The effects of relativistic down shift frequency on the determination of the electron temperature profile from electron cyclotron emission (ECE) in a large helical device(LHD) have been studied by a numerical calculation, since the value of toroidal magnetic field has maximum at the magnetic axis, along the sight line in the LHD, the emissivity at the magnetic axis decreases due to the relativistic effect. We found an apparent drop in electron temperature around the magnetic axis due to the relativistic effect. The parametric dependences of the relativistic effect on electron temperature measurement have been studied. When the electron temperature is higher and/or optical thickness is thinner, the gap from the nonrelativistic electron cyclotron frequency is wider. In the case of Te(0)=5 keV and ne= 1 × 1019 m -3, the frequency gap is about 0.5 GHz. Calculation shows that the electron temperature at the central region (ρ<0.1) apparently decreases less than one-half even at 5 keV, in the case of ne=1 × 10 19 m-3 due to the its unique magnetic configuration. Therefore, it is necessary for precise measurement of the electron temperature profile to take into account the relativistic downshift frequency in LHD plasma. To verify the relativistic effect on electron temperature measurement from ECE in the LHD experimentally, the parameter region is evaluated numerically. It is possible to verify the relativistic effect in LHD experimentally.
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