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Plunk G.G.,Max Planck Institute for Plasma Physics (Greifswald)
Physics of Plasmas | Year: 2017

We demonstrate that the scaling properties of slab ion and electron temperature gradient driven turbulence may be derived by dimensional analysis of a drift kinetic system with one kinetic species. These properties have previously been observed in gyrokinetic simulations of turbulence in magnetic fusion devices. © 2017 EURATOM.

Plunk G.G.,Max Planck Institute for Plasma Physics (Greifswald) | Xanthopoulos P.,Max Planck Institute for Plasma Physics (Greifswald) | Helander P.,Max Planck Institute for Plasma Physics (Greifswald)
Physical Review Letters | Year: 2017

In the complex 3D magnetic fields of stellarators, ion-temperature-gradient turbulence is shown to have two distinct saturation regimes, as revealed by petascale numerical simulations and explained by a simple turbulence theory. The first regime is marked by strong zonal flows and matches previous observations in tokamaks. The newly observed second regime, in contrast, exhibits small-scale quasi-two-dimensional turbulence, negligible zonal flows, and, surprisingly, a weaker heat flux scaling. Our findings suggest that key details of the magnetic geometry control turbulence in stellarators. © 2017 American Physical Society.

Plunk G.G.,Max Planck Institute for Plasma Physics (Greifswald)
Physics of Plasmas | Year: 2013

To address the problem of Landau damping in kinetic turbulence, we consider the forcing of the linearized Vlasov equation by a stationary random source. It is found that the time-asymptotic density response is dominated by resonant particle interactions that are synchronized with the source. The energy consumption of this response is calculated, implying an effective damping rate, which is the main result of this paper. Evaluating several cases, it is found that the effective damping rate can differ from the Landau damping rate in magnitude and also, remarkably, in sign. A limit is demonstrated in which the density and current become phase-locked, which causes the effective damping to be negligible; this result offers a fresh perspective from which to reconsider recent observations of kinetic turbulence satisfying critical balance. © 2013 American Institute of Physics.

Helander P.,Max Planck Institute for Plasma Physics (Greifswald)
Reports on Progress in Physics | Year: 2014

The theory of plasma confinement by non-axisymmetric magnetic fields is reviewed. Such fields are used to confine fusion plasmas in stellarators, where in contrast to tokamaks and reversed-field pinches the magnetic field generally does not possess any continuous symmetry. The discussion is focussed on magnetohydrodynamic equilibrium conditions, collisionless particle orbits, and the kinetic theory of equilbrium and transport. Each of these topics is fundamentally affected by the absence of symmetry in the magnetic field: the field lines need not trace out nested flux surfaces, the particle orbits may not be confined, and the cross-field transport can be very large. Nevertheless, by tailoring the magnetic field appropriately, well-behaved equilibria with good confinement can be constructed, potentially offering an attractive route to magnetic fusion. In this article, the mathematical apparatus to describe stellarator plasmas is developed from first principles and basic elements underlying confinement optimization are introduced. © 2014 IOP Publishing Ltd.

Helander P.,Max Planck Institute for Plasma Physics (Greifswald)
Physical Review Letters | Year: 2014

It is shown that magnetically confined electron-positron plasmas can enjoy remarkable stability properties. Many of the microinstabilities driving turbulence and transport in electron-ion plasmas are absent if the density is so low that the Debye length is significantly larger than the gyroradius. In some magnetic configurations, almost complete linear stability may be attainable in large parts of the parameter space. © 2014 American Physical Society.

Mynick H.E.,Princeton Plasma Physics Laboratory | Pomphrey N.,Princeton Plasma Physics Laboratory | Xanthopoulos P.,Max Planck Institute for Plasma Physics (Greifswald)
Physical Review Letters | Year: 2010

Up to now, the term "transport-optimized" stellarators has meant optimized to minimize neoclassical transport, while the task of also mitigating turbulent transport, usually the dominant transport channel in such designs, has not been addressed, due to the complexity of plasma turbulence in stellarators. Here, we demonstrate that stellarators can also be designed to mitigate their turbulent transport, by making use of two powerful numerical tools not available until recently, namely, gyrokinetic codes valid for 3D nonlinear simulations and stellarator optimization codes. Two initial proof-of-principle configurations are obtained, reducing the level of ion temperature gradient turbulent transport from the National Compact Stellarator Experiment baseline design by a factor of 2-2.5. © 2010 The American Physical Society.

Bird T.M.,Max Planck Institute for Plasma Physics (Greifswald) | Hegna C.C.,University of Wisconsin - Madison
Nuclear Fusion | Year: 2013

A mechanism is presented that suggests shielded 3D magnetic perturbations can destabilize microinstabilities and enhance the associated anomalous transport. Using local 3D equilibrium theory, shaped tokamak equilibria with small 3D deformations are constructed. In the vicinity of rational magnetic surfaces, the infinite-n ideal MHD ballooning stability boundary is strongly perturbed by the 3D modulations of the local magnetic shear associated with the presence of near-resonant Pfirsch-Schlüter currents. These currents are driven by 3D components of the magnetic field spectrum even when there is no resonant radial component. The infinite-n ideal ballooning stability boundary is often used as a proxy for the onset of virulent kinetic ballooning modes and associated stiff transport. These results suggest that the achievable pressure gradient may be lowered in the vicinity of low order rational surfaces when 3D magnetic perturbations are applied. This mechanism may provide an explanation for the observed reduction in the peak pressure gradient at the top of the edge pedestal during experiments where edge localized modes have been completely suppressed by applied 3D magnetic fields. © 2013 IAEA, Vienna.

Vilbrandt R.,Max Planck Institute for Plasma Physics (Greifswald)
Fusion Engineering and Design | Year: 2011

To control all the work and test steps during assembly of Wendelstein 7-X for each major assembly task Quality Assurance and Assembly Plans are used as the central managing instrument. These documents ensure that the order of all steps is carried out as planned and that the envisaged quality will be met. The confirmation of a successful working step often is done by tests and measurement. For each test special instructions were prepared to ensure reproducible and correct results. The tests are either carried out by the certified QA inspectors of the project or by specially qualified internal inspectors. The most important tests and measurements are outlined briefly. All quality deviations are assessed in relation of consequences for later operation. © 2011 Elsevier B.V. All Rights Reserved.

Wegener L.,Max Planck Institute for Plasma Physics (Greifswald)
IEEE Transactions on Applied Superconductivity | Year: 2012

Wendelstein 7-X represents the continuation of fusion experiments of the stellarator type at the Max-Planck-Institute for Plasma Physics. The size of device (725 t, height of 5 m, diameter 16 m) and the superconductive magnet system distinguish W7-X from earlier stellarators at the Max-Planck Institute. The paper describes the technologies and methods used for the assembly of the magnet system and it compiles the experiences gained. The assembly of the W7-X facility will be accomplished in 2014. © 2011 IEEE.

Hergenhahn U.,Max Planck Institute for Plasma Physics (Greifswald)
International Journal of Radiation Biology | Year: 2012

Purpose: The paper gives an introduction into Interatomic and Intermolecular Coulombic Decay (ICD). ICD is an autoionization process, which contrary to Auger decay involves neighbouring sites of the initial vacancy as an integral part of the decay transition. As a result of ICD, slow electrons are produced which generally are known to be active in radiation damage. The author summarizes the properties of ICD and reviews a number of important experiments performed in recent years. Materials and methods: Intermolecular Coulombic Decay can generally take place in weakly bonded aggregates in the presence of ionizing particles or ionizing radiation. Examples collected here mostly use soft X-rays produced by synchrotron radiation to ionize, and use rare-gas clusters, water clusters or solutes in a liquid jet to observe ICD after irradiation. Results: Intermolecular Coulombic Decay is initiated by single ionization into an excited state. The subsequent relaxation proceeds via an ultra-fast energy transfer to a neighbouring site, where a second ionization occurs. Secondary electrons from ICD have clearly been identified in numerous systems. ICD can take place after primary ionization, as the second step of a decay cascade which also involves Auger decay, or after resonant excitation with an energy which exceeds the ionization potential of the system. Conclusions: ICD is expected to play a role whenever particles or radiation with photon energies above the ionization energies for inner valence electrons are present in weakly bonded matter, e.g., biological tissue. The process produces at the same time a slow electron and two charged atomic or molecular fragments, which will lead to structural changes around the ionized site. © 2012 Informa UK, Ltd.

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