Polarimetry measurements of current density profile and fluctuation changes during lower hybrid experiments on Alcator C-Mod
Author(s)Xu, Peng, Ph. D. Massachusetts Institute of Technology
Massachusetts Institute of Technology. Department of Physics.
Earl S. Marmar and James H. Irby.
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As a possible method to achieve steady-state plasma operation in a tokamak fusion reactor, Lower Hybrid Current Drive (LHCD) has been extensively studied on Alcator C-Mod. The measurement of current density profiles (or poloidal magnetic field or safety factor) is essential to the understanding of the LH physics. A polarimeter measures the line-integral of the electron density times the magnetic field parallel to a chord through the plasma, and is capable of deriving the current density profile with multi-chord measurements. A three-chord poloidally viewing far-infrared (FIR) polarimeter has therefore been developed for C-Mod. FIR optics, FIR detectors, electronics and mechanical components were custom designed and fabricated. Various sources of vibrations and systematic errors have been carefully dealt with. A synthetic polarimeter signal has been derived from density profiles from Thomson scattering and magnetic field profiles from EFIT. The polarimeter results confirm that the magnetic field profiles from Kinetic EFIT (with Motional Stark Effect and plasma pressure constraints) is much more accurate than those from normal EFIT (no internal constraints, central safety factor qo~0.9 5) during the LHCD. The polarimeter data were used to constrain qO, and the result shows that the current density profile is flattened, and qo increases to ~2 with strong LHCD. Experiments during LHCD with a plasma density scan, a current scan, and LH power and phase scans have been carried out. The Polarimetry observations confirm the off-axis current drive, and strongly decreasing driven current when the density exceeds a limit of about 1.Ox 102 0/m3 . They also indicate that the driven current is closer to the plasma edge for Ip ~820 kA than for I, ~580 kA, and that lower LH phasing is preferred for higher current drive efficiency. The polarimeter is the first diagnostic to measure core magnetic fluctuations in Alcator C-Mod. These include broadband magnetic fluctuations during EDA H-modes that are suppressed with the application of LH power.
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, September 2013.Cataloged from PDF version of thesis.Includes bibliographical references (pages 179-186).
DepartmentMassachusetts Institute of Technology. Department of Physics
Massachusetts Institute of Technology