Case Studies

Transit-Time Oscillations Inside a Hall Effect Thruster

Tecplot is used to animate simulated transit-time oscillations inside the discharge channel of a Hall Effect thruster onboard an Air Force satellite. Oscillations can reduce the overall efficiency of the thruster and decrease its life. This movie represents the first successful 2-D simulation of this detrimental phenomena.

By quickly visualizing the highly-nonlinear oscillations, Dr. Fife's team can rapidly focus on parameters of interest, and immediately see their spatial extent. The movie also sheds light on the physics, and helps explain why certain fixes dampen the oscillation effect. Sophisticated 2-D animations like this are only possible using Tecplot's versatile suite of plotting and data visualization tools.

The Engineer
John Michael Fife, Ph.D., is a research scientist and group leader of the Electric Propulsion Laboratory at the U.S. Air Force Research Laboratory (AFRL). AFRL develops technology for future Air Force missions.

Dr. Fife's group specifically develops advanced maneuvering thrusters used onboard Air Force satellites called Electric Propulsion (EP). EP thrusters use electric power, typically from solar arrays, to boost rocket efficiency to extremely high levels (five times or more efficient than chemical rockets). Some examples of EP are Hall Effect thrusters, Ion Thrusters, Pulsed Plasma Thrusters, and Magnetoplasmadynamic Thrusters.

Hall Effect Thrusters
A Hall Effect thruster is an electric rocket engine which accelerates ions in crossed electric and magnetic fields in a quasi-neutral plasma. Hall Effect satellite thruster performance is important to the reduction of the overall size and electrical power needs of an orbiting spacecraft or satellite.

High-powered Hall Effect thrusters give satellites greater maneuverability and longer lives. In the future, this will be required for efficient orbit maintenance and repositioning of orbiting spacecraft. However, long-duration use of Hall thrusters is currently limited by wear on thruster components, especially near the plasma acceleration zone. Many organizations are currently focusing research and development efforts on minimizing component wear induced by accelerating plasma. When accomplished, it will dramatically increase the lifetime of the propulsion system.

Satellites using Hall Effect thrusters are five times as effective as their chemical counterparts. This translates into five times more maneuverability for the spacecraft for the same amount of propellant. The down side is that the thrust of EP systems is limited by how much electric power is available. For typical spacecraft, this means that EP systems may have only fractions of a pound of thrust, but they are designed to run for years (versus minutes for chemical rockets) at low thrust to accomplish the same goal.

The Plot and Simulation
The movie represents the first successful 2-D simulation of a common detrimental Hall Effect thruster oscillation first observed in the 1960s. Transit-time oscillations were initially a very big problem to Hall Effect thruster designers. They reduced the overall efficiency, complicated the power processing system, and decreased its life due to premature erosion of the insulator from ion bombardment.

Although the transit-time oscillation phenomenon was, and still is, not very well understood, a simple fix was discovered by Russian scientists through a combination of analysis and trial and error. This simple fix involved reducing the magnetic field strength near the anode (on the left side of the simulation) using a pair of magnetic screens.

The data used to create the movie was generated using a code named HPHall, which is a hybrid Particle-In-Cell code developed jointly by AFRL and the Massachusetts Institute of Technology. Dr. Fife modified HPHall to export simulation time-steps readable by Tecplot. He then wrote a Tecplot macro to load the time-steps sequentially into a Flash movie.

   

The difference in the pulsing dense plasma is obvious when you compare a simulation with magnetic screens versus one without.

Tecplot
Dr. Fife and his group typically use Tecplot to generate all of their scientific plots. This consists mostly of plasma physics phenomena in both 2- and 3-D. He feels Tecplot excels at plotting multiple data sets simultaneously, generating 3-D surface plots, and creating contours with continuous color ranges.

Dr. Fife adds, "Tecplot allows us to explore many types of XY, 2- and 3-D data to look for consistency between computational model results, and what we expect from physics."

Tecplot is used to visualize the results for a 4.5 kW Hall Effect thruster plume test case.

Tecplot is used to display a Hall Effect thruster and plume diagnostic instrumentation inside a laboratory vacuum chamber. The left image shows a plasma density slice, the middle image displays the surface sputtering rate, and the right image shows redeposition rate of aluminum.

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