Patent Publication Number: US-2018051679-A1

Title: Thruster

Description:
TECHNICAL FIELD 
     The present disclosure relates to a thruster. The thruster provides energised particles to provide thrust that may be used for manoeuvring a vehicle. 
     BACKGROUND 
     One method of moving an object is to provide the object with a thruster. By ejecting mass in a specified direction from the thruster, this imparts an equal and opposite momentum to the object. Thrusters may include rocket engines that burn a propellant fuel to create a jet of energised propellant gas that is exhausted from a nozzle of a thruster. Another form of thruster may include ejecting pressurised fluid (such as pressurised gas) from a nozzle. Yet another form of thruster includes electric propulsion that ejects particles that have been accelerated by electromagnetic fields. 
     One form of electric propulsion includes an ion propulsion system where a gas is ionised to provide ionised particles. The ionised particles are then accelerated by the electrodes and the accelerated ionised particles are subsequently neutralised by a neutralising apparatus. Neutralisation may be achieved by injecting electrons into the ion plume so that the charge on a vehicle will remain neutral. Neutralisation is important as there may otherwise be a build-up of negative charge on the vehicle that will eventually stop the exit of ions. 
     Injection of electrons into the ion plume may be provided by an electron gun mounted external to the thruster to neutralise the exiting ions. The electron gun is an additional power-consuming component that adds to the mass and power consumption of a vehicle. 
     SUMMARY 
     A thruster comprising: a chamber to contain a fluid; a plurality of nozzles to exhaust neutral particles derived from the fluid in the chamber, wherein each nozzle has a converging section and the converging section includes a first electrode; a second electrode located distal to the first electrode to provide a voltage differential between the first and second electrodes sufficient to create plasma ions from the fluid and the voltage differential accelerates the plasma ions on a flow path through the converging section, and wherein at least one or more of the accelerated plasma ions are neutralised to form the neutral particles by charge exchange with other neutral particles, or by recombination with electrons, on the flow path. 
     A thruster comprising: a chamber to contain a fluid; a plurality of nozzles to exhaust particles of the fluid from the chamber, wherein each nozzle has a converging section and the converging section includes a first electrode; a second electrode located distal to the first electrode to provide a voltage differential between the first and second electrodes sufficient to create plasma ions from the fluid and the voltage differential accelerates the plasma ions on a flow path through the converging section, and wherein at least one or more of the accelerated plasma ions are neutralised by charge exchange with neutral particles, or by recombination with electrons, on the flow path. 
     The thruster may have the plurality of nozzles arranged in an array. The array may include a two-dimensional array with regular spacing between each nozzle in the plurality of nozzles. The thruster may include a nozzle element having the plurality of nozzles in an array. 
     At least a portion of the nozzle element, having the plurality of nozzles in an array, may be substantially planar. The nozzle element may be formed of an electrically conductive material and the nozzle element forms at least part of the first electrode. 
     The nozzle may include an electrically conductive lining at the converging section to form at least part of the first electrode. The nozzle element may be formed of a conductive material and the nozzle further having the electrically conductive lining. In one alternative the nozzle element may be formed of a non-conductive material and the nozzle further having the electrically conductive lining. In yet another example, the nozzle element is formed of a semi-conductive material and the nozzle further having the electrically conductive lining. 
     The nozzle element may be formed of any one or more of a conductive, non-conductive or semi-conductive material, and wherein the nozzle includes an electrically conductive lining at the converging section to form at least part of the first electrode. 
     The converging section of each of the nozzles may converge towards a respective nozzle axis, and wherein the respective nozzle axis of each of the plurality of nozzles is substantially parallel. 
     In the thruster, the converging section may define a nozzle aperture that is frustoconical. The nozzle aperture may have a generator angle of between about 5 degrees and about 45 degrees from a nozzle axis of the nozzle aperture. In some examples, the nozzle aperture may have a generator angle of between about 15 degrees and about 45 degrees from a nozzle axis of the nozzle aperture. The frustoconical nozzle aperture may have a circular inlet and a circular outlet diameter, wherein the inlet has an inlet diameter in the range of about 1 to about 20 millimetres and the outlet has an outlet diameter in the range of about 0.1 to about 8 millimetres. In some examples, the frustoconical nozzle aperture may have a circular inlet and a circular outlet diameter, wherein the inlet has an inlet diameter in the range of about 0.5 to about 4 millimetres and the outlet has an outlet diameter in the range of about 0.1 to about 0.8 millimetres. The distance between the inlet diameter and outlet diameter along the nozzle axis may be in the range of about 1 to about 20 millimetres. In some examples, the distance between the inlet diameter and outlet diameter along the nozzle axis may be in the range of about 5 to about 20 millimetres. 
     In the thruster, the plurality of nozzles may be disposed at a first end of the chamber and the second electrode may be disposed at a second end of the chamber, wherein at least one chamber wall formed of non-conductive material separates the first and second ends. 
     The thruster may further include at least one shield located in the chamber proximal to a chamber wall, wherein the shield is electrically isolated from the first and second electrode. 
     The thruster may further include a cover to define at least part of the chamber. The cover may be formed of an electrically conductive material and is at least part of the second electrode. 
     In the thruster, the second electrode may be formed of an electrically conductive material. 
     The thruster may be a substantially rectangular cuboid. 
     The thruster may further include a voltage source connected to the first and second electrodes so that the first electrode is a cathode and the second electrode is an anode. 
     The thruster may further include a third electrode located adjacent to a path of the exhausted particles, wherein the third electrode is a second anode. 
     The thruster may further include a fluid inlet to supply fluid to the chamber, wherein the fluid inlet is located proximal to the second electrode. The fluid inlet may further include a plurality of inlets to distribute fluid entering the chamber. 
     The fluid inlet may, in some alternatives, include a plurality of nozzles to distribute fluid entering the chamber. 
     The thruster may further include a fluid flow control means to control the fluid flow into the chamber, wherein the fluid flow control means provide fluid to maintain an operating pressure inside the chamber in accordance with the formula: 
     
       
      
       P=K/D  
      
     
     where 
     P is the pressure inside the chamber in milliTorr; 
     D is the distance between the first and second electrodes in millimetres; and 
     K is a constant between 200 and 200000 milliTorr mm, and is preferably around 6000 milliTorr mm. 
     The thruster may have a length of the chamber between the first and second electrode of at least about 20 millimetres, and preferably of at least about 25 millimetres. 
     The thruster may have a width of the chamber in the range of about 10 to 50 about millimetres. 
     The thruster may have a flow rate in the range of about 0.2 to about 6 standard cubic centimetres per minute. 
     The thruster may have a number of nozzles in the plurality of nozzles in the range of 3 to 1000. In some examples, the thruster may have a number of nozzles in the plurality of nozzles in the range of 10 to 1000. 
     The fluid used in the thruster may include alcohol, water or a combination thereof. The alcohol may be one or more of methanol, ethanol, propanol (including n-propanol and isopropanol), and butanol (including n-butanol and t-butanol) and isopropyl alcohol or mixtures thereof. In one embodiment, the alcohol is isopropyl alcohol or similar. 
     The thruster may further include a permanent magnet to provide a magnetic field in the chamber. The magnetic field may assist in intensifying the plasma density. 
     The thruster may include at least one of the chamber and nozzle to be constructed of one or more silicon wafers. 
     A satellite including at least one thruster as described above. 
     A method of manufacturing a thruster described above, including the steps of: etching a first pattern on a first substrate; etching a second pattern on a second substrate; bonding the first and second substrate to form at least one of the chamber, plurality of nozzles, first electrode and second electrode of the thruster. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Examples of the present disclosure will be described with reference to: 
         FIG. 1  is a cross-sectional view of a thruster according to a first embodiment; 
         FIG. 2  is an enlarged cross-sectional view of a portion of the thruster of  FIG. 1 ; 
         FIGS. 3 a  and 3 b    show exploded perspective views of the thruster of  FIG. 1 ; 
         FIG. 4 a    is a perspective view of the thruster of  FIG. 1 ; 
         FIG. 4 b    is an alternative perspective view of the thruster of  FIG. 1 ; 
         FIG. 5 a    is a perspective view of a nozzle element of the thruster of  FIG. 1 ; 
         FIG. 5 b    is an alternative perspective view of the nozzle element of  FIG. 1 ; 
         FIG. 6  is a front view of a variation of a nozzle element; 
         FIG. 7 a    illustrates a cross-sectional side view of a thruster according to a second embodiment; 
         FIG. 7 b    illustrates the process of wafer etching nozzles in the nozzle element; 
         FIG. 8  illustrates a cross-sectional side view of a thruster according to a third embodiment; 
         FIG. 9  illustrates a cross-sectional side view of a thruster according to a fourth embodiment; 
         FIG. 10  illustrates a cross-sectional side view of a thruster according to a fifth embodiment; 
         FIG. 11  illustrates a cross-sectional side view of a thruster according to a sixth embodiment; 
         FIG. 12  illustrates an exploded cross-sectional view of the thruster in  FIG. 11 ; 
         FIG. 13  illustrates a schematic of a fluid system for supplying fluid to a thruster; 
         FIG. 14 a    is a perspective view of a fluid control means; 
         FIG. 14 b    is a cross-sectional side view of a fluid control means of  FIG. 14 a    along the length of a groove; 
         FIG. 15  is a perspective view of a satellite; 
         FIG. 16  is a perspective view of a nozzle element according to a seventh embodiment; and 
         FIG. 17  illustrates a cross-sectional side view of a thruster according to an eighth embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Overview 
     Referring to  FIGS. 1 to 3 , there is provided a thruster  1  for generating thrust by exhausting a flow of particles  8  from the thruster  1 . The thruster  1  has a chamber  5  to contain a fluid  7 . A plurality of nozzles  9  allow neutral particles  14 , derived from the fluid, to be exhausted from the chamber  5  and each nozzle  9  has a converging section  11  that includes a first electrode  13 . A second electrode  15  is located distal to the first electrode  13  so that a sufficient voltage differential provided between the first and second electrodes  13 ,  15  creates plasma ions  10  from the fluid  7  inside the chamber  5 . The voltage differential between the first and second electrodes  13 ,  15  also accelerates the plasma ions  10  in a flow path through the converging section  11 . At least one or more of the accelerated plasma ions  12  are neutralised to form the neutral particles by charge exchange with other neutral particles  16 , or by recombination with electrons, on the flow path. 
     The accelerated plasma ions  12  that are neutralised and exhausted from the thruster  1  provide neutral exhaust particles  14 . Particles  8  that include accelerated ions  12  and/or neutral exhaust particles  14 , flow in direction A to provide thrust to the thruster  1  in an opposing direction B. 
     The neutralisation of accelerated ions by charge exchange with neutral particles or by recombination with electrons  18  in the flow path may reduce or ameliorate the need to have a neutralising apparatus, such as an electron gun, for neutralising the ions in the exhaust plume. The configuration of the thruster  1  with a plurality of nozzles for exhausting particles from a common chamber (or in some embodiments multiple chambers) may provide a thruster  1  to have dimensions and a form factor that is compact and space efficient for the given thrust output. This may be useful for applications where space and mass are at a premium such as vehicles in space. The thruster  1  may have application with smaller satellites known as “CubeSats” or “nanosatellites”, in which volume and mass of components are particularly important. However, the thruster  1  may have application with larger satellites and it is to be appreciated that multiple thrusters  1  could be used and/or the thruster  1  may be scaled to a larger size to suit performance requirements. 
     Operation 
     The operation of the thruster  1  will now be described with reference to  FIG. 2  which shows an enlarged section of the thruster  1  shown in  FIG. 1 . 
     The fluid  7  is introduced into the chamber  5  via a fluid inlet  23  as gaseous neutral particles  16 . The voltage differential provided between the first and second electrodes  13 ,  15  (so that they become cathodes and anodes respectively) cause at least some of the gaseous neutral particles  16  to ionise to plasma ions  12 . The ions  12  are positively charged and are accelerated in a direction from the second electrode  15  (being an anode) towards the first electrode  13  (being a cathode). This is shown as accelerated ions  12 . Since the first electrode  13  is at least part of the converging section  11  of the nozzle  9 , the accelerated ions  12  move towards the region of the converging section  11 . 
     The converging sections  11  operate to restrict particles to freely flow from the chamber  5 . This configuration firstly facilitates maintenance of pressure inside the chamber  5  by reducing the path that particles, including neutral particles  16 , that may exit the chamber  5 . Secondly, the converging sections  11  channel the accelerated ions  12  in a path towards the nozzle axis  33 . The effect of this is to increase the probability of neutralisation of the accelerated ions  12  by undergoing charge exchange with neutral particles  16  in the flow path of the accelerated ions  12 . This may also increase the probability of neutralisation of the accelerated ions  12  by receiving secondary electrons  18 . 
     Furthermore, the converging section  11  including the first electrode  13  facilitates generation of an electric field for acceleration of the ions. The converging section  11  generates an electric field directed along a nozzle axis  33  from a relatively wider inlet  35  to a narrower outlet  37 , which results in the acceleration of the ions along the nozzle axis  33  in direction A. Some ions may also be accelerated in the chamber  5 , but the electric field (for example in the area halfway between the second electrode  15  and first electrode  13 ) may not be as strong as the electric field in the converging section  11  as discussed above. 
     The velocity of the accelerated ions  12  may depend on a number of factors. One factor is the strength of the electric field that the accelerated ion  12  is exposed to. Secondly, the velocity is also a function of time. Accelerated ions  12  that are relatively closer to the second electrode  15  would have a lower velocity in direction A as these ions have been exposed to a weaker electric field and for a shorter duration of time. This in contrast with the velocity of accelerated ions  12  that are in the converging section  11  that may have had the benefit of acceleration from travelling across the chamber  5  (such as from the second electrode  15  up to entering the converging section  11 ) as well as the relatively higher electric field in the converging section  11 . 
     The probability of neutralisation of an accelerated ion  12  by charge exchange with a neutral particle  16  increases with the velocity of the accelerated ion  12 . Since the accelerated ions  12  in the converging section  11  generally have a higher velocity than the accelerated ions  12  generally in the chamber  5 , the probability of neutralisation in the converging section  11  is higher. 
     It is to be appreciated that although the probability of charge exchange may increase with higher velocity, this probability may reach a maximum at a particular velocity and this may be different for different ionised fluids. Therefore the design of the thruster  1 , including the geometry of the converging section, chamber dimensions, voltages and electric fields may need to be adjusted to suit the characteristics of the fluid. 
     The accelerated ions  12  that have been neutralised become neutral exhaust particles  14  that travel generally in direction A. It is to be appreciated that in some embodiments, not all of the accelerated ions  12  may be neutralised when they are exhausted from the nozzles  9 . 
     The Chemical Equations 
     An example of the chemical equations during neutralisation will be described below. In this example, the fluid  7  is hydrogen. 
     During a first charge exchange, an accelerated ion  12  (denoted as H+) undergoes charge exchange with a neutral hydrogen molecule (denoted as H 2 ) as shown in Equation 1. 
       H + +H 2 ---&gt;H*+H 2   +   (Equation 1)
 
     The result on the right hand side of the equation is that the accelerated ion  12  is neutralised to become a neutral atom (denoted as H*) and exhausted as neutral exhaust particle  14 . The former neutral hydrogen molecule on losing a negative charge becomes a positively charged particle (denoted as H 2   + ). This new positively charged particle (H 2   + ) may then undergo the process of acceleration towards the first electrode  13  and neutralisation as discussed below. 
     The new positively charged particle (H 2   + ) is accelerated toward the first electrode  13  and may undergo a second charge exchange with a neutral hydrogen molecule (H 2 ). A chemical equation of this neutralisation is provided below in Equation 2. 
       H 2   + +H 2 ---&gt;H 2   + H 2   +   (Equation 2)
 
     The result at the right hand side of this equation is that the accelerated hydrogen molecule is neutralised (denoted as H 2 *) which may then be exhausted as neutral exhaust particle  14 . The former neutral hydrogen molecule (H 2 ) on the left hand side of the equation becomes a positively charged and may itself be accelerated and neutralised as discussed above. 
     As discussed above, some of the accelerated ions  12  (denoted as H+) may be neutralised by electrons (denoted as e − ), such as secondary electrons, that are in the flow path of the accelerated ions  12 . This may be shown by Equation 3 below. 
       H+ +   e   − ---&gt;H*  (Equation 3)
 
     The result at the right hand side of this equation is that the accelerated ion  12  is neutralised to provide a neutral exhaust particle  14  (denoted as H*). 
     Similarly a positively charged hydrogen molecule (H 2   + ) may also be neutralised by an electron according to Equation 4 below. 
       H 2   +   +e   − ---&gt;H 2*   (Equation 4)
 
     The result at the right hand side of this equation is that the hydrogen molecule is neutralised (denoted as H 2 *) which may then be exhausted as neutral exhaust particle  14 . 
     Design Considerations 
     One design consideration is maintaining pressure in the chamber  5 . Providing the converging section  11  reduces the likelihood of particles that have not been accelerated from leaving the chamber  5 . Maintaining the pressure also increases the density of neutral particles  16  in the chamber and in nozzle apertures  31  that are defined by the converging section  11  and consequently the likelihood of the accelerated ions  12  to be neutralised by charge exchange with the neutral particles  16 . 
     However, another consideration is that a higher pressure may increase the chance of collisions of the accelerated ions  12  and/or the neutral exhaust particles  14  with other particles in the flow path. Such collisions may reduce the velocity of the accelerated ions  12  and/or the neutral exhaust particles  14  in direction A, which may reduce the thrust generated by the thruster  1 . 
     Furthermore, providing converging sections  11  so that the accelerated ions  12  are directed towards a plurality of narrower outlets  37  of the nozzle apertures  31  may increase the velocity of the accelerated ions  12  compared to an alternative configuration where the nozzle apertures  31  are cylindrically shaped. The increase in velocity may increase the likelihood (i.e. cross-section) of the accelerated ions  12  undergoing charge exchange with neutral particles  16  in the flow path. 
     Another consideration is that in at least some instances, a change in geometry of the nozzle apertures  31  may result in a corresponding reduction in likelihood that an accelerated ion  12  would undergo charge exchange with neutral particles  16 . A change in geometry of the nozzle  9 , in particular the converging section  11 , will result in a change in the spatial distribution of the electric field that is generated inside the converging section  11 . This in turn will affect the distance over which the ions are accelerated and the number of charge exchange events that may occur. Similarly, a reduction in the distance between the first and second electrode  13 ,  15  may also reduce the likelihood of an accelerated ion  12  undergoing charge exchange with neutral particles  16  as the accelerated ions  12  would travel a shorter distance and hence pass by fewer neutral particles  16  in the flow path. 
     In light of at least some of the above mentioned considerations, the thruster  1  may be designed in accordance with the following formula: 
         P=K/D   (Equation 5)
 
     where
         P is the pressure inside the chamber in milliTorr;   D is the distance between the first and second electrodes in millimetres; and   K is a constant between 200 and 200000 milliTorr mm.       

     In some embodiments, it may be desirable to have a constant (K) around 6000 milliTorr mm. A constant (K) of 6000 milliTorr mm may be suitable for a fluid  7  that includes hydrogen (H 2 ). 
     However, it is to be appreciated that other constants (K) may be suitable depending on other factors including fluid  7  composition, voltage applied to the electrodes  13 ,  15  and/or shape and configuration of components of the thruster  1 . 
     A further and/or an alternative consideration is a minimum distance between the first electrode  13  and the second electrode  15  which may affect the electrostatic field in the chamber  5 . In one embodiment the distance between the first electrode  13  and the second electrode  15  is at least about 20 millimetres. In further embodiments, the distance between the first and second electrodes  13 ,  15  may be at least about 25 millimetres or more. In some embodiments the distance between the first and second electrodes  13 ,  15  is in the range of about 20 to about 100 millimetres. The chamber  5  may have a width in the range of about 10 to about 50 millimetres. In one example, the chamber width is about 30 millimetres. 
     The voltage potential provided between the first and second electrodes  13 ,  15  should be at a level that ionises the selected fluid  7  and provides acceleration of the accelerated ions  12 . The voltage required may also be dependent on the other design considerations, such as the distance between the first and second electrodes  13 ,  15  and the respective materials. In one embodiment, the voltage potential is in the range of 1.0 to 5 kV. In a further embodiment, the voltage potential is in the range of about 1.0 to 2.0 kV. In an alternative embodiment, the voltage potential is in the range of about 2.0 to 4.9 kV. 
     The above considerations may provide a thruster  1  in the described embodiments to have a flow rate of fluid through the thruster  1  in the range of about 0.2 to 6 standard cubic centimetres per minute. In some embodiments, the flow rate is less than 1 standard cubic centimetres per minute. 
     However it is to be appreciated that the thruster  1  may be scaled to smaller or larger sizes that include corresponding changes in dimensions, voltages, configurations and/or flow rates. 
     Description of a First Embodiment 
     The thruster  1  according to a first embodiment will now be described with reference to  FIGS. 3 to 5, 12 and 15 . 
     Enclosure  3   
     Referring to  FIGS. 3 a  and 3 b   , an enclosure  3  that defines the chamber  5  includes three main components: a nozzle element  17 , a spacer  19  and a cover  21 . The nozzle element  17  is disposed at a first end of the enclosure  3  and the cover  21  is at a second end of the enclosure and with the spacer  19  substantially there between. 
     When assembled, these components form the enclosed chamber  5 . A fluid inlet  23  allows fluid  7  to enter the chamber  5 , in which the fluid  7  is ionised and the ions accelerated, and the nozzles  9  allow particles  8  to be exhausted from the chamber  5  to provide thrust. A plurality of thrusters  1 , as shown in  FIG. 15 , could be used on a space vehicle to provide thrust in various axes or to generate torque. For example, the thrusters  1  could be used individually or in combination to achieve attitude manoeuvres. 
     One consideration for the shape of the enclosure  3  may include maximising the use of space or other spatial considerations in the application of the thruster  1 . The thruster  1  shown in  FIG. 4  has an enclosure  3  that is a substantially rectangular cuboid. This configuration may provide a compact configuration to maximise the use of space. For example, the thruster  1  may be used in a satellite  900  having a substantially rectangular cuboid shape, as illustrated in  FIG. 15 , and providing an enclosure  3  of a substantially rectangular cuboid may maximise the use of space in the satellite. However it is to be appreciated that in some alternatives, the enclosure  3  may have other shapes. For example, the enclosure  3  may have a substantially cylindrical shape as illustrated in  FIG. 12 . 
     The Cover  21   
     The cover  21  forms part of the perimeter at the second end of the enclosure  3  to define the chamber  5 . The cover  21  encloses at least part of the chamber  5  to prevent unwanted leakage of the fluid  7  from the chamber  5 . This is important in use to maintain pressure of the fluid  7  inside the chamber  5 . 
     Furthermore, the cover  21  includes the second electrode  15  at the side of the cover  21  facing the chamber  5 . In the illustrated embodiment, the second electrode  15  and cover  21  are substantially planar and oppose the first electrodes  13  at the other end of the enclosure  3 . For a cuboid chamber  5  of fixed dimensions, this configuration maximises the distance between the first and second electrode  13 ,  15 . 
     The cover  21  may be formed of an electrically conductive material so that a surface of the cover  21  forms the second electrode  15 . A conductive material for the cover  21  may include titanium, aluminium or gold. 
     In one alternative embodiment, the cover  21  may include a first substrate (of one or more of a conductive, non-conductive, or semi-conductive material) and a second substrate of conductive material whereby the second substrate faces the chamber  5  to form the second electrode. The first substrate may include a ceramic material, silicon, glass, etc. A conductive material for the second substrate may include doped silicon, titanium, aluminium and gold. Such conductive material may include silicon gold alloy. In yet another embodiment, the second electrode  15  may be a separate component from the cover  21 . 
     The cover  21  also includes a fluid inlet  23 , which is in the form of an aperture fluidly connected to an inlet pipe. The fluid inlet  23 , which is provided proximal to the second electrode  15  supplies fluid  7  to be ionised and accelerated. Providing the fluid  7  proximal to the second electrode  15  may provide a longer path for acceleration of ions from the second electrode  15  to the first electrode  13 , thereby providing greater impulse to the ions and resulting in greater velocity of the particles  8 . This may result in a greater chance of the accelerated ions  12  to be neutralised. It is to be appreciated that more than one fluid inlet  23  may be provided and that in alternative embodiments the fluid inlet  23  may enter the chamber through other components of the enclosure such as through the spacer  19 . 
     The Spacer  19   
     The spacer  19  also forms part of the perimeter of the enclosure  3  to define the chamber  5  and maintain pressure within the chamber  5 . The spacer also functions to separate the nozzle element  17  (having the first electrode  13 ) and the cover  21  (having the second electrode  15 ). The spacer  19  may be provided such that the length of the chamber  5  from the first electrode  13  to the second electrode  15  is at least 20 mm, or alternatively 25 mm or greater. In some embodiments the chamber has a length in the range of 20 to 100 millimetres and a width in the range of 10 to 50 mm. It is to be appreciated that these dimensions are in accordance with some embodiments and that other dimensions may be considered. 
     To provide ionisation and acceleration, a voltage difference is provided between the first electrode  13  and the second electrode  15 . Therefore it is important to provide good electrical insulation between the first electrode  13  and the second electrode  15 . This is facilitated by providing a spacer  19  made of non-conductive material. Non-conductive material may include one or more of: SiNx, SiO 2 , ceramic, polytetrafluoroethylene, or other polymers. 
     As shown in  FIGS. 3 a  and 3 b   , the spacer  19  includes four chamber walls  25  surrounding the chamber  5 . However it is to be appreciated that other configurations of the spacer  19  may be suitable, for example a cylindrical side wall surrounding the chamber  5  as shown in  FIG. 12 . 
     The Nozzle Element  17   
     The nozzle element  17  will now be described with reference to  FIGS. 3 and 5 . The nozzle element  17  is substantially planar and includes a plurality of nozzles  9  arranged in an array. The nozzles  9  may be arranged in a two-dimensional array with regular spacing between each of the nozzles. In some embodiments, regular spacing may assist in providing predetermined thrust characteristics or assist in calculation of thrust characteristics. In alternative embodiments, the plurality of nozzles  9  may be arranged with irregular spacing between nozzles. The plurality of nozzles  9  may be arranged to provide specified thrust characteristics. For example, one or more of the plurality of nozzles  9  may have a nozzle axis  33  that is different to another nozzle to impart a spin on the object having the thruster  1 . 
     In  FIG. 5  the nozzle element  17  includes sixteen nozzles  9  in a four-by-four array. In some embodiments the number of nozzles  9  in the plurality of nozzles is in the range of 3 to 1000. In some further embodiments the number of nozzles  9  in the plurality of nozzles is in the range of 10 to 1000. 
     In some embodiments, the nozzles  9  are arranged in predetermined circle packing patterns (e.g. triangular tiling as shown in  FIG. 6 ) to provide maximum density of nozzles  9  for the surface of the planar nozzle element  17 . However, it is to be appreciated that in other embodiments, the configuration of the plurality of nozzles may be in alternative patterns. 
     The nozzle element  17  may be formed of an electrically conductive material having the plurality of nozzles  9 , whereby at least part of the nozzle elements  17  form the at least one electrode  13 . In an alternative embodiment, the nozzle element  17  includes a substrate formed from one or more of a conductive, non-conductive or semi-conductive material and further including an electrically conductive lining at the converging section  11  of the nozzles to form the first electrode  13 . 
     In the embodiment illustrated in  FIGS. 3 a  and 3 b   , the entire chamber facing surfaces of nozzle element  17  may form a cathode. However, in some embodiments, only the converging section  11  (or part of the converging section  11 ), form a cathode. This is discussed below in the some of the other embodiments where parts of the cathode forming structure are masked to limit exposure of the cathode to the converging section  11 . 
     The Nozzles  9   
     The nozzles  9  will now be described with reference to  FIGS. 1 and 5 . Generally, each nozzle  9  has a nozzle aperture  31  formed by an inlet  35  that converges to a relatively narrower outlet  37  by the converging section  11 . This constricts the flow of particles from the chamber  5  and assists in maintenance of pressure inside the chamber  5 . 
     In the illustrated embodiment, each nozzle  9  has a nozzle aperture  31  with a frustoconical shape. Each nozzle aperture  31  has a nozzle axis  33  and extends from an inlet  35  (at the chamber side) to an outlet  37  (at the exhaust side). The inlet  35  and outlet  37  may be substantially circular. Between the inlet  35  and the outlet  37  is the converging section  11  which in this case is a generally conical surface that converges towards the nozzle axis  33  from the inlet  35  to the outlet  37 . 
     In some embodiments, conical surface of the converging section  11  has a generator angle of between 5 and 45 degrees from the nozzle axis  33  of the nozzle aperture  31 . In some further embodiments the generator angle is between 5 degrees and 45 degrees. 
     In some embodiments, the inlet  35  may have an inlet diameter of between 1 to 20 millimetres. In some further embodiments, the inlet diameter may be between 0.5 and 4 millimetres. The circular outlet  37  may have an outlet diameter of between 0.1 and 8 millimetres. In some further embodiments, the outlet diameter may be between 0.1 and 0.8 millimetres. In some embodiments, the distance between the inlet  35  and the outlet  37  along the nozzle axis is in the range of 1 to 20 millimetres. In some further embodiments, the distance between the inlet  35  and the outlet  37  along the nozzle axis is in the range of 5 to 20 millimetres. 
     It is to be appreciated that in some other embodiments, the nozzles apertures  31  may be defined by inlets  35 , outlets  37  and converging sections  11  in different configurations. In one example, the converging sections  11  may be formed of a plurality of planar surfaces that converge towards the nozzle axis  33  from the inlet  35  to the outlet  37  to form a nozzle aperture in the shape of a frustum of a pyramid as illustrated in  FIG. 7 . Such pyramid shapes may include triangular pyramids, square pyramids, rectangular pyramids, hexagonal pyramids, etc. In yet other embodiments, the converging sections  11  may include a curved surface where a cross-section in a plane through the centre axis  33  provides a converging section  11  edge with a curve. 
     Description of a Second Embodiment 
     Another embodiment of a thruster  101  will now be described with reference to  FIGS. 7 a  and 7 b   . In this embodiment the thruster is constructed from multiple layers of silicon wafers  150  processed by wet etching (as illustrated in  FIG. 7 b   ), lithography and thin film coating. Techniques for creating such multiple layers may include techniques the same as, or similar to, those used in the semiconductor industry. The layers of silicon wafers  150  may be bonded together with epoxy. 
     Referring to  FIG. 7 a   , the thruster  101  includes a plurality of chambers  105 , where each chamber  105  is provided with a respective nozzle  109 .  FIG. 7 a    shows a cross section of the thruster  101  with multiple nozzles arranged in a row. However, it is appreciated that additional nozzles may be provided so that the thruster  101  may include an array of nozzles as described above. The thruster  101  is constructed from multiple layers that will now be described in order. 
     A first cover layer  121  includes an aperture to form a common fluid inlet  123 . An intermediate chamber layer  128  provided at the outer perimeter of the enclosure  103  forms an intermediate fluid chamber  130 . The intermediate fluid chamber  130  aids distribution of the fluid  7  to the multiple chambers  105 . The next layer is a fluid inlet layer  138  provided with multiple apertures to form individual fluid inlets  140  for each of the plurality of chambers  105 . The apertures forming the individual inlets  140  may be etched, as illustrated in  FIG. 7 b   , to form apertures having a shape of a frustum of a square pyramid. It is to be appreciated that alternatively, other aperture shapes may be used, such as a frustoconical shape, a cylindrical shape, rectangular shapes, etc. 
     The next layer is an anode layer  115 . This layer may be formed of a conductive material such as titanium, aluminium, copper, gold, doped silicon, etc. The anode layer  115  provides the second electrode that is in communication with the chamber  105  and functions similarly to the second electrode  15  described above. 
     The next layer is a spacer layer  125  that includes a plurality of apertures to form the plurality of chambers  105 . The apertures may be rectangular, circular, or other shapes. The spacer layer  125  may function to electrically insulate the second electrode layer to the first electrode  113  (discussed below). Therefore the spacer layer  125  may be constructed of a functionally electrically non-conductive material. 
     The next layer is a nozzle layer  117 . The nozzle layer  117  includes a plurality of apertures that are defined by converging surface(s). The apertures may be shaped as a frustum of a pyramid or have a frustoconical shape. The nozzle layer  117  is provided with a cathode layer  113  that forms an electrode functionally similar to the first electrode  13  described above. The cathode layer  113 , which overlies the surfaces of the apertures in the nozzle layer  117 , also forms the converging sections  111  that define the apertures  109  and are functionally similar to the converging sections  11  that define apertures  9  described above. 
     The next layer is an end layer  122  that includes a plurality of apertures  160 . The apertures  160  allow passage for the particles  8  to be exhausted from the chamber  105 . The apertures  160 , in this embodiment, include diverging surface(s). Similar to the nozzles  9 , the apertures  160  may be shaped as a frustum of a pyramid, a frustoconical shape, etc. However, it is to be appreciated that alternative embodiments may include other shapes such as a cylindrical or square bore. 
     The next layer is a second anode layer  120  that forms a third electrode. The second anode layer  120  includes a plurality of apertures to allow particles  8  to exhaust from the thruster  101 . The second anode layer  120  may be provided with a voltage differential (to the cathode) so that electrons may be attracted to the region of the flow of particles  8 . In some embodiments, not all particles  8  that pass through the apertures  109  and the nozzle layer  117  are neutralized. The second anode layer  120 , by attracting electrons may facilitate providing electrons in the path of the particles  8  so that accelerated ions  12  (or other positively charged particles) may be neutralized. 
     In one embodiment, the second anode layer  120  may attract secondary electrons. 
     Description of a Third Embodiment 
       FIG. 8  illustrates a third embodiment of a thruster  201 .  FIG. 8  illustrates one chamber  205  and nozzle  209  to improve clarity. It is to be appreciated that in this embodiment, multiple chambers  205 , nozzles  209  and other relevant features may be provided on the layers so that the thruster  201  includes a plurality of nozzles, including an array arrangement as discussed above. The thruster  201  is constructed from multiple layers that will now be described in order. 
     The first layer is a cover layer  221  constructed of a ceramic material. The cover layer  221  may be the first layer that forms a base on which subsequent layers  250  of silicon, or other material, is fabricated on. The cover layer  221  includes an aperture to provide a fluid inlet  223 . 
     The next layer is a fluid inlet layer  238  with a passage to allow communication with the chamber  205 . The next layer is the anode layer  215  that provides the second electrode. The next layer is an intermediate chamber layer  228  that is provided at the outer perimeter of the enclosure  203  to form an intermediate chamber  230 . As illustrated in  FIG. 8  the intermediate chamber  230  is substantially wider than the chamber  205 . The intermediate chamber  230  also accommodates the large anode layer  215 . This configuration may provide greater surface area for the anode layer  215  to be in contact with the fluid  7  in the intermediate chamber  230 . 
     The next layers are the spacer layers  225 , similar to the spacer layers described above that include apertures to form the chamber  205 . The spacer layers  225  may be made of silicon wafers stacked on each other. In one embodiment, the layers provide a chamber length in the range of 20 to 35 millimetres. The apertures in the spacer layers  225  may provide chambers with a width of approximately 1.5 millimetres. 
     The next layer is a cathode layer  213 , which overlies the surfaces of the apertures in a nozzle layer  217 . The cathode layer  213  forms the converging sections  211  that define the apertures  209  and are functionally similar to the converging sections  11  described above. The cathode layer  213  is, in part, sandwiched between the spacer layer  225  and nozzle layer  217  to reduce the cathode layer  213  from being exposed. This may reduce the chance of charged particles from being inadvertently attracted or repelled by the cathode layer  213 . 
     The next layer is a nozzle layer  217 . The nozzle layer  117  includes an aperture that is defined by converging surface(s). The apertures may include various shapes as described above. The small exit diameter of the apertures in the nozzle  209  may have a width of approximately 0.1 millimetres. 
     The next layers are end layers  222  that include an aperture  260 . The aperture  260  allows passage for the particles  8  exhausted from the chamber  205 . The apertures  160 , in this embodiment, include a bore with a straight surface, although alternatives such as the other shapes described above may be used. 
     The next layer is a second anode layer  220 . In this embodiment, the second anode layer covers at least part of the bore of the aperture  260 . The second anode layer  220  may function similar to the second anode layer  120  described above to neutralize positively charged particles. The second anode layer  220 , by covering at least part of the bore of the aperture  260  may provide improved attraction of electrons in the area of the aperture  250 . 
     In one embodiment, the minimum distance between the cathode layer  213  and the second anode layer is approximately 0.5 millimetres. 
     One or more layers  250  may include silicon wafers. The silicon wafers may have a thickness in the range of 0.5 to 2 millimetres thick. The layers of silicon may also be oxidised on the surface to provide insulation. In one example, the silicon oxide layer is around 5 micrometres thick. 
     Description of a Fourth Embodiment 
       FIG. 9  illustrates a fourth embodiment of a thruster  301 . In this embodiment, the thruster  301  includes a common chamber  305   a  that leads to a plurality of individual chambers  305   b  for each respective nozzle  309 .  FIG. 9  illustrates one individual chamber  305   b  and nozzle  309  that is fluidly connected to the common chamber  305   a . However, it is to be appreciated that thruster  301  includes a common chamber  305   a  that leads to multiple individual chambers  305   b  and respective nozzles  309  which may be arranged in an array as discussed above. 
     The thruster  301  is constructed from multiple layers  350  that will now be described in order. 
     The first layer is an anode layer  315  that provides the second electrode and also functions to cover the end of the enclosure  303 . The anode layer  315  includes an aperture to for a fluid inlet  323 . 
     The next layer is a spacer  315  to provide a volume of a common chamber  305   a . The spacer  315  may be made of a non-conductive material such as glass. 
     The next layer is a spacer layer  325  that includes apertures to form respective individual chambers  305   b . In this embodiment, the aperture in the spacer layer  325  is smaller than the width of the common chamber  305   a  at the glass spacer  315 . The spacer layer  325  may be made of a non-conductive material. In one embodiment, the spacer layer  325  is made of one or more layers of silicon wafers. 
     The next layer is a cathode layer  313 , which overlies the surfaces of the apertures in a nozzle layer  317 . The cathode layer  313  forms the converging sections  311  that define the apertures  309  and are functionally similar to the converging sections  11  described above. The cathode layer  313  is, in part, sandwiched between the spacer layer  325  and nozzle layer  317 . 
     The arrangement of the spacer  315  and the spacer layer  525  may provide a combined chamber length of the common chamber  305   a  and individual chambers  305   b  to be longer. In one example the combined chamber length may be up to and including 110 millimetres. 
     Description of a Fifth Embodiment 
       FIG. 10  illustrates a fifth embodiment of a thruster  401  that includes multiple layers  450 . In this embodiment, the thruster  401  includes a chamber  405  that is fluidly connected to a plurality of nozzles  409 . A common fluid inlet  423  is fluidly connected to an intermediate fluid chamber  430 . The intermediate fluid chamber  430  leads to a plurality of individual fluid inlets  440  that in turn distribute fluid entering the chamber  405 . 
     The plurality of individual fluid inlets  440  may improve distribution of fluid  7  into the chamber  405 . In particular, this may assist uniform distribution to allow uniform plasma formation and thrust through the nozzles  409 . This may be advantageous for thrusters with a larger array of nozzles  409 , such as those with an array larger than 60 by 60 millimetres. 
     The thruster  401  is constructed from multiple layers  350  that will now be described in order. 
     The first layer is a cover layer  421  that includes an aperture to form a common fluid inlet  423 . This is followed by an intermediate chamber layer  428  that defines the intermediate chamber  430 . The next layer is a fluid inlet layer  438  which is provided with a plurality of apertures that form a plurality of individual fluid inlets  440  to allow the flow of fluid  7  into the chamber  405 . The fluid inlets  440  may be arranged in various patterns, including arrays, to achieve a desired distribution. The fluid inlet layer  438  may be made of a non-conductive material to mask at least part of the adjacent anode layer  415  discussed below. The masking of the anode layer  415  may reduce the chance of the anode layer  415  inadvertently influencing particles in the intermediate fluid chamber  430 . 
     The next layer is the anode layer  415  that includes a plurality of apertures to facilitate flow of fluid from the intermediate chamber  430  to the chamber  405 . The anode layer  415  is functionally similar to the second electrode described above. 
     The next layer is the spacer layer  425  provided to form the chamber  405 . The spacer layer  425  may be made of a non-conductive material. In one embodiment the spacer layer  425  includes a double side wall made of silicon. 
     The next layer is a cathode mask layer  456  that includes a plurality of apertures each leading to respective nozzles  409 . The cathode mask 456 layer is made of a non-conductive material and is provided to mask parts of the cathode layer  413  from the chamber  405 . In the illustrated embodiment the cathode mask layer  456  masks the cathode layer  413  so that only parts of the cathode layer  314  that form converging sections  411  are exposed to the chamber  405 . This configuration may facilitate acceleration of positively charged particles, such as ions  12 ,  16 , towards the nozzles  409 . 
     The nozzle layer  417  includes a plurality of apertures and supports the cathode layer  413  to provide the nozzles  409  similar to the nozzle layers discusses above. 
     The plurality of nozzles  409  lead to a common exhaust chamber  470 . An exhaust chamber layer  472  provides the common exhaust chamber  470 . An end layer  422  includes a plurality of apertures to provide a plurality of exhaust apertures  460 . In this embodiment, exhaust apertures  460  are provided on the nozzle axis  33  of respective nozzles  9 . This facilitates flow of particles from the nozzles  409  through the exhaust apertures  460 . 
     The next layer is the second anode layer  420 . The second anode layer  420  may function similar to the second anode layer  120 ,  220  discussed above to attract electrons for neutralising positively charged particles that may be exhausted through the nozzles  409 . 
     Description of a Sixth Embodiment 
       FIG. 11  illustrates a sixth embodiment of the thruster  501 . The thruster in this embodiment includes shields  580  adjacent to the walls  525  of the chamber  505 . 
     During operation, some of the accelerated ions  12  may collide with the first electrode  13  (illustrated in  FIG. 11  as cathode  513 ). Such collisions may cause sputtering, whereby the sputtered atoms may coat the walls  525  of the chamber  505  with a conductive layer. A conductive layer on the chamber wall may detrimentally cause leakage current between the first and second electrodes  13 ,  15  and reduce the power efficiency of the device. Providing shields  580  may prevent or reduce the effects of sputtering by shielding at least part of the wall of the chamber  505  from the sputtered atoms. The walls  525  and the shields  580  are made of a non-conductive material. This may include ceramics, polymers or other non-conductive materials described herein. 
     The embodiment in  FIG. 11  also includes an anode layer  515 , cover layer  521 , fluid inlet  523 , cathode mask layer  556 , cathode layer  513 , nozzles  509  and nozzle layer  517  similar to those described above. An end layer  522  is provided with a plurality of exhaust apertures  560  for each of the nozzles  509 . 
     The anode layer  515  may be formed of a layer of copper over an aluminium substrate. In one example, this may include a cover layer  521  made of aluminium with a copper anode layer  515 . This may include electroplating the copper to the aluminium substrate. The cathode layer  513  may include an aluminium substrate with a layer of titanium coated, plated or otherwise bonded to the aluminium substrate. In one example, this may include a nozzle layer  517  with a titanium cathode layer  513 . 
       FIG. 12  illustrates a variation of the thruster  601  that is substantially cylinder shaped. A spacer  619  includes a substantially tube shaped body wherein the hollow forms the chamber  5 . A cover  21  and a nozzle element  617  are substantially disc-shaped and are disposed at the ends of the tubular spacer  619 . The inner surface of the cover  621  forms an anode  615  and is made from a conductive material and may contain surface augmentation  666 , in the form of pyramid shaped protrusions, to promote the striking of plasma by the creation of a sharp electric field. In some embodiments, the nozzle element  617  may be constructed of aluminium or titanium, doped silicon, silicon gold alloy or other conductive material. The spacer  619  is constructed of an insulate, glass, ceramic, etc. As shown in  FIG. 12 , an aperture is provided in the side of the spacer  619  to form a fluid inlet  668 . The nozzle element  617  may be formed of a conductive material so that the region of the nozzle  9 , in particular the converging section, is conductive. The converging section may form a cathode as described above. The nozzles  9  may have a converging section  11  that defines an aperture having a frustoconical shape, with circular outlet having a diameter less than 1 millimetre. A cylindrical ring anode (not shown), functionally similar to the second anode  220  in  FIG. 8 , may be provided downstream of the nozzles  9 . 
     Description of a Seventh Embodiment 
     A seventh embodiment of the thruster includes a nozzle element  717  that is formed by milling a plurality of nozzles  9  from a based material. In one examples, the base material is a block of metal. The metal may include aluminium, titanium, and/or other metals and alloys. 
     An example of the nozzle element  717  of the seventh embodiment is illustrated in  FIG. 16 , although it is to be appreciated that it may be in a form similar to those illustrated in  FIGS. 3, 5   a , and  6 . In one example, the nozzle element  17  may be manufactured using a CNC (computer numerical control) machine to mill out material to create the nozzles  9 . 
     The nozzle element  717  also includes a milled channel  724  around the plurality of nozzles  9 . In one example, the milled channel  724  may receive a seal, such as an O-ring (not illustrated) made of rubber, silicon or other appropriate material. When the thruster is assembled, the O-ring also contacts the spacer to form a hermetic seal between the spacer and nozzle element  17  when joined. 
     In some alternatives the nozzle element  717  may be formed by additive manufacturing. This may include 3D printing of the nozzle element  717  or portions thereof. The 3D printed nozzle element  717  may be printed with the features of the plurality of nozzles  9  and or channel  724 . In some examples, further manufacturing processes may be used to finish the nozzle element  717  to provide such features. In some examples, the 3D printed nozzle element  717  includes one or more of the base metals described above. 
     It is to be appreciated that the spacer and the nozzle element may be joined in a number of ways. In one example, the nozzle element and spacer may be fastened to one another by fasteners, such as a bolt. As illustrated in  FIG. 16 , the nozzle element  717  may have apertures  726  to receive fasteners. In some examples, the spacer and nozzle element may be joined together by bonding, such as with an adhesive, chemical and/or cement. 
     The insulating spacer  19  may, in some alternatives, be manufactured with additive manufacturing. In some examples, this may include 3D printing of insulating material to form the insulated spacer  19 . 
     Description of an Eighth Embodiment 
       FIG. 17  illustrates an eighth embodiment of the thruster  801 . In this illustrated example, the thruster  801  also includes a nozzle element  817  and cover  821  with respective first and second electrodes  813 ,  815 , and a spacer  819 . It is to be appreciated that further variations may include features from the other embodiments described herein. 
     The thruster  801  in this embodiment includes a magnet  866  located outside the walls  825  of the chamber  805 . The magnet provides a magnetic field which in part, passes through the chamber, to influence the plasma as described below. It is to be appreciated that other variations may include a magnet located inside the chamber  805 . 
     In one example, the magnet  866  is an annular permanent magnet (e.g. a “ring magnet”) that is located to surround the walls  825  of the spacer  819 . Thus the magnet  866  encircles the chamber  805 . The magnet includes a north pole  874  and a south pole  876 . The magnet  866  provides a magnetic field that is represented by magnetic field lines  878  which, in part, passes through the chamber  805 . The magnetic field passes through the chamber  805  approximately along the electric field direction between the anode (in this case the second electrode  815 ) and the cathode (in this case the first electrode  813 ). This may assist confining electrons in around the magnetic field inside the chamber  805 . This may, in turn, assist in intensifying the plasma density and to produce more ions. This may result is enhanced thrust which, with greater efficiency, may reduce the fluid  7  or rate of fluid that needs to be consumed. 
     Fluid System  60   
       FIG. 13  shows a schematic of the fluid system  60  for supplying fluid  7  to the chamber  5  in the enclosure  3 . The fluid system  60  includes a fluid tank  61  that is in fluid communication, via fluid conduits  69 ,  71 ,  73 , to the fluid inlet  23  of the thruster  1 . In between the fluid tank  61  is a valve  67 , to allow or stop the flow of fluid  7  and a fluid flow control means  41  to control the flow rate of the fluid  7 . It is to be appreciated that the valve  67  could be placed anywhere between the fluid flow path between the fluid tank  61  and the thruster  1 . 
     The fluid  7  in the fluid tank  61  may be stored, in part, in a liquid state. The fluid  7  in the fluid tank  61  may be pressurised relative to the surroundings (either the surrounding atmosphere or the vacuum of space). This pressurisation may be due to the vapour pressure of the liquid fluid  7  and/or the gas pressurisation of gaseous fluid  7 . This relative pressurisation of fluid  7  in the fluid tank  61  causes the fluid  7  to flow from the fluid tank  61  to the chamber  5  and subsequently towards the nozzle apertures  31  that leads to the lower pressure surrounding atmosphere or vacuum of space. Thus this configuration may not require a fluid pump to supply the fluid  7  from the fluid tank  61  to the chamber  5 . However it is to be appreciated that in alternative embodiments, a fluid pump may be provided to facilitate supply of the fluid  7 . 
     A voltage source  75  is also illustrated which provides the voltage potential to the first and second electrodes  13 ,  15  by electrical leads  77  and  79  respectively. 
     Fluid Flow Control Means  41   
     Maintaining the pressure in the chamber  5  at a desired level or range of pressures, depends on one or more interrelated factors that may include the flow rate of fluid  7  into the chamber  5 , the dimensions and/or shape of the chamber  5 , the dimensions and shape of the nozzle apertures  31 , the number of nozzles  9 , the flow rate of particles  8  out of the thruster  1  and the voltage difference applied to the first and second electrodes  13 ,  15 . 
     The fluid flow control means  41 , that controls the fluid flow into the chamber  5  will now be described with reference to  FIGS. 14 a  and 14 b    that shows a perspective view of the fluid control means  41  and a cross-sectioned view of the fluid control means  41  along the length of a groove  45 . The fluid control means  41  include a first substrate  43  provided with a groove  45 . A second substrate  47  is provided to cover the groove  45  so that the groove  45  defines a fluid passage  46 . An inlet  49  is fluidly connected to one end of groove  45  and an outlet  51  is fluidly connected to another end of the groove  45 . 
     The flow rate of the fluid  7  through the fluid flow control means  41  may be dependent on the pressure difference between the inlet  49  and outlet  51 , the working temperature and specific properties of the fluid  7 . The flow rate is also dependent on the dimensions and structural configuration of the fluid passage  46 , including the cross-sectional area of the passage, the length of the fluid passage  46 , the area of the passage walls and the material properties of the first and second substrate  43 ,  47  that define the passage wall. 
     In the illustrated embodiment, the first and second substrates  43 ,  47  are substantially planar. The first and second substrates  43 ,  47  may be made of one or more of silicon wafer and/or glass. In one embodiment, the first substrate  43  is made of a silicon wafer with the second substrate  47  may be made of glass plate attached thereto. 
     To define the fluid passage provided by groove  45  when the first and second substrates  43 ,  47  are connectedly engaged, the first and second substrates  43 ,  47  may be bonded together. Bonding may be achieved by using an adhesive. In one example a thermally conductive epoxy may be used to bond the first and second substrates  43 ,  47  and to seal the fluid control means  41  assembly. In another embodiment, a glass substrate may be bonded to a silicon substrate by anodic bonding. 
     The first substrate  43  may have a groove  45  cut with a dicing saw. In one alternative, the groove may be created with a dry etching process. 
     The groove  45  may have a cross-sectional dimension and length that is, in part, dictated by the required flow rate and other factors as discussed above. In one example, the groove  45  has a cross-section of about 40 micrometres wide by 20 micrometres deep. In other examples the groove  45  has a cross-sectional dimension in the range of about 3 by 3 micrometres to about 10 by 10 micrometres. 
     The fluid flow control means  41  may advantageously provide precise fluid flow rates to the chamber  5  of the thruster  1 . In some application, such as in miniature satellites, the fluid flow rate is small, such as in the order of 1 standard cubic centimetre per minute or less. Such flow rates require precise control of fluid  7  that can be achieved by the characteristics of the fluid passage defined by groove  45 . 
     When determining characteristics of the fluid control means  41  and the thruster  1 , the mass flow, volume flow and leak rate (through the fluid control means  41 ) may be determined by the followings formulas: 
     
       
         
           
             
               
                 
                   
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                   ) 
                 
               
             
           
         
       
     
     Fluid Tank  61   
     The fluid tank  61  includes a fluid chamber  63  to store the fluid  7 . The fluid  7  in the fluid chamber  63  may be in a liquid state. Generally, fluid  7  stored in a liquid state may be advantageous as it allows the maximum storage of fluid  7  for a given volume of the fluid chamber  63 . That is, it may provide the most efficient use of space which is at a premium for satellite applications. 
     However, when providing the fluid  7  into the chamber  5  for ionisation and acceleration, it may be desirable to have the fluid  7  in a gaseous form. The fluid  7  in the gaseous form may assist the ionisation process as it may require less energy to ionise gaseous fluid compared to liquid fluid. Furthermore, if the fluid  7  flows through the conduits  69 ,  71 ,  73  in liquid form, this may result in an undesirably large amount of fluid  7  to flow into the chamber  7  that may affect the efficient operation of the thruster  1 . 
     To prevent or ameliorate the fluid  7  in liquid form from flowing out of the fluid tank  61 , a membrane  65  is provided to form a liquid barrier. The membrane  65  may include properties, such as microscopic apertures, to allow gaseous fluid  7  to pass from the fluid chamber  63  to the conduit  69 . In one example, the microscopic apertures in the membrane  65  may be in the range of 0.3 to 5 micrometres. It is to be appreciated that the membrane material and/or aperture size may be selected to suit the type of fluid  7  to achieve the above mentioned function. 
     The Fluid  7   
     The fluid  7  is of a type that can be ionised in the thruster  1 . The fluid  7  may be homogenous or alternatively a heterogeneous mixture. 
     One fuel may include hydrogen, where in the molecular form H 2 , is supplied into the chamber  5  via the fluid inlet  23 . At least some of the hydrogen is then ionised and accelerated as discussed in this description. 
     A gas, liquid or solid that can be atomised and strike plasma between the anode and cathode in the chamber may also be suitable for the thruster. For example, some other fluids may include water, isopropyl alcohol, methanol, ethanol, propanol (including n-propanol and isopropanol), and butanol (including n-butanol and t-butanol). It is also to be appreciated that the fluid  7  may be a mixture of fluids and, in one example, may include a mixture of isopropyl alcohol and water. In one embodiment, the alcohol is isopropyl alcohol or similar. 
     Satellite  900   
     One application for a thruster is for manoeuvring a spacecraft. The efficiency of spacecraft propulsion may be determined by the change in momentum (impulse) per unit weight of propellant, which is known as specific impulse. Greater propulsion efficiency is achieved by increasing the specific impulse. Electric propulsion methods are desirable as they produce high specific impulse compared to other known technologies. This makes them desirable for spacecraft where mass and space considerations are important and may allow a reduced amount of propelled to be carried. 
       FIG. 15  illustrates a satellite  900  including a thruster  1 . The thruster  1  may be used for one or more of the following, including attitude control of the satellite  900 , formation flying with other satellites, orbit station keeping by applying thrust to maintain altitude and extend orbit life and deep space exploration. 
     In one embodiment, the satellite  900  also includes additional thrusters  901 . Having two or more thrusters, in particular when directed in different directions, may be facilitate attitude control of the satellite  900 . 
     It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the above-described embodiments, without departing from the broad general scope of the present disclosure. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.