The last concept to discuss is the magnetic dipole, a concept that has so far received limited attention but which, based on present theoretical analyses, also shows promising potential to produce high-,3 plasmas [Hasegawa, 1987].
Astrophysical observations show that an equilibrium configuration consisting of a simple dipole field exhibit remarkable MHD stability properties (e.g., beta exceeding unity in the Jupiter magnetosphere). Interchange modes can indeed be shown to be stable if the pressure profile decreases sufficiently slowly toward the low-field region. Furthermore, if the equilibrium density and temperature gradients are sufficiently weak, as required by MHD stability, these free energy sources are incapable of driving small-scale instability, and the unwanted consequences of turbulent transport may be expected to be benign. In particular, the diamagnetic frequency tends to be smaller than the magnetic drift frequency, resulting in a strong stabilizing effect (see, e.g., [Kesner et al., 1998]).
A dipole configuration is produced by a large central coil levitated against gravity or local acceleration by a set of other coils that create a vertical field (see Figure B.22). The combined field produces a magnetic separatrix. Outside the separatrix a natural divertor configuration is formed. The presence of a magnetic separatrix can enhance MHD stability close to the separatrix and also by locally destabilizing drift waves,
Figure B.22. Levitated dipole (from [Teller et al., 1992]).
Figure B.22. Levitated dipole (from [Teller et al., 1992]).
although the latter could also be stabilized by edge-sheared flows, similar to those observed in tokamaks, in conjunction with improved confinement regimes.
Very little is known experimentally about dipole configurations. The Levitated Dipole Experiment (LDX), a facility with a superconducting ring of 0.4m radius, constructed at MIT [Kesner et al., 1998] aimed at exploring plasmas with 300 eV temperature and up to 1018 m-3 density. LDX operation began at the end of 2004.
The use of an internal coil surrounded by plasma is the major drawback to the dipole configuration since no external cooling (or power supply) can be applied. Following an early suggestion by Dawson, the assumption usually made is that radiative cooling from the ring surface balances heat input to the ring (from radiation, heat conduction, and neutrons). The power needed to cool the superconducting ring may be extracted from this heat flux by different energy conversion schemes. Note that, since the surface heat temperature is limited by structural materials (e.g., 2,700 K for tungsten), the above assumption sets a limit on the power that can reach the ring's surface, and therefore on the fusion power per unit volume.
A space propulsion application for levitated dipoles was considered in [Teller et al., 1992]. This levitated dipole scheme has a major radius of 6 m and a minor radius of 2 m. The magnetic field on the conductor is 15 T. The total fusion power (using D- 3He fuel) is 2GW, with 60% available for thrust. With a total ring mass of 1,1801, the resulting specific power is close to 1 kW/kg. Although conceptually of interest for space propulsion, such numbers are still too low. Improvements may come from optimizing the coil mass and from new materials capable of higher surface temperature and radiated power (ultra high-temperature ceramics, UHTC).
The design of the superconducting coil includes a 1 mm thick tungsten surface layer, capable of radiating 1 MW/m2 at 2,700 K, for a total radiated power of 400 MW, followed by a shield of C-C fiber composite (about 30% of the total ring mass) that reduces 90% of neutron flux (total neutron power is about 60 MW), This first shield is thermally insulated by a second shield, a steel structure containing two layers of B-H2O (with a radial width/working temperature of 0.24m/900K and 0.66m/300K, respectively) reducing neutron flux by a factor of 5,600. Only 467 W reaches the superconducting magnet working at 4.2 K. Extracting heat from these sources of power at their working temperatures and feeding it to the surface temperature (at 2,700 K) requires, ideally, about 10 MW of electric power, available by converting the 400 MW of input power to the ring. The Teller concept needs in fact additional in-depth work.
B.4 FURTHER STUDIES ON FUSION FOR SPACE APPLICATION B.4.1 Technology
A number of assumptions made in this study are based on zero-order physics awaiting further refinements, as discussed below.
The blanket (together with the magnet) can be a heavy component of the reactor core. Research performed for the SOAR conceptual design [Kulcinski et al., 1987] has pointed out that minimum mass is achieved by using LiH. On the basis of experience gained in the last ten years in design and R&D into blankets for fusion reactor applications, a detailed neutronic and thermal analysis should be made to assess the potential of this solution.
The magnet (together with the blanket) can be the heaviest component of the reactor core. Detailed designs exist for magnets to be used in tokamak reactors, although these designs have not considered the constraints arising from the low-mass requirements of space propulsion applications. A detailed design of a magnet for open magnetic field configurations should be made to benchmark the [sometimes questionable] figures found in generic fusion rocket studies, both for superconducting and actively cooled copper magnets. Use of high-temperature superconductors should be considered.
All fusion concepts that have been investigated rely on auxiliary systems for heating plasmas and on cryoplants to cool superconducting magnets. The assumptions made for the sake of illustration in generic fusion rocket studies (1,000 kg per kilowatt of heat extracted for the cryoplant and 2.5 kg per kilowatt of auxiliary power) definitely need a second look and assessment. A substantial amount of R&D has been carried out in international fusion program(s) on heating methods (neutral beam injection, ion cyclotron resonance heating, electron cyclotron resonant heating, and others). The capability of low-mass systems should be investigated together with high efficiency for power generation.
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