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Title: Reflectometer Sensing of RF Waves in front of the HHFW antenna on NSTX

Abstract

The ability to measure RF driven waves in the edge of the plasma can help to elucidate the role that surface waves and Parametric Decay Instabilities (PDI) play in RF power losses on NSTX. A microwave reflectometer has recently been modified to monitor RF plasma waves in the scrape-off layer in front of the 30 MHz High Harmonic Fast Wave (HHFW) antenna array on NSTX. In RF heated plasmas, the plasma-reflected microwave signal exhibits 30 MHz sidebands, due to the modulation of the cutoff layer by the electrostatic component of the heating wave. Similarly, electrostatic parametric decay waves (when present) are detected at frequencies below the heating frequency, near 28, 26, MHz, separated from the heating frequency by harmonics of the local ion cyclotron frequency of about 2 MHz. In addition, a corresponding frequency matched set of decay waves is also detected near the ion cyclotron harmonics, at 2, 4, MHz. The RF plasma-wave sensing capability is useful for determination of the PDI power threshold as a function of antenna array phasing (including toroidal wavelength), outer gap spacing, and various plasma parameters such as the magnetic field and the plasma current.

Authors:
 [1];  [1];  [1];  [1];  [2];  [2];  [2];  [2];  [2];  [2]
  1. ORNL
  2. Princeton Plasma Physics Laboratory (PPPL)
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
1110836
DOE Contract Number:
DE-AC05-00OR22725
Resource Type:
Conference
Resource Relation:
Conference: 16th Topical Conference on High-Temperature Plasma Diagnostic (HTPD), Williamsburg, VA, USA, 20060507, 20060511
Country of Publication:
United States
Language:
English

Citation Formats

Wilgen, John B, Ryan, Philip Michael, Hanson, Gregory R, Swain, David W, Bernabei, Stefano, Greenough, Nevell, DePasquale, Steve, Phillips, Cynthia, Hosea, Joel, and Wilson, Randy. Reflectometer Sensing of RF Waves in front of the HHFW antenna on NSTX. United States: N. p., 2006. Web.
Wilgen, John B, Ryan, Philip Michael, Hanson, Gregory R, Swain, David W, Bernabei, Stefano, Greenough, Nevell, DePasquale, Steve, Phillips, Cynthia, Hosea, Joel, & Wilson, Randy. Reflectometer Sensing of RF Waves in front of the HHFW antenna on NSTX. United States.
Wilgen, John B, Ryan, Philip Michael, Hanson, Gregory R, Swain, David W, Bernabei, Stefano, Greenough, Nevell, DePasquale, Steve, Phillips, Cynthia, Hosea, Joel, and Wilson, Randy. Sun . "Reflectometer Sensing of RF Waves in front of the HHFW antenna on NSTX". United States. doi:.
@article{osti_1110836,
title = {Reflectometer Sensing of RF Waves in front of the HHFW antenna on NSTX},
author = {Wilgen, John B and Ryan, Philip Michael and Hanson, Gregory R and Swain, David W and Bernabei, Stefano and Greenough, Nevell and DePasquale, Steve and Phillips, Cynthia and Hosea, Joel and Wilson, Randy},
abstractNote = {The ability to measure RF driven waves in the edge of the plasma can help to elucidate the role that surface waves and Parametric Decay Instabilities (PDI) play in RF power losses on NSTX. A microwave reflectometer has recently been modified to monitor RF plasma waves in the scrape-off layer in front of the 30 MHz High Harmonic Fast Wave (HHFW) antenna array on NSTX. In RF heated plasmas, the plasma-reflected microwave signal exhibits 30 MHz sidebands, due to the modulation of the cutoff layer by the electrostatic component of the heating wave. Similarly, electrostatic parametric decay waves (when present) are detected at frequencies below the heating frequency, near 28, 26, MHz, separated from the heating frequency by harmonics of the local ion cyclotron frequency of about 2 MHz. In addition, a corresponding frequency matched set of decay waves is also detected near the ion cyclotron harmonics, at 2, 4, MHz. The RF plasma-wave sensing capability is useful for determination of the PDI power threshold as a function of antenna array phasing (including toroidal wavelength), outer gap spacing, and various plasma parameters such as the magnetic field and the plasma current.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Sun Jan 01 00:00:00 EST 2006},
month = {Sun Jan 01 00:00:00 EST 2006}
}

Conference:
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  • The ability to measure rf driven waves in the edge of the plasma can help to elucidate the role that surface waves and parametric decay instabilities (PDIs) play in rf power losses on NSTX. A microwave reflectometer has recently been modified to monitor rf plasma waves in the scrape-off layer in front of the 30 MHz high harmonic fast wave antenna array on NSTX. In rf heated plasmas, the plasma-reflected microwave signal exhibits 30 MHz sidebands, due primarily to the modulation of the cutoff layer by the electrostatic component of the heating wave. Similarly, electrostatic parametric decay waves (when present)more » are detected at frequencies below the heating frequency, near 28, 26,... MHz, separated from the heating frequency by harmonics of the local ion cyclotron frequency of about 2 MHz. In addition, a corresponding frequency matched set of decay waves is also detected near the ion cyclotron harmonics at 2, 4,... MHz. The rf plasma-wave sensing capability is useful for determination of the PDI power threshold as a function of antenna array phasing (including toroidal wavelength), outer gap spacing, and various plasma parameters such as the magnetic field and the plasma current.« less
  • The 30 MHz high harmonic fast wave (HHFW) antenna array for NSTX consists of 12 current straps, evenly spaced in the toroidal direction. Each pair of straps is connected as a half-wave resonant loop and will be driven by one transmitter, allowing rapid phase shift between transmitters. A decoupling network using shunt stub tuners has been designed to compensate for the mutual inductive coupling between adjacent current straps, effectively isolating the six transmitters from one another. One half of the array, consisting of six full-scale current strap modules, three shunt stub decouplers, and powered by three phase-adjustable rf amplifiers hadmore » been built for electrical testing at ORNL. Low power testing includes electrical characterization of the straps, operation and performance of the decoupler system, and mapping of the rf fields in three dimensions.« less
  • Experimental results have shown that the high harmonic fast wave (HHFW) at 30 MHz can provide substantial plasma heating and current drive for the NSTX spherical tokamak operation. However, the present antenna strap design rarely achieves the design goal of delivering the full transmitter capability of 6 MW to the plasma. In order to deliver more power to the plasma, a new antenna strap design and the associated coaxial line feeds are being constructed. This new antenna strap design features two feedthroughs to replace the old single feed-through design. In the design process, CST Microwave Studio has been used tomore » simulate the entire new antenna strap structure including the enclosure and the Faraday shield. In this paper, the antenna strap model and the simulation results will be discussed in detail. The test results from the new antenna straps with their associated resonant loops will be presented as well.« less
  • The single-feed, end-grounded straps of the NSTX 12-strap HHFW antenna array have been replaced with double-feed, center-grounded straps to reduce the voltages in the vicinity of the Faraday shield (FS) for a given strap current. The strap spacings to the FS and to the back plate were increased by 3 mm to decrease the electric fields for a given voltage. The electric fields near the FS have been roughly halved for the same strap currents, permitting a direct examination of the roles that internal fields play in determining antenna power limits in plasmas. Extensive RF/plasma conditioning of the antenna wasmore » required to remove enough of the evaporated Li deposits from prior wall conditioning to permit coupling in excess of 4 MW to L- and H-mode plasmas in 2009. Most arcs were associated with expulsion of Li from the FS/antenna frame surfaces. The center-grounded straps were less susceptible to arcing during ELMing Hmode plasmas. Reliable operation above 2 MW was difficult after the installation of the Liquid Lithium Divertor (LLD) in 2010. Li-compound dust was found in the antennas after this run and is believed to have contributed to the reduced power limit.« less
  • Two important topics in the tokamak ELM control, using the non-axisymmetric (3D) magnetic perturbations, are studied in NSTX and combined envisioning ELM control in the future NSTX-U operation: Experimental observations of the edge harmonic oscillation in NSTX (not necessarily the same as EHOs in DIII-D), and theoretical study of its external drive using the high harmonic fast wave (HHFW) antenna as a 3D field coil. Edge harmonic oscillations were observed particularly well in NSTX ELM-free operation with low n core modes, with various diagnostics confirming n = 4 ~ 6 edge-localized and coherent oscillations in 2 ~ 8kHz frequency range.more » These oscillations seem to have a favored operational window in rotational shear, similarly to EHOs in DIII-D QH modes . However, in NSTX, they are not observed to provide particle or impurity control, possibly due to their weak amplitudes, of a few mm displacements, as measured by reflectometry. The external drive of these modes has been proposed in NSTX, by utilizing audio-frequency currents in the HHFW antenna straps. Analysis shows that the HHFW straps can be optimized to maximize n = 4 ~ 6 while minimizing n = 1 ~ 3. Also, IPEC calculations show that the optimized configuration with only 1kAt current can produce comparable or larger displacements than the observed internal modes. If this optimized external drive can be constructively combined, or further resonated with the internal modes, the edge harmonic oscillations in NSTX may be able to produce sufficient particle control to modify ELMs.« less