Stabilization and translation of a plasma with field-reversed configuration

A theoretical analysis and its experimental test are carried out on multipole-field suppression of the n=2 rotational instability. The theoretical analysis based on the MHD approximation gives a stability criterion for quadrupole field, Bsub(s)>=Bsub(sc)=1/2rsub(s)|Ω|√μsub(0)rhosub(0)-bar, where rsub(s), rhosub(0), and Ω are the radius, the mass density and the rotational angular velocity of the plasma column, respectively, Bsub(s) is the field strength at r=rsub(s) without a plasma column present, Bsub(sc) is its threshold, and μsub(0) is the vacuum permeability. The experiments show that the experimental stability threshold obeys the scaling rsub(s)|Ω|√rhosub(0)-bar, but is about 25 to 30% of the theoretical value. The n=2 rotational instability is also suppressed by using helical quadrupole fields produced by the helical winding. For a perturbation uniform in the z-direction, the stabilization effect of the helical field is not uniform along the z-axis so that there is always a region in the plasma column where the stabilization effect is large. Therefore, the rotational instability is considered to be more effectively suppressed by using a helical field. Experiments show that the stabilization threshold current J supplied to the helical winding decreases with the pitch angle α(=2π/L; L is the pitch length of the winding). Typically, the current J in α=6 is less than a fifth of that in α=0 (straight quadrupole field). To examine the possibility of extending the lifetime of a field-reversed-configuration (FRC) plasma by increasing the ratio xsub(s) of the plasma radius to the conducting-wall radius, experiments are carried out on the translation of the FRC plasma from the formation region into the confinement region (metal vessel) with mirror field. The ratio xsub(s) increases up to 0.7 and the lifetime of the translated plasma is 80 μs, while the ratio xsub(s) and the lifetime are 0.4 and 30 μs, respectively, when the FRC plasma is confined in the formation region. (author)