Electrostatic ion thruster

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.[1]

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Method of operation

1. Propellant atoms are injected into the propulsion chamber. The propellant atoms are bombarded with electrons from a hollow cathode, causing the atoms to lose electrons of their own and become ionized, thus forming positive ions. The thruster walls and grid absorb the lost electrons.
2. The positively charged ions move towards the exit of the chamber due to diffusion. Ions will leak into a plasma sheath just upstream of the positively charged grid.
3. Once ions enter the sheath they are in the positive and negative grids at the exit of the chamber, they are electrostatically accelerated away from the positive grid and towards the negative one.
4. The positive grid is at a much higher potential than the negative grid, thus the negative grid pulls on the positive ions. As the ions approach the negative grid they are electrostatically focused through the apertures of the negative grid and out into space at a high speed.
5. The expelled ions propel the ship in the opposite direction according to Newton's 3rd law.
6. Electrons are shot from a cathode, called the neutralizer, towards the ions behind the ship 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.

Performance

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 or caesium 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 (1.8 years), 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.

Variants

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.

See also

• Spacecraft propulsion
• Ion thruster
• Field Emission Electric Propulsion
• Hall effect thruster
• Magnetoplasmadynamic thruster
• Pulsed inductive thruster
• Variable specific impulse magnetoplasma rocket
• Vacuum tube