MSE Spectroscopy in CHS Heliotron/Torsatron

The spectroscopic measurement using the motional Stark effect (MSE) has been recognized to be a useful tool to measure the direction of the magnetic field inside the plasma. The MSE arises from the Lorentz Electric Field, E = v × B, in the atom's rest frame, which is induced when the neutral beam, having the velocity v , crosses the magnetic field B. The spectrum with the motional Stark splitting in the Hα emission consists of 15 components. The spectra which are circularly polarized perpendicular to the Lorentz field are called as σ (0σ, ＋1σ and -1σ lines) components, while the spectra which are linearly polarized parallel to the Lorentz field, namely perpendicular to the magnetic field are called as π (±2π，±3π and ±4π lines) components. Four kinds of spectra with the polarization angle of 0, 45, 90 and 135 degrees are measured to derive the polarization angle of the circularly polarized σ component, and eliminate the overlap between π components and σ components which are perpendicular each other. The diagnostic neutral beam (DNB) is injected almost perpendicularly to the magnetic field line into the Compact Helical System (CHS) heliotron/torsatron device in order to maximize the Lorentz field, namely the Stark shift. The line broadening due to a beam divergence angle results in overlapping between the adjacent lines. Therefore, the smaller beam divergence angle is desirable in DNB for the MSE measurement. The beam divergence angle of DNB is 0.65 degrees, and it gives enough separation between the measured σ and π components in MSE spectroscopy. Although the one-third energy (E/3) component has the smaller motional Stark splitting than the full energy component, the intensity of E/3 component is larger than that of full energy component. Therefore, the one-third energy component is used to derive the pitch angle in CHS.<br />　　The new polarization sensitive spectroscopy, which consists of a polarizer and a ferroelectric liquid crystal (FLC) cell, has been developed to measure the polarization angle with good accuracy. The FLC cell functions as a switchable half-wave retarder, in which polarization angle of the light is rotated by 90 degrees. By using the FLC, the polarization angle can be measured with the error bar of less than ±1 degrees.<br />　　Because the FLC has fast response, the spectra with the polarizer angle of 0 and 90 degrees or -45 and 45 degrees can be measured with the fast modulation of 50 Hz. New CCD detector, where the charse of each pixel is shifted up and down (Dig Dag) synchronized with two sets of FLC modulation, has been developed for MSE measurements. <br />　　The pitch angle of the magnetic field line is derived from the polarization angle of measured σ and π components of the Hα line with the polarization sensitive spectroscopy. The radial profile of a pitch angle of a local magnetic field is measured for the plasma with the magnetic field of 0.88 T and the magnetic axis of 92.1cm in CHS. The spectra of the σ and π components of the Hα line with the motional Stark effect are measured with the polarization sensitive spectroscopy. The major radius, where the local pitch angle is zero, shifts outboard from that calculated with coil current alone (no plasma) by 28 ± 16 mm. This shift is consistent to that predicted by the effect of finite beta due to bulk plasma and fast ions of heating neutral beam.

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総研大甲第349号

博士論文

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博士論文