arXiv subjects

Viktor Decyk

Publications and source records attributed to Viktor Decyk.

5 recordsLinked to original sources

arXiv2022

Fast inverse transform sampling of non-Gaussian distribution functions in space plasmas

Non-Gaussian distributions are commonly observed in collisionless space plasmas. Generating samples from non-Gaussian distributions is critical for the initialization of particle-in-cell simulations that investigate their driven and undriven dynamics. To this end, we report a computationally efficient, robust tool, Chebsampling, to sample general distribution functions in one and two dimensions. This tool is based on inverse transform sampling with function approximation by Chebyshev polynomials. We demonstrate practical uses of Chebsampling through sampling typical distribution functions in space plasmas.

physics.plasm-ph↗

arXiv2020

Unified view of nonlinear wave structures associated with whistler-mode chorus

A range of nonlinear wave structures, including Langmuir waves, unipolar electric fields and bipolar electric fields, are often observed in association with whistler-mode chorus waves in the near-Earth space. We demonstrate that the three seemingly different nonlinear wave structures originate from the same nonlinear electron trapping process by whistler-mode chorus waves. The ratio of the Landau resonant velocity to the electron thermal velocity controls the type of nonlinear wave structures that will be generated.

physics.space-ph↗

arXiv2017

Electrostatic and whistler instabilities excited by an electron beam

The electron beam-plasma system is ubiquitous in the space plasma environment. Here, using a Darwin particle-in-cell method, the excitation of electrostatic and whistler instabilities by a gyrating electron beam is studied in support of recent laboratory experiments. It is assumed that the total plasma frequency $ω_{pe}$ is larger than the electron cyclotron frequency $Ω_e$. The fast-growing electrostatic beam-mode waves saturate in a few plasma oscillations by slowing down and relaxing the electron beam parallel to the background magnetic field. Upon their saturation, the finite amplitude electrostatic beam-mode waves can resonate with the tail of the background thermal electrons and accelerate them to the beam parallel velocity. The slower-growing whistler waves are excited in primarily two resonance modes: (a) through Landau resonance due to the inverted slope of the beam electrons in the parallel velocity; (b) through cyclotron resonance by scattering electrons to both lower pitch angles and smaller energies. It is demonstrated that, for a field-aligned beam, the whistler instability can be suppressed by the electrostatic instability due to a faster energy transfer rate between beam electrons and the electrostatic waves. Such a competition of growth between whistler and electrostatic waves depends on the ratio of $ω_{pe}/Ω_e$. In terms of wave propagation, beam-generated electrostatic waves are confined to the beam region whereas beam-generated whistler waves transport energy away from the beam.

physics.plasm-ph↗

arXiv1997

Simulating Plasma Turbulence in Tokamaks

A challenging and fundamental research problem is the better understanding and control of the turbulent transport of heat in present-day tokamak fusion experiments. Recent developments in numerical methods along with enormous gains in computing power have made large-scale simulations an important tool for improving our understanding of this phenomena. Simulating this highly non-linear behavior requires solving for the perturbations of the phase space distribution function in five dimensions. We use a particle-in-cell approach to solve the equations. The code has been parallelized for a variety of architectures (C90, CM-5, T3D) using a 1-D domain decomposition along the toroidal axis, for which the number of particles in each cell remains approximately constant. The quasi-uniform distribution of particles, which minimizes load imbalance, coupled with the relatively small movement of particles across cells, which minimizes communications, makes this problem ideally suited to massively parallel architectures. We present the performance of the program for different numbers of processors and problem sizes. In addition, we discuss some recent scientific results obtained from the code.

physics.plasm-ph↗

arXiv1996

Massively Parallel Computing of Turbulent Transport in Tokamaks

With the advent of the gyrokinetic formalism, recent developments in low-noise nonlinear $δf$ methods, and enormous gains in computing power, large-scale gyrokinetic simulations have become an important tool for improved understanding of anomalous transport in tokamaks. Simulating the non-linear behaviour requires solving for the perturbations of distrbution function in five dimensions. We use a particle-in-cell approach to solve the equations via the non-linear characteristic method. The code has been parallelized for a variety of architecures (C90, CM-5, T3D) using a 1-D domain decomposition along the torroidal axis, for which the number of particles in each cell remains approximately constant. The quasi uniform distribution of particles, which minimizes load imbalance, coupled with the relatively small movement of particles across cells, which minimizes communications, makes this problem ideally suited to massively parallel architectures. We present the performance of the program as a function of number of processors and problem size, which demonstrates the near perfect scalability of the code. In addition, we discuss the scientific results obtained from the code and the types of problems that will be addressable as the next generation of super-computers become available.

physics.plasm-ph↗