Equilibrium and stability measurements in a high beta tokamak

The Columbia High Beta Tokamak (HBT) utilizes fast formation techniques to produce plasmas capable of testing theoretically predicted equilibrium and stability boundaries. At very high values of poloidal beta (εβ_p ≳ 1.7), internal magnetic diagnostics show the encroachment of an inboard separatrix followed by rapid loss of confinement, suggesting the existence of an equilibrium limit. Plasmas formed at lower β_p are observed to become unstable at values of q* and β in accordance with the Kruskal-Shafranov and Troyon relations. Internal and external magnetic field measurements identify the modes as long wavelength external kinks that grow on an Alfven time scale. Ideal MHD numerical simulations reproduce qualitative features of the mode structure, but predict slower growth rates for q* > 1 plasmas. A description of HBT-EP is given. This tokamak is designed to test the effects of an adjustable conducting shell on the external mode stability of a longer-pulse length, higher-temperature plasma.