A. F. Falkowski

Multipoint Thomson scattering diagnostic for the Madison Symmetric Torus reversed-field pinch.

The multipoint Thomson scattering diagnostic on the Madison Symmetric Torus (MST) is now fully operational with 21 spatial points, which cover the entire minor radius. Four full electron temperature profiles can be obtained during each MST discharge, with a variable delay between each profile. This system overcomes challenges that arise from the unique machine design, location, and plasma characteristics of MST. The machine design limits the maximum porthole diameter to 11.4 cm, requiring a compact, re-entrant, seven element lens for scattered light collection. Limited space near MST necessitates a long beam path for the two Nd:YAG lasers requiring a remote beam line adjustment system to suppress drift in the beam position due to thermal expansion of the building. Due to the remote location of the laser head, substantial design effort was put into the creation of a set of safety interlocks for the laser system. The dynamic nature of MST plasmas and the wide range of operating space require a versatile scattered light detection system consisting of filter polychromators with temperature controlled avalanche photodiode detectors. We also implement an insertable integrating sphere, which travels along the laser beam path through the vacuum vessel, for the alignment of both the fiber optics and the lasers.

Calibration of a Thomson scattering diagnostic for fluctuation measurements

Detailed calibrations of the Madison Symmetric Torus polychromator Thomson scattering system have been made suitable for electron temperature fluctuation measurements. All calibrations have taken place focusing on accuracy, ease of use and repeatability, and in situ measurements wherever possible. Novel calibration processes have been made possible with an insertable integrating sphere (ISIS), using an avalanche photodiode (APD) as a reference detector and optical parametric oscillator (OPO). Discussed are a novel in situ spatial calibration with the use of the ISIS, the use of an APD as a reference detector to streamline the APD calibration process, a standard dc spectral calibration, and in situ pulsed spectral calibration made possible with a combination of an OPO as a light source, the ISIS, and an APD used as a reference detector. In addition a relative quantum efficiency curve for the APDs is obtained to aid in uncertainty analysis.

Improvements to the calibration of the MST Thomson scattering diagnostic.

Calibration of the Madison Symmetric Torus Thomson scattering system has been refined to improve temperature fluctuation measurements. Multiple avalanche photodiodes have been directly calibrated for use as reference detectors during calibration, improving accuracy and ease of use. From the absolute calibration we calculate corrections to the gain for variation in detector operating temperature. We also measure the spatial uniformity of detector responsivity for several photodiodes, and present a method of accounting for non-uniformity in the calibration process. Finally, the gain and noise enhancement are measured at multiple wavelengths to improve temperature and uncertainty measurements.

Advances in time-resolved measurement of magnetic field and electron temperature in low-magnetic-field plasmas

Abstract Internal time-resolved measurement of magnetic field and electron temperature in low-field (≤ 1 T) plasmas is a difficult diagnostic challenge. To meet this diagnostic challenge in the Madison Symmetric Torus reversed-field pinch, two techniques are being developed: 1) spectral motional Stark effect (MSE) and 2) Fast Thomson scattering. For spectral MSE, the entire Stark-split Hα spectrum emitted by hydrogen neutral beam atoms is recorded and analyzed using a newly refined atomic emission model. A new analysis scheme has been developed to infer both the polarization direction and the magnitude of Stark splitting, from which both the direction and magnitude of the local magnetic field can be derived. For Fast Thomson scattering, two standard commercial flashlamp-pumped Nd:YAG lasers have been upgraded to “pulse-burst” capability. Each laser produces a burst of up to fifteen pulses at repetition rates 1–12.5 kHz, thus enabling recording of the dynamic evolution of the electron temperature profile and electron temperature fluctuations. To further these capabilities, a custom pulse-burst laser system is now being commissioned. This new laser is designed to produce a burst of laser pulses at repetition frequencies 5–250 kHz.

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.