The Ω phase of the liquid sodium α-Ω dynamo experiment at New Mexico Institute of Mining and Technology in cooperation with Los Alamos National Laboratory has demonstrated a high toroidal field B(ϕ) that is ≃8×B(r), where B(r) is the radial component of an applied poloidal magnetic field. This enhanced toroidal field is produced by the rotational shear in stable Couette flow within liquid sodium at a magnetic Reynolds number Rm≃120. Small turbulence in stable Taylor-Couette flow is caused by Ekman flow at the end walls, which causes an estimated turbulence energy fraction of (δv/v)(2)∼10(-3).
Local linear analysis shows that magneto-rotational instability can be excited in laboratory rotating plasmas with a density of 1019m−3, a temperature on the order of 10eV, and a magnetic field on the order of 100G. A laboratory plasma annulus experiment with a dimension of ∼1m, and rotation at ∼0.5 sound speed is described. Correspondingly, magnetic Reynolds number of these plasmas is ∼1000, and magnetic Prandtl number ranges from about one to a few hundred. A radial equilibrium, ρUθ2∕r=d(p+Bz2∕2μ0)∕dr=K0, with K0 being a nonzero constant, is proposed for the experimental data. Plasma rotation is observed to drive a quasisteady diamagnetic electrical current (rotational current drive) in a high-β plasma annulus. The rotational energy depends on the direction and the magnitude of the externally applied magnetic field. Radial current (Jr) is produced through biasing the center rod at a negative electric potential relative to the outer wall. Jr×Bz torque generates and sustains the plasma rotation. Rotational current drive can reverse the direction of vacuum magnetic field, satisfying a necessary condition for self-generated closed magnetic flux surfaces inside plasmas. The Hall term is found to be substantial and therefore needs to be included in the Ohm’s law for the plasmas. Azimuthal magnetic field (Bθ) is found to be comparable with the externally applied vacuum magnetic field Bz, and mainly caused by the electric current flowing in the center cylinder; thus, Bθ∝r−1. Magnetic fluctuations are anisotropic, radial-dependent, and contain many Fourier modes below the ion cyclotron frequency. Further theoretical analysis reflecting these observations is needed to interpret the magnetic fluctuations.
A Nine-Electrode Probe (NEP) has been developed for simultaneous measurement of all terms in the ideal Ohm’s law E+U×B=0 in the radial (r̂) direction in cylindrical geometry, where E is the electric field, U is the plasma flow velocity, and B is the magnetic field. The probe consists of two pairs of directional Langmuir probes (“Mach” probes) to measure the axial (ẑ) and azimuthal (θ̂) plasma flows, two pairs of floating Langmuir probes at different radial positions to measure the radial electric field, and two B-dot coils to measure the axial and azimuthal magnetic field. The measurement is performed in the Flowing Magnetized Plasma (FMP) experiment. Two flow patterns are identified in the FMP experiment by the NEP. The peak-to-peak values of radial electric field fluctuation is 1.5–4 times of the mean values. Comparisons of ∣U×B∣r and Er show that Er+ ∣U×B∣r is not zero within some periods of discharge. This deviation suggests non-ideal effects in Ohm’s law can not be neglected.
Turbulent transport in rapidly rotating shear flow very efficiently transports angular momentum, a critical feature of instabilities responsible both for the dynamics of accretion disks and the turbulent power dissipation in a centrifuge. Turbulent mixing can efficiently transport other quantities like heat and even magnetic flux by enhanced diffusion. This enhancement is particularly evident in homogeneous, isotropic turbulent flows of liquid metals. In the New Mexico dynamo experiment, the effective resistivity is measured using both differential rotation and pulsed magnetic field decay to demonstrate that at very high Reynolds number rotating shear flow can be described entirely by mean flow induction with very little contribution from correlated velocity fluctuations.
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