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This article focuses on electrostatic ion thrusters - for a more general description, refer to electric propulsion.
An 'ion thruster' (more specifically an electrostatic ion thruster) is one of several types of spacecraft propulsion, specifically electric propulsion. It uses beams of ions — electrically charged atoms or molecules — for propulsion. The precise method for accelerating the ions may vary, but all designs take advantage of the charge-to-mass ratio of ions to accelerate them to very high velocities using a high electric field. Ion thrusters are therefore able to achieve high specific impulse, reducing the amount of reaction mass required, but increasing the amount of power required compared to chemical rockets. Ion thrusters can generally deliver one order of magnitude greater propellant efficiency than traditional liquid fuel rocket engines, but are constrained to very low accelerations by the small mass of the accelerated ions.
The principles of Ion thrusters go back to the concepts developed by the German/Austrian physicist Hermann Oberth which were published in his famous 1929 work "Die Rakete zu den Planetenräumen" (The rocket to planetary space).
The first ion thrusters, known as Kaufman-type ion thrusters, were developed by Harold R. Kaufman, working for NASA in the 1960s, and were based on the Duoplasmatron.
Ion engines have enjoyed possibly the most "Hollywood press" of electric propulsion systems, most notably appearing as the propulsion system of the TIE fighters in the Star Wars universe, among other works. However, the maneuvering capabilities displayed in those fictional works are well beyond current ion engine technology in the real world, as these engines typically only produce accelerations. Ion thrusters are operated over several hours to months to provide the desired change in velocity, delta-V, to compensate for this.

Contents

Types of ion thruster

General design

Energy usage

Thrust

Lifespan

Missions

SERT

Smart 1

Artemis

Deep Space 1

Dawn

Hayabusa

LISA Pathfinder

Development

See also

References

Types of ion thruster

The term ion thruster usually refers to Electrostatic ion thrusters, the earliest form of spacecraft electric propulsion. It can though be used to describe other types of spacecraft electric propulsion, that also use ions to accelerated by a high voltage grid to provide the change in momentum. Some of the types of ion thruster are:

★ Electrostatic ion thrusters

★ Hall effect thrusters

★ Colloid thrusters. Generally though colloid thrusters use large charged droplets, rather than ions. They can produce ions though under certain conditions.

★ Field Emission Electric Propulsion (FEEP)

★ DS4G also known as the ''Dual-Stage 4-Grid''^{[1] [2]}
Other forms of high-efficiency electric thruster have also been proposed; see spacecraft propulsion.

General design

In an electrostatic ion thruster, atoms of argon, mercury or xenon are ionized by exposure to electrons provided by a cathode filament. The ions are accelerated by passing them through highly charged grids. Electrons are also fired into the ion beam downstream of the grids as the positively charged ions leave the thruster. This keeps the spacecraft and the thruster beams neutral electrically. The acceleration uses up very little reaction mass (i.e., the specific impulse, or ''I''_sp, is very high). In the 1970s and 1980s, research of ion propulsion first began with cesium, but this was found to erode the grid. After that, the mid-noble gases were mainly used as a propulsion source.

Energy usage

A major consideration is the amount of power required to run the thruster, partly to ionize the materials, but mostly to accelerate the ions to the extremely high speeds required to have any useful effect. Exhaust speeds of 30 km/s are not uncommon, which is far faster than the 3–4.5 km/s for chemical rockets, or the speed of the vehicle. This makes for notably low propellant usage.
The exhaust velocity attained by ions when they are accelerated inside an electric field can be calculated from the kinetic and electric protential energy:
:
:$E\_p\; =\; QV$
Where $v\_i$ is the velocity of the accelerated ion,
$Q$ is the charge of the ion,
$m\_i$ is the mass of the ion,
and $V$ is the potential difference across with the electric field.
Rearranging gives a term for the exhaust velocity;
:$v\_i\; =\; sqrt\{2VQ\; over\; m\_i\}$

Thrust

Assuming that the amount of ions leaving the thruster is constant, then the thrust is given by;
:
where:
:''T'' = thrust (force) generated,
: mass flow rate.
In practice, with currently practical energy sources of perhaps a few tens of kilowatts, and given a typical ''I''_sp of 3000 seconds (30 kN·s/kg), ion thrusters give only extremely modest forces (often tenths or hundredths of a newton). Large ion propulsion engines require large electric power sources. Ion engines typically provide space craft acceleration rates of from 10⁻⁵ ''g'' to 10⁻³ ''g'' (0.000098 m/s² to 0.0098 m/s²).

Lifespan

Given the low thrust, the life of the thruster becomes important. Ion thrusters have to be kept running a large part of the time to allow the milligee acceleration to gain a useful velocity.
In the simplest ion thruster design, an electrostatic ion thruster, the ions often hit the grids, which leads to erosion of the grids and their eventual failure. Smaller grids lower the chance of these accidental collisions, but decrease the amount of charge they can handle, and thus lower the thrust.

Missions

Of all the electric thrusters, ion thrusters have been the most seriously considered commercially and academically in the quest for interplanetary missions and orbit raising maneuvers. Ion thrusters are seen as the best solution for these missions as they require very high change in velocity overall that can be built up over long periods of time.

SERT

Several spacecraft have operated with this technology. The first was SERT ^[3] in the 1970's.

Smart 1

The Hall effect thruster is a type of ion thruster that has been used for decades for station keeping by the Soviet Union and is now also applied in the West: the European Space Agency's satellite Smart 1 used it. This satellite completed its mission on September 3, 2006, in a controlled collision on the Moon's surface, after a trajectory deviation to be able to see the 3 meter crater the impact created on the visible side of the moon.

Artemis

On 12 July 2001, the European Space Agency failed to launch their Artemis telecommunication satellite, and left it in a decaying orbit. The satellite's chemical propellant supply was sufficient to transfer it to a semi-stable orbit, and over the next 18 months the experimental onboard ion propulsion system (intended for secondary stationkeeping and maneuvering) was utilized to transfer it to a geostationary orbit. ^[4]

Deep Space 1

NASA has developed an ion thruster called NSTAR for use in their interplanetary missions. This thruster was tested in the highly successful space probe Deep Space 1. Hughes has developed the XIPS (Xenon Ion Propulsion System) for performing stationkeeping on geosynchronous satellites. These are electrostatic ion thrusters and work by a different principle than Hall effect thrusters.

Dawn

Dawn is to be launched in September 2007 to explore the dwarf planet Ceres and the asteroid Vesta. To cruise from Earth to its targets it will use three Deep Space 1 heritage Xenon ion thrusters (firing only one at a time) to take it in a long outward spiral. An extended mission in which Dawn explores other asteroids after Ceres is also possible.

Hayabusa

The Japanese space agency's Hayabusa, which was launched in 2003 and successfully rendezvoused with the asteroid 25143 Itokawa and remained in close proximity for many months to collect samples and information, is powered by four xenon Ion Engines. It is using xenon ions generated by microwave ECR, and a Carbon / Carbon-composite material for acceleration grid which is resistant to erosion.^[5]

LISA Pathfinder

LISA Pathfinder is an ESA spacecraft to be launched in 2009. It will not use ion thrusters as its primary propulsion system, but will use both coloid thrusters and FEEP for very precise attitude control - the low thrusts of these propulsion devices make it possible to move the spacecraft incremental distances very accurately. It is a test for the possible LISA mission.

Development

In 2003 NASA ground-tested a new version of their ion thruster called High Power Electric Propulsion, or HiPEP. The HiPEP thruster differs from earlier ion thrusters because the xenon ions are produced using a combination of microwave energy and magnetic fields. The ionization is achieved through a process called electron cyclotron resonance (ECR). In ECR, a uniform magnetic field is applied to a chamber holding xenon gas. The small number of free electrons present in the neutral gas orbit around the magnetic field lines at a fixed frequency called the cyclotron frequency. Microwave radiation is applied that is carefully tuned to this frequency, supplying energy to the electrons, which then ionize more xenon atoms through collisions. This process is a highly efficient means of creating a plasma in low density gases. Previously the electrons required were provided by a hollow cathode.

See also

★ Electric propulsion

★ Spacecraft propulsion

★ Nuclear electric rocket

★ Hall effect thruster

★ Magnetoplasmadynamic thruster

★ Field Emission Electric Propulsion

★ Pulsed inductive thruster

★ VASIMR

★ Electrodeless plasma thruster

★ EmDrive

References

★ Plasma Propulsion in Space, , Eric J., Lerner, The Industrial Physicist, 2000

★ ElectroHydroDynamic Thrusters (EHDT), RMCybernetics.
1.
2. ANU and ESA make space propulsion breakthrough ANU Space Plasma, Power & Propulsion Group (SP3)
3. Space Electric Rocket Test
4. Artemis team receives award for space rescue ESA
5. 小惑星探査機はやぶさ搭載イオンエンジン (Ion Engines used on Asteroid Probe Hayabusa) ISAS