The perturbation in a glow discharge in hydrogen is studied theoretically and experimentally. In an ordinary glow discharge a perturbation has been produced by applying a small sinusoidal high frequency electric field. This modulates the electron energy and hence the number density of atoms in excited states; The relative phase and amplitude of light emitted in the first four lines of the Balmer series have been measured. A theory based on the work of Bates et al, is developed in this thesis and the results predicted give better agreement with the experimental values than do alternative theories which have been published recently. These if absorption of line radiation by the discharge is shown to have a critical effect on the behaviour of the plasma. The limitations and validity of assumptions are discussed and a modified experiment is suggested. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

A "gradient probe" consisting of two search coils has been developed to measure the current density in a pulsed plasma. This probe measures both the magnitude and the gradient of the magentic field simultaneously enabling more accurate measurements than the conventional magnetic probe which has only one coil. It has been used to measure the current densities and the magnetic fields in z-pinch discharges in helium at pressures between 500μ and 4 mmHg. The collapse curves obtained agreed with the predictions of a modified snow-plow equation which allowed for the loss of particles from the collapsing current shell. The flow of current in the plasma is distorted by the presence of a probe. Such an effect spoils the spatial resolution so that the measured values of the current density Jp are averages of the true current density J₀ over a finite region. To investigate this, a correction formula which relates Jp to J₀ has been developed. Our error analyses showed that any scatter in Jp due to experimental errors was magnified twenty times in J₀. For a pulsed plasma, therefore, one should try to reduce the perturbation of the probe instead of relying on the correction procedure. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Plasmas confined by a dipole magnetic field exhibit interchange and entropy mode turbulence causing bursty intermittent transport of particles and energy [1]. On the Collisionless Terrella Experiment (CTX), this turbulence is dominated by low-frequency, long-wavelength modes with amplitudes and phases that vary chaotically in time [2]. We present a new paradigm for characterizing this turbulence by measuring the time-evolution of the fluctuation power spectrum and the instantaneous bispectrum using the continuous wavelet transform [3, 4] and computing the statistical properties of turbulent wave kinetics. We observe that both the fluctuation power and the energy transfer by three-wave coupling, or bispectrum, between these fluctuations can be intermittent. When antenna are used to actively launch waves into the turbulence, the intermittency of the driven waves decreases, while the intermittency of other waves increases. Similarly, application of active feedback [5] to amplify the turbulence decreases the intermittency of the wave energy, while suppressing feedback increases this intermittency. Measurements based on this new paradigm show that the transfer of wave energy to larger and smaller scales in a turbulent plasma is not steady but occurs in short and intense bursts, analogous to the better-known short bursts of particle transport in magnetized plasma.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Nuclear Engineering, 1977 / Vita. / Includes bibliographical references. / Ph. D. / Ph. D. Massachusetts Institute of Technology, Department of Nuclear Engineering

The physics of the biasing induced L-H transition and the mechanism for E×B shear flow suppression of turbulence are investigated on HBT-EP. Detailed measurements of the transverse length scales, behavior, and propagation direction of the edge turbulence match what is expected for the ion temperature gradient (ITG) mode. In the scrape-off layer (SOL), radially propagating blob-filament turbulence is identified and characterized, with velocities, sizes, and distributions comparable to measurements on other devices. Through systematic studies of the effect of applied shear flow on the turbulence, it is found that the E×B suppression of turbulence matches what is expected by the spectral shift model [Staebler et al. 2013 Phys. Rev. Lett. 110 055003]. Namely, the application of shear flow tilts the turbulent eddies and shifts the mean radial wavenumber ⟨kr⟩ of the turbulence spectrum from near zero to finite values, leading to a reduction in the turbulence intensity. The investigation also shows that both the decorrelation model and quench rule are able to reproduce the measured reduction of the turbulence intensity with applied shear flow when appropriate parameters are chosen. However, the decorrelation model fails to explain the increase in the shear-wise correlation length measured with increasing applied shear, and the quench rule fails to capture the suppression of the turbulence to a finite intensity at high shear. It is found that the same shearing effect that tilts the eddy structures and shifts ⟨kr⟩, enhances the gradient in the Reynolds stress at the edge and suppresses the blob-filament turbulence in the SOL. Although the biasing levels leading up to the transition are shown to enhance the Reynolds stress in a radially varying manner, it is found that the high flow shear in the H-mode state completely quenches the Reynolds stress. A careful examination of the spatial structure and temporal dynamics of the forcing terms in both dithering and one-step transitions reveals that the biasing induced L-H transition is caused by a reduction in poloidal viscosity at high flow velocity, in agreement with neoclassical theory. Nevertheless, the Reynolds force is measured to be comparable to the force from the electrode current, allowing the turbulence driven stress to work synergistically (or antagonistically) with forces from the probe to achieve the critical poloidal flow velocities. The similarities between the transition criteria on HBT-EP and other devices indicate that reduction of poloidal viscosity leading to the transition to improved confinement regimes may be a universal trait among toroidal confinement devices. The application of resonant magnetic perturbations (RMPs) is shown to both reduce the Reynolds stress and increase the biasing threshold for the transition. The observed reduction in the Reynolds stress stems from a reduction in the intensity of the underlying turbulence; namely, a decrease in the amplitude of velocity fluctuations in regions where the Reynolds stress is high without an applied RMP. This study has therefore expanded the current understanding of transport barrier formation in magnetic confinement devices.

The aim of this work is to develop a deeper understanding of the nuances involved in designing and optimizing the performance of gas/liquid plasma reactors for chemical synthesis. The design of such reactors requires integrating knowledge from a number of scientific disciplines including mechanical engineering (reactor construction), electrical engineering (power network design and electrical diagnostics), physics (plasma formation and diagnostics), chemical engineering, (reactor design, transport phenomena / modeling), and chemistry (chemical analysis). Due to the complicated nature of such a multidisciplinary study, complete analysis of a single reactor system is difficult and rarely performed with accuracy, especially for a variety of operating conditions. In this work, a novel continuously flowing liquid film plasma reactor was developed and fully characterized under the above criteria for a range of operating conditions in order to better understand which variables most significantly impact the generation of hydrogen peroxide from pure water and argon gas. This work shows that increases in the energy yield for hydrogen peroxide with pulsed plasma discharge is possible by variation of the plasma properties to reduce the amount of "wasted" energy which does not contribute to desired chemical reactions. In addition, increases in the production rate of hydrogen peroxide without a loss in energy yield is shown to be possible by increasing the pulse frequency while simultaneously decreasing the gas phase residence time. The high concentration of hydroxyl radicals produced by this system was also used to partially oxidize simple organic compounds into higher value products. / A Dissertation submitted to the Department of Chemical and Biomedical Engineering in partial fulfillment of the Doctor of Philosophy. / Fall Semester 2016. / November 10, 2016. / Discharge, Hydrogen Peroxide, Hydroxyl radical, Plasma / Includes bibliographical references. / Bruce R. Locke, Professor Directing Dissertation; Igor Alabugin, University Representative; Ravindran Chella, Committee Member; Danial Hallinan, Committee Member.

In this work, we develop an effective field theory program for many-body systems out of finite temperature equilibrium. Building on recent work, we combine powerful mathematical tools such as the Schwinger-Keldysh closed-time-path formalism, the coset construction, and Wilsonian effective field theory to construct novel actions that describe a wide range of many-body systems out of finite-temperature equilibrium. Unlike ordinary actions, these non-equilibrium actions account for dissipation and statistical and quantum fluctuations. The novel actions constructed include those for solids, supersolids, nematic liquid crystals, smectic liquid crystals in phases A, B, and C, chemically reacting fluids, quasicrystals, higher-form dual theories of superfluids and solids, and plasmas that can support large charge density. In order to construct these actions, we propose a new kind of coset construction with a total of four distinct types of inverse Higgs constraints. We extend the coset construction to account for higher-form symmetries and investigate the relationship between two kinds of ’t Hooft anomalies and spontaneous symmetry breaking.

This thesis is a work divided into two parts on aspects of three-dimensional (3D) magnetic fields: (I) magnetic reconnection treated from a strictly 3D viewpoint and (II) the design of coils for producing the 3D magnetic fields of optimized stellarators. In astrophysical settings, magnetic fields are generically 3D. 3D divergence-free fields have rich topological structures such as magnetic nulls and chaotic field line structures. Standard reconnection literature identifies magnetic nulls as locations of magnetic reconnection, and that intense currents will build up around them. This idea is explored with a key realization that by placing a vanishingly small sphere around the null, boundary conditions on field lines passing through the sphere may be sorted out. The main result here is (1) the dismissal of the notion that nulls are crucial places for magnetic reconnection and current accumulation, instead identifying separatrices of topological type on the boundaries of null-passing field lines to be crucial. Standard reconnection literature dismisses chaotic flows yet 3D fields generically have chaotic flows. An inherent property of chaotic flows is exponentiation. The main result here is (2) the identification of exponentiation as a natural mechanism for magnetic reconnection and that the associated current builds up linearly in time in contradiction to standard results requiring the formation of high-density current sheets. The magnetic fields of optimized stellarators are intricate, producing complex 3D magnetic surfaces. These fields are conventionally generated by non-planar electromagnetic coils, though these coils are costly to manufacture, slow device assembly, and hinder stellarator maintenance. Part II of this thesis explores methods of stellarator coil simplification that do not involve modular coils. All of this work uses current potentials, which are stream functions of the current sheets that produce magnetic surfaces. We begin with a result found using analytic methods on current potentials that (1) there may be an inherent limitation in the ability of modular coils to produce fields at a distance. This result is not surprising, though further analysis is necessary to work out some complexities of the result. Next, (2) a novel method to produce localized patches of current potential, representative of patches of current sheets, is developed and used to identify crucial locations of current placement for shaping magnetic surfaces. Most notably, these current sheet patches are able to produce much of the surface shaping while occupying a small fraction of the winding surface, resulting in good open-access stellarator coil configurations. Continuing the trend away from modular coils, (3) helical coils are optimized to support stellarator magnetic fields. This work agrees with related work on the optimization of helical coils, finding them unsuitable to the precise production of equilibria generated by modular coils. To improve this result, we use coil sets of mixed-type: helical coils with windowpane coils or permanent magnets, to mitigate field error left behind by the helical coils. Finally, (4) the development of a generalized method to cut modular, helical, and windowpane coils out of current potentials and to identify the associated coil currents is developed and used in coil optimization.

This thesis presents a comprehensive investigation into the observations and suppression of sawtooth instabilities on the High Beta Tokamak-Extended Pulse (HBT-EP) device. The principle and design of a new tangential multi-energy extreme ultraviolet and soft x-ray (ME-EUV/SXR) diagnostic system is presented. This system enables the clear detection of sawtooth events for the first time on HBT-EP. It is the first multi-energy tangential-view system designed to work in a temperature range below 200 eV in a tokamak, which enables measurements of the electron temperature and the examination of mode dynamics. By employing a combination of 0.1 um aluminum and 0.2 um titanium filters, the system allows measurements of electron temperature profiles through reconstruction of the emission profile using the standard ``double-foil'' technique. Using the tangential ME-EUV/SXR diagnostic system, the thesis reports on the first detailed observations of sawtooth events on HBT-EP, analyzing their features and comparing the findings with results from other devices. It investigates the phenomenon of discharge scenario bifurcation, where plasma exhibits distinct behaviors under similar parameters. The study examines the correlation between the amplitude of the edge mode and the strength of sawtooth events, along with the role of the conducting wall system in this context. It was found that when the normalized wall radius 𝒃/𝒂 is within a critical value, the edge mode can be stabilized and strong sawtooth events occur. In-depth analysis is performed on the modes present during sawtooth-suppressed stages, with a particular focus on the coupling between the 1/1 helical core (HC), 2/1 tearing mode (TM) and the 3/1 external kink mode (𝐗𝐊). Evidence is provided to support the effectiveness of magnetic flux pumping in suppressing sawtooth instabilities when the 3/1 𝐗𝐊 exhibits a significant amplitude. Conversely, the suppression of the 3/1 𝐗𝐊 due to stabilization by the conducting wall leads to the weakening of magnetic flux pumping, resulting in the occurrence of strong sawtooth events. In conclusion, this thesis contributes to the understanding of sawtooth instabilities on the HBT-EP tokamak and highlights the role of magnetic flux pumping as a mechanism for sawtooth suppression. It broadens the understanding of flux pumping across various tokamak operational regimes and demonstrates the potential of sawtooth suppression through external mode manipulation. This contributes to the future development of sawtooth control strategies, improving plasma stability and advancing fusion energy research.

Experiments on the DIII-D device in the negative triangularity (NT) regime of tokamak operation demonstrate core conditions that offer advantageous stability properties. Long duration, stationary discharges in this scenario maintain performance metrics that scale to viable reactor gain. Deleterious global modes of toroidal mode number n=1 are infrequent in these plasmas, which operate free of core instability cycles that can kick off global instabilities. These plasmas operate free of edge instability cycles that would damage reactor components, as do all strongly shaped NT plasmas. Reproducible access to high-power stationary states was developed at two values of q95, the edge magnetic winding number or “safety factor”. Core MHD instabilities manifest in one form of internal ideal mode, the quasi-interchange mode (QI), found to be consistent with modeling of the profiles and parameter space in which NT operates. The GATO and DCON ideal MHD codes are used to characterize the limits to normalized pressure in NT, finding global kink modes with strong poloidal harmonic m=1,2 components at normalized plasma pressure βN=3-3.5. Limits to β_N are predicted to be mostly insensitive to plasma boundary shape in NT and similar at both q95 values obtained in experiments. Average triangularity is shown to affect ideal limits, when modified at the outer midplane. A similar result is obtained with the RDCON resistive MHD code, which is used to characterize the stability to resistive “tearing modes”. Experimental NT equilibria and equilibria across shape scans were investigated. Only outer midplane modifications affected tearing calculations. Ideal kink modeling and experimental observations of sporadic QI mode provide an explanation for current diffusion not predicted by neoclassical theory. This effect is found in experiments at q95=3, analyzed with the ONETWO transport code’s facility to evolve magnetic flux over a discharge consistently with measured profiles and reconstructed magnetic flux surfaces. This result is compared with GATO calculations and ONETWO flux diffusion analysis of a conventional shape, ITER baseline demonstration discharge that is shown to have an intrinsically 3D core. Radiation from accumulated plasma impurities seems to alter the core q profile. This makes unstable a QI mode that spurs formation of a helical core, sustained by anomalous magnetic flux diffusion. NT experiments at q95=4 are limited in energy confinement by poor fast ion confinement, as a result of nondisruptive core 3/2 tearing modes. Analysis with ONETWO shows agreement with neoclassical flux diffusion predictions in these cases, corresponding to a removal of core instabilities and elevation of minimum safety factor values qmin to unity. This understanding of the core MHD, performance, and operational limits of NT scenarios in DIII-D advances the development of negative triangularity scenarios and informs the core phenomena observed in experiments spanning the regime.