Lecture 13-14 Electrostatic Thrusters 1 Introduction Electrostatic thrusters c ion engines")are the best developed type of electric propulsion device, dating in conception to the 50's, )and having been demonstrated in space in 1964 on a suborbital flight of the SERT I spacecraft(2). The early history and concepts are well documented(),(3), and evolved through progressive refinements of various types of ion beam sources used in Physics laboratories, the and long life for these sources to be used in space. Of the various configurations improvements being essentially dictated by the needs for high efficiency, low mas discussed for example in Ref. 3(ca. 1973), only the electron bombardment noble gas type, plus(in Europe) the radio-frequency ionized thruster 4and(in Japan)th Electron Cyclotron Resonance thruster, have survived. Other interesting concepts such as Cesium Contact thrusters and duo-plasmatron sources have been largely abandoned, and one new special device, the field Emission Electrostatic(5)thruster has been added to the roster the electron bombardment thruster itself has evolved in the same time interval from relatively deep cylindrical shapes with uniform magnetic fields produced by external coils and with simple thermoionic cathodes, to by permanent magnets, and with hollow cathode plasma bridges used as cathode ed shallow geometrics using sharply nonuniform magnetic field configurations, prod and neutralizer. Where a typical ion production cost was quoted in Ref. (3)as 400 600 ev for Hg at 80% mass utilization fraction, recent work with ring-cusp thrusters has yielded for example a cost of 116 ev in Xenon at the same utilization o. Such reductions make it now possible to design for efficient operation(above 80% with environmentally acceptable noble gases at specific impulses below 3000 sec, a goal that seemed elusive a few years back. The major uncertain issues in this field seem now reduced to lifetime(measured in years of operation in orbit)and integration problems, rather than questions of cost and physical principle or major technological hurdles. Extensions to higher power(tens of kw)and higher specific impulse(to 7,000-8,000 s)are now being pursued by NASa for planetary missions requiring high△V 2 Principles of Operation Electrostatic thrusters accelerate heavy charged atoms(ions) by means of a purely electrostatic field Magnetic fields are used only for auxiliary purposes in the ionization chamber. It is well known that electrostatic forces per unit area(or energies per unit volume)are of the order of =c E, where e is the strength of the field(volts/m)and E, the permittivity of vacuum E,=8.85x10-12 Farad ypical maximum fields, as limited by vacuum breakdown or shorting due to imperfections are of the order of 10 V/m, yielding maximum force densities of roughly 5N/m2=5x105 atm This low force density is one of the major drawbacks of electrostatic engines and can be compared to force densities of the order of 10 N/m in self-magnetic devices such as MPD thrusters, or to the typical gas pressures of 10-10'N/m in chemical rockets. Simplicity and efficiency must therefore compensate for this disadvantage. 16.522, Space Propulsion Lecture 13-14 Page 2 of 2516.522, Space Propulsion Lecture 13-14 Prof. Manuel Martinez-Sanchez Page 2 of 25 Lecture 13-14 Electrostatic Thrusters 1 Introduction Electrostatic thrusters (“ion engines”) are the best developed type of electric propulsion device, dating in conception to the ‘50’s,(1) and having been demonstrated in space in 1964 on a suborbital flight of the SERT I spacecraft(2). The early history and concepts are well documented(1),(3), and evolved through progressive refinements of various types of ion beam sources used in Physics laboratories, the improvements being essentially dictated by the needs for high efficiency, low mass and long life for these sources to be used in space. Of the various configurations discussed for example in Ref. 3 (ca. 1973), only the electron bombardment noble gas type, plus (in Europe) the radio-frequency ionized thruster(4) and (in Japan) the Electron Cyclotron Resonance thruster, have survived. Other interesting concepts, such as Cesium Contact thrusters and duo-plasmatron sources have been largely abandoned, and one new special device, the Field Emission Electrostatic(5) thruster has been added to the roster. The electron bombardment thruster itself has evolved in the same time interval from relatively deep cylindrical shapes with uniform magnetic fields produced by external coils and with simple thermoionic cathodes, to shallow geometrics using sharply nonuniform magnetic field configurations, produced by permanent magnets, and with hollow cathode plasma bridges used as cathode and neutralizer. Where a typical ion production cost was quoted in Ref. (3) as 400- 600 eV for Hg at 80% mass utilization fraction, recent work with ring-cusp thrusters has yielded for example a cost of 116 eV in Xenon at the same utilization(6). Such reductions make it now possible to design for efficient operation (above 80%) with environmentally acceptable noble gases at specific impulses below 3000 sec, a goal that seemed elusive a few years back. The major uncertain issues in this field seem now reduced to lifetime (measured in years of operation in orbit) and integration problems, rather than questions of cost and physical principle or major technological hurdles. Extensions to higher power (tens of kW) and higher specific impulse (to 7,000 – 8,000 s) are now being pursued by NASA for planetary missions requiring high ∆V . 2 Principles of Operation Electrostatic thrusters accelerate heavy charged atoms (ions) by means of a purely electrostatic field. Magnetic fields are used only for auxiliary purposes in the ionization chamber. It is well known that electrostatic forces per unit area (or energies per unit volume) are of the order of 1 2 E 2 0 ε , where E is the strength of the field (volts/m) and 0 ε the permittivity of vacuum 12 Farad 8.85 10 m − 0 ⎛ ⎞ ε= × ⎜ ⎟ ⎝ ⎠. Typical maximum fields, as limited by vacuum breakdown or shorting due to imperfections, are of the order of 106 V/m, yielding maximum force densities of roughly 2 -5 5 N m = 5×10 atm. This low force density is one of the major drawbacks of electrostatic engines, and can be compared to force densities of the order of 104 N/m2 in self-magnetic devices such as MPD thrusters, or to the typical gas pressures of 106 -107 N/m2 in chemical rockets. Simplicity and efficiency must therefore compensate for this disadvantage