Higher beta at higher elongation in the DIII-D tokamak

Abstract

A theoretical and experimental evaluation of axisymmetric stability and axisymmetric control has led to a modification of the vertical position control in the DIII-D tokamak, which now allows operation to within a few percent of the ideal magnetohydrodynamic (MHD) n=0 limit. It is found that the onset the departure from rigid shift behavior in D-shaped plasmas limits plasma elongation to 2.5 in DIII-D. The possibility of avoiding the vertical instability in future tokamaks with highly elongated plasmas is discussed. Recent experiments have focused on utilizing this capability for axisymmetric control to construct plasma shapes optimized to increase the achievable beta. Operation near the axisymmetric stability limit allows an increase in the achieved normalized current Ip/aBT, where Ip is the total plasma current, a is the minor radius, and BT is the toroidal field. Based on stability calculations, an equilibrium was developed to achieve marginal stability simultaneously to axisymmetric, kink, and ballooning instabilities. In the experiment, the shape was altered to higher elongation during the high-beta phase as the current profile broadened. A record high beta for DIII-D of 11% was achieved. The high-beta phase of the discharge lasted 40 msec, approximately one confinement time.