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  • American Institute of Physics (AIP)  (2)
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  • 1
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Electrical resistivity at the magnetic axis η(R) and at the separatrix η(rs) of a field-reversed configuration (FRC) plasma produced by theta pinch machines are estimated from detailed interferometric measurements, assuming that the temperature is uniform within the separatrix. The ratio f0[=η(R)/η(rs)] increases with the beta value at the separatrix βs, which is consistent with the fact that f0 decreases when xs (separatrix radius normalized by the inner radius of a theta pinch coil) increases [Phys. Fluids 28, 888 (1985)]. This tendency is a natural consequence of the transport properties of the FRC plasma when the particle confinement time is nearly equal to the decay time of the trapped reversed magnetic flux, as is normally the case. Theoretical expectations of the anomaly of η(rs) over the classical resistivity η0 increases and decreases slightly with βs for the case of the lower hybrid drift and the low-frequency drift instability, respectively. On the other hand, the observed η(rs)/η0 decreases substantially with βs. [The value of η(rs)/η0 changes from about 15 to 5 as βs changes from 0.45 to 0.6.]
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: 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.
    Type of Medium: Electronic Resource
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