I am an experimental plasma physicist. My interest in plasma physics spans a broad area, but I am most interested in the basic questions of how a plasma may be magnetically confined, what are the physical mechanisms which drive particle, momentum and energy transport and what is the non-linear evolution of a magnetically confined plasma with transport.
My current research focuses on plasmas confined within a magnetic dipole. The Levitated Dipole Experiment (LDX) offered me a unique opportunity to study plasma confinement in an unorthodox "magnetic bottle". I joined the LDX group as the Experimental Head in 1998 and I have been involved in all levels of design of the facility and diagnostics as well as in developing an increased understanding of the physics of plasma confinement and stability in a dipole field.
The dipole is an ubiquitous natural example of high-b magnetic plasma confinement. In planetary magnetospheres, spacecraft have reported evidence of stable equilibria with local plasma b significantly greater than unity. Plasma b, the ratio of plasma kinetic energy to magnetic energy (b &Mac186; 2m0p/B2), is a gross measure of how efficient a magnetic configuration confines plasma. For comparison, many laboratory plasma confinement devices, including the tokamak, are limited by instabilities at b < 10%.
LDX, a collaboration between Columbia University and MIT, is currently nearing completion of its construction phase at the MIT Plasma Science and Fusion Center. LDX was designed to provide an experimental test of our physical understanding of how equilibrium and stability is maintained in a dipole configuration at high-b, and to evalutate the configuration as a possible fusion device. In order to achieve its goal b ~ 50% as well as to study plasma transport, the dipole field in LDX is provided by a levitated superconducting circular coil. By eliminating conductor leads, magnetic pole faces and other field line crossing supports, we eliminate the main loss of plasma particles and energy in magnetospheres: the loss of plasma along field lines to the poles which give rise to planetary aurora. (Coincidently, the study of aurora plasma was a focus of my postdoctoral research.)
Plasma stability is maintained by a unique mechanism in the magnetic dipole. The most fundamental magnetohydrodynamic (MHD) instability, the interchange mode, which is closely analogous to the Raleigh-Taylor instability in fluid dynamics, occurs when the magnetic field curvature is in the same direction as the plasma pressure gradient. Most toroidal confinement devices rely on rotational transform, or helically nested magnetic surfaces, to provide linkages between good and bad curvature regions. It is the loss of of the average good curvature equilibrium that imposes a limit on maximum achievable b in these devices.
Dipole plasmas, whose outer regions have everywhere bad curvature, rely instead on plasma compressibility for stability. To understand the effect, consider two tubes of unit magnetic flux within a dipole plasma that interchange. In bad curvature, the inner flux tube has higher plasma pressure and moves outward to larger radius and gains volume. In doing so, the contained plasma expands adiabatically and its pressure is reduced. The opposite is true for the outer flux tube moving inward. We may choose an initial pressure profile that then remains constant, allowing no free energy to drive the interchange instability. The marginally stable profile is given by the adiabatic condition, pVg = constant, and indicates a flat entropy profile within the plasma. Understanding compressibility stabilized interchange and related ballooning mode instabilities is a key goal of the LDX experiment and also a current area of magnetospheric space plasma research.
When the condition for MHD stability is met and similar interchange stationary density and temperature profiles are chosen, microscopic scale turbulent instabilities, which typically drive transport in plasma confinement devices, are also stabilized. However, without rotational transform to short out electric fields on magnetic surfaces, large scale convective cells are possible in dipoles and other closed field line systems. Interestingly, for constant entropy profiles convective cells transport particles without necessarily convecting energy. Convective cells may be formed by external non-axisymmetric heating and fuelling sources, such as the case for the large magnetospheric convection patterns caused by the influence of the solar wind, or through the non-linear evolution of interchange mode instabilities. As a consequence transport in a convective cell is generally not proportional to the local instability drive, an assumption commonly made in plasma physics. Such "non-local" transport and non-linear equilibria evolution has received little attention until relatively recently. (An example of non-local transport that has received recent attention are zonal flows in tokamaks, or areas of rapid transport separated by transport barriers, similar to the gas flow evident on surface of Jupiter.) Because of the capability to explore possible stabilization of microturbulence, formation of large scale convective cells, and non-local transport phenomena, LDX will provide a unique testbed for exploring the basic physics of plasma transport.
The LDX experiment has inspired substantial interest in dipole physics from groups at UCLA, University of Maryland, UC San Diego, University of Texas, Austin, University of Washington, University of Tokyo, and the Kurchatov Institute (Moscow). With excellent diagnostic capabilities and the ability to modify the plasma equilibrium and profiles, LDX is an exceptional facility for the study of dipole confinement and other questions of magnetospheric physics.
| Darren T. Garnier | dg276@columbia.edu |