A. Krämer-Flecken
A5046416769- Magnetic confinement fusion research
- Ionosphere and magnetosphere dynamics
- Laser-Plasma Interactions and Diagnostics
- Fusion materials and technologies
- Particle accelerators and beam dynamics
- Superconducting Materials and Applications
- Plasma Diagnostics and Applications
- Solar and Space Plasma Dynamics
- Nuclear physics research studies
- Atomic and Molecular Physics
- Nuclear Physics and Applications
- Laser-induced spectroscopy and plasma
- Laser Design and Applications
- Nuclear reactor physics and engineering
- Atomic and Subatomic Physics Research
- Geophysics and Gravity Measurements
- Advanced NMR Techniques and Applications
- Gyrotron and Vacuum Electronics Research
- Astro and Planetary Science
- Scientific Research and Discoveries
- Distributed and Parallel Computing Systems
- Metallurgical Processes and Thermodynamics
- Electron and X-Ray Spectroscopy Techniques
- Advanced Data Storage Technologies
- Quantum chaos and dynamical systems
Forschungszentrum Jülich
2016-2025
Max Planck Society
2015-2019
Max Planck Institute for Plasma Physics
2003-2019
Stadtwerke Jülich (Germany)
2016-2018
National Institute for Fusion Science
2016-2018
Princeton Plasma Physics Laboratory
2016-2018
Huazhong University of Science and Technology
2018
Institute of Plasma Physics
2018
Chinese Academy of Sciences
2018
The Graduate University for Advanced Studies, SOKENDAI
2018
After completing the main construction phase of Wendelstein 7-X (W7-X) and successfully commissioning device, first plasma operation started at end 2015. Integral start-up using electron cyclotron resonance heating (ECRH) an extensive set diagnostics have been completed, allowing initial physics studies during operational campaign. Both in helium hydrogen, breakdown was easily achieved. Gaining experience with vessel conditioning, discharge lengths could be extended gradually. Eventually,...
The optimized, superconducting stellarator Wendelstein 7-X went into operation and delivered first measurement data after 15 years of construction one year commissioning. Errors in the magnet assembly were confirmend to be small. Plasma was started with 5 MW electron cyclotron resonance heating (ECRH) power five inboard limiters. Core plasma values keV, keV at line-integrated densities achieved, exceeding original expectations by about a factor two. Indications for core-electron-root found....
Stationary high energy confinement is observed on TEXTOR-94 for times limited only by the flux swing of transformer using strong edge radiation cooling. Necessary tools are feedback control radiated power and plasma content. At highest densities obtained (up to 1.2 Greenwald limit), exceeds edge-localized-mode-free H-mode scaling ITERH93-P more than 20%. β limits reached with fH89/qa≈0.6. No detrimental effect seeded impurity seen. These discharges meet many conditions necessary a fusion...
The magnetic-field perturbation produced by the dynamic ergodic divertor in TEXTOR changes topology of magnetic field plasma edge, creating an open chaotic system. spectrum contains only a few dominant harmonics and therefore it can be described analytical model. modeling is performed vacuum approximation without assuming backreaction does not rely on any experimentally obtained parameters. It shown that this predicts many details observed structure. Several experiments have been to prove...
The dynamic ergodic divertor (DED) on the TEXTOR tokamak allows for creation of static and rotating helical magnetic perturbation fields. In 3/1 configuration strong m/n = 2/1 sideband excites a locked tearing mode above critical field strength. onset threshold depends strongly plasma fluid rotation with respect to mode. Rotation in current direction destabilizes certain range frequencies, whereas counter-rotation has stabilizing influence. shows minimum when frequency external equals MHD
The geodesic acoustic mode (GAM) is a high frequency branch of zonal flows, which observed in toroidal plasmas. Because curvature effects, density fluctuations are excited, investigated with the $O$-mode correlation reflectometer at TEXTOR. This Letter reports on poloidal distribution GAM induced fluctuation and compares them theoretical predictions. influence flows ambient turbulence studied, too.
The geodesic acoustic mode (GAM) is a coherently oscillating zonal flow that may regulate turbulence in toroidal plasmas. Uniquely, the complete poloidal and structure of magnetic component turbulence-driven GAM has been mapped TCV tokamak. Radially localized measurements fluctuating density, ECE radiative temperature show fully coherent, radially propagating wave. These observations are consistent with electrostatic, gyrokinetic simulations.
This letter reports on quasi-coherent (QC) modes observed in fluctuation spectra from Tore Supra and TEXTOR reflectometers. QC have characteristics between coherent broad-band fluctuations as they oscillate around a given frequency but wide spectrum. They are ballooned at the LFS midplane appear usually ranging 30 to 120 kHz. In ohmic plasmas both tokamaks, detected only linear confinement (LOC) regime disappear saturated (SOC) regime. Linear simulations predict that LOC SOC regimes...
The TCV tokamak is augmenting its unique historical capabilities (strong shaping, strong electron heating) with ion heating, additional heating compatible high densities, and variable divertor geometry, in a multifaceted upgrade program designed to broaden operational range without sacrificing fundamental flexibility. rooted three-pronged approach aimed at ITER support, explorations towards DEMO, research. A 1 MW, tangential neutral beam injector (NBI) was recently installed promptly...
The results from the investigation of neoclassical core transport and role radial electric field profile (Er) in first operational phase Wendelstein 7-X (W7-X) stellarator are presented. In plasmas, details Er expected to have a strong effect on both particle heat fluxes. Investigation is important understanding validation calculations. closely related perpendicular plasma flow (u⊥) through force balance equation. This allows be inferred measurements velocity, which can measured using x-ray...
The transport of heavy impurities has been investigated at the Wendelstein 7-X stellarator during core electron root confinement (CERC) experiments. Iron atoms were injected via laser blow-off technique and analyzed by VUV x-ray spectrometers. amount iron does not change global plasma parameters but yields strong enough line radiation for detailed studies based on impurity code STRAHL. latter is supplied with neo-classical diffusion convection profiles from drift kinetic equation solver...
The first results of the Dynamic Ergodic Divertor in TEXTOR, when operating m/n=3/1 mode configuration, are presented. deeply penetrating external magnetic field perturbation this configuration increases toroidal plasma rotation. Staying below excitation threshold for m/n=2/1 tearing mode, rotation is always direction current, even if projection rotating opposite direction. observed consistent with a radial electric field, generated by an enhanced electron transport ergodic layers near...
Controlled experiments on the suppression of m/n = 2/1 tearing mode with electron cyclotron heating and current drive in TEXTOR are reported. The was produced reproducibly by an externally applied rotating perturbation field, allowing a systematic study its suppression. Heating inside island is shown to be dominant mechanism these experiments. An extrapolation findings ITER indicates that projected system for could significantly more effective than present estimates indicate, which only...
The mechanism governing the impact of mass isotope on plasma confinement is still one main scientific conundrums facing magnetic fusion community after more than thirty years intense research. We have investigated properties local turbulence and long-range correlations in hydrogen deuterium plasmas TEXTOR tokamak. Experimental findings shown a systematic increasing amplitude during transition from to dominated plasmas. These results provide first direct experimental evidence importance...
Wendelstein 7-X, a superconducting optimized stellarator built in Greifswald/Germany, started its first plasmas with the last closed flux surface (LCFS) defined by 5 uncooled graphite limiters December 2015. At end of 10 weeks long experimental campaign (OP1.1) more than 20 independent diagnostic systems were operation, allowing detailed studies many interesting plasma phenomena. For example, fast neutral gas manometers supported video cameras (including one fast-frame camera frame rates...
The radiative improved (RI) mode is a tokamak regime offering many attractive reactor features. In the article, RI of TEXTOR-94 shown to follow same scaling as linear ohmic confinement and thus identified one most fundamental operational regimes. current understanding derived from experiments modelling conditions necessary for sustaining reviewed, are mechanisms leading L-RI transition. article discusses compatibility high impurity seeding with low central power density burning reactor, well...
Improved confinement is achieved on TEXTOR under high power conditions (up to 4 MW of additional heating with NBI-co+ICRH, NBI-co+counter or NBI-co+counter+ICRH) edge radiative cooling employing silicon neon as the radiating impurities. It shown that in quasi-stationary up 85% input can be radiated. Such fractions offer possibility utilizing these techniques facilitate exhaust problem for a Tokamak reactor. Discharges exhibit enhanced properties at densities, e.g. central line averaged...
With the dynamic ergodic divertor in TEXTOR fundamental effects of coupling external magnetic field perturbations to confined plasma have been studied. The non-linear between (m/n = 12/4) and internal modes 3/1) has investigated. critical perturbation for excitation an m/n 2/1 tearing mode depends not only on magnitude but also direction toroidal angular momentum input by neutral beam injection (NBI). Below threshold this a spin-up observed, which strength field. It is independent both...
Long period sawteeth have been observed to result in low-β triggering of neo-classical tearing modes, which can significantly degrade plasma confinement. Consequently, a detailed physical understanding sawtooth behaviour is critical, especially for ITER where fusion-born α particles are likely lead very long periods. Many techniques developed control, and particular destabilize the sawteeth. The application counter-current neutral beam injection (NBI) JET has resulted shorter periods than...
Experimental investigations of sawteeth interaction with fast ions measured by collective Thomson scattering on TEXTOR are presented. Time-resolved measurements localized 1D fast-ion distribution functions allow us to study dynamics during several sawtooth cycles. Sawtooth oscillations interact strongly the population in a wide range plasma parameters. Part ion phase space density oscillates out oscillation hydrogen neutral beam injection (NBI). These most likely originate from energies...
Wendelstein 7-X (W7-X) is a large optimized stellarator (B=2.5T, V=30m3) aiming at demonstrating the reactor relevance of stellarators. In 2015 W7-X will begin its first operation phase (OP1.1) with five inertially cooled inboard limiters made graphite. Assuming heat loads can be spread out evenly between limiters, 1 second discharges 2 MW ECRH heating power could run in OP1.1. The expected plasma parameters sufficient to demonstrate readiness installed diagnostics and even physics program....