A free-standing wire scattering technique to monitor calibration variations of the DIII-D density profile reflectometer

Journal Article · Wed Oct 31 00:00:00 EDT 2018 · Review of Scientific Instruments

DOI:https://doi.org/10.1063/1.5036581· OSTI ID:1609194

Zeng, L.  ^[1]; Wang, G. ^[1]; Rhodes, T. L. ^[1]; Peebles, W. A. ^[1]; Sung, C. ^[1]; Lantsov, R. ^[1]

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1. Univ. of California, Los Angeles, CA (United States)

Real-time phase calibration of the ITER profile reflectometer is essential due to the long plasma duration and expected waveguide path length changes during a discharge. Progress has been recently made in addressing this issue by employing a phase calibration technique on DIII-D that monitors calibration variations that occur during each plasma discharge. By installing a thin free-standing metallic wire (1 mm diameter) near the end of the overmoded waveguide transmission system (oriented perpendicular to the waveguide axis), the round-trip phase shift from the wire is detected simultaneously with the plasma phase shifts. Variations in the reflectometer round trip path length (-26 m) are then calculated after each DIII-D plasma discharge, allowing the calibration phase to be accurately monitored and updated. The round-trip reflectometer path length is observed to vary by -3 mm (root mean square value) during a typical DIII-D discharge. Using the variations in calibration phase, the density profile measurement accuracy can be improved. Since the wire retro-reflected power is -0.01 of the plasma signal, minimal effect is observed on the reflected signal from the plasma. Importantly, through a suitable choice in wire diameter, the calibration signal can be made approximately independent of the V-band reflectometer launch polarization. Furthermore, this is particularly important on DIII-D since orthogonal X- and O-mode polarized beams are coupled into the same transmission waveguide and launch antenna.

• Research Organization: General Atomics, San Diego, CA (United States); Univ. of California, Los Angeles, CA (United States)
• Sponsoring Organization: USDOE Office of Science (SC)
• Grant/Contract Number: FC02-04ER54698; FG02-08ER54984
• OSTI ID: 1609194
• Alternate ID(s): OSTI ID: 1467944
• Journal Information: Review of Scientific Instruments, Vol. 89, Issue 10; ISSN 0034-6748
• Publisher: American Institute of Physics (AIP)
• Country of Publication: United States
• Language: English

Related Subjects

71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
instruments & instrumentation
physics
plasma confinement
reflectometry
electron density
real-time process
scattering theory
phase detector
plasma discharges