Ralf Schneider - Academia.edu (original) (raw)

Papers by Ralf Schneider

Applied Sciences, 2018

In this work, the equations of motion for table-tennis balls were numerically solved on graphics ... more In this work, the equations of motion for table-tennis balls were numerically solved on graphics processing units (GPUs) using Compute Unified Device Architecture (CUDA) for systematical statistical studies of the impact of ball size and weight, as well as of net height, on the distribution functions of successful strokes. Half a billion different initial conditions involving hitting location, initial spin, and velocities were analyzed to reach sufficient statistical significance for the different cases. In this paper, an advanced statistical analysis of the database generated by the simulation is presented.

Nuclear Fusion, 2004

... 73 107–24 [7] Reiman AH and Greenside HS 1988 Numerical solution of three-dimensional magneti... more ... 73 107–24 [7] Reiman AH and Greenside HS 1988 Numerical solution of three-dimensional magnetic differential equations J. Comput. ... 1990) vol 2 (Vienna: IAEA) p 603 [10] Borchardt M., Riemann J., Schneider R. and Bonnin X. 2001 W7-X modelling with the 3D SOL fluid code ...

International Journal of Modern Physics C, 2009

Quantum states can be described equivalently by density matrices, Wigner functions, or quantum to... more Quantum states can be described equivalently by density matrices, Wigner functions, or quantum tomograms. We analyze the accuracy and performance of three related semiclassical approaches to quantum dynamics, in particular with respect to their numerical implementation. As test cases, we consider the time evolution of Gaussian wave packets in different one-dimensional geometries, whereby tunneling, resonance, and anharmonicity effects are taken into account. The results and methods are benchmarked against an exact quantum mechanical treatment of the system, which is based on a highly efficient Chebyshev expansion technique of the time evolution operator.

AIP Conference Proceedings, 2007

AIP Conference Proceedings, 2007

Plasma edge physics is one of the major challenges in fusion plasmas. The need for power and part... more Plasma edge physics is one of the major challenges in fusion plasmas. The need for power and particle exhaust for any reactor inspired a lot of theoretical and experimental work. Understanding this physics requires a multi-scale ansatz bringing together also several physics and numerical models. The plasma edge of fusion experiments is characterized by atomic and molecular processes. Hydrogenic ions and neutrals hit material walls with energies from several eV up to 1000s of eV. They saturate the wall materials and due to physical or chemical processes neutrals are released from the wall, both atomic and molecular. They determine via interaction with the plasma strongly its properties. These processes can be beneficial for a fusion experiment by using radiation losses to minimize the power load problem of target plates, but also can create severe problems if the dilution of the plasma gets too large or condensation radiation instabilities can be created. A complete physics model for the plasma-wall interaction processes alone is already rather challenging (and still missing): it requires e.g. inclusion of collision cascades, chemical formation of molecules, diffusion in strongly 3D systems. A full description needs a multi-scale model combining quite different numerical techniques like molecular dynamics, binary collisions, kinetic Monte Carlo and mixed conduction/convection equations in strongly anisotropic systems.

AIP Conference Proceedings, 2008

A 3-dimensional Particle-Particle Particle-Mesh (P3M) code is applied to study the charging proce... more A 3-dimensional Particle-Particle Particle-Mesh (P3M) code is applied to study the charging process of micrometer size dust grains confined in a capacitive RF discharge. In our model, particles (electrons and ions) are treated kinetically (Particle-in-Cell with Monte Carlo Collisions (PIC-MCC)). In order to accurately resolve the plasma particles' motion close to the dust grain, the PIC technique is supplemented with

プラズマ 核融合学会年会予稿集, Nov 18, 2003

Plasma edge physics is one of the major challenges in fusion plasmas. The need for power and part... more Plasma edge physics is one of the major challenges in fusion plasmas. The need for power and particle exhaust for any reactor inspired a lot of theoretical and experimental work. Understanding this physics requires a multi-scale ansatz bringing together also several physics and numerical models. The plasma edge of fusion experiments is characterized by atomic and molecular processes. Hydrogenic ions and neutrals hit material walls with energies from several eV up to 1000s of eV. They saturate the wall materials and due to physical or chemical processes neutrals are released from the wall, both atomic and molecular. They determine via interaction with the plasma strongly its properties. A complete physics model for the plasma-wall interaction processes alone is already rather challenging (and still missing): it requires e.g. inclusion of collision cascades, chemical formation of molecules, diffusion in strongly 3D systems. A full description needs a multi-scale model combining quite different numerical techniques like molecular dynamics, binary collisions, kinetic Monte Carlo and mixed conduction/convection equations in strongly anisotropic systems.

The coupled B2–Eirene multi-fluid code has been used to simulate the plasma evolution for a densi... more The coupled B2–Eirene multi-fluid code has been used to simulate the plasma evolution for a density-ramp deuterium discharge in ASDEX–Upgrade starting from the low density region which is hard to realize experimentally up to the density limit. In particular, the two critical phenomena, detachment and marfe formation are considered in detail, both beeing related to the negative derivative of the impurity radiation characteristic and the nonlinear dependence of the parallel thermal conductivity on the electron temperature. During detachment the plasma changes from a high temperature divertor state to a low temperature one (bifurcation effect / 1 /); marfe formation – indicating the density limit – is connected to closed flux surfaces and leads to an unstable plasma state. The latter, however, can dynamically be stabilized by appropriately controlling the density. The calculations refer to the MHD equilibrium and the plasma parameter region of the discharge #7758. The actual discharge is simulated by the steady states of the plasma in the corresponding density range.

A newly developed 2d3v Particle-in-Cell code with Monte Carlo (MC) Collisions is used to simulate... more A newly developed 2d3v Particle-in-Cell code with Monte Carlo (MC) Collisions is used to simulate the operation of the PPPL 100 W cylindrical Hall thruster (CHT). The model includes all relevant collisional processes (Coulomb collisions, electron-neutral elastic, ionization and excitation collisions; ion-neutral momentum transfer and charge exchange collisions). The dynamics of the background gas is self-consistently resolved with direct MC simulation. Secondary electron emission at the thruster walls is accounted through a probabilistic MC model. The anomalous electron transport perpendicular to magnetic field is included in the simulation via Bohm-type diffusion. The computational domain includes the thruster channel and the near-field plume region. The selfsustained and the current overrun modes of the CHT operation are studied and compared with experiments.

A new 3 dimensional Particle-in-Cell code was applied to simulate the operation of a 100 W CHT th... more A new 3 dimensional Particle-in-Cell code was applied to simulate the operation of a 100 W CHT thruster. In the simulation a spoke moving with a velocity of about 0.8 cm/µ µ µ µs was observed. The initial position of the spoke was found to be strongly correlated with the cathode placement. The simulation has shown that the depletion of neutral gas can lead to azimuthal asymmetry of the discharge and possibly to the spoke phenomenon.

ABSTRACT The Particle-in-Cell (PIC) method was used to study two different ion thruster concepts ... more ABSTRACT The Particle-in-Cell (PIC) method was used to study two different ion thruster concepts –Stationary Plasma Thrusters (SPT) and High Efficiency Multistage Plasma Thrusters (HEMP-T), in particular the plasma properties in the discharge chamber due to the different magnetic field configurations. Special attention was paid to the simulation of plasma particle fluxes on the thrusters' channel surfaces. In both cases, PIC proved itself as a powerful tool, delivering important insight into the basic physics of the different thruster concepts. The simulations demonstrated that the new HEMP thruster concept allows for a high thermal efficiency due to both minimal energy dissipation and high acceleration efficiency. In the HEMP thruster the plasma contact to the wall is limited only to very small areas of the magnetic field cusps, which results in much smaller ion energy flux to the thruster channel surface as compared to SPT. The erosion yields for dielectric discharge channel walls of SPT and HEMP thrusters were calculated with the binary collision code SDTrimSP. For SPT, an erosion rate on the level of 1 mm of sputtered material per hour was observed. For HEMP, thruster simulations have shown that there is no erosion inside the dielectric discharge channel.

Applied Sciences, 2018

In this work, the equations of motion for table-tennis balls were numerically solved on graphics ... more In this work, the equations of motion for table-tennis balls were numerically solved on graphics processing units (GPUs) using Compute Unified Device Architecture (CUDA) for systematical statistical studies of the impact of ball size and weight, as well as of net height, on the distribution functions of successful strokes. Half a billion different initial conditions involving hitting location, initial spin, and velocities were analyzed to reach sufficient statistical significance for the different cases. In this paper, an advanced statistical analysis of the database generated by the simulation is presented.

Nuclear Fusion, 2004

... 73 107–24 [7] Reiman AH and Greenside HS 1988 Numerical solution of three-dimensional magneti... more ... 73 107–24 [7] Reiman AH and Greenside HS 1988 Numerical solution of three-dimensional magnetic differential equations J. Comput. ... 1990) vol 2 (Vienna: IAEA) p 603 [10] Borchardt M., Riemann J., Schneider R. and Bonnin X. 2001 W7-X modelling with the 3D SOL fluid code ...

International Journal of Modern Physics C, 2009

Quantum states can be described equivalently by density matrices, Wigner functions, or quantum to... more Quantum states can be described equivalently by density matrices, Wigner functions, or quantum tomograms. We analyze the accuracy and performance of three related semiclassical approaches to quantum dynamics, in particular with respect to their numerical implementation. As test cases, we consider the time evolution of Gaussian wave packets in different one-dimensional geometries, whereby tunneling, resonance, and anharmonicity effects are taken into account. The results and methods are benchmarked against an exact quantum mechanical treatment of the system, which is based on a highly efficient Chebyshev expansion technique of the time evolution operator.

AIP Conference Proceedings, 2007

AIP Conference Proceedings, 2007

Plasma edge physics is one of the major challenges in fusion plasmas. The need for power and part... more Plasma edge physics is one of the major challenges in fusion plasmas. The need for power and particle exhaust for any reactor inspired a lot of theoretical and experimental work. Understanding this physics requires a multi-scale ansatz bringing together also several physics and numerical models. The plasma edge of fusion experiments is characterized by atomic and molecular processes. Hydrogenic ions and neutrals hit material walls with energies from several eV up to 1000s of eV. They saturate the wall materials and due to physical or chemical processes neutrals are released from the wall, both atomic and molecular. They determine via interaction with the plasma strongly its properties. These processes can be beneficial for a fusion experiment by using radiation losses to minimize the power load problem of target plates, but also can create severe problems if the dilution of the plasma gets too large or condensation radiation instabilities can be created. A complete physics model for the plasma-wall interaction processes alone is already rather challenging (and still missing): it requires e.g. inclusion of collision cascades, chemical formation of molecules, diffusion in strongly 3D systems. A full description needs a multi-scale model combining quite different numerical techniques like molecular dynamics, binary collisions, kinetic Monte Carlo and mixed conduction/convection equations in strongly anisotropic systems.

AIP Conference Proceedings, 2008

A 3-dimensional Particle-Particle Particle-Mesh (P3M) code is applied to study the charging proce... more A 3-dimensional Particle-Particle Particle-Mesh (P3M) code is applied to study the charging process of micrometer size dust grains confined in a capacitive RF discharge. In our model, particles (electrons and ions) are treated kinetically (Particle-in-Cell with Monte Carlo Collisions (PIC-MCC)). In order to accurately resolve the plasma particles' motion close to the dust grain, the PIC technique is supplemented with

プラズマ 核融合学会年会予稿集, Nov 18, 2003

Plasma edge physics is one of the major challenges in fusion plasmas. The need for power and part... more Plasma edge physics is one of the major challenges in fusion plasmas. The need for power and particle exhaust for any reactor inspired a lot of theoretical and experimental work. Understanding this physics requires a multi-scale ansatz bringing together also several physics and numerical models. The plasma edge of fusion experiments is characterized by atomic and molecular processes. Hydrogenic ions and neutrals hit material walls with energies from several eV up to 1000s of eV. They saturate the wall materials and due to physical or chemical processes neutrals are released from the wall, both atomic and molecular. They determine via interaction with the plasma strongly its properties. A complete physics model for the plasma-wall interaction processes alone is already rather challenging (and still missing): it requires e.g. inclusion of collision cascades, chemical formation of molecules, diffusion in strongly 3D systems. A full description needs a multi-scale model combining quite different numerical techniques like molecular dynamics, binary collisions, kinetic Monte Carlo and mixed conduction/convection equations in strongly anisotropic systems.

The coupled B2–Eirene multi-fluid code has been used to simulate the plasma evolution for a densi... more The coupled B2–Eirene multi-fluid code has been used to simulate the plasma evolution for a density-ramp deuterium discharge in ASDEX–Upgrade starting from the low density region which is hard to realize experimentally up to the density limit. In particular, the two critical phenomena, detachment and marfe formation are considered in detail, both beeing related to the negative derivative of the impurity radiation characteristic and the nonlinear dependence of the parallel thermal conductivity on the electron temperature. During detachment the plasma changes from a high temperature divertor state to a low temperature one (bifurcation effect / 1 /); marfe formation – indicating the density limit – is connected to closed flux surfaces and leads to an unstable plasma state. The latter, however, can dynamically be stabilized by appropriately controlling the density. The calculations refer to the MHD equilibrium and the plasma parameter region of the discharge #7758. The actual discharge is simulated by the steady states of the plasma in the corresponding density range.

A newly developed 2d3v Particle-in-Cell code with Monte Carlo (MC) Collisions is used to simulate... more A newly developed 2d3v Particle-in-Cell code with Monte Carlo (MC) Collisions is used to simulate the operation of the PPPL 100 W cylindrical Hall thruster (CHT). The model includes all relevant collisional processes (Coulomb collisions, electron-neutral elastic, ionization and excitation collisions; ion-neutral momentum transfer and charge exchange collisions). The dynamics of the background gas is self-consistently resolved with direct MC simulation. Secondary electron emission at the thruster walls is accounted through a probabilistic MC model. The anomalous electron transport perpendicular to magnetic field is included in the simulation via Bohm-type diffusion. The computational domain includes the thruster channel and the near-field plume region. The selfsustained and the current overrun modes of the CHT operation are studied and compared with experiments.

A new 3 dimensional Particle-in-Cell code was applied to simulate the operation of a 100 W CHT th... more A new 3 dimensional Particle-in-Cell code was applied to simulate the operation of a 100 W CHT thruster. In the simulation a spoke moving with a velocity of about 0.8 cm/µ µ µ µs was observed. The initial position of the spoke was found to be strongly correlated with the cathode placement. The simulation has shown that the depletion of neutral gas can lead to azimuthal asymmetry of the discharge and possibly to the spoke phenomenon.

ABSTRACT The Particle-in-Cell (PIC) method was used to study two different ion thruster concepts ... more ABSTRACT The Particle-in-Cell (PIC) method was used to study two different ion thruster concepts –Stationary Plasma Thrusters (SPT) and High Efficiency Multistage Plasma Thrusters (HEMP-T), in particular the plasma properties in the discharge chamber due to the different magnetic field configurations. Special attention was paid to the simulation of plasma particle fluxes on the thrusters' channel surfaces. In both cases, PIC proved itself as a powerful tool, delivering important insight into the basic physics of the different thruster concepts. The simulations demonstrated that the new HEMP thruster concept allows for a high thermal efficiency due to both minimal energy dissipation and high acceleration efficiency. In the HEMP thruster the plasma contact to the wall is limited only to very small areas of the magnetic field cusps, which results in much smaller ion energy flux to the thruster channel surface as compared to SPT. The erosion yields for dielectric discharge channel walls of SPT and HEMP thrusters were calculated with the binary collision code SDTrimSP. For SPT, an erosion rate on the level of 1 mm of sputtered material per hour was observed. For HEMP, thruster simulations have shown that there is no erosion inside the dielectric discharge channel.