FORMATION AND DECAY OF A SPHEROMAK PLASMA

The magnetic properties of the spheromak configuration produced by a c ombination of slow theta and Z discharges in the University of Marylan d Spheromak experiment (MS) are reported. The magnetic structure of th e plasma in MS has been mapped out by arrays of passive magnetic picku p coils. The Taylor relaxation process is observed during the formatio n phase. The magnetic profile evolves in such a way that the ratio of poloidal current I(p) to poloidal flux psi in the plasma approaches a constant value, where mu0I(p) = k(el)psi. When the spheromak is formed , the magnetic field configuration is close to Taylor's minimum energy state, mu0j = kB. This constant k is related to the size of the spher omak 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 (6-8 X 10(20) m-3 ) and the presence of low-Z impu rities (mainly carbon and oxygen), the plasma is radiation dominated w ith electron temperature less-than-or-equal-to 15 eV. The magnetic fie ld decays exponentially during the decay phase. Axisymmetric equilibri um states that could exist in the configuration are calculated with a Grad-Shafranov equilibrium code. Comparison of the numerical calculati on with the experimental measurements indicates that the magnetic-fiel d structure stays close to the equilibrium state as the plasma decays.