UMD Plasma Seminar

Plasma Physics Seminar

Fall 2018 Schedule

September 5, Wednesday 3:30PM ERF 1207, Large Conference Room	Dr. George Wilkie, Chalmers University Fast spectral solution method for the nonlinear Boltzmann equation with applications Recent advances in applied mathematics (Gamba and Rjasanow, JCP 2018) make direct solution of the Boltzmann equation particularly accessible. A recently-developed implementation, LightningBoltz, has been optimized, parallelized, and rigorously benchmarked. Features include general large-angle, inelastic, and/or self-collisions. Collision matrices are pre-computed and stored on an online database, making time advancement remarkably efficient. I will also discuss possible future applications for this tool including discharges, reentry blackout, tokamak divertors, runaway electrons, carbon sequestration, and more.
September 12, Wednesday 3:30PM ERF 1207, Large Conference Room	C.S. Liu, University of Maryland Nonlinear Transformation from Convective to Absolute Stimulated Raman Backscattering Instability and Inflation of Laser Reflectivity in Laser Fusion Experiment Convective and absolute nature of parametric instabilities will be reviewed. Convective Raman Backscatter Instability can be transformed into absolute Raman Backscatter instability by trapped electrons in the Langmuir wave, leading to enhancement of reflectivity by several orders of magnitude with small change of pump laser power inflation. The relevance to the experimental observation of Raman Reflectivity in Trident laser plasma experiment will be discussed.
September 19, Wednesday 3:30PM ERF 1207, Large Conference Room	Open
September 26, Wednesday 3:30PM ERF 1207, Large Conference Room	Open
October 3, Wednesday 3:30PM ERF 1207, Large Conference Room	Open
October 10, Wednesday 3:30PM ERF 1207, Large Conference Room	Prof. Saskia Mordijck, William and Mary Changes in particle transport as a result of resonant magnetic perturbations Resonant magnetic perturbations (RMPs) have successfully been employed to suppress Edge Localized Modes (ELMs) in multiple Tokamak devices and are being considered as the main technique for mitigating and suppressing ELMs on ITER. RMPs are small steady-state magnetic perturbations that add a radial component to the equilibrium magnetic field. These small magnetic fields are optimized to be resonant with the rational surfaces at the plasma edge. RMPs reduce the large gradients at the plasma edge to values below the stability limit that is responsible for the creation of ELMs. However, adding RMPs to H-mode plasmas in DIII-D results in a strong reduction of the overall particle confinement, a so-called `density pump-out'. The effects of RMPs are thus not just limited to the plasma edge. In this talk, I will first show how these RMPs create a stochastic edge and how a comparison between neoclassical theory and experimental observations can tell us something about the extent of the stochastic layer. Next, I will show that the creation of a stochastic layer is not sufficient to explain the changes in particle confinement. Finally, I will show that the increases in turbulent transport are substantial and contribute to the overall decrease in particle confinement, not just to a reduction in confinement at the plasma edge.
October 17, Wednesday 3:30PM ERF 1207, Large Conference Room	Open
October 24, Wednesday 3:30PM ERF 1207, Large Conference Room	Open
October 29, Monday 3:30PM ERF 1207, Large Conference Room	Rogerio Jorge, Swiss Plasma Center - EPFL A gyrokinetic model for the tokamak periphery We present a new gyrokinetic model that retains the fundamental elements of the plasma dynamics at the tokamak periphery, namely electromagnetic fluctuations at all scales, comparable amplitudes of background and fluctuating components, and a large range of collisionality regimes. Such model is derived within a gyrokinetic full-F approach, describing distribution functions arbitrarily far from equilibrium, and projecting the gyrokinetic equation onto a Hermite-Laguerre velocity space polynomial basis, obtaining a gyrokinetic moment hierarchy. The treatment of arbitrary collisionalities is performed by expressing the full Coulomb collision operator in gyrocentre phase space coordinates, and providing a closed formula for its gyroaverage in terms of the gyrokinetic moments. In the electrostatic regime and long-wavelength limit, the novel gyrokinetic hierarchy reduces to a drift-kinetic moment hierarchy that in the high collisionality regime further reduces to an improved set of drift-reduced Braginskii equations, which are widely used in scrape-off layer simulations. First insights on the linear modes described by our novel gyrokinetic model will be presented.
November 7, Wednesday 3:30PM ERF 1207, Large Conference Room	APS-DPP meeting: No seminar
November 14, Wednesday 3:30PM ERF 1207, Large Conference Room	Joel Dahlin, Goddard Space Flight Center The Role of Reconnection in Magnetic Self-Organization and Catastrophic Energy Release in the Solar Corona Explosive solar activity in the form of coronal mass ejections and eruptive flares is generally agreed to be powered by the explosive ejection of highly stressed coronal magnetic fields. Magnetic reconnection has long been understood to be the primary driver for the explosive energy release. However, recent studies suggest that reconnection may also play an important role in both the formation and destabilization of the pre-eruptive field. We report on new 3D MHD numerical simulations that definitively demonstrate three distinct roles of reconnection in the evolution of an eruptive flare. The initial configuration consists of a current-free (potential) magnetic field with a coronal null point, and energy and helicity are injected into the corona via small-scale surface flows. A reconnection-mediated inverse helicity cascade concentrates highly sheared magnetic fields above photospheric reversals in the radial magnetic field. The resulting localized magnetic pressure deforms the coronal null point into a current sheet that eventually reconnects, destabilizing quasi-static force balance by removing restraining tension. The configuration expands outward slowly, stretching the magnetic field lines to form a flare current sheet. Onset of fast reconnection at this sheet expels the accumulated shear and drives rapid energy release. These results have important implications for magnetic self-organization and catastrophe in astrophysical plasmas.
November 19, Monday 3:30PM ERF 1207, Large Conference Room	Haoming Liang, West Virginia University Kinetic Entropy as a Diagnostic for Magnetic Reconnection Kinetic entropy is the entropy defined using kinetic theory for plasmas that are not necessarily in local thermodynamic equilibrium. Entropy is a natural metric of irreversible dissipation since it is conserved in ideal isolated systems and increases only when there is dissipation. It should be especially important in many heliospheric systems, where collisions can be rare so that plasmas are not in thermodynamic equilibrium. This suggests kinetic entropy can address important unsolved questions on the nature of irreversible dissipation in fundamental plasma processes such as magnetic reconnection, plasma turbulence and collisionless shocks. While entropy is often investigated in fluid and gyrokinetic systems, it is vastly underutilized in fully kinetic systems. In this work, we carry out an initial study to develop and apply the kinetic entropy diagnostic in particle-in-cell (PIC) simulations. We start with 2.5D collisionless anti-parallel reconnection. In the simulations, we calculate the commonly-used kinetic entropy written as the phase space integral of - f ln f, where f is the distribution function, and the full Boltzmann entropy related to the logarithm of the number of microstates for a specific macrostate. By decomposing kinetic entropy into a sum of velocity space and position space entropies, we show that position space entropy decreases while velocity space entropy increases during magnetic reconnection. We find that total kinetic entropy in the simulations is preserved quite well (better than three percent) and use the departure from exact conservation to quantify the effective numerical dissipation. Electrons and ions have slightly different effective collision frequencies. Finally, we use kinetic entropy to identify regions with non-Maxwellian distributions and compare the results with other approaches. Note, our work uses collisionless simulations, so we cannot yet address physical dissipation mechanisms; nonetheless, the infrastructure developed here will be useful for future studies in weakly collisional systems. It is being applied to Magnetospheric Multiscale (MMS) data.
November 28, Wednesday 3:30PM ERF 1207, Large Conference Room	Dhairya Malhotra, Courant Institute, New York University A boundary integral solver for computing force-free fields in stellarators We present a new boundary integral equation solver for computing force-free fields (Taylor relaxed states) in toroidal geometries. Such fields can be used to compute magneto-hydrodynamic (MHD) stepped pressure profile equilibria in magnetically confined plasmas in tokamaks and stellarators. Our method uses the generalized Debye source formulation for the time-harmonic Maxwell's equations, which results in a well-conditioned second-kind boundary integral equations. Compared to traditional methods, which require discretization of the entire volume, our method requires significantly fewer unknowns since we only discretize the boundary and this results in significant savings in work. However, the computation of the boundary integral operator requires efficient high-order quadratures for singular and near singular integrals on complex three-dimensional surfaces. We will discuss fast algorithms for such quadratures. We will present numerical results to show accuracy, efficiency and scalability of our method for several challenging geometries. We will also show preliminary results for computing stepped pressure profile equilibria using our solver.
December 5, Wednesday 3:30PM ERF 1207, Large Conference Room	Stefan Buller, Chalmers University Transport of collisional impurities with flux-surface density variation in stellarator plasmas Highly charged non-fuel ions can enter fusion plasmas through interactions at plasma-facing components. Due to their high charge, such impurities radiate strongly, leading to prohibitively large power losses if they are allowed to accumulate in the core of the fusion plasma. Additionally, their high charge makes these impurities susceptible to slight variations in the electrostatic potential that arise along the magnetic field in fusion devices, which cause them to develop density variation along the magnetic field lines. We here present an analytic calculation that shows how the transport of these heavy impurities are affected by such density variations in stellarators, and apply it to find optimal density variations that reduce or prevent the accumulation of impurities.
December 12, Wednesday 3:30PM ERF 1207, Large Conference Room	Open

Contact Information: Marc Swisdak or Matt Landreman