Patent Grant
11473568
The invention relates to electric space propulsion systems, and specifically, to the construction and operation of a heaterless hollow cathode (hereinafter HHC) for such systems. Recent developments in the technology of HHC's for use in electric space propulsion systems are described in a non-patent reference entitled “Heaterless Hollow Cathode Technology—A Critical Review,” Proceedings of the Space Propulsion 2016 Conference, 2-6 May 2016, Rome, Italy, SP2016_3125366, which is hereby incorporated herein by reference in its entirety.
The operation of an HHC consists of three phases: (1) ignition through high-voltage breakdown of a neutral gas, (2) cathode heating either by glow discharge and/or by electric arc, and (3) continuous self-sustained cathode emission. The stochastic physical nature of discharge phenomena in gases may lead to failure of the HHC during any one of the three phases of operation. A large body of academic research has succeeded in identifying many of the failure mechanisms that have been observed during HHC operation.
In the prior art, an HHC having a configuration known as an “Open-End Emitter—Orificed Keeper” has been disclosed in U.S. Pat. No. 4,475,063 to Graeme Aston, entitled “Hollow Cathode Apparatus,” dated Oct. 2, 1984. This configuration has been tested experimentally and various failure mechanisms have been observed.
For example, erosion of the hollow electron emitter by ion bombardment may reduce the overall HHC lifetime. In some cases, catastrophic destruction of the emitter may be caused by high current arcing between the emitter and keeper.
Another source of HHC failure is connected with the mass flow of the gas propellant. When the inner diameter of the emitter is small, the HHC is susceptible to plugging. When the inner diameter of the emitter is large, the gas pressure may be too low for ignition. If the gas pressure is increased by increasing the mass flow of the gas propellant, the increased mass flow rapidly depletes the supply of gas propellant and also may lead to plasma instability during the third phase of HHC operation. If, instead of increasing mass flow, the ignition voltage, thus energy, is increased to compensate for low gas pressure, there is an increased risk of emitter damage by high-voltage arcing between the emitter and keeper.
Still another source of premature failure occurs during the third phase of operation, i.e. the continuous self-sustained cathode emission phase. It is believed that such failures are due to thermal stresses and melting of the emitter caused by excessively high operating temperatures (e.g. greater than 2,500 degrees Celsius). Such high temperatures are often reached in order to attain thermionic emission from emitters having a high work function (e.g. above 4 eV), such as those made of pure Tantalum or pure Tungsten. In an effort to lower their work function, emitters are sometimes impregnated with dopant materials, such as Barium Oxide or Scandium Oxide. However, even doped emitters have been observed to fail prematurely because of thermal stresses and melting. It is thought that this is may be due to depletion of the dopant material after extended operation of the emitter at a high electron surface current density. Once the dopant has been depleted, the emitter work function increases to the level of the pure metal, and excessively high temperatures are again needed to achieve thermionic emission, together with the attendant risk of thermal failure.
The present invention is an apparatus and method for operating a heaterless hollow cathode.
According to the teachings of an embodiment of the present invention, there is provided a heaterless hollow cathode apparatus including:
According to one feature of certain preferred implementations of the apparatus, the emitter holder includes an emitter holder neck which encapsulates the electron emitter.
According to a further feature of certain preferred implementations of the apparatus, the area of the keeper orifice is between 5% and 25% of the area of the emitter orifice.
According to a further feature of certain preferred implementations of the apparatus, the keeper includes a change in thickness.
According to a further feature of certain preferred implementations of the apparatus, the electron emitter is either a refractive ceramic material or a refractive metal impregnated with an oxide.
According to a further feature of certain preferred implementations of the apparatus, the electron emitter has a work function less than 2.2 electron volts.
According to a further feature of certain preferred implementations of the apparatus, the controller detects initiation of the discharge between the emitter assembly and the keeper by identifying a sudden sharp increase in the emitter-keeper current.
According to a further feature of certain preferred implementations of the apparatus, the above predetermined current value is in a range of 100 to 150 milli-amperes.
According to a further feature of certain preferred implementations of the apparatus, the above predetermined voltage threshold is in a range of 50 to 100 volts.
According to a further feature of certain preferred implementations of the apparatus, the above predetermined minimum time duration is in a range of 1 to 3 seconds.
There is also provided according to the teachings of an embodiment of the present invention, a method for operating a heaterless hollow cathode including the steps of:
According to one feature of certain preferred implementations of the method, the controller detects initiation of the discharge between the emitter assembly and the keeper by identifying a sudden sharp increase in the emitter-keeper current.
According to a further feature of certain preferred implementations of the method, the above predetermined current value is in a range of 100 to 150 milli-amperes.
According to a further feature of certain preferred implementations of the method, the above predetermined voltage threshold is in a range of 50 to 100 volts.
According to a further feature of certain preferred implementations of the method, the above minimum time duration is in a range of 1 to 3 seconds.
The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:
The present invention is an HHC apparatus and method of operation. The principles of the present invention may be better understood with reference to the drawings and the accompanying description.
Referring now to the drawings,
Power supply 170 provides electrical voltages and currents required by controller 180. Controller 180 contains at least one processor and at least one data storage element, a timing mechanism, and a multiplicity of electrical interfaces to sensors and actuators. The electrical interfaces associated with anode element 140, HHC gas distributor 190, and HHC 200 are represented schematically in
During the operation of system 100, a stream of electrons exits HHC 200 and travels towards anode element 140 in response to an applied electric field. The latter, in conjunction with an orthogonal magnetic field established by magnetic poles 160, creates a circulating Hall electron current. The latter collides with and ionizes neutral propellant gas molecules flowing through anode element 140. The ionized propellant gas molecules are accelerated to high velocities by an applied electric field and pass through discharge channel 150. On exiting system 100, the ionized propellant gas molecules are neutralized by electrons emitted by HHC 200.
System 100 is known to those skilled in the art as a Hall effect thruster, and is mentioned here as one preferred but non-limiting example of electric space propulsion systems which employ an HHC. The scope of the present invention is not limited to such an example, and includes embodiments employing any and all types of electric space propulsion system which utilize an HHC apparatus and/or method of operation according to this invention.
The structure of an exemplary implementation of HHC 200 is illustrated in
The thermal properties and design of HHC 200 are preferably configured to avoid premature failure due to thermal stress or melting. The material composition of keeper 310 is preferably a refractory metal having a low thermal conductivity (e.g. less than or equal to 180 watts per meter per degree Celsius at a peak temperature of 500 degrees Celsius). Tungsten, Molybdenum, and Tantalum are examples of such refractory metals. To avoid thermal cracking at high temperatures, keeper 310 should not consist of brazed components, but rather should be a single machined structure. Keeper thickness change 315 reduces the flow of heat away from the emitter assembly by reducing the cross-sectional area for heat conduction through keeper 310. The material composition of emitter base 330 and emitter holder 340 is preferably a refractory metal, which may be the same as or different from that of keeper 310. Coefficients of thermal expansion are preferably matched between the refractory metals so as to minimize thermal stress. With proper thermal design as described (choice of materials, wall thickness etc.), the HHC of this invention has been shown to provide continuous self-sustained cathode emission, at a stable electron current, for more than 1500 hours of operation.
Electron emitter 350 is preferably a material having a threshold temperature for thermionic emission below 1800 degrees Celsius and a melting point above 2000 degrees Celsius. Typically, this can be achieved with an emitter material having a work function which is less than 2.2 electron volts. One such material is the refractory ceramic Lanthanum Hexaboride; another is Tungsten impregnated with an oxide dopant, such as Barium Oxide or Scandium Oxide.
While waiting for the gas pressure to stabilize, or preferably after the gas pressure has already stabilized, as shown in block 620 of
Within a short time of detecting plasma breakdown, typically 100 microseconds or less, controller 180 implements emitter-keeper current control, as shown in block 650. The switch to emitter-keeper current control is preferably performed quickly in order to prevent continued ramping of Vke, which may lead to excessively high values of Ike and to depletion of dopant material in electron emitter 350. In block 650, controller 180 monitors the emitter-keeper current Ike, and adjusts the applied emitter-keeper voltage Vke, so as to keep Ike at a predetermined current value which preferably is in a range of 100 to 150 milli-amperes. At this current level, there is a plasma glow discharge, which gradually heats electron emitter 350 to a temperature threshold needed for thermionic emission. Typical values of Vke during this step are from 200 to 300 volts.
It is important to note that heating of electron emitter 350 must be accomplished gradually by means of plasma glow discharge, and not suddenly, by electric arcing. Although the latter would require less time, it has been found to lead to emitter damage by overheating and melting at one or more points on the surface of electron emitter 350, causing cratering and erosion which lead to premature failure of electron emitter 350.
In block 660 of
Once thermionic emission has been achieved, controller 180 activates main discharge control, as indicated in block 670. Controller 180 applies a discharge voltage to initiate the flow of ionized gas through discharge channel 150. After several seconds, once the current of the main discharge circuit has stabilized, controller 180 sets Vke to zero, and HHC 200 continues to operate in its third mode of operation, namely, continuous, self-sustained cathode emission, as shown in block 680 of
It will be appreciated that the above descriptions are intended only to serve as examples, and that many other embodiments are possible within the scope of the present invention as defined in the appended claims.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IL2017/051341 | 12/12/2017 | WO |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2019/116361 | 6/20/2019 | WO | A |
| Entry |
|---|
| Lev “Development of a Low Current Heaterless Hollow Cathode for Hall Thrusters” (Year: 2015). |
| Han “Physical Processes in Hollow Cathode Discharge” (Year: 1990). |
| Trent “Control of the Electron Energy Distribution Function (EEDF) in a Hall Thruster Plasma” (Year: 2016). |
| Number | Date | Country | |
|---|---|---|---|
| 20210071650 A1 | Mar 2021 | US |
