Abstract:
Systems and methods may be provided for cylindrical Hall thrusters with independently controllable ionization and acceleration stages. The systems and methods may include a cylindrical channel having a center axial direction, a gas inlet for directing ionizable gas to an ionization section of the cylindrical channel, an ionization device that ionizes at least a portion of the ionizable gas within the ionization section to generate ionized gas, and an acceleration device distinct from the ionization device. The acceleration device may provide an axial electric field for an acceleration section of the cylindrical channel to accelerate the ionized gas through the acceleration section, where the axial electric field has an axial direction in relation to the center axial direction. The ionization section and the acceleration section of the cylindrical channel may be substantially non-overlapping.

Description:
STATEMENT OF GOVERNMENT INTEREST 
       [0001]    This invention was made with government support under Grant No. DE-ACO2-09CH11466 awarded by the Department of Energy. The government has certain rights in this invention. 
     
    
     FIELD OF THE INVENTION 
       [0002]    Embodiments of the invention relate generally to propulsion systems, and more particularly, to systems and methods for cylindrical Hall thrusters with independently controllable ionization and acceleration stages. 
       BACKGROUND OF THE INVENTION 
       [0003]    Propulsion systems are utilized in many low-power space applications. One such type of propulsion system is a cylindrical Hall thruster, which may also be referred to as a Hall effect thruster or a Hall current thruster. Traditional Hall thrusters utilize an anode and cathode to provide for both ionization of gases and acceleration of the ionized gases. Because the same anode and cathode are utilized to control both ionization and acceleration, there are various considerations and tradeoffs between or among power consumption, ionization amount, and acceleration rate. Accordingly, there is an opportunity for systems and methods for cylindrical Hall thrusters with independently controllable ionization and acceleration stages. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0004]    According to an example embodiment of the invention, there is a cylindrical Hall thruster. The cylindrical Hall thruster may include a cylindrical channel having a center axial direction, a gas inlet for directing ionizable gas to an ionization section of the cylindrical channel, an ionization device that ionizes at least a portion of the ionizable gas within the ionization section to generate ionized gas, and an acceleration device distinct from the ionization device. The acceleration device may provide an axial electric field for an acceleration section of the cylindrical channel to accelerate the ionized gas through the acceleration section, where the axial electric field may have an axial direction in relation to the center axial direction. The ionization section and the acceleration section of the cylindrical channel may be substantially non-overlapping, according to an example embodiment of the invention. 
         [0005]    According to another example embodiment of the invention, there is a method for a cylindrical Hall thruster. The method may include: providing a cylindrical channel having a center axial direction; directing ionizable gas to an ionization section of the cylindrical channel; ionizing, by an ionization device, at least a portion of the ionizable gas within the ionization section to generate ionized gas; and accelerating, by an acceleration device distinct from the ionization device, the ionized gas through an acceleration section of the cylindrical channel. The acceleration device may provide an axial electric field for the acceleration section, where the axial electric field may have an axial direction in relation to the center axial direction. The ionization section and the acceleration section of the cylindrical channel are substantially non-overlapping, according to an example embodiment of the invention. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0006]    Reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein: 
           [0007]      FIG. 1  illustrates an example system for a two-stage cylindrical Hall thruster utilizing electron cyclotron resonance (ECR) ionization, according to an example embodiment of the invention. 
           [0008]      FIG. 2  illustrates an example system for a two-stage cylindrical Hall thruster utilizing inductive ionization, according to an example embodiment of the invention. 
           [0009]      FIG. 3  illustrates an example satellite utilizing an example two-stage cylindrical Hall thruster in accordance with an example embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0010]    Embodiments of the invention now will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. 
         [0011]    Example embodiments of the invention may provide for two-stage cylindrical Hall thrusters for use in a variety of spacecraft propulsion systems, including satellite propulsion. The two-stage cylindrical Hall thrusters in accordance with example embodiments of the invention may have an ionization stage and an acceleration stage. The ionization stage and the acceleration stage may be operated independently of each other. According to an example embodiment of the invention, the ionization stage and the acceleration stage may be substantially non-overlapping in physical positioning. The ionization stage may provide or support the ionization of gases to generate ionized gases. The acceleration stage may accelerate the ionized gases to generate higher velocity exhaust, thereby generating propulsion for the associated spacecraft. 
         [0012]    By providing a first ionization stage and a second acceleration stage, the ionization and acceleration can be decoupled. The decoupling of the ionization and acceleration may allow for operation of the cylindrical Hall thruster with a variety of propellant gases, including those that may be difficult to ionize or that may have a low molecular weight. For instance, an example cylindrical Hall thruster in accordance with an example embodiment of the invention can operate with a variety of gases, whether obtained or derived from a closed source (e.g., container having gas or matter from which gas can be derived) or from an external environment. These gases can include inert gases such as xenon and other gases found in planetary atmospheres, including low molecular weight gases or other molecular gases. The decoupling of the ionization and acceleration can also allow for broadening the operating envelope/parameters for the cylindrical Hall thruster. Indeed, an example cylindrical Hall thruster in accordance with example embodiments of the invention may be able to operate under various pressures, and with ion accelerating voltages that are different from the ionization voltages, thereby providing a broader possible range of operating pressures and ion accelerating voltages. Furthermore, an example cylindrical Hall thruster in accordance with an example embodiment of the invention may provide for increased operating efficiency by providing narrow ion energy distribution and/or reducing ion beam divergence. These features and yet other features may be available in accordance with example embodiments of two-stage cylindrical Hall thrusters. 
         [0013]      FIG. 1  illustrates an example system  100  for a two-stage cylindrical Hall thruster utilizing electron cyclotron resonance (ECR) ionization, according to an example embodiment of the invention. In  FIG. 1 , the system  100  may include a cylindrical chassis, which may be comprised of a first cylindrical chassis  105   a  and a second cylindrical chassis  105   b . The chassis  105   a ,  105   b  may be formed of any variety of materials, including metal (e.g., aluminum, steel, alloys, etc.), ceramic, plastic, or a combination thereof. In an example embodiment of the invention, the first cylindrical chassis  105   a  and the second cylindrical chassis  105   b  may be joined together with respective chassis flanges  106   a ,  106   b . In an alternative embodiment of the invention, the first cylindrical chassis  105   a  and the second cylindrical chassis  105   b  may be respective portions of a same single cylindrical chassis. 
         [0014]    The first cylindrical chassis  105   a  may house or include an ionization source or device within its interior walls or interior portion. In an example embodiment of the invention, the ionization source or device may be an example electron cyclotron resonance (ECR) ionization source. The ECR ionization source or device may be comprised of a radio frequency (RF)/microwave source  110 , and a transmission line  112  and/or antenna  113  for delivering or radiating the electromagnetic fields, energy, or waves (e.g., microwaves) generated from the RF/microwave source  110 . The RF/microwave source  110  may include virtually any radiation source, including vacuum tube devices (e.g., magnetron, klystron, gyrotron, traveling wave tube, and the like) and solid state devices (e.g. transistors, diodes, etc.). The transmission line  112  may include a microstrip, a coaxial transmission line, a waveguide, or the like. In some example embodiments of the invention, the transmission line  112  can serve as or include an antenna for delivering or radiating the electromagnetic fields or waves (e.g., microwaves). In an alternative embodiment of the invention, the transmission line  112  can be connected to another antenna  113  for delivering or radiating the electromagnetic fields or waves. 
         [0015]    According to an example embodiment of the invention, the ionization source housed or provided in the interior of the first cylindrical chassis  105   a  may be separated from the interior of the second cylindrical chassis  105   b  via one or more dielectric windows  115 . The dielectric window  115  may operate to prevent plasma or other gases, including ionizable gases, from the interior of the second cylindrical chassis  105   b  from contacting the ionization source housed or provided in the interior of the first cylindrical chassis  105   a . The dielectric window  115  may be formed of ceramic, glass, plastic, Plexiglas, resins, or another suitable dielectric material. In addition, the first cylindrical chassis  105   a  may include a magnet  125  around its exterior. The magnet  125  may be a permanent magnet, an electromagnet, or any other magnetic device, according to an example embodiment of the invention. The magnet  125  may impose, provide, or support a magnetic field inside the chassis  105   a  and/or chassis  105   b /ceramic discharge channel  130 , where the magnetic field may establish the conditions utilized for electron cyclotron resonance, and may impede the flow of electrons from an externally mounted cathode  150  to an anode  145  located inside the channel  130 . In this regard, the magnet  125  may provide a magnetic field having substantial axial as well as radial components. The magnetic field provided by the magnet  125  can also enhance ionization of at least a portion of the ionizable gas within the ionization stage  120 , and support an axial electric field within the acceleration stage  135 , as likewise discussed herein. It will be appreciated that the extent of ionization provided by the ionization stage  120  may be controlled by varying one or both of the magnetic field strength provided by magnet  125  or the microwave/electromagnetic radiation frequency of the RF/microwave source  110 . 
         [0016]    Turning now to the second cylindrical chassis  105   b , there may be provided a cylindrical ceramic discharge channel  130 . At or near a first end of the cylindrical ceramic discharge channel  130  closest to the RF/microwave source  110  may be ionization stage  120 . At or near the opposite end of the cylindrical ceramic discharge channel  130  near the discharge opening may be an acceleration stage  135 . The ionization stage  120  and the acceleration stage  135  of the cylindrical ceramic discharge channel  130  may be substantially non-overlapping. A gas inlet  140  may be arranged with respect to the cylindrical chassis  105   b /discharge channel  130  (or chassis  105   b ) to direct ionizable gas to or near the ionization stage  120  of the interior of the cylindrical chassis  105   b . For example, in  FIG. 1 , the gas inlet  140  may be provided through a portion of the cylindrical chassis  105   b  and the ceramic discharge channel  130 . However, the gas inlet  140  can be provided in various other positions, configurations or arrangements with respect to the chassis  105   a ,  105   b  and/or the ceramic discharge channel  130  or dielectric window  115  without departing from example embodiments of the invention. For example, the positions of the gas inlet  140  and the RF/microwave source  110  could be swapped without departing from example embodiments of the invention. In an example embodiment of the invention, the gas inlet  140  may include a valve, including a one-way or directional valve, or a through hole without departing from example embodiments of the invention. If a valve is utilized for the gas inlet  140 , then the valve can be controlled or adjusted to direct a desired amount or rate of ionizable gas to or near the ionization stage  120 , according to an example embodiment of the invention. Additionally or alternatively, the flow rate from the source of the ionizable gas can be adjusted to obtain the desired amount or rate of ionizable gas through the gas inlet  140 , according to an example embodiment of the invention. In addition, it will be appreciated that ionizable gas provided for gas inlet  140  can be obtained or derived from either (i) an external environment or (ii) a container having ionizable gas. 
         [0017]    As mentioned above, an acceleration stage  135  may be located at or near the opposite end of the second cylindrical chassis  105   b  near the discharge opening. The operation of the acceleration stage  135  may be supported by an arrangement or configuration of an acceleration device. In an example embodiment of the invention, an example acceleration device may be comprised of an anode  145  that is electrically connected to a cathode  150  via a DC power source  155 . In general, the arrangement or configuration of the anode  145  and the cathode  150  may create a voltage differential between the anode  145  and the cathode  150 , thereby providing at least an axial electric field in the acceleration stage  135 . The axial electric field may support the acceleration of ionized gas through the acceleration stage  135  as one or more ion beams to the discharge opening of the second cylindrical chassis  105   b , thereby providing thrust or propulsion for the cylindrical Hall thruster. To provide at least an axial electric field, an anode  145  may be located inside the channel  130  immediately prior to the acceleration stage  135 , and the cathode  150  may be provided external to the second cylindrical chassis  105   b  near its discharge opening, thereby creating an axial electrical field through the acceleration stage  135  towards the discharge opening. The magnitude of the axial electric field may be adjusted by adjusting the voltage and/or current level of an adjustable DC power source  155 , according to an example embodiment of the invention. In an example embodiment of the invention, the anode  145  may be formed cylindrically or annularly in, near, or adjacent to the inner portion of the ceramic discharge channel  130  immediately prior to the acceleration stage  135 . The cathode  150  may supply electrons which neutralize the ion beams discharged through the discharge opening, and localize the anode  145 -cathode  150  potential drop inside the channel  130 . The neutralization of the ion beams, through interaction with the applied magnetic field of magnet  125 , may result in the anode  145 -cathode  150  potential drop to be localized within or near the acceleration stage  135  that is located near the exit of the discharge channel  130 , according to an example embodiment of the invention. 
         [0018]    It will be appreciated that the ionization source and the acceleration device may be operated independently of each other. For example, the ionization source may control the intensity, rate, or amount of RF/microwave power that is provided for ionizing the ionizable gas from the gas inlet  140  at the ionization stage  120 . As another example, the acceleration device can control the magnitude of the axial electric field provided by the anode  145 /cathode  150 , thereby controlling the amount of acceleration provided for the ionized gas through the acceleration stage  135 . By decoupling the operations of the ionization stage  120  and the acceleration stage  135 , the amount of ionization and/or acceleration can be individually controlled without the need to balance the ionization and acceleration required by conventional cylindrical Hall thrusters. Likewise, the decoupling of the ionization and acceleration may allow for operation of the cylindrical Hall thruster of  FIG. 1  with a variety of propellant gases, including those that may be difficult to ionize or that may have a low molecular weight. Example propellant gases or ionizable gases may include N 2 , O, O 2 , or other gases found in planetary atmospheres. Furthermore, an example cylindrical Hall thruster in accordance with an example embodiment of the invention may provide for increased operating efficiency by providing narrow ion energy distribution and/or reducing ion beam divergence. 
         [0019]    During an example operation of the cylindrical Hall thruster of  FIG. 1 , the RF/microwave source  110  may supply microwave power or other electromagnetic energy to transmission line  112  and/or antenna  113  for radiating ions at a frequency resonant with electron gyromotion, which can ionize the ionizable gas or other propellant gas. Ions that are too large or massive to be influenced by the magnetic field provided by magnet  125  may be accelerated in the acceleration stage  135  having the anode  145 -to-cathode  150  (discharge) potential drop. As discussed herein, electrons supplied by the cathode  150  may neutralize the ion beam and localize the discharge potential drop within the channel  130 , according to an example embodiment of the invention. 
         [0020]    These features and yet other features may be available for the example cylindrical Hall thruster described with respect to  FIG. 1 . Indeed, many variations of the cylindrical Hall thruster of  FIG. 1  are available. For example, there may be variations in the configurations in the location or application of the magnetic field and the RF/microwave source  110 . According to one example variation, ECR ionization of ionizable gas or plasmas may be generated in configurations employing multi-polar magnetic fields. According to another example, the antenna  113  for the RF/microwave source  110  may be positioned or configured radially instead of axially, as shown in  FIG. 1 . Likewise, in another variation, no dielectric window  115  may be necessary such that the transmission line  112  and/or antenna  113  may be directly immersed in the ionizable gas or plasma. Many variations of  FIG. 1  are available without departing from example embodiments of the invention. 
         [0021]      FIG. 2  illustrates an example system  200  for a two-stage cylindrical Hall thruster utilizing inductive ionization, according to an example embodiment of the invention. In  FIG. 2 , the system  200  may be a cylindrical chassis, which may be comprised of a first cylindrical chassis  205   a  and a second cylindrical chassis  205   b . The chassis  205   a ,  205   b  may be formed of any variety of materials, including metal (e.g., aluminum, steel, alloys, etc.), ceramic, plastic, or a combination thereof. In an example embodiment of the invention, the first cylindrical chassis  205   a  and the second cylindrical chassis  205   b  may be joined together with respective chassis flanges  206   a ,  206   b . In an alternative embodiment of the invention, the first cylindrical chassis  205   a  and the second cylindrical chassis  205   b  may be respective portions of a same single cylindrical chassis. 
         [0022]    According to an example embodiment of the invention, the interior of the first cylindrical chassis  205   a  may be separated from the interior of the second cylindrical chassis  205   b  via a ceramic separator disk  228 . The ceramic separator disk  228  may include or be configured with a gas inlet  240  to allow for ionizable gas to be provided from or directed to or near the ionization stage  220  of the interior of the second cylindrical chassis  205   b . The source of the ionizable gas may be provided in the interior of the first cylindrical chassis  205   a . In an example embodiment of the invention, the gas inlet  240  may include a valve, including a one-way or directional valve, or a through hole without departing from example embodiments of the invention. If a valve is utilized for the gas inlet  240 , then the valve can be controlled or adjusted to direct a desired amount or rate of ionizable gas to or near the ionization stage  220 , according to an example embodiment of the invention. Additionally or alternatively, the flow rate from the source of the ionizable gas can be adjusted to obtain the desired amount or rate of ionizable gas through the gas inlet  240 , according to an example embodiment of the invention. It will be appreciated that ionizable gas provided for gas inlet  240  can be obtained or derived from either (i) an external environment or (ii) a container having ionizable gas. 
         [0023]    In addition, the first cylindrical chassis  205   a  may include a magnet  225  around its exterior. The magnet  225  may be a permanent magnet, an electromagnet, or any other magnetic device, according to an example embodiment of the invention. The magnet  225  may provide a magnetic field having substantial axial as well as radial components to support the movement of ionizable gas along a central longitudinal axis towards the ionization stage  220  of the second chassis  205   b . The magnetic field provided by the magnet  225  can also enhance ionization of at least a portion of the ionizable gas within the ionization stage  220 , and support an axial electric field within the acceleration stage  235 , as described herein. 
         [0024]    Turning now to the second cylindrical chassis  205   b , there may be provided a cylindrical ceramic discharge channel  230 . In some example embodiments of the invention, the ceramic discharge channel  230  can also include the ceramic separator disk  228 , which may be formed substantially perpendicular to the ceramic separator disk  228 . At or near a first end of the cylindrical ceramic discharge channel  230  closest to the gas inlet  240 , may be ionization stage  220 . At or near the opposite end of the cylindrical ceramic discharge channel  230  near the discharge opening may be an acceleration stage  235 . The ionization stage  220  and the acceleration stage  235  of the cylindrical ceramic discharge channel  230  may be substantially non-overlapping. 
         [0025]    As introduced above, a gas inlet  240  may be arranged with respect to the ceramic separator disk  228  to direct ionizable gas to or near the ionization stage  220  of the interior of the cylindrical chassis  205   b . In addition, an ionization source may also be provided near the ionization stage  220 . As shown in  FIG. 2 , the ionization source may be an inductive ionization source comprising an RF power source  210  coupled to an inductive coil  212 . The inductive coil  212  may be positioned cylindrically or annularly between the chassis  205   b  and the ceramic discharge channel  230  such that the inductive coil generally surrounds at least a portion of the ionization stage  220 . Accordingly, when the ionizable gas is provided through the gas inlet  240 , the RF power source  210  can operate the inductive coil  212  to ionize the gas and generate ionized gas. In an example embodiment of the invention, the RF power source  210 /inductive coil  212  may ionize the gas via fluctuating electric field strengths. 
         [0026]    In addition, an acceleration stage  235  may be located at or near the opposite end of the second cylindrical chassis  205   b  near the discharge opening. The operation of the acceleration stage  235  may be supported by an arrangement or configuration of an acceleration device. In an example embodiment of the invention, an example acceleration device may be comprised of an anode  245  that is electrically connected to a cathode  250  via a DC power source  255 . The operation of the anode  245  and the cathode  250  is substantially similar to that described with respect to the anode  145  and the cathode  150  of  FIG. 1 , and need not be discussed in further detail with respect to  FIG. 2 . 
         [0027]    It will be appreciated that the ionization source and the acceleration device in  FIG. 2  may be operated independently of each other. For example, the ionization source may control the intensity, frequency, or amount of one or more electric fields that are provided for ionizing the ionizable gas from the gas inlet  240  at the ionization stage  220 . As another example, the acceleration device can control the magnitude of the axial electric field provided by the anode  245 /cathode  250 , thereby controlling the amount of acceleration provided for the ionized gas through the acceleration stage  235 . By decoupling the operations of the ionization stage  220  and the acceleration stage  235 , the amount of ionization and/or acceleration can be individually controlled without the need to balance the ionization and acceleration required by conventional cylindrical Hall thrusters. Likewise, the decoupling of the ionization and acceleration may allow for operation of the cylindrical Hall thruster of  FIG. 2  with a variety of propellant gases, including those that may be difficult to ionize or that may have a low molecular weight. Furthermore, an example cylindrical Hall thruster in accordance with an example embodiment of the invention may provide for increased operating efficiency by providing narrow ion energy distribution and/or reducing ion beam divergence. These features and yet other features may be available for the example cylindrical Hall thruster described with respect to  FIG. 2 . 
         [0028]    It will be appreciated that many variations of the cylindrical Hall thrusters of  FIGS. 1 and 2  are available without departing from example embodiments of the invention. 
         [0029]      FIG. 3  illustrates an example satellite  300  utilizing an example two-stage cylindrical Hall thruster in accordance with an example embodiment of the invention. The example satellite  300  may include a satellite bus  305 , which may include a collimator  310 , a diffuser  315 , and a cylindrical Hall thruster  320 . As shown in  FIG. 3 , external environmental gas may be moved through the collimator  310  to produce parallel beams of external environmental gas to provide thermalized gas. The thermalized gas is directed by the diffuser  315  into a mixing chamber or gas inlet of the cylindrical Hall thruster  320 . The cylindrical Hall thruster  320  can operate substantially the same as that described with respect to  FIGS. 1 and 2 , where the ionization stage generates ionized gas from the thermalized gas, and the acceleration stage accelerates the ionized gas, which is discharged from the discharge opening of the discharge channel, thereby resulting in high velocity ionized exhaust and generating propulsion. It will be appreciated that the external environmental gas can include N 2 , O, O 2 , or other gases found in planetary atmospheres. 
         [0030]    Many modifications and other embodiments of the invention set forth herein will be apparent having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.