Upgrades to improve the usability, reliability, and spectral range of the MST Thomson scattering diagnostic.

The Thomson scattering diagnostic on MST records both equilibrium and fluctuating electron temperature with a range capability of 10 eV-5 keV. Standard operation with two modified commercial Nd:YAG lasers allows measurements at rates of 1 kHz-25 kHz. Several subsystems of the diagnostic are being improved. The power supplies for the avalanche photodiode detectors (APDs) that record the scattered light are being replaced to improve usability, reliability, and maintainability. Each of the 144 APDs will have an individual rack mounted switching supply, with bias voltage adjustable to match the APD. Long-wavelength filters (1140 nm center, 80 nm bandwidth) have been added to the polychromators to improve capability to resolve non-Maxwellian distributions and to enable directed electron flow measurements. A supercontinuum (SC) pulsed white light source has replaced the tungsten halogen lamp previously used for spectral calibration of the polychromators. The SC source combines substantial brightness produced in nanosecond pulses with a spectrum that covers the entire range of the polychromators.

Identification and mitigation of stray laser light in the Thomson scattering system on the Madison Symmetric Torus (MST).

The Thomson scattering diagnostic on the Madison Symmetric Torus (MST) records excessive levels of stray Nd:YAG laser light. Stray light saturates the 1064 nm spectral channel in all polychromators, which prevents absolute electron density measurements via Rayleigh scattering calibration. Furthermore, stray light contaminates adjacent spectral channels for r/a ≥ 0.75, which renders the diagnostic unable to make electron temperature measurements at these radii. In situ measurements of stray light levels during a vacuum vessel vent are used to identify stray light sources and strategies for reduction of stray light levels. Numerical modeling using Zemax OpticStudio supports these measurements. The model of the vacuum vessel and diagnostic includes synthetic collection optics to enable direct comparison of measured and simulated stray light levels. Modeling produces qualitatively similar stray light distributions to MST measurements, and quantifies the mitigation effects of stray light mitigation strategies prior to implementation.

Turbulent diamagnetism in flowing liquid sodium.

The nature of Ohm's law is examined in a turbulent flow of liquid sodium. A magnetic field is applied to the flowing sodium, and the resulting magnetic field is measured. The mean velocity field of the sodium is also measured in an identical-scale water model of the experiment. These two fields are used to determine the terms in Ohm's law, indicating the presence of currents driven by a turbulent electromotive force. These currents result in a diamagnetic effect, generating magnetic field in opposition to the dominant fields of the experiment. The magnitude of the fluctuation-driven magnetic field is comparable to that of the field induced by the sodium's mean flow.

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.

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.

Sensitivity of surveillance cultures for the detection of methicillin-resistant Staphylococcus aureus in a nursing-home-care unit.

This study compared the sensitivity of nasal culture alone versus multiple-site cultures and single versus duplicate sampling for the detection of methicillin-resistant Staphylococcus aureus (MRSA)-colonized individuals in a nursing-home population. Repeat culture of 68 specimens collected from 35 colonized subjects yielded identical results for 57 specimens, (84%), and 89% of the colonized residents (31 of 35) were identified by the first culture of multiple sites. A single nares culture detected 27 (77%) of 35 (first screen) and 29 (83%) of 35 (second screen) residents colonized with MRSA at any site. The most cost-effective screening would consist of a nasal culture only or combined with a gastrostomy tube site, if applicable. To identify all colonized individuals, however, it would be necessary to culture more than one specimen from multiple sites on each resident.