High-performance double-filter soft x-ray diagnostic for measurement of electron temperature structure and dynamics

Abstract

A new soft x-ray (SXR) T(e) and tomography diagnostic has been developed for MST that can be used for simultaneous SXR spectrum measurement, tomographically reconstructed emissivity, and reconstructed and line-of-sight electron temperature. The diagnostic utilizes high-performance differential transimpedance amplifiers (gain 10(5)-10(9)) to provide fast time response (up to 125 kHz), allowing for the study of plasma structure dynamics. SXR double-foil T(e) measurements are consistent with Thomson scattering. SXR brightness through a variety of filter thicknesses has been combined with charge exchange recombination spectroscopy (CHERS) impurity density measurements to determine the plasma energy spectrum. Magnetic pickup from the fluctuating magnetic fields in the plasma (B̃∼20 gauss at 10-20 kHz) has been dramatically reduced by improving the detector and housing design, so that nanoampere diode currents are now measured without interference from the substantial fluctuating magnetic field incident on the plasma facing surface of the probe.

Effect of resonant magnetic perturbations on three dimensional equilibria in the Madison Symmetric Torus reversed-field pinch

“…The possibility of locking the helix in any desired orientation allows us to position it favorably for MST's major diagnostics, including Thomson scattering, 39 far infrared (FIR) interferometry/polarimetry, 40 and soft-x-ray (SXR) tomography. 41,42 The importance of controlling the orientation of the helical structure can be seen in Figure 2. Here, there are two different electron temperature profile measurements.…”

Effect of beryllium filter purity on x-ray emission measurements

“…These measurements are tomographically reconstructed into a poloidal emissivity map. Additionally, the paired measurements through different beryllium filters are compared to determine electron temperature (T e ) using the double-filter technique [9].…”

“…A mixed foil configuration including beryllium foils from two separate material lots, installed on MST in December 2012 (after the publication of [9]), resulted in systematic differences in the brightness measured from different poloidal angles, despite nominally identical detectors and electronics. All the detectors use stacks of ∼80 µm beryllium foils to achieve identical total filter thickness.…”

Design and initial operation of a two-color soft x-ray camera system on the Compact Toroidal Hybrid experiment

“…Given measurements of multiple chords, the spatial profile of the electron temperature can be found with the use of tomographic inversion techniques. 7 The two-foil technique has been used on a number of current-carrying axisymmetric plasmas including reversed field pinch, [8][9][10] tokamak, 11,12 and stellarator plasmas. 13 Similar systems using multiple (more than two) foils have also been developed recently.…”