Evaluation of a combined electrostatic and magnetostatic configuration for active space-radiation shielding

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Abstract

Developing successful and optimal solutions to mitigating the hazards of severe space radiation in deep space long duration missions is critical for the success of deep-space explorations. A recent report (Tripathi et al., 2008) had explored the feasibility of using electrostatic shielding. Here, we continue to extend the electrostatic shielding strategy and examine a hybrid configuration that utilizes both electrostatic and magnetostatic fields. The main advantages of this system are shown to be: (i) a much better shielding and repulsion of incident ions from both solar particle events (SPE) and galactic cosmic rays (GCR), (ii) reductions in the power requirement for re-charging the electrostatic sub-system, and (iii) low requirements of the magnetic fields that are well below the thresholds set for health and safety for long-term exposures. Furthermore, our results show transmission levels reduced to levels as low as 30% for energies around 1000 MeV, and near total elimination of SPE radiation by these hybrid configurations. It is also shown that the power needed to replenish the electrostatic charges due to particle hits from the GCR and SPE radiation is minimal.

Highlights

► Alternative strategy combines electrostatic and magnetostatic shielding for optimal benefits. ► SPE radiation almost eliminated; GCR transmission reduced to 30% for energies as high as 1000 MeV. ► System below the 0.4 Tesla safety threshold.

Introduction

One of the biggest obstacles to human space exploration of the solar system is the risk posed by prolonged exposure to space radiation. Space crews traveling aboard interplanetary spacecraft will be exposed to a constant flux of galactic cosmic rays (GCR), as well as intense fluxes of charged particles during solar particle events (SPEs). While the particle flux for SPE falls off steeply with increasing energy and is typically below 100 MeV per nucleon, the GCR spectrum peaks around 800 MeV per nucleon before falling off (Yao et al., 2006). Furthermore, it is generally agreed that particles with energies of 1–4 GeV per nucleon are the most damaging to humans (Schimmerling and Cucinotta, 2006). Naturally as a result, a strong focus on the safety of the missions and the crew to guard against space radiation for long duration missions has begun to emerge as a critical component of planning and design. Passive techniques essentially entail using solid material to create a shield which prevents particles from penetrating a given region by attenuating their harmful effects. Based on such conventional radiation shielding strategies, an interplanetary spacecraft would require substantial shielding of 50 g/cm2 of aluminum by one estimate if the 50 mSv limit for astronaut exposure was not to be exceeded (Wilson et al., 2001). In this connection though, more extensive requirements and analyses for health and safety standards have also been published (NCRP No. 142, 2002). Though other materials (including carbon nanotube-based shielding) might reduce the weight somewhat, these passive shielding strategies amount to adding ‘‘dead mass’’ to a spacecraft which is not an economically viable solution (Tripathi et al., 2001). Furthermore, collisions of the incoming ionized particles with the passive material lead to secondary radiation, thereby introducing an additional detrimental aspect from the standpoint of health and safety.

Active shielding, on the other hand, eliminates the need for massive shields and relies on using electric (Townsend, 1984, Sussingham et al., 1999, Smith et al., 2006), or magnetic fields (Adams et al., 2005, Cocks, 1991, Rossi et al., 2004, Spillantini, 2010) to deflect particles from a region surrounding the spacecraft. However, a different set of issues both technical and practical can potentially arise for these shielding strategies. The most serious is the safety concern associated with the presence of exceptionally large voltages (>1 kilo-Volts) or large magnetic fields (>1 Tesla) that would be required for shielding, in close proximity to the spacecraft occupants (e.g., Townsend, 2000). For example, the recommended exposure limits against magnetic fields for longtime exposures has been set to 0.4 T for the general public (ICNIRP Guidelines, 2009). Magnetic fields can set up damaging currents within blood vessels, as preliminary data seems to suggest (Kinouchi et al., 1996), and lead to other adverse effects on biological tissues (Kawakubo et al., 1999; Nittby et al., 2008). Very simply, blood contains hemoglobin which has iron (Fe). Since iron is a ferromagnetic substance, it can be affected by strong external magnetic fields. Hence, any attempt at using magnetic fields for active shielding must necessarily reduce the magnetic field intensities down to safe levels.

In this context of revolutionary technology for space radiation shielding, detailed analysis was recently presented for a new configuration of electrostatic active shielding (Tripathi et al., 2008). Electrostatic shielding based on charged spheres has been discussed previously (e.g., Smith et al., 2006). Numerical techniques for evaluating the effectiveness of such spherical geometries have also been reported (e.g., Metzger and Lane, 2009). The new electrostatic configuration, based on two sets of negatively (outer) and positively (inner) spheres, still led to transmission levels of about 80% at high energies in excess of 1800 MeV which are typical of GCR spectra. Additionally, the need to lower the requisite electrostatic potential for the active shielding for a variety of reasons, and to improve upon such active shielding strategies even further, remains. For example, by reducing the electrostatic potential, the energy requirement for powering up the system could be lowered. For a similar reason, any power expended in re-charging the electrostatic structure due to ionized particle strikes would be reduced if the electrostatic potential were smaller.

Though the magnetic and electrostatic shielding approaches appear to have advantages over the traditional passive shielding, the technological feasibility of achieving an even more optimized result in terms of alternate configurations needs to be investigated. The use of a single technology alone (whether electrostatic or magnetostatic) could have some serious drawbacks. For example, shields which rely strictly on static magnetic fields to deflect charged particles via the Lorentz force, generally utilize current carrying wires (or coils) that are located at large distances from the spacecraft. With these configurations, though the amount of electrical current needed to sustain a given magnetic dipole moment decreases with the coil size, the shielding capacity is also reduced significantly to the extent that almost no shielding occurs in a region near the center of the coil (Shepherd and Kress, 2007). On the other hand, if the coils were to be located in close proximity to the spacecraft living for better shielding, then the magnetic field-strengths necessary for protection against GCR particles would be well above 10 T (Shepherd and Shepherd, 2009).

In the best case scenario, a combination of the electrostatic and magnetostatic shielding might present a far superior alternative from the standpoint of radiation protection. A coupled, dual-approach could conceivable lower the field requirements, and thus be a safer alternative. The lower field intensities would have the added advantage of reduced power requirements. Here therefore, we probe one such hybrid configuration that uses twelve electrostatic spheres (similar to a configuration proposed by Tripathi et al., 2008), in concert with a current-carrying superconducting ring for a superimposed magnetic field. The geometry is shown in Fig. 1, and consists of six outer spheres held at a negative potential (−Vneg), six inner spheres held at a positive potential (Vpos), and superconducting ring (carrying a loop current I) for providing the magnetic field. The innermost blue sphere denotes the volume to be shielded. The six outer negatively charged spheres are designed to play a role in repelling the free electrons from the solar wind (Townsend, 1984). Without such protection against the negative charge, three potential problems could arise: (a) electrons accelerated due to the positive potential would become dangerous to the astronauts and pose substantial health risks, (b) the acceleration would lead to excessive Bremsstrahlung levels, and (c) the electron current collected by the shielding elements (e.g., the toroidal rings) held at positive potential would annihilate the charge and lead to enhanced power requirements to maintain an effective electrostatic shield.

For completeness, it may be mentioned that our goal here is simply to make a case, with quantitative analyses, for a hybrid structure which would have advantages over either electrostatic or magnetic shielding alone. Details such as the cross-section of a current-carrying ring could be optimized or further designed to suitably diminish the magnetic strengths. Besides, other structures might also need to be evaluated eventually from the standpoint of reducing the overall mass.

In this contribution, the effectiveness of the dual electrostatic–magnetostatic shielding configuration will be studied through numerical simulations. In particular, the energy-dependent transmission probability of SPE and GCR ionized particle penetration into an inner spherical volume (denoted by the blue region in Fig. 1) will be evaluated. A Monte Carlo simulation will be used to mimic the trajectories of incoming ions, with characteristics corresponding to SPE and GCR environments taken from the literature. The results for this dual configuration will be compared against an earlier all-electrostatic active-shielding approach. Furthermore, the power requirements for replenishing the electrostatic charges on the twelve-sphere shielding configuration due to particle hits from the GCR and SPE radiation will be computed.

Section snippets

Simulation method

For a given potential configuration, the influence that the electrostatic fields have on incident charged particles via the collective Coulomb forces can lead to regions of space within which particles below some energy are unable to enter. These “forbidden” regions of space are said to be shielded from the incoming particles. In the simplest configuration of Fig. 1, there are six charged spheres, each at potential Vj and having an associated charge Qj on the surface. The equation of motion of

Results and discussion

Simulation results were obtained for the configuration of Fig. 1 that included twelve charged spheres and one current-carrying ring. The six outer negatively charged spheres were taken to be at a −100 MV potential, while the six inner positively charged spheres were each set at 100 MV. These voltages, incidentally, are much lower than the values of +300 MV chosen for the all-sphere, electrostatic scheme by Tripathi et al. The lower voltage values reduce the energy requirement, and so as such

Summarizing conclusion

We conducted detailed numerical studies to evaluate the potential for a hybrid system for active shielding against space radiation. Most previous approaches have been based on a single concept alone, such as the magnetic, electrostatic or plasma shielding. However, all of the conventional approaches (including passive shielding) have potential problems. The hybrid configuration included a set of electrostatic spheres as was proposed recently, and combined a simple current-carrying ring to

Acknowledgements

This work was supported in part by NASA Innovative Advanced Concept (NIAC) Program. The team at Old Dominion University (RPJ and HQ) acknowledges the NASA Langley Research Center for partial support through grant No. NNX11AG71G. Useful discussions with Prof. G. Flandro’s group (Univ. Tennessee Space Institute) are also greatly appreciated.

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