Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Magnet structure or material
Patent
1995-02-16
1995-12-12
Picard, Leo P.
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Magnet structure or material
60202, 3133591, 3133611, 3133621, 31511121, 31511141, 31511181, H01J 152, F03H 100, H05H 100
Patent
active
054753546
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to plasma accelerators applied in particular to space propulsion and more particularly it relates to plasma accelerators of the closed electron drift type also referred to as stationary plasma accelerators or, in the USA, as "Hall current accelerators".
PRIOR ART
Electrical accelerators are intended essentially for space propulsion applications. As sources of ions or of plasma, they are also used for terrestrial applications, in particular for ion machining. Because of their high specific impulse (1500 s to 6000 s) they enable considerable mass savings to be achieved on satellites compared with accelerators that make use of chemical propulsion.
One of the typical applications of accelerators of this type is north-south control of geostationary satellites, where a mass saving of 10% to 15% can be achieved. They may also be used for drag compensation in low orbit, for maintaining a heliosynchronous orbit, and for primary interplanetary propulsion.
Ion thrusters can be divided into several categories.
A first type of ion thruster, also known as a Kaufman accelerator, is thus constituted by an accelerator in which ionization is performed by bombardment. Examples of thrusters of that type are described in particular in Documents EP-A-0 132 065, WO 89/05404 and EP-A-0 468 706.
In an accelerator making use of ionization by bombardment, atoms of propellant gas are injected under low pressure into a discharge chamber where they are bombarded by electrons emitted by a hollow cathode and collected by an anode. The ionization process is increased by the presence of a magnetic field. A certain number of atom-electron collisions cause a plasma to be created whose ions are attracted by the acceleration electrodes (outlet grids) themselves at a negative potential relative to the potential of the plasma. The electrodes concentrate and accelerate the ions which leave the thruster in the form of widely spreading radiation. The ion radiation is then neutralized by a flow of electrons emitted from an external hollow cathode, referred to as a "neutralizer".
The specific impulse (Isp) obtained from a thruster of that type can be of the order of 3000 seconds or more.
The power requirement is of the order of 30 W per mN of thrust.
Other types of ionization accelerator are constituted by accelerators using radiofrequency ionization, accelerators using ionization by contact, or field emission accelerators.
Those various ionization accelerators, including accelerators where ionization is obtained by bombardment, have in common the fact that the functions of ionization and of accelerating the ions are clearly separate.
They also have in common the fact of presenting a current density in the ion optics which is limited by the space charge phenomenon, with density being limited in practice to 2 mA/cm.sup.2 to 3 mA/cm.sup.2 in accelerators where ionization is obtained by bombardment, thus presenting thrust per unit area that is quite low.
In addition, such accelerators, and bombardment accelerators in particular, require a certain number of electrical feeds (between 4 and 10), thereby leading to the implementation of rather complex electronic circuits for conversion and control.
A 1974 article by L. H. ARTSIMOVITCH et al. on the stationary plasma engine (SPD) development program and on tests performed using the "METEOR" satellite, discloses accelerators of the closed electron drift type, also known as stationary plasma accelerators which differ from the other categories of accelerators by the fact that ionization and acceleration are not separate and the acceleration zone includes the same number of ions and of electrons, thereby making it possible to eliminate any space charge phenomenon.
A closed electron drift accelerator as proposed in the above article by L. H. ARTSIMOVITCH et al. is described below with reference to FIG. 2.
An annular channel 1 defined by a part 2 made of insulating material is placed in an electromagnet comprising outer and inner annular pole pieces 3 and 4
REFERENCES:
patent: 5218271 (1993-06-01), Egorov et al.
patent: 5359258 (1994-10-01), Arkhipov et al.
"Open Single-Lens Hall-Current Accelerator", V. N. Dem'yanenko, et al., vol. 21, No. 8, Aug. 1976, New York, pp. 987-988, Soviet Physics Technical Physics.
"Local Plasma Properties in a Hall-Current Accelerator with an Extended Acceleration Zone", A. M. Bishaev, et al, vol. 23, No. 9, Sep. 1978, New York, pp. 1055-1057, Soviet Physics Technical Physics.
Bougrova Antonina
Morozov Alexei
Valentian Dominique
Barrera Raymond M.
Picard Leo P.
Societe Europeenne de Propulsion
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