Physics Highlights, 2012
Distribution of plasmoids in High-Lundquist-Number Magnetic Reconnection
November 1, 2012
We have carried out statistical studies of the plasmoid distribution with respect to the magnetic flux content of plasmoids, with high-Lindquist-number simulations up to S =10^7. We find that over an extended range of J , the plasmoid distribution function f(J) follows a power law f (J) ~ J−1 . We further develop a kinetic model of plasmoid dynamics that reproduces the observed distribution. Figure 5 shows plasmoid distributions from direct numerical simulations and our kinetic model.
High velocity turbulent MHD wind tunnel on SSX
September 16, 2012
Simulation of Dynamo and Flow Generation in the Reversed Field Pinch
July 11, 2012
Our two-fluid simulation-based study of magnetic tearing and relaxation in pinch profiles has produced three new theoretical results. The first concerns the influence of finite gyroradius effects from warm ions, where stresses from the variation in the magnitude of magnetic field and from magnetic curvature in the poloidal direction lead to drifting and partial stabilization of linear tearing modes. The net effect is analogous to previous drift- tearing results, but others have considered configurations where drifts are from pressure gradients. The importance of ''grad-B'' and poloidal curvature drifts had not been appreciated for pinches. The second finding is that the ion gyroviscous stresses are also important for large, nonlinearly saturated magnetic islands. They are not diminished by nonlinear transport effects, unlike pressure-based drift effects, and the residual forces are balanced by net Lorentz forces that maintain a Hall dynamo effect. Our third result concerns warm-ion effects when multiple tearing fluctuations are active. The magnetic relaxation that results from the Hall dynamo effect from net Lorentz forces is accompanied by changes in the plasma flow profile, and the magnitude and direction of the computational results are consistent with measurements in the Madison Symmetric Torus RFP (Figure). These results confirm our understanding of the physics, and help to validate the NIMROD code. [This work is leveraged by support from U.S. Dept. of Energy grant DE-FG02-06ER54840 for theoretical research in fusion energy science.]
Stirring Unmagnetized Plasma
May 15, 2012
This past year, CMSO researchers experimentally demonstrated a new concept for spinning unmagnetized plasma, marking an important first step towards laboratory studies of a wide variety of phenomenon in plasma astrophysics. The ability to produce a hot, fast flowing, magnetic field-free plasma (in contrast to highly magnetized plasmas used for fusion energy research) may help us better understand the dynamo, a process thought to be responsible for the creation of magnetic fields in stars in galaxies. Flows can be adjusted in the experiment to mimic those of accretion disks, making it possible to study, for the first time, the magneto-rotational instability in a plasma, a mechanism of interest for its roles in providing the fuel for high energy emission from compact objects, and aiding in the formation of stars and planets.
In the experiment, plasma is confined by a cylindrical “bucket” assembly of permanent magnets, arranged in rings of alternating polarity, to form an axisymmetric cusp magnetic field. The field is localized to the boundaries, leaving a large, unmagnetized plasma in the bulk. The plasma is stirred using JxB torques, where current is driven by electrostatically biased electrodes in the magnetized edge region. 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 can be adjusted by simply increasing the bias voltage of the electrodes, and the addition of electrodes at the inner boundary will allow studies of shear flow profiles, including the Keplerian-like flows in accretion disks, where v_φ /sqrt(r). Flows as high as 6 km/s have been observed in subsequent experiments. See C. Collins et al. Phys. Rev. Lett. 108, 115001 (2012).
Effect of Magnetic Shear on Hall – Mediated Magnetic Reconnection
March 13, 2012
As shown in this figure, we observed that the addition of guide field substantially reduces the reconnection rate, and we confirm that the Hall currents in the reconnection plane determine the reconnection electric field (the reconnection rate) over a wide range of applied guide field strengths. Also we have measured quantitative dependence of the reconnection characteristics and the reconnection rate on guide field by systematically applying an external guide field using central conductor coil which creates controlled guide field. Our results will be quantitatively compared with 2-D numerical simulations in the future.