Abstract:
A Liquid Metal Ion Thruster (LMIT) has a substrate having a plurality of pedestals, one end of the pedestal attached to the substrate, and the opposing end of the pedestal having a tip, the pedestals having grooves and the substrate also having grooves coupled to each other and to a source of liquid metal. An extractor electrode positioned parallel to the substrate and above the pedestal tips provides an electrostatic extraction field sufficient to accelerate ions from the tips of the pedestals through the extractor electrode. A series of focusing electrodes with matching apertures provides a flow of substantially parallel ion trajectories, and an optional negative ion source provides a charge neutralization to prevent space charge spreading of the exiting accelerated ions. The assembly is suitable for providing thrust for a satellite while maintaining high operating efficiencies.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a thruster for propulsion of a satellite. In particular, the invention relates to a Liquid Metal Ion Thruster (LMIT) formed from an array of wetted tips. 
     BACKGROUND OF THE INVENTION 
     Since the mid 1960s, near-earth space has been populated by ever larger spacecraft, typically today in the ˜1000 kilogram category, launched and boosted into Geosynchronous Earth Orbit (GEO) for communications, or Low Earth Orbit (LEO) for mapping and defense purposes. There is a rapidly growing commercial demand for small satellites of 100-200 Kg mass in sun-synchronous Low Earth Orbit (LEO) of approximately 200 nautical miles, but a significant reduction in the cost of access to orbit for small payloads is essential for success of the emerging commercial space industry. Today&#39;s high costs are justified mostly by defense needs, or by launching large satellites into GEO where their useful life is long enough to justify amortized life-cycle costs. Modern satellite launches into LEO use chemical propulsion systems, such as liquid or solid propellant single-stage and multi-stage rockets. Innovative approaches not yet made practical include launching at high altitude from airborne platforms. Advances in structural materials will be one key enabling technology to meeting the challenging cost-to-LEO target. Nano-composite materials could increase the strength/mass ratio of rocket structures and lead to single-stage to orbit with higher payload and hence reduced cost/kilogram delivered to LEO. Exotic chemical propellants that generate higher exhaust velocities would also increase payload delivered to LEO for a given launch pad mass, and hence reduce launch costs. 
     Small satellites in LEO will place more demands on micro-spacecraft in the 1 kg-10 kg class and small satellites in the ˜100 kg class. With increasing ability to integrate cameras and sophisticated communications systems, the demand for propulsion systems for small (and inherently power limited) spacecraft will grow. 
     In modern satellites, high thrust for rapid maneuvers has been provided to spacecraft by chemical propulsion, such as hydrazine and other rocket motors. The exhaust velocity of such chemical rockets is limited by the inherent specific energy released by combustion, to ˜2500-3000 m/s. Due to this limited speed, chemical rockets burn up more propellant to effect an orbital maneuver than would other forms of propulsion that offer higher exhaust speeds. These include electro-thermal rockets and electric propulsion. In electro-thermal rockets, the chemical energy released by the propellant is augmented by additional energy input via an external heater. The higher exhaust speeds possible are limited by the temperature at which the rocket nozzle may be safely operated. Electric propulsion is the most efficient in terms of propellant utilization, as it offers much higher exhaust speeds. This is possible because electric rockets add energy to passive propellants via external means and contain the high energy propellant ions or plasma in electromagnetic fields, so that they are not in contact with material walls. Thus the usual limitation on propellant temperature is removed. At high temperatures, exhaust speeds in the 10,000-30,000 m/s range are possible for plasma rockets, while electrostatic ion engines may boost the exhaust speed of ions to still higher velocities, (&gt;100,000 m/s) limited only by breakdown of vacuum gaps at high voltages. Such an order-of-magnitude higher exhaust speed for electric rockets makes them far more efficient in terms of propellant utilization for in-space maneuvers. To illustrate this by example, consider a 100 kg satellite that must be moved in its orbit by a change in orbital velocity of 2000 m/s. If a chemical rocket with 100N of thrust and an exhaust speed of 3000 m/s is used, the orbital maneuver would take about 24 minutes to complete, with a fuel consumption of 49 kg which implies that only half of the initial 100 Kg spacecraft mass would arrive at the destination. By contrast, for an electric propulsion engine with thrust of only 1N, but having an exhaust speed of 30,000 m/s, the same orbital maneuver would take 54 hours but consume only 6.5 kg of propellant, so nearly 94% of the initial mass would be delivered to its destination. The cost/kg of useful payload delivered would be half as much as with the chemical propulsion, in exchange for a longer mission duration. As the required velocity change becomes larger and larger relative to the exhaust speed, chemical propulsion becomes far less efficient. For example, if in the above example, the velocity change were increased from 2 km/s to 4 km/s, the chemical rocket would deliver only 26 kg of the original 100 kg to its destination, vs. 88 kg for the electric rocket. This factor of 3.3 higher useful payload could significantly reduce costs to move objects in space. The above example is illustrative of the general advantage of higher exhaust speed in space. However, the example also shows that the price paid for higher speed electric rockets is often a much longer mission duration, due to the typically much lower thrust offered by such engines, relative to their chemical counterparts. For a given efficiency, the thrust T and exhaust speed are inversely related via: 
             T   =       2   ⁢       P   e     ·   η         u   exhaust             
with P e  being the power into the thruster, η the overall thruster efficiency and u exhaust  the exhaust velocity of the rocket engine. As the above example illustrates, chemical rockets have given high thrust but at low exhaust velocity, while electric rockets have given low thrust at high velocities. Orbital maneuvers in space could be dramatically improved if a single propulsion engine were available that offered variable exhaust speed and thrust for a fixed power input at high efficiency. With such an engine, one could operate at high thrust and lower velocity for rapid maneuvers that consume more fuel, but reduce to low thrust at very high velocity, to accomplish slower missions far more efficiently. Rather than carrying two completely different types of engine on board to accomplish this (as is done today) one could utilize a single electric engine to do both tasks.
 
     A new type of electric thruster is known as a Liquid Metal Ion Thruster (LMIT). LMITs offer the advantage that they can be integrated into Micro-Electro-Mechanical-System (MEMS) structures, very similar to current systems being used for field emitters in plasma displays. An LMIT works by producing a high velocity ion current via field emission from a liquid metal source. A high voltage is applied between an extractor electrode at cathode potential and a liquid metal coated field enhancing structure like a small (micron radius) sharp tip. The high voltage leads to the formation of tiny micro tips protruding from the liquid metal surface, known as Taylor cones. These Taylor cones enhance the applied electric field further, leading to a condition where ions can “tunnel” out of the liquid phase into vacuum. The applied extraction voltage accelerates the ions to a velocity u, 
     
       
         
           
             
               u 
               exhaust 
             
             = 
             
               
                 
                   2 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   eV 
                 
                 
                   m 
                   ion 
                 
               
             
           
         
       
     
     where: 
     e as the elementary charge (1.6×10−19 Coulomb), 
     V is the extraction voltage, and 
     m ion  is the mass of the individual ion. 
     In LMIT systems with increasing extractor voltage, the velocity and the number of ions extracted increase and essentially more thrust is produced. 
     PRIOR ART 
     U.S. Pat. No. 4,328,667 by Valentian et al. describes a liquid metal ion thruster assembly having a plurality of hollow-cone tips coupled to a reservoir of liquid metal, where the metal ions are drawn from the tip by the electrostatic force generated by an adjacent electrode. 
     U.S. Pat. Nos. 6,097,139 and 6,741,025 by Tuck et al. describe the use of impurities on a surface for the formation of enhanced electric fields for use as composite field emitters. 
     U.S. Pat. Nos. 6,516,024 by Mojarradi et al. and 6,996,972 by Song describe a hollow tip liquid ion extractor assembly for generation of thrust. 
     U.S. Pat. No. 7,059,111 describes a thruster whereby liquid metal ions are boiled from a reservoir and electro-statically attracted through a cylindrical ring, thereby generating thrust. 
     U.S. Pat. Nos. 6,531,811 and 7,238,952 describe an ion extractor having a reservoir opposite a needle tip and an extractor electrode. 
     OBJECTS OF THE INVENTION 
     A first object of the invention is a liquid metal ion thruster having an array of pedestals, each pedestal having one end attached to a substantially planar substrate, and an opposite end tapering to a tip located on a pedestal axis, an extractor electrode co-planar to said substrate and located above said pedestal tip, the extractor electrode having an aperture for the emission of ions at the intersection of each pedestal axis with the extractor electrode, the pedestals and planar substrate having a wetted surface for the conduction of a liquid metal suitable for ionization and extraction from the tip of each pedestal. 
     A second object of the invention is a liquid metal ion thruster having an array of pedestals, attached to a substantially planar substrate, each pedestal having one or more grooves on the surface of the pedestal which are substantially parallel to the axis of each pedestal, the grooves carrying liquid metal from the substrate to the tip of the pedestal, an extractor electrode co-planar with the planar substrate for drawing ions from the pedestals, the extractor electrode having a plurality of apertures located on the axis of each pedestal, and a plurality of focusing electrodes located co-planar to the extractor electrode and forming the extracted ions into a stream of flows substantially parallel to the axis of the associated pedestal. 
     A third object of the invention is a liquid metal ion thruster having an array of pedestals, one end of each pedestal attached to a substantially planar substrate, and the other end formed into a tip having an axis, the axis of each tip forming an ion flow axis, an extractor electrode co-planar to the substrate and having apertures for each ion flow axis, one or more focusing electrodes co-planar to the extractor electrode and having an aperture for each ion flow axis, the focusing electrode producing a substantially co-linear flow of extracted ions, and a source of neutralizing electrons applied to the co-linear flow of extracted ions to produce a neutral charge to ensure continued co-linear ion flow beyond the extent of the focusing electrodes. 
     SUMMARY OF THE INVENTION 
     In a first embodiment of the invention, a substrate is formed which includes a plurality of pedestals, each pedestal having an axis, each pedestal having one end attached to the substrate, and the pedestal having an opposite end which tapers to a tip. Parallel to the planar substrate and located above the pedestal tips is a substantially planar extraction electrode which has apertures located at the intersection points of the axis of each pedestal and the extraction electrode. The surface of the substrate and the pedestals is wetted with a liquid metal such that in operation, a film of liquid metal coats the substrate and pedestals. Upon application of an electric potential between the extraction electrode and the pedestals, which are connected to each other and the substrate through the liquid metal, the liquid metal at the tips of the pedestals forms an ion beam. The reaction force of the ions results in the generation of a thrust. 
     In another embodiment of the invention, the pedestals have surface grooves which are parallel to the pedestal axis and provide a conduit for liquid metal. 
     In another embodiment of the invention, the pedestals have surface grooves or channels which are parallel to the pedestal axis, and additionally the substrate has surface grooves or channels which interconnect the pedestals to each other and also to a reservoir of liquid metal. 
     In another embodiment of the invention, the pedestals have surface roughness on their tapered opposing end segments that allows surface capillary flow of liquid metal from the surface grooves or channels which are parallel to the pedestal axis, to the tips of the pedestals. Without such surface roughness, capillary flow will be inhibited from moving along the tapered opposing end portions of the pedestals towards their sharp points. 
     In another embodiment of the invention, the pedestals have surface grooves or channels which connect at one end, to the surface grooves or channels that are along the sides of the pedestals, and at the other end converge to the tip of the pedestal. These converging channels along the tapered opposing end surface must have channel widths that decrease towards the point and are designed to allow continuous and stable flow of liquid metal from the surface grooves or channels which are parallel to the pedestal axis, to the tips of the pedestals. Without such decreasing channel width along the tapered opposing end of the pedestals, capillary flow will be inhibited from moving along the tapered opposing end portions of the pedestals towards their sharp points. 
     In another embodiment of the invention, the extraction electrode has a parallel set of planar electrostatic focusing electrodes such that each pedestal generates a flow of ions that is substantially parallel to the pedestal axis. 
     In another embodiment of the invention, the extractor electrode is coated with an insulator. 
     In another embodiment of the invention, the region below the pedestal tips and planar substrate contains an insulating material. 
     In another embodiment of the invention, the liquid metal is indium and the extractor electrode is insulated. 
     In another embodiment of the invention, the liquid metal on the substrate is indium at a sufficient temperature to keep it in the liquid state, and the extractor electrode is coated with indium at a sufficient temperature to keep it in a solid state. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a cross sectional view of a liquid metal ion thruster (LMIT). 
         FIG. 2  shows a detailed view of a pedestal tip of  FIG. 1 . 
         FIG. 3A  shows Indium applied to an un-wetted surface. 
         FIG. 3B  shows Indium applied to a wetted surface. 
         FIG. 3C  shows a cross section view of bulk Indium applied to a wetted surface. 
         FIG. 3D  shows a cross section view of bulk Indium applied to a wetted surface as in  FIG. 3C , after being heated to its melting temperature. 
         FIG. 4  shows the sectional view of an LMIT with an insulated extraction electrode. 
         FIG. 5  shows a sectional view of an LMIT with ion beam focusing electrodes and ion charge neutralizer. 
         FIG. 6  shows a thruster having an array of pedestal tips. 
         FIG. 7  shows a cross sectional view of  FIG. 6 . 
         FIG. 8  shows the plan view of a pedestal tip. 
         FIG. 9  shows a sectional view of the pedestal tip, substrate, and liquid metal reservoir. 
         FIG. 10  shows the side view of a liquid metal ion thruster with a cold cathode electron emitter for space charge neutralization. 
         FIG. 11  shows extractor voltage waveforms for proportional control of thruster force. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows one embodiment of a liquid metal ion thruster (LMIT) according to the present invention, the thruster having a substrate  102 , a substrate pedestal  118  with a central axis  114 , where the pedestal  118  has one end formed from or attached to the substrate  102 , and the pedestal has a tapered opposing end  116 . Covering the substrate and pedestal is a film of liquid metal  104  at a potential such as 0V with reference to a voltage source  120 . A negative potential is applied to an extractor electrode  110  which draws liquid metal ions from the tip to form an ion stream  112  which passes through an aperture  122  centered about the pedestal axis  114 . 
     The particular liquid metals which are good candidates for use in a liquid metal ion thruster include Indium, Gallium, alloys of these metals, and more broadly, metals with a melting point in the range of 300° K. to 700° K. 
     The pedestal typically has circular symmetry about its axis  114 , and may include features such as grooves or scallops cut into the pedestal outer surface to act as channels for the conduction of liquid metal from the planar substrate  102  up the walls of the pedestal  118  via the grooved channels, and to the emission tip  108 . The liquid metal travels along patches of surface roughness or along the narrowing channels that converge to the pedestal tip, where the separating ions respond to the concentrated electric field by forming a Taylor cone.  FIG. 2  shows magnified details of the structure of  FIG. 1 , including Taylor cone  202 , which forms the critical region where the liquid metal forms a tip further concentrating the enhanced electric field density and such that ions are drawn from the surface of the Taylor cone and accelerated by the electric field potential of the extractor electrode  110  to form ion streams  122 . 
     The choice of a substrate is often governed by the ease of machining, often using chemical etching or chemical machining, as is known in the art of Micro-Electro-Mechanical Systems (MEMS), where features are etched on a substrate material such as Silicon. Although easily etched, one difficulty of Silicon is that it is not easily wetted by liquid metal candidates such as Indium or Gallium, so the substrate may require a metal coating, such as Titanium, Molybdenum, or Tungsten over Nickel, or any combination of these metals. Finally, a surface roughening may be applied to the layer of metal coating in contact with the liquid metal of the thruster, which may improve liquid metal flow over this surface. 
     There are several operational requirements for optimum operation of the device shown in  FIGS. 1 and 2 : 
     1) It is critical that a continuously wetted surface be available from the substrate surface to the tip of the emitter, particularly in the regions responsible for feeding liquid metal to the Taylor cone. 
     2) Ionic emissions from the Taylor cone should be accelerated by the electric field formed by extraction electrode  110 , but the ions should pass through aperture  122  such that the ions do not deposit on the extractor electrode  110 , as this would reduce the thrust efficiency and operational lifetime of the thruster. 
     3) Due to the small size of the pedestal and its features, it is desired to form the substrate and pedestal from silicon using machining techniques such as those used in MEMS, where the substrate may be silicon and the features of the substrate including the pedestal etched using photolithographic or direct erosion etching techniques, or any chemical machining technique known in the art of MEMS device fabrication. 
     4) It may be advantageous to apply a surface metallization after machining the substrate and pedestals such that the surface metallization will ease the initial application of a liquid metal propellant with a low melting temperature, such as Indium or Gallium, which also has an undesirably high surface tension and tends to resist forming initial conformal coatings, instead forming isolated spherical depositions when sputtered. 
     In addition to the grooved features of the pedestals, enhanced liquid metal flow results where the pedestal grooves or scallops, and optionally substrate grooves or depressions, have surface roughness on their tapered opposing end segments that allows surface capillary flow of liquid metal from the surface grooves or channels which are parallel to the pedestal axis, with the surface roughness continuing up the sides of the pedestal to the tip. Without such surface roughness, capillary flow will be inhibited from moving along the tapered opposing end portions of the pedestals towards their sharp points. In the best mode, the surface roughness Ra for the conduction of liquid Indium is on the order of 5μ (in the range of 0.1μ to 5μ), however the roughness may be varied to improve liquid metal flow according to the flow characteristics of the particular metal or metal alloy being supported. Surface roughening of the coating which will be in contact with the liquid metal may be accomplished by chemical etching, method of sputtering, or any technique which results in the required surface roughness Ra. 
       FIG. 3A  shows the substrate  301  which has been coated  304  with a thin layer of Tungsten or Molybdenum applied using a sputter coating technique, or any technique which provides for a roughened surface for mechanical bonding of the later-applied Indium to the roughened surface. Typically, the substrate  301  must be coated with a metallization  304  thickness of at least 10,000 Å, which allows the metallized surface to be etched and roughened in preparation for the introduction of Indium  302 . As Indium has a high surface tension, depositions of this material tend to gather in spherical depositions  302 , where the degree of surface tension may be measured by the angle between the substrate and the contact surface of the material to be wetted  302 , shown as 126 degrees.  FIG. 3B  shows the result of applying a plasma jet to  FIG. 3A , whereby the spherical metal  302  spreads sphere  302  over the roughened surface  304  of the substrate  301  and the metal is spread, with the surface tension angle reduced to 9 degrees. In one embodiment of the invention, a plasma jet is used to spread the Indium over the surface, thereby resulting in an initial wetting which allows for continuous liquid flow to replace the ions which are accelerated away in the Taylor cone tip.  FIG. 3C  shows an alternate method of Indium application, where bulk Indium  308  is applied to roughened metallic surface  304  over substrate  301 , and after heating the Indium flows over the original extent of application, leaving an uneven deposition  310 , the excess of which may be removed such as by centrifugal force while the Indium is in a liquid state, thereby realizing an Indium wetted surface. The roughened metallization  304  may be heated directly or indirectly such as by magnetic induction, or any method which results in a reliable wetted interface between the Indium  306 / 310  and metallization surface  304 .  FIG. 4  shows another embodiment of the thruster, whereby the extraction electrode  406  is coated with an insulating material such as a 1000 Å thick coating  404  of aluminum oxide, or alumina. This insulating coating reduces the likelihood of Indium ions depositing on the insulated surface of the electrode  406  during operation. 
       FIG. 5  shows another embodiment of the invention, whereby the ions form a Taylor cone  408  and are accelerated to the exit aperture  512  about the pedestal axis  514  by the extraction electrode  506 , and then focused into parallel paths  516  using electrostatic potentials applied by focusing electrodes  504  and  502 . The number of focusing electrodes and inter-electrode spacing may vary according to the application, but the objective of the focusing electrodes  504  and  502  is to produce substantially parallel ion trajectories at the exit point of the thruster. Optionally, it may also be helpful to remove residual charges in the emitted ions by spraying a source of negatively charged ions or electrons  510  at the accelerated Indium ions  516 , thereby minimizing the space charge effect, whereby the like-charged Indium ions tend to repel each other and generate a larger plume of ions than would be produced by charge neutralized ions. One objective of the charge neutralization electrode  518  and electrostatic focusing electrodes  502  and  504  is to reduce the number of ions returning to the thruster and assemblies around the thruster, such as satellite antennas. 
       FIG. 6  shows an ion thruster which comprises an array of pedestal emitters  606  on a substrate  604  with liquid metal feed channels  602  formed into the substrate and coupled to a dispenser  614  of  FIG. 7 , where the dispenser may contain a low melting point metal which is heated to slightly above the melting point, such as Indium impregnated into a dispenser  614  formed from porous tungsten, or any other dispenser means for storing Indium and allowing it to wick through the substrate surface channels  602 . Positioned substantially parallel to the surface of the substrate is an ion extractor electrode  608 , and first focus electrode  610  and second focus electrode  612 , each electrode at a different potential provided by voltage sources V 1   702  referenced to the liquid metal on the substrate surface, and the electrodes having an aperture corresponding to each pedestal axis such as  706  for attracting, forming, and focusing the ion beam. The liquid metal reservoir  614  is coupled to the feed channels  602  such that the liquid metal flows from the reservoir  614  along the wetted channels  602  which form wells  640  around the pedestals such as  606 , such that the liquid metal is drawn up the grooves or channels  606  of each pedestal to the tip region where the Taylor cone is formed and ions are electrostatically attracted and released by the electric field produced by extractor electrode  606 , the ion stream is focused by electrodes  610  and  612 , and the ion stream is neutralized by charge balance injector  615 . The charge balance injector  615  is coupled to the negative terminal of a voltage source V 2   704 , and generates a cloud of negatively charged particles to neutralize the positively charged metal ions that have been accelerated from the tips such as  606 . The provision of this negative charge into the ion cloud results in the reduction of space charge spreading of the emitted ion stream.  FIG. 7  also shows a side view of the extractor electrode  608 , focus electrodes  610  and  612 , which are formed from substantially planar sheets of metal with apertures disposed above the center axis of each pedestal emitter such as  606 , and co-planar with the substrate surface  604 . 
       FIG. 8  shows a detailed view of the pedestal and grooves  620  of  FIG. 6 . The substrate surface grooves  602  are coupled to the reservoir through an aperture  802 , such that liquid metal flows  804  as shown through the channel  602  and at each pedestal is drawn upwards to the grooves disposed in each pedestal  606 .  FIG. 9  shows a side detail view, including liquid metal reservoir  614 , aperture  802 , channel  602 , and pedestal  606 . Reservoir  614  may store the liquid ion propellant such as Indium in bulk, or alternatively, reservoir  614  may be a porous material with a high melting point such as porous tungsten. 
     In an alternate embodiment for the extractor electrode  902  shown in  FIG. 9 , the extractor electrode  904  may be coated with a thin layer of the same material used for the liquid ion source, such as Indium. By maintaining the temperature of electrode  902  such that the surface coating of Indium  902  is in the solid phase, the emitted plasma jet of Indium from the Taylor cone region of the tip of the pedestal  606  which deposits onto the Indium surface  902  of the extraction electrode  902  is unable to deposit extractor electrode material  902  back to the pedestal tip  606  through high temperature atomic sputtering, which contamination would cause failure of the Taylor cone associated with pedestal tip  606 , and eventually, failure of the entire thruster array. Therefore, coating the extractor electrode  902  with material identical to the liquid metal ion source greatly reduces the failure caused by electrode material  902  sputtering back to the pedestal tip  606 . The Indium-coated extractor electrode  902  may be used in combination with any of the other pedestal tips and channels geometries previously described or in the prior art. Alternatively, in applications where the extractor electrode is likely to achieve high temperatures and the Indium coating  904  would be in the undesirable liquid phase, the coating  904  may be Indium oxide instead, which has a much higher melting temperature than elemental Indium. 
     One problem of an ion thruster is that the ion stream tends to spread and disperse over distance because of internal space charge effects, where the similarly charged ions repel away from each other. Furthermore, in the absence of a charge neutralizing cloud, the positively charged ion cloud leaving the spacecraft will cause the potential on the spacecraft to become negative, eventually applying a braking force on the ion cloud and pulling it back towards the spacecraft. This space charge effect will reduce the thrust to zero, as there will be no net flow of positive ions away from the spacecraft. This problem may be reduced or eliminated by using a source of electron injection as was shown in the charge neutralizing structure  615  of  FIG. 7 .  FIG. 10  shows another embodiment of the present invention where an ion thruster pedestal tip  1014  provides a source of liquid metal ions which form a Taylor cone  1006  at the tip, and the ions are attracted to an extraction electrode  1020  with the potential difference V 1  provided by voltage source  1004 , as was described earlier. An electron emitter pedestal  1016  which is electrically isolated from the liquid metal ion pedestal  1014  may contain a small amount of metal sufficient to form a Taylor cone  1008  when the pedestal  1016  is elevated to a temperature sufficient for the metal at the tip of pedestal  1016  to flow and form a Taylor cone  1008  in response to a voltage V 2  applied to source  1010 . During the Taylor cone forming phase, the pedestal  1008  would have an electrical potential that is positive with respect to extraction electrode  1020 . Then, during a second cooling phase, the potential V 2   1010  would remain while the electron emission pedestal  1016  is cooled until the liquid metal Taylor cone is preserved as a solid metal tip. During a third operational phase, the electron emission pedestal  1016  and hardened Taylor cone tip  1008  may be used as a cold cathode electron emitter, which produces a stream of electrons  1012  which are attracted by extraction electrode  1020  which is now has a positive voltage V 2  with respect to the pedestal  1016 . The volume of the stream of electrons  1012  from the hardened Taylor cone  1008  is selected to mix with and neutralize the stream of charged ion thrust particles  1002  to eliminate the space charge of the plume of ions, thereby preventing ions from returning to the thruster and structures nearby the thruster. 
     In another embodiment of the invention, the ion extraction voltage sources such as V 1   1004  of  FIG. 10 , V 1   702  of  FIG. 7 , V 1  of  FIG. 5 , and voltage source  120  of  FIG. 1  may be time-varying voltages. The nature of the extraction voltage over time may be a duty cycle modulated voltage that steps to a constant voltage level for varying amounts of time as shown in waveform  1100 , or it may step from a first voltage to a varying second voltage at fixed amounts of time as shown in waveform  1102 , or it may step to a fixed voltage for varying amounts of time on a fixed period, as shown in the waveform  1104 , or any combination thereof which allows fine control of the ion stream strength while allowing the optimal operation of the thruster, particularly in the formation of the critical Taylor cones at the tips of the pedestals. It is also clear to one skilled in the art that the voltages of other structures, such as the voltages applied to the focusing electrodes and electron generation structures would vary accordingly to achieve the objectives of ion beam collimation and charge neutralization, respectively. 
     The particular embodiments described herein are for example only. It is clear that the various embodiments can be practiced separately or in combination. In particular, the various forms of liquid metal reservoir, the various liquid metals used as ion sources, the coatings or insulations applied to the extractor electrode, the types and number of focusing electrodes, the various structures of the ion charge neutralizers, and the manner in which voltage is applied to the ion extractor electrode to regulate and control the amount of thrust are each independent variations of the thruster invention which may be practiced alone or in combination.