Conference Paper

Commissioning of the Lower Hybrid Current Drive System on Alcator C-Mod

Plasma Sci. & Fusion Center, MIT, Cambridge, MA
DOI: 10.1109/FUSION.2005.252948 Conference: Fusion Engineering 2005, Twenty-First IEEE/NPS Symposium on
Source: IEEE Xplore


A lower hybrid current drive (LHCD) system has been developed for current profile control of advanced tokamak experiments on Alcator C-Mod. LHCD along with Ion Cyclotron Radio Frequency (ICRF) heating will be used to develop regimes with high confinement, high betan and high bootstrap fraction and extend them to quasi-steady-state conditions. This paper will describe the commissioning and initial operation of the LHCD system that includes a 50 kV, 208 A pulsed-power supply, twelve 250 kW Klystron transmitters, a 96 waveguide launcher and required control, protection and data acquisition systems

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Available from: James Randall Wilson, Oct 04, 2015
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    ABSTRACT: Current profile evolution will be controlled and sustained in the Alcator C-Mod advanced tokamak lower hybrid current drive experiment by use of 3 MW of 4.6 GHz lower hybrid current drive (LHCD) now being installed and tested. LHCD and an existing 5 MW ICRH capability are to be used to develop regimes with high confinement, high β<sub>n</sub> and high bootstrap fraction and extend them to quasi-steady-state conditions. This paper will describe the design, installation and testing of the low power microwave active control system used in the experiment. The LHCD low power microwave active control system uses vector modulators to provide a phase and amplitude controlled driver for each of twelve 4.6 GHz, 250 kW klystrons. Phase and power output of each klystron are monitored by an I-Q detector and the resulting signals are used in digital controllers for closed-loop control of the klystron phase and amplitude to preset values.
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    ABSTRACT: A combined model for current profile control and MHD stability analysis has been used to identify stable operating modes near the ideal stability limit (βN 3) in the Alcator C-Mod tokamak. These discharges are characterized by relatively high fractions of bootstrap current (fBS = 0.70) and non-monotonic profiles of the safety factor with qmin > 2. In the absence of a conducting shell, stability was determined by the onset of the low (n = 1) external kink mode. In these studies, current profile control in the plasma periphery (r/a 0.5) was provided by 2.5-3.0 MW of LHCD power. Internal and edge transport barriers were introduced into the model calculations in the form of density transitions. Excellent wave accessibility and absorption were still found in the presence of an H-mode-like edge density barrier. However, the presence of these barriers resulted in about a 10% decrease in the stability limit, from βN 3 to βN 2.7.
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    ABSTRACT: Princeton Plasma Physics Laboratory and the Massachusetts Institute of Technology are preparing an experiment of current profile control using lower hybrid waves to produce and sustain advanced tokamak regimes in steady-state conditions in Alcator C-Mod. Unlike the Joint European Torus, ToreSupra, and JT60 couplers, the C-Mod lower hybrid coupler does not employ the now conventional multijunction design but will have similar characteristics, compactness, and internal power division while retaining full control of the antenna element phasing. This is achieved by using 3-dB vertical power splitters and a stack of laminated plates with the waveguides milled in them. Construction is simplified and allows easy control and maintenance of all parts. Many precautions are taken to avoid arcing. Special care is also taken to avoid the recycling of reflected power, which could affect the coupling and the launched n.
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