THE PLASMA CORE SHIELD
Cosmic ray shielding is needed for all human habitats in space. The obvious solution is to use mass as shielding, but mass is expensive. Thus if a different means of radiation protection is possible and is compatible with the other requirements of a productive habitat, it should be used. Such a possibility is offered by the class of devices called "plasma radiation shields" (ref. 31). However, these devices are speculative.
A plasma core shield is a variant of the plasma radiation shield discussed in reference 31. Figure 4-20 shows a toroidal habitat with an "electron well" at the hub. Inside this well about 10^3 C of electrons spiral along lines of magnetic force, and hold the metallic habitat at a positive potential of 15 billion volts. The enormous electrostatic potential repels the protons and other cosmic ray nuclei from the habitat, and cuts off the cosmic ray spectrum for energies below 7.5 GeV/nucleon (15 GeV for protons). With this cutoff the net radiation dose, including secondary production, is below the acceptable dose of 0.5 rem/yr.
The critical advantage of the plasma core shield over earlier plasma shields is that the fringing fields at the lips of the electron well keep the electrons electrostatically confined to the well's interior. Thus there are no electrons near the exterior surfaces of the habitat. This feature enormously simplifies construction, operations, and even theoretical analysis (e.g., the electron plasma in this device is cylindrically symmetric instead of toroidal). In essence this device is a "bolt-on" shield, since any metallic structure in electrostatic contact with the electron well is protected - provided it stays well within the last magnetic flux line which passes through the electron well but does not touch the well's metal sides (any line that does touch is "shorted out").
The shield is energized by operating a 10 GeV Electron accelerator to shoot high energy electrons away from the habitat. Electrons form in the well when electrons from the solar plasma, attracted to the ever more positive habitat, are drawn along magnetic lines into the well. The main energy term in the system's energy budget is electrostatic energy, and this may exceed 10^13 J of energy for a habitat sized like the Stanford torus (this energy is equal to 100 MW of power stored up over 1 day). This much energy could easily be transformed into penetrating radiations should a subsystem fail - for example, the magnetic cryogenic system. A safe procedure for dumping 10^13 J of energy in a small fraction of a second is essential if the plasma core shield is to be usable.
One procedure is to accelerate positive ions away from the habitat. The electron cloud charge of 10^3 C is only about 10 mm of of particles, thus 1 percent of a mole of hydrogen ionized outside the metal structure would be enough to neutralize the habitat once the habitat's electric field had accelerated the ions away. In effect, the "charge" account is balanced by absorbing the electrons contributed by the ions - now receding from the habitat at great speed. Of course the electrons in the well are affected by this rearrangement. As the well's fringing electric fields die away the well electrons repel themselves along the lines of magnetic flux - arranged not to touch the space habitat. Thus, in perhaps a millisecond, 10^13 J and 10^3 C of electrons are safely neutralized. Obviously some more work should be done to verify this possibility.
Because a practical shield must remain in operation essentially 100 percent of the time, it must be possible to gain entrance or exit from the habitat at will - without turning off the shield. Since there are essentially no electrons external to the well this is not a difficult feat. It is only necessary to achieve varying levels of charge on objects being transferred from the habitat to the unshielded zone and back again. A device called a "shuttle shell" does this quite easily.
The shuttle shell is a Faraday cage equipped with electron/ion guns and a thruster unit. As the shuttle shell nears the habitat its electron gun bleeds off enough electrons (which go into the well) to equalize the potential between its cargo and the habitat. In reversing the operation the shuttle shell emits positive ions (which head for infinity) to neutralize its cargo. A subtlety of the shuttle shell's operation arises from the fact that like charges repel. Thus, a highly positive shuttle shell approaching a highly positive habitat feels a stiff "electric wind." To avoid excessive thrust requirements two shuttle shells might be used connected on either side of the docking port by cables which are winched in to draw the two shells to the dock, rather like cable cars.
Because the essential dynamic component of the plasma core shield is an electron plasma, plasma instabilities are to be expected. Experiments have shown that these can probably be controlled by varying the electron density as a function of radius. The real source of likely problems is the detailed systems engineering necessary to wed this device to a functioning habitat. Until extensive work is done to study all these ramifications the plasma core shield cannot be claimed as a practical solution to the radiation problem in space.
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