Stability of Small Aspect Ratio Toroidal Hybrid Devices

The stability properties of small aspect ratio toroidal hybrid (SMARTH) devices are investigated. SMARTH configurations are a hybrid between a stellarator and a tokamak: the poloidal field is provided by both external modulated coils as well as plasma current. Initial studies of these devices focused on configurations with a simple set of modulated circular toroidal field coils.(B. A. Carreras, V. E. Lynch, and A. S. Ware, ORNL/TM-13252, 1996) These configurations have a magnetic well throughout the plasma volume and β limits for these devices are set by ideal ballooning modes. Here, we focus on the effect of finite plasma current on the ideal ballooning stability limit for SMARTH configurations. Preliminary results indicate that adding plasma current raises the stability β threshhold. More recent investigation have focused on configurations optimized solely for maximal particle confinement.(D. A. Spong, et al., 1997 Sherwood Theory Meeting, Poster 3C08) Confinement optimization, however, was deleterious to stability. We will report on progress to include a `stability criteria' in further confinement optimization studies.