Internal disruptions in Heliotron E*

In Heliotron E [K. Uo, Nucl. Fusion 25, 1243 (1985)] shifted-in vacuum magnetic field configuration, the q profile varies from just above 2 at the magnetic axis to 0.4 at the plasma edge. For low-β plasmas, resistive interchange modes are the dominant low-n instabilities at the plasma core. They saturate at low fluctuation levels. Above a threshold value, the ideal m/n=2/1 modes become unstable. They can be resonant or nonresonant modes depending on the value of q(0). In either case, their nonlinear evolution leads to a sudden crash of the pressure within the r/a=0.3 radius (sawtooth oscillation). When beta is increased further, the q=2 surface moves out of the plasma, and the ideal m/n=2/1 modes are effectively stabilized when q(0)<1.85. As a result, the sawtooth oscillations are suppressed.