Session BM9: Mini-conference: The Future of the Field I

Validating gyrokinetic simulations of plasma turbulence in the Texas Helimak

Using the computational plasma framework Gkeyll, we present the first continuum gyrokinetic simulations of plasma turbulence in the Texas Helimak, a simple magnetized torus experiment [1,2]. The device has features similar to the scrape-off layer region of tokamaks, such as bad-curvature-driven instabilities and sheath boundary conditions, which we include in our model. A bias voltage can be applied across conducting plates to drive $E \times B$ flow and study the effect of velocity shear on turbulence suppression. We performed simulations of grounded and limiter-biased scenarios, which produced equilibrium profiles and fluctuation amplitudes that approach experimental values. Comparison with experimental data also illustrated some important quantitative differences, and we discuss how including additional physical and geometric effects in our model improves agreement with experiment. [1] Bernard et al., PoP 26(4), 042301 (2019). [2] Bernard, UT Austin PhD thesis (2019).   

Electromagnetic full-$f$ continuum gyrokinetics in the tokamak scrape-off layer

We present the first electromagnetic continuum gyrokinetic simulations of turbulence on open field lines in the tokamak SOL. Gkeyll, a full-$f$ continuum gyrokinetic code, is being developed to study turbulence in the edge region of fusion devices. The edge region involves large-amplitude fluctuations, electromagnetic effects, and plasma interactions with material walls, making it more computationally challenging than the core region. Gkeyll models the turbulence by solving the 5D full-$f$ gyrokinetic system in Hamiltonian form using an energy-conserving high-order discontinuous Galerkin scheme. The Gkeyll code has been extended to include self-consistent electromagnetic perturbations using a symplectic ($v_\parallel$) formulation. We present linear benchmarks of kinetic Alfv\'en waves and kinetic ballooning mode instability calculations that illustrate the success of the electromagnetic scheme and the avoidance of the Amp\`ere cancellation problem. We then present nonlinear electromagnetic turbulence simulations in a model helical SOL geometry with sheath boundary conditions on open field lines. These simulations use parameters from NSTX SOL measurements. We make comparisons between the electromagnetic simulations and electrostatic simulations with the same setup.   

Parametric Decay Instabilities during Electron Cyclotron Resonance Heating of Fusion Plasmas

Three-wave interactions are ubiquitous in media with quadratic nonlinearities, including plasmas, fluids, and optical crystals. While their applications, e.g., generation of entangled photons, underlie many recent advances in quantum optics and metrology, they also play a crucial role in the onset of turbulence in plasmas and fluids. We consider parametric decay instabilities (PDIs), in which a monochromatic pump wave decays to two daughter waves when its amplitude exceeds a nonlinear threshold, during electron cyclotron resonance heating (ECRH) of plasmas. Such PDIs are of importance in fusion and ionospheric modification (IM) experiments. We demonstrate that these PDIs can generate strong microwave bursts near half the pump frequency when magnetic islands are present in fusion plasmas at the ASDEX Upgrade tokamak. These bursts have recently damaged essential plasma diagnostics such as electron cyclotron emission radiometers. Our results validate present theories of PDIs during ECRH of fusion plasmas and illustrate the necessity of accounting for PDIs in current and future fusion devices with significant ECRH power. They may even form a basis for testing IM PDI theories in a controlled laboratory setting.   

New theory of magnetic pumping as applied to spacecraft observations of particle heating

Energetic particle generation is an important component of a variety of astrophysical systems, from seed particle generation in shocks to the heating of the solar wind. Starting from the drift kinetic equation, we have derived a magnetic pumping model, where particles are heated by the largest scale turbulent fluctuations. We have shown that for a spatially-uniform flux tube, this is an effective heating mechanism up to $v\leq\omega/k$, and naturally produces power-law distributions like those observed in the solar wind, as verified by particle-in-cell simulations [1]. When this model is extended to a spatially-varying flux tube, magnetic trapping renders magnetic pumping an effective Fermi heating process for particles with $v\gg\omega/k$. To test this, we used satellite observations of the strong, compressional magnetic fluctuations near the Earth's bow shock from the Magnetospheric MultiScale mission and found strong agreement with our model. Given the ubiquity of such fluctuations in different astrophysical systems, this mechanism has the potential to be transformative to our understanding of how the most energetic particles in the universe are generated. [1] E. Lichko, J. Egedal, W. Daughton, and J. Kasper. Astrophys. J. Lett. 2, 850 (2017)   

Studying Magnetic-Pressure-Driven Bow Shocks at OMEGA

We present data and analysis from a campaign to study astrophysically relevant, magnetic-pressure-driven bow shocks at the OMEGA laser facility. The system consists of a slow, low-density plasma flow impinging on the external azimuthal magnetic field around a current-carrying wire. The spatially resolved optical Imaging Thomson Scattering diagnostic provides quantitative measurements of electron number density and temperature across a shock. Proton images also indicate the formation of an MHD shock at a standoff distance from the wire. We simulate the experiment using the FLASH code and compare these results to the data.   

Shock Driven, Discrete Vortices on Oblique Interfaces

A shock incident on an interface between two materials will deposit baroclinic vorticity. This vorticity will typically cause any perturbations on the pre-shock interface to grow. The vorticity distribution along the post-shock interface often determines which process dominates the post-shock evolution. Here, we will show that growth dominated by discrete vortices arises from the interaction of a supported shock with a staircase perturbation. We will present theory, xRAGE simulations, and preliminary experimental results in support of this result.   

Ionization and heating trends observed in a laboratory photoionized plasma experiment at Z

An experimental effort is ongoing to create and study laboratory photoionized plasmas relevant to the extreme conditions in x-ray binaries and active galactic nuclei. Astronomers seeking to understand such objects rely on photoionization models developed mainly from theory because this regime has long been experimentally inaccessible and is only beginning to be thus examined with devices such as the Z-Machine at Sandia National Laboratories. The experiment employs the intense broadband x-ray flux emitted during the collapse of a Z-pinch to drive and backlight a neon photoionized plasma contained within a cm-scale gas cell with atom number densities of 10$^{17}$ to 10$^{18}$ cm$^{-3}$. At the available gas cell positions, the x-ray flux reaches a peak of order 10$^{12}$ W/cm$^{2}$. Combinations of these parameters afford an order of magnitude range in ionization parameter, allowing for the study of trends in astrophysically relevant photoionized plasmas. Through K-shell line absorption spectroscopy, the resulting plasma conditions (i.e. ion areal densities, charge state distribution, and electron temperature) are determined, which can be compared with simulation results to test atomic kinetics and heating models for photoionized plasmas.   

Thomson Scattering on Laboratory Plasma Jets to Study Current Polarity Effects

Thomson scattering measurements have been performed on plasma jets created from a 15 $\mu$m thick radial Al foil load on COBRA, a 1.2 MA pulsed power machine with 100 ns rise time, to study current polarity effects on the jet. The ion acoustic wave (IAW) spectrum was recorded with a streak camera, while the electron plasma wave (EPW) spectrum was recorded on a gated camera. The Thomson scattering laser had a maximum energy of 10 J at 526.5 nm and a 2.2 ns full width at half maximum duration. Previous work showed that current polarity affects jet formation due to extended magnetohydrodynamic (XMHD) effects such as the Hall effect. Experiments show that jets with current flowing radially outward (“reverse polarity”) through the foil were taller and denser than jets with current flowing radially inward (“standard polarity”). The IAW feature with 0.5 J or 1 J of laser energy showed $T_e$ to be 15 eV in both polarity’s jets, while scattering with higher laser energies showed more heating in the reverse polarity jets due to the higher density. The EPW feature measures $n_e$ outside of the jet to be around $5\times 10^{17}$ cm$^{-3}$ while inside the jet $n_e$ was at least $2\times 10^{18}$ cm$^{-3}$. Comparing these results with XMHD simulation can help to validate the simulations.   

Magnetic field impact on the temperature distribution in MagLIF laser heating

Laser-only experiments have been performed at Z with beryllium liners filled with argon-doped deuterium to investigate the laser pre-heat stage of Magnetized Liner Inertial Fusion (MagLIF). Time integrated, axially-resolved spectra of the Ar K-shell emission were recorded. The spectra are sensitive to electron temperature $T_{e}$ and contain line emission from the He$\alpha$ and intercombination line in He-like Ar, as well as associated Li-like satellites. Via the individual analysis of the spatially resolved spectra, axially resolved temperature distributions $T_{e}(z)$ were extracted for multiple experiments. Changes to the laser beam profile and entrance window thickness were reflected in the magnitude and shape of the extracted profiles. The results from two identical experiments, with and without an external magnetic field, show that the inclusion of the magnetic field increased both $T_{e}$ and the axial extent of the laser heated region. Radiation hydrodynamic simulations of these experiments were performed and post-processed. Analysis of the modeled spectra revealed that the simulations under-predicted $T_{e}$ and the differences were larger for the magnetized case\footnote{K. R. Carpenter \textif{et al.}, Phys. Plasmas, Submitted for publication}.