4. Co-Workers
• Dr. Steve Adams
• Dr. Brad Sommers
• Allen Tolson
• Amber Hensley
• Joel Summerfield
• Andy Kremer
• Me
5. 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
7. 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
8. Air Force Study of
Gas 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.
9. Paschen’s Law
The Theory
• Theoretical Breakdown Voltage, 𝑉 =
𝐵𝑃𝑑
ln 𝐴𝑃𝑑 −ln (ln(1+γ−1)
• 𝐴 and 𝐵 are constants of the gas (Argon for us)
• γ is the secondary electron emission coefficient
• The probability that an electron will emit from the
electrode as an ion hits the electrode.
• 𝑉 𝑚𝑖𝑛 =
𝑒𝐵
𝐴
ln(
1+γ
γ
)
– the minimum breakdown voltage
• 𝑃𝐷 𝑚𝑖𝑛 =
𝑒
𝐴
ln(
1+γ
γ
)
– PD with minimum breakdown voltage
10. 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
Ground
Vacuum
chamber
11. 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
12. Comparing Experimental Data to
Theoretical Fit
• The γ 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.
13. • 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).
Reasons for Differences Between
Experiment and Theory
14. 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.
Voltage of 200 V and
PD of 3 cm-Torr:
No Breakdown
Voltage of 300 V and
PD of 1 cm-Torr:
Breakdown
15. 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 γ ‘s to previous work
• Reliable γ values are difficult to find in the literature that
match our operating conditions
16. 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.
17. 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.
