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H. Li (LANL), R.V.E. Lovelace (Cornell), S.A. Colgate (LANL)
Powerful narrow jets from many active galaxies might arise from twisting magnetic fields threading a differentially rotating accretion disk around a black hole. We model the formation of astrophysical jets by numerically solving the Grad-Shafranov equation with axisymmetry. An initial flux distribution is set up on a Keplerian disk, which is regarded as perfectly conducting and massive. We use the differential rotation of the disk as the input control parameter and calculate the current on each flux surface self-consistently. Since the greatest winding of the flux surfaces takes place at the smallest radius, we show that most of the magnetic energy is concentrated near the disk axis, resulting in the formation and strong collimation of force-free magnetic helix. Furthermore, we show that the increasing differential winding gives an axially growing magnetic tower. We discuss how these results might explain the distribution of magnetic fields in galaxy clusters. (Supported by the DOE.)