The electrostatic ion thruster is a kind of design for ion thrusters (a kind of highly-efficient low-thrust spacecraft propulsion running on electrical power). These designs use high voltage electrodes in order to accelerate ions with electrostatic forces. A variant of the duoplasmatron, they were initially developed by Harold R. Kaufman at NASA in the early 1960s, but they were rarely used before the late 1990s. NASA has produced practical electrostatic ion thrusters, notably the NSTAR engine that was used successfully on Deep Space 1. Hughes Aircraft Company has developed the XIPS (Xenon Ion Propulsion System) for performing station keeping on geosynchronous satellites. NASA is currently working on a 20-50 kW electrostatic ion thruster called HiPEP which will have higher efficiency, specific impulse, and a longer lifetime than NSTAR. Aerojet 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.
Method of operation
- Propellant atoms are injected into the discharge chamber and get ionized by electron bombardement forming a plasma. There are several ways of producing the energetic electrons for the discharge: (1) The electrons are emitted from a hollow cathode 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 a 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 ship 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 ship from gaining a net negative charge.
The ion optics are constantly bombarded by propellant 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. Mercury
atoms were used as propellants during tests in the 1960s and 1970s, but these propellants adhered to, and eroded the grids. Xenon
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
), and test are still ongoing for double this lifetime. Electrostatic ion thrusters have also achieved a specific impulse
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, together with the Australian National University, have announced successful testing of an improved electrostatic ion engine 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.
The chief variable in electrostatic ion thrusters is the method of ionizing the fuel atoms. New techniques such as using microwaves to heat the fuel atoms into a plasma
(thus ionizing them) are under development; the advantage of such a technique is the lack of a cathode that would wear out or erode, increasing thruster life.
Other designs of ion thruster have also been developed in an effort to circumvent the problems of the electrostatic ion thruster. The chief focus of attention has been the grid, since grid wear is a major limiting factor in engine lifetime.