Physics Highlights, 2011
Control of Turbulent Diffusion in a Liquid Metal Dynamo
December 2, 2011
Comparison of the power in magnetic modes in the Madison Dynamo Experiment without baffles (blue triangles) and with baffles (red squares). The dark red ovals show that the turbulent magnetic power is reduced by the baffles while the green ovals show that the power in the large scale field is enhanced without the baffles.
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Planetary, stellar, and probably galactic magnetic fields are sustained by dynamos, in which energy is transferred from the flows in the system – usually rotation, convection, and turbulence – to the magnetic field. But theory predicts that turbulence can also destroy the field. Destruction of a large scale field by small scale turbulence is known as the β effect. We have deployed small scale equatorial baffles in the liquid sodium Madison Dynamo Experiment. The baffles selectively damp the largest turbulent eddies. This results in reduced energy in the small scale magnetic field and increased energy in the large scale field, as predicted by dynamo theory.

MHD dissipation range turbulence
October 9, 2011
The magnetic turbulence spectrum measured in the Madison Symmetric Torus, as a function of toroidal wavenumber k⊥ (in green), in the direction with maximum fluctuation power. The spectrum is well fit (overlying blue curve) by a theoretical model only if both power law and exponential factors are included. The exponential decay becomes important at smaller k⊥ than expected for classical dissipation, hinting at a kinetic dissipation process.
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Measurements of magnetic turbulence in laboratory, space, and astrophysical plasmas are accessing small scales where dissipation can become dynamically important. To help interpret spectra measured at such scales, a theory for dissipation range MHD turbulence has been derived for the first time. Like hydrodynamic dissipation range turbulence the spectra consist of a product of a power law and an exponential falloff that scales with the dissipative (Kolmogorov) wavenumber. Unlike hydrodynamics, the values of exponential and power law indices depend on whether the turbulence is aligned or unaligned, and on the ratio of viscous to magnetic diffusivity, or magnetic Prandtl number. There are also multiple dissipative wavenumbers. (See P.W. Terry and V. Tangri, Phys. Plasmas 16, 082305 (2009).]

Energetic ion creation during magnetic reconnection in the RFP
August 3, 2011
The neutral particle flux escaping the MST plasma, before and after a magnetic reconnection event (of short duration). The blue curves represent the bulk thermal ions with Ti ~ 300 eV, well described as a Maxwellian (straight lines). The red curves characterize the tail ions, well described by a power-law energy dependence.
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Particle heating is an important process related to magnetic reconnection. A large part of a system’s magnetic energy can be transferred to thermal energy during the reconnection process. In the reversed field pinch, the ions are well established to be heated in this way. Past measurements of the ion temperature have focused on the bulk, thermal portion of the ion distribution. New measurements reported here using a neutral particle diagnostic reveal that there is, in addition, a significant population of ions that form an energetic tail, as shown in the figure below. Ions in the plasma undergo charge-exchange with background neutrals and are allowed to cross the magnetic field confining the plasma. The escaping neutral flux detected by the particle analyzer is energy-resolved and therefore reflects the ion distribution in the plasma. There is an excess of neutrals (i.e., ions) at high energy relative to the thermal Maxwellian bulk. This tail is enhanced during the magnetic reconnection process, and is well fit to a power-law energy dependence. Such power-law particle distributions are commonly observed in astrophysical plasmas, and represent an important clue as to the origin of the ion energization process.

Magnetic field amplification in galaxy clusters
June 19, 2011
Distribution of magnetic fieldstrengths in a turbulent galaxy cluster simulation. Strong filaments are embedded within the weaker turbulent field. The range of fieldstrengths is similar to what is observed.
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Galaxy clusters are observed to be pervaded by hot, tenuous gas, which is strongly magnetized. While some of this gas was apparently stripped from galaxies, much of it is primordial, and simple estimates show that the magnetic field is too strong to simply have been torn from galaxies with their interstellar gas – further amplification is required. We have performed state of the art cosmological magnetohydrodynamic simulations, with adaptive mesh refinement to show that magnetic fields injected by active galaxies can be amplified by intracluster medium turbulence through a small scale dynamo process. Such turbulence is generated and sustained by mergers of galaxies or groups of galaxies with the cluster.

Spiral Magnetic Instabilities: Observation of a spiral magnetic instability in the Princeton MRI experiment
April 27, 2011
The azimuthal velocity in a spiral mode spontaneously excited in the MRI experiment, relative to the angle-averaged mean. Although the mean flow varies with radius, the spiral shear is steady.
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The MRI experiment, a cylindrical channel of magnetized, liquid gallium confined between two rotating cylinders, was designed to study instabilities relevant to angular momentum transport and magnetic field amplification in astrophysical disks. We discovered that when a sufficiently strong magnetic field is imposed on a hydrodynamically stable flow, coherent oscillations emerge in both azimuthal and radial flow components. We detected this through measurements by Ultrasonic Doppler Velocimetry (UDV) mounted at various heights and azimuthal angles. Initially, the oscillations exhibit higher azimuthal mode numbers, but later evolve into an m=1 mode, appearing as a spiral structure in the azimuthal velocity as shown in Figure.

The dependence on the imposed magnetic field strength shows similarities with the predictions for MRI (Magnetorotational Instability), but the instability persists at much lower Reynolds numbers than the predictions. By comparing with MHD simulations in a similar geometry at similar parameters, it has been suggested that the observed spiral instability is a hydrodynamic instability either of a sheared azimuthal flow induced by an axial magnetic field over the speed gap between two end rings, or of a poloidal circulation (Ekman circulation) enhanced by the axial field. In either case, however, the instability is qualitatively different from the MRI. On the other hand, the discovery of this new instability has directly relevance to geophysics since both strong axial field and similar boundary conditions exist there.

Stellar Dynamo Simulations
February 11, 2011
There's a Little Black Spot on the Sun Today
Researchers employ Kraken to study solar magnetism, stellar evolution.

To the naked eye, the Sun comes and goes each day exactly as it appeared the day before - round, yellow-orange, and bright, perhaps mankind’s most reassuring and longest-running companion.

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