Momentum Transport Research Plan

The Center Activities will be concentrated on the following topics:

1. Momentum transport by stochastic magnetic fields Magnetic fields in both laboratory and space often have well-ordered components on large scales and random or stochastic components on smaller scales. This is the case in all Center experiments, is observed in MHD computation of accretion disks with MRI, and is likely the case in astrophysical jets and in the sun. As particles and/or waves travel along the real field lines, they can wander in space and lead to transport of particles, energy, and momentum.

2. Momentum transport by Maxwell Stress from current-driven instabilities Magnetic fluctuations can directly affect plasma flow by producing Maxwell Stresses. In experiment, strong tearing mode activity is presumed to be responsible for rapid changes in plasma momentum, and coupled modes can produce a nonlocal transport of momentum from one region of the plasma to another. Astrophysical jets have a magnetic geometry that is similar to the Center experiments and may be susceptible to similar tearing or kink type instabilities.

3. Momentum transport by Maxwell Stress from Magnetorotational Instability The leading candidate for momentum transport in accretion disks is the Maxwell Stress caused by the Magnetorotational Instability (MRI). Here a weak magnetic field couples with strong differential rotation giving rise to instability. A possibly important effect which has not yet been fully explored is that of a magnetized corona above and below the disk which couples to the disk plasma by magnetic field lines and may significantly alter the momentum transport by MRI. The MRI has never been observed in the laboratory but is being pursued in both plasma and liquid metal experiments.

4. Generation and relaxation of momentum as part of a two-fluid form of magnetic relaxation In single-fluid MHD, it has been shown that some systems will relax to a state of minimum magnetic energy subject to the constraint of total magnetic helicity. This idea has been a guiding principle in Center experiments, many of which undergo this type of relaxation to at least some degree. In two-fluid systems, a similar relaxation may occur with consequences for both the arrangement of the magnetic fields (or currents) and the plasma momentum. Although theory predicts that this should occur in some systems, the principle has never been confirmed in experiment and never been applied in astrophysics.

5. Momentum transport in the sun Recent observations of differential rotation in the solar interior pose new challenges to models for solar dynamics. A striking feature is the region of intense flow shear at the tachocline (the interface between the radiative and convective zones). Momentum transport in part determines the flow profile in the sun which in turn gives rise to dynamo action and produces the large-scale magnetic fields we observe throughout the solar system.


A National Science Foundation Physics Frontier Center,
established in coordination with the Department of Energy.