Physics

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Proton Energization during Magnetic Reconnection in Macroscale Systems
(2024) Yin, Zhiyu; Drake, James; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Magnetic reconnection is a widespread process in plasma physics that is crucial for the rapid release of magnetic energy and is believed to be a key factor in generating non-thermal particles in space and various astrophysical systems. In this dissertation, a set of equations are developed that extend the macroscale magnetic reconnection simulation model kglobal to include particle ions. The extension from earlier versions of kglobal, which included only particle electrons, requires the inclusion of the inertia of particle ions in the fluid momentum equation. The new equations will facilitate the exploration of the simultaneous non-thermal energization of ions and electrons during magnetic reconnection in macroscale systems. Numerical tests of the propagation of Alfvén waves and the growth of firehose modes in a plasma with anisotropic electron and ion pressure are presented to benchmark the new model. The results of simulations of magnetic reconnection accompanied by electron and proton heating and energization in a macroscale system are presented. Both species form extended powerlaw distributions that extend nearly three decades in energy. The primary drive mechanism for the production of these nonthermal particles is Fermi reflection within evolving and coalescing magnetic flux ropes. While the powerlaw indices of the two species are comparable, the protons overall gain more energy than electrons and their powerlaw extends to higher energy. The power laws roll into a hot thermal distribution at low energy with the transition energy occurring at lower energy for electrons compared with protons. A strong guide field diminishes the production of non-thermal particles by reducing the Fermi drive mechanism. In solar flares, proton power laws should extend down to 10's of keV, far below the energies that can be directly probed via gamma-ray emission. Thus, protons should carry much more of the released magnetic energy than expected from direct observations. In Encounter 14 (E14), the Parker Solar Probe encountered a reconnection event in the heliospheric current sheet (HCS) that revealed strong ion energization with power law distributions of protons extending to 500keV. Because the energetic particles were streaming sunward from an x-line that was anti-sunward of PSP, the reconnection source of the energetic ions was unambiguous. Using upstream parameters based on the data observed by PSP, we simulate the dynamics of reconnection applying kglobal and analyze the resulting spectra of energetic electrons and protons. Power law distributions extending nearly three decades in energy develop with proton energies extending to 500keV, consistent with observations. The significance of these results for particle energization in the HCS will be discussed.
Radiative Plasmas in Pulsar Magnetospheres
(2024) Chernoglazov, Alexander; Philippov, Alexander; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Pulsars are highly magnetized rotating neutron stars known for their periodic bursts of radio emission. Decades of astronomical observations revealed that pulsars produce non-thermal radiation in all energy bands, from radio to gamma rays, covering more than 20 decades in photon energy. Modern theories consider strongly magnetized relativistic electron-positron plasmas to be the source of the observed emission. In my Thesis, I investigate physical processes that can be responsible for plasma production and the observed high-energy emission in the wide range of photon energies, from eV to TeV. In the first Chapter of my Thesis, I investigate relativistic magnetic reconnection with strong synchrotron cooling using three-dimensional particle-in-cell kinetic plasma simulations. I characterize the spectrum of accelerated particles and emitted synchrotron photons for varying strengths of synchrotron cooling. I show that the cutoff energy of the synchrotron spectrum can significantly exceed the theoretical limit of 16 MeV if the plasma magnetization parameter exceeds the radiation reaction limit. Additionally, I demonstrate that a small fraction of ions present in the current sheet can be accelerated to the highest energies, making relativistic radiative reconnection a promising mechanism for the acceleration of high-energy cosmic rays. In the second Chapter, I present the first multi-dimensional simulations of the QED pair production discharge that occurs in the polar region of the neutron star. This process is believed to be the primary source of the pair plasma in pulsar magnetospheres and also the source of the radio emission. In this work, I focus on the self-consistently emerging synchronization of the discharges in different parts of the polar region. I find that pair discharges on neighboring magnetic field lines synchronize on a scale comparable to the height of the pair production region. I also demonstrate that the popular “spark” model of pair discharges is incompatible with the universally adopted force-free magnetospheric model: intermittent discharges fill the entire polar region that allows pair production, leaving no space for discharge-free regions. My findings disprove the key assumption of the spark model about the existence of distinct discharge columns. In the third Chapter, I demonstrate how the key findings of two previous chapters can provide a self-consistent explanation of the recently discovered very-high-energy, reaching 20TeV, pulsed emission in Vela pulsar. Motivated by the results of recent global simulations of pulsar magnetospheres, I propose that this radiation is produced in the magnetospheric current sheet undergoing radiative relativistic reconnection. I show that high-energy synchrotron photons emitted by reconnection-accelerated particles efficiently produce electron-positron pairs. The density of secondary pairs exceeds the supply from the polar cap and results in a self-regulated plasma magnetization parameter of $\sim 10^7$. Electrons and positrons accelerate to Lorentz factors comparable to $\sim 10^7$ and emit the observed GeV radiation via the synchrotron process and ~10 TeV photons by Compton scattering of the soft synchrotron photons emitted by secondary pairs. My model self-consistently accounts for the ratio of the gamma-ray and TeV luminosities.
Optimization of high-beta fusion devices against linear instabilities
(2023) Gaur, Rahul; Dorland, William; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Magnetic confinement fusion is a technique in which a strong magnetic field is used tocontain a hot plasma, which enables nuclear fusion. In terms of overall energy efficiency, the two most promising magnetic confinement concepts are tokamaks (axisymmetric devices) and stel- larators (nonaxisymmetric devices). The power P produced by a magnetically confined nuclear fusion device is proportional to Vβ2B4, where V is the volume of the device, β is the plasma pressure - magnetic pressure ratio, and B is the magnetic field strength. Most tokamaks and stellarators currently in operation are low-β devices. In general, there are three ways to increase P , one may increase the operating β, the magnetic field or the volume of the device. The cost of these devices is proportional to V , making large enough devices expensive. Similarly, a large magnetic field (>10T) requires superconducting magnets that, even after the recent innovations in HTS (High-Temperature Superconductors), are expensive to manufacture. High-β devices are an attractive idea to efficiently produce fusion energy. However, a high-β generally also implies a large gradient in plasma pressure that can be a source of numerous instabilities. If fusion devices could be optimized against such instabilities, high-β operation would become an attractive approach compared to high field or large-volume reactors. Therefore, this thesis explores the optimization of high-β tokamak and stellarator equilibrium equilibria against linear instabilities. We will start by investigating the stability of high-β tokamaks and stellarator equilibria against the infinite-n ideal ballooning mode, an important pressure-driven MHD instability. We stabilize these equilibria against the ideal ballooning mode. To achieve this, we formulate a gradient-based adjoint technique and demonstrate its speed and effectiveness by stabilizing these equilibria. We also explain how this technique can be easily extended to low-n ideal-MHD modes in both tokamaks and stellarators. After demonstrating the adjoint technique for stabilizing against ideal MHD modes, wefirst analyze the kinetic stability of a sequence of axisymmetric equilibria. We study this by nu- merically solving the δf gyrokinetic model, a simplified version of the Vlasov-Maxwell model. Since these kinetic instabilities are driven by temperature and density gradients, we explore them by scanning multiple values of the plasma β, temperature and density gradients, and plasma boundary shapes, discovering interesting relationships between equilibrium-dependent quantities and growth rates of these instabilities. We then repeat the same process for two recently pub- lished stellarator equilibria with quasisymmetry — a favorable hidden symmetry in stellarators. With this study, we verify that our observations from high-β tokamaks can be generalized to quasisymmetric stellarators. From our microstability study, we find that electromagnetic effects are important for high-βdevices. Hence, using the numerical tools and knowledge derived from the previous chapters we build an optimization framework that searches for stable equilibria. Due to the similarity between axisymmetry and quasisymmetry, we then use the microstability optimizer to search for ideally and kinetically-stable, quasisymmetric, high-β stellarators.
Increasing Helicity towards Dynamo Action with Rough Boundary Spherical Couette Flows
(2022) Rojas, Ruben; Lathrop, Daniel P; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The dynamo action is the process through which a magnetic field is amplified and sustained by electrically conductive flows. Galaxies, stars and planets, all exhibit magnetic field amplification by their conductive constituents. For the Earth in particular, the magnetic field is generated due to flows of conductive material in its outer core. At the University of Maryland, our Three-meter diameter spherical Couette experiment uses liquid sodium between concentric spheres to mimic some of these dynamics, giving insight into these natural phenomena. Numerical studies of Finke and Tilgner (Phys. Rev. E, 86:016310, 2012) suggest a reduction in the threshold for dynamo action when a rough inner sphere was modeled by increasing the poloidal flows with respect to the zonal flows and hence increasing helicity. The baffles change the nature of the boundary layer from a shear dominated to a pressure dominated one, having effects on the angular momentum injection. We present results on a hydrodynamics model of 40-cm diameter spherical Couette flow filled with water, where torque and velocimetry measurements were performed to test the effects of different baffle configurations. The selected design was then installed in the 3-m experiment. In order to do that, the biggest liquid sodium draining operation in the history of the lab was executed. Twelve tons of liquid sodium were safely drained in a 2 hours operation. With the experiment assembled back and fully operational, we performed magnetic field amplification measurements as a function of the different experimental parameters including Reynolds and Rossby numbers. Thanks to recent studies in the hydrodynamic scale model, we can bring a better insight into these results. Torque limitations in the inner motor allowed us to inject only 4 times the available power; however, amplifications of more than 2 times the internal and external magnetic fields with respect to the no-baffle case was registered. These results, together with time-dependent analysis, suggest that a dynamo action is closer than before; showing the effect of the new baffles design in generating more efficient flows for magnetic field amplification. We are optimistic about new short-term measurement in new locations of the parameter space, and about the rich variety of unexplored dynamics that this novel experiment has the potential to reach. These setups constitute the first experimental explorations, in both hydrodynamics and magnetohydrodynamics, of rough boundary spherical Couette flows as laboratory candidates for successful Earth-like dynamo action.
Mid-Infrared Laser Driven Avalanche Ionization and Low Frequency Radiation Generation
(2022) Schwartz, Robert Max; Milchberg, Howard; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
In this dissertation, we discuss the applications of intense mid-infrared laser interactions in three main topics. First, we demonstrate and discuss the remote detection of radioactive materials using avalanche breakdowns driven by picosecond, mid-infrared laser pulses. In the presence of radioactive materials, an enhanced population of free electrons and weakly bound ions are created in air. Laser driven avalanche ionization is a powerful tool for amplifying and detecting this weak signature, allowing for detection at standoff distances beyond the stopping distance of the radioactive particles. This technique can be applied more generally to the detection of any low density plasma. In the second section, we apply a similar method to measure laser ionization yields in atmospheric pressure gas across an extremely wide range. Finally, we demonstrate and discuss the generation of THz and low harmonics from two-color mid-infrared laser pulses. This technique allows for the generation of highly efficient, ultra-broadband coherent radiation.
Laser Wakefield Acceleration in Optical Field Ionized Plasma Waveguides
(2021) Feder, Linus; Milchberg, Howard; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Laser wakefield accelerators (LWFA) can support acceleration gradients orders of magnitude higher than conventional radio frequency linear accelerators. This gives them the potential to drive the next generation of accelerators for high energy physics, as well as compact accelerators for many other applications. However, in order to reach higher energies and improve electron beam quality, LWFA requires the development of plasma waveguides. This thesis demonstrates two new all optical techniques for the creation of plasma waveguides. The first, “two-Bessel” technique uses a ?0 Bessel beam to form the core of the waveguide and a higher order ?? Bessel beam to form the cladding. In the second, “self-waveguiding” technique, the guided beam itself forms the cladding of the waveguide. Preliminary electron acceleration results using the self-guiding technique, as well electron acceleration simulations are also presented.
Electron Acceleration during Macroscale Non-Relativistic Magnetic Reconnection
(2021) Arnold, Harry; Drake, James; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
In this thesis we developed the new model {\it kglobal} for the purpose of studying nonthermal electron acceleration in macroscale magnetic reconnection. Unlike PIC codes we can simulate macroscale domains, and unlike MHD codes we can simulate particles that feedback onto the fluids so that the total energy of the system is conserved. This has never been done before. We have benchmarked the model by simulating Alfv\'en waves with electron pressure anisotropy, the growth of the firehose instability, and the growth of electron acoustic waves. We then studied the results of magnetic reconnection and found clear power-law tails that can extend for more than two decades in energy with a power-law index that decreases with the strength of the guide field. Reconnection in systems with guide fields approaching unity produce practically no nonthermal electrons. For weak guide fields the model is extremely efficient in producing nonthermal electrons. The nonthermals contain up to $\sim80\%$ of the electron energy in our lowest guide field simulation. These results are generally consistent with flare observations and specifically the measurements of the September 10, 2017, flare.
Turbulent and Collisional Transport in Optimized Stellarators
(2020) Martin, Mike; Dorland, William; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The stellarator is a fusion energy concept that relies on fully three-dimensional shaping of magnetic fields to confine particles. Stellarators have many favorable properties, including, but not limited to, the ability to operate in steady-state, many optimizable degrees of freedom, and no strict upper limit on the plasma density. Due to the three-dimensional character of stellarators, theoretical and computational studies of stellarator physics are challenging, and they also possess some disadvantages compared with tokamaks. Namely, particle confinement and impurity control are problems in generic stellarator magnetic fields that must be addressed with optimized magnetic fields. Further, simulations will require a substantial increase in grid points because of the three-dimensional structure, leading to more expensive computations. This thesis will address both topics, by first exploring the behavior of impurity particle transport in optimized stellarators, and then introducing a boundary condition to reduce the cost of stellarator turbulence simulations. Impurity temperature screening is a favorable neoclassical phenomenon involving an outward radial flux of impurity ions from the core of fusion devices. Quasisymmetric magnetic fields lead to intrinsically ambipolar neoclassical fluxes that give rise to temperature screening for low enough $\eta^{-1}\equiv d\ln n/d\ln T$. Here we examine the impurity particle flux in a number of approximately quasisymmetric stellarator configurations and parameter regimes while varying the amount of symmetry-breaking in the magnetic field. Results indicate that achieving temperature screening is possible, but unlikely, at low-collisionality reactor-relevant conditions in the core. Further, in configurations optimized for quasisymmetry, results suggest that neoclassical fluxes are small compared with a gyro-Bohm estimate of turbulent fluxes. Calculating these turbulent fluxes is generally done by solving the gyrokinetic equation in a flux tube simulation domain, which employs coordinates aligned with the magnetic field lines. The standard ``twist-and-shift'' formulation of the boundary conditions 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 of this generalization is to rely on integrated local shear, allowing one significantly more freedom when choosing the extent of the simulation domain in each direction. Simulations of stellarator turbulence that employ the generalized parallel boundary conditions allow for lower resolution to be used compared with simulations that use the (incorrect, axisymmetric) standard parallel boundary condition.
Study and Mitigation of Transverse Resonances with Space Charge Effects at the University of Maryland Electron Ring
(2020) Dovlatyan, Levon; Antonsen, Thomas M; Beaudoin, Brian L; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Research at the intensity frontier of particle physics has led to the consideration of accelerators that push the limits on achievable beam intensities. At high beam intensities Coulomb interactions between charged particles generate a space charge force that complicates beam dynamics. The space charge force can lead to a range of nonlinear, intensity- limiting phenomena that result in degraded beam quality and current loss. This is the central issue faced by the next generation of high-intensity particle accelerators. An improved understanding of the interaction of the space charge forces and transverse particle motion will help researchers better design around these limiting issues. Furthermore, any scheme able to mitigate the impacts of such destructive interactions for space charge dominated beams would help alleviate a significant limitation in reaching higher beam intensities. Experimental work addressing these issues is presented using the University of Maryland Electron Ring (UMER). This dissertation presents experimental studies of space charge dominated beams, and in particular the resonant interaction between the transverse motion of the beam and the periodic perturbations that occur due to the focusing elements in a circular ring. These interactions are characterized in terms of the tune shifts, Qx and Qy, that are the number of transverse oscillations (in and out of the plane of the ring) per trip around the ring. Resonances occur for both integer and half-integer values of tune shift. Particle tune measurement tools and resonance detection techniques are developed for use in the experiment. Results show no shift for either the integer (Qx = 7.0, Qy = 7.0) or half-integer (Qx = 6.5, Qy = 6.5) resonance bands as a function of space charge. Accepted theory predicts only a shift in the half-integer resonance case. A second experiment testing the potential mitigation of transverse resonances through nonlinear detuning of particle orbits from resonance driving terms is also presented. The study included the design, simulation, and experimental test of a quasi-integrable accelerator lattice based on a single nonlinear octupole channel insert. Experiments measured a nonlinear amplitude dependent tune shift within the beam on the order of ∆Qx ≈ 0.02 and ∆Qy ≈ 0.03. The limited tolerances on accelerator steering prevented measuring any larger tune shifts.
Adjoint methods for stellarator shape optimization and sensitivity analysis
(2020) Paul, Elizabeth; Dorland, William; Landreman, Matthew; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Stellarators are a class of device for the magnetic confinement of plasmas without toroidal symmetry. As the confining magnetic field is produced by clever shaping of external electro-magnetic coils rather than through internal plasma currents, stellarators enjoy enhanced stability properties over their two-dimensional counterpart, the tokamak. However, the design of a stellarator with acceptable confinement properties requires numerical optimization of the magnetic field in the non-convex, high-dimensional spaces describing their geometry. Another major challenge facing the stellarator program is the sensitive dependence of confinement properties on electro-magnetic coil shapes, necessitating the construction of the coils under tight tolerances. In this Thesis, we address these challenges with the application of adjoint methods and shape sensitivity analysis. Adjoint methods enable the efficient computation of the gradient of a function that depends on the solution to a system of equations, such as linear or nonlinear PDEs. Rather than perform a finite-difference step with respect to each parameter, one additional adjoint PDE is solved to compute the derivative with respect to any parameter. This enables gradient-based optimization in high-dimensional spaces and efficient sensitivity analysis. We present the first applications of adjoint methods for stellarator shape optimization. The first example we discuss is the optimization of coil shapes based on the generalization of a continuous current potential model. We optimize the geometry of the coil-winding surface using an adjoint-based method, producing coil shapes that can be more easily constructed. Understanding the sensitivity of coil metrics to perturbations of the winding surface allows us to gain intuition about features of configurations that enable simpler coils. We next consider solutions of the drift-kinetic equation, a kinetic model for collisional transport in curved magnetic fields. An adjoint drift-kinetic equation is derived based on the self-adjointness property of the Fokker-Planck collision operator. This adjoint method allows us to understand the sensitivity of neoclassical quantities, such as the radial collisional transport and self-driven plasma current, to perturbations of the magnetic field strength. Finally, we consider functions that depend on solutions of the magneto-hydrodynamic (MHD) equilibrium equations. We generalize the well-known self-adjointness property of the MHD force operator to include perturbations of the rotational transform and the currents outside the confinement region. This self-adjointness property is applied to develop an adjoint method for computing the derivatives of such functions with respect to perturbations of coil shapes or the plasma boundary. We present a method of solution for the adjoint equations based on a variational principle used in MHD stability analysis.

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