Physics Highlights, 2014
Confinement of unmagnetized plasmas in MPDX
April 15, 2014
The scaling of (a) electron temperature, (b) density, and (c) radiated power measured while varying the input power from 4 to 135 kW into 6.5 × 10−5 Torr helium in MPDX. The data are used to predict the radiated and equilibrium power required using a confinement a model, which is compared to the input power (d) by the LaB6 cathodes.
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posted by M.D. Nornberg

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.

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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).

Stirring Plasmas
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
Dynamo activity as a function of latitude and time.
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posted by F. Cattaneo

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.

Large Scale Fast Dynamos
March 13, 2014
posted by Steve Tobias and Fausto Cattaneo

Nature, Volume 497, Issue 7450, pp. 463-465 (2013)