Gridded ion thruster
Image:Electrostatic [ion thruster-en.svg|thumb|right]
The gridded ion thruster is a common design for ion thrusters, a highly efficient low-thrust spacecraft propulsion method running on electrical power by using high-voltage grid electrodes to accelerate ions with electrostatic forces.
History
The ion engine was first demonstrated by German-born NASA scientist Ernst Stuhlinger, and developed in practical form by Harold R. Kaufman at NASA Lewis 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 thrusters used mercury as the reaction mass. The first was SERT-1, launched July 20, 1964, which 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 gridded ion thrusters through the 1980s, developing the NASA Solar Technology Application Readiness engine, that was used successfully on the Deep Space 1 probe, the first mission to fly an interplanetary trajectory using electric propulsion as the primary propulsion. It later flew on the Dawn asteroid mission.
Hughes Aircraft Company has developed the XIPS for performing station keeping on its 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.
In 2006, Aerojet completed testing of a prototype NEXT ion thruster.
Beginning in the 1970s, radio-frequency ion thrusters were developed at Giessen University and ArianeGroup. RIT-10 engines are flying on the EURECA and ARTEMIS. Qinetiq has developed the T5 and T6 engines, used on the GOCE mission and the BepiColombo mission. From Japan, the μ10, using microwaves, flew on the Hayabusa mission.
In 2021, DART launched carrying a NEXT-C xenon ion thruster.
In 2021, ThrustMe reported satellite orbit changes using their NPT30-I2 iodine ion thruster.
Method of operation
Propellant atoms are injected into the discharge chamber and are ionized, forming a plasma.There are several ways of producing the electrostatic ions for the discharge chamber:
- electron bombardment by a potential difference between a hollow cathode and anode
- radio frequency oscillation of an electric field induced by an alternating electromagnet, which results in a self-sustaining discharge and omits any cathode
- microwave heating
The positively charged ions diffuse towards the chamber's extraction system. After ions enter the plasma sheath at a grid hole, they are accelerated by the potential difference between the first and second grids. The ions are guided through the extraction holes by the powerful electric field. The final ion energy is determined by the potential of the plasma, which generally is slightly greater than the screen grids' voltage.
The negative voltage of the accelerator grid prevents electrons of the beam plasma outside the thruster from streaming back to the discharge plasma. This can fail due to insufficient negative potential in the grid, which is a common ending for ion thrusters' operational life. The expelled ions propel the spacecraft in the opposite direction, according to Newton's 3rd law.
Lower-energy electrons are emitted from a separate cathode, called the neutralizer, into 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, which would attract ions back toward the spacecraft and cancel the thrust.
Performance
Longevity
The ion optics are constantly bombarded by a small amount of secondary ions and erode or wear away, thus reducing engine efficiency and life. 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 thruster for over 16,000 hours and NEXT thruster for over 48,000 hours.In the extraction grid systems, minor differences occur in the grid geometry and the materials used. This may have implications for the grid system operational lifetime.