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The dipole magnetic field is the simplest and most common magnetic field configuration in the universe. It is the magnetic far-field of a single, circular current loop, and it represents the dominate structure of the middle magnetospheres of magnetized planets and neutron stars. The use of a dipole magnetic field generated by a levitated ring to confine a hot plasma for fusion power generation was first considered by Akira Hasegawa after participating in the Voyager 2 encounter with Uranus [1]. Hasegawa recognized that the inward diffusion and adiabatic heating that accompanied strong magnetic and electric fluctuations in planetary magnetospheres represented a fundamental property of strongly magnetized plasmas not yet observed in laboratory fusion experiments. For example, it is well-known that global fluctuations excited in laboratory fusion plasmas result in rapid plasma and energy loss. In contrast, large-scale fluctuations induced by sudden compressions of the geomagnetic cavity (due to enhancements in solar wind pressure) or by unsteady convections occurring during magnetic substorms energize and populate the energetic electrons trapped in the Earth's magnetosphere [2]. The fluctuations induce inward particle diffusion from the magnetospheric boundary even when the central plasma density greatly exceeds the density at the edge. Hasegawa postulated that if a hot plasma having pressure profiles similar to those observed in nature could be confined by a laboratory dipole magnetic field, this plasma might also be immune to anomalous (outward) transport of plasma energy and particles.

The dipole confinement concept is based on the idea of generating pressure profiles near marginal stability for low-frequency magnetic and electrostatic fluctuations. For ideal MHD, marginal stability results when the pressure profile, p satisfies the adiabaticity condition, d(pV^g ) = 0, where V is the flux tube volume (V = §dl/B) and g = 5/3. This condition leads to dipole pressure profiles that scale with radius as r^-20/3, similar to energetic particle pressure profiles observed in the Earth's magnetosphere. Since the magnetic field of a dipole is poloidal, there is no drift off of flux surfaces and therefore no "neoclassical" degradation of confinement as seen in a tokamak. It has been pointed-out that a plasma that satisfies the MHD interchange stability requirement may be intrinsically stable to drift frequency modes. Stability of low frequency modes can be evaluated using kinetic theory and a Nyquist analysis permits an evaluation of stability boundaries with a minimum of simplifying assumptions. Using kinetic theory we have shown that when the interchange stability requirement (for small Larmor radius) becomes w_*p > w_d with w_*p the diamagnetic drift frequency and w_d the curvature drift frequency and this result is consistent with MHD [3]. This property implies that the pressure scale length exceed the radius of curvature, which is a physical property that distinguishes a dipole confined plasma from other approaches to magnetic fusion plasmas. Additionally, when the interchange stability criterion is satisfied, it can be shown that localized collisionless trapped particle modes and dissipative trapped ion modes become stable. Low frequency modes that are driven by parallel dynamics (i.e.) the universal instability) also tends to be stable due to the requirement that the parallel wavelength of the mode fit on the closed field lines. Recent theoretical work on anomalous inward diffusion (towards the ring) due to high frequency, drift-cyclotron instability supports the view that both stability and confinement can be extremely good in a levitated dipole [4].

By levitating the dipole magnet end losses can be eliminated and conceptual reactor studies [5,6] supported the possibility of a dipole based fusion power source that utilizes advanced fuels. The ignition of an advanced fuel burning fusion reactor requires high beta and good energy confinement. Additionally advanced fuels require steady state and efficient ash removal. A levitated dipole may provide uniquely good properties in all of these areas. The chief drawback of the dipole approach is the need for a levitated superconducting ring internal to the plasma and this provides a challenge to the engineering of the device. A fusion reactor based on a levitated dipole has been explored in several studies [5-7]. Recent advances in high temperature superconductors coupled with an innovative design concept of Dawson [8] on the maintenance of an internal superconducting ring in the vicinity of a fusion plasma lead us to believe that this issue is technologically solvable.

The dipole confinement approach can be tested in a relatively modest experiment which profits from the development of the technology of superconductors, gyrotrons and pellet injectors. A concept exploration experiment is presently being developed jointly by Columbia University and MIT. This experiment is LDX.

1. A. Hasegawa, Comm Pl Phys & Cont Fus, 1, (1987) 147.
2. A. L. Vampola, 1. Symp. on Natural and Manmade Rad. in Space p. 538, (Wash., D.C., 1972).
3. J. Kesner, Phys. Plasmas 4 (1997) 419.
4. V. Pastukhov and A. Yu. Sokolov, Nuc. Fusion 32 (1992) 1725.
5. A. Hasegawa, L. Chen and M. Mauel, Nuclear Fus. 30, (1990) 2405.
6. A. Hasegawa, L. Chen, M. Mauel, H. Warren, S. Murakami, Fusion Technology 22 (1992) 27.
7. E. Teller, A. Glass, T.K. Fowler et al., Fusion Technology 22, (1992) 82.
8. J. Dawson, Private communication, see [7].