RESUMEN
This work describes a novel optical heterodyne detection scheme that significantly extends the frequency response of the phase contrast imaging method to detect electron density fluctuations in tens of megahertz frequency range. The system employs a variable frequency electro-optic modulator to allow operation at any frequency in the range 10-40 MHz without the need to realign the system. The frequency coverage of the system makes it suitable to measure the radial structure of the electron density component of ion cyclotron emission on devices having confining magnetic field of a few tesla, thus extending the purely temporal measurements provided so far by magnetic probes.
Asunto(s)
Ciclotrones , Diagnóstico por Imagen , MagnetismoRESUMEN
We report on experiments and modeling on a rotating confined liquid that is forced by circumferential jets coaxial with the rotation axis, wherein system-scale secondary flows are observed to emerge. The jets are evenly divided in number between inlets and outlets and have zero net mass transport. For low forcing strengths the sign of this flow depends on the sign of a sloped end cap, which simulates a planetary ß plane. For increased forcing strengths the secondary flow direction is insensitive to the slope sign, and instead appears to be dominated by an asymmetry in the forcing mechanism, namely, the difference in radial divergence between the inlet and outlet jet profiles. This asymmetry yields a net radial velocity that is affected by the Coriolis force, inducing secondary zonal flow.
RESUMEN
A phase contrast imaging (PCI) diagnostic has been developed for the Wendelstein 7-X (W7-X) stellarator. This diagnostic, funded by the U.S. Department of Energy through the Office of Fusion Energy Sciences, is a collaboration between the Max Planck Institute for Plasmaphysics, MIT, and SUNY Cortland. The primary motivation for the development of the PCI diagnostic is measurement of turbulent fluctuations, such as the ion temperature gradient, electron temperature gradient, and the trapped electron mode instabilities. Understanding how the magnetic geometry and other externally controllable parameters, such as the fueling method and heating scheme, modify the amplitude and spectrum of turbulence is important for finding operational scenarios that can lead to improved performance at fusion-relevant temperatures and densities. The PCI system is also sensitive to coherent fluctuations, as may arise from Alfvén eigenmodes or other MHD activity, for example. The PCI method creates an image of line-integrated variations in the index of refraction. For a plasma, the image created is proportional to the line-integral of electron density fluctuations. This paper provides an overview of some key features of the hardware and the optical system and presents two examples of recent measurements from the W7-X OP1.2a experimental campaign.
RESUMEN
We present fluid velocity measurements in a modified Taylor-Couette device operated in the quasi-Keplerian regime, where it is observed that nearly ideal flows exhibit self-similarity under scaling of the Reynolds number. In contrast, nonideal flows show progressive departure from ideal Couette as the Reynolds number is increased. We present a model that describes the observed departures from ideal Couette rotation as a function of the fluxes of angular momentum across the boundaries, capturing the dependence on Reynolds number and boundary conditions.
RESUMEN
Experiments in a modified Taylor-Couette device, spanning Reynolds numbers of 10(5) to greater than 10(6), reveal the nonlinear stability of astrophysically relevant flows. Nearly ideal rotation, expected in the absence of axial boundaries, is achieved for a narrow range of operating parameters. Departures from optimal control parameters identify centrifugal instability of boundary layers as the primary source of turbulence observed in former experiments. By driving perturbations from a series of jets we demonstrate the robustly quiescent nature of quasi-Keplerian flows, indicating that sustained turbulence does not exist.
RESUMEN
Groups of frequency chirping modes observed between sawtooth crashes in the Alcator C-Mod tokamak are interpreted as reversed shear Alfvén eigenmodes near the q=1 surface. These modes indicate that a reversed shear q profile is generated during the relaxation phase of the sawtooth cycle. Two important parameters, q_{min} and its radial position, are deduced from comparisons of measured density fluctuations with calculations from the ideal MHD code NOVA. These studies provide valuable constraints for further modeling of the sawtooth cycle.
