Formation and decay of a spheromak plasma

The magnetic properties of the spheromak configuration produced by a combination of slow theta and Z discharges in the University of Maryland Spheromak experiment (MS) are reported. The magnetic structure of the plasma in MS has been mapped out by arrays of passive magnetic pickup coils. The Taylor relaxation process is observed during the formation phase. The magnetic profile evolves in such a way that the ratio of poloidal current I_p to poloidal flux ψ in the plasma approaches a constant value, where μ₀I_p=k_elψ. When the spheromak is formed, the magnetic field configuration is close to Taylor's minimum energy state, μ₀j=kB. This constant k is related to the size of the spheromak produced. A spheromak with 1.0 T maximum field, corresponding to 650 kA poloidal current, has been produced in MS. However, due to the high plasma density (6-8X10²⁰ m^-3) and the presence of low-Z impurities (mainly carbon and oxygen), the plasma is radiation dominated with electron temperature ≤15 eV. The magnetic field decays exponentially during the decay phase. Axisymmetric equilibrium states that could exist in the configuration are calculated with a Grad-Shafranov equilibrium code. Comparison of the numerical calculation with the experimental measurements indicates that the magnetic-field structure stays close to the equilibrium state as the plasma decays.