Hall thrusters operate on a variety of propellants, the most common being xenon. Other propellants of interest include krypton, argon, bismuth, magnesium, and zinc.
Hall thrusters are able to accelerate their exhaust to speeds between 10–80 km/s, with most models operating between 15-30 km/s. 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.
2 kW Hall thruster in operation as part of the Hall Thruster Experiment at the Princeton Plasma Physics Laboratory.
History
Hall thrusters were studied independently in the US and the USSR in the 1950s and '60s. However, the concept of a Hall thruster was only developed into an efficient propulsion device in the former Soviet Union, whereas in the US, scientists focused instead on developing gridded ion thrusters.
Two types of Hall thrusters were developed in the Soviet Union:
- Thrusters with wide acceleration zone, SPT (Stationary Plasma Thruster) at Design Bureau Fakel.
- Thrusters with narrow acceleration zone, TAL (Thruster with Anode Layer), at the Central Research Institute for Machine Building (TsNIIMASH).
Soviet and Russian SPT Thrusters The common SPT design was largely the work of A. I. Morozov. The first SPT to operate in space, an SPT-50 launched on the Soviet Meteor spacecraft, was launched December 1971. They were mainly used for satellite stabilization in North-South and in East-West directions. Since then until the late 1990s 118 SPT engines completed their mission and some 50 continued to be operated. Thrust of the first generation of SPT engines, SPT-50 and SPT-60 was 20 and 30 mN respectively. In 1982 SPT-70 and SPT-100 were introduced, their thrusts being 40 and 83 mN, respectively. In the post-Soviet Russia high-power (a few kilowatts) SPT-140, SPT-160, SPT-200, T-160 and low-power (less than 500 W) SPT-35 were introduced. Soviet and Russian TAL-type thrusters include the D-38,D-55, D-80,and D-100. Soviet-built thrusters were introduced to the West in 1992 after a team of electric propulsion specialists from NASA's Jet Propulsion Laboratory and Glenn Research Center, under the support of the Ballistic Missile Defense Organization, visited Soviet 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 of a Hall thruster has ever occurred on orbit. Hall thruster 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 1990's, Hall thrusters have been the subject of a large number of research efforts throughout the United States, France, Italy, Japan, and Russian (with many smaller efforts scattered in various countries across the globe). Hall thruster research in the US is conducted at several government laboratories, universities and private companies. Government centers include NASA's Jet Propulsion Laboratory, NASA's Glenn Research Center and the Air Force Research Laboratory (Edwards AFB, CA). Universities include the University of Michigan, Stanford, MIT, Princeton, Michigan Tech, and Georgia Tech. A considerable amount of development is being conducted in industry, such as Aerojet and Busek Co. in the USA, SNECMA in France and Alta in Italy. The first use of Hall thrusters outside of Earth's orbit was on the European Space Agency (ESA) lunar mission SMART-1 in 2003. Hall thrusters were first demonstrated on a western satellite 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. Aerojet has flight qualified the BPT-4000, which will first fly when the Advanced EHF military spacecraft is launched. Several countries worldwide continue efforts to qualify Hall thruster technology for commercial uses. Operation 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 a few tens of milliteslas is used to hold the electrons in place, where the combination of the magnetic field and an attraction to the anode force a fast circulating electron current around the axis of the thruster and only a slow axial drift towards the anode occurs. 
Hall Thrusters are Largely Axially Symmetric
This is a cross-section containing that axis.
A schematic of a Hall thruster is shown in the image to the right. An electric potential on the order of 300 volts is applied between the anode and cathode.
The central spike forms one pole of an electromagnet and is surrounded by an annular space and around that is the other pole of the electromagnet, with a radial magnetic field in-between.
The propellant, such as xenon gas is fed through the anode, which has numerous small holes in it to act as a gas distributor. Xenon propellant is used because of its high molecular weight and low ionization potential. As the neutral xenon atoms diffuse into the channel of the thruster, they are ionized by collisions with high energy circulating electrons (10–20 eV or 100,000 to 250,000 °C). Once ionised the xenon ions typically have a charge of +1 though a small fraction (~10%) are +2.
The xenon ions are then accelerated by the electric field between the anode and the cathode. The ions quickly reach speeds of around 15,000 m/s for a specific impulse of 1,500 seconds (15 kN·s/kg). Upon exiting however, the ions pull an equal number of electrons with them, creating a plume with no net charge.
The axial magnetic field is designed to be strong enough to substantially deflect the low-mass electrons, but not the high-mass ions which have a much larger gyroradius and are hardly impeded. The majority of electrons are thus stuck orbiting in the region of high radial magnetic field near the thruster exit plane, trapped in E×B (axial electric field and radial magnetic field). This orbital rotation of the electrons is a circulating Hall current and it is from this that the Hall thruster gets its name. Collisions and instabilities allow some of the electrons to be freed from the magnetic field and they drift towards the anode.
About 30% of the discharge current is an electron current which does not produce thrust, which limits the energetic efficiency of the thruster; the other 70% of the current is in the ions. Because the majority of electrons are trapped in the Hall current, they have a long residence time inside the thruster and are able to ionize almost all (~90%) of the xenon propellant. The ionization efficiency of the thruster is thus around 90%, while the discharge current efficiency is around 70% for a combined thruster efficiency of around 63% (= 90% × 70%).
The magnetic field thus ensures that the discharge power predominately goes into accelerating the xenon propellant and not the electrons, and the thruster turns out to be reasonably efficient.
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.
However, Hall thrusters operate at the high specific impulses that is achieved with ion thrusters. One particular advantage of Hall thrusters, as compared to an ion thruster, is that the generation and acceleration of the ions takes place in a quasi-neutral plasma and so there is no Child-Langmuir charge (space charge) saturated current limitation on the thrust density, and thus thrust is high for electrically accelerated thrusters.
Another advantage is that these thrusters can use a wider variety of propellants supplied to the anode, even oxygen, although something easily ionised is needed at the cathode. One propellant that is starting to be used is liquid bismuth due to its low cost, high atomic mass and low partial pressure.
Applications
Hall thrusters have been flying in space since December 1971 when the Soviets launched an SPT-50 on the Meteor satellite. Over 240 thrusters have flown in space since that time with a 100% success rate. Hall thrusters are now routinely flown on commercial GEO communications satellite where they are used for orbit insertion and stationkeeping.
On October 23, 1998, the first Hall thruster to fly on a western satellite was the Russian D-55 built by TsNIIMASH on the NRO's STEX spacecraft. On September 28, 2003, the first Hall thruster used outside of geosynchronous Earth orbit began as the European Space Agency's SMART-1 spacecraft started its journey to the moon using a Snecma PPS-1350.
The solar electric propulsion system of the European Space Agency's SMART-1 spacecraft used a Snecma PPS-1350-G Hall thruster. SMART-1 was a technology demonstration mission that orbited the moon. The use of the PPS-1350-G was the first use of a Hall thruster outside of geosynchronous earth orbit (GEO). Unlike most Hall thruster propulsion systems used in commercial applications, the Hall thruster on SMART-1 could be throttled over a range of power, specific impulse, and thrust:
- Discharge Power: 0.46-1.19 kW
- Specific Impulse: 1100-1600 s
- Thrust: 30-70 mN
- Thruster operating time: 5000 h
- Xenon throughput: 82 kg
- Total Impulse: 1.1 MN-s
- Total ΔV: 3.9 km/s
- Thruster operating time: 10,500 h
- Total impulse: 3.39 MN-s
- Start/Stop Cycles: 7309
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