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Hall effect thruster

Hall effect thruster

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In spacecraft propulsion
Spacecraft propulsion
Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. There are many different methods. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. However, most spacecraft today are propelled by forcing a gas from the...

, a Hall thruster is a type of ion thruster
Ion thruster
An ion thruster is a form of electric propulsion used for spacecraft propulsion that creates thrust by accelerating ions. Ion thrusters are categorized by how they accelerate the ions, using either electrostatic or electromagnetic force. Electrostatic ion thrusters use the Coulomb force and...

 in which the propellant
Propellant
A propellant is a material that produces pressurized gas that:* can be directed through a nozzle, thereby producing thrust ;...

 is accelerated by an electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...

. Hall thrusters trap electrons in a magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

 and then use the electrons to ionize propellant, efficiently accelerate the ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

s to produce thrust
Thrust
Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system....

, and neutralize the ions in the plume. Hall thrusters are sometimes referred to as Hall Effect
Hall effect
The Hall effect is the production of a voltage difference across an electrical conductor, transverse to an electric current in the conductor and a magnetic field perpendicular to the current...

 Thrusters
or Hall Current Thrusters. Hall thrusters are often regarded as a moderate specific impulse
Specific impulse
Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...

 (1600 s) space propulsion technology, the Hall effect thruster has benefited from considerable theoretical and experimental research since the 1960’s.
Hall thrusters operate on a variety of propellants, the most common being xenon
Xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...

. Other propellants of interest include krypton
Krypton
Krypton is a chemical element with the symbol Kr and atomic number 36. It is a member of Group 18 and Period 4 elements. A colorless, odorless, tasteless noble gas, krypton occurs in trace amounts in the atmosphere, is isolated by fractionally distilling liquified air, and is often used with other...

, argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...

, bismuth
Bismuth
Bismuth is a chemical element with symbol Bi and atomic number 83. Bismuth, a trivalent poor metal, chemically resembles arsenic and antimony. Elemental bismuth may occur naturally uncombined, although its sulfide and oxide form important commercial ores. The free element is 86% as dense as lead...

, magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...

, and zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...

.

Hall thrusters are able to accelerate their exhaust to speeds between 10–80 km/s (1000-8000 s specific impulse
Specific impulse
Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...

), with most models operating between 15–30 km/s (1500-3000 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.

History


Hall thrusters were studied independently in the US and the USSR in the 1950s and 1960s. However, the 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 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).



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
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...

 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
Jet Propulsion Laboratory
Jet Propulsion Laboratory is a federally funded research and development center and NASA field center located in the San Gabriel Valley area of Los Angeles County, California, United States. The facility is headquartered in the city of Pasadena on the border of La Cañada Flintridge and Pasadena...

, Glenn Research Center
Glenn Research Center
NASA John H. Glenn Research Center at Lewis Field is a NASA center, located within the cities of Brook Park, Cleveland and Fairview Park, Ohio between Cleveland Hopkins International Airport and the Cleveland Metroparks's Rocky River Reservation, and has other subsidiary facilities in Ohio...

, and the Air Force Research Laboratory
Air Force Research Laboratory
The Air Force Research Laboratory is a scientific research organization operated by the United States Air Force Materiel Command dedicated to leading the discovery, development, and integration of affordable aerospace warfighting technologies; planning and executing the Air Force science and...

, under the support of the Ballistic Missile Defense Organization
Ballistic Missile Defense Organization
The Ballistic Missile Defense Organization was an agency of the United States Department of Defense that began on 20 May 1974 with the responsibility for all U.S. ballistic missile defense efforts. It evolved from the SAFEGUARD System Organization. The original mission of BMDO was comparable to...

, 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
Space Systems/Loral
Space Systems/Loral , of Palo Alto, California, is the wholly owned manufacturing subsidiary of Loral Space & Communications. It was acquired in 1990 for $715 million by Loral Corp. from Ford Motor Company as the Space Systems Division of Ford Aerospace...

, 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). Hall thruster research in the US is conducted at several government laboratories, universities and private companies. Government centers include NASA's Jet Propulsion Laboratory
Jet Propulsion Laboratory
Jet Propulsion Laboratory is a federally funded research and development center and NASA field center located in the San Gabriel Valley area of Los Angeles County, California, United States. The facility is headquartered in the city of Pasadena on the border of La Cañada Flintridge and Pasadena...

, NASA's Glenn Research Center
Glenn Research Center
NASA John H. Glenn Research Center at Lewis Field is a NASA center, located within the cities of Brook Park, Cleveland and Fairview Park, Ohio between Cleveland Hopkins International Airport and the Cleveland Metroparks's Rocky River Reservation, and has other subsidiary facilities in Ohio...

 and the Air Force Research Laboratory
Air Force Research Laboratory
The Air Force Research Laboratory is a scientific research organization operated by the United States Air Force Materiel Command dedicated to leading the discovery, development, and integration of affordable aerospace warfighting technologies; planning and executing the Air Force science and...

 (Edwards AFB, CA). Universities include the US Air Force Institute of Technology, University of Michigan
University of Michigan
The University of Michigan is a public research university located in Ann Arbor, Michigan in the United States. It is the state's oldest university and the flagship campus of 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
Aerojet
Aerojet is an American rocket and missile propulsion manufacturer based primarily in Rancho Cordova, California with divisions in Redmond, Washington, Orange, Gainesville and Camden, Arkansas. Aerojet is owned by GenCorp. They are the only US propulsion company that provides both solid rocket...

 and Busek Co. in the USA, SNECMA
Snecma
Snecma is a major French manufacturer of engines for commercial and military aircraft, and for space vehicles. The name is an acronym for Société Nationale d'Étude et de Construction de Moteurs d'Aviation .In 2005, the Snecma group, which included 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
SMART-1
SMART-1 was a Swedish-designed European Space Agency satellite that orbited around the Moon. It was launched on September 27, 2003 at 23:14 UTC from the Guiana Space Centre in Kourou, French Guiana. "SMART" stands for Small Missions for Advanced Research in Technology...

 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
TacSat-2
TacSat-2 is an experimental satellite built by the USAF's Air Force Research Laboratory with an operational life expected to be not more than one year as part of the 'Advanced Concept Technology Demonstration' program.-Purpose:The TacSat series of experimental spacecraft are designed to allow...

 technology demonstration spacecraft. The first flight of an American Hall thruster on an operational mission, was the Aerojet
Aerojet
Aerojet is an American rocket and missile propulsion manufacturer based primarily in Rancho Cordova, California with divisions in Redmond, Washington, Orange, Gainesville and Camden, Arkansas. Aerojet is owned by GenCorp. They are the only US propulsion company that provides both solid rocket...

 BPT-4000, which launched August 2010 on the military Advanced Extremely High Frequency
Advanced Extremely High Frequency
Advanced Extremely High Frequency is a series of communications satellites operated by the United States Air Force Air Force Space Command. The spacecraft will be used to relay secure communications for the Armed Forces of the United States, the British Armed Forces, the Canadian Forces and the...

 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.

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 hundred gauss
Gauss (unit)
The gauss, abbreviated as G, is the cgs unit of measurement of a magnetic field B , named after the German mathematician and physicist Carl Friedrich Gauss. One gauss is defined as one maxwell per square centimeter; it equals 1 tesla...

 (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 azimuthally, forming the Hall current from which the device gets its name.


A schematic of a Hall thruster is shown in the image to the right. An electric potential between 150 and 800 volts is applied between the anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....

 and cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...

.

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
Xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...

 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
Ionization potential
The ionization energy of a chemical species, i.e. an atom or molecule, is the energy required to remove an electron from the species to a practically infinite distance. Large atoms or molecules have a low ionization energy, while small molecules tend to have higher ionization energies.The property...

. As the neutral xenon atoms diffuse into the channel of the thruster, they are ionized by collisions with high energy circulating electrons (typically 10–40 eV, or about 10% of the discharge voltage). Once ionized, the xenon ions typically have a charge of +1, though a small fraction (~20%) are +2.

The xenon ions are then accelerated by the electric field between the anode and the cathode. For discharge voltages of 300 V, the ions 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 radial 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
Hall effect
The Hall effect is the production of a voltage difference across an electrical conductor, transverse to an electric current in the conductor and a magnetic field perpendicular to the current...

, and it is from this that the Hall thruster gets its name. Collisions with other particles and walls, as well as plasma instabilities, allow some of the electrons to be freed from the magnetic field, and they drift towards the anode.

About 20–30% of the discharge current is an electron current, which does not produce thrust, which limits the energetic efficiency of the thruster; the other 70–80% 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 of the xenon propellant, allowing for mass utilizations of 90–99%. The mass utilization 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%). Modern Hall thrusters have achieved efficiencies as high as 75% through advanced designs.

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 impulse
Specific impulse
Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...

s that is typical of 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 and so there is no Child-Langmuir charge (space charge)
Space charge
Space charge is a concept in which excess electric charge is treated as a continuum of charge distributed over a region of space rather than distinct point-like charges...

 saturated current
Saturation current
Saturation current is a term used in relation to semiconductor diodes. It is more fully, and accurately, named reverse saturation current and is "part of the reverse current in a diode caused by diffusion of minority carriers from the neutral regions to the depletion region...

 limitation on the thrust density. This allows for 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.

Cylindrical Hall Thrusters


Although conventional (annular) Hall thrusters are efficient in the kilowatt power regime, they become inefficient when scaled to small sizes. This is due to the difficulties associated with holding the performance scaling parameters constant while decreasing the channel size and increasing the applied magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

 strength. This led to the design of the cylindrical Hall Thruster. The cylindrical Hall thruster can be more readily scaled to smaller sizes due to its nonconventional discharge-chamber geometry and associated magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

 profile. The cylindrical Hall thruster more readily lends itself to miniaturization and low-power operation than a conventional (annular) Hall thruster. The primary reason for cylindrical Hall thrusters is that it is difficult to achieve a regular Hall thruster that operates over a broad envelope from ~1 kW down to ~100 W while maintaining an efficiency of 45-55%.

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.

The first Hall thruster to fly on a western satellite was a Russian D-55 built by TsNIIMASH, on the NRO's STEX
STEX
STEX was an experimental NRO satellite built by Lockheed Martin. It was launched on October 3, 1998. One of the experiments was ATEx , which was deployed on 22. January 1999, and subsequently jettisoned.- External links :* at Gunter's Space Page...

 spacecraft, launched on October 3, 1998.

The solar electric propulsion system of the European Space Agency
European Space Agency
The European Space Agency , established in 1975, is an intergovernmental organisation dedicated to the exploration of space, currently with 18 member states...

's SMART-1
SMART-1
SMART-1 was a Swedish-designed European Space Agency satellite that orbited around the Moon. It was launched on September 27, 2003 at 23:14 UTC from the Guiana Space Centre in Kourou, French Guiana. "SMART" stands for Small Missions for Advanced Research in Technology...

 spacecraft used a Snecma PPS-1350
Pps-1350
PPS-1350 is a Hall effect thruster, a kind of ion propulsion system for spacecraft. It was used in the SMART-1 mission to the moon. It creates a stream of electrically charged ions. The PPS-1350 was created by Snecma, a French aerospace firm, under license from Fakel, who designed the SPT-100, on...

-G Hall thruster. SMART-1 was a technology demonstration mission that orbited the moon. This use of the PPS-1350-G, starting on September 28, 2003, 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


In 2005, SMART-1 exhausted its xenon supply after flawlessly operating the thruster and establishing new records for Hall thruster operation in space
  • Thruster operating time: 5000 h
  • Xenon throughput: 82 kg
  • Total Impulse: 1.1 MN-s
  • Total ΔV: 3.9 km/s


In parallel to the flight demonstration, a qualification model (QM) PPS-1350-G has also undergone wear testing on the ground. Through 2007, the QM model has demonstrated:
  • Thruster operating time: 10,500 h
  • Total impulse: 3.39 MN-s
  • Start/Stop Cycles: 7309

External links