WO2019222549A1

WO2019222549A1 - Apparatus, systems and methods for spacecraft scale magnetosphric protection from galactic cosmic radiation

Info

Publication number: WO2019222549A1
Application number: PCT/US2019/032740
Authority: WO
Inventors: John Slough
Original assignee: Msnw Llc
Priority date: 2018-05-16
Filing date: 2019-05-16
Publication date: 2019-11-21

Abstract

Systems and methods employ a Plasma Magnetic Shield (PMS) to provide for active magnetic shielding for protection of a spacecraft, its occupants, and cargo from galactic cosmic radiation (GCR).

Description

APPARATUS, SYSTEMS AND METHODS FOR SPACECRAFT SCALE MAGNETOSPHRIC PROTECTION FROM GALACTIC COSMIC RADIATION

PRIORITY CLAIM

[0001] This patent application is a non-provisional of U.S. provisional application entitled“SPACECRAFT SCALE MAGNETOSPHRIC PROTECTION FROM GALACTIC COSMIC RADIATION,” having application serial number 62/672,369, filed May 16, 2018, which is incorporated herein by reference in its entirety.

BACKGROUND

[0002] Traditional chemical-based propulsion systems may be used to launch and maneuver space vehicles. Once aloft, an additional amount of chemicals, or fuel, is required to provide thrust. Further, additional fuel may be required to generate electrical power. However, such chemical-based propulsion systems are inherently limited by the amount of fuel that is transported into space along with the vehicle. At some point during the operating life of the space vehicle, the fuel will become depleted and will thus render the space vehicle unusable. [0003] It is well known that exposure to the ionizing radiation of galactic cosmic- rays (GCR) and solar energetic particles (SEP), referred to herein genetically as radiation, is an important concern for the health of the crew for long duration interplanetary missions. With legacy systems, it is not feasible to mitigate this exposure in a practical manner. The SEP events are characterized by the emission of high fluxes of lower energy particles, which last on the order of hours to days. Although the intensity of the radiation is much greater than the GCR, passive shielding (i.e. shielding provided by the slowing down and absorption of the charged panicles in inert materials) can be effective for SEP as the duration and penetration distance for these lower energy particles is quite short. The GCR flux however is near isotropic and is relatively constant, being modulated somewhat by the solar cycle. Periods of maximum solar activity result in the decrease of the low energy GCR flux due to their interaction with the higher particle flux emitted by the Sun. Previous analyses have pointed out the futility of attempting to employ a material barrier for the GCR as the increase in shield thickness creates a mass penalty that is far too great to be practical.

[0004] Even partial shielding is not an effective option as the radiation hazard can actually be greatly enhanced in this case as the "daughter" particles generated during the slowing down process of the GCR in the material. This is particularly true for the high charge and energy (HZE) ions such as iron which can create several high energy secondary' and tertiary high energy particles. The radiation risk arises from the damage caused by the energy lost from these charged particles in human tissue. The GCR fluxes represent a longer-term risk exposing the crew members to lower life expectancy due to radiation induced cancers.

[0005] Of all the legacy active methods that have been suggested to take the place of passive (i.e. material) shielding, the most promising from a technological point of viewy and the most studied, is the use of magnetic fields to deflect the GCR away from the human space habitat. The basis for this approach is given by the partial shielding provided by the Earth's magnetic field, however it is not Earth's magnetic field but rather the bulk of the Earth's atmosphere that protects us from the full brunt of the OCR. The physics behind the concept is simple and is based on the fact that a charged particle in traversing a transverse magnetic field will experience a transverse acceleration to its direction of motion due to the Lorentz force field will experience a transverse acceleration to its direction of motion due to the Lorentz force:

[0006] F = qvx B (1)

[0007] As this is a non-central force, the kinetic energy of the particle is undiminished and cannot be slowed down. Only the direction of motion can be changed. In a uniform, transverse (to the particle motion) magnetic field B the particle executes a circular motion at a radius I_L (Lamior radius) given by:

m v me

[0008] ^{¾ :=} ir ^=: _qi^" (2)

[0009] where it is recognized that these high energy ( ¾ GeV/nucleon) particles will be moving close to the speed of light. In order for a magnetic field to deflect a charged particle in a variable magnetic field, such as occurs in a magnetospheric dipole field, the integral of the magnetic field over the path of the particle motion within the magnetic shield must be sufficiently large (i.e. comparable to the gyro-radius of the particle), or:

[0011] where q is the charge of the particle, m is the relativistic mass, and the subscript L denotes quantities perpendicular to the magnetic field.

[0012] Any particle with w < w dipois will be deflected, irrespective of its initial pitch angle. For shielding of MeV protons and < lO MeV electrons, Eq. (1) then requires that ]B wdr be of the order of 0.1 Tesla-meter (T~m). Deflection of GeV would then require that |B v±dr be of the order of 3 T~m while solar wind particles require 0.001 T-m. This criterion is a necessary condition but it turns out to not be a sufficient condition.

Magnetic shielding must be verified by single particle tracking techniques.

[0013] For scaling purposes though the simple deflection picture is adequate. In a spatially uniform magnetic field, the distance L the particle must travel transverse to this B field to be deflected by an angle 8 is given roughly by:

[0015] Clearly one wishes to maximize Band L. The other kev variable is the charge to mass ratio q/m. This is where active magnetic deflection has a great advantage over passive shielding. This ratio is almost constant at roughly 0.5 coulomb/nucleon as one goes from helium to iron. While hydrogen has a somewhat more favorable ratio (unity), the magnetic field deflection of HZE ions like iron will be as effective as that for deflecting lighter ions with the same charge to mass ratio. This of course is with the caveat that these HZE ions do not encounter material objects. In that case the tissue damage from the subsequent particle cascade can more than make up for their much lower GCR population.

[0016] There have been several groups that have presented active shield designs based on superconducting magnet configurations. Virtually all have assumed shield configurations composed of toroidal or soienoidal magnets. There is a very good reason for this. With the advent of high -temperature superconductors (HTSC), it is possible to achieve very high magnetic flux densities at operating temperatures (25 °K) that do not require the use of liquid helium thereby alleviating the technical difficulties in providing for the stability of a cryogenic system in space. There have been three main studies, all employing HTSC shields and all assuming B field current coils partially surrounding the space habitat with the high magnetic fields concentrated in a layer radially outward from the habitat. The idea being that a high B-L product can be achieved at least for GCR flux coming in the radial direction toward the habitat.

[0017] The legacy HTSC shields were as follows: Yttrium-Barium-Copper Oxide (YBCO) and Magnesium Diboride (MgB2), and were evaluated based on the results obtained with 3 -dimensional Monte Carlo simulations that propagate the charged particles in the magnet field, and generate interactions of the particles in the materials of the current coils and support structures of the magnet shield. Even with extrapolations of existing technology for the materials of the current coils and support structures of the magnet shield, the results were not encouraging to say the least. The results from the calculation can be found in Table 1 below:

Table 1. The performance of the NIAC 6 + 1 extendable solenoid (ES) and the SR2S continuous coil toroid (CCT) shields.

[0018] The performance is expressed in terms of the contribution of the field to the overall dose reduction of the shield, including the passive shielding elements, and the combined shield plus habitat dose level with respect to the habitat dose level. As can be seen, the role that the magnetic field plays in reducing GCR is paltry at best. The NIAC numbers are within error bars of having no effect at all. Even with up to 100 Mt of magnet material, the spacecraft habitat itself provided most of the meager shielding that was obtained.

[0019] The reason lies in the magnetic topology of the approach taken in all three legacy efforts. It is clear from equation ( l) that to deflect the GCR particle, the magnetic field must be perpendicular to the direction of motion. Since particles can come from any direction, deflecting only those coming in perpendicular to the primarily axial or azimuthal B field will hardly suffice. This“hole” on-axis makes what shielding there is almost worthless much like a large hole in the bottom a boat makes any craft unseaworthy regardless of how watertight the rest of the hull might be. In fact, GCR particles will be deflected into this region by the very fields meant to keep them out! The results from the Monte Carlo calculations clearly illustrate the problem.

[0020] The radiation protection with regard to these “closed” magnetic field configurations is actually worse than just the lack of adequate coverage for particle deflection. A major concern is the lack of any magnetic shielding for the current coils themselves as these are exposed to the GCR flux unabated. The cure now becomes the disease as the current coils become the passive source of a cascade of daughter particles. The reduction in primary GCR ions due to the shield is more than made up for by secondary emission for all but the highest BT_ product technologically feasible.

[0021] Accordingly, there is a need in the arts for an improved system and method of providing spacecraft shielding of cosmic radiation.

SUMMARY

[0022] Systems and methods of establishing a.

[0023] In another embodiment, .

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Preferred and alternative embodiments are described in detail below with reference to the following drawings:

[0025] Figure 1 is conceptual diagram showing an example embodiment of a Plasma Magnetic Shield (PMS) system disposed on a leading portion of a spacecraft; [0026] Figure 2 are graphs of Modulus B as a function of z on axis and at the torus radius R; and

[0027] Figure 3 is a block diagram of a Plasma Magnetic Shield (PMS) system embodiment.

DETAILED DESCRIPTION

[0028] Embodiments of the Plasma Magnetic Shield (PMS) system 100 and method provide for active magnetic shielding for protection of a spacecraft, its occupants, and cargo from galactic cosmic radiation (GCR) and/or solar energetic particles (SEP).

Figure 1 is conceptual diagram showing an example embodiment of a toroid-shaped Plasma Magnetic Shield (PMS) system 100 disposed around a spacecraft habitat area 102 that is suitable for habitation by one or more humans and that is located in an example embodiment on a leading portion of a spacecraft 104. However, the PMS system 100 and habitat area may be located at any desired portion of the spacecraft 104 in alternative embodiments.

[0029] For convenience, a spacecraft propulsion system 106 is illustrated at the distal portion (at the end) of the spacecraft 104. Energy to power the PMS system 100 is provided by a power system 108. In an exemplary embodiment, power system 108 is preferably a renewable energy source, such as a solar panel array or the like. Alternatively, or additionally, power may be provided by other suitable power sources (not shown). Further, the power system 108 may optionally provide power to the spacecraft propulsion system 106.

[0030] In the various embodiments, at a minimum for active magnetic shielding to be a viable option, the following criteria are to be met:

[0031] (1) The magnetic shield must deflect the vast ma_jority of GCR ions including IIZE ions.

[0032] (2) The magnets must not create additional high energy particles from passive absorption . [0033] (3) The structural, mass, and power requirements for the shield must not become "the tail that wags the dog".

[0034] (4) The shield development roadmap must allow for experimental validation at much reduced cost and. scale prior to full deployment.

[0035] Adequate shielding in all directions, as well as protection of the magnets themselves, can be accomplished by employing a dipolar magnetic field generated by the PMS system 100. The various embodiments of the PMS system 100 employ shielding magnets to generate dipolar magnetospheric shielding provided by a generated toroidal current Ic in a toroidal magnet 110. Some embodiments may employ high-temperature superconductors (HTSC) magnet located external to and around the habitat area 102 of the spacecraft 104. The toroidal magnet 110 fully surrounds the habitat area 102 for cancellation of all magnetic fields within the habitat area 102.

[0036] The habitat area 102 takes on any suitable form within the shielding toroidal magnet 110 depicted in Figure 1. The dipole currents Ic all flow through current coils disposed around a toroid shell, or that are arranged to form a toroid shell, surrounding the habitat area 102. The current, interchangeably referred to herein as a toroidal current, flows in each toroidal current coil in the same toroidal direction. That is, the current coils form a toroidal geometry around the habitat area 102, wherein current flows only in the toroidal direction in the toroidal magnet 110 thereby creating a toroidal field only on the outside of the toroid. The dipolar field created by the unidirectional toroidal currents in the shell all add to the strength of the external dipole field B outside the space habitat area 102. In this scenario, there is no need for providing additional current coils to cancel the internal fields because the toroidal currents are arranged to flow so as to maintain the shell as a constant flux boundary where the poloidal flux cp = 0.

[0037] To illustrate, consider a cylindrical tube carrying a uniform axial current along its length. The field created using a cylindrical tube is purely azimuthal (i.e. poloidal) and vanishes inside the wall of the cylindrical conductor, which can be easily understood by a simple application of Ampere's law at any radius inside the cylinder. The, bend this cylindrical tube into a torus. The toroidal currents in this case however do not flow at a constant magnitude at ail azimuthal angles around shell periphery as it does for the cylinder. Only for torii at very large aspect ratio a (torus diameter/habitat diameter) does it approach uniformity. For a typical aspect ratio a ~ 10, a larger current must flow on the inner wall of the torus to maintain the f = 0 state inside the habitat area 102.

[0038] The magnetic field strength is characterized by the magnetic field B¥ at r = z = 0. Plots of the magnetic field on axis and at r = 10 meters (m) as a function of z on axis is shown in Figure 2 which clearly shows how the field (and thus IF) is significantly higher on the inboard side of the torus for a toroid with an a ~ 10 as is the case in this example embodiment. The characteristic axial field strength is B¥ = 1.5 T for this embodiment. Accordingly, in the various embodiment both the space habitat and toroidal current coils are located as illustrated in Figure 1 , thereby providing complete protection from GCR bombardment.

[0039] In the various embodiment, a Plasma Magnetic Shield (PMS) generates shielding provided by the magnetic geometry created by plasma currents in the force free configuration of a spheromak. A spheromak is an arrangement of plasma formed into a toroidal shape similar to a smoke ring. The spheromak contains large internal electric currents and their associated magnetic fields arranged so that the magnetohydrodynamic forces within the spheromak are nearly balanced, resulting in long-lived (microsecond) confinement times without external fields. Spheromaks belong to a type of plasma configurations referred to as compact toroids.

[0040] In an example embodiment, configuration as the plasma equilibrium has the highest magnetic content provided by plasma currents alone. A PMS-based embodiment creates a larger version of the terrestrial laboratory plasmas that have been generated in meter scale vacuum chambers. Magnetic fields produced in this manner have several advantages . First, because the magnetic field is carried by plasma, the system would be less massive by many orders of magnitude than their superconducting counterparts. The predicted plasma confinement, based on the observed confinement in the laboratory, and a conservative estimation of the confinement scaling observed in virtually all laboratory plasma confinement studies predicts a plasma mass replacement of only grams/day. Second the magnetic field is primarily at a large distance from the spacecraft 104. Astronauts are never subject to intense magnetic fields and the weak nearby fields can be easily screened locally. There is also no large energy storage or voltages in the near vicinity of the astronauts. Finally, there are no material interactions so that issues associated with the generation of secondaries is not a problem and the shielding can be turned on within minutes enabling SEP shielding as soon as the first energetic particles are detected.

[0041] After considerable modeling effort, as well as some thought as to what really provides for the magnetic deflection of a high energy ion, an unexpected result was realized that the magnetic geometry was as critical an element as field magnitude. Even though the magnetic geometry was simply connected (where the field lines remained within a contiguous volume without connection to any bounding surface) the direction and magnitude of the magnetic field produced a particle deflection that was not correlated with modulus B. In fact, the closed nature of the flux surfaces tended to confine as well as deflect the incoming GCR ions. The regions of maximum shielding aligned with surfaces of constant flux. The net result provided by the various embodiments was a decrease in the effective shielding as normalized to the simple picture of particle motion in a dipolar magnetic field.

[0042] This idealization of an example embodiment can be used in for determining estimations for magnetic field deflection of the GCR. However, such idealized estimations can be misleading as there can never be a magnetic field whose geometry is uniformly perpendicular to the influx of GCR ions. Accordingly, GCR“leaks” can occur. Careful design can direct such GCR leaks to where the crew compartment in the habitat area 102 is not located.

[0043] For the case of a large scale magnetic field created by volumetric plasma currents, the crew compartment is preferably located in the region well shielded, but unlike the toroidal surface coil currents of the various embodiments, the habitat area 102 would interrupt the plasma currents that must flow there as these are volumetric currents. The power and energy requirements are reasonable for even as large a product B-L ~ 10 T-m. For the plasma torus (i.e. spheromak), the actual field-distance product needed appeared to be roughly 3-5 times larger than the simple dipolar current coil product. This resulted in an energy requirement that was up to 125 times larger. The magnitude of the magnetic field for the PMS configuration implied a total stored energy in the terajoule range and thus became a significant obstacle for any virtually any formation scheme, thus violating criteria (3) and (4) noted above.

[0044] Several different magnetic configuration were evaluated during conception of the various embodiments, taking into consideration the criteria (3) and (4) of feasibility. For“open” systems where the magnetic field is not confined by a conducting boundary (B ~ l/r^3÷) the best candidate is the magnetic dipole. For a closed magnetic geometry, the use of nested toroids appeared to provide the best shielding. There are two reasons that this option was not investigated to any great depth. It was compact, but required magnetic fields at the conductor boundary that are close to 10 T. This approached the threshold for high temperature superconductors. It was also found to not shield ail particles from the interior compartment of the habitat area 102. Again, intuition misleads and it took the full three dimensional (3-D) isotropic flux calculations to show how the field can be readily breached. [0045] From both a confinement and shielding perspective, as well as the obvious choice for simplicity, shielding from a nested set of simple circular loops of wire proposed here was found to be the most favorable. A code to calculate the 3-D B field was developed and tested, and was found to be quite accurate. It is possible however to do some analytic estimates that reproduce with reasonable accuracy the results from the 3-D B field numerical calculations. Since it is easy to calculate the field from a single loop (or sets of loops as in the toroidal shield), the shielding effectiveness can be characterized by the B_z field on axis at the center (B₀₀) and the far-field dipole magnetic field B_z(r) where:

[0047] where it was found that g = 0.5. B₀₀ is typically less than the field strength at the toms boundary but serves the purpose of relating the field strength to the shielding effectiveness. Given that an incoming particle penetration distance must he less than a characteristic deflection distance (see Eq. 4), equation (6) results.

[0049] Where the relativistic nature of the GCR particle is explicitly expressed. Integrating using Eq. (5), equation (7) results.

[0051] Where the approximation assumes that smallest characteristic deflection radius r₀ is assumed to be roughly that of the toms radius R. For a 1 GeV proton one has gn ^:= 5.4x10^s m/s. For a loop radius of 10 meters, the B₀₀ i s 2.2 T. This turns out to be somewhat more restrictive than the detailed particle calculations, but close enough for most purposes such as scaling, current, and structural requirements. The results from the Magnetospberic Dipolar Toms (MDT) with a = 10. Accordingly, substantially or complete shielding can be obtained for higher energy nucleons by increasing the magnetic field, and partial shielding is possible even at higher energies although the effectiveness diminishes rapidly for these higher energy GCR particles.

[0052] In an example embodiment, a copper current coil is used to produce, at least transiently, the necessary B L product for a significant reduction in GCR particles. For example, with a zeroth order evaluation of a one-meter toms magnet generating a one Tesla field, a key parameter is the stored energy in a dipolar magnetic field, which can be shown to be:

[0054] While this is clearly not an insignificant amount of required energy, it should be noted that modem Lithium-based batteries for electric vehicles hold vastly larger stores of energy. For instance, an 85 kWh car battery pack contains over 300 MJ of stored energy. A much smaller energy Lithium storage unit would be used by the various embodiments for charging the MDT. The MDT coil set would be wound to match the battery voltage output at a safe current discharge rate. The coil cable would be of an appropriate gauge to result in a total winding thickness of roughly 5 em. This will assure a very long L/R current (i.e. B field) decay time of many seconds. For an N turn coil one has.

[0056] where N is the number of turns, r is the minor radius of the torus and d is the effective thickness of the copper wire. For a reasonable number of N = 10, r = .01 m, and d = 0.013, the TL/R ~ 2.5 seconds. Here, power requirements for continuous operation might be too high to be practical (2.5 MJ/2.5 sec ~ 1,000 kW). However, 100 kW is not to of the reach of some embodiments that access a supplemental power source. Such power would allow for a pulsed operation of the MDT with a duty cycle of approximately 10%, which would be sufficient for some applications [0057] Figure 3 is a block diagram of a Plasma Magnetic Shield (PM 8) system 100 embodiment, wherein the toroidal magnet 110 and the habitat area 102 are shown in a cross sectional view. The power source 108 provides power to the plasma generation system 302 and the toroidal magnet 110. In an example embodiment, the toroidal magnet 110 is a HTSC magnet, or is optionally operable as a HTSC magnet. A plurality of current coils 304 arranged in a toroidal fashion carry the toroidal current Ic. For example, during operation, the power source 108 may provide the total Ic current (the sum of the individual currents Ic through each of the plurality of current coils 304, or å Ic).

[0058] Operation of the plasma generation system 302 and the HTSC magnet 110 embodiment is controlled by the controller 306. One or more detectors 308 detect GCR and/or SEP events about the spacecraft 104. Based on the detected levels of GCR and/or SEP, the detectors 308 provide information to the controller 306. The controller 306 then modulates the strength on the shield 310 (B) by regulating Ic in the current coils. Desired current levels Ic are determined based on current levels of detected GCR and/or SEP. Accordingly, the control of the strength of the shield 310 may be controlled in real time, or in near real time.

[0059] In deep space where SEP is not likely to be encountered at significant levels of concern, if at all, the plasma generation system 302 may not be required to provide plasma that is injected into the plasma region 312 within the toroidal magnet 110. Preferably, the plasma is a low temperature plasma. In the various embodiments, when a stronger shield 310 is needed, the toroidal magnet 110 may be configured to operate in a HTSC mode. Here, the plasma generation system 302 generates a plasma that is generated into the plasma region 312 such that the toroidal magnet 110 operates as a HTSC magnet.

[0060] In a preferred embodiment, the power source 108 is a renewable energy source, such as solar panels or the like. At times when there is no GCR, or when there are relatively small amounts of GCR, excess energy generated by the power source 108 may be optionally stored in the power storage system 314 for later use. Any suitable energy storage system may be used. For example, but not limited to, conventional electric power batteries may be used.

[0061] It is appreciated that the power requirements of the spacecraft 104 may exceed the power capacity of the power source 108. Or, transient power requirements of the PMS system 100 may exceed the energy capacity of the power source 108 and/or the power storage system 314. Accordingly, an optional power source 316 onboard the spacecraft 104 may be used to supplement the power and energy needs of the spacecraft 104 and/or the PMS system 100. Any suitable source of power, such as chemical fuels and/or nuclear fuel, may be used. The unused energy provided optional power source 316 may be stored in the power storage system 314 for later use.

[0062] It should be emphasized that the above-described embodiments of the magnetic insulation fusion system 100 are merely possible examples of implementations of the invention. Many variations and modifications may be made to the above-described embodiments. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.

Claims

Claims:

1. A Plasma Magnetic Shield (PMS) system, comprising:

a toroidal magnet defined by a plurality of current coils that are each oriented in a toroidal direction about a toroid,

wherein the toroidal current coils enclose a habitat area of a spacecraft that is suitable for habitation by one or more humans; and wherein toroidal current injected into the toroidal current coils generates a magnetic shield disposed about the outside of the toroidal magnet that prevents galactic cosmic-rays (OCR) from entering into the habitat area; and

a power system that generates the toroidal current that is injected into the toroidal current coils.

2. The PMS system of Claim 1, further comprising:

at least one detector configured to detect levels of galactic cosmic-rays (GCR) and solar energetic particles (SEP); and

a controller that receives information corresponding to the detected levels of GCR and SEP,

wherein the controller operates the power source to control the toroidal current to adjust the magnetic shield to prevent the detected level of GCR and SEP from entering into the habitat area.

3. The PMS system of Claim 1, further comprising:

a plasma generation system configured to generate plasma, wherein when the generated plasma is injected into the toroidal magnet, the toroidal magnet operates as a high-temperature superconductors (HT8C) magnet.

4. The PMS system of Claim 3, further comprising: at least one detector configured to detect levels of galactic cosmic-rays (GCR) and solar energetic particles (SEP); and

wherein the controller operates the plasma generation system to control plasma injection and operates the power source to control the toroidal current to adjust the magnetic shield to prevent the detected level of GCR and SEP from entering into the habitat area.

5. A method of generating a toroidal shaped magnetic field about a spacecraft habitat area that is suitable for habitation by one or more humans, comprising:

injecting current into a plurality of toroidal current coils of a toroidal magnet; and establishing the toroidal shaped magnetic field in response to injecting the toroidal current into the toroidal current coils,

wherein the plurality of toroidal current coils are arranged to form a toroid that encloses the habitat area, and

wherein the established magnetic field prevents entry of galactic cosmic-rays (GCR) and solar energetic particles (SEP) into the habitat area.