Electrostatic ion thruster
Encyclopedia
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...
s (a highly-efficient low-thrust 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...
running on electrical power). These designs use high voltage electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
s in order to accelerate 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 with electrostatic forces.
History
The ion engine was first demonstrated by German-born NASANASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
scientist Ernst Stuhlinger
Ernst Stuhlinger
Ernst Stuhlinger was a German-born American atomic, electrical and rocket scientist. After being brought to the United States as part of Operation Paperclip, he developed guidance systems with Wernher von Braun's team at the US Army, and later, NASA...
, and developed in practical form by Harold R. Kaufman
Harold R. Kaufman
Harold R. Kaufman is an American physicist, noted for his development of electrostatic ion thrusters for NASA during the 1950s and '60s...
at NASA Lewis (now Glenn) Research Center from 1957 to the early 1960s.
The use of ion propulsion systems were first demonstrated in space by the NASA Lewis "Space Electric Rocket Test" (SERT) I and II. These thruster used mercury as the reaction mass. The first was SERT-1, launched July 20, 1964, successfully proved that the technology operated as predicted in space. The second test, SERT-II, launched on February 3, 1970, verified the operation of two mercury ion engines for thousands of running hours. Despite the demonstration in the 1960s and 70s, though, they were rarely used before the late 1990s
NASA Glenn continued to develop electrostatic ion thrusters through the 1980s, developing the NSTAR engine, that was used successfully on the Deep Space 1
Deep Space 1
Deep Space 1 is a spacecraft of the NASA New Millennium Program dedicated to testing a payload of advanced, high risk technologies....
probe, the first mission to fly an interplanetary trajectory using electric propulsion as the primary propulsion. It is currently flying the Dawn asteroid mission. Hughes Aircraft Company
Hughes Aircraft
Hughes Aircraft Company was a major American aerospace and defense contractor founded in 1932 by Howard Hughes in Culver City, California as a division of Hughes Tool Company...
(now L-3 ETI) has developed the XIPS (Xenon Ion Propulsion System) for performing station keeping on its geosynchronous satellites (more than 100 engines flying). NASA is currently working on a 20-50 kW electrostatic ion thruster called HiPEP which will have higher efficiency, 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 ,...
, and a longer lifetime than NSTAR. 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...
has recently completed testing of a prototype NEXT ion thruster.
At Giessen University and EADS the radio-frequency ion thrusters RIT were developed starting in the 1970s. RIT-10 engines are flying on ARTEMIS
Artemis
Artemis was one of the most widely venerated of the Ancient Greek deities. Her Roman equivalent is Diana. Some scholars believe that the name and indeed the goddess herself was originally pre-Greek. Homer refers to her as Artemis Agrotera, Potnia Theron: "Artemis of the wildland, Mistress of Animals"...
. Qinetiq (UK) has developed the T5 and T6 engines (Kaufman type), flying the GOCE
Goce
Goce is an opera composed by Kiril Makedonski in tribute to Gotse Delchev. The work was commissioned to be the very first opera performed by the Macedonian National Opera Company. It premiered on May 24, 1954 and it is the first opera to be written in the Macedonian language....
mission (T5) and is baselined for BepiColombo
BepiColombo
BepiColombo is a joint mission of the European Space Agency and the Japan Aerospace Exploration Agency to the planet Mercury, due to launch in 2014. The mission is still in the planning stages so changes to the current description are likely over the next few years...
mission (T6). In Japan, microwave engines µ10 flew on the Hayabusa
Hayabusa
was an unmanned spacecraft developed by the Japan Aerospace Exploration Agency to return a sample of material from a small near-Earth asteroid named 25143 Itokawa to Earth for further analysis....
mission.
Method of operation
- Propellant atoms are injected into the discharge chamber and are ionized by electron bombardment, forming a plasma. There are several ways of producing the energetic electrons for the discharge: (1) The electrons are emitted from a hollow cathodeElectron gunAn electron gun is an electrical component that produces an electron beam that has a precise kinetic energy and is most often used in television sets and computer displays which use cathode ray tube technology, as well as in other instruments, such as electron microscopes and particle...
and are accelerated on their way to the anode (Kaufman type ion thruster). (2) The electrons can be accelerated by the oscillating electric field induced by an alternating magnetic field of a coil, which results in a self-sustaining discharge and omits any cathode (radiofrequency ion thruster). (3) Microwave heating - The positively charged ions move towards the extraction system (2 or 3 multi-aperture grids) of the chamber due to diffusion. Once ions enter the plasma sheath at a grid hole they will be accelerated by the potential difference between the first (screen) and the second (accelerator) grid of the extraction system. The ions are ion-optically focused by the rather large electric field to pass through the extraction holes. The final ion energy is determined by the potential of the plasma (the plasma potential is a few volts larger than the screen grid voltage).
- The negative voltage of the accelerator grid prevents electrons of the beam plasma outside the thruster from streaming back to the discharge plasma. Electron backstreaming occurs if the potential within the grid is not sufficiently negative, this can mark the end-of-life of the ion thruster. By increasing the negative voltage electron backstreaming can be avoided.
- The expelled ions propel the spacecraft in the opposite direction according to Newton's 3rd law.
- Electrons are emitted from a separate cathode placed near the ion beam, called the neutralizer, towards the ion beam to ensure that equal amounts of positive and negative charge are ejected. Neutralizing is needed to prevent the spacecraft from gaining a net negative charge.
Performance
The ion optics are constantly bombarded by a small amount of secondary ions and erode or wear away, thus reducing engine efficiency and life. Ion engines need to be able to run efficiently and continuously for years. Several techniques were used to reduce erosion; most notable was switching to a different propellant. MercuryMercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
or caesium
Caesium
Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C , which makes it one of only five elemental metals that are liquid at room temperature...
atoms were used as propellants during tests in the 1960s and 1970s, but these propellants adhered to, and eroded the grids. 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...
atoms, on the other hand, are far less corrosive, and became the propellant of choice for virtually all ion thruster types. NASA has demonstrated continuous operation of NSTAR engines for over 16,000 hours (1.8 years), and tests are still ongoing for double this lifetime. Electrostatic ion thrusters have also achieved a 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 ,...
of 30-100 kN·s/kg, better than most other ion thruster types. Electrostatic ion thrusters have accelerated ions to speeds reaching 100 km/s.
In January 2006, 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...
, together with the Australian National University
Australian National University
The Australian National University is a teaching and research university located in the Australian capital, Canberra.As of 2009, the ANU employs 3,945 administrative staff who teach approximately 10,000 undergraduates, and 7,500 postgraduate students...
, have announced successful testing of an improved electrostatic ion engine, the Dual-Stage 4-Grid
Dual-Stage 4-Grid
Dual-Stage 4-Grid is an electrostatic ion thruster currently under development by the European Space Agency, in collaboration with the Australian National University....
(DS4G), that showed exhaust speeds of 210 km/s, reportedly four times higher than previously achieved, allowing for a specific impulse which is four times higher. Conventional electrostatic ion thrusters possess only two grids, one high voltage and one low voltage, which perform both the ion extraction and acceleration functions. However, when the charge differential between these grids reaches around 5 kV, some of the particles extracted from the chamber collide with the low voltage grid, eroding it and compromising the engine's longevity. This limitation is successfully bypassed when two pairs of grids are used. The first pair operates at high voltage, possessing a voltage differential of around 3 kV between them; this grid pair is responsible for extracting the charged propellant particles from the gas chamber. The second pair, operating at low voltage, provides the electrical field that accelerates the particles outwards, creating thrust. Other advantages to the new engine include a more compact design, allowing it to be scaled up to higher thrusts, and a narrower, less divergent exhaust plume of 3 degrees, which is reportedly five times narrower than previously achieved. This reduces the propellant needed to correct the orientation of the spacecraft due to small uncertainties in the thrust vector direction.
Variants
The largest difference in the many electrostatic ion thrusters is the method of ionizing the propellant atoms - electron bombardment (NSTAR, NEXT, T5, T6), radiofrequency (rf) excitation (RIT 10, RIT 22, µN-RIT), microwave excitation (µ10, µ20). Related to this is the need of a cathode and required effort for the power supplies. Kaufman type engines require at least supplies to the cathode, anode and chamber, whereas the rf and microwave types require an additional rf generator, but no anode and cathode supplies.In the extraction grid systems minor differences occur in the grid geometry and the materials used, which may have implications for the grid system lifetime.