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

Abstract

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.

Impurity transport studies in the Madison Symmetric Torus reversed-field pinch during standard and pulsed poloidal current drive regimes

“…Inputs to the code are T e (r = 0, t) measured by Thomson scattering diagnostic (central chord [17]) and n e (r = 0, t) from the central line-averaged density measured by the CO 2 interferometer [18]; electron temperature profile, T e (r), follows the indications from the multi-chord Thomson scattering measurements; electron density profile, n e (r), is assumed on the basis of FIR interferometry measurements [19]. In standard conditions T e (r) and n e (r) are fixed in shape and only their central values evolve in time according to the measurements.…”

A Thomson scattering diagnostic on the Pegasus Toroidal experiment

“…These absorbing plates are oriented at the Brewster angle to reduce stray light, based on previous implementations. 6 Passive vibration isolation is implemented for the laser head and beam line optics. Optical components with damage thresholds 10-20 J/cm 2 were obtained from CVI Melles Griot.…”

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

“…The MST Thomson scattering (TS) diagnostic uses scattered photon spectra to measure electron temperature fluctuations. 1 Traditionally data have been fit to Maxwellian distributions when determining the temperature. The recent diagnosis of energetic tails in the electron distribution function via MST's fast x-ray detector 2 has motivated adding the capability to appropriately fit non-Maxwellian and shifted distribution functions.…”