastic collisions allow for the build up of electron energy
PA is the mean power transfer per electron

8. 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

9. 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

10. 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

11. 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)

12. Video plasma flame free radicals/active species skin pore
virus/bacteria
skin pore