AIP Digital Archive
In order to obtain a detailed density profile of a field-reversed configuration (FRC) plasma, fast-response multichannel heterodyne quadrature interferometers are constructed. Using these interferometers and assuming a rigid-body radial shift motion of the plasma, a spatially fine-grained line integrated density (∫ n dl) profile at its axial midplane is measured. A radial density profile n(r) is reduced from spline fitting of ∫ n dl. The n(r) is found to be nearly an even function of u(=r2/R2−1, R is the magnetic axis radius) as expected. The n(r) is also obtained by the fitting of a line integral of a model n(r) consisting of a modified rigid rotor (RR) profile which can describe the density steepening near the separatrix of the FRC plasma. When the plasma is fat (xs =separatrix radius/coil inner radius=0.63), the density profile is very near to the RR profile itself given by sech2 (Ku), where K is a constant. When the plasma is slender (xs =0.43), the modification is somewhat pronounced. In both cases n(r) at r=R is flatter but near to the RR profile, and the scale length of the density gradient at the separatrix is about twice the ion gyroradius. Detailed error analyses of the fitting parameters are done to show the range of allowed profiles. Although the fitting is accomplished very well (root-mean-square excursion of the fitted ∫ n dl from the measured one is from 1.9% to 2.5%), much variation of n(r) is still possible.
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