A period-one unstable periodic orbit (UPO) is identified in the inter–edge-localized-mode (ELM) intervals of a high-performance quasisteady state discharge of JT-60U using UPO identification based on their recurrence properties and the so-called fixed point transform. This result is tested by use of surrogate data sets. This diagnostics invariant interpretation of the ELM confinement state gives detailed and applicable quantitative information since the UPOs in general form the skeleton of dynamical invariants and have in numerous systems been shown to be controllable by weak parameter perturbations.

• Received 6 January 1999

DOI:https://doi.org/10.1103/PhysRevLett.83.1339