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Plasma-based thrusters can reduce the cost of many satellites and space missions by reducing the mass of propellant that must be carried on-board. In a chemical rocket, the energy to accelerate the propellant comes from chemical reactions, limiting the attainable exhaust velocity to about 4 km/s. In an electrostatic thruster, the propellant gas is first ionized, and then accelerated across an electric potential, which is set-up by biasing a configuration of grids relative to the chamber wall, to velocities around 100 km/s. Since the required power is the product of thrust and exit velocity, for a fixed power supply, the thrust produced by this type of thruster is quite small. Also, the maximum thrust density is fundamentally limited by the ion space charge, since the accelerated ions repel one-another.

The Hall thruster overcomes this limitation by replacing the grid system with a radial magnetic field, where field lines form approximate equipotential surfaces. The field traps electrons in azimuthal ExB orbits inside the thruster channel, neutralizing the ion space charge and allowing the Hall thruster to produce a much higher thrust density. With a typical propellant velocity around 20 km/s, and thrust from 10-1000 mN, Hall thrusters are attractive for orbit station-keeping, orbit insertion, and inner-planetary missions.

An active area of Hall thruster research is the development of efficient low-power miniaturized thrusters for micro-spacecraft applications. The annular design of a conventional Hall thruster has a high surface-to-volume ratio, which results in large wall losses when miniaturized. In addition, miniaturization renders it impossible to maintain the optimal magnetic field profile.

The Hall thruster group at Princeton has developed a cylindrical Hall thruster with a shortened center magnetic pole to overcome these challenges. Ionization takes place in the short annular region, while acceleration occurs in the cylindrical region with a lower surface-to-volume ratio. The magnetic field topology is fundamentally different: a hybrid magneto-electrostatic trap confines electrons axially as they undergo azimuthal orbits. Ions are accelerated away from the walls, reducing wall losses, but unfortunately also increasing the divergence of the plasma plume. Recent experimental results have shown that plume divergence can be significantly reduced by increasing electron emission from the cathode neutralizer. The focus of my experimental project was on understanding the coupling of the cathode neutralizer to the performance of the thruster discharge.

Hall thrusters also serve as a practical device for basic plasma physics research. As with high temperature fusion plasmas, electron transport across magnetic field lines in the Hall thruster cannot be explained by classical collisions but is largely driven by fluctuations in the plasma potential and density. This ''anomalous'' transport is especially dominant in the CHT. Understanding the mechanisms that cause turbulent oscillations will ultimately enable the design of better devices for propulsion, plasma confinement, and other applications.

2007 Student Seminar presentation pdf.

2007 APS Division of Plasma Physics poster pdf.

"Cathode effects in cylindrical Hall thrusters", J. Appl. Phys. 104, 103302 (2008); pdf.

PPPL Hall Thruster group webpage http://htx.pppl.gov/