9 research outputs found
The Parallel Boundary Condition for Turbulence Simulations in Low Magnetic Shear Devices
Flux tube simulations of plasma turbulence in stellarators and tokamaks
typically employ coordinates which are aligned with the magnetic field lines.
Anisotropic turbulent fluctuations can be represented in such field-aligned
coordinates very efficiently, but the resulting non-trivial boundary conditions
involve all three spatial directions, and must be handled with care. The
standard "twist-and-shift" formulation of the boundary conditions [Beer,
Cowley, Hammett \textit{Phys. Plasmas} \textbf{2}, 2687 (1995)] was derived
assuming axisymmetry and is widely used because it is efficient, as long as the
global magnetic shear is not too small. A generalization of this formulation is
presented, appropriate for studies of non-axisymmetric, stellarator-symmetric
configurations, as well as for axisymmetric configurations with small global
shear. The key idea is to replace the "twist" of the standard approach (which
accounts only for global shear) with the integrated local shear. This
generalization allows one significantly more freedom when choosing the extent
of the simulation domain in each direction, without losing the natural
efficiency of field-line-following coordinates. It also corrects errors
associated with naive application of axisymmetric boundary conditions to
non-axisymmetric configurations. Simulations of stellarator turbulence that
employ the generalized boundary conditions require much less resolution than
simulations that use the (incorrect, axisymmetric) boundary conditions. We also
demonstrate the surprising result that (at least in some cases) an easily
implemented but manifestly incorrect formulation of the boundary conditions
does {\it not} change important predicted quantities, such as the turbulent
heat flux
Optimization of Nonlinear Turbulence in Stellarators
We present new stellarator equilibria that have been optimized for reduced
turbulent transport using nonlinear gyrokinetic simulations within the
optimization loop. The optimization routine involves coupling the
pseudo-spectral GPU-native gyrokinetic code GX with the stellarator equilibrium
and optimization code DESC. Since using GX allows for fast nonlinear
simulations, we directly optimize for reduced nonlinear heat fluxes. To handle
the noisy heat flux traces returned by these simulations, we employ the
simultaneous perturbation stochastic approximation (SPSA) method that only uses
two objective function evaluations for a simple estimate of the gradient. We
show several examples that optimize for both reduced heat fluxes and good
quasisymmetry as a proxy for low neoclassical transport. Finally, we run full
transport simulations using T3D to evaluate the changes in the macroscopic
profiles
Highly Volcanic Exoplanets, Lava Worlds, and Magma Ocean Worlds:An Emerging Class of Dynamic Exoplanets of Significant Scientific Priority
Highly volcanic exoplanets, which can be variously characterized as 'lava
worlds', 'magma ocean worlds', or 'super-Ios' are high priority targets for
investigation. The term 'lava world' may refer to any planet with extensive
surface lava lakes, while the term 'magma ocean world' refers to planets with
global or hemispherical magma oceans at their surface. 'Highly volcanic
planets', including super-Ios, may simply have large, or large numbers of,
active explosive or extrusive volcanoes of any form. They are plausibly highly
diverse, with magmatic processes across a wide range of compositions,
temperatures, activity rates, volcanic eruption styles, and background
gravitational force magnitudes. Worlds in all these classes are likely to be
the most characterizable rocky exoplanets in the near future due to
observational advantages that stem from their preferential occurrence in short
orbital periods and their bright day-side flux in the infrared. Transit
techniques should enable a level of characterization of these worlds analogous
to hot Jupiters. Understanding processes on highly volcanic worlds is critical
to interpret imminent observations. The physical states of these worlds are
likely to inform not just geodynamic processes, but also planet formation, and
phenomena crucial to habitability. Volcanic and magmatic activity uniquely
allows chemical investigation of otherwise spectroscopically inaccessible
interior compositions. These worlds will be vital to assess the degree to which
planetary interior element abundances compare to their stellar hosts, and may
also offer pathways to study both the very young Earth, and the very early form
of many silicate planets where magma oceans and surface lava lakes are expected
to be more prevalent. We suggest that highly volcanic worlds may become second
only to habitable worlds in terms of both scientific and public long-term
interest.Comment: A white paper submitted in response to the National Academy of
Sciences 2018 Exoplanet Science Strategy solicitation, from the NASA Sellers
Exoplanet Environments Collaboration (SEEC) of the Goddard Space Flight
Center. 6 pages, 0 figure
Towards continuum gyrokinetic study of high-field mirrors
High-temperature superconducting (HTS) magnetic mirrors under development
exploit strong fields with high mirror ratio to compress loss cones and enhance
confinement, and may offer cheaper, more compact fusion power plant candidates.
This new class of devices could exhibit largely unexplored interchange and
gradient-driven modes. Such instabilities, and methods to stabilize them, can
be studied with gyrokinetics given the strong magnetization and prevalence of
kinetic effects. Our focus here is to: a) determine if oft-used gyrokinetic
models for open field lines produce the electron-confining (Pastukhov)
electrostatic potential; b) examine and address challenges faced by gyrokinetic
codes in studying HTS mirrors. We show that a one-dimensional limit of said
models self-consistently develops a potential qualitatively reaching the
analytical Pastukhov level. Additionally, we describe the computational
challenges of studying high mirror ratios with open field line gyrokinetic
solvers, and offer a force softening method to mitigate small time steps needed
for time integration in colossal magnetic field gradients produced by HTS
coils, providing a 19X speedup