Impact of 3D Magnetic Fields on Hot Plasmas

643. WE-Heraeus-Seminar

22 May - 24 May 2017


Physikzentrum Bad Honnef

Scientific organizers:

Prof. Y. Liang, FZ Jülich • Prof. T. Sunn Pedersen, MPP Greifswald • Prof. O. Schmitz, U Wisconsin/USA • Prof. Sibylle Günter, MPP Garching

At present, the world relies predominantly on fossil fuels as our primary energy source. These fossil fuels represent a limited resource and are responsible for the emission of large quantities of carbon dioxide into the atmosphere, leading to potentially catastrophic climate change. Nuclear fusion, the reaction that powers stars, has the potential to provide mankind with a safe, environmentally friendly and virtually limitless supply of energy. Over the last six decades, research into controlled nuclear fusion through the magnetic confinement of hot plasmas has been pursued in many countries with the long-term aim of providing a practically inexhaustible, safe and environmentally friendly energy source, and considerable progress has been made towards this goal. The rate of the increase in the fusion product of density, temperature and confinement time has matched Moore’s law for the density of transistors on microchips.

To date, the most advanced concepts for fusion confinement are the tokamak and the stellarator. The latter is exemplified by the Wendelstein 7-X (W7-X) stellarator in Greifswald, Germany, which has recently completed its operational phase 1.1. The tokamak scheme is currently the leading variant for plasma confinement and is therefore being used for the next-generation device ITER and is planned for the DEMO reactor. However, reliably achieving and maintaining the stability of fusion plasmas remains an important fundamental issue that needs to be resolved.

In both tokamaks and stellarators, stochastic magnetic fields can arise and influence the interplay between three-dimensional (3D) magnetic topology and plasma confinement. Stellarator devices represent an inherent three-dimensional challenge. They make use of the island divertor concept, and stochasticity and magnetic topology therefore play a fundamental role in their operation. With the extended operational regimes pioneered on the Large Helical Device (LHD) and with W7-X, attention has been directed towards the challenge of 3D plasma equlibria, transport and plasma–surface interactions.

In the tokamak line, non-axisymmetric magnetic perturbations, which change the magnetic topology, are applied on the majority of large-scale tokamaks nowadays to control plasma edge stability and transport. Recent research has highlighted the significance of the role that stochasticity and 3D magnetic topology also play in this fundamentally 2D concept. Their influence can be seen in transport and energy confinement, in the nature of disruption events and in the control of various magnetohydrodynamic (MHD) instabilities, most notably edge-localized modes (ELMs), which expel considerable amounts of energy from the plasma and pose a risk of damaging plasma–facing components in ITER and other next-generation fusion devices.

The existence of these stochastic and 3D effects brings tokamak and stellarator physics closer together, and a holistic approach to studying them provides the most promising path to making good progress. Understanding these effects is essential for the success of future fusion devices, and they represent a hot topic in current fusion research. In addition, reversed field pinches offer access to these topics with unique features such as the bifurcation into self-generated 3D equilibria and multi-mode unstable plasma conditions with a high degree of magnetic field stochasticity. Joint discussions of these aspects across the three communities will foster progress on basic as well as applied understanding in these complex branches of high-temperature plasma physics. Therefore, it will be of great interest and scientific importance to share the most up-to-date theories and techniques and to provide a platform for discussion between leading experts in the field.

The 643th WEH Seminar is an attempt to discuss issues relating to impact of 3D magnetic fields on hot plasmas from all sides, bringing together experts from different devices (tokamaks, stellarators and reversed-field pinches) and from different fields (equilibrium and confinement, turbulence, MHD instabilities, transport and plasma–wall interactions). It will build upon the success of the 480th WEH Seminar entitled Active Control of MHD Stability (June 2011), the 531st WEH Seminar entitled versus 2D in Hot Plasmas (April–May 2013) and the 597th WEH Seminar entitled Stochasticity in Fusion Plasmas (September 2015). These previous seminars focused on integrating experimental methods of controlling MHD instabilities, on comparing the relative merits of 2D and 3D conceptualizations of hot plasmas, and on the underlying physics of stochastic effects. In contrast, the focus of this seminar will be on the latest experimental results from stellarators and from tokamaks with the capability of applying 3D magnetic perturbations. In particular, the seminar should be well timed to cover the first W7-X experiments with the new island divertor, the first LHD experiments using deuterium, and possibly also the first experiments from the spherical tokamak MAST-U after its comprehensive upgrade. This will naturally lead to a comparison of W7-X’s 3D island divertor with various different axisymmetric divertor configurations under the application 3D fields being a highlight of the seminar.

The success of these three previous WEH Seminars is exemplified by the publication of associated special issuees of the journal Nuclear Fusion. The 597th WEH Seminar absorbed the International Workshop on Stochasticity in Fusion Plasmas, which ran successfully for over a decade and had established itself as the foremost international workshop in this field. This brought the number of participants to 67, surpassing the preceding workshops, and we expect a similar number for the next seminar.

Discussing together and summarizing recent approaches will improve the physics understanding of various effects of 3D fields in magnetically confined plasmas. Analysing the influence of stochasticity and magnetic topology in fusion plasmas will be beneficial for research in the field and will guide the design of future fusion devices. A further major goal of this seminar is to give young scientists the opportunity to enter an active and growing field of research by interacting with world-leading experts.