Poloidal magnetic field profile measurements on the microwave tokamak experiment using far-infrared polarimetry
The measurement of plasma poloidal magnetic field (B(theta)) profiles on 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(theta) (or equivalently the current density J or safety factor q) 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(theta) measurements. A fifteen chord far-infrared (FIR) polarimeter has been developed to measure B(theta) 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(theta) 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 technique simplifies the implementation of large multi-chord systems, and minimizes the effect of amplitude variations caused by refraction. This instrument has been used successfully to measure B(theta) profiles for a wide range of experiments on MTX. For ohmic discharges, measurements of the safety factor on axis give q(0) approximately 0.75 during sawteeth and q(0) greater than 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. Heavy-metal impurity radiation is believed to be the cause of the initially hollow profile. Current-ramp experiments were done to perturb the J profile from the nominal Spitzer conductivity profile.
- Research Organization:
- California Univ., Davis, CA (United States)
- OSTI ID:
- 6173971
- Resource Relation:
- Other Information: Ph.D. Thesis
- Country of Publication:
- United States
- Language:
- English
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Poloidal magnetic field profile measurements on the microwave tokamak experiment using far-infrared polarimetry
Fifteen chord FIR polarimetry system on MTX
Related Subjects
FAR INFRARED RADIATION
FARADAY EFFECT
POLARIMETRY
MAGNETIC FIELDS
PLASMA DIAGNOSTICS
PLASMA RADIAL PROFILES
TOKAMAK DEVICES
CURRENT DENSITY
REFRACTION
CLOSED PLASMA DEVICES
ELECTROMAGNETIC RADIATION
INFRARED RADIATION
RADIATIONS
THERMONUCLEAR DEVICES
700320* - Plasma Diagnostic Techniques & Instrumentation- (1992-)