Seminar
Recent events from the MagnetoFluid Dynamics Seminar are listed below. To see the complete list of events, visit the seminar's main page, here.
Past Events

Tuesday, May 7, 201911AM, Warren Weaver Hall 905
A momentbased model for edge plasma dynamics at arbitrary collisionality
Rogerio Jorge, University of Maryland, College ParkSynopsis:
Despite significant development over the last decades, a model able to describe the periphery region of magnetic confinement fusion devices, extending from the edge to the far scrapeoff layer, is still missing. The lack of a proper model has undermined our ability to properly simulate the plasma dynamics in this region, which is necessary to predict the heat flux to the vessel wall of future fusion devices, the LH transition, and ELM dynamics. These are some of the most important issues on the way to a fusion reactor. In this work, a driftkinetic and a gyrokinetic model able to describe the plasma dynamics in the tokamak periphery are developed, which take into account electrostatic fluctuations at all relevant scales, allowing for comparable amplitudes of background and fluctuating components. In addition, the models implement a full Coulomb collision operator, and are therefore valid at arbitrary collisionality regimes. For an efficient numerical implementation of the models, the resulting kinetic equations are projected onto a HermiteLaguerre velocityspace polynomial basis, obtaining a momenthierarchy. The use of systematic closures to truncate the momenthierarchy equation, such as the semicollisional closure, allows for the straightforward adjustment of the kinetic physics content of the model. In the electrostatic high collisionality regime, our models are therefore reduced to an improved set of driftreduced Braginskii equations, which are widely used in scrapeoff layer simulations. The first numerical studies based on our models are carried out, shedding light on the interplay between collisional, using the Coulomb collision operator, and collisionless mechanisms. In particular, the dynamics of electronplasma waves and the driftwave instability are studied at arbitrary collisionality. A comparison is made with the collisionless limit and simplified collision operators used in stateoftheart simulation codes, where large deviations in the growth rates and eigenmode spectra are found, especially at the levels of collisionality relevant for present and future magnetic confinement fusion devices.

Tuesday, April 2, 201911AM, Warren Weaver Hall 905
Integrating guiding center motion in loop space
Joshua Burby, Los Alamos National LaboratorySynopsis:
In a strong, weakly inhomogeneous magnetic field, charged particles rapidly gyrate around magnetic field lines on the cyclotron timescale. On longer timescales, the center of gyration suffers a slow drift. Computing this longtimescale drift is known as the guiding center problem. The established method for tackling the guiding center problem involves first moving into a perturbed coordinate system that decouples the gyration from the drift. Leveraging this decoupling, standard numerical integration schemes for nonstiff systems are then applied in the new coordinates. The major shortcoming of this approach is that the perturbed coordinates are only known as difficulttocompute asymptotic series. Highorder terms in the series are especially bothersome because they involve highorder derivatives of the magnetic field. In this talk I will describe an alternative solution of the guiding center problem that does not involve asymptotic coordinate transformations. The crux of the new approach is the observation that drift dynamics may be identified with a slow manifold in the phase space for loops entrained in the Lorentz force flow. I will demonstrate that loop dynamics on the slow manifold may be computed efficiently using a fullyimplicit energyconserving integrator for deformable loops coupled with a method for preparing initial conditions on the slow manifold. In particular, I will show that the slow manifold structure may be exploited to cast each implicit timestep as a wellconditioned fixedpoint problem.

Tuesday, March 12, 201911AM, Warren Weaver Hall 905
Available energy of magnetically confined plasmas
Per Helander, Max Planck Institute for Plasma Physics, Greifswald, GermanySynopsis:
In this talk, the energy budget of a collisionless plasma subject to electrostatic fluctuations is studied. In particular, the excess of thermal energy over the minimum accessible to it under various constraints that limit the possible forms of plasma motion is considered. This excess measures how much thermal energy is “available” for conversion into plasma instabilities, and therefore constitutes a nonlinear measure of plasma stability. The “available energy” defined in this way becomes an interesting and useful quantity in situations where adiabatic invariants impose nontrivial constraints on the plasma motion. For instance, microstability properties of certain stellarators can be inferred directly from the available energy, without the need to solve the gyrokinetic equation. The technique also suggests that an electronpositron plasma confined by a dipole magnetic field could be entirely free from turbulence.

Tuesday, March 5, 201911AM, Warren Weaver Hall 905
An energyconserving and asymptoticpreserving time integrator for implicit PIC simulation of magnetized plasmas
Lee Ricketson, Lawrence Livermore National LaboratorySynopsis:
Tremendous progress has been made on implicit particleincell (PIC) schemes in recent years. They feature exact energy conservation and have been shown to be more robust to the finite grid instability than their explicit counterparts, making them quite powerful for longtime simulation. Simultaneously, there has been increasing interest in implicit, fullorbit kinetic simulation that steps over the gyration timescale as an alternative to gyrokinetics. Such an approach would have the noteworthy advantage of being able to handle widely varying levels of magnetization within the computational domain. We present a new fullorbit time integrator at the intersection of these two ideas. The integrator is built on CrankNicolson and preserves the crucial exact energy conservation property of implicit PIC, but it also reproduces all firstorder guiding center drifts and the correct gyroradius when stepping over the gyration timescale, all while converging to the full orbit dynamics for small timesteps. The key innovations are (1) the ability to capture the gradB drift and mirror force without breaking energy conservation and (2) a detailed understanding of the timestep restrictions on the scheme along with an adaptive timestepping strategy that ensures these restrictions are respected. Results from several test problems are presented that demonstrate the scheme’s effectiveness. Notably, we are able to predict trapped/passing boundaries, adiabatic invariance of magnetic moment, and behavior when passing through unmagnetized regions much more accurately than previous efforts.
Notes:
Work performed under the auspices of the U.S. Department of Energy by LLNL and LANL under contracts DEAC5207NA27344, DEAC5206NA25396, and supported by the Exascale Computing Project (17SC20SC), a collaborative effort of the U.S. Department of Energy Office of Science and the National Nuclear Security Administration.

Tuesday, February 26, 201911AM, Warren Weaver Hall 905
A flux balanced HasegawaWakatani model for collisional plasma edge turbulence
Di Qi, Courant Institute of Mathematical SciencesSynopsis:
We propose a new reduced fluid model for the study of the drift wave–zonal flow dynamics in
magnetically confined plasmas. Our model can be viewed as an extension of the classic Hasegawa
Wakatani (HW) model and is based on an improved treatment of the electron dynamics parallel to
the field lines, to guarantee a balanced electron flux on the magnetic surfaces. Our fluxbalanced
HW (bHW) model contains the same driftwave instability as previous HW models, but also has unique features which distinguish it from these models: 1) it converges exactly to the modified HasegawaMima model in the collisionless limit; 2) zonal structures are always present in the fluxbalanced model, even for high resistivity, and strongly reduce the level of particle and vorticity flux; 3) the more robust zonal jets also have a higher variability, which is further enhanced when the computational domain is chosen to be elongated in the radial direction.
When the computational domain is elongated in the radial direction, we furthermore observe complex multiscale dynamics, with multiple jets interacting with one another, and intermittent bursts. 
Tuesday, February 12, 201911AM, Warren Weaver Hall 905
A Multiscale, Conservative, Adaptive Implicit Algorithm for the Multispecies VlasovFokkerPlanckLandau Equation
Luis Chacon, Los Alamos National LaboratorySynopsis:
The VlasovFokkerPlanckLandau (VFP) model is considered a firstprinciples model for the simulation of collisional plasmas. It has realworld applications in the simulation of many laboratory and natural plasma systems of interest, including the Sun, Earth’s magnetosphere, and thermonuclear fusion experiments. As a mathematical model, however, VFP presents notable challenges for its efficient solution, including high dimensionality (6D+time), nonlinearity, extreme temporal and spatial scale disparity, as well as scale disparity in velocity space due to the presence of multiple species with disparate masses and temperatures. To address these challenges, we propose a novel algorithmic framework in 1D2V based on implicit timestepping with optimal multigridbased solvers, spatial and velocityspace adaptivity via mesh motion, and a new discrete treatment of both the collision operator (in the Rosenbluth form) and the Vlasov equation that ensures exact conservation of mass, momentum, and energy. As a result, this new treatment achieves optimal O(N) computational complexities while delivering unprecedented longterm simulation capabilities with moderate computational resources (<1000 cores). We demonstrate the utility of our approach with the simulation of a real Inertial Confinement Fusion (ICF) capsule in spherical geometry, where we show the impact of kinetic effects on the fusion reactivity of the capsule.