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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

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Motivation

• To understand the response of “thin” air to intense microwave fields

• To study the electrodynamic mechanism involved in Ozone formation

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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

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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:

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Uniform Electric Field Region

Cavity

oo

Dielectric

roro

dVEH

dVEH

ff)(

21

)(

122

22

2 2

Total EM energy

0

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NetworkAnalyzer

Amplifier Pirani GaugeVacuum Chamber

CryoRefrig

TemperatureController

Compressor

JakeThree Modules in this Experiment:•Microwave•Vacuum•Cryogenic

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at breakdown

Transmission of power through the resonator…

below breakdown

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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

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Ambient Temperature Effects

Water vapor makes breakdown more difficult at low pressure.

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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

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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

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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

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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