R. D. Kendrick

Observation of a Turbulence-Induced Large Scale Magnetic Field

An axisymmetric magnetic field is applied to a spherical, turbulent flow of liquid sodium. An induced magnetic dipole moment is measured which cannot be generated by the interaction of the axisymmetric mean flow with the applied field, indicating the presence of a turbulent electromotive force. It is shown that the induced dipole moment should vanish for any axisymmetric laminar flow. Also observed is the production of toroidal magnetic field from applied poloidal magnetic field (the omega effect). Its potential role in the production of the induced dipole is discussed.

Intermittent Magnetic Field Excitation by a Turbulent Flow of Liquid Sodium

The magnetic field measured in the Madison dynamo experiment shows intermittent periods of growth when an axial magnetic field is applied. The geometry of the intermittent field is consistent with the fastest-growing magnetic eigenmode predicted by kinematic dynamo theory using a laminar model of the mean flow. Though the eigenmodes of the mean flow are decaying, it is postulated that turbulent fluctuations of the velocity field change the flow geometry such that the eigenmode growth rate is temporarily positive. Therefore, it is expected that a characteristic of the onset of a turbulent dynamo is magnetic intermittency.

Measurements of the magnetic field induced by a turbulent flow of liquid metal

Initial results from the Madison Dynamo Experiment provide details of the inductive response of a turbulent flow of liquid sodium to an applied magnetic field. The magnetic field structure is reconstructed from both internal and external measurements. A mean toroidal magnetic field is induced by the flow when an axial field is applied, thereby demonstrating the omega effect. Poloidal magnetic flux is expelled from the fluid by the poloidal flow. Small-scale magnetic field structures are generated by turbulence in the flow. The resulting magnetic power spectrum exhibits a power-law scaling consistent with the equipartition of the magnetic field with a turbulent velocity field. The magnetic power spectrum has an apparent knee at the resistive dissipation scale. Large-scale eddies in the flow cause significant changes to the instantaneous flow profile resulting in intermittent bursts of nonaxisymmetric magnetic fields, demonstrating that the transition to a dynamo is not smooth for a turbulent flow.

The rotating wall machine: A device to study ideal and resistive magnetohydrodynamic stability under variable boundary conditions

The rotating wall machine, a basic plasma physics experimental facility, has been constructed to study the role of electromagnetic boundary conditions on current-driven ideal and resistive magnetohydrodynamic instabilities, including differentially rotating conducting walls. The device, a screw pinch magnetic geometry with line-tied ends, is described. The plasma is generated by an array of 19 plasma guns that not only produce high density plasmas but can also be independently biased to allow spatial and temporal control of the current profile. The design and mechanical performance of the rotating wall as well as diagnostic capabilities and internal probes are discussed. Measurements from typical quiescent discharges show the plasma to be high β (≤p>2μ(0)/B(z)(2)), flowing, and well collimated. Internal probe measurements show that the plasma current profile can be controlled by the plasma gun array.

On the Possibility of an Homogeneous MHD Dynamo in the Laboratory

The cause of spontaneous generation of magnetic fields in conducting bodies (such as plasmas) is a longstanding, major problem in plasma astrophysics, geophysics, and laboratory plasmas. It is observed that magnetic fields exist in the Earth, Sun and other stars (and perhaps in galaxies), that cannot be explained as surviving primordial fields, and generally believed that such magnetic fields are generated by plasma flow (or flow of liquid metal for the Earth). The question of how magnetic fields are generated by unconstrained flows of conducting fluids and plasma is referred to as the“dynamo” problem; theoretical research into dynamo mechanisms has been actively pursued for several decades. However, until quite recently our probing of the dynamo problem has been limited to analytic calculations, numerical modelling and observational studies; experimental validation (the critical test for any theory) of aspects of the theory and experimental studies of laboratory dynamos have been scarce.

Fluctuation-driven magnetic fields in the Madison Dynamo Experiment

Turbulent fluctuations in the velocity and magnetic fields of electrically conducting fluids have been experimentally shown to be capable of inducing large-scale magnetic fields. Here, simulations of the Madison Dynamo Experiment are used to qualitatively reproduce these experimental results. Due to the high magnetic Prandtl number of the simulations, Pm=0.08 vs Pm∼10−5 for liquid sodium, the simulations do not identically reproduce the fluctuation levels of the experiment’s magnetic and velocity fields. Nonetheless, the simulations reproduce the qualitative behavior of the fluctuation-induced large-scale magnetic field as a function of applied field magnitude and magnetic Reynolds number. The scaling of the induced dipole moment as a function of Reynolds number is also presented, demonstrating that the nature of the fluctuations in the simulations changes after a critical value of the Reynolds number is crossed, resulting in a change in the direction of the induced dipole moment. Experimental conditions ...

Current profile control and fluctuation reduction in MST via electrostatic current injection

Summary form only given, as follows. We have designed and are conducting a current profile control experiment aimed at reducing fluctuations and improving confinement in RFP plasmas. Profile modification will be achieved via electrostatic injection of electrons from /spl sim/30 miniature plasma sources. Each source produces a 1 kA electron beam which is dense (/spl sim/150 kA/cm/sup 2/ at 70 cm from the source), highly directional, and without large impurity content. An initial multi-injector experiment in which a total current of 5 kA was injected is completed. Results from this experiment will be discussed. One of the promising results is an apparent increase in the plasma density by 50% and associated increase in the particle confinement time. Low impurity influx from the sources is essential for scaling up the injection system. MHD computations predict a current of /spl sim/30 kA is necessary to suppress fluctuations in Madison Symmetric Torus (MST) and future experiments will employ an increasing number of injectors until this level is reached.