Fig. 2-1 Measurement of plasma current density profile using the motional stark effect (MSE) A schematic layout of the MSE measurement in JT-60 is shown. Beam atoms injected into a plasma are excited through collisions with ions and electrons in the plasma, emitting radiation of light. The emitted light is polarized due to the induced electric field that arises from the motion of beam atoms across the tokamak magnetic field. The local direction of the magnetic field, a quantity related to the plasma current distribution, is deduced from the measurement of the polarization of a spectral line. We can measure the polarization of the emitted light at fourteen points in the plasma simultaneously in JT-60.

Fig. 2-2 Plasma current waveform and time variation of current density profile in JT-60 While the measured current density profile is generally bell-shaped, a hollow profile at the plasma center is observed at early times (3.6 s for example) of the discharge.

The plasma current flowing in a tokamak such as JT-60 is one of the essential physical quantities that determine the basic plasma performance. To study stabilities and confinement properties of a tokamak plasma, it is necessary to know not only the magnitude of the plasma current but also its detailed distribution in the plasma. A diagnostic method of current density distribution (profile) based on the "motional stark effect" has recently been developed. The method is to measure the polarization of light emitted by energetic hydrogenic or deuterium atomic beams injected into the plasma to heat it up to a high temperature. The energetic atoms feel an induced electric field as they move across the tokamak magnetic field, and the induced electric field causes splitting and polarization of the spectral lines of the light emitted by them. The polarization is a physical phenomenon in which the direction of oscillations of light waves is biased to a certain specific direction. By measuring the direction of polarization, we can find a direction of the induced electric field. Since the velocity of the beam atoms and the tokamak magnetic field are known to us, it is possible to deduce the local magnetic field produced by the plasma current and hence its distribution (see Fig. 2-1). Figure 2-2 shows an example of the current density distribution as a function of plasma radius obtained in JT-60. We have improved the wavelength detection system of the emitted light and have attained a high precision in the measurement of the polarization angle. This diagnostic technique is now established and has proved to be indispensable for the detailed analysis of the effect of the current distribution on the plasma confinement, currently one of the most important subjects in tokamak fusion research.

Reference T. Fujita et al., Current Profile Measurements with MSE Polarimeter in JT-60U, Seventh International Toki Conference on Plasma Physics and Controlled Nuclear Fusion, Toki Japan (1995), Nucl. Eng. Des., 34-35, 289 (1997).

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