11-04-2021

The I-mode confinement regime is special because it shows enhanced energy confinement without enhanced particle confinement.Due to the lower pressure gradient at the edge, the large magnetohydrodynamic instabilities (so-called ELMs) do not occur, which would not be tollerable for a reactor. This makes the I-mode an interesting regime for a fusion reactor. However, relaxation processes at the plasma edge can also occur in the I-mode. In our recent paper in Physics of Plasmas we simulated the dynamics of relaxation processes in the I-mode. We are very pleased that the experimental data from ASDEX Upgrade made it to the cover of the October issue.   

Dr. Nils Fahrenkamp was recruited as the first member of the working group. Dr. Fahrenkamp studied in Rostock and received his PhD from the University of Greifswald in cooperation with IPP and CERN. There, the previous research focus was on the development of helicon plasma sources for linear particle accelerators. His expertise for laboratory operations is laser diagnostics. Now he will be responsible for the relocation, as well as the subsequent operation of the VINETA experiment.

Welcome to the Experimental Plasma Physics team.

17/05/2021

Accretion disks are rotating disks around central objects, which transport matter towards the center (accrete). They consist of differently strongly ionized gas (plasma). In order to accrete, there must be a special mechanism for angular momentum redistribution. We want to study this mechanism in the laboratory. In particular, in laboratory experiments, the two-dimensional geometry and magnetohydrodynamic properties of an accretion disk are difficult to reproduce. Here we propose the setup of a shallow water experiment in accretion disk geometry, where differential rotation is induced in a quasi-two-dimensional shallow water layer with an open surface, allowing direct measurement of radial and azimuthal fluid dynamics. The effect of the magnetic field is mimicked by the established equivalence of magnetohydrodynamic and viscoelastic fluids. In this work, we demonstrate the radial outward transport of angular momentum in a laboratory experiment near the accretion disk geometry. We hope that the proposed experimental setup will help us identify the origin of angular momentum redistribution in accretion disks.

This work has been published in

F. Günzkofer, P. Manz 'Outwards transport of angular momentum in a shallow water accretion disk experiment', Phys. Rev. Fluids 6, 054401 (2021)

31/05/2021

The efficient operation of a tokamak is limited by several constraints such as the transition to high confinement or the density limits occurring in both confinement regimes. These particular boundaries of operation are derived in terms of a combination of dimensionless parameters describing interchange-drift-Alfvén turbulence without any free adjustable parameter. Approaching the density limit in L-mode, the turbulence is in the regime of resistive ballooning modes.  The actual density limit is related to a transition from the electrostatic to the electromagnetic regime. The derived density limit is compared against the Greenwald scaling.  L-H transitions are restricted to the regime of drift-wave turbulence. The L-H transition occurs, when the energy transfer rate from the turbulence into the E×B flow exceeds the turbulent growth rate. The turbulent growth rate is carried by the ions, as the electron transport is strongly reduced by diamagnetic stabilization. The ExB shear is dominated by the ion background pressure gradient. Regarding the energy transfer, the physics of zonal flows can be applied to the background flow as well. Furthermore, the role of the L-H transition for the density limit is clarified. The dependence of the efficiency of energy coupling between turbulence and shear flow on abiabaticity is also considered. Most important, the derived boundaries describe the operational space at the separatrix of the ASDEX Upgrade tokamak which is presented in terms of an electron density and temperature existence diagram. The excellent agreement at the separatrix demonstrates the critical importance of the outermost edge region for better understanding of the overall tokamak system.

This work has been accepted for publication in Nuclear Fusion.

This work is a highlight of the devision E2M at IPP Garching