Michael James Burin

Hydrodynamic turbulence cannot transport angular momentum effectively in astrophysical disks

The most efficient energy sources known in the Universe are accretion disks. Those around black holes convert 5–40 per cent of rest-mass energy to radiation. Like water circling a drain, inflowing mass must lose angular momentum, presumably by vigorous turbulence in disks, which are essentially inviscid. The origin of the turbulence is unclear. Hot disks of electrically conducting plasma can become turbulent by way of the linear magnetorotational instability. Cool disks, such as the planet-forming disks of protostars, may be too poorly ionized for the magnetorotational instability to occur, and therefore essentially unmagnetized and linearly stable. Nonlinear hydrodynamic instability often occurs in linearly stable flows (for example, pipe flows) at sufficiently large Reynolds numbers. Although planet-forming disks have extreme Reynolds numbers, keplerian rotation enhances their linear hydrodynamic stability, so the question of whether they can be turbulent and thereby transport angular momentum effectively is controversial. Here we report a laboratory experiment, demonstrating that non-magnetic quasi-keplerian flows at Reynolds numbers up to millions are essentially steady. Scaled to accretion disks, rates of angular momentum transport lie far below astrophysical requirements. By ruling out purely hydrodynamic turbulence, our results indirectly support the magnetorotational instability as the likely cause of turbulence, even in cool disks.

On the transition to drift turbulence in a magnetized plasma column

Experimental results from a magnetized argon plasma column demonstrate a controlled transition to a turbulent state as the magnetic field (B) strength is increased. At lower B there is an onset of fluctuations in density and potential. These are shown to be due to drift waves that have been modified by flow shear. As B is increased the character of the fluctuations undergoes several changes. These changes include a general decrease of coherence, an increase in the phase lag (between density and potential), and a straightening of the observed dispersion relation. Concomitantly, the intensifying and broadening fluctuation spectra lead to significant cross-field radial particle transport. Other nonlinear dynamical activity is inferred during the transition, e.g., three-wave interactions, the formation of localized structures (that do not significantly contribute to the net particle transport), and energy transfer to the largest available scales.

On the nonlinear turbulent dynamics of shear-flow decorrelation and zonal flow generation

Sheared flows, thought to be generated by turbulence in magnetized fusion plasmas, are predicted to mediate the transport of mass, momentum, and heat across the shear flow region. In this paper we show that an examination of three-wave coupling processes using the bispectrum and bicoherence of turbulent fields provides an experimentally accessible test of the turbulence-generated shear flow hypothesis. Results from the Continuous Current Tokamak (CCT), Princeton Beta Experiment–Modified (PBX–M), and DIII–D tokamaks indicate that the relative strength of three-wave coupling increases during low-mode to high-mode (L–H) transitions and that this increase is localized to the region of strong flow and strong flow shear. These results appear to be qualitatively consistent with the turbulence-generated shear flow hypothesis.

Radially sheared azimuthal flows and turbulent transport in a cylindrical plasma

A radially sheared azimuthal flow is observed in a cylindrical helicon plasma device without any apparent external sources of angular momentum input. Broadband fluctuations combined with a chain of coherent structures are observed, turbulent particle transport across the shear layer is inhibited, and energy appears to be transferred from linearly unstable intermediate wave numbers into both larger and smaller spatial scales that are linearly stable. The shape of the radial plasma potential profile associated with the azimuthal flow is in agreement with published theory, and the flow magnitude is consistent with estimates of the turbulent Reynolds stress.

Development of a Couette–Taylor flow device with active minimization of secondary circulation

A novel Taylor-Couette experiment has been developed to produce rotating shear flows for the study of hydrodynamic and magnetohydrodynamic instabilities which are believed to drive angular momentum transport in astrophysical accretion disks. High speed, concentric, corotating cylinders generate the flow where the height of the cylinders is twice the radial gap width. Ekman pumping is controlled and minimized by splitting the vertical boundaries into pairs of nested, differentially rotating rings. The end rings and cylinders comprise four independently driven rotating components which provide flexibility in developing flow profiles. The working fluids of the experiment are water, a water-glycerol mix, or a liquid gallium alloy. The mechanical complexity of the apparatus and large dynamic pressures generated by high speed operation with the gallium alloy presented unique challenges. The mechanical implementation of the experiment and some representative results obtained with laser Doppler velocimetry in water are discussed.

Bispectral analysis of low- to high-confinement mode transitions in the National Spherical Torus Experiment

This paper will present an experimental study of the temporal and spatial characteristics of the autobicoherence calculated from light amplitude fluctuations measured in the edge plasma of the National Spherical Torus Experiment (NSTX) [M. Ono et al., Plasma Phys. Controlled Fusion 45, A335 (2003)] using data from the gas puff imaging (GPI) diagnostic [R. J. Maqueda et al., Rev. Sci. Instrum. 74, 2020 (2003); S. J. Zweben et al., Nucl. Fusion 44, 134 (2004)] obtained during a series of thirteen shots in which the NSTX plasma underwent spontaneous low- to high-confinement mode (L-H) transitions. The autobicoherence calculated from the available GPI chord signals in the region near the magnetic separatrix and just above the outer midplane indicates that there is no significant increase, i.e., outside the rms error, in the amount of nonlinear coupling between low frequency fluctuations and high frequency fluctuations during the 10ms before the transition. Limitations of bicoherence analysis are discussed.

Evidence for Reynolds-stress driven shear flows using bispectral analysis: theory and experiment

Spontaneous shear flow generation in magnetized fusion plasmas is thought to occur by an interaction of the turbulent Reynolds stress with the shear flow. This interaction can be viewed as a transfer of turbulent energy to the shear flow scales via a three-wave coupling process, which suggests that investigations of bispectral quantities may be of interest. In this paper, we discuss the theory of mean flow generation when described via the bispectrum. Simple theoretical analysis is used to demonstrate the connection between the bispectrum and the nonlinear amplification mechanism for zonal shear flows. We also discuss results from analysis of edge fluctuations from DIII-D during an L-H transition. These results indicate a transient rise and fall of three-wave coupling during the transition, which is spatially localized to inside the separatrix. Future efforts to determine the direction and rate of turbulent energy transfer during shear-flow formation will also be discussed.

Radially sheared azimuthal flows and turbulent transport in a cylindrical helicon plasma device

A radially sheared azimuthal flow is observed in a cylindrical helicon plasma device. The shear flow is roughly azimuthally symmetric and contains both time-stationary and slowly varying components. The turbulent radial particle flux is found to peak near the density gradient maximum and vanishes at the shear layer location. The shape of the radial plasma potential profile associated with the azimuthal E × B flow is predicted accurately by theory. The existence of the mean shear flow in a plasma with finite flow damping from ion–neutral collisions and no external momentum input implies the existence of radial angular momentum transport from the turbulent Reynolds-stress.

Magnetorotational Instability in a Short Couette Flow of Liquid Gallium

A concise review is given of an experimental project to study magnetorotational instability (MRI) in a short Couette geometry using liquid gallium. Motivated by the astrophysical importance and lack of direct observation of MRI in nature and in the laboratory, a theoretical stability analysis was performed to predict the required experimental parameters. Despite the long‐wavelength nature of MRI, local analysis agrees excellently with global eigenmode calculations when periodic boundary conditions are used in the axial direction. To explore the effects of rigidly rotating vertical boundaries (endcaps), a prototype water experiment was conducted using dimensions and rotation rates favored by the above analysis. Significant deviations from the expected Couette flow profiles were found. The cause of the discrepancy was investigated by nonlinear hydrodynamic simulations using realistic boundary conditions. It was found that Ekman circulation driven by the endcaps transports angular momentum and qualitatively ...

Some Daytime Activities in Solar Astronomy

This centurys transits of Venus (2004, 2012) captured significant public attention, reminding us that the wonders of astronomy need not be confined to the night. And while nighttime telescope viewing gatherings (a.k.a. “star parties”) are perennially popular, astronomy classes are typically held in the daytime. The logistics of coordinating students outside of class can often be problematic, leading to dark-sky activities that are relegated to extra credit for only those who can attend.