Page 5 - Development of a three-wave far-infrared laser interferometry and polarimetry diagnostic system for the C-2W field-reversed configuration plasmas

10B109-5
Deng et al.
Rev. Sci. Instrum. 89, 10B109 (2018)
FIG. 6. Time evolution of Abel inverted electron density profiles for shot 105094.
ends up with a centrally peaked profile when the plasma field reversal is lost. This trend of evolution is the same as C-2 and C-2U plasmas. From about 1 ms, a clear double trough structure is seen in the profiles. Similar to the double hump structure seen in C-2U plasmas,11 the density profile is reshaped due to the significant fast ion pressure.
C. Preparing for polarimetry operation
The interferometry phase under normal operation con-
ditions is >1000◦ which does not require super high phase
resolution. Therefore, most of the improvements described
in Sec. II are prepared for polarimetry operation, which
IV. SUMMARY
A full scale 14-chord three-wave FIR laser polarimetry and interferometry diagnostics is designed for the C-2W FRC experiments for simultaneous density and internal magnetic field measurements. It is based on the previous successful FIR polarimetry and interferometry diagnostics for C-2 and C-2U. Methods to address the challenges learned from the previous FIR diagnostics for FRC plasmas are implemented. The routine stable interferometry operation is achieved, and high quality interferometer data are obtained. A new FIR laser with improved stability design is assembled, which will soon be integrated into the system to provide simultaneous FIR polarimetry and interferometry measurements in C-2W FRC plasmas.
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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requires very high phase resolution.7 The small laser phase 6,7,9
noise has been demonstrated before. The vibration is minimized. The laser beam refraction effects are mitigated. EMI noise has been reduced to the digitizer bit noise level. From previous experience, fine tuning the collinear align- ment of the two polarimetry probe beams is critical to the success of polarimetry measurements in the high density gra- dient FRC plasma environments.7 The process can be pro- longed, and therefore, long term thermal stability of the laser systems described in Sec. II E is essential for polarimetry operation.