Poloidal magnetic field profile measurements on the microwave tokamak experiment using far-infrared polarimetry
The measurement of plasma poloidal magnetic field (B) profiles in tokamaks with good temporal and spatial resolution has proven to be a difficult but important measurement. A large range of toroidal confinement phenomena is expected to depend sensitively on the radial variation of B including the tearing instability, sawtooth oscillations, disruptions, and transport. Experimental confirmation of theoretical models describing these phenomena has been hampered by the lack of detailed B measurements. A fifteen chord far-infrared (FIR) polarimeter has been developed to measure B in the Microwave Tokamak, Experiment (MTX). Polarimetry utilizes the well known Faraday rotation effect, which causes a rotation of the polarization of an FIR beam propagating in the poloidal plane. The rotation angle is proportional to the component of B parallel to the beam. A new technique for determining the Faraday rotation angle is introduced, based on phase measurements of a rotating polarization ellipse. This instrument has been used successfully to measure B profiles for a wide range of experiments on MTX. For ohmic discharges, measurements of the safety factor on axis give q{sub 0} {approximately} 0.75 during sawteeth and q{sub 0} > 1 without sawteeth. Large perturbations to the polarimeter signals correlated with the sawtooth crash are observed during some discharges. Measurements in discharges with electron cyclotron heating (ECH) show a transition from a hollow to peaked J profile that is triggered by the ECH pulse. Current-ramp experiments were done to perturb the J profile from the nominal Spitzer conductivity profile. Profiles for initial current ramps and ramps starting from a stable equilibrium have been measured and are compared with a cylindrical diffusion model. Finally, the tearing mode stability equation is solved using measured J profiles. Stability predictions are in good agreement with the existence of oscillations observed on the magnetic loops.
- Research Organization:
- Lawrence Livermore National Lab., CA (United States)
- Sponsoring Organization:
- USDOE, Washington, DC (United States)
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 10103522
- Report Number(s):
- UCRL-LR-111863; ON: DE93002890
- Resource Relation:
- Other Information: DN: Thesis submitted to University of California, Davis; TH: Thesis (Ph.D.); PBD: Sep 1992
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
TOKAMAK DEVICES
MAGNETIC FIELD CONFIGURATIONS
POLARIMETRY
MEASURING METHODS
ECR HEATING
TEARING INSTABILITY
MHD EQUILIBRIUM
INFRARED RADIATION
DESIGN
PERFORMANCE
SAWTOOTH OSCILLATIONS
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PLASMA DIAGNOSTIC TECHNIQUES AND INSTRUMENTATION
PLASMA CONFINEMENT
PLASMA WAVES, OSCILLATIONS, AND INSTABILITIES