Yearbook of the Max Planck Society:
IPP articles
 

Energetic particles in plasmas are often the cause of interesting wave-particle interactions that fundamentally influence the stability of the plasma. In fusion plasmas enclosed by magnetic fields, for example, they can excite special forms of oscillation; the solar wind hitting the Earth's atmosphere can generate chorus waves. We are developing novel numerical methods to simulate such phenomena, combining efficient fluid models to describe the wave with more elaborate kinetic models to describe the particles.

When designing a stellarator fusion power plant, a "digital twin" should help to investigate the impact of new technologies, physical findings or uncertainties. This would allow an almost endless number of design alternatives to be investigated in parallel. In order to describe the interaction of system components, they must first be modeled individually. In recent years, IPP has succeeded - for the first time worldwide - in developing a number of these models for a stellarator power plant and combining them in a simulation platform.

The aim of fusion research is to use fusion of light atomic nuclei to generate energy in a power plant. A number of problems remain to be solved on this way, including understanding or control of large-scale plasma instabilities. These include Edge-Localized Modes, periodic instabilities at the plasma edge that can eject ten percent of the plasma energy in less than a millisecond. In numerical simulations, it has now been possible for the first time to calculate their full non-linear dynamics over several cycles, thereby reproducing most experimental observations.

The Wendelstein 7-X fusion device, an optimized stellarator working at IPP Greifswald since December 2015, will be put into operation step by step. In the first two experimental phases, the plasma was first bordered by a material limiter and then magnetically by an uncooled divertor. Currently, a water-cooling system is being installed. This is intended to enable half hour-long plasma discharges at high heating power in Wendelstein 7-X.

Transferring extreme powers from the hot plasma of a future fusion power plant to the surrounding material surfaces in a gentle manner is a central challenge for science. For this purpose, in experiments at the ASDEX Upgrade fusion device in Garching and JET in Culham/Great Britain, suitable plasma scenarios were developed. In those scenarios, the specific contamination of the hydrogen plasma by addition of impurity atoms plays an important role.

One of the greatest tasks of fusion research is to confine the hot fuel with good heat insulation. Energy losses, in particular due to turbulence, must remain as small as possible. In the Wendelstein 7-X stellarator, by injecting pellets made of frozen hydrogen a plasma state with low turbulence could be achieved.

A proven method of heating plasmas in fusion devices to many millions of degrees is to beam in radio waves of ion cyclotron frequency. This heating method, however, has involved disadvantages in plasma vessels with metal walls, as envisaged for a future power plant. The antenna beaming the waves into the plasma has now been successfully optimised to make radio-wave heating compatible with metallic walls.

The second operating phase of the Wendelstein 7-X nuclear fusion device lasted from July to November 2018. The newly installed divertor for removing particles and energy from the plasma was tested. High plasma density, plasma temperature, and energy content were attained, and long discharge times of up to 100 seconds achieved, these being record results for fusion devices of the stellarator type.

In addition to large experimental equipment, computer simulations on supercomputers have been playing an increasingly important role in fusion research in recent years. By combining tailored physical models with state-of-the-art numerical methods, it is possible to solve the complex basic equations of plasma physics on some of the world's most powerful computers. Thus, many important individual aspects of plasma dynamics can already be described quantitatively today.

Construction of the neutral particle heating for the Wendelstein 7-X fusion device will soon be completed. In addition to the existing microwave heating, the new device will be ready for use from summer next year. It will inject up to seven megawatts into the plasma.

By means of the so-called divertor – specially equipped and cooled plates at the bottom of the plasma vessel to which particles are deflected from the edge of the plasma – a part of the generated fusion energy is dissipated in a later fusion power plant. With the Divertor Manipulator DIM-II, this concept is prepared at the ASDEX Upgrade fusion device. With DIM-II, parts of the divertor can be examined and replaced without opening the plasma vessel. This allows for investigation of plasma-material interactions at the divertor plates as well as for concept studies for actively cooled plates.

A pair plasma consisting of electrons and positrons is of great interest both in basic plasma physics and in astrophysics. Here these plasmas are believed to exist in the vicinity of various astrophysical objects. Within the framework of the APEX project, a magnetically confined electron-positron plasma is to be generated in the laboratory for the first time. First positron experiments have already yielded important results.

Many properties of a plasma that are not, or not in detail, experimentally accessible can be systematically investigated only in computer simulations. Many codes, however, use numerical methods that insufficiently take into account important properties of mathematical equations. This results in important phenomena not being reproduced in simulations. So-called structure-preserving integration methods could be the remedy. These combine ideas from numerics, physics, and geometry and allow more realistic simulations than classical methods.

Following nine years of construction work and one year of technical preparations and tests on 10 December 2015 the first helium plasma was produced in the Wendelstein 7-X fusion device at the Max Planck Institute for Plasma Physics (IPP) in Greifswald. The first hydrogen plasma was to follow on 3 February 2016, this marking the start of scientific operation. Wendelstein 7-X, the world’s largest fusion device of the stellarator type, is to investigate this configuration’s suitability for use in a power plant.

After more then twelve years of research the small WEGA fusion device at IPP’s Greifswald branch end of 2013 has been shut down. The “Wendelstein training experiment at IPP Greifswald” is making room for the Wendelstein 7-X large-scale device, construction of which will be concluded next year. WEGA served as a training ground for students and junior scientists to bridge the gap till completion of Wendelstein 7-X. Inspite of its small dimensions WEGA achieved remarkable research results.

Special requirements have to be met in developing diagnostics for the ITER international experimental reactor, which is to produce an ignited, energy-yielding plasma. The bolometers – radiation detectors for measuring light ranging from radiant heat to X-rays, emerging from the ITER plasma – are being developed at the Max Planck Institute for Plasma Physics in Garching.

The experiment VINETA.II is designed for studies of magnetic reconnection. Due to the separation of plasma generation and reconnection drive a high degree of controllability and reproducibility is achieved. Special attention is paid to investigations of the spatial and temporal development of the reconnection current sheet on different scales. On the macroscopic scale the current sheet is mainly influenced by the geometry of the magnetic field. On the microscopic scale the current sheet develops turbulent fluctuations, which characteristics are determined by the electron dynamics.

Tokamaks can provide excellent confinement and high kinetic pressure of fusion plasmas due to an axisymmetric magnetic field. However, strong pressure gradients at the plasma edge cause repetitive instabilities leading to expulsion of hot plasma towards the surrounding wall. This instability is studied in detail at the tokamak ASDEX Upgrade. It has been found that the fast power loss from the plasma and the associated high peak power load onto the wall can be reduced by a small dedicated non-axisymmetric magnetic perturbation without compromising the favourable confinement properties.

After more then twelve years of research the small WEGA fusion device at IPP’s Greifswald branch end of 2013 has been shut down. The “Wendelstein training experiment at IPP Greifswald” is making room for the Wendelstein 7-X large-scale device, construction of which will be concluded this year. WEGA served as a training ground for students and junior scientists to bridge the gap till completion of Wendelstein 7-X. Inspite of its small dimensions WEGA achieved remarkable research results.

A new class of tungsten materials – tungsten fibre reinforced tungsten – is developed and investigated in the division „Plasma Edge and Wall“ of the MPI für Plasmaphysik. Tungsten fibres are combined with a tungsten matrix. Extrinsic mechanisms of energy dissipation in combination with a high ductility of the fibres lead to a strong toughness enhancement. A new method of chemical vapour infiltration for tungsten allows for the first time the fabrication of such material. The toughening mechanisms are shown by means of advanced experimental techniques such as x-ray microtomography.

In the high temperature plasmas of tokamak fusion devices, the radial transport is produced by micro-turbulence, at ion and electron Larmor radius scales. An essential element of the physical understanding of the turbulent transport is the identification of the relationship between theoretically predicted turbulent transport mechanisms and the macroscopically observed behaviors of the plasma profiles. The main results of this theoretical and experimental research performed at the Max-Planck-Institut für Plasmaphysik are presented.

The Max Planck Institute for Plasma Physics in Garching makes with the worldwide largest ion source test facility for negative hydrogen ions ELISE a major contribution for the success of the international fusion experiment ITER in Cadarache. After two years of construction ELISE will start operation in June 2012. The aim is to demonstrate the source parameters required for the heating and long pulse operation of the fusion plasma. The ion source is aimed to deliver a one hour negative deuterium ion beam of 20 A from a source with half the size of that for the ITER source.

The ultra-hot fusion plasma with extremely low density is confined in the fusion devices by magnetic forces without making contact with the vessel wall. The divertor is the only place in the plasma vessel that is touched by the plasma. For the Wendelstein 7-X research device, now being built at Greifswald, divertor components were developed that can permanently withstand the extremely high heat loads of 10 MW/ m².

The usually applied scheme of electron heating in fusion plasmas by millimeter waves is limited regarding the plasma density. At the ASDEX Upgrade tokamak, which is operated by Max-Planck-Institut für Plasmaphysik in Garching, new schemes have been developed that allow efficient electron heating at higher density. These schemes are not only important to extend the operational space of ASDEX Upgrade but could also be used in the Wendelstein 7-X stellarator that is currently being built by the Greifswald branch of Max-Planck-Institut für Plasmaphysik.

WEGA is a classical stellarator, which is operated at the Max-Planck-Institut für Plasmaphysik in Greifswald for educational training and basic research. New methods on the application of high frequency wave excitation, propagation and absorption at different frequencies are studied. For example a stationary plasma state was achieved which is characterized by a very high density and simultaneously a highly supra-thermal electron component.

New materials capable of withstanding very high loads are being developed by the ExtreMat (New Materials for Extreme Environments) research programme, an integrated project of the European Union. A European research and industrial consortium headed by Max Planck Institute for Plasma Physics at Garching is working on the development of innovative high-grade materials. These are to open up new areas of application in nuclear fusion, nuclear fission, electronics and space technology.

Wendelstein 7-X, currently under construction at the Max-Planck-Institut für Plasmaphysik in Greifswald, is a highly optimised stellarator. Within the rather flexible magnetic configuration space of Wendelstein 7-X, the different optimisation criteria are confirmed by predictive transport simulations, which can also contribute to the design and control of plasma discharges in the later experiments.

In order to qualify tungsten as surface material for a future fusion reactor, the plasma facing carbon tiles of ASDEX Upgrade were coated with tungsten. The first experimental campaign – without wall conditioning by deposition of boron – demonstrated a favourable behaviour of tungsten with regard to hydrogen retention. After boronization, the reduced impurity content caused low intrinsic radiation. Injection of nitrogen re-established a radiative level required to protect the divertor from thermal overload, resulting also in an (unexpected) improvement of the energy confinement.

The development of stationary plasma operation scenarios is of utmost importance in next step devices in view of a future fusion power station. A necessary prerequisite is a stationary magnetic field configuration, a continuously operating heating system and a stationary energy and particle control. Among the different heating methods heating with strong microwaves is attractive with respect to physics capabilities and technological properties. The most powerful microwave plant with steady state capability is presently under construction for the Wendelstein 7-X stellarator at the Max-Planck-Institut für Plasmaphysik (IPP) Greifswald.

The efficiency of a future fusion power plant depends on the confinement of the fusion products, i.e. the helium nuclei, in the magnetic configuration. Therefore, the investigation of the transport properties of this super-thermal particle population is of great scientific interest and will be one of main research areas at the international fusion experiment ITER. Especially large-scale internal and external magnetic perturbations and instabilities driven by the energetic particles can contribute critically to this transport.

We describe experiments on photoionisation of a free cluster jet with synchrotron radiation. Electron spectroscopy allows to map changes of the electronic structure upon condensation of monomers to clusters. Energy remaining in the cluster after photoionization can be released via emission of a secondary electron, which proceeds by an ultrafast energy transfer between neighbouring atoms or molecules within the cluster.

Various materials – tungsten, beryllium, and fibre-reinforced graphites – exhibiting different advantages and disadvantages are under discussion as wall materials for the plasma vessel of fusion devices, particularly ITER. The properties of graphite were investigated in detail. One disadvantage here is the deposition of amorphous hydrocarbon layers on the vessel walls. This, however, can be prevented.

An operation scenario suitable for a fusion power plant should feature high thermal insulation of the hot plasma in conjunction with good stability properties. In order to allow the two quantities to be optimised simultaneously, one has to understand the underlying mechanisms involved, i. e. the nonlinear interaction of turbulent particle and energy transport with large-scale instabilities. One example is the “Improved H-mode“ discovered in ASDEX Upgrade.

In the Materials Science Department of IPP processes of the plasma surface interaction in fusion devices are investigated. In this article, experiments in the device MAJESTIX are reported. This device is devoted to the investigation of microscopic processes relevant to the interaction of hydrogen, hydrocarbon radicals, and ions with carbon surfaces. These processes are of particular importance in plasma surface interaction in fusion devices.

Stellarators are characterised by a complex three-dimensional structure, distinguishing them from other toroidal magnetic confinement concepts. The investigation of instabilities, especially micro instabilities, and their development into turbulence is not well understood for stellarators. It is particularly interesting if turbulence in stellarators is different from those in other confinement configurations and if a manipulation by optimisation of the magnetic structure is possible. In the following, numerical simulations of the ion-temperature driven instability in the plasma core and of turbulence in the plasma edge for Wendelstein 7-X will be presented.

IPP’s Technology Division in Garching is conducting a development programme for the international test reactor ITER – a new ion source for plasma heating by neutral particle beams. In contrast to former devices for ITER negative ion beams are needed. Production, acceleration, and neutralisation of negative hydrogen ions, which in contrast to positive ions are very fragile objects, is accompanied by a series of challenging physics and technology problems. In addition, high particle energies and steady state is requested. The results up to now nevertheless indicate, that IPP’s ion source is well on the way to be chosen as a candidate for ITER.

At the Greifswald Branch of IPP the linear plasma generator VINETA (Versatile Instrument for Studies on Nonlinearity, Electromagnetism, Turbulence and Application) is operated to study the basic dynamics of a magnetised plasma, i.e. the behaviour of plasmas waves, turbulence and instabilities as well as basic questions of plasma edge physics. VINETA allows one to establish precisely the respective plasma modi and to conduct detailed experimental investigations.

Data analysis employing Bayesian probability theory constitutes one of the IPP contributions to the inter-institutional collaboration "Centre for Interdisciplinary Plasma Science" (IPP/MPE). The goal of this project is the optimal solution of ill conditioned or even underdetermined inverse problems. From the spectrum of activities we present examples from the field of plasma physics, astronomy and climatology.

Whether, or to what degree tungsten (W) can be used as plasma facing material is a key question for all future fusion devices, having in mind the complex chemistry carbon exhibits in the presence of hydrogen isotopes. Since 1999 the fusion experiment ASDEX Upgrade pursues the progressive increase of W plasma facing components (PFCs). Meanwhile 65 per cent of all PFCs are equipped with W coated tiles. The incremental approach in the transition to a W dominated device provides the opportunity to explore the influence of different W-components and to identify major carbon sources. New spectroscopic tools have been developed which allow detection of W concentrations below 10^-6. Poloidally resolved measurements of the W and C influx reveal that the W erosion is dominated by light impurities. At the same time a fast redistribution of C onto the tungsten surfaces is observed. The W concentrations remained well below 10^-5 in the reference scenario for a future reactor. The central impurity accumulation could be efficiently reduced by central heating and repetitive injection of cryogenic deuterium pellets. The results obtained so far open up optimistic prospects for the use of W in a reactor and future studies in ASDEX Upgrade will enlighten details of the operation of a carbon free device.

Wendelstein 7-X - when ready, the world’s largest fusion device of the stellarator type - is aimed at investigating the suitability of this concept for a power plant. With discharges lasting up to 30 minutes it is to demonstrate the essential property of stellarators, the capability of continuous operation. The first major components for the device have been delivered to IPP: two magnet coils, the first plasma vessel segments, vessel ports and two microwave transmitter for the plasma heating.