Topological bifurcations of the magnetic axis and the alternating-hyperbolic sawtooth C. B. Smiet 1,2 G. J. Kramer 1 S. R. Hudson 1 • When q 0 =2/3 the structure of the field can transition to an alternating-hyperbolic configuration. • In equilibria where q 0 ∼ 2/3 there is a strong ideal mode located on the axis. • The displacement associated with this mode drives the topological transition and causes magnetic chaos around the axis. • Sawteeth observed around q 0 ∼ 0.7 could thus be explained by this mode when q 0 reaches 2/3, and the stochastisation drives rapid reconnection that resets the cycle. 1 Princeton Plasma Physics Laboratory, Princeton, USA 2 Renaissance Fusion, Grenoble, France