Convective Cell Formation in a Levitated Dipole

A plasma that is confined in a closed field line geometry such as a levitated dipole may be subject to the formation of convective cells. Using a “Drift” fluid model we show that such flow patterns can represent an appropriate equilibrium in a dipole field that contains a plasma with a non-axisymmetric pressure profile. This can provide a means of fueling and heating a dipole confinement device. MIT Report PSFC/JA-99-35 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 [1, 2]. As a laboratory approach to controlled fusion a dipole configuration would generate the magnetic field with a circular magnet internal to the plasma. To avoid losses on supports the ring would need to be superconducting and it would be magnetically levitated within the vacuum chamber. Since a large flux expansion is necessary to obtain a fusion grade plasma for a fixed edge plasma the configuration requires a small coil that is levitated within a relatively large vacuum chamber. An initial test of this concept is embodied in the Levitated Dipole Experiment (LDX) which is being built jointly by Columbia University and MIT [3]. Since electric potential tends to be constant along magnetic field lines it can vary both radially and azimuthally in a closed field line system. This can lead to the formation of convective cells in closed field line configurations that lack magnetic shear [4, 5, 6]. Thus in a dipole configuration the magnetic field may be axisymmetric but field lines can charge up leading to steady state, non-axisymmetric E × B flow patterns. The conceptual view of convective cells, presented by Dawson and Okuda [4] is that in a plasma, stable modes will be excited by thermal fluctuations which, even in the absence of equilibrium flows, will develop non-linearly, into convective cells. Convective cells were observed in closed field line experiments (for example Ref. [6]) but the effective temperature was observed to be several orders of magnitude above the thermal level. In this work we show that when the plasma heating is non-axisymmetric, convective flows tend to develop in the equilibrium. In solving the equilibrium problem it is important to include the diamagnetic flows which are of comparable magnitude to the E×B (electricfield driven) flows. (The E×B flow is the only flow included in the magnetohydrodynamic (MHD) equations). In fact convective cells are seen to form in regions where the E × B and diamagnetic flows approximately cancel. It should be pointed out that convective cells lead to particle transport but not nec-