<<In spacecraft propulsion, a Hall-effect thruster (HET) is a type of ion thruster in which the propellant is accelerated by an electric field. Hall-effect thrusters trap electrons in a magnetic field and then use the electrons to ionize propellant, efficiently accelerate the ions to produce thrust, and neutralize the ions in the plume. Hall-effect thrusters (based on the discovery by Edwin Hall) are sometimes referred to as Hall thrusters or Hall-current thrusters. Hall thrusters are often regarded as a moderate specific impulse
(1,600 s = 15.69 km/s) space propulsion technology. Hall thrusters were studied independently in the United States and the Soviet Union. They were first described publicly in the US in the early 1960s. However, the Hall thruster was first developed into an efficient propulsion device in the Soviet Union. In the US, scientists focused instead on developing gridded ion thrusters.
The essential working principle of the Hall thruster is that it uses an electrostatic potential to accelerate ions up to high speeds. In a Hall thruster, the attractive negative charge is provided by an electron plasma at the open end of the thruster instead of a grid. A radial magnetic field of about 100–300 G (0.01–0.03 T) is used to confine the electrons, where the combination of the radial magnetic field and axial electric field cause the electrons to drift in azimuth thus forming the Hall current from which the device gets its name.
Compared to chemical rockets, the thrust is very small, on the order of 83 mN for a typical thruster operating at 300 V, 1.5 kW. For comparison, the weight of a coin like the U.S. quarter or a 20-cent Euro coin is approximately 60 mN. As with all forms of electrically powered spacecraft propulsion, thrust is limited by available power, efficiency, and specific impulse.
However, Hall thrusters operate at the high specific impulses that is typical for electric propulsion. One particular advantage of Hall thrusters, as compared to a gridded ion thruster, is that the generation and acceleration of the ions takes place in a quasi-neutral plasma, so there is no Child-Langmuir charge (space charge) saturated current limitation on the thrust density. This allows much smaller thrusters compared to gridded ion thrusters.
Another advantage is that these thrusters can use a wider variety of propellants supplied to the anode, even oxygen, although something easily ionized is needed at the cathode. Hall thrusters operate on a variety of propellants, the most common being xenon. Other propellants of interest include krypton, argon, bismuth, iodine, magnesium, and zinc.
Hall thrusters are able to accelerate their exhaust to speeds between 10 and 80 km/s (1,000–8,000 s specific impulse), with most models operating between 15 and 30 km/s (1,500–3,000 s specific impulse). The thrust produced by a Hall thruster varies depending on the power level. Devices operating at 1.35 kW produce about 83 mN of thrust. High-power models have demonstrated up to 3 N in the laboratory. Power levels up to 100 kW have been demonstrated by xenon Hall thrusters.
Two types of Hall thrusters were developed in the Soviet Union:
- thrusters with wide acceleration zone, SPT (Russian: СПД, стационарный плазменный двигатель; English: SPT, Stationary Plasma Thruster) at Design Bureau Fakel
thrusters with narrow acceleration zone, DAS (Russian: ДАС, двигатель с анодным слоем; English: TAL, Thruster with Anode Layer), at the Central Research Institute for Machine Building (TsNIIMASH).
Soviet-built thrusters were introduced to the West in 1992 after a team of electric propulsion specialists from NASA's Jet Propulsion Laboratory, Glenn Research Center, and the Air Force Research Laboratory, under the support of the Ballistic Missile Defense Organization, visited Russian laboratories and experimentally evaluated the SPT-100 (i.e., a 100 mm diameter SPT thruster). Over 200 Hall thrusters have been flown on Soviet/Russian satellites in the past thirty years. No failures have ever occurred on orbit. Hall thrusters continue to be used on Russian spacecraft and have also flown on European and American spacecraft. Space Systems/Loral, an American commercial satellite manufacturer, now flies Fakel SPT-100's on their GEO communications spacecraft.
Since their introduction to the west in the early 1990s, Hall thrusters have been the subject of a large number of research efforts throughout the United States, France, Italy, Japan, and Russia (with many smaller efforts scattered in various countries across the globe). The first use of Hall thrusters on lunar orbit was the European Space Agency (ESA) lunar mission SMART-1 in 2003.
On a western satellite Hall thrusters were first demonstrated on the Naval Research Laboratory (NRL) STEX spacecraft, which flew the Russian D-55. The first American Hall thruster to fly in space was the Busek BHT-200 on TacSat-2 technology demonstration spacecraft. The first flight of an American Hall thruster on an operational mission, was the Aerojet BPT-4000, which launched August 2010 on the military Advanced Extremely High Frequency GEO communications satellite. At 4.5 kW, the BPT-4000 is also the highest power Hall thruster ever flown in space. Besides the usual stationkeeping tasks, the BPT-4000 is also providing orbit raising capability to the spacecraft. Several countries worldwide continue efforts to qualify Hall thruster technology for commercial uses.>>