Plasma Detachment from Magnetic Nozzles
Magnetic nozzles consist of a converging-diverging magnetic field arrangement meant to supersonically accelerate a plasma. Since plasma streamlines tend to follow magnetic field lines, it can cool by expansion throughout the nozzle by a mechanism similar to that of a de Laval nozzle. This expansion converts the thermal energy of the electrons into bulk ion motion. Since magnetic nozzles do not require any electrodes, they are capable of performing on extended missions. They are also able to use a variety of propellants, potentially allowing in-flight refueling. While nozzles are being investigated for high-power applications, at PEPL we focus primarily on lower-power devices, ideal for small satellites or CubeSats.
Before magnetic nozzles can be reliably flown in space, however, the problem of detachment must be better understood. As the magnetic field lines of the nozzle will inevitably return to the thruster, the plasma may follow the lines back, negating thrust. We generally assume that ions are unmagnetized, having too high inertia to be affected by the magnetic fields directly. However, electrons may remain attached, expanding with the magnetic fields and generating radial electric fields that will cause the ions to appear attached as well. We still have no reliable way of predicting the detachment point in these devices, but multiple theories have been proposed. These include resistive detachment, electron inertia effects, electron demagnetization, magnetic field line stretching, and the onset of instabilities.
The Magnetic Detachment eXperiment (MDX) is a flexible testbed device designed to experimentally investigate the physics driving detachment and develop a consistent model over a wide operating parameter space. Xenon is flowed through a quartz liner roughly an inch in diameter and ionized from a radio-frequency antenna surrounding the liner. A 147-turn wire coil carries currents of up to 30 A to generate the magnetic nozzle.
We are investigating electron detachment in MDX. The first project currently using MDX is investigating electron-neutral collisions as a source for detachment within and just outside of the liner. Tim Collard is making novel use of Laser-Induced Fluorescence to track ion streamlines and determine where detachment occurs. .He is also implementing a suite of electrostatic probes to map plasma parameters and determine collision frequnecies throughout the plume with the goal of expanding his classical detachment theory. While Tim is finishing his thesis work, Shad is beginning his endeavors into the question of detachment. He is focusing more on the downstream detachment point, determining where the plasma will cut itself off from the magnetic field for good. He is investigating the onset of plasma instabilities and how they might cause electrons to rejoin ions.
Kinetic Method for Quasi-One-Dimensional Simulation of Magnetic Nozzle Plasmadynamics