More than 50 scientists from 10 countries came to Jülich (Germany) between March the 14th and 17th 2005 to discuss recent progress in the understanding of stochastic phenomena in nuclear fusion. The series of international workshops on “Stochasticity in Fusion Plasmas” (SFP) has been initiated by the Research Centre Jülich Association in October 2003 because of growing interest in the effects of magnetic field ergodisation within the magnetic confinement fusion community. TEXTOR’s Dynamic Ergodic Divertor (DED) is a pioneering experiment in this context.

In particular the role of ergodic magnetic fields for transport and stability is increasingly recognised. This is not only triggered by the hope to develop new methods for particle and heat exhaust in fusion reactors. In addition, a better understanding of transport around the stochastic tips (in jargon “tangles”) of magnetic islands due to tearing mode instabilities is strongly desired. One of the most prominent results from stochasticity research so far is that edge ergodisation can reduce or even suppress large type I ELMs – which is a very beneficial development for next generation devices. Respective experiments have been carried out at the DIII-D tokamak, and it is currently being investigated if and how far the physical findings can be transferred to JET and ITER.

Many contributions proved the importance of ergodisation not only for specific tokamaks but also for stellarators. Specially designed experiments have shown the chaotic transport of particle fluxes in the boundary layer of several fusion machines and the importance of ergodisation for disruptions. A major fraction of the presentations was devoted to the new results obtained from TEXTOR’s Dynamic Ergodic Divertor – covering divertor properties, transport in ergodic magnetic fields and the excitation of modes by externally imposed field perturbations.

The workshop has addressed these three topics:
• mitigation of ELMs by ergodisation of the edge magnetic field lines,
• experimental results from ergodised plasmas, and
• theory development for ergodic systems.

As the ergodised structures are of a three dimensional nature, advanced 3-D transport codes had to be developed which are based on Monte Carlo solution techniques for MHD equations. These codes have been applied successfully both to tokamaks and island divertors of stellarators. For the study of the nonlinear dynamics in ergodic systems new mapping techniques have been introduced. The mappings concern both the tracking of the magnetic field lines and that of the particle orbits. Another subject of interest was the penetration of external magnetic perturbation fields into the plasma – there leading to the excitation of tearing modes and to a force transfer from the external system to the plasma.

The next SFP workshop will be held in Jülich in 2007.
More information can be found at http://www.fzjuelich. de/sfp/, including all talks and contributions from the participants.