Transport Topical Group (TTG) under EFDA

The main aim of fusion research is to confine fusion plasma at high temperatures and at a high enough density whilst still ensuring sufficient insulation. The quality of the confinement is directly connected to transport which describes the motion of particles, momentum, and heat in the plasma. Transport in a radial direction from the centre to the periphery of the plasma is particularly important as this causes an unwanted loss of energy and particles. The physical understanding and the quantitative prediction of these transport processes play a crucial role within fusion research and in the design and operation of present and future fusion devices. EFDA established the EFDA Transport Topical Group to coordinate research work in this very important field.

Transport mechanisms

Larmor movement: Inside the plasma, particles follow the magnetic field lines and gyrate around them

A good method that can be used to explain transport phenomena is to look at particle transport. Heat or energy transport can be described in a similar way. Magnetic fusion devices create magnetic fields to confine plasma particles thus preventing them from touching the wall of the machine. Inside the plasma, the particles follow the magnetic field lines spiralling or gyrating around them (Larmor movement).

In the interior region of the plasma, these field lines are “closed”, they run around the doughnut shaped device in never ending loops. Near to the plasma edge, the field lines are “open”, meaning that they eventually intersect with the vessel walls. Particles on these field lines are lost to the plasma and will hit the wall. Rather than following magnetic field lines, which is called parallel transport, the particles can also start out in the “safe” and confined plasma core, and move out to the edge. This movement is called perpendicular transport since it is perpendicular to the magnetic field lines. To ensure confinement those plasma particles, which have not yet been involved in fusion reactions, must be prevented from exiting the inner plasma region.

There are several mechanisms by which these particles can move across the field lines. Firstly, collisions can cause transport. Plasma particles which gyrate around the field lines, collide with other particles, deviate from their original trajectory and start gyrating around other field lines.

The structure of the magnetic field is another possible cause of transport. For example, the strength of the magnetic field in the fusion devices decreases from the inner to the outer region and the field lines are bent as a result of the shape of the machines. This results in transport effects which can nevertheless be balanced by the helical magnetic structure of the devices. While the classical model describes transport based on the Larmor movement of the plasma particles and collisions between them, the so-called neoclassical theory also considers the geometrical considerations mentioned above.

Carlos Hidalgo received his PhD degree from the Madrid Complutense University in 1984 with his PhD work on structural defects in solids and positron annihilation spectroscopy.

His next area of research was related to plasma turbulence, transport and diagnostics in CIEMAT, where he is currently leading the Experimental Plasma Physics Division. He has worked in different international laboratories, initially as a PhD student (Technical University of Denmark, Nuclear Research Centre of Grenoble, Technical University of Helsinki) and later as a visiting scientist (Fusion Centre at Austin, Oak Ridge National Laboratory, JET, Max Planck Institute). Teaching experience, including his present involvement at the Plasma Physics and Nuclear Fusion EM Master, complements his research activity.

He was head of the European Transport Task Force on Turbulence between 1994 and 2000. And he has been head of the EPS Plasma Physics Division since 2008.

The neoclassical model is an effective tool that can be used to explain many of the phenomena taking place in fusion plasmas. Transport processes, for instance, which take place in the direction parallel to the magnetic field lines, are well described. However, in magnetic fusion devices, the observed perpendicular transport is significantly higher than estimations based on neoclassical transport theory. In particular, electron heat transport is measured at two orders of magnitude larger than the neoclassical predictions while ion heat transport is up to one order of magnitude larger. For many decades, this difference has been referred to as “anomalous” transport.

Computer simulation of microturbulence in tokamak plasma. The different colours correspond to different densities. Vortexes are aligned with the magnetic field lines and follow their twist. Image: J. Candy, R. Waltz, General Atomics

In the last two decades, increasing scientific research has been devoted to understand the main sources of this anomalous transport, both experimentally and theoretically. Attention has turned to the instabilities which create turbulence and thus contribute to transport. Temperature, density, and other plasma parameters are not uniform in the direction perpendicular to the magnetic field lines, but instead they have radial gradients. It has been found that, as a consequence of these gradients, instabilities can occur in the plasma, at both the ion Larmor radius as well as the electron Larmor radius scales. The scales are caused by the considerable difference in mass between deuterium, tritium nuclei and electrons. For deuterium ions and electrons, for example, these scales differ by a factor of 60.

The main physical process leading to instability arises from the fact that the drift motion of the particles depends on their velocity. Therefore, any fluctuation in temperature simultaneously produces a fluctuation in density. The density fluctuation causes a fluctuation in charge and thus a fluctuation in the electrostatic potential. This generates an electric field which introduces an additional transport and can enhance the initial perturbation, leading to an instability in the presence of a background gradient of the temperature. These micro-instabilities lead to the development of a turbulent, an eddy-like chaotic and stochastic, state in the plasma, in which fluctuating electromagnetic fields generate radial transport of particles, momentum and heat. Plasma physicists realised in the 80’s that turbulence processes at very small scales can be responsible for anomalous transport. Hence, the prediction of transport in a fusion plasma requires an understanding of the crucial aspects of plasma turbulence.

There are several models designed to describe these turbulent processes. All of them involve extensive calculations, thus, as a result of limited computer power, the models are mostly only solved under certain boundary conditions which restrict the number of unknown parameters. The different scales produce one of the most challenging problems in the present attempts to simulate plasma turbulence. A simplified description of transport in the core of the plasma by means of local quasi-linear theory has been found to be successful. Moving towards the periphery of the plasma the problem increases in complexity. In this extremely complex region at the edge of the plasma, the most important experimental observations remain even to be qualitatively explained and understood. So far, scientists are not able to describe transport phenomena across the entire plasma. The main issue open today is writing a code which calculates the entire plasma from the centre to the edge.

The question of how turbulent transport can be reduced and controlled during the plasma operation by reliable external means remains one of the most challenging open problems. The production of regimes of improved confinement is of extreme importance in present research on magnetic fusion, particularly in view of optimising the efficiency of a fusion reactor.

A practical example

One of the many practical aims of transport research is the determination of the power threshold of H-mode. Today, scientists assume that fusion devices will be operated in the so-called H-mode, at which the confinement time of the plasma is 100% longer than in normal L-mode. H-mode is characterised by a sharp temperature gradient near the plasma edge (the temperature pedestal) and an associated pressure gradient called the transport barrier. The transition from L-Mode into H-Mode is spontaneous and occurs when the heat flux exceeds a certain threshold. This power threshold rises with the plasma density or the magnetic field, or with the size of the machine. Both the heating power above which the transition from L- to H- mode occurs as well as the pressure produced at the top of the edge barrier, can thus far only be predicted by means of empirical scaling based on present experimental observations which still retain large uncertainties. Understanding the H-mode is a major challenge for fusion physicists.

Considerations like these are the basis for the specific tasks which the EFDA Transport Topical group has set out.

Clarisse Bourdelle did her PhD at CEA Cadarache between 1997 and 2000. Her work, under the supervision of Xavier Garbet and Tuong Hoang, was on the linear stability analysis of improved confinement discharges: internal transport barriers in Tore Supra and radiative improved mode in TEXTOR. After her PhD, she spent 2 years at Princeton working on NSTX low aspect ratio, high beta plasmas transport understanding and collaborating with Ed Synakowski and Bill Dorland. Since 2002, she has been working at CEA Cadarache on transport, at the interface between theory and experience. Her latest work is on quasi-linear transport modelling for energy, particleobtained his PhD degree at CRPP, EPFL Lausanne (CH) at the end of 2001, under the supervision of Olivier Sauter. During the PhD, his research was dedicated to neoclassical transport, electron heat transport modelling of TCV plasmas as well as the integrated modelling of sawtooth oscillation in both TCV and JET plasmas.

The role of the EFDA Transport Topical
Group (TTG)

The investigators of transport phenomena in a plasma work with many unknown quantities. There are indeed too many variables to be calculated, unknown parameters to be defined, or effects of known parameters to be understood. Because of the openness of the task, the different scientific groups use a variety of approaches. The TTG addresses these open scientific questions with the practical challenge of developing the knowledge to design and optimise a fusion reactor. The TTG provides an overview over the different approaches, results, models and experiments. It provides a common framework, monitors the scientific activities in the area of transport, encourages cooperation between the individual groups whenever it looks promising and provides a platform for all transport investigators to discuss their ideas. The creation of specific task oriented research projects will be a key element in the TTG activities when there is a need to focus efforts on a specific question. The group will also provide an interface, at the European level, to other connected fields by means of the existing European collaborations under EFDA. On an international level, the TTG is an interface to work on transport worldwide, and in particular, to the existing and future ITPA groups and the US Transport Task Force. Carlos Hidalgo chairs the TTG with Clemente Angioni and Clarisse Bourdelle as vice-chairs.

Within the EFDA Transport Topical Group, both the requirements of the experimental observation and identification of basic plasma behaviours and the related understanding in terms of first principle theory are considered concurrently. The TTG takes into account longterm challenges as well as short-term answers to specific open questions relevant for ITER. The successful development of TTG Task oriented research projects will require an improvement in the diagnostics development and implementation on the EU devices, as well as strong links with modelling/theory activities.

Clemente Angioni obtained his PhD degree at CRPP, EPFL Lausanne (CH) at the end of 2001, under the supervision of Olivier Sauter. During the PhD, his research was dedicated to neoclassical transport, electron heat transport modelling of TCV plasmas as well as the integrated modelling of sawtooth oscillation in both TCV and JET plasmas. After his PhD, he did his Post Doc on experimental and theoretical aspects of particle and heat transport at IPP Garching with a Marie Curie Individual Fellowship supervised by A.G. Peeters.

Since 2004, he has been working as staff scientist in the theory division at IPP Garching

The TTG research programme for 2008/09 has been organized on the basis of four research areas:

  • Edge and scrape off layer (SO) transport physics
  • Core heat and particle transport
  • Core and edge momentum transport
  • MHD and fast particles interactions with transport

A pilot TTG research project was started on April 2008 on “Long-range correlations and transport barrier physics”, which is coordinated by Carlos Hidalgo. It is intended to address the question of understanding and determining the H-Mode power threshold as discussed above. The project is evolving successfully and showing promising results. Another three projects were subsequently launched in 2008. The first TTG Meeting was held in Denmark in September 2008. The second meeting will be held at JET on 16–18 September 2009.

The chairs and vice-chairs of TTG supported the work on this article