Stable high beta spheromak equilibria using concave flux conservers

Ideal magnetohydrodynamic stability to the n=1 and n=2 modes are calculated for spheromak equilibria in flux conserver shapes, which include a midplan e gap, conforming outer wall, and concave side walls. The equilibria are fo rce-free (del xB=lambda(psi)B) but not minimum energy states and, therefore , have nonuniform lambda(psi) profiles. For each flux conserver shape, the equilibrium with the most hollow linear lambda(psi) profile is found that i s stable to the n=1 and n=2 modes. The Mercier beta limit is calculated for each flux conserver shape using the most hollow current profile constraine d by the stability boundary. The results show that a stable high <beta > (> 10%) spheromak equilibrium can be produced using a concave flux conserver, improving energy confinement times and plasma performance. (C) 2000 Americ an Institute of Physics. [S1070-664X(00)04707-8].