Bulletin of the American Physical Society
59th Annual Meeting of the APS Division of Plasma Physics
Volume 62, Number 12
Monday–Friday, October 23–27, 2017; Milwaukee, Wisconsin
Session PI3: DPP Award Session and Legacy of Kaw |
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Chair: Amitava Bhattacharjee, Princeton Plasma Physics Laboratory Room: 103ABC |
Wednesday, October 25, 2017 2:00PM - 2:30PM |
PI3.00001: Thomas H. Stix Award for Outstanding Early Career Contributions to Plasma Physics Research: MHD Stability and control in tokamak plasmas Invited Speaker: Ian Chapman Highly energetic magnetised plasmas are subject to various magnetohydrodynamic instabilities, which often limit the global fusion performance, and in many cases have the potential to damage the reactor vessel. Consequently, understanding these performance limits and establishing ways to control the instabilities is vital to the success of ITER and fusion reactors which follow it. This talk will summarise understanding of the stability limits governing sawtooth instabilities, Resistive Wall Modes and Edge Localised Modes, and discuss ways to control each of these. [Preview Abstract] |
Wednesday, October 25, 2017 2:30PM - 3:00PM |
PI3.00002: Katherine E. Weimer Award: X-ray light sources from laser-plasma and laser-electron interaction: development and applications Invited Speaker: Felicie Albert Bright sources of x-rays, such as synchrotrons and x-ray free electron lasers (XFEL) are transformational tools for many fields of science. They are used for biology, material science, medicine, or industry. Such sources rely on conventional particle accelerators, where electrons are accelerated to gigaelectronvolts (GeV) energies. The accelerated particles are wiggled in magnetic structures to emit x-ray radiation that is commonly used for molecular crystallography, fluorescence studies, chemical analysis, medical imaging, and many other applications. One of the drawbacks of these machines is their size and cost, because electric field gradients are limited to about 100 V/M in conventional accelerators. Particle acceleration in laser-driven plasmas is an alternative to generate x-rays via betatron emission, Compton scattering, or bremsstrahlung. A plasma can sustain electrical fields many orders of magnitude higher than that in conventional radiofrequency accelerator structures. When short, intense laser pulses are focused into a gas, it produces electron plasma waves in which electrons can be trapped and accelerated to GeV energies. X-ray sources, driven by electrons from laser-wakefield acceleration, have unique properties that are analogous to synchrotron radiation, with a 1000-fold shorter pulse. An important use of x-rays from laser plasma accelerators is in High Energy Density (HED) science, which requires laser and XFEL facilities to create in the laboratory extreme conditions of temperatures and pressures that are usually found in the interiors of stars and planets. To diagnose such extreme states of matter, the development of efficient, versatile and fast (sub-picosecond scale) x-ray probes has become essential. In these experiments, x-ray photons can pass through dense material, and absorption of the x-rays can be directly measured, via spectroscopy or imaging, to inform scientists about the temperature and density of the targets being studied. [Preview Abstract] |
Wednesday, October 25, 2017 3:00PM - 3:30PM |
PI3.00003: Marshall N. Rosenbluth Outstanding Doctoral Thesis Award: Magnetorotational turbulence and dynamo Invited Speaker: Jonathan Squire Accretion disks are ubiquitous in astrophysics and power some of the most luminous sources in the universe. In many disks, the transport of angular momentum, and thus the mass accretion itself, is thought to be caused by the magnetorotational instability (MRI). As the MRI saturates into strong turbulence, it also generates ordered magnetic fields, acting as a magnetic dynamo powered by the background shear flow. However, despite its importance for astrophysical accretion processes, basic aspects of MRI turbulence---including its saturation amplitude---remain poorly understood. In this talk, I will outline progress towards improving this situation, focusing in particular on the nonlinear shear dynamo and how this controls the turbulence. I will discuss how novel statistical simulation methods can be used to better understand this shear dynamo, in particular the distinct mechanisms that may play a role in MRI turbulence and how these depend on important physical parameters. [Preview Abstract] |
Wednesday, October 25, 2017 3:30PM - 4:00PM |
PI3.00004: A New Scaling for Divertor Detachment Invited Speaker: Robert Goldston The ITER design and future fusion power plant designs depend on divertor detachment, whether partial, pronounced or complete, both to limit heat flux to plasma-facing components and to limit surface erosion due to sputtering. Generally the parallel heat flux, estimated as proportional to $P_{sep}/R$ or $P_{sep}B/R$, is used as a proxy for the difficulty of achieving detachment. Here we argue that the impurity cooling required for detachment is strongly dependent on the upstream separatrix density, which is limited by Greenwald scaling. Taking this into account self-consistently, along with the Heuristic Drift (HD) model for the SOL width, and using a Lengyel radiation model that includes non-coronal effects, we find\footnote{RJ Goldston, ML Reinke, JA Schwartz, Plasma Phys. Control. Fusion 59 (2017) 055015} that the relative impurity concentration, $c_z \equiv n_z/n_e$, required for detachment scales dominantly as $c_z \propto {P_{sep}}/B_p (n_{sep}/n_{GW})^2}$. The absence of any explicit favorable size scaling is concerning, as $P_{sep}$ must increase by an order of magnitude from present experiments to an economic fusion power system, while increases in the poloidal magnetic field strength are limited by magnet technology and MHD stability. This result should not be surprising, as it follows from the simplest scaling, $P_{sep} \propto c_z n_e^2 V_{SOL}$, taking into account the Greenwald density limit and the HD SOL volume scaling. Reinke\footnote{ML Reinke, Nucl. Fusion 57 (2017) 034004} has combined a similar approach with the requirement to maintain H-mode, which sets a lower limit on $P_{sep}$, and also arrives at an incentive for high field and disincentive for large size. These results should be challenged by comparison with 2D divertor codes and with measurements on existing experiments. In particular measurements are required for extrinsic divertor impurity concentration over a range of power and density conditions far from the regime where detachment can be achieved with deuterium puffing and intrinsic impurities alone. Nonetheless, these results suggest that higher magnetic field, stronger shaping, double-null operation, “advanced” divertor magnetic and baffle configurations, as well as lithium vapor targets merit greater attention. [Preview Abstract] |
Wednesday, October 25, 2017 4:00PM - 4:30PM |
PI3.00005: Effects of Density and Impurity on Edge Localized Modes in Tokamaks Invited Speaker: Ping Zhu Plasma density and impurity concentration are believed to be two of the key elements governing the edge tokamak plasma conditions. Optimal levels of plasma density and impurity concentration in the edge region have been searched for in order to achieve the desired fusion gain and divertor heat/particle load mitigation. However, how plasma density or impurity would affect the edge pedestal stability may have not been well known. Our recent MHD theory modeling and simulations using the NIMROD code have found novel effects of density and impurity on the dynamics of edge-localized modes (ELMs) in tokamaks. First, previous MHD analyses often predict merely a weak stabilizing effect of toroidal flow on ELMs in experimentally relevant regimes. We find that the stabilizing effects on the high-$n$ ELMs from toroidal flow can be significantly enhanced with the increased edge plasma density [1]. Here $n$ denotes the toroidal mode number. Second, the stabilizing effects of the enhanced edge resistivity due to lithium-conditioning on the low-$n$ ELMs in the high confinement (H-mode) discharges in NSTX have been identified. Linear stability analysis of the experimentally constrained equilibrium suggests that the change in the equilibrium plasma density and pressure profiles alone due to lithium-conditioning may not be sufficient for a complete suppression of the low-$n$ ELMs. The enhanced resistivity due to the increased effective electric charge number $Z_{\rm eff}$ after lithium-conditioning provides additional stabilization of the low-$n$ ELMs [2]. These new effects revealed in our theory analyses may help further understand recent ELM experiments and suggest new control schemes for ELM suppression and mitigation in future experiments. They may also pose additional constraints on the optimal levels of plasma density and impurity concentration in the edge region for H-mode tokamak operation. [1] S.-K. Cheng, P. Zhu, and D. Banerjee, Enhanced toroidal flow stabilization of edge localized modes with increased plasma density, submitted to Phys. Plasmas (2017). [2] D. Banerjee, P. Zhu, and R. Maingi, Stabilizing effects of resistivity on low-n edge localized modes in NSTX, Phys. Plasmas 24, 054501 (2017). [Preview Abstract] |
Wednesday, October 25, 2017 4:30PM - 5:00PM |
PI3.00006: Ion acoustic instability, turbulence, anomalous resistivity and enhanced laser light absorption in ICF plasmas Invited Speaker: Wojciech Rozmus Hot plasmas with strong temperature gradients in inertial confinement fusion (ICF) experiments are examined for ion acoustic instabilities and kinetic effects produced by electron heat flux. Return current instability (RCI) due to neutralizing current of cold electrons arising in response to large electron heat flux is investigated as a source of the stationary levels of ion acoustic turbulence (IAT). Two mechanisms of anomalous laser light absorption on IAT: due to enhanced anomalous collisionality and mode conversion into Langmuir waves at the critical density are described in terms of effective absorption rates and applied to hohlraum plasmas with ZT$_{\mathrm{e}}$/T$_{\mathrm{i}}$\textgreater \textgreater 1. The RCI threshold and growth rates are derived in the nonlocal regime of the thermal transport. They are compared with results of Vlasov-Fokker-Planck (VFP) simulations. Quasi-stationary state of the IAT produced by the RCI is achieved in VFP simulations. Nonlinear saturation of the RCI involves the mechanisms of the quasi-linear evolution and induced scattering of ions on IAT. In this talk, these topics will be explored in light of Professor Kaw's enduring research results on anomalous resistivity, enhanced laser light absorption and parametric instabilities in laser produced plasmas [1]. [1] P.K. Kaw, J.M. Dawson, Phys. Fluids \textbf{12}, 2586 (1969); P.K. Kaw, E. Valeo, J.M. Dawson Phys. Rev. Lett. \textbf{25}, 430 (1970); P.K. Kaw, J. M. Dawson, Phys. Fluids \textbf{14}, 792 (1971). [Preview Abstract] |
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