Bulletin of the American Physical Society
58th Annual Meeting of the APS Division of Plasma Physics
Volume 61, Number 18
Monday–Friday, October 31–November 4 2016; San Jose, California
Session NO6: Simulation Methods and Applications |
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Chair: Carl Sovinec, Univ. Wisconsin-Madison Room: 230 C |
Wednesday, November 2, 2016 9:30AM - 9:42AM |
NO6.00001: Monte-Carlo finite elements gyrokinetic simulations of Alfven modes in tokamaks Alberto Bottino, Alessandro Biancalani, Francesco Palermo, Natalia Tronko The global gyrokinetic code ORB5 [S. Jolliet et al., Comp. Phys. Comm., 177, 409 (2007)] can simultaneously include electromagnetic perturbations, general ideal MHD axisymmetric equilibria, zonal-flow preserving sources, collisions, and the ability to solve the full core plasma including the magnetic axis. In this work, a Monte Carlo Particle In Cell Finite Element model, starting from a gyrokinetic discrete Lagrangian, is derived and implemented into the ORB5 code. The variations of the Lagrangian are used to obtain the time continuous equations of motion for the particles and the Finite Element approximation of the field equations. The Noether theorem for the semi-discretised system, implies a certain number of conservation properties for the final set of equation. Linear and nonlinear results, concerning Alfv\'en instabilities, in the presence of an energetic particle population, and microinstabilities, such as electromagnetic ion temperature gradient (ITG) driven modes and kinetic ballooning modes (KBM), will be presented and discussed. Due to losses of energetic particles, Alfv\'en instabilities can not only affect plasma stability and damage the walls, but also strongly impact the heating efficiency of a fusion reactor and ultimately the possibility of reaching ignition. [Preview Abstract] |
Wednesday, November 2, 2016 9:42AM - 9:54AM |
NO6.00002: MOVED TO POSTER |
Wednesday, November 2, 2016 9:54AM - 10:06AM |
NO6.00003: Interactive, multiobjective Bayesian optimization of tokamak scenarios Jakub Urban, Jean-Fran\c{c}ois Artaud Bayesian optimization is applied to tokamak scenario optimizations. The key advantages are 1) a reduced number of objective function evaluations, 2) no need for derivatives, and 3) the possibility to include a prior knowledge. This is of a great value for optimizing tokamak scenarios, where several (competing) objectives with often unknown magnitudes exist and the number of parameters is large (>10). The first two properties imply that Bayesian optimization is well suited for heavy, complex objective functions. Reusing previous iterations as priors for next optimization steps effectively enables interactive, multiobjective optimizations, regardless of whether a human decision maker is included or not. We show that these features make Bayesian optimization an outstanding tool for optimizing tokamak scenarios. Objective functions and constraints, targeting, e.g., fusion gain, flux consumption, coils currents limits or q-profile, can be assembled interactively. The optimized parameter vector may include actuators like plasma current or heating waveforms. We demonstrate the capabilities on optimizing ITER and DEMO-like scenarios, simulated by the METIS code. [Preview Abstract] |
Wednesday, November 2, 2016 10:06AM - 10:18AM |
NO6.00004: Performance Evaluation of the Electrostatic Particle-in-Cell Code hPIC on the Blue Waters Supercomputer Rinat Khaziev, Ryan Mokos, Davide Curreli The newly-developed hPIC code is a kinetic-kinetic electrostatic Particle-in-Cell application, targeted at large-scale simulations of Plasma-Material Interactions. The code can simulate multi-component strongly-magnetized plasmas in a region close to the wall, including the magnetic sheath/presheath and the first surface layers, which release material impurities. The Poisson solver is based on PETSc conjugate gradient with BoomerAMG algebraic multigrid preconditioners. Scaling tests on the Blue Waters supercomputer have demonstrated good strong-scaling up to 262,144 cores and excellent weak-scaling (tested up to 64,000 cores). In this presentation, we will make an overview of the on-node optimization activities and the main code features, as well as provide a detailed analysis of the results of the verification tests performed. [Preview Abstract] |
Wednesday, November 2, 2016 10:18AM - 10:30AM |
NO6.00005: Capabilities of Fully Parallelized MHD Stability Code MARS Vladimir Svidzinski, Sergei Galkin, Jin-Soo Kim, Yueqiang Liu Results of full parallelization of the plasma stability code MARS will be reported. MARS calculates eigenmodes in 2D axisymmetric toroidal equilibria in MHD-kinetic plasma models. Parallel version of MARS, named PMARS, has been recently developed at FAR-TECH. Parallelized MARS is an efficient tool for simulation of MHD instabilities with low, intermediate and high toroidal mode numbers within both fluid and kinetic plasma models, implemented in MARS. Parallelization of the code included parallelization of the construction of the matrix for the eigenvalue problem and parallelization of the inverse vector iterations algorithm, implemented in MARS for the solution of the formulated eigenvalue problem. Construction of the matrix is parallelized by distributing the load among processors assigned to different magnetic surfaces. Parallelization of the solution of the eigenvalue problem is made by repeating steps of the MARS algorithm using parallel libraries and procedures. Parallelized MARS is capable of calculating eigenmodes with significantly increased spatial resolution: up to 5,000 adapted radial grid points with up to 500 poloidal harmonics. Such resolution is sufficient for simulation of kink, tearing and peeling-ballooning instabilities with physically relevant parameters. [Preview Abstract] |
Wednesday, November 2, 2016 10:30AM - 10:42AM |
NO6.00006: Kinematic determination of Electron-Hole velocities Ian H Hutchinson, C Zhou Coherent self-sustaining BGK potential structures, like the electron holes that often form during nonlinear electrostatic instabilities and are frequently observed in space plasmas, have ``kinematic'' momentum conservation properties that determine their velocity. The electron and ion momentum, both internal and external to the hole, must be included. Momentum changes arise from hole acceleration and from hole depth growth, by energization processes we call jetting; and these must balance any additional external forces on the particles. Comprehensive analytic expressions for the contributions have been calculated for holes of arbitrary localized potential form. Using these, we can deduce velocity changes in various interesting situations such as the self-acceleration of electron holes during formation, the circumstances under which holes accelerate at the rate of the electrons in a background electric field, the influence of the ion stream pushing and pulling holes to higher or lower speeds, and the trapping of hole velocity between the velocity of two ion streams. The predictions are in excellent quantitative agreement with targeted PIC simulations. The kinematic theory thus explains why isolated holes behave the way they do. [Preview Abstract] |
Wednesday, November 2, 2016 10:42AM - 10:54AM |
NO6.00007: Electron hole tracking PIC simulation Chuteng Zhou, Ian Hutchinson An electron hole is a coherent BGK mode solitary wave. Electron holes are observed to travel at high velocities relative to bulk plasmas. The kinematics of a 1-D electron hole is studied using a novel Particle-In-Cell simulation code with fully kinetic ions. A hole tracking technique enables us to follow the trajectory of a fast-moving solitary hole and study quantitatively hole acceleration and coupling to ions. The electron hole signal is detected and the simulation domain moves by a carefully designed feedback control law to follow its propagation. This approach has the advantage that the length of the simulation domain can be significantly reduced to several times the hole width, which makes high resolution simulations tractable. We observe a transient at the initial stage of hole formation when the hole accelerates to several times the cold-ion sound speed. Artificially imposing slow ion speed changes on a fully formed hole causes its velocity to change even when the ion stream speed in the hole frame greatly exceeds the ion thermal speed, so there are no reflected ions. The behavior that we observe in numerical simulations agrees very well with our analytic theory of hole momentum conservation and energization effects we call ``jetting''. [Preview Abstract] |
Wednesday, November 2, 2016 10:54AM - 11:06AM |
NO6.00008: Largescale Long-term particle Simulations of Runaway electrons in Tokamaks Jian Liu, Hong Qin, Yulei Wang To understand runaway dynamical behavior is crucial to assess the safety of tokamaks. Though many important analytical and numerical results have been achieved, the overall dynamic behaviors of runaway electrons in a realistic tokamak configuration is still rather vague. In this work, the secular full-orbit simulations of runaway electrons are carried out based on a relativistic volume-preserving algorithm. Detailed phase-space behaviors of runaway electrons are investigated in different timescales spanning 11 orders. A detailed analysis of the collisionless neoclassical scattering is provided when considering the coupling between the rotation of momentum vector and the background field. In large timescale, the initial condition of runaway electrons in phase space globally influences the runaway distribution. It is discovered that parameters and field configuration of tokamaks can modify the runaway electron dynamics significantly. Simulations on 10 million cores of supercomputer using the APT code have been completed. A resolution of 10$^{\mathrm{7}}$ in phase space is used, and simulations are performed for 10$^{\mathrm{11}}$ time steps. Largescale simulations show that in a realistic fusion reactor, the concern of runaway electrons is not as serious as previously thought. [Preview Abstract] |
Wednesday, November 2, 2016 11:06AM - 11:18AM |
NO6.00009: MHD simulations of Plasma Jets and Plasma-surface interactions in Coaxial Plasma Accelerators Vivek Subramaniam, Laxminarayan Raja Coaxial plasma accelerators belong to a class of electromagnetic acceleration devices which utilize a self-induced Lorentz force to accelerate magnetized thermal plasma to large velocities ($\sim $40 Km/s). The plasma jet generated as a result, due to its high energy density, can be used to mimic the plasma-surface interactions at the walls of thermonuclear fusion reactors during an Edge Localized Mode (ELM) disruption event. We present the development of a Magnetohydrodynamics (MHD) simulation tool to describe the plasma acceleration and jet formation processes in coaxial plasma accelerators. The MHD model is used to study the plasma-surface impact interaction generated by the impingement of the jet on a target material plate. The study will characterize the extreme conditions generated on the target material surface by resolving the magnetized shock boundary layer interaction and the viscous/thermal diffusion effects. Additionally, since the plasma accelerator is operated in vacuum conditions, a novel plasma-vacuum interface tracking algorithm is developed to simulate the expansion of the high density plasma into a vacuum background in a physically consistent manner. [Preview Abstract] |
Wednesday, November 2, 2016 11:18AM - 11:30AM |
NO6.00010: PIC simulation of reactive radio-frequency plasma Paul Matthias, Daniel Kahnfeld, Karl Lueskow, Gunnar Bandelow, Ralf Schneider, Stefan Kemnitz, Julia Duras Reactive plasmas are important for industrial applications. For sputter processes and plasma etching especially asymmetric capacitively coupled plasmas with a radio-frequency modulated voltage are used. The latest experimental results show an unexpected high-energy peak of negative ions at the grounded anode, depending on the cathode material. Here the Particle-in-Cell (PIC) method was used to simulate this experiment. The main mechanism for the effect is identified as the production of negative ions near the surface of the cathode. In a one dimensional simulation the negative ions are trapped inside the plasma because of the symmetric potential. Thus it was shown that these high-energy peaks of negative ions at the anode only appear in asymmetric discharges, due to the self-bias voltage. To reproduce the asymmetry a two dimensional model will be used in the future. [Preview Abstract] |
Wednesday, November 2, 2016 11:30AM - 11:42AM |
NO6.00011: Particle-In-Cell simulation concerning heat-flux mitigation using electromagnetic fields Karl Felix L\"uskow, Julia Duras, Stefan Kemnitz, Daniel Kahnfeld, Paul Matthias, Gunnas Bandelow, Ralf Schneider, Detlev Konigorski In space missions enormous amount of money is spent for the thermal protection system for re-entry. To avoid complex materials and save money one idea is to reduce the heat-flux towards the spacecraft. The partially-ionized gas can be controlled by electromagnetic fields. For first-principle tests partially ionized argon flow from an arc-jet was used to measure the heat-flux mitigation created by an external magnetic field. In the successful experiment a reduction of 85\% was measured. In this work the Particle-in-Cell (PIC) method was used to simulate this experiment. PIC is able to reproduce the heat flux mitigation qualitatively. The main mechanism is identified as a changed electron transport and by this, modified electron density due to the reaction to the applied magnetic field. Ions follow due to quasi-neutrality and influence then strongly by charge exchange collisions the neutrals dynamics and heat deposition. [Preview Abstract] |
Wednesday, November 2, 2016 11:42AM - 11:54AM |
NO6.00012: Calculations of the Raman photoionization cross section for bound-free transitions in Neon-like Iron Michael Kruse, James Gaffney, Carlos Iglesias, Brian Wilson The recent higher-than-expected solar opacity measurements of Bailey {\em et al} on the Sandia Z-machine have raised questions over the accuracy of theoretical opacity models near the solar convection-radiation boundary\footnote{J.E. Bailey {\em et al}, Nature {\bf 517}, 56-59}. Of concern in particular are the Iron opacities for which discrepancies of 30\%-400\% were found between theory and experiment. Naturally the question has been raised whether theoretical models have neglected to include all the relevant atomic physics processes. In this talk we discuss the effects of the hitherto neglected two-photon ionization cross section for bound-free transitions in Neon-like Iron (a prominent charge state in the solar convection-radiation region). The calculations proceed by solving the Schroedinger equation for an electron moving in a parameterized mean-field potential that has been fitted to experimental data. The required dipole transition strength is calculated by the Dalgarno and Lewis method which exactly recovers the summation over the infinite set of intermediate states between the initial and final state. Conclusions are given with respect to opacity models. [Preview Abstract] |
Wednesday, November 2, 2016 11:54AM - 12:06PM |
NO6.00013: A newly discovered role of self-electric fields to ohmic breakdown in a Toakamk Min-Gu Yoo, Jeongwon Lee, Young-Gi Kim, Yong-Su Na The ohmic breakdown is one of the general methods to initiate the plasma in a tokamak. During the process of inducing the toroidal electric fields to generate electron avalanche, time-varying complicated electromagnetic structures are produced in the tokamak. The physical mechanism of the ohmic breakdown has not been revealed clearly due to these complexities. Especially, self-electric fields, produced by space-charges in the plasma, have been paid little attention so that their magnitude has been assumed to be negligible. During the ohmic breakdown, however, the exponentially growing plasma density becomes high enough to produce strong self-electric fields much larger than the external toroidal electric fields. Therefore, the self-electric fields must be considered to understand the ohmic breakdown physics properly. For this purpose, a particle simulation code BREAK has been developed and applied to various ohmic breakdown scenarios to investigate unrevealed physical mechanisms self-consistently and systematically under the realistic complex circumstances. As a result, significant roles of the self-electric fields, such as decrease of the plasma density growth rate and enhancement of the perpendicular transports by ExB drifts, are newly discovered. [Preview Abstract] |
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