Page 2 - Development of a Zeff diagnostic using visible and near-infrared bremsstrahlung light for the C-2W field-reversed configuration plasma

10D130-2 Nations et al. processes near cool-edge regions.
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The near-infrared spec-
Rev. Sci. Instrum. 89, 10D130 (2018)
transmit only at a narrow spectral region. Irradiance calibra- tion is applied, and measurements of bremsstrahlung intensity are Abel-inverted to get local emissivity. The critical compo- nents of the experimental apparatus used here are described in Subsections III A–III C.
A. Optical mount and collection optics
The integrated diagnostic system utilizes a multi-purpose optical mount (see Fig. 2) that holds a total of 15 focusing plano-convex spherical lenses (f/2, Ø1′′). The lenses sample quasi-cylindrical plasma volumes at multiple lines-of-sight and couple emitted light into specialized optical fiber bun- dles. These bundles enable multiple diagnostics (VIS/NIR bremsstrahlung, Dα, i-CHERS, etc.) to share the same view- ing geometry. Each bundle cable is ∼2 m long and consists of 7 high-grade fused silica optical fibers in a round configu- ration combined in an SMA 905 connector. Each fiber has a Ø600 μm fiber core and a numerical aperture (NA) of 0.22. In addition, a 2′′ long stainless steel sleeve is added at the end of each bundle for durability and additional bending protec- tion. On the opposite end, fiber bundles fan-out and connect to a patch panel located near the vessel. From there, long (∼30–35 m) optical fibers (Ø600 μm fiber core, 0.22 NA) route emission signals from the patch panel to either one of four screen rooms or the diagnostics lab, depending on the location of the diagnostic system. The fibers used in this work have an attenuation of ∼13 dB/km near 523 nm and ∼8 dB/km near 1000 nm; this corresponds to ∼91% and ∼95% transmission for visible and near-infrared light, respectively.
Additionally, the optical mount has several SMA connec- tors to mount optical fibers (Ø600 μm fiber core, 0.22 NA) for survey spectrometers. Here, an array of four spectrome- ters (Avantes part number AvaSpec-ULS2048L-EVO) enables full-range measurements of impurity emission lines from the vacuum ultraviolet to the near-infrared (250–1100 nm; 1.2 ms integration time; ∼0.3–0.9 nm resolution). The spectrome- ters sample a conical volume that encompasses most of the FRC, providing a good picture of the plasma composition. Extrusions on both vertical walls of the optical mount allow
FIG. 1. Cross-sectional view of the confinement vessel (0.8 m radius) show- ing the multi-purpose optical mount with 15 viewing chords and experimen- tal layout. An example FRC separatrix structure (0.4 m radius) is also shown for reference.
trum, although less affected by the aforementioned issues, has
weaker bremsstrahlung signals and thus requires sensors with
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ing Zeff values, in particular, in the low-density region (ne < 1 × 1013 cm−3) near the walls, a more flexible approach was chosen. Here, both visible (VIS) and near-infrared (NIR) bremsstrahlung emission signals were measured simultane- ously for the same viewing geometry. One of the key advan- tages of using two different spectral regions to measure the bremsstrahlung continuum is that they have different pol- lutant sensitivities, rendering an integrated diagnostic more robust, given the highly non-uniform plasma profile (inside and outside of the FRC separatrix, where the dominant type of pollutant emission can differ).
In this work, the VIS bremsstrahlung diagnostic system targets a narrow spectral region near 523 nm. The system12 was inherited from TAE’s previous devices (namely, C-213 and C-2U14), and the design underwent only minor alterations. The NIR bremsstrahlung diagnostic system targets a spectral region near 1000 nm, and it was designed from scratch specifically for the plasma conditions expected in C-2W.
III. MEASUREMENT METHODOLOGY AND EXPERIMENTAL SETUP
Measurements are made near the axial mid-plane of the confinement vessel (z ∼ 0.24 m, where z in cylindrical coor- dinates is the distance from the center “A-plane” along the machine axis) at multiple chords spanning from the FRC cen- ter to the edge. Figure 1 shows a schematic cross-sectional view of C-2W outlining the viewing chords; impact parame- ters (the perpendicular distance between a viewing chord and the center of the vessel at r = 0) range from 0 to 73.6 cm (i.e., r /r wall ∼ 0.92). Emission signals are collected by an array of lenses, and optical fibers route the measured light to a separate room where the detection system is located. Before reach- ing the detectors, light passes through bandpass filters which
higher sensitivity.
Given the expected complications of accurately determin-