Plasma Physics Seminar ( Phys 769)

Dr. Carlos Romero Talamas, LLNL Measurements and phenomenological modeling of magnetic flux buildup in spheromak plasmas Measurements of spheromak formation and evolution at the Sustained Spheromak Physics Experiment [E. B. Hooper, L. D. Pearlstein, R. H. Bulmer, Nucl. Fusion 39, 863 (1999)] are conducted using a variety of experimental and numerical tools. These are aimed at understanding the processes by which currents and magnetic fields in the plasma self-organize to form the spheromak, an emerging candidate for the magnetic confinement of fusion energy plasmas. The measurements are analyzed in the context of a phenomenological model of magnetic helicity based on the topological constraint of minimum helicity in the open flux (i.e., intercepting the chamber walls), before reconnecting to form linked closed flux. Two stages are analyzed: (i) the initial spheromak formation, i. e. when all flux surfaces are initially open and reconnect to form closed flux in the toroidal average sense, and (ii) the stepwise increase of closed flux when operating the gun on a new mode that can apply a train of high-current pulses to the plasma. In the first stage, large kinks in the open flux surfaces areobserved in the high-speed images taken shortly after plasma breakdown, and coincide with large magnetic asymmetries recorded in a fixed insertable magnetic probe that spans the flux conserver radius. Closed flux appears shortly after this. This stage is also investigated using resistive magnetohydrodynamic simulations. In the second stage, a time lag in response between open and closed flux surfaces after each current pulse is interpreted as the time for the open flux to build helicity, before transferring it through magnetic reconnection to the closed flux. Large asymmetries are seen during these events, which then relax to a slowly decaying spheromak before the next pulse. Return to main page