New insights into using “the process that powers the stars”

Turbulence simulations, carried out in the framework of the PRACE Research Infrastructure, have provided new insights into the fundamental processes of turbulent transport. This research is contributing to the understanding and optimisation of magnetic confinement in fusion devices. Ultimately, the goal is to be able to make predictions for ITER, an international flagship fusion experiment being built in Southern France.

“The research conducted in our team is part of a world-wide effort to understand and predict turbulent transport in magnetically confined fusion plasmas. Our work is contributing to a better grasp of the underlying processes and helping to establish predictive and control capabilities,” tells Professor Frank Jenko at the Max Planck Institute for Plasma Physics (IPP) in Garching, Germany.Professor Jenko headed the team from several European research institutes, including the IPP, the Ecole Polytechnique Fédérale in Lausanne in Switzerland and the University of Oxford in England.

CO2 free energy

Nuclear fusion is a promising carbon-free option to meet the increasing global demand for electricity in the future. A world-wide consortium including the European Union is currently constructing the flagship fusion facility ITER, one of the most challenging scientific projects ever undertaken. Construction of the facility began in 2007 and the first plasma is expected in 2019.“We hope that ITER will, in a few years, help us to demonstrate the physical feasibility of power plants based on fusion which is the process that powers the stars. Given the predicted rise in energy demands over the next few decades, the world will need safe, carbon-free power plants. The fusion of magnetically confined hydrogen plasma at a temperature of 100 million degrees offers a very interesting way of meeting these challenges.”In order for ITER to achieve its main goal – the creation of a self-sustaining or “burning” plasma – the unavoidable heat losses induced by turbulent flows have to be kept in check.

Left: Snapshot from an ab initio simulation of the ITER-like fusion device ASDEX Upgrade with the sophisticated plasma turbulence code GENE.

Turbulence to be unravelled

Understanding turbulent flows and their behaviour is a challenging task. “Turbulence has often been called one of the most important unsolved problems of classical physics,” states Professor Jenko.Many famous physicists have attempted to tackle turbulence using analytical tools but with only limited success. Today, modern supercomputers offer a different approach to investigating these phenomena.“We can make progress in this area by state-of-the-art simulations on some of the world’s most powerful supercomputers. Simulations are especially useful for studying turbulence in hot plasmas, which must be described by means of five-dimensional kinetic models.”A single simulation aiming to be as physically comprehensive as possible could take several months or even a year, if run on medium-size computers. With PRACE offering the most powerful hardware available today in Europe, investigation of turbulence becomes feasible in a reasonable time. “We have performed plasma turbulence ab initio simulations with the sophisticated plasma turbulence GENE code on the JUGENE supercomputer, which is one of the Tier-0 systems of the pan-European Supercomputer Research Infrastructure created by PRACE. It lets us investigate important physical effects at a level that had not been possible before. These simulations are closely linked to the ITER experiment.”

Correlation between turbulent transport and size of device

“For the first time, physically comprehensive turbulence simulations of the fusion experiment device ASDEX Upgrade in Garching could be performed over time scales that are comparable to the duration of an actual discharge. ASDEX Upgrade is conceptually very similar to the ITER facility, although smaller by a factor of four in its linear dimensions,” Professor Jenko points out.A further set of simulations helped to develop a better understanding of how turbulent transport depends on the size of the device. “This is a crucial question because transport sets the energy confinement time, which is a key factor when determining the expected performance of the fusion device.”

Left: Professor Frank Jenko, Max Planck Institute for Plasma Physics

Ongoing work

“Following up on these successful simulation campaigns and building on the ongoing improvements in the employed software and hardware, the time seems right to finally tackle a long-standing enigma of plasma turbulence research. This is the formation of so-called transport barriers – radially extended regions in which the turbulent transport is suppressed by about an order of magnitude. This effect greatly improves plasma confinement and is therefore highly desirable. However, it currently still lacks a satisfying theoretical explanation, which is necessary for reliable predictions to ITER.”

© Minna Keinonen@PRACE

See also an article at ISGTW: http://www.isgtw.org/feature/small-…

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