Page 5 - Development of a Zeff diagnostic using visible and near-infrared bremsstrahlung light for the C-2W field-reversed configuration plasma

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Nations et al.
Rev. Sci. Instrum. 89, 10D130 (2018)
FIG. 7.
and NIR (open triangles) diagnostics. The dotted line at Zeff = 1 corresponds to a theoretical minimum for the case of a pure hydrogen isotope plasma. The hatched area indicates the radial location where Zeff profile analysis is difficult.
Figure 7 shows measured Z eff profiles inside of the separa- trix at t = 2 ms, and both bremsstrahlung diagnostics show good agreement; an average Z eff (inside the FRC core) of 1.33 ± 0.24 and 1.54 ± 0.28 was calculated for the VIS and NIR systems, respectively. In FRCs, radial variation in electron temperature and density are small inside the separatrix (compared to toka- maks where these quantities change by an order of magnitude). Consequently, the charge state distribution of impurities does not change significantly and Zeff profiles inside the separatrix are expected to be rather uniform (within the measurement uncertainty). Here, what most contributes to Zeff profiles is the accuracy of the inversion methods used to obtain both the electron density and bremsstrahlung emissivity profiles. The former, in fact, can have a significant impact in the attained Z eff profile due to its strong 1/ne2 dependence. At r > 35 cm, the noticeable rise in measured Zeff can be due to edge pollution from neutrals creeping inside the separatrix and/or impurity transport from the SOL (a phenomenon which remains an active field of research for FRCs).
It is important to emphasize that, for the low temperature
regime where these measurements were made (T
impurity ions present inside of the FRC core are not fully ionized, which also explains the relatively low Zeff values mea-
ACKNOWLEDGMENTS
We thank our shareholders for their support and trust, and all fellow TAE staff for their dedication, excellent work, and extra efforts.
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13
Furthermore, the preliminary Zeff values shown here serve to emphasize the effect of neutral pollutants (line
sured in a low electron density regime (∼2–3 × 10
For plasmas at higher electron temperature regimes such as tokamaks (Te ∼ 1–10 keV), impurities are fully ionized and bremsstrahlung signals are significantly weaker (ε ∝ T e 0.15 ) and thus more sensitive to light pollution. As a result, Zeff val- ues can be substantially higher than the values shown here for similar electron densities. Note that when the plasma is not fully ionized, Zeff can vary even if the overall impurity content of the plasma does not change significantly.
e
∼ 65 eV),
cm
).
−3
emission from edge-localized impurities and electron-neutral bremsstrahlung continuum in the core) on measured e–i bremsstrahlung emission. Note that in Fig. 7, Z eff from the NIR diagnostic is systematically higher than estimates obtained using the VIS system. This could be an artifact of edge-light pollution as the wider NIR filter is more prone to leaks from dim emission lines of neutrals and singly ionized impurities (e.g., C-i, C-ii, O-i, O-ii, Ti-i, Ti-ii, etc.) that exist near the walls.
Future work will focus on the removal of pollutant
emission (especially for chords near the edge) to improve
Measured Z eff profiles inside the FRC core for both VIS (open circles)
lished techniques. Improved accuracy in determining Zeff profiles will also be possible with an integrated analysis of multiple diagnostics using Bayesian statistics, which will reduce the uncertainties associated with traditional Abel- inversion methods. In addition, survey spectrometer data will be used to search for different spectral regions in the visible and near-infrared with less line emission inter- ference and better suited for a “clean” bremsstrahlung measurement.
Zeff estimates. This will be attempted using previously pub- 4