Physics Highlights, 2014
Confinement of unmagnetized plasmas in MPDX
April 15, 2014
Initial measurements of the temperature and density in MPDX establish a power-balance model for the cusp-confinement strategy employed to create hot, unmagnetized plasmas for plasma astrophysics research. The model provides estimates of the plasma parameters at the design input power of 300 kW and shows the device's capability of generating the conditions needed for a magnetohydrodynamic dynamo.
Investigation of the RMI in Magnetized Plasmas
March 21, 2014
posted by S. Heinz
Shock propagation through inhomogeneous media is a common astrophysical scenario. When the interface between different fluid phases is misaligned with the shock normal, the passage of the shock will lead to the generation of vorticity through the so-called Richtmyer-Meshkov instability (RMI), and, in the case of contained volume of roughly spherical shape, to the creation of a vortex ring. This is particularly relevant in galaxy clusters, where shocks driven by cluster mergers and active galactic nuclei pass over pockets of relativistic plasma that is injected into the intra-cluster gas by jets from growing black holes. Evidence for the operation of this effect can be found in radio relics (synchrotron emitting structures in the vicinity of cluster merger shocks, some of which are thought to be fossil radio plasma inflated by jets in the past, re-energized by the shock and turned into vortex rings).
March 21, 2014
posted by C. Forest
The cylindrical Plasma Couette Experiment (PCX) was quickly assembled to demonstrate that plasma can be confined and stirred. Measurements show that the azimuthal flow viscously couples momentum from the magnetized edge (where the plasma viscosity is small) into the unmagnetized bulk (where the viscosity is large) so that the bulk rotates like a solid body. Flow speeds as high as 10 km/s are attained through control of the bias voltage of the electrodes. A recent addition of electrodes at the inner cylinder allows studies of sheared-flow profiles, including quasi-Keplerian flow, for studies of unstable Couette flow and MRI.
Shear Driven Dynamo Waves
March 21, 2014
Since dynamo waves were introduced by E. Parker over 50 years ago, they have provided a paradigm for the theoretical modeling of cyclic activity in late type stars. However, although dynamo waves emerge from so-called mean field theories, the construction of an explicit system with dynamo wave solutions at high magnetic Reynolds numbers (Rm) has remained elusive. In this limit, small-scale eddies can amplify the field much more rapidly that the large-scale ones, and thus the averages, i.e. the mean field behavior is swamped by small-scale fluctuations. In a typical system, mean field behavior is observed at small to moderate values of Rm, but then it is lost as Rm is further increased. We have successfully constructed a system consisting of a large-scale shear and small-maximally helical flows that convincingly display mean-field behavior in the form of propagating dynamo waves even at very high values of Rm. The breakthrough came from a combination of exploiting a symmetry of the he lical flows that can be used to reduce the problem from three to two dimensions, and the use of large-scale super-computer resources to explore the high Rm regime.