Spring 2023
January 30, 2023 (Monday) 4:00-5:00p.m.
Speaker: Yan Yang, University of Delaware
Hosts: S. Mordijck and M. Sher
Title: Multiscale Nature of Turbulence in Space Plasmas
Abstract: Turbulence enters into space plasmas in many guises. The complexity and variability of the behavior of plasma turbulence is in large part due to the involvement of dynamics at many scales, ranging from macroscopic fluid to sub-electron scales. Based on what plasma properties we are interested in studying, be they dominant at small or large scales, a plasma can be treated as tractable models in various limits, such as the kinetic theory and magnetohydrodynamic (MHD) theory. Turbulence flows are characterized by the nonlinear transfer of energy and other quantities across a huge range of scales. Observed turbulence in space is expected to involve cross-scale energy transfer and subsequent dissipation and heating. Space plasmas are frequently taken to be weakly collisional or collisionless. Therefore, an explicit form of viscous dissipation as in collisional (e.g., MHD) cases cannot be easily defined. A variety of approaches have attempted to characterize specific mechanisms (e.g., magnetic reconnection, wave-particle interaction and turbulent-driven intermittency) and to quantify the dissipation. However, the community has not come to a consensus solution applicable to all systems. In this talk I will first give an overview of some basic properties for turbulence. Then I will briefly review turbulence theory application in space plasmas. I will discuss in detail how to disentangle multiscale properties, how plasma dynamics bridges multiple scales, what new ingredients are introduced in cross-scale transfer as models progress from fluid to kinetic, and how to identify key steps in energy transfer and estimate energy dissipation rate in weakly collisional plasmas. These also motivate several unresolved issues that may be addressed by future studies. Where feasible, examples are given from MHD, Particle in Cell, and hybrid Vlasov-Maxwell simulations, and from Magnetospheric Multiscale (MMS) observations.
February 13, 2023 (Monday) 4:00-5:00p.m.
Speaker: Rogerio Jorge, IST, University of Lisbon
Hosts: S. Mordijck and M. Sher
Title: Recent nuclear fusion breakthroughs using stellarator optimization
Abstract: The success of magnetic confinement nuclear fusion requires magnetic fields with optimal properties. Namely, these should provide the necessary plasma performance to sustain high-temperature plasmas for a long enough time. However, there are still important outstanding issues in magnetic fusion, such as the understanding and control of plasma density, fast particles, and turbulence. To achieve good confinement, recent experiments such as HSX and W7-X have been designed using optimization based on the calculation of MHD equilibria at each evaluation of the objective function. Here, we show how new design methods have been able to reduce the computational cost of such optimization efforts by orders of magnitude, while also providing new insights into the space of solutions. These methods enable a direct geometrical construction of magnetic fields with extremely good confining properties such as quasisymmetry and quasi-isodynamic fields. Thanks to the reduced computational cost, we are able to achieve good particle confinement with much higher accuracy than reported before and perform wide surveys over parameter space. Furthermore, many figures of merit can be now calculated directly, including MHD stability and heat flux, making the design of a fusion machine optimized for turbulent transport and fast particles a possibility in the near future.
February 20, 2023 (Monday) 4:00-5:00p.m.
Speaker: James Juno, Plasma Physics National Laboratory
Hosts: S. Mordijck and M. Sher
Title: A look at the plasma universe through phase space
Abstract: Ninety-nine percent of the luminous universe exists in the plasma state of matter. Closer to home, the study of plasmas has diverse applications to the development of fusion energy and semi-conductor manufacturing. A commonality amongst all of these diverse plasmas is they are “weakly collisional” and thus best described by kinetic theory. The fundamental object of kinetic theory is the particle distribution function, a statistical description of the individual particle trajectories, and the fundamental equation for the evolution of this particle distribution function is the Boltzmann equation. One of the major obstacles to solving these kinds of problems is computational — the Boltzmann equation is notoriously difficult to integrate numerically owing to its complexity and high dimensionality.
Motivated by the wealth of data in the particle distribution function, I will present a one-of-a-kind grid-based Boltzmann solver implemented within the Gkeyll simulation framework. Results will be presented which demonstrate the wide applicability of this computational tool, from plasma-material interactions to fundamental physics studies of instabilities in space and astrophysical plasmas. These studies demonstrate the utility of the code, both the novel analysis in position-momentum phase space we can perform with this approach and the solver’s ability to overcome numerical challenges present in other algorithms. I will briefly discuss the algorithmic advances discovered through this work and look ahead to future applications of this unique tool, including applications to high energy astrophysics and magnetized plasma systems such as fusion and laboratory plasmas.
February 22, 2023 (Wednesday) 4:00-5:00p.m.
Speaker: Surabhi Jaiswal, Princeton University
Hosts: S. Mordijck and M. Sher
Title: Exploring low temperature plasma fundamentals and applications
Abstract: Plasma is generally defined as a fourth state of matter which is an ionized gas consisting of charged particles dominated by electromagnetic forces. In low temperature plasma (LTP), only a small fraction of gas is ionized while the mean energy of electrons (a few to 10 eV) is much larger than the temperature of ions and neutrals which can be as low as room temperature. LTP technology has shown outstanding capability in a variety of fields in recent decades and new technologies are continuously being invented to solve modern societal problems. While applications have been explored, understanding the fundamental physics of LTP is primarily important. This seminar will cover our ongoing research on charged microparticles dynamics in plasma as a function of externally applied magnetic field and discuss the coupling of microparticle with plasma filamentation at high magnetic field and its control mechanism. Understanding these issues on a firm basis is especially important for practical applications. Recent research on LTP at atmospheric pressure for studying auroral formation and diagnostics capabilities will also be discussed. Lastly, future collaborative plasma research in these related areas will be discussed with a focus on the impact for students.
March 3, 2023 (Friday) 4:00-5:00p.m.
Speaker: Andrew Jackura, Jefferson Lab
Hosts: I. Novikova/J. Dudek
Title: Nuclear Reactions & QCD Spectroscopy
Abstract: It is widely accepted that all observed nuclear phenomena emerge from the strong interactions between quarks and gluons as governed by Quantum ChromoDynamics (QCD). Reactions between hadrons, the bound states of quarks and gluons, are responsible for most nuclear processes observed in nature. Quantifying low-energy nuclear reactions directly from QCD, however, remains challenging due to the non-perturbative nature of theory. I will present an overview of an on-going program which aims to connect QCD to nuclear reactions. I will focus on the challenges of accessing the excited hadron spectrum, and illustrate how newly developed theoretical tools allow us to compute few-body nuclear reactions from first principles QCD.
March 6, 2023 (Monday) 4:00-5:00p.m.
Speaker: Andrew Hanlon, Brookhaven National Laboratory
Hosts: I. Novikova/J. Dudek
Title: The Nature of Strongly Interacting Matter: Connecting Theory to Experiment
Abstract: Theory and experiment go in tandem. Although experiment gives us an incredible window into the vast structure of strongly interacting matter, theory provides us with a much-needed understanding to build a wider view of this rich phenomena. It is well-accepted that quantum chromodynamics (QCD) is the correct theory of the strong nuclear force. Despite this, there remain various challenges in using QCD directly. Fortunately, there exists a numerical method, lattice QCD, which is a systematically-improvable first-principles approach to performing calculations in QCD. This approach can be used in combination with effective field theory, among other tools, to make connections to experiment. The past decade has seen a significant maturity in these methods to address several puzzles in QCD. In this talk, I will show how lattice QCD can be used to make progress towards resolving open problems — e.g. through calculations of baryon resonances, two-baryon interactions, and three-body forces — and therefore increase our understanding of how the complex nature of strongly interacting matter arises from QCD.
March 24, 2023 (Friday) 4:00-5:00p.m.
Speaker: Paul Black, Wake Forest University
Host: M. Sher
Title: Innovations in Surface Guidance Techniques for Radiotherapy Applications
Abstract: About half of all cancer patients receive radiation therapy, which is delivered in a prescribed number of fractionated doses. As such, radiation therapy delivery requires accurate and repeatable patient positioning, ideally to sub-millimeter accuracy, for each fraction. This need for reproducible, consistent patient positioning becomes even more important as the plan complexity or the dose per fraction increases. Conventionally, patient positioning is verified before radiation delivery, but not actively monitored for changes during treatment. Positioning verification performed in real-time, concurrent with treatment, stands to improve on this methodology. Recently, groups have been working towards establishing methods for real-time verification of radiation treatment delivery. The two techniques presented use optical light to track patient positioning and verify radiation delivery.
When particles of sufficient energy travel through human tissue, light emission, known as Cherenkov radiation, is observable on the irradiated patient skin. This has been shown to correlate with ionizing radiation dose delivery in solid tissue, allowing real-time treatment verification. Cherenkov light images were acquired during radiation delivery to standard and anthropomorphic phantoms. Two clinical scenarios were tested: 1) observation of field overlaps or gaps in matched radiation fields and 2) patient positioning shifts during modulated dose delivery. The second technique investigated is Optical Surface Guidance (OSG), in which an optical light field is projected onto the patient surface and monitored using a multi-angle camera system. This technology can reliably monitor the patient surface in real-time, but cannot visualize radiation dose.
The detectability limit for determining radiation field placement and phantom position was investigated. For matched radiation fields, measurements of Cherenkov images agreed with known field separations. Detection of sub-millimeter positioning shifts was also demonstrated. For Cherenkov imaging, the major confounding factors were radiation angle of incidence, beam energy, and radiation type. For the OSG investigation, we verified sub-millimeter accuracy of the system, including rotational position changes. The combination of Cherenkov detection and OSG can be used in the development and refinement of a real-time patient and radiation delivery monitoring technique for clinical radiation dose delivery.
March 31, 2023 (Friday) 4:00-5:00p.m.
Speaker: Arkaitz Rodas, Jefferson Lab
Hosts: I. Novikova/J. Dudek
Title: The QCD spectrum: Challenges and prospects
Abstract: From quarks and gluons, the emergence of hadrons is governed by the theory of strong interactions, quantum chromodynamics (QCD). However, its non-perturbative nature prevents us from determining the hadron spectrum algebraically. The way these basic constituents arrange into the matter we observe remains unsolved. Furthermore, studying some of these hadron interactions is very challenging from an experimental perspective, which has led to conflicting results in the past.
In this talk, I will explain the role that modern amplitude-analysis and first-principles techniques play in hadron physics. I will first highlight how these principles allow us to solve longstanding puzzles like the determination of hadrons that go beyond the quark-model predictions. Then, I will summarize how Lattice QCD could enable us to decipher fundamental questions about their formation.
April 10, 2023 (Monday) 4:00-5:00p.m.
Speaker: Mikhail Mihasenko, ORIGINS Excellence Cluster, Munich, Germany
Hosts: Dudek & Stevens
Title: Three-Body Problems in Hadron Physics
Abstract: Understanding strong interaction is an essential challenge in particle physics. In recent years, researchers have discovered various exotic hadrons, such as pentaquarks, tetraquarks, and others. These particles need to be classified and better understood. In this seminar, we will explore techniques for analyzing hadron resonances, with a special focus on three-body problems, which present challenges across various fields of particle physics.
We will discuss recent studies on hadronic three-body amplitudes and their application in characterizing new types of particles, like doubly charmed tetraquarks. I will show how our growing understanding of three-body physics enhances the study of charm baryons and opens new avenues in Beyond the Standard Model searches using the polarimetry vector field. We will also explore how this knowledge might help resolve challenges in interpreting hadron correlation functions, connecting lattice QCD and experimental studies of hadron interactions through the femtoscopy technique.
I will also touch upon machine learning methods, and advances in computational techniques, like differentiable programming, that help with these analyses.
April 14, 2023 (Friday) 4:00-5:00p.m.
Speaker: Anton Burkov, University of Waterloo
Host: E. Rossi
Title: Emergent symmetries, Luttinger's theorem and topology of metals.
Abstract: Luttinger's theorem connects a basic microscopic property of a given metallic crystalline material, the number of electrons per unit cell, to the volume, enclosed by its Fermi surface, which defines its low-energy observable properties. Such statements are valuable since, in general, deducing a low-energy description from microscopics, which may perhaps be regarded as the main problem of condensed matter theory, is far from easy.
In this talk I will describe a unified framework, which allows one to discuss Luttinger's theorems for ordinary metals, as well as closely analogous exact statements for topological semimetals, whose low-energy description contains either discrete point or continuous line nodes. This framework is based on the concept of a 't Hooft anomaly of the emergent symmetry, which characterizes the low-energy description of a given metallic phase.
The 't Hooft anomaly relates the low-energy theory of a metal to the topological response of a higher-dimensional symmetry-protected topological insulator.
April 21, 2023 (Friday) 4:00-5:00p.m.
Speaker: Biao Lian, Princeton University
Host: E. Rossi
Title: Magic flat bands of electrons in moiré graphene
Abstract: The stacking of two similar lattices produces a moiré pattern which is a large spatial period superlattice. Experimentally, a large class of 2D moiré materials can be created using this method, which has attracted extensive interests recently. A paradigm is the twisted bilayer graphene (TBG), which generates topological flat bands of electrons at the so-called magic angle (1.1 degrees), and exhibits superconductivity and correlated insulator phases. I will talk about the systematic theory of TBG with Coulomb interactions, which has an approximate U(4) symmetry and interacting Chern insulator ground states at integer fillings. I will further present alternative setups of moiré models of graphene hosting magic flat bands, such as twisted trilayer graphene, and lattice mismatch moiré graphene systems which exhibits topological flat bands in effective kagome and honeycomb moiré lattices. These systems provide promising platforms for studying novel strongly correlated phases of matter.
