Wright Scholar Outbriefing

XuHai (Jack) Huang2014-2015

Background

• University of Pittsburgh• Interested in physics and electrical engineering.

Where I Worked:AFRL/RQQE Energy Sciences Facility

Building 23

Co-Workers

• Dr. Steve Adams • Dr. Brad Sommers• Allen Tolson• Amber Hensley• Joel Summerfield• Andy Kremer • Me

Group Objectives

• To investigate the electrical breakdown in gases from the application of high voltage

• To apply these measurements to models in order to optimize AF systems • fuel ignition• power electronics• thermal management

• To use optical spectroscopy to study ionized gases or plasma

My Project Gas Breakdown and Paschen’s Law

Gas Breakdown Experiment 680

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Vol

tage

in V

olts

2824201612840Pressure*distance in torr*cm

0.5cm 1cm 2cm 4cm 6cm

Aluminum

Paschen Curve Data

Gas Breakdown

• When gas is exposed to high voltage, free electrons within the gas will “avalanche” if the voltage is high enough. This event is also known as “breakdown”.

Examples of gas breakdown:Lightning Fluorescent lights

Air Force Study ofGas Breakdown and Paschen’s Law

• Various AF technologies rely on gas breakdown• Spark ignition • Plasma film deposition• Plasma decontamination

• Paschen’s Law• A theoretical equation that predicts the breakdown

voltage for a given pressure times distance (PD).• Paschen Curve

• A plot of breakdown voltage vs PD for a certain gas with specific electrode materials.

Paschen’s Law The Theory

• Theoretical Breakdown Voltage, • and are constants of the gas (Argon for us) • is the secondary electron emission coefficient

• The probability that an electron will emit from the electrode as an ion hits the electrode.

• )– the minimum breakdown voltage

• – PD with minimum breakdown voltage

Gas Breakdown Experiment

• Vacuum chamber contains argon gas• Two metal disc electrodes are centered in

the chamber. One electrode is grounded, and a variable voltage is applied to the other

• The voltage between electrodes is increased until breakdown is detected and a glow discharge is formed.

• Pressure is then stepped by 0.1 Torr while the distance remains constant

High Voltage

GroundVacuum chamber

Experimental

• The material of the electrodes are interchangeable

• Aluminum

• Stainless steel

• Graphite

• Copper• Chamber must be clean• Experimental process:

• Start the pump

• Lower the pressure

• Input specific values for the run

• Run program in LabView

• Let the program run overnight

Comparing Experimental Data to Theoretical Fit

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1086420PD

1cm 2 1cm 2cm fit

Gamma Value=0.0015A=12B=180min,1.2cm*torr and 265Voltage

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tage

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6543210 Pressure*Distance torr*cm

1cm 2 1 cm 2 cm fit

• The value determined in this theoretical fit is 0.0015, which is smaller than expected, but reasonable.

• The curve fit agrees with the experimental data at lower PD, but diverges dramatically when the PD is over 5 Torr*cm.

• The theoretical prediction of Breakdown Voltage in argon is known to be only valid between the PD of 1 – 3 Torr*cm (due to Ar ionization rate)

• Theory assumes electrodes are infinitely wide. Our electrodes are only 2 inches wide, but very close together (which is a large aspect ratio).

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Vol

tage

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4.03.53.02.52.01.51.00.5Pressure*Distance torr*cm

fit gamma=0.0045 fit gamma=0.0014 graphite 1cm graphite 0.4cm Aluminum 1cm Aluminum 0.5cm

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ge V

3.02.52.01.51.00.5Pressure*Distance torr*cm

0.4cm 0.4cm 1cm 1cm fit Gamma=0.0015 2cm 4cm 6cm

Graphite

Reasons for Differences Between Experiment and Theory

Improvements to Theoretical Fit

• As an alternative to Paschen’s Law, we developed a computer model to simulate gas breakdown.• The program uses the known ionization rate for argon to

simulate the electron avalanche process.• Plots show total electron production vs time.• If the second derivative is positive, this indicates breakdown.

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Voltage of 200 V and PD of 3 cm-Torr:No Breakdown

Voltage of 300 V and PD of 1 cm-Torr:

Breakdown

Experimental and Analytical Issues

• The purity of the electrodes can affect data.• Contamination can reduce the breakdown voltage by as

much as 50% as that of clean electrodes.

• There was difficulty comparing our ‘s to previous work• Reliable values are difficult to find in the literature that

match our operating conditions

Summary of My Experience

• I learned a lot of physics, including the science behind gas discharge phenomena.

• I learned to be patient while doing research.• Results do not always appear instantly.

• My contribution was part of an on-going project.• The next step is to complete the computer model.• Improve the accuracy of the model.• Fully automate the data acquisition program on the

experiment so it will plot a full Paschen curve.

Acknowledgements

I would like to thank Debbie Miller for making this year’s Wright Scholar program possible. I would also like to thank the speakers who took their time to give lectures. Special thanks to Dr. Steve Adams, Allen Tolson, Amber Hensley, Dr. Brad Sommers and Joel Summerfield for teaching me how to use the equipment in the lab.