1/12 Dielectric Breakdown in Air Components at Microwave Frequencies under Stratospheric Conditions...
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Transcript of 1/12 Dielectric Breakdown in Air Components at Microwave Frequencies under Stratospheric Conditions...
1/12
Dielectric Breakdown in Air Components at Microwave Frequencies under
Stratospheric Conditions
S.K. Remillard, A. Hardaway*, Hope College, Department of PhysicsBrian Mork, Jake Gilliland*, Hope College, Department of Chemistry
*students
This work is funded by the Michigan Space Grant Consortium
Michigan Space Grant Consortium Conference, Oct 18, 2008, Ann Arbor
2/12
Motivation
• To understand the response of “thin” air to intense microwave fields
• To study the electrodynamic mechanism involved in Ozone formation
3/12
The Microwave Plasma Generator
Swept Signal Generator
Power Amplifier
tran
smis
sio
n frequency
Microwave Resonant Cavity
Network Analyzer
1.8 GHz
Quarter wave resonator
Frequency tuner
~1W
4/12
Resonance and Perturbation Analysis
• Intense electric field in a resonator:
E=10,000 V/m for Pin=700 mW
• Perturbation measurement of the electric field in the gap using a PTFE insert:
/4
Uniform Electric Field Region
Cavity
oo
Dielectric
roro
dVEH
dVEH
ff)(
21
)(
122
22
2 2
Total EM energy
0
5/12
NetworkAnalyzer
Amplifier Pirani GaugeVacuum Chamber
CryoRefrig
TemperatureController
Compressor
JakeThree Modules in this Experiment:•Microwave•Vacuum•Cryogenic
6/12
at breakdown
Transmission of power through the resonator…
below breakdown
7/12
8000
12000
16000
20000
24000
0.1 1 10 100
Presure (torr)
Bre
ak
do
wn
Fie
ld (
V/m
)Oxygen Nitrogen Air
Air and its Principal Components 1.8 GHz and 290 K
Paschen
minimum
8/12
Ambient Temperature Effects
Water vapor makes breakdown more difficult at low pressure.
9/12
Phenomenological Model #1
P
EU
eEU
So,
P
1But
U
2U
Mean Electron Energy
ℓ=Mean Free PathP=pressure
ℓ (A.U.)
U
(A.U
.)
Nitrogen at 294 K and 1.8 GHz
Bre
akdo
wn
10/12
Phenomenological Model #2
What should be the breakdown E field?
m
c
ceff NEE
22
222
Effective field for energy transfer
Collision Rate P
Number density P
Free fit parameter
Does this fit the data?
2
2
)(1
PBCPE m
At breakdown
11/12
Phenomenological Model #2
For a plasma in free space, m=1. (Gurevich, 1997)
m=0.43
m=1
High R2 =7:
• Model doesn’t account for very low pressure• Underestimated uncertainties
dotted line: force m to be one
12/12
Conclusion and Follow-up• Next two variables: 1. Microwave frequency
2. Characteristic Diffusion Length
• Another goal is to relate ozone production in air discharge to the electromagnetic wave properties:
1. A residual gas analyzer gas sampler has been constructed
2. Currently designing a resonator that mounts directly to the gas sampler.
Follow-up
This work is funded by the Michigan Space Grant Consortium