Abstract:
A thrust producing system includes an RF ion thruster with a discharge chamber having a gas inlet and an outlet, and a coil about the discharge chamber. The system further includes an RF cathode proximate the discharge chamber outlet of the RF ion thruster for ion beam neutralization. The RF cathode includes a discharge chamber having a gas inlet and an outlet and a coil about the discharge chamber. A tank for containing iodine in solid form and a heater associated with said tank to produce iodine vapor. A feed subsystem fluidly couples the tank with the RF ion thruster discharge chamber gas inlet and with the RF cathode discharge chamber gas inlet.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims benefit of and priority to U.S. Provisional Application Ser. No. 62/319,550 filed Apr. 7, 2016, under 35 U.S.C. §§119, 120, 363, 365, and 37 C.F.R. §1.55 and §1.78, which is incorporated herein by this reference. 
     
    
     GOVERNMENT RIGHTS 
       [0002]    This invention was made with U.S. Government support under Contract No. NNX15CC90C awarded by the National Aeronautics and Space Administration (NASA). The Government may have certain rights in the subject invention. 
     
    
     FIELD OF THE INVENTION 
       [0003]    The subject invention relates to radio frequency ion thrusters. 
       BACKGROUND OF THE INVENTION 
       [0004]    Radio frequency ion thrusters typically operate on a propellant such as xenon and include a hollow cathode to neutralize the ion beam emitted by the discharge chamber of the ion generator. See U.S. Pat. No. 9,060,412 incorporated herein by this reference. 
         [0005]    The hollow cathode includes an oxide insert which is consumed when the cathode is operated limiting the life of the cathode. Also, a heater is required. A hollow cathode with a C12A7 electride insert is proposed in U.S. Published Application No. 2014/0354138 incorporated herein by this reference and purportedly a heater is not required. Still, in testing, a heater was required for proper operation and such a hollow cathode design may impart constraints for certain types of missions where power is limited or rapid start of the cathode is required. 
         [0006]    Because of the difficulty to integrate high-pressure xenon gas vessels onboard small spacecraft, iodine has been proposed for use in electric propulsion devices such as RF ion thrusters. See U.S. Pat. No. 6,609,363 incorporated herein by this reference. Still, a hollow cathode was used as the neutralizer. 
         [0007]    The notion of an RF neutralizer for an ion thruster has been studied. See T. Hatakeyama et al., “Preliminary Study on Radio Frequency Neutralizer for Ion Engine,” LEPC-2007-226 (2007) incorporated herein by this reference. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    Until now, no known RF ion thruster using iodine as a propellant includes an RF cathode as the neutralizer that also uses iodine as a source of fuel. In the subject invention, both the RF ion thruster and the RF cathode are designed to use iodine in order to, respectively, create ions and neutralizing electrons. As a result, high pressure gas vessels are not required to operate the ion thruster or cathode. 
         [0009]    Featured is a system comprising an RF ion thruster including a discharge chamber having a gas inlet and an outlet and a coil about the discharge chamber. An RF cathode is located proximate the discharge chamber outlet of the RF ion thruster and includes a discharge chamber having a gas inlet and an outlet and a coil about the discharge chamber. The system further includes a tank for containing iodine in solid form, a heater associated with said tank to produce iodine vapor, and a feed subsystem fluidly coupling said tank with the RF ion thruster discharge chamber gas inlet and also with the RF cathode discharge chamber gas inlet. 
         [0010]    The system further may include an igniter associated with the RF cathode discharge chamber inlet and, in one example, an igniter associated with the RF ion thruster discharge chamber inlet. The RF cathode discharge chamber inlet may include a conduit and the igniter includes spaced conductive electrodes in the conduit coupled to a voltage source and a voltage bias source. 
         [0011]    The RF ion thruster may include a conductive grid subsystem proximate the RF ion thruster discharge chamber outlet. In one example, the conductive grid subsystem of the RF ion thruster includes at least two conductive plates with orifices therethough, one said plate supplied with a positive voltage and the other said plate supplied with a negative voltage. 
         [0012]    Preferably, the RF cathode includes an electron extractor proximate the RF cathode discharge chamber outlet. In one example, the electron extractor includes a conductive grid subsystem with at least one conductive plate with orifices therethough and a plurality of rearwardly extending members received at least partially in the RF cathode discharge chamber to increase the electron output of the RF cathode. In another example, the electron extractor includes a ring internal to the RF cathode discharge chamber. One preferred ring includes spaced fingers extending therefrom. Further included may be an orifice plate at the RF cathode discharge chamber outlet. 
         [0013]    Preferably, the RF ion thruster, the RF cathode, the tank, and the feed subsystem are made of materials resistant to iodine. In one example, the discharge chamber of the RF ion thruster and the discharge chamber of the RE cathode are made of ceramic material. The tank may be made of thermoplastic and includes a metal internal coating to uniformly distribute heat. In one example, the heater includes tape heaters bonded to the coating. Also featured is a novel RF cathode system comprising a ceramic discharge chamber having a gas inlet and an outlet, a coil about the discharge chamber, a tank for containing iodine in solid form, a heater associated with said tank to produce iodine vapor, a feed subsystem fluidly coupling the tank with the RF cathode discharge chamber gas inlet, an igniter associated with the RF cathode discharge chamber inlet including spaced conductive electrodes coupled to a voltage source and a voltage bias source, and an electron extractor proximate the discharge chamber outlet. 
         [0014]    Also featured is a method of generating thrust. Iodine is headed to produce iodine vapor. The iodine vapor is fed to an RF ion thruster to produce an ion beam which is neutralized by feeding iodine vapor to an RF cathode producing electrons directed to the ion beam output by the RF ion thruster. 
         [0015]    The method may further include increasing the electron output of the RF cathode by placing at least one grid plate proximate the RF cathode and increasing the surface area of the grid plate. There may be two conductive plates with orifices therethough, one plate supplied with a positive voltage and the other plate supplied with a negative voltage. 
         [0016]    The method may further include generating seed electrons drawn into the RF cathode. In one example, generating seed electrons includes locating an igniter upstream of the RF cathode. In some embodiments, the method may further include generating seed electrons drawn into the RF ion thruster by locating an igniter upstream of the RF ion thruster. 
         [0017]    The method may further include extracting electrons from the RF cathode. In one example, extracting electrons includes placing a ring with fingers extending therefrom inside the RF cathode. 
         [0018]    The subject invention, however, in other embodiments, need not achieve all these objectives and the claims hereof should not be limited to structures or methods capable of achieving these objectives. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0019]    Other objects, features and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings, in which: 
           [0020]      FIG. 1  is a highly schematic view of an example of an RF ion thruster system in accordance with the invention; 
           [0021]      FIG. 2  is a schematic view of a conductive grid plate for the RF cathode shown in  FIG. 1 ; 
           [0022]      FIG. 3  is a schematic view showing an example of an igniter for the RF cathode shown in  FIG. 1 ; 
           [0023]      FIG. 4  is a schematic view of an example of another RF cathode useful in connection with the system of  FIG. 1 ; 
           [0024]      FIG. 5  is a cross sectional view of the RF cathode of  FIG. 4 ; 
           [0025]      FIG. 6  is a schematic three dimensional cut away view of an example of an iodine storage tank shown in  FIG. 1 ; and 
           [0026]      FIG. 7  is a schematic three dimensional partially cut away view showing another example of an iodine storage tank. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    Aside from the preferred embodiment or embodiments disclosed below, this invention is capable of other embodiments and of being practiced or being carried out in various ways. Thus, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. If only one embodiment is described herein, the claims hereof are not to be limited to that embodiment. Moreover, the claims hereof are not to be read restrictively unless there is clear and convincing evidence manifesting a certain exclusion, restriction, or disclaimer. 
         [0028]      FIG. 1  shows an exemplary RF ion thruster system  10  with RF ion thruster  12  including ceramic discharge chamber  14  having a gas inlet  16  and an outlet  18 . Coil  20  is disposed about chamber  14  and conductive grid subsystem  22  proximate discharge chamber outlet  18  typically includes conductive plates  24   a ,  24   b  with orifices therethrough. In operation, plate  24   a  may be supplied with a positive voltage (e.g., 1.5-2.5 kV) and plate  24   b  may be supplied with a negative voltage (e.g., −100 to −200 V). 
         [0029]    RF cathode  30  is disposed proximate the discharge chamber outlet  18  of RF ion thruster  12 . Cathode or neutralizer  30  includes ceramic discharge chamber  32  with gas inlet  34  and outlet  36 . Coil  28  is disposed about discharge chamber  32 . A conductive grid subsystem  40  disposed at the outlet  36  may include a conductive plate  42   a  and optional conductive plate  42   b  both including orifices therethrough. Plate  42   a  may be supplied with a negative voltage (e.g., −50 to −200 V) and plate  42   b  if used is typically at reference potential, 0 volts. 
         [0030]    The ceramic material of the discharge chambers  14  and  32  is preferably highly resistant to iodine. In one example, the ceramic material used was alumina. In another example, the ceramic material was Macor. The RF coils  20  and  38  are preferably made of highly conductive material with extremely thin insulation layer for maximum inductance at a 1-10 MHz frequency range. In one example the coil was made of pure silver wire with 0.005″-thick PTFE tubing. In another example the coil was made of pure silver wire with 0.001″-thick polyamide tape or coating for insulation. 
         [0031]    Tank  50  stores solid iodine  52  therein (e.g., powder, a solid block of iodine, iodine chips, or the like). Heater  51  associated with tank  50  is used to heat the solid iodine and produce iodine vapor supplied by feed subsystem  54  to the inlet  16  of ion thruster  12  and to the inlet  34  of cathode  30 . Tank  50  may be formed from a machined polyetherimide (PEI, trade name Ultem) box with an epoxied or ultrasonically-welded Ultem lid, or a stamp formed thin Hastelloy or Inconel sheet with a welded lid, or a fully welded box made of thin Hastelloy or Inconel sheets. Heater  51  is bonded to the tank exterior and an outer layer of insulation  53  may be included. The heater  51  may also be bonded to the tank  50  interior, with or without additional outer layer of insulation  53 . 
         [0032]    The various valves, sensors, heaters, and controls associated with a feed system fluidly connecting the tank  50  with the ion thruster and the cathode are not shown in  FIG. 1 . But, aspects of the propellant management subsystem of U.S. Pat. No. 8,610,356, incorporated herein by this reference, may be used. Also, aspects of the vapor distribution subsystem disclosed in U.S. Provisional Patent Application No. 62/259,779 incorporated herein by this reference, may be used. The various conduits used may be PTFE, PFA, Nylon, Hastelloy or Inconel, or silica-coated metal gas lines. 
         [0033]    Preferably, conductive grid plate  42   a  of the cathode,  FIG. 2  includes rearwardly extending members such as posts  60  proximate the periphery of plate  42   a  and extending at least partially into chamber  32 ,  FIG. 1  to increase the electron output of RF cathode  30  by increasing the corresponding ion collection surface area. 
         [0034]    Also, an igniter  70 ,  FIG. 1  is preferably associated with RF cathode  30  discharge chamber inlet  34 . In one example, the igniter  70 ,  FIG. 3  includes two spaced conductive ring electrodes  72   a ,  72   b  inside inlet conduit  74  coupled to a voltage source  76  and a bias voltage source  77  relative to reference potential at 0 Volts. The voltage source  76  can be either DC or AC powered and its main function is to create a pilot discharge that generates seed electrons, which are then drawn into the cathode&#39;s discharge chamber  32  to ignite the plasma in the RF cathode  30 . The electrodes represented by  72   a  and  72   b ,  FIG. 3 , in another example were made of a wire spaced from a metal gas fitting. The said igniter  70  can also be incorporated into the RF ion thruster  12  at the discharge chamber gas inlet  16  with the same voltage sources  76  and  77 . 
         [0035]    Because of the corrosiveness of iodine, any component in the system exposed to iodine is preferably made of materials resistant to iodine. Thus, the interior of tank  50 , the interior of the conduits of the feed subsystem  54 , the interior of chambers  14  and  32 , the plates of the conductive grid subsystems  22  and  40 , the electron extractor  39 ,  FIGS. 4-5 , and the igniter electrodes are made of materials resistant to iodine. In one example, all the gridded plates were made of molybdenum. Other suitable iodine resistant materials and coatings includes certain plastics such as Ultem, PEEK, or the like, certain metals such as gold, Hastelloy, Inconel, tungsten, platinum, nickel, nickel sulfamate, molybdenum, MolRe alloys, ceramics or glass such Alumina, Macor, silica and even ruby or sapphire materials. 
         [0036]    Another example of an RF cathode  30 ′ is shown in  FIGS. 4-5 . The RF cathode shown in  FIG. 1  employs active electron extraction because the two grid plates  42   a  and  42   b  urge electrons out of the discharge chamber. RF cathode  30 ′,  FIGS. 4-5 , in contrast, employs passive electron extraction because it relies on the positively charged plume generated by RF thruster  12 ,  FIG. 1  to withdraw electrons from the RF cathode discharge chamber to achieve neutralization. Power is saved because only one cathode grid is required. 
         [0037]    The RF cathode of  FIGS. 4-5  features ceramic orifice plate  35  at outlet of the RF cathode discharge chamber. The orifice  37  is shown in  FIGS. 4-5 . RF cathode  30 ′ also features negatively biased internal grid electrode  39  (i.e. an ion collection electrode). Electrode  39  preferably includes spaced fingers  41  extending from ring portion  43  which may include flange  45  disposed between the end of chamber  32  and the inside of orifice plate  35 . Thus, the ring portion  43  and the fingers  41  abut the interior of the chamber  32 . The fingers extend readwardly in the chamber. The ring and fingers may be made of Hastelloy, Inconel, Stainless Steel, or Molybdenum. 
         [0038]    In one example, iodine tank  50 ′,  FIG. 6  is made of a plastic material  80  (e.g., Ultem) internally coated with a layer of gold or nickel  82  to uniformly distribute the heat supplied by tape style heaters  84  bonded to layer  82 . The exterior of plastic tank  80  can be reinforced with a fiberglass coating, a carbon fiber wrap, or metal bands  86 . Lid  88  is epoxied or ultrasonically welded to the tank. 
         [0039]    In another example, tank  50 ″,  FIG. 7  includes stamp-formed, fully welded, or 3-D-printed Hastelloy or Inconel tank, exterior heater  100 , and thermal insulation layer  102 . 
         [0040]    Although specific features of the invention are shown in some drawings and not in others, this is for convenience only as each feature may be combined with any or all of the other features in accordance with the invention. The words “including”, “comprising”, “having”, and “with” as used herein are to be interpreted broadly and comprehensively and are not limited to any physical interconnection. Moreover, any embodiments disclosed in the subject application are not to be taken as the only possible embodiments. Other embodiments will occur to those skilled in the art and are within the following claims. 
         [0041]    In addition, any amendment presented during the prosecution of the patent application for this patent is not a disclaimer of any claim element presented in the application as filed: those skilled in the art cannot reasonably be expected to draft a claim that would literally encompass all possible equivalents, many equivalents will be unforeseeable at the time of the amendment and are beyond a fair interpretation of what is to be surrendered (if anything), the rationale underlying the amendment may bear no more than a tangential relation to many equivalents, and/or there are many other reasons the applicant cannot be expected to describe certain insubstantial substitutes for any claim element amended.