| 1. |
- Kuldkepp, Matias, et al.
(author)
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Accurate polarization measurements with a dual photoelastic modulator
- 2005
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In: Applied Optics. - 1559-128X .- 2155-3165. ; 44:28, s. 5899-5904
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Journal article (peer-reviewed)abstract
- Measurements of the polarization effects in multimirror experiments by using a dual photoelastic modulator are described. The effect of single and multiple mirrors in polarization measurements in two and three dimensions is discussed, and experimental results show how symmetrical placement of mirrors in three-dimensional geometry can eliminate changes in the polarization. Calibration procedures for a dual photoelastic modulator and potential error sources such as misalignment of analyzer, signal dc offset, and neglect of aperture size are presented. Mirror-surface evolution and how it can disturb the polarization measurement are also addressed.
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| 2. |
- Kuldkepp, Matias, et al.
(author)
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First mirror contamination studies for polarimetry motional Stark effect measurements for ITER
- 2004
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In: Review of Scientific Instruments. - : AIP Publishing. - 0034-6748 .- 1089-7623. ; 75, s. 3446-3448
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Journal article (peer-reviewed)abstract
- The motional Stark effect (MSE) diagnostic on the International Thermonuclear Experimental Reactor will need to guide the light through a labyrinth of mirrors to provide neutron shielding. Knowledge of how the mirrors change the polarization is essential for accurate determination of the q profile. The optical properties of the plasma facing mirror are also expected to change with time due to deposition/erosion. For the purpose of examining this experimentally a detector system, identical to the JET MSE system, using twin photoelastic modulators was constructed. Measurements have been performed on freshly prepared mirrors, on mirrors after exposure to plasmas in Tore Supra, and labyrinth designs. The result shows a significant effect on the optical properties and demonstrate the need for in situ monitoring. The measured properties of the labyrinth closely follow the Mueller matrix formalism. With a correct choice of material the angle change introduced by the four mirrors furthest away from the plasma will be below 1 degrees.
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| 3. |
- Kuldkepp, Matias, et al.
(author)
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Motional Stark effect diagnostic pilot experiment for MAST
- 2006
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In: Review of Scientific Instruments. - : AIP Publishing. - 0034-6748 .- 1089-7623. ; 77:10, s. 10E905-
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Journal article (peer-reviewed)abstract
- Exploiting the motional Stark effect (MSE) in the low magnetic fields of spherical tokamaks such as MAST is complicated by the Doppler smearing of the relatively closely spaced Stark components. Extensive modeling of MSE spectra and the subsequent polarized fraction (similar to 20%) of spectrally filtered light and signal to noice ratios have been performed taking account of real experimental conditions including neutral beam parameters, port sizes, optical losses, filter characteristics, etc. A design is selected which uses high throughput interference filters (0.1 nm bandpass ) for separation of the spectral components. An accuracy of similar to 0.5 degrees S compared with typically 15 degrees is estimated for field angle measurements. The design allows for early implementation, starting with a pilot two chord system, and for an economic expansion to a multiplicity of chords. Matching the Doppler shifted D-alpha from the beam neutrals will be accomplished by a combination of filter selection and fine-tuning of the beam voltage. Avoiding filter tuning in the design greatly simplifies the diagnostic. Calibration results of the diagnostic support the calculations.
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