Page 4 - First time-resolved electron density measurements in the C-2W advanced field-reversed configuration plasmas from long-path compact second-harmonic interferometer

10B113-4 Beall, Sheftman, and TAE Team
Rev. Sci. Instrum. 89, 10B113 (2018)
FIG. 7. Plot of a single-sided formation plasma traveling through the inner divertor region and into the confinement vessel, showing broadening of an originally dense CT into a more elongated plasmoid.
delay between when the system magnets are fired and when the resulting motion propagates to the optics, allowing measure- ments such as in Fig. 7 to be seen before the system vibrations shown in Fig. 8 kick in. While the severity of the vibrations is much less significant for the radial interferometer, the signal level picking up near the end of the core plasma is also quite low, restricting the usefulness of the data except in special cases.
Contamination of the signal due to electromagnetic inter- ference is relatively small, due to the placement of most of the electronics in shielded rooms far from the machine. However, failures of piezo-controllers and Peltier modules used to sta- bilize the SHG temperatures have been tentatively attributed to the shot noise and plasma contact with the vessel wall during FRC collision/merger. Both effects appear largely mit- igated for the axial interferometer system, which has more robust electrical insulation and for which all electronics are disconnected during plasma operation.
FIG.8. Largevibrationobservedonaxialinterferometer(DICV2),beginning around 1 ms and ramping up at 2 ms.
FIG. 9. Plot of dispersion interferometer data from C-2U comparing noise levels of the system mounted directly to vessel (dotted-dashed line), system supported independently on a post (solid line), and average size of plasma signal (thick bar).
V. FUTURE WORK
Several possible options to upgrade the systems have been considered and examined. Given that the mechanical stability of the system seems to be the primary limiting factor, the first three consist of increasing the structural integrity of the enclo- sure, detaching the optical units from the vacuum vessel, and increasing the robustness of the optical train. Building a more robust box is only pending design time, and the use of an inde- pendent post mount for the optics has already seen significant success, as shown in Fig. 9.
The remaining challenge is to determine a method of
reducing the vibrational susceptibility of the optics themselves.
No obvious solution immediately presents itself, but some
possibilities include the redesign of the lenses for improved
tolerance, thorough examination of the system for spurious
dispersion sources, or possibly pulling the optical units back
from the experimental area entirely and coupling the light to
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Early density measurements by a pair of compact, mobile, turn-key second harmonic interferometers provided critical information for early phases of C-2W experiments. The size and simplicity of the system allowed for the characteriza- tion and analysis of preliminary device operation when more sophisticated diagnostics would not yet be available. Further, the opportunity is provided to make relative quick changes to the installation configuration to study areas of interest. While the system lacks an exceptional signal to noise ratio, some possible improvements are available and are discussed.
ACKNOWLEDGMENTS
We thank our shareholders for their support and trust and all fellow TAE staff for their dedication, excellent work, and extra efforts.
and from the device via fibers, as suggested by Yasuhara.
VI. SUMMARY