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The propulsion of choice for science fiction writers has become the propulsion of choice for scientists and engineers at NASA. The Hall thruster, which uses gases like xenon and krypton as propellants, can enable modern spacecraft to travel farther, faster, and cheaper than chemical propulsion technologies. Hall thrusters are currently used for stationkeeping, orbit raising, and life extension on satellites and for main propulsion on deep space probes. Hall thrusters are a gridless type of ion thruster that expels ions to create thrust. They provide higher thrust than other types of ion propulsion.

What Is an Ion?
An ion is simply an atom or molecule that is electrically charged. Ionization is the process of electrically charging an atom or molecule by adding or removing electrons. Ions can be positive (when they lose one or more electrons) or negative (when they gain one or more electrons). A gas is considered ionized when some or all the atoms or molecules contained in it are converted into ions. Plasma is an electrically neutral gas in which all positive and negative charges from neutral atoms, negatively charged electrons, and positively charged ions add up to zero. Plasma exists everywhere in nature; it is designated as the fourth state of matter (the others are solid, liquid, and gas). It has some of the properties of a gas but is affected by electric and magnetic fields and is a good conductor of electricity. Plasma is the building block for all types of electric propulsion, where electric and/or magnetic fields are used to push on the electrically charged ions and electrons to provide thrust. Examples of plasmas seen every day are lightning and fluorescent light bulbs. Hall thrusters use a method of ionizing propellant atoms called electron bombardment. When a high-energy electron (negative charge) collides with a propellant atom (neutral charge), a second electron is released, yielding two negative electrons and one positive ion. The ionization process in a xenon Hall thruster is shown below:

e- + Xe0=>Xe+ + 2e-

Hall Thruster Operation
Hall thrusters use a hollow cathode located on the downstream perimeter of the thruster to generate electrons. The anode (or channel) of the Hall thruster (shown in the diagram at the bottom of the last page) is charged to a high positive potential by the thruster's power supply. The electrons are attracted to the channel walls and accelerate in the upstream direction. As the electrons move toward the channel, they encounter a magnetic field generated by the thruster's powerful electromagnets. The high-strength magnetic field traps the electrons, causing them to form into a circling ring at the downstream end of the thruster channel. The Hall thruster derives its name from this flow of electrons, called the Hall current (after Dr. Edwin Hall). The propellant, usually an inert gas such as xenon or krypton, is injected into the thrusterís channel. Since Hall thrusters use inert gas for propellant, there are no explosion risks as there are with chemical rockets. Some of the trapped electrons in the channel collide with the propellant atoms, creating ions.

When the propellant ions are generated, they experience the electric field produced between the channel (positive) and the ring of electrons (negative) and accelerate out of the thruster, creating an ion beam. The thrust is generated from the force that the ions impart to the electron cloud. This force is transferred to the magnetic field, which, in turn, is transmitted to the magnetic circuit of the thruster. The electrons are highly mobile and attracted to the ions in the beam, causing an equal amount of electrons and ions to leave the thruster at the same time. This keeps the electrical charge of the thruster neutral.

Hall thrusters are generally classified in two groups: the stationary plasma thruster (SPT) and the thruster with anode layer (TAL). The major difference between them is that the TAL has a metallic channel wall, while the SPTís channel wall is coated with a ceramic material.

The Electric Propulsion System
The electric propulsion system consists of five main parts: the power source, power processing unit (PPU), propellant management system (PMS), the control computer, and the Hall thruster. The power source can be any source of electrical power, but solar and nuclear are the primary options. A solar electric propulsion system (SEP) uses sunlight and solar cells for power generation. A nuclear electric propulsion system (NEP) uses a nuclear heat source coupled to an electric generator. The PPU converts the electrical power generated by the power source into the power required by each component of the Hall thruster. It generates the high voltages required by the Hall thruster channel and the high currents required for the hollow cathode. The PMS controls the propellant flow from the propellant tank to the thruster and hollow cathode. Modern PMS units have evolved to a level of sophisticated design that no longer requires moving parts. The control computer controls and monitors system performance. The Hall thruster then processes the propellant and power to perform work.

Modern Hall thrusters are capable of propelling a spacecraft up to 50,000 meters per second (about 112,000 miles per hour (mph)). To put that into perspective, the space shuttle is capable of a top speed of around 18,000 mph. The tradeoff for this high top speed is low thrust (or low acceleration). Thrust is the force that the thruster applies to the spacecraft. Modern Hall thrusters can deliver up to 3 Newtons (0.7 pounds) of thrust, which is equivalent to the force you would feel by holding 54 U.S. quarters in your hand. To compensate for low thrust, the Hall thruster must be operated for a long time for the spacecraft to reach its top speed.

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Last Updated: 10/02/07