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Wright Scholar Outbriefing_Huang
- 1. Wright Scholar Outbriefing XuHai (Jack) Huang 2014-2015
- 2. Background • University of Pittsburgh • Interested in physics and electrical engineering.
- 3. Where I Worked: AFRL/RQQE Energy Sciences Facility Building 23
- 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
- 6. My Project Gas Breakdown and Paschen’s Law Gas Breakdown Experiment Aluminum Paschen Curve Data
- 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.