Highlights 2013
Research news from the division Plasma Edge and Wall
H-mode density limit, M. Bernert
Nov 2013
H-mode density limit, M. Bernert Nov 2013 Future fusion reactors will most likely be operated in the high confinement mode (H-mode). It is desired to operate these devices at plasma densities as high as possible in order to increase the produced fusion power. However, this is limited by the H-mode density limit.
Classification of the H-mode density limits into 4 phases, here measurements of the energy and density of the plasma
The H-mode density limit was recently studied at the tokamak ASDEX Upgrade at the IPP Garching. Four phases are identified on the approach towards the H-mode density limit, which affect the plasma energy and density differently. These phases are a stable H-mode (green) followed by a degrading H-mode (yellow). The phase of the breakdown of the H-mode (red) finally leads to the L-mode (low confinement mode) (blue). Along with this classification, a new way to explain the H-mode density limit was found. This new description is in line with observations of previous experiments. It involves the coupling of two effects, an increased heat transport at the edge of the plasma and the ionization of the neutral gas outside of the confined plasma.
Experiments at other tokamaks, such as JET, the currently largest tokamak in operation in Culham, UK, are going to be analyzed using this new method in order to make predictions for ITER and other future tokamaks.
With this work Matthias Bernert graduated at the LMU München on the 23rd of October 2013.
http://edoc.ub.uni-muenchen.de/16262/
General Meeting of the Max Planck Society
Otto Hahn Medal for Young Scientist from IPP
Gregor Birkenmeier's doctoral studies have made a fundamental contribution to turbulence research in magnetized plasmas. His work has now been honored by the Max Planck Society.
At the general meeting in Potsdam on 5 June, the Max Planck Society awarded the young scientists the Otto Hahn Medal for his outstanding scientific achievements. Dr. Gregor Birkenmeier, scientist in the Plasma Edge and Wall Division at IPP in Garching, was one of the 30 recipients. He did his doctoral studies at the TJ-K stellarator experiment, located at the University of Stuttgart, and received the medal for precise experimental investigation of the three-dimensional microscopic structure of turbulence in magnetized plasmas.
Turbulence and the concomitant transport of particles and energy are omnipresent in neutral fluids and plasmas. In magnetized astrophysical plasmas and fusion plasmas, turbulent transport plays a key role. Gregor Birkenmeier's doctoral studies presented the first detailed experimental investigations of the spatial structure of turbulent plasma transport in a three-dimensional magnetic field. He clearly demonstrated that the curvature of the magnetic field acts on the turbulent transport as a dominant parameter. This confirmed previous theoretical models.
Gregor Birkenmeier has also conducted detailed investigations of the interaction between self-generated shear flows and turbulence. He verified that turbulently generated zonal flows can sustainably reduce transport, and he showed that this reduction is caused by a spatial phase shift between potential and density fluctuations.
The annual presentation of the Otto Hahn Medal is intended to motivate especially gifted junior scientists and researchers to pursue a future university or research career.
Julia Sieber
Related publications:
G. Birkenmeier, M. Ramisch, P. Manz, B. Nold, and U. Stroth, Phys. Rev. Lett. 107, 025001 (2011)
G. Birkenmeier, M. Ramisch, G. Fuchert, A. Köhn, B. Nold, and U. Stroth, Plasma Phys. Control. Fusion 55 (2013) 015003
G. Birkenmeier, M. Ramisch, B. Schmid, and U. Stroth, Phys. Rev. Lett. 110, 145004 (2013)
High-accuracy radial electric field measurements at ASDEX Upgrade
The understanding of the physics relevant to the edge transport barrier (ETB) of an H-mode fusion plasma is of crucial importance as it leads to steep gradients at the plasma edge which implies a confinement gain at the boundary of the plasma. This improvement propagates into the plasma core, where a hot and dense plasma is required for fusion. The ETB is thought to be caused by a sheared plasma flow perpendicular to the magnetic field which is equivalent to a sheared radial electric field Er. In the present work this mechanism has been confirmed as the location of the steepest ion pressure gradient ∇pi was shown with unprecedented accuracy to match the position of the largest Er shear.
The installation of a new edge charge exchange recombination spectroscopy (CXRS) diagnostic at ASDEX Upgrade (AUG) enables high temporally and radially resolved measurements of the poloidal and toroidal rotation velocities, densities and temperatures of impurity ions. Thus, it provides all measurements for deriving Er from the radial force balance equation. The new CXRS measurements, combined with the unique edge diagnostic suite available at AUG, allowed for the high-accuracy localization (2–3 mm) of the Er profile based on an established alignment procedure. Using this technique it has been found that the maximum in the ExB shearing rate (ωExB) coincides with the steepest ∇pi and lies inside the position of the minimum of the Er well (see figure 1). This suggests that the negative shear region is the important region for the formation of the pedestal.
In the radial force balance of impurities the poloidal rotation contribution yields the dominant term in the evaluation of Er at the plasma edge. For the main ions, the Er minimum coincides with the maximum pressure gradient term ∇pi/eni supporting that the Er well is created by the main ion species. The fact that ∇pi/eni matches Er in the ETB is consistent with the main ion poloidal flow being at neoclassical levels. Quantitative comparisons between neoclassical predictions and experimental measurements of both impurity and main ion poloidal rotation show that the sign and the magnitude are in remarkably good agreement.
Brittle material becomes pseudo ductile: Tungsten fibre reinforced Tungsten
Due to a unique property combination tungsten is a favoured candidate for the use in plasma facing components in a future fusion reactor. However its inherent brittleness and the subsequent lack of damage tolerance strongly restrict its application. In particular the poor resistance against operation embrittlement due to recrystallization or irradiation damage is up to now an unsolved problem.
A possible solution is the generation of structures which allow local energy dissipation and thus generate an enhanced fracture resistance and therefore toughness. This is called extrinsic, i.e. externally applied toughening or pseudo toughness. Examples for such mechanisms are fibre bridging, crack deflection or the ductile fibre deformation.
This idea is investigated and implemented within a research project in the division „Plasma Edge and Wall“. Here tungsten is reinforced by coated long fibres made of drawn tungsten wire. The toughening mechanisms have been identified and their stability against embrittlement was shown by means of high energy synchrotron tomographic experiments. In bending test on larger samples stable crack propagation and a doubling of the load bearing capability have been observed. Analytic calculations revealed the high potential of tungsten fibre reinforced tungsten. Compared to other tungsten materials the new composite features a real damage tolerance at room temperature as well as the possibility of local energy dissipation and therefore an increased toughness in the as produced and embrittled state.
With this work Johann Riesch graduated at the TU München with summa cum laude on November the 19th 2012.