Patent Application
20100213851
1. Field of the Invention
This invention pertains generally to plasma sources that ionize gas and heat plasma using RF antennas, applied magnetic fields, plasmas and coaxial gas confinement tubes. Applications comprise plasma production for doping and testing materials in plasma processing applications, break-down of toxic gas streams, and sterilization of materials by bombardment with plasma. Space applications comprise using these systems for rocket engines to provide thrust by discharging ionized particles in a particular direction. More specifically, the invention relates to improvements to antennas, magnet geometries, and thermal solutions for the steady state production and heating of plasmas.
2. Description of the Related Art
The application that covers the broadest implementation of these arts is the Variable Specific Impulse Rocket (VASIMR). The VASIMR is fundamentally an electromagnetic plasma accelerator that borrows heavily from the physics and technology of magnetic confinement fusion research. In the jargon of the fusion community it is called a “magnetic mirror,” an open magnetic system or “linear machine” because unlike the toroidal Stellerator or Tokamak, the topology of the magnetic field is open. Linear machines were pursued heavily in the 1970s as potential fusion devices, but due to their weak axial plasma confinement, lost favor in the United States over closed topology magnetic systems such as the Tokamak and the Stellerator. The weakness of the mirror machine as a plasma confinement device is the strength of the VASIMR as a plasma thruster. Plasma in these systems is radially confined, but free to flow axially out of the device to provide rocket propulsion. Therefore, the bulk of the prior art on open ended magnetic confinement systems could be relevant to furthering the technology of the VASIMR.
Three linked magnetic stages perform specific interrelated functions in VASIMR. The first stage handles the main injection of propellant gas and its ionization; the second, also called the “RF booster” acts to further energize the plasma; the third stage is a magnetic nozzle, which converts the energy of the fluid into directed flow. VASIMR is a radio frequency (RF) driven device where the ionization of the propellant is done by a helicon-type discharge. The plasma ions are further accelerated in the second stage by ion cyclotron resonance heating (ICRH), a well-known technique, used extensively in magnetic confinement fusion research.
It is known in the art that plasma may be accelerated by a series of antennas to generate thrust in a rocket engine. U.S. Pat. No. 6,334,302 describes a variable specific impulse magnetoplasma rocket (VASIMR) using two antennas to deliver energy to a gas stream. First, a helicon antenna is used as part of a helicon plasma generator to impart radio frequency (RF) power to the gas stream exciting the gas atoms to an ionized state.
Downstream of the helicon antenna, the resulting plasma is subjected to additional RF power imparted by an Ion Cyclotron Radio Heating (ICRH) antenna to excite ion cyclotron resonance on the plasma. The power imparted to the plasma by the antennas is converted to kinetic energy when the ions are thereafter ejected through a magnetic nozzle to provide the desired thrust.
Overall system efficiency, neglecting any ambipolar contribution to thrust, can be expressed as a ratio of exhaust kinetic energy to electrical power input, where part of the electrical power input goes to the helicon plasma generator and part goes to ICRH antenna. Lower mass flow has higher flow velocity, demonstrating variable specific impulse control technique.
In a VASIMR rocket, neutral gas is first injected into a tube with RF compatible dielectric properties. As the gas flows downstream, plasma is generated as the gas is ionized by a helicon antenna. At this stage the temperature of the plasma may be about 60,000 Kelvin. As the plasma flows further downstream it is further heated by an ion cyclotron resonance heating (ICRH) antenna where it could reach temperatures in the millions of Kelvin. The engine's surrounding surfaces are protected from direct contact high temperature plasma by a magnetic field acting on the plasma. However, considerable heat is still transferred between the hot plasma and the antennas, primarily 15 through radiation from the plasma.
A heat pipe is a passive device for heat removal. The extremely high temperatures associated with the plasma require that the plasma be contained by a magnetic field. Heat transferred by radiation or other mechanisms from the plasma to the surrounding surfaces must be removed in steady-state operation if the surrounding surfaces are to maintain their structural integrity.
The use of a heat pipe to transfer heat efficiently from a hot location to a cold location is known in the art (See U.S. Pat. No. 2,350,348). Generally a heat pipe consists of a vacuum tight envelope, a wick structure and a working fluid. The heat pipe is evacuated and then back-filled with a small quantity of working fluid, just enough to saturate the wick. The atmosphere inside the heat pipe is set by an equilibrium of liquid and vapor. As heat enters the heat pipe at the hot end, (the evaporator), this equilibrium is upset generating vapor at a slightly higher pressure. This higher pressure vapor travels to the cold end (the condenser) where the slightly lower temperatures cause the vapor to condense giving up its latent heat of vaporization. The condensed fluid is then pumped back to the evaporator by the capillary forces developed in the wick structure.
The present inventors are aware of no prior art where thermal management of a helicon—especially antennas and ionization chamber tubing—is achieved through innovative use of heat pipes, coolant flow, heat exchangers, and/or thermally conductive materials with low RF dielectric losses such as CVD diamond. The present inventors are aware of no prior art where gas density and flow rate, antenna design, and magnetic field shape, including the choke design, are all optimized to move the hot plasma downstream for steady-state plasma source operation.
A plasma source that ionizes gas and heats plasma using optimized RF antennas, applied magnetic fields, plasmas, coaxial gas confinement tubes, and waste heat management methods comprising means for carrying heat axially to a heat exchanger or heat sink for disposal.
The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings in which:
a depicts the plasma ion flux versus time in accordance with the VX-100 embodiment of the present invention.
b depicts the energy investment required to extract an electron-ion pair in the plasma stream as a function of the RF power coupled to the first stage of the VX-100 embodiment of the present invention.
In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, that the present invention may be practiced without these specific details.
Certain embodiments of the present invention provide for efficient plasma stream production. Such a plasma stream is suitable for space propulsion. High efficiency in terms of total power utilization to produce the plasma is desirable for electric propulsion systems such as VASIMR engines. Another feature of certain embodiments of the present invention is the production of a plasma stream in one region with a subsequent flowing of that plasma into a different region for further modification of plasma properties. An apparatus in accordance with an embodiment of the present invention may have various uses other than space propulsion.
An apparatus in accordance with an embodiment of the present invention comprises a radio frequency (RF) based, gridless plasma source suitable for extracting plasma streams and subsequently tailoring the spatial and energy distribution of those streams. Such an apparatus could provide for control of the total plasma flux, the ionization fraction, the spatial distribution, or the energy distribution of the flowing plasma. Such control is useful for many applications in addition to VASIMR. Various embodiments of the present invention can be used for material surface modification, materials testing, waste decomposition, and other purposes.
Various gas species require a minimal amount of energy to ionize because of line radiation losses and the energy of ionization inherently stored when an electron is removed directly, or through a series of energy level steps, from a neutral atom at the ground state. For example, the minimum, averaged over all excitation channels, energy to produce an argon ion from the neutral ground state is roughly 40 eV. However, extraction and flow of these ions into a separate stage for further independent processing is significantly more difficult. For an RF plasma source that provides extraction and flow to a separate stage, ions can be extracted for energies between 40 and 100 eV each from a single ended RF plasma source to produce a flowing, quasi-neutral plasma stream. Simultaneous extraction from both ends to form two streams can be configured to approach the minimal energy cost values for ion production. Alternatively, the energy required to extract ions using traditional grid-extraction techniques is typically much higher than 100 eV and the extracted current density is greatly constrained by ion and plasma interactions with the grid itself. Gridless configurations do not have these gridded extraction limitations.
The helicon as a stand-alone plasma generator can efficiently ionize heavier propellants, such as Neon, Nitrogen, Argon and Xenon, as well as lighter propellants, such as Hydrogen, Deuterium, and Helium. Preliminary experiments with most of these gases have been conducted in Ad Astra's laboratory with significant success. Further experiments are planned that also include chemical mixtures such as ammonia.
Neutral gas is injected into the system by a propellant injection assembly (typically an off-the-shelf programmable mass flow controller) that delivers the appropriate flow of new propellant to the ionization chamber. There are important considerations that must be taken into account in accomplishing this function effectively as discussed below.
The gas flow rate in these systems can range from a few hundred standard cubic centimeters per minute (SCCMs) to thousands of SCCMS, depending on the type of gas and power being used. This is equivalent to a range from fractions of a milligram/second to several 10 s of milligrams/second. The gas is injected into a volume (the ionization chamber) which is initially at vacuum and rapidly achieves a steady state operating pressure ranging from a few tens of mtorr before ionization to up to 1 ton after the plasma is established. Operation in space can take advantage of the virtually “infinite” vacuum pumping capability of the natural environment, while an accumulation of downstream pressure requires adjustments for operation in terrestrial environments. The results in the laboratory vacuum chamber are adjusted to be representative of the expected behavior over a broad range of vacuum conditions and indicate that the axial distribution of the plasma and gas pressure will be strongly dependent on the ionization chamber and magnetic field geometry and affected by the plasma itself. Once the discharge is initiated, the magnetic field indirectly aids in the trapping of the incoming gas by creating a plasma plug downstream.
For space applications, a VASIMR 200 kilowatt experimental prototype, the VX-200, is envisioned to verify operation of a system qualified for space use. Variations in subsystem configuration that depart from this initial design are contemplated depending on the applications and are currently under study.
There have recently been four fundamental advances in the state-of-the-art. The first is the production of a quasi-neutral plasma stream from an inflowing stream of neutral gas using radio frequency coupling techniques. The second is the extraction of the flowing quasi-neutral plasma for further processing of that plasma and any additional neutral gas in the extraction region. The third is the optional tailoring of the energy distribution of the extracted plasma by additional RF heating that is selectively tailored to affect ions, electrons, or both. The fourth is the thermal management of various components to enable long-pulse or steady-state operation with minimal and/or controllable damage by plasma-surface interactions.
Conventional magnetized RF discharges typically use a recycling fill of gas that allows neutral material to bypass the plasma ionization region, or they generate nearly stagnant plasmas surrounded by neutral gas. However, it is possible for a plasma source to allow the option of using multiple and/or simultaneous RF techniques at differing power levels to allow new regimes of electromagnetic operation. The first stage of such an ion source is capable of ionizing nearly any gas or vaporized neutral material. A first stage, in accordance with an embodiment of the present invention, could use a helicon-like RF source as is shown in
The production of a flowing plasma stream that can be guided into and through a region of very high magnetic field strength allows a unique capability to further modify the energy content and distribution of the plasma stream with ICH for light or even heavy ion species. A helicon-like first stage 132 and an ICH second stage 131, further comprising an RF coupler for ions 130 and magnets 120, in accordance with an embodiment of the present invention, is shown in
Relative to conventional ionization sources, improvements to a plasma source stage 132, in accordance with an embodiment of the present invention, could comprise self-consistent integration of an electromagnetic RF coupler, a static magnetic field used to channel and contain the plasma flow, a neutral injection system, and/or thermal management systems. Embodiments of the present invention could comprise various combinations of these features. Such features could allow an embodiment of the present invention to operate in previously unobtainable regimes. For example, a new regime could efficiently develop a stable fully-flowing plasma stream by ionizing up to 100% of the incoming neutral gas or particles. Another novel feature made possible by such a comprehensive design is provision for control of plasma impact along field lines between the region of plasma production and the region where the plasma stream is utilized further downstream. Such control can allow the power and ionization efficiencies to be optimized in conjunction with the electromagnetic performance of the RF coupling system. Such features can also allow the lifetime of plasma-facing structural components in the source to be maximized by controlling ion impact on these structural components using static magnetic shielding. Furthermore, such features can minimize the introduction of impurities into the plasma stream from erosion of structural materials used to make the source.
For an ionization process in accordance with an embodiment of the present invention, a helicon-like system can be used for a first stage in which a concentrated pattern of RF electric fields are produced in a region of axial inhomogeneity. Such a configuration can self-consistently create a region of high electric field localized axially and near the center. This power heats electrons that ionize neutral gas primarily though electron impact ionization and excitation processes. Power dissipation is proportional to the square of the electric field, leading to damping of the waves near the axis in the region of wave convergence. To minimize gas throughput while maximizing plasma output, magnetic field geometry can somewhat counter intuitively be strengthened on the downstream end (the plasma outlet). Concentrating RF power in this converging magnetic field region causes much of the ionization to occur in the downstream section where plasma production is desired. Such a magnetic geometry also allows for a closely fitting solid wall to be protected from the plasma by the magnetic field, while forming a geometrical trap to contain gas, which is not affected by the magnetic field and might otherwise escape downstream without being ionized. Geometric trapping of the neutral gas can be especially useful during initiation of the plasma when high-vacuum conditions exist downstream of the source, such as those encountered in space.
An electromagnetic simulation in accordance with an embodiment of the present invention is shown in
An embodiment of the present invention could also allow injection of the plasma stream into regions of static magnetic field with a wide range of field strengths including much higher magnetic fields than previously possible. Plasma sources lacking features of the present invention could be limited when operating in high magnetic fields because of instabilities and/or the loss of control of the plasma that is produced. Methods and apparatus in accordance with the present invention can provide for stable operation with either stagnant or flowing plasma at very high throughput using very high static magnetic field strengths. High magnetic field strengths allow efficient modification of the ion energy distribution in a second stage using a second RF technique based on ion cyclotron resonant (ICH) interactions between the ions and RF waves in the plasma, even for heavy ions. Stable, high density and high plasma flux conditions have been experimentally demonstrated well in excess of 1 Tesla with a plasma source in accordance with an embodiment of the present invention, and operation at much higher fields is possible. Very high overall power efficiency for high magnetic field is possible for embodiments of the invention using super-conducting magnet coils. Embodiments of the present invention using combinations of super-conducting and/or conventional magnet coils, and/or permanent magnets, and/or materials with high permeability are also possible depending on overall design requirements for the system.
Overall power efficiency is useful for energy conservation in all applications and especially so in space applications. This efficiency must include the technology required to generate the RF power. An ionization stage in accordance with an embodiment of this invention allows the use of RF power in a frequency regime well below the FM and industrial 13.56 MHz frequencies that are typically used in other helicon-like discharges. The ability to use frequencies much less than 13.56 MHz allows the use of solid state, or other amplifiers that are very efficient in converting DC to RF power. The ability of an embodiment of the present invention to operate at lower frequencies is useful for minimizing total power consumption required to ionize the neutral feedstock and to generate the desired plasma properties. An embodiment of the present invention having a choke region geometry similar to that shown in
In accordance with an embodiment of the present invention, optimizing the design of various components both separately and synergistically allows advances that are not possible with previous plasma source technologies. These features allow optimization and integration of the electromagnetic coupling configuration, the static magnetic field geometry, the structural components, the gas injection system, and/or thermal management systems. Features of certain embodiments of the present invention are organized and expanded upon below. The first discussed relate to the basic electromagnetic design. Second is integration to exploit synergies between any of the following: electromagnetic design, static magnetic field geometry, structural components, and gas injection. Third comes integration of compatible thermal management to improve long-pulse or steady-state operation.
For electromagnetic wave geometry in accordance with an embodiment of the present invention, low frequency waves do not typically propagate far from the launching structure without the presence of the plasma, which is produced self-consistently by absorption of those waves. Plasma dielectric properties are nonlinear in terms of the coupled RF power, which in turn affects the geometry of plasma dielectric required for the coupler to function efficiently. The magnetization of the plasma further complicates its dielectric properties and introduces strong asymmetries compared with communications or other applications with relatively simple dielectrics. The plasma response must be represented by a full dielectric tensor that is dependent on the coupled RF power. Thus, an RF coupling design in accordance with an embodiment of the present invention is more complicated than an antenna used in the traditional communications, radar, or simple dielectric heating applications. Rather, an RF coupler in accordance with an embodiment of the present invention performs an antenna-like function only in the presence of the self-consistent plasma state produced by the coupling system. Furthermore, plasma cannot typically come into contact with the current carrying components of the coupler without shorting it out. The effects of non-radiated near-fields play a critical role in the initiation of the plasma and in the efficient coupling of the RF power across an evanescent gap. Winding techniques, more akin to those used for transformers can greatly improve an RF coupler's performance in coupling power to the plasma. This performance is affected by the effective plasma resistive load in the RF circuit. Winding an antenna also modifies the inductance of the overall circuit that is used to resonate the antenna. Thus, an RF coupling system in accordance with an embodiment of the present invention shares certain characteristics of a transformer, an inductive circuit element, and an antenna. The geometry of the self-consistently produced plasma in a static magnetic field also plays a role in determining the coupling of electromagnetic waves and the required geometry of the launching structure. Although an RF coupling system may sometimes be referred to as an “antenna” by analogy and for simplicity, the actual integrated RF coupling process with plasma is much more complex than processes exemplified by typical antenna applications.
Another embodiment of the present invention comprises an RF coupler comprising an electrically insulating winding mandrel 127 having channels 135, as shown in
The (helicon) RF antenna launches electromagnetic waves into the ionization chamber to provide the basic plasma source. These waves interact with the background gas and produce an ionization cascade. Once a plasma is formed, the waves couple primarily to electrons in the plasma, which ionize the gas primarily by electron impact, generating dense (10 raised to the 20th power particles per cubic meter) and cold (electron temperature of 5 ev and ions at room temperature) plasma. The wave power must be efficiently absorbed by the self generated plasma so that the “cost of ionization,” typically measured in ev/electron-ion pair, is operationally acceptable and does not levy an undue tax on the overall power budget for the system. Operation can also be constrained by the removal of the wasted energy from the walls of the ionization chamber and other system components. Our studies indicate that ionization costs above 200 ev/e-i pair would result in generally low efficiency for rocket applications in space, but may be acceptable for many terrestrial applications. Present designs and experimental results with Argon indicate ionization costs of less than 100 ev/e-i pair can be achieved. Ionization costs improve with higher power.
The helicon waves propagate from the antenna radially inward and downstream towards the axis of the ionization chamber. As they do so, they are absorbed by the plasma (damping mainly on the electrons). The wave absorption can be influenced by the local gas/plasma pressure so, depending on the application, it is important to insure that optimal conditions exist downstream of the helicon antenna for the wave energy to be readily absorbed while simultaneously delivering the desired plasma downstream.
The RF subsystem comprises the RF transmitters, the impedance matching circuits, the transmission lines and the RF antennae. Both solid state and tube-based RF technology are used in the laboratory and for terrestrial applications. All solid state technology will be used for the VX-200, the first flight-like version of the VASIMR. These will now be discussed sequentially.
The RF transmitters convert input DC power at moderate voltages (a few hundred 5 volts) into RF power that is delivered to the plasma by the antennae.
The basic building block of the RF system is a Metal Oxide Silicon Field Effect Transistor (MOSFET) module capable of up to a kilowatt of power. These units have been produced commercially for the AM radio market and other applications. These units are driven in clusters and integrated in sub-modules that, together, reliably generate the required RF power. Two such modules will drive the helicon and ion cyclotron stages respectively. The RF transmitters will operate at two different frequencies to power the helicon and RF booster stages respectively.
Impedance matching between the source and the load is required to efficiently couple the RF power to the plasma. This is accomplished by tuned transmission lines with a possible intermediate matching circuit with the appropriate capacitance and inductance. Because perfect matching may not be practical throughout the plasma startup and/or other transients, the transmitter module and related circuitry must be robust enough to tolerate brief (millisecond) off-match conditions where the power reflected back into the source may be considerable. Moreover, if off-match conditions persist because of other fault or off-normal conditions, the transmitter system should be capable of automatic shutdown with no damage to the hardware. These features are feasible with modern RF technology.
A transmission line, typically an insulated high voltage coaxial central conductor sheathed in a grounded outer jacket, or a stripline delivers the RF power to the antenna. To minimize line losses, the transmission line should be as short as practical, putting the RF system as close as possible to the plasma. It should also incorporate impedance characteristics that assist in the matching process.
The helicon antenna transmits power to start-up the plasma and heats the plasma which further ionizes the gas in the ionization chamber. This plasma can then be used for the desired application. In the VASIMR system or any application requiring further plasma heating, the plasma moves downstream to the ion cyclotron resonance antenna.
The ion cyclotron resonance antenna is the core of the VASIMR second stage, also known as the “RF booster.” The antenna launches ion cyclotron waves onto the flowing plasma. The waves damp by resonating with the cyclotron motion of the ions, a well known plasma heating mechanism used in controlled fusion research. The waves are launched in a region where the natural cyclotron frequency of the plasma ions is above the wave frequency, as a result, the waves do not damp on the surface of the plasma but are able to penetrate radially inwards as well as propagate downstream to a region of lower magnetic field where the wave frequency does match the ion cyclotron frequency exactly. This axial position is called the cold resonance and waves are able to couple energy to the plasma beyond this point. In practice however, the actual resonance differs somewhat from the cold resonance due to Doppler effects caused by the plasma flow velocity.
Some of the wave energy in the second plasma heating stage of systems that require such heating is also delivered to the plasma electrons. The partition of wave energy between ions and electrons is driven by antenna design features, such as the antenna length, twist and number of straps. While the RF power given to the electrons is not considered useful in our calculations of rocket performance for VASIMR, the electron heating may indeed provide useful thrust by increasing the electron temperature and hence the “ambipolar” electric field that is naturally established by the physics requirement that the ions and electrons leave the system at the same rate. As a consequence, the calculations of rocket performance are considered conservative estimates.
One unique aspect in the present VASIMR configuration is the delivery of the RF energy in a single ion pass through the system. This is in contrast to classic ion cyclotron heating, which relies on multiple ion passes under the antenna field as they bounce in a magnetic well. The present VASIMR RF booster does not require such trapping and has demonstrated sufficient wave energy absorption in a single ion transit thus rendering a considerably simpler magnetic structure.
The helicon antenna twist, length and diameter are design parameters that stem directly from AARC's plasma models as well as experimental results from the physics demonstrator experiment. The efficient operation of the antenna also depends on the magnetic field geometry and the plasma density.
There are important considerations on the installation of the antenna that have a bearing on potential plasma bombardment and erosion onto the insulating inner surface of the plasma chamber. This effect is produced by high voltages associated with plasma sheath rectification. There are a number of simple design and antenna assembly measures that can be implemented in accordance with an embodiment of the invention to reduce these effects. These include mounting the antenna onto the dielectric with a small physical separation, which increases its stand-off voltage capability. In addition, the antenna resonant circuit can be designed with a “floating ground” that can greatly reduce the sheath voltage issue. Other measures include the use of Faraday shields to reduce direct electron bombardment onto the antenna straps.
A problem with having a solid metal antenna is that the RF is only carried near the outer surface giving locally very high current densities and the rest of antenna cross-section is wasted. In accordance with an embodiment of the invention, a LITZ wire weave (an oblong bundle of micro wires) can be used in the ICRH antenna—perhaps also in the helicon antenna in the future, though the frequencies in the helicon may typically be too high for LITZ wire. The weave should route wire from inner surface to outer surface so that RF currents can be distributed over the entire cross section of the current carrying material. This wastes less RF power in the antenna and yields more ions per Joule. A thermally conductive electrical insulator, such as aluminum nitride, can be used to carry heat away from a Litz wire pack.
The ICRH antenna twist and number of straps and proximity to the plasma are important considerations in a successful design. The antenna loading is strongly dependent on the total antenna current as seen by the plasma and its proximity to it. Since the antenna current only flows on the surface of the conductor, one way to maximize loading in accordance with an embodiment of the invention is through the radial layering of very thin antenna straps embedded in an insulator matrix. Ad Astra Rocket Company has begun testing experimental versions of these designs in the physics demonstrator experiment in Houston. Initial experiments with Argon have shown promising results. In general: the greater the number of straps, the greater the plasma loading.
Also, by increasing the twist of the straps one can increase (at the expense of loading) the energy coupled to the ions. Proximity to the plasma also increases the plasma loading but can lead to greater damage to the antenna structure due to plasma bombardment and heating. These physics considerations play an important role in the engineering of the system and are carefully evaluated when making design choices. The design team makes considerable use of AARC's plasma models and experimental results from the physics demonstrator experiment in carrying out design optimization of all these parameters.
High performance and high efficiency of the source can be achieved by exploiting synergies between the RF wave pattern that is excited by a coupling device and the plasma that is produced by the absorption of RF power from this wave pattern. An integrated approach is used to design an RF coupler so that it remains compatible with other constraints while meeting the demands of the desired application, and especially those for the VASIMR application. The dielectric response of the plasma depends on the plasma that is produced, which thereby affects the wave pattern that can be efficiently excited. The initiation of the plasma depends on the ability of an RF coupling system to couple relatively small amounts of power during imperfect transient conditions until the self-generated plasma can evolve into the intended configuration. This configuration is also affected by device geometry, static magnetic field conditions, and by the electromagnetic design of the RF coupling structure. The final plasma-electromagnetic coupling state can be tailored to suit the specific needs of the application.
An integrated embodiment comprising a first stage used to ionize the incoming neutral gas stream and direct its flow into a region of high magnetic field is shown in
A key factor in antenna performance is overall helicon geometry. A counter intuitive benefit of a helicon configuration is that it produces much of the hot plasma downstream of the helicon antenna in the region of increasing magnetic field. A stronger magnetic field downstream of the antenna creates a narrow chokepoint that the plasma and gas must flow through. It also changes the characteristics of the RF waves in this region. Most physicists would expect the magnetic pinch to push the hot plasma upstream, but instead it works in a fashion analogous to a lens to focus the plasma production downstream where the ambipolar potential and gas pressure push the plasma through the magnetic choke. This effect is a feature of the high flow, high power, and high efficiency helicon. It is a key design driver in the helicon. Gas density and flow rate, antenna design, and magnetic field shape are all optimized to move the hot plasma downstream. Depending on the application, for the optimum magnetic pinch geometry, the ratio of the field strength at the choke point to the field strength at the helicon is usually greater than two, and is typically 4 or 5.
An integrated plasma source according to an embodiment of the present invention, as shown in
An integrated plasma source according to an embodiment of the present invention comprising a helicon-like first stage 132 and a second RF system 131 suitable for ion cyclotron resonance heating of the flowing plasma stream, is shown in
These systems rely on a DC magnetic field to produce, confine, guide and accelerate the plasma. For space applications or any application that requires minimal power consumption, this field is produced by a cluster of superconducting electromagnets. Superconducting magnets can be manufactured commercially by a limited number of companies worldwide.
The VASIMR magnet comprises four sub assemblies: (1) the helicon first stage magnet, (2) the choke coil, (3) the ICRH or booster magnet and (4) the nozzle magnet. Design of these space-relevant systems requires careful evaluation of thermal and structural issues within a package of manageable weight. Terrestrial applications may not be so restricted and can be water cooled, although advanced, space-relevant solutions may still be used if they are economically viable.
The helicon magnet is an integrated assembly of magnet coils designed to provide minimal plasma interaction with the walls of the ionization chamber. A smaller diameter thermal shield is used as a 300° K. barrier for superconducting magnet designs, enabling the cryostat to handle the low cryogenic temperatures of the magnet through a cryogenic thermal management system. The helicon field is typically less than 1 Tesla hence the magnet can be rather long and thin. The two magnets, when combined with the choke magnet, produce the required field profile near the helicon antenna. Superconducting magnets typically require that they be attached to other parts of the system by struts with low thermal conductivity.
The choke magnet is a short annular structure producing the highest field in the system, and may generate fields of 1 Tesla or more depending on the application. This field has two purposes: (1) separate the helicon plasma from the ICRH by means of the high magnetic mirror and (2) provide a narrowing of the plasma column to fit inside the ICRH antenna at the proper density for RF absorption. The choke magnet works with the RF booster magnet to provide the magnetic beach where the ion cyclotron waves are absorbed.
The booster magnet is a longer solenoid structure producing a fairly flat axial field profile at high magnetic field. The booster magnet encloses the ICRH antenna and is designed to provide sufficient absorption of the RF waves in the flowing plasma for applications that require additional heating beyond that provided by the helicon source.
Transition coils may also be needed to tailor the plasma exhaust for a specific application. In the case of VASIMR, the transition is performed by a magnetic nozzle to properly shape the field for (1) efficiently convert the ion perpendicular motion into axial motion and hence useful thrust and (2) provide sufficient expansion of the of the field to enable effective detachment of the plasma (and the field) from the portion of the field that remains attached to the rocket. Considerable experimental research is currently ongoing on the topic of detachment. Our present models are able to predict nozzle performance under a variety of approximations, conditions and magnet geometries. The nozzle magnet may be designed as a single unit or as an assembly of discrete rings that will provide the appropriate field shaping.
An additional key driver in the plasma source design is the alignment of the magnetic field tangential to the components of the system. This alignment minimizes the heating and erosion from plasma impingement on the surfaces of the source components.
Geometry is optimized for a particular gas. Magnetic field strength at the helicon antenna is matched to the size of the ionization chamber. One option to allow the use of different fuels in a VASIMR engine is to swap the ionization chamber for one sized for a different gas and then change the geometry of the magnetic field to match the new chamber, then adjust overall magnetic field geometry to optimize it for the new gas. A preferred embodiment would have the entire engine optimized for a particular gas, but allow for cartridge changes and magnetic field adjustments to use different gases.
Moving hot plasma downstream allows the gas injection plate/lid to be moved closer to the helicon antenna, and can provide flexibility for the materials to be used. Dense gas blocks ions from hitting the plate. Moving the gas injection plate shortens the magnet and reduces overall engine length and weight for space applications. Adjustments of the plate position can also change the gas flow characteristics through the system, depending on the application and method used for gas injection.
The solid choke that keeps non-ionized gas in the helicon to get a higher ionization percentage can be made of ceramic, metal, or other materials depending on the application's requirements.
The helicon antenna twist, length and diameter are design parameters that stem directly from AARC's plasma models as well as experimental results from the physics demonstrator experiment. The efficient operation of the antenna also depends on the magnetic field geometry and the plasma density.
A possible propellant starvation is believed to be caused by relatively hot plasma being created from the relatively cold feedstock gas. This condition is promoted by the higher transport velocity of the plasma compared to that of the cold gas. In order to reduce this effect for some applications, we may “force feed” the propellant directly to the location where it is needed rather than allowing it to flow on its own from the upstream axial injection point currently in use. Proper gas injection may reduce ionization cost and hence increase efficiency for some regimes of operation.
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The VASIMR presents several thermal challenges to the designer as heat must be removed at various temperature ranges. The subsystem is envisioned to encompass three distinct temperature ranges: high, intermediate and cryogenic.
The construction of an RF coupling structure and thermal management can be achieved by various active and/or passive means. Waste heat generated by an RF circuit can be minimized by using materials that are good conductors of RF. For frequencies low enough to take advantage of Litz wire's enhanced RF conductivity, very high efficiency can be achieved using multiple wraps of Litz wire to minimize Joule heating of the RF coupler by distributing the RF current over the cross section of the Litz wire. The wrapping of the coupler also determines the inductance of the antenna, allowing control of the remainder of the resonant circuit design for the second stage.
Convective cooling, such as by air surrounding the system is a possible cooling option for some plasma sources according to certain embodiments of the present invention. However, such traditional cooling is not always compatible or available for some applications. A plasma source according to an embodiment of the present invention may comprise alternative solutions for actively and/or passively cooling the source components during long-pulse or steady-state operation.
In one embodiment of the invention, the metal antenna is simply covered on at least one surface with a diamond film layer using a chemical vapor deposition (CVD) process. The metal portions of the antenna can thus be protected because heat can be quickly conducted through the diamond layer. Although other materials, such as quartz or aluminum nitride, could be used, diamond is desirable because of its combination of unique physical properties including an extremely high thermal conductivity and transparency to the RF energy being supplied to the gas stream. The heat carried away from the antenna may be disposed of remotely through any type of heat exchanger.
In further embodiments, the antenna may have alternate geometries: The entire antenna may be embedded in a hollow cylinder made of either diamond or other RF compatible ceramics; the antennas may have various numbers of straps and twists; the antennas may be made of woven LITZ wires; the straps themselves may have rectangular or alternate cross sections; and any number of surfaces on the straps may be covered with a thermally conducting layer.
In another embodiment of the invention, the antenna 100 may comprise a plurality of metal substrate layers 102 alternating with layers of thermally conducting electrical insulators 104. The thickness of the substrate 102 and thermal conductors 104 may be varied as necessary to achieve optimum levels of heat transfer and delivery of energy to the gas stream 134. Voltage through the electrically insulating layers 104 is low, so they could be made very thin.
In further embodiments, the antenna may have alternate geometries: The entire antenna may be embedded in a hollow cylinder made of either diamond or other RF compatible ceramics; the antennas may have various numbers of straps and twists; the antennas may be made of woven LITZ wires; the straps themselves may have rectangular or alternate cross sections; and any number of surfaces on the straps may be covered with a thermally conducting layer.
A plasma source 100 according to an embodiment of the present invention comprising an integrated thermal management solution using conduction cooling through thermally conductive, electrically insulating material 127 is shown in
A plasma source according to an embodiment of the present invention comprising an integrated thermal management solution using conduction cooling of a mandrel 135 used for winding the RF coupler is shown in
As shown in the article titled “Comparing experiments with modeling for light ion helicon plasma sources” in Physics of Plasmas Volume 9, Number 12, page 5097 (2002), the RF antennae are typically helical conducting straps integrated in an insulated sleeve that also serves as the primary gas chamber. The antenna conductor may be actively or passively cooled to remove resistive (I2 R) power losses and heat soak-back from the plasma chamber itself. Because the circuit resistive losses are dependent on temperature, keeping the antenna straps as cold as possible is highly desirable. The key performance parameter for coupling RF power to the plasma is referred to as the “antenna loading.” In that sense, the plasma represents a desirable resistive load onto the resonant circuit.
If the structural support and conductor cooling are provided otherwise, the antenna straps need not be unnecessarily thick. Rather, since RF currents only propagate within one skin depth of the conductor surface, in accordance with an embodiment of the invention, the conducting straps could literally be “painted” on the insulator substrate. These generally involve very thin conducting straps deposited on an insulator substrate. In accordance with an embodiment of the invention, multiple layers are also being considered to maximize the antenna current near the plasma. At the present time aluminum nitride appears to be an acceptable candidate for insulator substrate with gold as the potential conductor material. The assembly could be cooled passively by conduction heat transfer or actively by a flowing liquid or gas. The materials issues are critical in these designs, as their operating temperature can affect the I2R losses as well as the so called loss tangent, a measure of the RF power absorbed by the material itself (which ultimately also ends up as waste heat).
A plasma source 100, according to an embodiment of the present invention, comprising an integrated thermal management solution comprising an RF feed 136 and a re-entrant fluid cooling loop at the RF ground point 138 is shown in
An antenna such as those described above may also require additional thermal protection from other heat sources. The thermal protection must be compatible with the RF energy going into the plasma and to minimize RF losses in the thermal protection components. Although the antenna may be isolated from the ionized plasma by a magnetic field, heat is still radiated back to the antennas. Sometimes, in the Helicon section, a cold neutral atom gives an electron to a hot ion. Because the resulting hot neutral is not affected by the magnetic field, it can travel radially, deposit its energy on nearby structures, heat those structures, and thereby waste the energy that otherwise would have been used for propulsion. The ICRH antenna of the VASIMR, on the other hand, can heat plasma to many millions of degrees Kelvin, but generally, the plasma in this section is completely ionized so there are very few neutrals to donate electrons and there are very few hot neutrals escaping the magnetic field. A tube having reasonably transparent properties with respect the RF, is positioned coaxially between the plasma stream and the antenna to contain neutral gases and can carry some of the radiated heat away from the antennas. Another possible thermal protection means involves the use of a heat pipe. In this case the antennas are imbedded in a fluid filled tube wherein the fluid carries heat from the antennas. An antenna within a heat pipe must be embedded in an electrical insulator so that the signal is not grounded out. Thermal protection means for the antenna must be transparent to the RF energy that passes from the antenna to the target gas and plasma.
An RF coupler according to an embodiment of the present invention is shown in
Experimental results from one embodiment of the present invention, known as VX-100, are given in
The plasma flux of the VX-100 embodiment was measured using an array of 10 probes (not shown), positioned across the outgoing plasma stream, drawing ion saturation current. At maximum power, the ion flux was estimated to exceed 1021 ions/second, ionizing 100% of the injected neutral gas as shown in
Efficient ICH coupling to the plasma stream was also demonstrated for argon ions in these experiments with the VX-100 embodiment of the present invention. The efficiency was measured using Q measurements of the second stage with and without plasma to isolate the plasma loading and circuit efficiencies. The results are contained in
Here we present a concept for removing the waste heat that is deposited into the walls of the ionization chamber. A significant fraction of the RF power transmitted into the ionization chamber is deposited into the chamber walls either in the form of radiation or convection of the plasma. This heat must be removed in order to keep the material temperatures low enough to maintain structural integrity and to keep the loss tangent of the gas containment tube (normally a ceramic) low. Since the antenna 100 should be placed as close as possible to the ionization chamber, the thickness of the gas containment tube is practically limited to about 1 cm. The only material that might be able to conduct the heat axially along the tube for the length of the antenna is diamond. An embodiment of the present invention is a more practical solution—an ionization chamber in which the walls are actively cooled. As shown in
These, typically ceramic, tubes must be chosen based on having a low loss tangent at the operating temperature, thermal conductivity, and strength. The fluid then flows into a downstream manifold 117 where it exits the helicon section through the flow exit 116 to reject the heat.
This innovative combination of a high-power helicon antenna and thermal control system allows for plasma mass flow throughput and densities never before achievable in steady state.
Since the fluid must pass between the helicon antenna and the plasma, it must also have a low loss tangent at the operating temperature. To reduce the thickness of the tubes, the fluid must also have a low vapor pressure and viscosity at the operating temperature. We have found that silicon oils can meet these requirements, although other fluids may also be used, and some heat-pipe concepts may be possible.
Various antenna properties have been tested for use with the VASIMR. The optimal geometry for the helicon antenna, for example, appears to be a half-twist double helix with the gas stream running through the center. Additional plasma coupling can be obtained as needed by connecting electrically isolated antennas in series. Early ICRH antennas generally comprised a series of short cylindrical tubes or rings spaced a distance from one another along the axis of the gas stream, although helical antennas provide the optimum ICRH performance for many applications. The antennas need not be bulky, but they must be formed from a good conductor at the RF frequencies used. These antennas must be cooled for steady state operation because of the heat dissipated in them by RF currents flowing through them and other possible heat sources. Both fluid loop and heat-pipe cooling techniques are feasible depending on the application.
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The working fluid is drawn to the relatively low pressure at the condenser end of the heat pipe where it condenses again to a liquid 110 and enters the wick 112. The arrows indicate the flow of the working fluid within the heat pipe. The working fluid should be transparent to the RF energy transmitted through the antenna, so ammonia and nitrogen are good choices.
In another embodiment of the invention, the antenna comprises a heat pipe for carrying heat away to a heat exchanger. In this case, the antenna has a hollow cross section and comprises a fluid filled tube having a wick inside and the working fluid carries heat from the antennas to a heat exchanger. Heat from the plasma evaporates liquid working fluid in the wick and draws it into the vacuum tight hollow interior of the tube as a vapor. The vapor working fluid is drawn to the relatively low pressure at the condenser end of the heat pipe where it condenses again to a liquid and enters the wick.
It should be appreciated that although embodiments of the invention have often been described in the context of a VASIMR engine, the present invention has much broader potential application, and could be useful in any circumstance where efficient production of plasma is desirable.
In the foregoing specification, embodiments of the invention have been described with reference to numerous specific details that may vary from implementation to implementation. Thus, the sole and exclusive indicator of what is the invention, and is intended by the applicant to be the invention, is the set of claims that issue from this application, in the specific form in which such claims issue, including any subsequent correction. Any definitions expressly set forth herein for terms contained in such claims shall govern the meaning of such terms as used in the claims. Hence, no limitation, element, property, feature, advantage or attribute that is not expressly recited in a claim should limit the scope of such claim in any way. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US2008/002381 | 2/19/2008 | WO | 00 | 8/17/2009 |
| Number | Date | Country | |
|---|---|---|---|
| 60902016 | Feb 2007 | US |
