Plasma Wall Interaction Task Force

“The boundary edge is where the stellar world of hot plasmas meets the earthly world of cold solids. Understanding the complex interaction of these two worlds is essential for operating a fusion reactor successfully”. This is how Wojtek Fundamenski, leader of the Task Force dealing with plasma edge issues for JET, describes Plasma Wall Interaction.

In a fusion reactor, fusion takes place in the extremely hot (above 100 million °C) centre plasma, while the edge plasma continuously exhausts large heat and particle fluxes (in future reactors it will also exhaust helium produced by deuteriumtritium reactions). To control the impact on the wall, two types of magnetic configurations have been developed: Limiters and divertors, which limit the plasma interaction with material surfaces to dedicated strike areas. These components are able to handle significant power loads of up to 20 MW/m2 and are equipped with pumping systems that remove the particles. Despite the magnetic field confining the plasma, moderate heat and particle fluxes also impinge the vessel walls outside these dedicated strike areas. On top of these steady state loads, additional transient heat fluxes arise from Edge Localised Modes (ELMs), which occur in the improved confinement regime, the socalled H–mode (at which most modern tokamaks operate), and from anomalous events such as disruptions (see FN May 2009). ELMs generate periodic bursts of high heat and particle flux whereas during disruptions all the energy stored in the plasma is lost on very short time scales.

The basic processes


Heat flux patterns on the vessel walls during a disruption in JET. The red areas correspond to the plasma facing components heated by the disruption. The divertor is at the bottom of the vessel. (Photo: EFDA JET)

Particle and heat fluxes lead to the erosion of Plasma Facing Components (PFCs) by means of a number of processes:

• Physical sputtering: Incident particles eject atoms from the wall when their energy is above a given threshold.

• Chemical erosion: Wall materials react with plasma particles to form chemical compounds. For carbon based materials, this occurs even at low energy (no threshold), leading to the formation of hydrocarbons. There is no chemical erosion from tungsten PFCs.

• Melting: Metals melt and carbons sublimate when the heat flux exceeds the limits of the materials, for example, during transients such as ELMs and disruptions.

All these erosion processes determine the lifetime of the PFCs. But the PFCs take their revenge: Eroded impurities can travel into the plasma and degrade the fusion performance by diluting the fuel and cooling down the plasma core via radiative energy losses. These radiative losses are linked to different atomic physics processes, such as the excitation of the electrons of the impurities which take thermal energy away from the plasma. The excitation process continues until the impurities are fully stripped from their electrons. Light elements with few electrons – like carbon – are completely ionised in the plasma edge, and therefore do not radiate in the plasma core. For heavy elements like tungsten, however, this process continues in the plasma core. As a result some carbon may be tolerated in the plasma (in ITER up to a few percent), while tungsten levels must be kept drastically low (the levels of purity required for ITER are around 10-5, and have been achieved on ASDEXUpgrade, a European tokamak which is fully equipped with tungsten PFCs).

Material migration

After entering the plasma, the eroded impurities are eventually redeposited on the vessel walls, both close to and far away from their starting point, sometimes even in remote areas like gaps between tiles or pumping ducts. If they are still contained in an area of strong plasma wall interaction, they may be re-eroded and start another erosion – transport – redeposition cycle. If they are shadowed from the plasma, layers of redeposited materials may build up. If the vessel walls are made of various materials, mixed materials can form which may become a problem if these have degraded properties when compared to the original materials. Beryllium-tungsten alloys, for instance, have a lower melting temperature than pure tungsten.

Fuel retention

The build up of deposited layers also leads to fuel retention in the vessel walls as the deuterium or tritium fuel may be co-deposited along with the eroded materials. This is in particular true for carbon, which has a chemical affinity for hydrogen isotopes such as deuterium or tritium. While fuel retention is not a problem in present day tokamaks that are using deuterium plasmas, it would become an issue for next step devices, where the allowable in vessel tritium inventory is limited, in particular if carbon were used as the dominant PFC. This is the main driver for developing the use of tungsten PFCs in view of the deuterium-tritium phase of ITER.

Dust production

When the deposited layers reach a significant thickness they may flake and form dust. The impact of strong transient events like ELMs and disruptions on PFCs may also produce dust. This may lead to bursts of impurities penetrating the plasma and can become an operational difficulty. In next step devices, where dust may contain activated materials or tritium, the allowable in vessel dust inventory is limited.

After completing a Master in engineering at Cornell University (USA), Emmanuelle Tsitrone received her PhD in fusion plasma physics in 1995 at the Institute of Research on Magnetic Fusion (IRFM) of CEA. She then worked on experiments and modelling of a new concept for particle exhaust in tokamaks. After completing her PhD, she joined the Plasma Facing Components group of CEA as an officer responsible for designing and manufacturing the new pump system during the last upgrade of the Tore Supra tokamak. Once the device was installed in Tore Supra, she returned from technology to physics, taking part in the edge modelling project at JET, while specialising in the field of fuel retention, and became the deputy leader of the Plasma Edge Physics group in CEA. She is now coordinating research programmes in her field, in close collaboration with the ITER International Organisation, as the leader of the EFDA Task Force on Plasma Wall Interactions and the co-chair of the International Tokamak Physics Activities (ITPA) group which is investigating divertors and scrape-off-layers.


The challenges for ITER

In terms of plasma wall interactions, ITER is a significant scale up from today’s devices. A full power nominal shot of 400 s in ITER is roughly equivalent to one JET operational year in terms of energy input, and more like 3 – 4 years in terms of particle fluence in the divertor. Handling power flux to walls is a very challenging task. Specific plasma scenarios – based on impurity injection intended to spread the power by radiation from the plasma edge – have been developed to maintain the heat loads in an acceptable range for the PFCs (10 – 20 MW/m2 instead of 40 MW/m2 without radiation). The main challenges for ITER are:

• PFC lifetime: While handling the steady state loads should not constitute a major challenge, transient events like ELMs and disruptions are an issue if they remain unmitigated. Promising techniques to mitigate ELMs have been developed, for instance by perturbing the magnetic fields at the plasma edge, as on the US device DIII-D, and are contemplated for ITER. Several techniques are also developed to ensure that disruptions can be efficiently mitigated. Electron Cyclotron Resonant Heating (ECRH) is used to avoid disruptions by keeping the instability at stake under control. Massive Gas Injection (MGI) can mitigate the impact of disruptions on PFCs by spreading the power and preventing the formation of runaway electrons i.e. extremely highly energetic and potentially damaging electron beams. PWI research evaluates these measures by investigating their impact on the PFCs.

• Fuel retention: To keep within the ITER regulatory limit, the tritium inventory build up in ITER must be controlled. The build-up is extrapolated from present day machines and no serious inventory issues are foreseen if tungsten PFCs are used. Techniques for tritium removal and tritium inventory measurements are under active development since they could be needed in the case carbon PFCs are used during deuterium-tritium experiments.

•Dust production: Just as for fuel retention, research is being conducted to predict accurately dust build up. Dust removal techniques and dust inventory measurements are developed and tested as well.

Wall materials

In order to cope with all the constraints in a tokamak environment, a plasma facing material must have outstanding thermo-mechanical properties, a low erosion yield and a low fuel retention capability. It should have a minimum impact on fusion performance (no core plasma radiation losses, low fuel dilution) and should be easy to machine into complex shapes. Schematically, there are two families of materials, each with pros and cons. Light elements (called low-Z, due to their low atomic number), such as carbon or beryllium, are generally good candidates for a low impact on core plasma performance. Carbon also has excellent thermo-mechanical properties and, unlike the metals, it does not melt, instead it sublimates. However, low-Z elements have a significant erosion yield, impacting their lifetime and leading to fuel retention. On the other hand, the heavy (or high Z) elements such as tungsten, have a low erosion yield, negligible tritium retention but can degrade the plasma performance. The article about the tungsten coating in this issue describes the material scenarios for ITER.

Work programme of the PWI Task Force

Interior view of ASDEX Upgrade in its final full tungsten configuration. The progressive year-byyear transition from a carbon configuration into a tungsten one has enabled insights to be gathered into tokamak operation with this plasma facing material. The ITER-like wall project at JET (see the article in this issue) will also be a major step allowing PWI studies to investigate a tungsten-beryllium mix similar to ITER. (Photo: IPP)

The Plasma Wall Interaction Task Force (PWI TF) was implemented in 2002 making it the oldest of the EFDA Task Forces and Topical Groups. It gathers together the efforts of 23 European Associations in the field in order to tackle the most urgent PWI-related issues for ITER. The PWI TF is organized into seven Special Expert Working Groups (SEWGs).

While four of these groups directly address ITER priorities (SEWG transient heat loads, SEWG fuel retention, SEWG fuel removal, SEWG dust), the other three groups deal with material related issues (SEWG material migration, SEWG High Z materials and liquid metals, SEWG mixed materials). The main activities of the SEWGs are described on the PWI TF website (see link below). The various groups meet once a year to discuss their results and plan collaborative work. Their leaders report scientific highlights at the annual PWI TF meeting where the main orientations of the future work programme are debated.

The Task Force’s present work programme reflects the priorities set by the ITER project and includes:

• The characterisation of the transient heat loads in present day tokamaks for extrapolation to ITER, as well as their mitigation. In particular, the use of massive gas injection to mitigate the impact of disruptions on the vessel walls is being examined in several European tokamaks.

• A strong PWI programme with tungsten PFCs. Besides experiments on ASDEX Upgrade with its full tungsten configuration, laboratory studies in dedicated PWI simulators are being carried out to study the fundamental processes such as erosion and fuel retention in a well controlled environment.

• An effort to establish multi machine scaling for fuel retention and dust production from the results of present day tokamaks in order to refine the predictions for ITER. Moreover, the basic processes involved in fuel retention and dust production are investigated.

• The development of different methods for fuel and dust removal from PFCs, including laboratory studies, as well as tests in the harsh environments of tokamaks.

• The investigation of the formation of mixed materials. In this field, a fruitful bilateral collaboration has been established with the PISCES B linear device (USA), a unique facility able to operate with beryllium. In this framework, EU scientists are regularly sent abroad on a one year contract.

• Understanding material migration. Tracer experiments, for instance, are performed in several European tokamaks in which a trace impurity is injected during plasma operation and located on the PFCs thereafter.

Rudolf Neu did his PhD degree in nuclear physics at University of Tübingen in 1992. After his habilitation in experimental physics in 2004, he became a private lecturer at the University of Tübingen. Between 1992 and 1994 he was a post doctorate at IPP Garching in the ASDEX Upgrade spectroscopy group. Since 1995 he has held a permanent position at IPP, working on spectroscopy and plasma-wall interaction. In 2006, he was appointed head of the group dealing with scrape-off-layers, divertors and wall at ASDEX Upgrade. Since 1994, he has been Session Leader and, since 1996, he has coordinated the Tungsten Programme at ASDEX Ugrade. Between 2003 and 2008 he led the special expert working group High-Z/liquid metal PFCs and in 2008 he was appointed deputy leader of EFDA PWI Task Force. He has recently been appointed leader of the JET taskforce which will conduct the first JET experiments with the new ITER-like wall.

In addition, the PWI TF collaborates with other EFDA groups, like Fusion Materials TG, the MHD TG (on ELMs and disruptions) and the EFDA Emerging Technologies Group (on dust and tritium).
Thanks to Emmanuelle Tsitrone, CEA, Rudolf Neu, IPP and Roman Zagôrski, EFDA for their input and support

More information about the Plasma Wall Interaction Task Force can be found on the web site: