A free-standing wire scattering technique to monitor calibration variations of the DIII-D density profile reflectometer
- 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)Copyright Statement
- Country of Publication:
- United States
- Language:
- English
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