Page 3 - First time-resolved electron density measurements in the C-2W advanced field-reversed configuration plasmas from long-path compact second-harmonic interferometer

10B113-3 Beall, Sheftman, and TAE Team
Rev. Sci. Instrum. 89, 10B113 (2018)
III. PRELIMINARY DATA
Initial measurements of electron density focused on
observing the quality of the generated plasmoid as it trav-
eled through the inner divertor region. In order to characterize
the gas ionization and translation in the formation region, gas
pressures were scanned to observe the effect on the gener-
ated compact toroids (CTs), as shown in Fig. 5(a). Using data
from magnetics to calculate a volume, an approximate parti-
cle count can be compared to the expected number of particles
injected, reaching as high as 40%. With the addition of veloc-
ity measurements, a total kinetic energy could be calculated
to diagnose performance of the formation process, details of
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This information provided a starting point for optimizing the formation process, and once the generated CTs trans- lated through the inner divertor consistently, they could also be observed as they passed into the CV, and the quality of the translation could be examined. An example of a success- ful translation is shown in Fig. 5, where the plasma can be
FIG. 5. (a) Confirmation that the pressure of the gas injected into formation correlates linearly with the observed density of the translating plasma. (b) Calculated kinetic energy of the plasmoid compared to the applied voltage in the formation pinch.
which are discussed elsewhere.
the increase in the CT kinetic energy with the voltage applied to the formation pinch.
As a proxy, Fig. 5(b) shows
FIG. 6. Plot of early time FRC collision/merger from two interferometer units, compared with magnetics calculations of excluded-flux radius and single point density measurement from the Thomson scattering system.
seen to compress in the high mirror fields around the inner divertor and broaden out in the more flat field in the confine- ment region.
Once it was concluded that translation had been suffi- ciently characterized and the plasma merger and equilibrium could be performed, the system was removed from its posi- tion on the inner divertor and aligned to view the CV from a near-axial position (13◦ off), as shown in Fig. 1(b) as DI CV2. Assuming an elliptical FRC core plasma with semi- minor and semi-major axes determined by magnetic flux measurements, the line-integrated density signals from the dispersion interferometer system can be normalized for time spent in the core plasma to estimate the averaged electron density.
Figure 6 shows initial data of two interferometers in early FRC collision/merging experiment on C-2W. Plotted with the Thomson scattering data and magnetics, while approximation used to determine an averaged electron density may be over- estimating slightly, interferometry data are in good agreement with the Thomson scattering data. Around t ∼ 0.35 ms, there is an oscillation in density and a drop in excluded-flux radius, which indicates an instability of FRC plasma without sufficient edge control and stabilization.
IV. KNOWN ISSUES
Despite the convenience and mobility of the diagnostic, the system is not without complications. Many of them seem to be primarily the result of having the optics directly con- nected to the vacuum vessel, subjecting them, their mounts, and the SHG electronics to high levels of electromagnetic and mechanical noise. While the two-color nature of the dis- persion interferometer should make it immune to mechanical vibrations, it is found that very high noise levels are still a problem. This is likely due to the effect of dispersive elements in the optical chain, such as the compensator and the lenses. Although they are designed to be achromatic, when the beam is deflected away far enough for the paraxial approximation to fail, significant dispersion may occur. Fortunately, there is a