Experimental Apparatus for Investigating Plasma Sheaths on Hypersonic Re-Entry Vehicles
Open Access
- Author:
- Umashankar, Mohnish
- Graduate Program:
- Aerospace Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- June 17, 2024
- Committee Members:
- Sven G Bilén, Thesis Advisor/Co-Advisor
Sean David Knecht, Committee Member
Amy Pritchett, Program Head/Chair
Mark A Miller, Committee Member
Jesse Kane Mc Ternan, Special Signatory - Keywords:
- Hypersonics
Atmospheric Re-entry
Plasma
Langmuir Probes
Plasma Diagnostics - Abstract:
- When a spacecraft re-enters Earth’s atmosphere, the air surrounding the vehicle compresses due to the high velocity and heats up. This compression heating, along with some skin friction, causes the air to ionize, creating a plasma environment around the vehicle. This hypersonic plasma environment creates multiple challenges such as thermal loading and communication disruption. The plasma densities and temperatures of these hypersonic plasmas can range from 10^8–10^15 cm^(−3) and approximately 2–5 eV respectively depending on altitude and location of the plasma on the vehicle. Over the years there have been a number of facilities that have been examining these plasma environments in a laboratory setting. NASA Ames has developed and improved upon a miniature arc jet aero-thermal testing facility to examine thermal protection systems and their interaction with a supersonic plasma. The University of Michigan developed a test bed using a helicon source to produce high electron density and low electron temperature plasmas similar to that of plasmas found on re-entry vehicles. This thesis developed and characterized a small-scale hypersonic test system using a small vacuum chamber and modified plasma torch. To characterize the vacuum chamber, a pump test was completed to determine the ultimate pressure and how fast the chamber could reach this value. The chamber is capable of reaching 500 mTorr within a maximum time of 10 minutes. A number of tests characterized the plasma torch system. The plasma torch is capable of operating in a pressure range from 760 torr to 200 torr. The arc voltage and flowrate of the torch was monitored. Atmospheric operation conditions supply the torch with approximately 90 V peak-to-peak and 130 slm of gas. This 90 V limit was considered to be the threshold for a plasma to form. In a higher vacuum, the flowrate drops to 10-30 slm, causing the power supply to limit the amount of voltage the torch is receiving. Occasionally, the voltage would spike above this voltage threshold, creating a small plasma burst. Nozzle erosion was monitored on the torch for documentation purposes because this was the first time a plasma torch had been used in the Space Propulsion and Environments Lab. After about 40 tests with the torch, each lasting about 5 seconds, nozzle erosion was not a major concern for the system. Finally, the plasma density of the plasma plume was taken using a double Langmuir probe setup. Three tests were completed to examine if plasma measurements were possible and how the measurements were impacted in different environments. The three operating conditions were atmosphere, low vacuum and high vacuum conditions. Both the atmosphere and low vacuum tests presented a plasma density of approximately 10^10 cm^(−3). The electron temperature during atmospheric conditions was about 2 eV higher than when a vacuum was introduced to the plasma. The low vacuum test provided an electron temperature that fell within the 2–5 eV range. This implied that, in a higher vacuum, the electron temperature could potentially be lower. These results were only achieved in a low vacuum environment and over a 5 second interval. The intention of the apparatus will be to operate the torch on the order of minutes in a higher vacuum environment. However, the main objective of this research was achieved, which was producing electron densities and temperatures similar to that of hypersonic plasmas. Future work for this experimental apparatus includes modifications to the vacuum chamber and plasma source. The two immediate modifications needed are to be able to operate the plasma torch in a higher vacuum as well as increase the size of the vacuum chamber. Additional modifications should include thermal monitoring or regulation for the torch and chamber to increase life span during long duration tests. Future testing will include a more in-depth characterization of the plasma plume using a higher precision Langmuir probe and examining different locations of the plume. Finally, the plasma source can also be modified to be re-purposed as a thermal testing bed to increase the versatility of the testing facility.