Session JP13: Poster Session: Undergraduate Research (2:00pm - 5:00pm)

Analytic and Computational Exploration of Stellarator Coil Optimization

Designing stellarator coils which meet both physics and engineering constraints remains an important area of stellarator research. In particular coil robustness to manufacturing errors and geometric simplicity has become a critical issue in the design of new experiments. Through a combination of both analytic and numerical methods, we seek to explore the connection between the magnetic axis, quasisymmetric magnetic field, and coil complexity of quasiaxisymmetric stellarator configurations, with a particular eye towards developing a more sophisticated understanding in how coil complexity and robustness is coupled to the magnetic field and axis geometry.   

EPPS measurements of particle acceleration due to magnetically driven reconnection using laser-powered capacitor coils

Magnetic reconnection is a ubiquitous plasma phenomenon characterized by the rapid breaking and reconnecting of magnetic field lines, during which magnetic energy is converted into plasma flow, thermal, and nonthermal energy. Reconnection has been studied in laboratory experiments; however, it is difficult to measure the accelerated particle spectra due to a short particle mean free path relative to detector distance. To bypass this limitation, we have developed a reconnection platform at the Jupiter Laser Facility composed of a pair of U-shaped Cu coils joining two parallel Cu plates irradiated by a ~250 J IR nanosecond square pulse from the Titan laser. Using this, we have obtained particle spectrometer data as part of a broader study of the reconnection particle acceleration mechanism(s). To isolate reconnection-accelerated particles, we compare data from a reconnection (2-coil) case to a control (1-coil) case. We also investigate angular dependence by comparing data between five particle spectrometers placed at various angles relative to the driven target.   

Investigation of Powder Injection Using Segmented Electrodes

Injection Boron powders were demonstrated to be an effective tool for wall conditioning and recycling reduction in tokamak devices. Presently, boron powder is injected using the Impurity Powder Dropper (IPD) which relies on gravity for delivery in the plasma. This scheme results in velocity limited by gravitational acceleration. It is proposed that by electrically charging boron, transit times can be reduced by at least a factor of ten. Additionally, injection can become directionally independent, removing the requirement for installation above a tokamak and allowing for greater system flexibility. Dust charging via lamp based photoemission processes were studied to set physics constraints for powder dynamics held at vacuum pressure. Various regimes of powder granularity and lamp energy were investigated to guide design of a segmented electrode system for improved injection control and velocity. Control of particle radius and injection velocity also opens avenues for experimental studies of ablation characteristics in the edge of tokamaks.   

Generating Stellarator fields using complex hypersphere coordinates

Magnetic fields for plasma confinement are divergence free vector fields that lie on a foliation of nested toroidal surfaces. We show how to generate a wide class of such fields using the stereographically projected complex coordinates on the hypersphere, $S^3$. The configurations included knotted and twisted fields. However, these fields originally derived to generate solutions to Maxwells equations, can still carry current and are therefore not suitable as stellarator fields. We explore different analytical methods to adapt these fields to generate current-free (or parallel current) configurations.   

Temperature Effects on Relief Pressure of Helium Bubbles in Tungsten

Tungsten, the current material of choice for tokamak divertors, is known to develop a network of tendrils, dubbed ``fuzz,'' when exposed to helium plasma. The formation of helium bubbles below the surface is an important part of the process of fuzz formation. This study utilizes molecular dynamics to analyze the effects of temperature on the pressure at which dislocation loop-punching and/or helium bubble bursting occurs. As expected, raising the temperature lowers the pressure at which bubbles of a given size at a given depth from the surface will burst or loop-punch, but the magnitude of the change decreases as the temperature increases. The relief pressure also falls off near the surface: the relief pressure at all tested temperatures is generally well-described by an empirical equation of the form $P_r = P_b(1-e^{-Cd}),$ where $P_r$ is the relief pressure, $P_b$ is the bulk loop-punching pressure at the same temperature, $d$ is the depth of the bubble, and $C$ is a constant to be determined.   

Improved Vertical Stability Model for NSTX-U

Establishing stable plasma discharges at large plasma elongation ($\kappa $ $\ge $ 2.5) with real-time vertical position control is critical for the mission of the National Spherical Torus Experiment Upgrade (NSTX-U) to realize large normalized $\beta $ ($\beta_{\mathrm{N}}$ \textgreater 4) concurrently with large non-inductive current fraction. A database of NSTX-U discharges is utilized to establish the maximum controllable open-loop vertical instability growth rate realized during operations in 2016. A closed-loop simulation of the real-time vertical position control on NSTX-U was developed to investigate the potential for improvements of the vertical control strategy and to quantify the impact of the poloidal field coil power supplies on the vertical position control near the controller limits. Specifically, the impact of switching transients on the actuators and measurements will be investigated in order to potentially motivate improvements to the power supplies or magnetic measurements.   

Modeling Atomic and Molecular Plasma Processes During Startup of PFRC-2

To study how initial conditions of PFRC-2, a reversed-field configuration device at Princeton, affect startup and the relative importance of different processes during startup, we constructed and solved a 0D model as an initial value problem. Incorporated into the model are hydrogen processes using collisional radiative rate coefficients taken from EIRENE, charged particle loss due to flow parallel to B, enhanced confinement from mirror fields and the FRC, electron interactions with the ends of the machine, and subsequent generation of nonthermal, high energy electrons. By solving the model we obtain electron density and energy as a function of time and can determine the delay to densification. Additionally, we present trends in these outputs as functions of major machine inputs, namely: $P_{in}$, $n_H$, $\tau_e$, and $B$.   

Two Fluid, Ten Moment Simulations of Temperature Anisotropy Driven Instabilities in the Solar Wind

The solar wind is a stream of plasma that continuously flows from the sun and expands outward into the heliosphere. This expansion leads to anisotropy in pressure and temperature along and across magnetic fields lines that drive instabilities, such as the proton and electron firehose, mirror, and whistler instabilities. Here we explore these instabilities using the two fluid, ten moment model in the Gkeyll simulation framework. We attempt to expand upon hybrid fluid models that have been used to study these instabilities by retaining more dynamical information about the electrons, i.e., the full electron pressure tensor, while also exploring the limits of the ten-moment model to capture the physics of these kinetic instabilities. Using extended fluid models which capture these dynamically important instabilities provides a promising path forward for global modeling given the computational constraints inherent to kinetic modeling.   

Testing H-mode pedestal transport models through predictive simulations

A number of theoretical transport and stability mechanisms are predicted to impact the structure of the tokamak pedestal -- the boundary region with steep temperature and density gradients that provide good energy confinement. While collisional diffusion provides a minimum for transport, various turbulent mechanisms and MHD instabilities limit the total temperature, density, and pressure gradients by allowing relatively efficient particle and energy transport. We are particularly interested in these microinstabilities since unraveling their role in setting these gradients remains a key research area for predicting fusion performance. This work focuses on predicting temperature and density gradients in the pedestal through modeling multiple transport mechanisms to test if they can adequately explain experimental observations. This is accomplished by numerically solving several coupled, time-dependent transport equations using simplified transport models based on first-principles simulations. Key results will be to clarify and illustrate how the nonlinear mechanisms interact to determine the pedestal structure, including how the pedestal density and temperature evolve depending on relative source rates or following various transient perturbations.   

Electron Temperature Measurement using Collisional Radiative Model in PFRC-II

The PFRC-II experiment is a plasma containment device that aims to improve odd-parity rotating magnetic field (RMF) heating. This method drives electrical current and is theoretically capable of heating a plasma to fusion temperatures. To achieve this goal, a myriad of different parameters must be assessed during a pulse that lasts only milliseconds long. The primary objective of my research was to study the electron temperature values as a function of time observed in each discharge of the PFRC-II experiment. A non-intrusive method of studying electron temperature is examining specific wavelengths of visible light that are emitted during an RMF pulse. These spectra are produced due to electron-impact excitation of neutral hydrogen followed by radiative deexcitation. The spectroscopic data central to studying electron temperature is the three lowest energy spectra in the Hydrogen Balmer Series. To ensure a strong level of precision, an Ocean Optics Spectrometer was utilized to measure all three spectra and background radiation simultaneously. Using the empirically generated spectroscopic data and a collisional radiative model's approximations, electron temperature values were produced without impeding the heating process of the PFRC\textunderscore II experiment.   

Langmuir probe data interpretation with a neural network

Particle diagnostics of tokamak edge plasmas are commonly performed with Langmuir probes, which are swept at several kHz and produce a large volume of data to be processed for each shot of plasma. Each probe trace is manually truncated and fitted to find corresponding plasma parameters. However, because standard Langmuir probe characteristics are well-understood for a near-Maxwellian plasma and can be simulated easily as a piecewise function with added noise, an opportunity arises to develop an automated, neural network-based workflow to extract relevant plasma parameters. The NN is trained on simulated Langmuir probe data, then put to work on real data taken from tokamak experiments.   

Large Scale Simulations of Plasma Facing Component Boronization

Boronization of fusion reactor walls has emerged as a crucial technique in improving the performance of fusion devices and remains a topic requiring further understanding. Previous studies in this area have either made assumptions about the structure of the reactor wall, such as randomized atom placement, or have been limited by simulation size. Thus, in this study, classical molecular dynamics was used to simulate a large oxygenated graphite structure undergoing boronization and deuterium bombardment, starting from an ideal graphite crystal. Simulations show the boronization process to occur in phases. First, a layer of boron adheres to the graphite surface. After continued bombardment, the top layers of the graphite fragment and form an amorphous mixture of carbon, oxygen, and boron that rests upon the remaining graphite layers. This behavior may help explain the deuterium retention and sputtering behavior previously cataloged in both simulations and experiments.   

GPU-Accelerated 2D Kinetic Modeling of Transport in a Hall Thruster Channel

The causes of anomalous electron transport across the magnetic barrier in Hall thrusters is an area of ongoing research. An understanding of the mechanisms causing this transport would allow for the development of turbulence models for this process Since the transport is kinetic, the 2D particle-in-cell code LTP-PIC serves as a fitting numerical tool to carry out this study. Such simulations may also be used to study dominant modes using a spectral diagnostic. The simulation is extended azimuthally to observe periodic structures. Given that PIC codes are computationally expensive, requiring a large number of particles and time steps, adapting this MPI $+$ OpenMP portable code to GPU using the OpenACC standard decreases runtime while maintaining a single code base. Currently, LTP-PIC is limited by the speed of the field solver and particle-push. To further improve runtime, we explore using Ampere's law to evolve the electric field rather than solving Poisson's equation.   

Machine Learning for Shot Classification

This research examines preliminary machine learning approaches to classifying shots on the DIII-D tokamak to establish a metric of similarity between shots, classify different modes without human intervention, and indicate previously unexplored regions of phase space. The scope of this project encompasses a database of nearly 12,000 experiments on the machine. A subset of these experiments contain readily distinguishable modes, which form the basis for classification using neural networks, decision trees, and other models. We provide metrics for the accuracy of these different machine learning approaches. Based on the resulting clusters of shots, we discuss parameters most essential to establishing shot similarity, and which parameter combinations warrant further exploration.   

Behavior of the Solar Near-Surface Shear Layer

My study uses the Pencil Code to focus on the characteristics and behavior of the solar Near Surface Shear Layer (NSSL). This study uses a mean-field model with forcing to try and reproduce the NSSL and focuses on comparing Cartesian with spherical coordinates. To perform these computational simulations, my study varies a set of different parameters that may be able to explain the behavior of the layer. The parameters that are examined more thoroughly in my research are the forcing strength, location, and width of the layer where the forcing is applied, and the solar rotation. To analyze the NSSL, we simplified the problem by implementing the local box approximation; this attempts to allow for the entirety of the Sun to be modelled by a single local area of the Sun. With modifications of the code, the Sun is able to be simulated in spherical coordinates. This allows for the behavior of the Sun as a whole to be modelled more realistically.   

Progress in numerical implementation of metaplectic geometrical optics

Radiofrequency waves are widely used for auxiliary heating and current drive in fusion plasmas. The design and optimization of such systems is often performed using ray tracing codes, which rely on the geometrical-optics (GO) approximation. However, GO is known to fail at wave cutoffs and caustics. To accurately model the wave behavior in these regions, more advanced and computationally expensive ``full-wave" simulations are typically used, but this is not strictly necessary. A new, generalized formulation, called metaplectic geometrical optics (MGO), has been proposed that reinstates GO near caustics [Lopez \& Dodin, arXiv: 2004.10639]. The MGO framework yields an integral representation of the wave field, but evaluating the corresponding integral in the general case must be done numerically. We present a survey of numerical integration methods for MGO, including Gaussian quadrature and numerical steepest descent. These methods are benchmarked against analytical solutions in special cases when such solutions are available.   

Position Tolerance of Permanent Magnets and Reduction of Magnetic Islands in the Stellarator MUSE

We adapt techniques developed for coil optimization to the design of permanent magnet stellarators. Permanent magnets were recently proposed to address the challenge of building optimized stellarators by simplifying complex modular coils. MUSE is a table-top stellarator experiment using permanent magnets and will be built at Princeton Plasma Physics Laboratory. The permanent magnets for MUSE are designed by using the FAMUS code.\footnote{Zhu et al. arXiv:2005.05504 (2020).} We are adapting the shape-gradient method\footnote{Landreman & Paul, \textit{Nuclear Fusion} \textbf{58}(7), 076023 (2018).} and the Hessian matrix method\footnote{Zhu et al. \textit{Plasma Physics and Controlled Fusion}, \textbf{60}(5), 054016 (2018).} to calculate the position tolerance of permanent magnets in MUSE. The target figures of merit that we are going to evaluate are the normal field error on the target plasma boundary, the quasi-symmetry of the produced magnetic field and the magnetic island width. In doing so, we can help construct MUSE to an acceptable precision and reduce the magnetic islands for MUSE.   

Studies of Ion Incident Angle Distributions for Different Plasma Configurations

High particle and heat fluxes from a fusion plasma cause damage to components and vessel walls. The ion impact angle distribution (IAD) for both polar and azimuthal angles at the wall surface is an important parameter for understanding plasma-material interactions (PMI) such as erosion, deposition, and migration within fusion devices. IADs are not yet fully understood because ions are strongly affected by the Chodura sheath. Ion trajectories in the sheath are calculated using a non-collisional kinetic model for different plasma configurations relevant to linear plasma devices, e.g. PISCES-A (UCSD), and tokamak devices, e.g. LTX-$\beta$ and NSTX-U (PPPL). Micro-trench samples will be designed to verify the calculated IAD results for those plasma devices. Micro-trench samples have been used to measure IADs using DiMES on DIII-D (GA) by analyzing impurity deposition patterns on the trench bottom shadowed from incoming ions by its wall. Calculations using a Monte-Carlo model, micro-patterning and roughness (MPR) code, are planned to simulate sputtering behavior on the micro-trench samples to verify its design.   

Measuring Electromagnetic Fields in Laboratory Plasmas Using Laser-Wakefield-Accelerated Electrons

Particle radiography is a diagnostic method to obtain crucial information about the magnetic field of plasmas. Radiography involves sending a beam of particles through an electromagnetic field and imaging the resulting deflections of the particles on a detector. Laser-Wakefield-Accelerated (LWFA) electrons may be an accessible, high-energy source for particle radiography diagnostics. Compared to traditional proton radiography, the relativistic speeds associated with the LWFA electrons allow a wider range of magnetic field strengths to be probed. Additionally, LWFA electrons can penetrate thicker material filters and are compatible with high-repetition-rate applications. In this work, electron radiography is studied through synthetic calculations to determine the required energy, fluence, and source size, to properly probe the electromagnetic fields of laboratory plasmas using LWFA electrons.   

3D Coil Design Using Splines for Simplified Maintenance in Fusion Devices.

Difficulty in fabricating non-planar coils is one of the main challenges for stellarators. Many efforts have been undertaken in recent years to simplify their coil designs. The FOCUS code (Caoxiang Zhu et. al. 2018 Nucl. Fusion 58 016008) gets rid of the toroidal ”winding” surface used in previous approaches and uses an arbitrary closed curve in 3D to represent the coils, such that more possible solutions can be found. Until now, the coils were described by the Fourier series. In this work, the FOCUS code has been expanded to use a spline representation for the coils. By using a spline representation it is possible to implement real space constraints more easily by acting on the control points describing the spline. More importantly, this new development will allow FOCUS to design straight out-leg coils with improved accessibility and simplified maintenance, which would be vital for future fusion reactors.   

Building a Fokker-Planck Solver using the MFEM Finite Element Library

Driving toroidal currents by auxiliary power is necessary for magnetic confinement in tokamaks. The process of driving these currents by radio frequency (RF) waves and the corresponding evolution of the electron velocity distribution function is described as a diffusion by the Fokker-Planck (FP) equation. An efficient FP solver fully utilizing the emerging computing architecture is desired for modeling the evolution of tokamak discharge. In this work, we show a new FP solver based on MFEM (www.mfem.org) finite element method library developed at Lawrence Livermore National Laboratory. Initial results on RF waves in the lower hybrid wave frequency regime will be discussed. Output of this solver is compared with the results of existing code. Additionally, we explore the potential for this solver to work on a modern GPU computing architecture.   

Machine learning modeling and analysis of temperature and density profiles on NSTX and NSTX-U

Accurate real-time fusion plasma modeling is critical to implementing reliable control systems in present-day and future tokamak reactors. Physics-based approaches such as TRANSP can accurately model profile evolution of tokamak plasmas, yet are too computationally intensive for use in real-time controls. To address this, a machine learning based approach to modeling density and temperature profiles has been developed and tested on the NSTX experimental database. The model predicts the shape of the electron density and temperature profiles based on scalar quantities that can be measured and/or predicted in real-time, e.g, plasma boundary shaping and plasma current. This approach is orders of magnitude faster than comparable TRANSP predictions and shows potential for use in optimization algorithms. Different neural network architecture designs are evaluated for accuracy and the number of samples required for model convergence is determined. Challenges in predicting scenarios beyond the training space, such as predicting future shots based on past data, are discussed and potential solutions are tested.   

Driven steady states and neutral-induced dynamics in flowing plasmas.

The collisional transport of particles, momentum, and heat in magnetized plasmas, even in a slab geometry, exhibits rich dynamics. A new code, MITNS, has been recently created to explore these dynamics. In its current form, however, the code has reflecting boundary conditions, and no sources or sinks of particles or heat---it thus can only model an isolated system, in its approach to thermodynamic equilibrium. In plasma applications and experiments, however, we are often examining a driven steady state: for instance, in linear discharges, an energy source constantly heats the gas in a chamber to produce a hot plasma. Modeling such scenarios requires the inclusion of source terms and non-reflecting boundary conditions. These will be added to the code, and the resulting steady-states analyzed, with a particular focus on rotating plasmas. As a possible extension, sources consistent with the ionization of neutrals will be added, since neutral-induced charge transport is critical in understanding the rotation profiles of many laboratory devices.   

Not Participating

We focus on research and examination of various types of centrifuges used in industries around the world to determine the effectiveness of implementing the Advanced Annular Couette Centrifuge (AACC) technology developed by researchers at PPPL. The AACC method was developed to enhance the separation efficiency of liquid centrifuges by modifying a Taylor-Couette (TC) device. TC flow is the flow of a fluid between two co-axial differentially rotating cylinders. The PPPL method limits secondary flows and turbulence by having end-cap rings spinning at intermediate speeds so that the inner cylinder spinning at higher speeds can create higher effective gravity. The modified TC device can promote mixing or separation by keeping the fluid in one container and adjusting the two rings' speed. An engineering study examines the current applications of traditional centrifuges in agricultural and chemical engineering processes, energy applications, and the separation of sub-micron-sized particles in liquids. This research identifies specific markets where AACC technology can reduce process times and increase cost-efficiency. This new method can be applied to optimize processes and applications of liquid centrifuges used in industries around the world.   

Calculation of Runaway Electron Formation in Tokamak Thermal Quench Including Electron-Electron Collisions~~

Runaway Electrons (REs) are a great concern in tokamak plasmas, particularly in ITER due to the potential for highly energetic runaways due to large current. The complex dynamics of the electron-electron collisions, including small angle collisions, is typically not included in analytic predictions nor in previous numerical techniques relying on the linear collision operator such as CODE. Here, we simulate REs using the NORSE (Non-linear Relativistic Solver for Electrons) code, which treats collisions in the plasma relativistically and non-linearly. We calculate runaway electron generation under varying initial temperatures and thermal quench rates for ITER like conditions, two important parameters affecting runaway generation. We compare NORSE calculations of the driven runaway current to predictions from analytical work in the literature and previous work with CODE.~~   

A Practical Method for Extraction of Orbital Energy

Approximately 5000 manmade satellites are in orbit around the Earth as of today, about 1900 of which are still operational. The orbits of these satellites about the Earth can be altered, and even brought to ground via technology that extracts gravitational potential energy and stores that energy in a capacitor. Analysis of the orbits of coupled masses in the presence of a large central body~suggest~the potential to recover energy from the orbits of these bodies. By absorbing energy from the epicyclic orbits of these bodies, the entire system gradually falls toward the central body, while the lost energy is gained in the form of electricity, allowing the operator(s) of the satellite to recover some of the initial investment of putting the satellite into orbit.~Creating~a numerical simulation for this process that accounts for the dynamical interactions in the system allows for many configurations to be tested in search for the ideal energy extraction method, and facilitates an improved understanding of the basic principles involved.~   

Analyzing Proton Radiographs of Turbulent Transport in Magnetized HED Plasmas

Many laboratory experiments use magnetic fields to control high energy density (HED) plasmas, including studies of inertial fusion energy, magnetized shocks, and magnetic reconnection. Therefore, it is important to understand the coupling and behavior of magnetic fields and HED plasmas -- especially turbulent or anomalous transport of plasma relative to the magnetic field. We present experiments at the OMEGA laser facility to study the interaction of a flowing plasma generated from the ablation of a CH target with an external magnetic field powered by MIFEDS. The plasma-field interaction was diagnosed with 2D proton radiography, which measures magnetic fields through the deflections of the protons. A mesh placed between the proton source and plasma served to break the proton beam into quantifiable beamlets. In this work we establish a system for analysis of this data, including algorithms to detect beamlet locations and automatically calculate their deflections from a reference image, which provides information about the evolution of the magnetic field.   

Not Participating

We have developed a Sample Exposure Probe (SEP), which is a sample translator and ultrahigh vacuum suitcase, for wall surface analysis in the Lithium Tokamak eXperiment-$\beta $ (LTX-$\beta )$, which tests the performance of lithium-coated plasma facing components (PFCs). The SEP contains a heating element that can be used for temperature programmed desorption (TPD) measurements to identify the chemical composition of gaseous species that are desorbed (evolved) from a lithium-covered stainless steel sample after H plasma discharges in LTX-$\beta $. H$_{\mathrm{2}}$ evolution and other chemical species evolved from O and C impurities can be characterized and quantified using TPD. These results are important to help understand the chemical and physical state of lithium-coated PFCs, which affects H retention and plasma performance. The SEP upgrade involves a new configuration and installation of a residual gas analyzer (RGA) for use in TPD. A LabVIEW module was written, and integrated with the RGA software provided by the manufacturer, to control the temperature of the heating element in the SEP and obtain TPD data.   

Effectiveness of ELM mitigation techniques in reducing tungsten erosion rates during ELMs

The intense heat and high-density particle bursts on the plasma-facing components (PFCs) of a tokamak that occur during edge localized modes (ELMs) may be the most challenging plasma-materials interaction issue for ITER. ELMs in high confinement mode (H-mode) plasmas erode PFCs and lead to impurities in the core, reducing confinement. ITER must understand the level of material erosion caused by ELMs and develop solutions to mitigate their effects. We analyze erosion that occurred to tungsten (W) PFCs during the Metal Rings Campaign (MRC), in which tungsten-coated tiles were installed in the DIII-D divertor for three weeks in summer 2016. From these data, the effectiveness of ELM mitigation techniques, including resonant magnetic perturbations, pellet pacing, and QH-mode, in reducing (W) erosion rates during ELMs is evaluated. Standard D-alpha and WI filterscope channels were used to detect ELM start times and to infer the gross erosion rate of the tungsten PFCs respectively. The data obtained from these filterscopes are also compared to simulations of the `free-streaming plus recycling model' to assess validity of the model to extrapolate results and predictions to future tokamaks such as ITER.   

Correlation of divertor heat flux widths with upstream SOL profiles in DIII-D

Measurements of divertor target heat flux profiles are compared to upstream Scrape-Off-Layer (SOL) profiles to test models of SOL energy transport. Determination of the heat flux width from upstream profiles is important for scaling dissipative divertor solutions to future devices. The divertor target heat flux width is characterized by IR camera measurements and tested for consistency with data from target Langmuir probes and divertor Thomson scattering. Midplane SOL profiles are measured with Thomson scattering for electron density and temperature and CER for ion temperature. The assumption of parallel electron thermal conduction is tested by comparing the upstream profiles with the divertor target measurements. This correlation is compared for variation in the assumed role of heat flux transport via convection.   

Stability Analysis for Advanced Tokamak Plasmas on DIII-D

The advanced tokamak is a high-performance plasma regime intended to be the operational steady-state scenario for tokamak fusion. High $q_{min}$ plasmas are such a regime and are currently being developed on DIII-D. These high $q_{min}$ plasmas typically have $q_{min}>1.5$. Some of these high-performance discharges are destabilized by magnetohydrodynamic (MHD) tearing modes, spoiling the high performance of the plasma. A good understanding of the stability of these high $q_{min}$ plasmas and the onset of these tearing modes is crucial to the success of this scenario. Stability analysis has been performed on many high $q_{min}$ discharges on DIII-D. EFIT and CORSICA are used to generate equilibria for the ideal DCON stability code. Beta limits are found by calculating the stability of equilibria after systematically increasing the plasma pressure. In this work, scans of toroidal and poloidal grid resolution, edge q, and whether to fix current density or q after increasing the plasma pressure are used to determine optimal parameters. No-wall and ideal-wall beta limits are then correlated to the onset of tearing modes in high $q_{min}$ plasmas. A stability database for these discharges has also been generated to assist in predictions for the onset of MHD tearing modes.   

Extended Atomic Physics in FISASIM.

FIDASIM is a code that models signals produced by charge-exchange reactions between neutrals and ions (both fast and thermal) in magnetically confined plasmas. With the ion distribution function as input, the current version of the code predicts the efflux to a neutral particle analyzer diagnostic and the photon radiance of Balmer-alpha light to a fast-ion D-alpha or charge-exchange recombination diagnostic. Three extensions to FIDASIM are reported. First, the code now predicts the radiance from Lyman-alpha transitions. Second, Zeeman splitting is included. Third, Stokes parameters are an optional output.   

Passive 3D Coil Design to Protect Tokamaks from the Threat of Runaway Electrons

A passive 3D helical coil was modeled and optimized to passively de-confine runaway electrons (RE) in the DIII-D tokamak. RE are created during tokamak disruptions when the rapid plasma current drop induces an electric field that accelerates plasma electrons to relativistic speeds. A beam of REs can cause significant damage to plasma facing components. In order to prevent this threat, the helical coil would create a 3D magnetic field to enhance the radial drift and deconfinement of the REs. The coil is inherently passive in that no external power is needed; the disruption itself would provide the required current to create the needed 3D field due to the changing magnetic flux causing an emf within the coil. An in-house electromagnetic code (BSharm) was developed and used to create various coil designs and calculate the 3D field induced by each design. The inductive coupling between the DIII-D tokamak and the various coil designs were calculated using the TokSys simulation suite. The coil designs were then tested for their efficacy by comparing vacuum field harmonic amplitudes, and an optimized design for the 3D coil was selected. Finally, this optimized coil was tested using MARS to model the complete plasma response to the 3D fields induced by the coil.   

The Impact of Resonant Magnetic Perturbation Strength on the L-H Power Threshold

We report a DIII-D database study of H-mode power threshold in the presence of Resonant Magnetic Perturbations (RMPs) compared to the Martin L-H power scaling [Y.R. Martin, et al., J. Phys. Conf. Ser. 123, 012033 (2008).]. Since ELM control is critical for ITER, this study is important for assessing the impact of RMP ELM suppression on the L-H power over a range of DIII-D plasma conditions, which can eventually be projected to ITER. The L-H transition is a complex phenomenon with several key control parameters, some of which are altered by the RMP field. In order to understand the effects of RMPs on the L-H threshold, a scaling model is being constructed that includes the amplitude and phase of various 3D toroidal modes due to field-error correction and RMP coils in DIII-D. In addition to the loss power used in the Martin scaling (P$_{\mathrm{L}} \quad =$ P$_{\mathrm{ohm}}$ $+$ P$_{\mathrm{abs}}$ -- dW/dt -- P$_{\mathrm{f-loss}})$ we include the radiated power loss from the core, which has a significant impact of the scaling in DIII-D and is expected to be important in ITER. Preliminary results indicate a systematically higher power needed to cross into H-mode when RMPs are present when compared to the Martin scaling.   

Wave Redux: Multiple Passes of EC Waves through the DIII-D Tokamak

A model is being developed to track electron cyclotron (EC) waves as they pass multiple times through the plasma in the DIII-D tokamak. EC waves are a leading candidate to heat and drive current in tokamak reactor plasmas. Currently, the second harmonic X-mode (X2) polarization is favored due its high absorption in the plasma, but sometimes the plasma density is too high to allow the X2 waves to reach the plasma center. Second harmonic O-mode (O2) waves have twice the density limit of the X2 mode but are only partially absorbed on their first pass through the plasma. The O2 waves will reflect off the conductive graphite tiles and pass a second time through the plasma with possibly significant absorption. In this work, the TORAY ray tracing code has been extended to track multiple passes of EC waves through the plasma, including changes in polarization, to track secondary heating and current drive effects. By modulating gyrotron power and analyzing the electron temperature response, predictions from TORAY will be compared to experiments for which multiple passes may be important. Using the improved understanding of EC wave ``bouncing'' and resulting secondary power deposition, future experiments and reactors can use moderately absorbed EC waves more effectively.   

Machine Learning Noise-Reduction Signal Processing Techniques for DIII-D LLAMA Diagnostic

A tokamak is a challenging environment for accurate plasma diagnosis. For example, measurements from plasma diagnostics may be dominated by spurious signal, or noise, due to the strongly varying electric and magnetic fields. In the case of the Lyman-Alpha Measurement Apparatus, or LLAMA, at DIII-D, the measurements are polluted by a large pick-up noise from the poloidal field coils' power supply. This noise prevents accurate measurements of transient plasma phenomena which require sub-millisecond resolution. The projects goal is to apply noise-reduction signal processing techniques based on neural networks to develop a time series prediction of the expected noise for the LLAMA diagnostic. The predicted noise can be subtracted from the raw diagnostic data to yield a signal with the desired resolution. By overcoming the large amplitude of noise due to the tokamak's magnetic coils, this analysis technique has great potential to study the physics of neutral deuterium at the plasmas edge. Specifically, sub-millisecond precision will allow closer study of deuterium ionization at the plasmas edge and how this impacts the behavior of edge localized modes (ELMs).   

Prediction of DIII-D pedestal density structure from externally controllable parameters

The plasma pedestal is an increase in pressure at the edge of the plasma in a tokamak's high confinement mode (H-mode). The pedestal improves plasma and fusion performance, so the relationship between controllable parameters and the pressure of the pedestal must be understood or predicted to optimize the tokamak as an energy source. The relationship between the pedestal density and externally controllable parameters is complex, and current models are limited in practicality and speed. A neural network (NN) is designed to predict the pedestal density from controllable variables such as plasma shape, heating method and power, and gas puff. The NN is trained on data from the DIII-D tokamak, which provides pedestal data for a wide variety of operational parameters over multiple run campaigns. This data was first pre-processed to discard edge localized modes (ELMs), building a database of pedestal parameters from pre-ELM conditions. Then the NN will be trained, tuned, and tested on data subsets from the database. Envisaged improvements of the NN will help to avoid overfitting by incorporating measurement uncertainties and applying pre-processing of features.   

Optimization of accelerator grid potentials in the DIII-D neutral beam source using numerical simulations

The Neutral Beam Injection system of the DIII-D tokamak consists of eight ion sources based on the US Common Long Pulse Source (CLPS), with a total output power of 20 MW. The ion source is a filament driven magnetic bucket design and the accelerator is a slot and rail tetrode design with vertical focusing achieved through tilted grids. Precursor ion beam divergence is one of the most important quantities that determine the quality of a beam in a neutral beam injector. Factors that affect the divergence of a neutral beam include ion and electron temperatures in the source, accelerator extraction geometry, stray electric and magnetic fields in the extraction region, grid potential distribution, and ion species composition. The accelerator is modeled electrostatically with finite element analysis using the beam transport code IGUN. The results are used to find the optimal settings that minimize divergence and maximize the brightness of the precursor ion beam in the DIII-D neutral beam source. Additionally the output from IGUN is used to establish the initial conditions for a particle tracking code used to predict the interaction of the beam and beamline components.   

Simulating Re-ionization of the Neutral Beams in the Drift Duct Region of the DIII-D Tokamak

The results of simulation and evaluation of the re-ionization of neutral beams in the DIII-D neutral beam injection (NBI) system are presented in this work. NBI provides plasma heating, non-inductive current drive, toroidal rotation, and fueling in the DIII-D tokamak. To generate neutral beams, first, deuterium ions are produced in the ion source then accelerated electrostatically to up to 85 keV, then neutralized through charge exchange collisions with deuterium gas. This neutralization process has efficiency of 61 \% at 85 keV. The residual ions are diverted by a magnet and the neutral beam proceeds to the entrance to the tokamak. However, due to background gas molecules downstream of the magnet, a portion of the beam is re-ionized, and is subsequently subject to the effect of magnetic field in the drift duct region, which curves the path of the re-ionized particles. Damage to the drift duct is expected due to the bombardment of these particles on the wall. To evaluate the damage and develop the operation limit, the path and the final position of the particles must be understood. Simulation of the re-ionized beams is carried out with a particle tracking code and maps of the particle paths are created.   

Automated Hohlraum Inspection

Components for Inertial fusion energy (IFE) and high-energy density physics (HED) targets need to be manufactured to micron-level precision and require inspection for accuracy of machined features as well as verification of surface conditions. General Atomics, a provider of precision components and subassemblies for the IFE and HED community, has started to use robotics and automation to assist in some of the most repetitive and time-consuming assembly and metrology steps. Robotic cells are being developed in an effort to reduce the man-hours required to perform the inspections necessary to ensure delivery of precision machined parts to specifications. Inertial fusion utilizes hohlraums to encapsulate hydrogen isotopes that is to undergo fusion. Hohlraums are prone to manufacturing defects such as grain boundaries, delamination, fractures, etc. This project investigates the use of robotics and visual software to create 2D and 3D mapping of hohlraum surfaces to sufficiently identify these defects. The success of this project would considerably reduce the number of hours and processes needed in order to adequately inspect hohlraums for imperfections.   

Image Processing for the Analysis and Optimization of 2PP Additive Manufactured Microstructures

General Atomics' Inertial Fusion Technologies (IFT) division employs a specific AM method known as two-photon-polymerization (2PP), among other techniques, in its target manufacturing activities. In 2PP, focused, ultrashort laser pulses are directed into a volume of photosensitive material or photoresist. Currently, the 2PP additive manufacturing process provides unique opportunities to create geometries that are unavailable to alternative fabrication techniques. However, further development of modeling and metrology techniques is needed for full utilization of the extensive capabilities that the 2PP print process allows. The goal of this work is to develop part measuring and model scaling techniques in order to achieve higher geometric tolerances as well as develop modeling and metrology techniques to fabricate and characterize parts with internal density gradients.   

Spatial-mode Analysis of Blast-wave-driven Hydrodynamic-instability Growth

Hydrodynamic instabilities in fluids are a heavily studied phenomenon, and in plasmas this topic remains a comparatively lighter research area due to its additional complexity. Hydrodynamic instabilities can cause energy cascades to smaller spatial scales via turbulent behavior. The transition to turbulent behavior in plasmas is important in many astrophysical objects, such as supernova remnants (SNRs). While SNRs provide beautiful illustrations of plasma turbulence, laser-driven shock-tube experiments enable a more practical approach due to their controlled manner. Experiments using the X-ray Free Electron Microscope (XFEL) at the Spring-8 Angstrom Compact free electron LAser (SACLA) in Japan allow for significant resolution enhancement of high energy density (HED) plasma images compared to previous radiographic techniques. These high resolution images allow for a more precise analysis of plasma motion down to an approximately 1um spatial scale. Spatial-mode analysis of experimental data will be shown and discussed as it relates to classical turbulence theory.   

Optimization of Polishing Techniques to Reduce Foil Thickness Variation.

Thin foils of Beryllium (Be) and Depleted Uranium (DU) are used in detection applications for plasma and radiation experiments. To improve foil performance, a new standard method for polishing foils was created to reduce thickness variation to $+$/-2 $\mu $m and produce repeatable foil thicknesses. Using non-hazardous metals as surrogates to Be and DU, the results from increased automation and varying polishing times {\&} pressures were compared against previous methods, whose thickness variation neared $+$/-10 $\mu $m. Metal foils of Aluminum (Al), Tin (Sb), Silver (Ag) and Gold (Au) with diameters from 3 mm to 12.5 mm were polished to target thicknesses of 5 $\mu $m, 10 $\mu $m, and 40 $\mu $m. Topographies of the foils were inspected to determine reproducibility of thickness variation within specification using Dual Confocal Microscope (DCM) imaging and White-light Interferometry Microscope (WIM) imaging.   

Utilizing Python and Computer Vision to Automate X-ray Image Acquisition of Capsules used at the National Ignition Facility

An X-ray microscope instrument, Xradia 510 Versa, is used to collect radiographs of High Density Carbon (HDC) capsules that are used for the indirect drive laser approach of Inertial Confinement Fusion (ICF). The radiographs of the capsules are analyzed to record quantitative measurements of properties to be used in ICF simulations and to ensure the quality of the capsules. Current analysis of the capsules requires manual centering in the X-ray microscope's (XRM's) field-of-view, which leads to increased operation time.~To enhance the efficiency and the quality of the data, Python Application Programming Interface (API) will be utilized to automate the centering, acquisition and analysis of the capsule radiographs.   

Particle Confinement Structures in Relaxed Taylor States

We study the orbits of particles confined in a relaxed Taylor state plasma. We seek to characterize the surfaces along which these particles move, which are significantly less studied than those in axisymmetric field configurations. We simulate motion for particles with many varying initial conditions of position and velocity, then characterize the surfaces upon which their orbits lie. We evaluate the magnetic field by solving the eigenvalue equation $\nabla \times \mathbf{B} = \lambda \mathbf{B}$ with the PSI-Tet program. We then simulate particle motion by using the Boris algorithm to solve the Lorentz force law equation of motion. The Boris code has been verified by simulating particle orbits in axisymmetric configurations with known paths (wire, dipole, spheromak).   

Studying the Relaxation and Merging of Taylor State Plasma with the Dedalus Computational Framework

Here we present magnetohydrodynamic (MHD) simulations of three different plasma configurations using the Dedalus project as the computational framework of choice. First, we demonstrate the validity of our computational approach by presenting simulations of Hartmann flow, a well-known problem in MHD with analytical solutions. Secondly we will show a simulation of the evolution of a Taylor state plasma in a cylindrical flux conserver. Lastly we will show the time evolution of a system with two merging Taylor states. Magnetic reconnection has been observed at the Swarthmore Spheromak Experiment (SSX) for the last configuration. The goal of our simulations is to present both the effectiveness and limitations of using MHD numerical simulations to study plasma configurations that involve magnetic reconnection.   

Particle Tracing and Confinement Analysis in the Harris Sheet

We are interested in plasma particle properties in the Harris Sheet geometry, which is a good approximation for the interface of merging two relaxed Taylor states in SSX. We then seek to characterize the confinement properties of the Harris Sheet. We first use the Boris Algorithm to solve for the motion of the particle due to static electric and magnetic fields, and verify the calculation using an axisymmetric spheromak configuration. After generating $\sim 10^5$ protons with $10^2$ velocities drawn from the Maxwellian distribution and applying Boris Algorithm, we find particles generally gain energy as they sample the sheet electric field, and a small fraction of the particles stay confined.   

Sub-Nanosecond Pulsed Power Source for an Atmospheric Pressure Plasma Jet

We present the design for a circuit that supplies pulsed power to an Atmospheric Pressure Plasma Jet. Sub-nanosecond pulsed power sources are becoming more accessible to smaller groups but have not been extensively applied to Atmospheric Pressure Plasma Jets. The short duration of the nanosecond pulse allows for a strongly non-equilibrium plasma, which in turn yields a higher efficiency energy transfer to electrons. In an effort to create a low-cost high repetition rate pulse generator we plan to combine Linear Transformer Driver (LTD) design with Drift Step Recovery Diodes (DSRDs), enabling picosecond rise times, 6 kV peak voltage, and khz repetition rates. LTD setups allow for a modular design, making current and voltage easily customizable, while the DSRDs compress the signal to give desired rise times and pulse duration.   

Optical Emission Spectroscopy Measurement of Atmospheric Pressure Plasma Species

We refurbish a Jarrell-Ash Model 82-000 monochromator for the purpose of observing parameters in an atmospheric pressure plasma jet. Adapting the device to our requirements presented a series of obstacles which were overcome by applying elements of plasma spectroscopy literature and simple electronics. We plan to use emission spectroscopy to identify species and plasma specifications, such as electron temperature and line ratios, to help optimize effectiveness of PFAS chemical breakdown. The motorized Czerny-Turner monochromator was fitted with a digitized data acquisition system, while the plasma was created using argon gas and three different commercially-available plasma lighter circuits. Our setup represents a straightforward, low-budget method with which to perform optical emission spectroscopy for diagnosing atmospheric pressure plasma.   

Organic Molecule Degradation by Atmospheric Pressure Plasma Jets Using Commercially Available Circuitry

This study aims to reveal the efficacy of commercially available arc lighter circuits used in atmospheric pressure plasma jets through the degradation of long-chain, organic dyes. Three affordable arc lighters were chosen off of an online public marketplace and were modified accordingly to fit the developed jet designs. Testing chambers were created to include an aerator fueled by argon gas in order to increase transport of the dye to the plasma-water interface. The results measured through a spectrophotometer reveal the efficiency of this degradation through varying time intervals. While we have drawn on previous research centered around the development and improvement of low-temperature APPJs to create our jet geometries, this study aims to be a precursor for the capability of easily produced APPJs to mineralize other organic contaminants in water, specifically PFAS chemicals. We have chosen a long-chain, organic dye because of its basic similarities in structure to compounds of interest.   

Study of formation of solar coronal jets by 3D MHD simulations

Spheromak stability concepts are used to explain solar physics events, namely solar coronal jets emerging from half-dome shaped magnetic structures on the surface of the sun. These structures are observed to remain stable for a long time, before suddenly erupting in a coronal jet, releasing their stored energy through the magnetic reconnection. 3D MHD simulations are used to study the stability properties of the spheromak line-tied to a conducting surface. A stability threshold is calculated for the tilt instability in asymmetrically line-tied conditions, depending on the elongation and the fraction of the line-tied flux of the spheromak. A high-resolution non-linear simulation demonstrates a current sheet formation between the spheromak tilted closed field lines and the ambient magnetic field at the top of the dome. This leads to reconnection and the plasma jet release from the dome. Numerical results support a model of coronal jet eruptions where the structure grows through flux emergence on the solar surface, tilts, reconnects, and erupts. The strong effect of line-tying in the simulations also suggests that in order for eruptive coronal jets to occur there must be magnetic reconnection on the bottom of the spheromak, between the magnetic dome and the solar surface.   

Exploring the Experimental Parameters of a Future Radiative Shock/Shear Experiment

The Shock/Shear experimental platform was created to study turbulence in the High-Energy-Density (HED) regime (Flippo et al. 2016). The design allows two counter propagating shock waves to cross at the center of an aluminum tracer strip. By isolating shear induced mixing caused by shock waves, the platform helps highlight and study turbulent effects. This study aims to extend the Shock/Shear platform to the radiative regime in order to increase understanding of how radiation affects turbulence. This research will use the Eulerian radiation-hydrodynamics code (CRASH) developed at the University of Michigan which includes block adaptive mesh refinement, multigroup diffusive radiation transport, and electron heat conduction. Characterization of this experiment under different computational parameters such as ablator thickness, foam density, and laser drive energy will increase understanding of the environment necessary to produce a radiative shock, and thus aid the design and development of a radiative Shock/Shear experiment at the National Ignition Facility.   

Harnessing Orbital Energy

Approximately 5000 manmade satellites are in orbit around the Earth as of today, about 1900 of which are still operational. The orbits of these satellites about the Earth can be altered, and even brought to ground via technology that extracts gravitational potential energy and stores that energy in a capacitor. Analysis of the orbits of coupled masses in the presence of a large central body~suggest~the potential to recover energy from the orbits of these bodies. By absorbing energy from the epicyclic orbits of these bodies, the entire system gradually falls toward the central body, while the lost energy is gained in the form of electricity, allowing the operator(s) of the satellite to recover some of the initial investment of putting the satellite into orbit.~Creating~a numerical simulation for this process that accounts for the dynamical interactions in the system allows for many configurations to be tested in search for the ideal energy extraction method, and facilitates an improved understanding of the basic principles involved.~   

Terahertz Radiation Produced in Mid-IR Laser-driven Electron Acceleration

Strong long wavelength infrared (LWIR) and terahertz (THz) radiation sources have become an increasing topic of interest for applications such as nonlinear optics and spectroscopy. Laser wakefield acceleration provides a new source of high field LWIR/THz, since the overlap of the drive laser and refractive index profile of the excited plasma wave leads to a frequency downshift of the high energy drive pulse. Mid-IR lasers start with a lower frequency and can therefore achieve more efficient conversion to long wavelengths into the THz range. This poster will present Particle-In-Cell (PIC) simulations that provide insight into THz yield by analyzing the output frequency spectrum of the electric field in mid-IR laser driven electron acceleration, as well as preliminary experimental results.   

Production of synthetic phase contrast images for comparison with CRASH radiograph output

We plan to use the BELLA Hundred TW Thompson laser at the Lawrence Berkeley National Laboratory to perform experiments evaluating shock wave propagation in high-energy-density (HED) plasma research. The laser produces betatron oscillations of a laser-wakefield accelerated electron beam to act as an X-ray source for the experiments. The University of Michigan's Center for Radiative Shock Hydrodynamics (CRASH) software is used to simulate shock propagation through a 120-micron-radius water target at the point of impact of the 1-2 J laser pulse. The output from these CRASH simulations is incorporated into an algorithm developed for Phase Contrast Imaging to obtain synthetic images of the shock front at a distance of 490 cm. These images may be compared to the synthetic radiographs of similar phenomena produced by CRASH in earlier experiments in order to capture finer details of the dynamic evolution of shock waves propagating in HED plasma environments.   

A Non-Invasive Method to Measure Magnetic Field from Emission Diagnostics in WIRX

The Wheaton Impulsive Reconnection Experiment (WIRX) is a laboratory experiment for the study of magnetized plasma arcades. We present a method for deriving the global magnetic field from light emission diagnostics. An ICCD camera provides 2D images with good spatial resolution (1 mm) but at only two points in time. Meanwhile, a custom 1D photodiode camera provides emission data with coarse spatial resolution (10 mm) but good temporal resolution (1 $\mu s$). Combining the two diagnostics allows us to characterize the emission as a function of both space and time. We postulate that the current density $J$ and emissivity $E$ are related by $J\propto E^{\alpha}$. The direction of the current density vector at each point is set by constraining the current to flow along a volume-filling set of circular arcs that intersect the two electrodes. Since the diagnostics integrate the emission along the line of sight, an inversion algorithm is applied to extract the emissivity on each circular arc. The magnetic field is calculated from the current density using the Biot-Savart Law. The validity of this emission-based model and the best choice of exponent $\alpha$ are examined by comparing the magnetic field predicted by the model with actual magnetic probe data at select locations.   

A Simple XUV Spectrometer for Z-Pinch Experiments

A XUV spectrometer was designed and built in order to measure the spectrum of light emitted from plasma created in Z-pinch experiments. The spectrometer uses a simple and compact design with a conical opening towards the plasma and rectangular enclosure to hold the grating and reduce outside light. It is based on a 1200 grooves/mm grating with a radius of curvature of 5650 mm and a blazing angle of 3 degrees. The grating is placed 237 mm away from the plasma and 235 mm from the detector surface, making this instrument suitable for small vacuum chambers. The detector used is an imaging plate. \\ The design of the instrument and initial measurements will be presented. The purpose of this device is to study the light emitted in the wavelength range 20-150 mm from Z-pinch plasmas on the recently commissioned CESZAR driver at UCSD, capable of up to 0.9 MA in 150 ns. The measured spectra will provide information on the density and temperature of the plasma, particularly for gas puff Z-pinches with low-atomic-number elements such as hydrogen, oxygen and neon.   

Neutron Diagnostic System Redesign

System overhaul and redesign for an outdated data acquisition system for Experimental Physics Industrial and Control System software (EPICS). This redesign will be using Modern National instruments hardware as a replacement for the older outdated CAMAC 404 timing module. EPICS is used to observe the data acquired from the National Spherical Torus eXperiment - Upgrade (NSTX-U). The NSTX-U uses 4 Neutron Flux Monitors (Thermo Fisher Scientific TR-10-5) units which measure neutron count. This data is then transferred using Labview DAQmx and NI CompactDAQ (cDAQ). Models NI-9402, NI-9220, NI-9425, and NI 9184 will be used in various ways to read and store data. The data is read and stored by Labview to the MDS Plus tree which then takes the raw data and uses python functions to calculate the neutron rate. Neutrons are a by-product of plasma which is used to determine the effectiveness of the shot, and the higher number of neurons recorded from a shot, the more effective the NSTX-U was at creating plasma.   

Ion Particle Transport in DIII-D H-Mode Plasmas

This paper investigates ion particle transport as a function of turbulence using a perturbative deuterium gas puff modulation in DIII-D H-mode plasmas. The carbon6+ impurity response to the modulation is analyzed using the charge exchange recombination (CER) spectroscopy diagnostic system. From this data, we can extract perturbative transport coefficients for the carbon ions and compare them to prior results looking at electron transport in Ion Temperature Gradient (ITG) versus Trapped Electron Mode (TEM) turbulence regimes (S. Mordijck et al 2015 Nucl. Fusion 55 113025). Theoretical models predict turbulent transport should be larger for ions than electrons in the ITG regime, while the opposite is true in the TEM regime (C. Bourdelle et al 2018 Nucl. Fusion 58 076028). The calculated transport coefficients from carbon6+ measurements partially agree with this model; coefficients are smaller for ions than electrons in the TEM discharge, however the ion coefficients were smaller for the ITG discharge as well. STRAHL simulations will be performed to evaluate how closely the perturbative analysis method approaches steady state expectations for transport, and NEO simulations will be performed to compare the neoclassical contributions to turbulent contributions in carbon6+ transport.   

Performance of Components of the DIII-D ECCD Top Launch System

A new extension of the ECH/ECCD system on DIII-D consists of the addition of a section of transmission line and a launcher in the top launch (TL) configuration. Experiments at DIII-D have showed that increased current drive efficiency was achieved with this vertical launch geometry [1]. Power at 110 GHz or 117.5 GHz produced by single-frequency gyrotrons has been injected into the plasma using the TL system. A waveguide switch installed in the transmission line allows the rf power to be directed either to a steerable low field side launcher or to the fixed top launcher. As part of the TL installation, a new DC break was designed and tested to provide electrical isolation for the TL branch of the transmission line. The measured rf leakage at the DC break gap in the waveguide line was compared with theoretical predictions. An absorbing cylinder was fitted around the DC break to reduce the rf leakage from the insulating gap. A description of the installation, top launch transmission line performance, and COMSOL simulation results is presented. [1] Chen, X., \textit{et al.,} 61st Annual Meeting of the APS Division of Plasma Physics, Volume 64, Number 11 (2019)   

Energy Dissipation at the onset of polarity switching in the PK-4 experiment

The behavior micron-sized particles (dust) in a plasma system is of great interest and, because of the low charge to mass ratio of these particles, the dynamic time scales of the dust grains are long and easily accessible from an experimental perspective. However, the high mass leads to sedimentation effects in ground-based experiments. To reduce sedimentation effects, it is necessary to perform experiments in a free-fall (“microgravity”) environment, such as in the experiment facility “Plasma-Kristall-4” (“PK-4”) on parabolic flights, where the effects of gravity are reduced. In the PK-4 facility, particles are injected into a dc glow discharge plasma and flow along an axial electric field. Upon the application of a periodic oscillation of the electric field (polarity switching), a sudden change in the bulk motion of the dust and a redistribution of kinetic energy of the dust particles can be observed. This poster will present the results of a study comparing this distribution of energy on the macroscopic level and the microscopic level in ground-based and parabolic flight experiments at the onset of polarity switching.   

Numerical studies of energy transport in dusty plasma monolayer

Here we present a many-body simulation of a dusty plasma monolayer in the moderate coupling (liquid) regime, where coplanar and out-of-plane laser perturbations result in the excitation of instabilities. The goal of this study is to determine how the energy dissipation throughout the structure is linked to the presence of anomalous dust particle diffusion. Different diffusion regimes are simulated using a numerical thermal bath which provides `kicks' of various strength and direction to each dust grain. Modified Gaussian and L\'{e}vy distributions are employed as probability distribution functions for these kicks, which allows us to model both local and nonlocal interactions with gas and plasma particles. For each set of parameters, the observed relation between global dynamics and dust diffusion is verified against the predictions of the Fractional Laplacian Spectral code, which calculates the probability for energy transport as a function of random defects and nonlocal interactions in the medium. These phenomena are also compared to video data from experiments performed in the CASPER lab at Baylor University.   

Analysis of Dust Acoustic / Dust Density Waves in Magnetized Plasmas

The dust acoustic / dust density wave is a well-documented phenomenon that is a commonly occurring feature in many complex/dusty plasmas. Recent studies have shown that in the presence of a magnetic field, the observed waves can become modified, often times losing their semblances of uniformity due to changes in the background plasma or the overall stability of the dusty plasma cloud. This presentation reports on recent experimental studies of dust acoustic/dust density waves performed using the Auburn University Magnetized Dusty Plasma Experiment (MDPX). Changes in the spatial characteristics of the dust waves are investigated under varying magnetic field strengths up to 2 Tesla. Additionally, measurements of the evolution of the wave dispersion relation as a function of the magnetic field will be discussed.   

Reversal of microparticle motion at the onset of polarity switching in ground-based PK-4 experiments

Due to microparticles' large mass relative to ions and electrons, gravity plays an important role in complex/dusty plasmas.~ In order to isolate the role of interparticle forces from the dominating role of gravity, several complex plasma experiments have thus been performed under microgravity conditions on the International Space Station over the last two decades.~ The current experiment, PK-4, produces flowing complex plasma in a dc discharge plasma.~ A periodic oscillation of this electric field referred to as ``polarity switching'' is used to trap the particles in the plasma.~ This presentation discusses the differences between microgravity and ground experiments, the observations of spatial variances in the paths of dust grains at the onset of polarity switching and provides insights into the forces that are acting on the particles.   

Smart dusty plasma liquids on the International Space Station

This study examines dusty plasma liquids using data from the PK-4 facility on board the ISS. Similar to smart materials which change their properties in response to externally applied stimuli, microgravity dusty plasmas are highly sensitive to changes in the discharge conditions. Here we report the observation of dust acoustic waves at lower pressure and the formation of long dust chains (electrorheology) at higher pressures in a DC neon discharge with polarity switched field. The driving mechanism of the waves is studied by comparison to analytically derived dispersion relations corresponding to different scenarios, including strong coupling effects and fast ion drift speeds. Strong coupling of the dust grains is expected to deviate from the usual Yukawa form due to the formation of dust-ion-wakefield proxy dipoles. The possibility of large ion drift speeds in the bulk plasma can result from high frequency ionization waves due to the polarity switching of the DC field. These phenomena are further compared to data from on-ground experiments performed in the PK-4 analogue facility at Baylor University.   

Using Reduced Order Modeling to Understand the Physics of Injection in Laser Wakefield Acceleration

Laser Wakefield Acceleration (LWFA) is a process by which plasmas are excited by a laser leading to the acceleration of electrons. The process is highly nonlinear, leading to difficulties in developing an accurate theoretical model for~\textit{a priori}~prediction. Recent experiments at the Rutherford Appleton Laboratory's (RAL) Central Laser Facility (CLF) in the United Kingdom using the 20 TW, 5Hz repetition rate Astra-Gemini laser has produced new results in LWFA research, that can allow unprecedented exploration of the parameter-space of laser and target conditions. Experimental measurements can inform scaling laws for the creation of more robust prediction and control models. With the new data constraining previous scaling laws, models can be extended into new ranges. These data allow the construction of reduced order models that can make predictions without the need for full scale simulation.~ ~~   

Hydroscaling and Alpha Heating in High Adiabat Layered Implosions

High adiabat implosions in inertial confinement fusion (ICF) are designed to be more robust to detrimental plasma and hohlraum physics than their lower adiabat counterparts. They drive a strong first shock into the ablator as well as into the DT fuel, reducing the sensitivity of the integrated system to uncertainties in shock-timing, preheat, and instabilities in the ablator and at the fuel-ablator interface. The higher adiabat enables a short pulse which simplifies hohlraum physics by limiting the extent of difficult to model dynamics such as gold bubble expansion, plasma filling of the hohlraum, and stagnation and interpenetration of the wall, capsule and gas interfaces and subsequent laser propagation through those regions. We report on DT layered implosions used to test the level of alpha heating driven in these high adiabat implosions and the hydroscaling of these implosions between two scales, scaled by x1.125. We present hydroscalings of the hotspot parameters which are shown to scale differently with the scale factor than no alpha heating analytic theory predicts. *This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

Broadband Microwave Emission from Laser Plasmas Generated in Water Droplets

A high-power ultrashort laser pulse focused in air generates a plasma that radiates broadband electromagnetic waves. The transient current source responsible for the radiation remains an open area of study. A laser plasma is generated in the presence of water droplets produced by a humidifier to investigate the physics governing the amplification of radiation at microwave frequencies in comparison to radiation quantities created by laser plasmas generated in air. A microwave horn is used to measure the radial pattern of radio frequencies (RF) from 2 to 13.6 GHz produced by the laser plasma. The extinction coefficient of the water vapor-based fog is determined by the transmittance of continuous wave lasers through the interaction region. Our research demonstrates a relationship between the density of the water droplets and the laser plasma radiation mechanism.   

Geant4 Simulations for MRS Neutron Spectrometers on OMEGA and the NIF

In Inertial Confinement Fusion (ICF), a tiny pellet of fuel filled with a deuterium-tritium (D-T) mix is bombarded with high energy lasers to induce implosion. Neutrons generated in the D-T reactions are then measured to determine implosion conditions, such as yield, ion temperature, areal density, and asymmetries. The measurement of these neutrons is accomplished via a deuterated polyethylene (CD2) foil in the blast chamber, which has a well-studied conversion rate from incident neutrons to deuterons. A magnetic recoil spectrometer (MRS) measures these deuterons, from which we can infer the initial neutron spectrum using an instrument response function (IRF) calculated with Geant4. The primary focus of this project was to update the IRF simulations for the MRS's located at both OMEGA and the NIF. Previous measurements from the MRS at the NIF revealed a feature in the low energy tail of the measured deuteron spectrum. This could be a signature of inherent asymmetries in the implosion, but more rigorous simulations are needed to rule out an instrumental effect. To address this, we have worked on implementing the Ta-W blast shield and the Ta foil holder present at the NIF into the relevant Geant4 simulations to verify their impact on the response function.   

Simulations advancing 2D Laser Driven Shock Experiments

Laser driven cylindrically or spherically focused shocks in fluids are a topic of great interest due to their wide variety of applications, including studies of materials in extreme conditions, sonoluminescence, synthesis of new materials, and controlled fusion. A novel platform for generating 2D focusing shocks uses a laser to shock a liquid or soft solid layer between two glass plates [T. Pezeril et al., PRL 2011]. This platform was demonstrated to generate pressure in excess of 30 GPa at shock convergence. In this work, we investigate methods to increase the maximum pressure at the shock focus through simulations of the experiments. The effect of changing the geometry of the glass was investigated. Utilizing a lensed glass shape increased the pressure at the center of the shock wave by over 30 times. The thickness of the liquid layer and the radius of the shocked region were also varied to find optimal conditions for maximum pressure. These simulations are guiding ongoing experiments that will study high pressure materials and fusion sources in a 2D geometry. This work was supported in part by the U.S. DOE and the MIT/NNSA CoE.   

Optimization of Supersonic Gas Jet Density Profiles for Laser-Plasma Target Production

Gas jet profiles are essential in fields involving laser-plasma interaction, especially in the creation of high gradient, compact laser-plasma accelerators. In many such experiments, a gas jet is the medium that the intense laser propagates into in order to create the plasma. For laser-plasma interactions, in many cases, a uniform density profile with sharp transitions at the profile edges, often labeled as a ``flat-top'' profile, is desired at the laser-plasma interaction line. Supersonic gas jets, which are used to create such density profiles, are produced from gas flowing through converging-diverging (CD) nozzles. In this presentation, preliminary density profile results towards meeting accelerator needs, extracted from computational fluid dynamics (CFD) simulations of various CD nozzle geometries, are presented. The nozzle geometry parameters including the throat size, the exit half-angle, and the curvature of the section between the throat and nozzle exit, were investigated for further tailoring of the desired density profiles.   

CT Analysis of Double Shell Targets

Double shell capsules are an alternative to the single shell method of inertial confinement (ICF). Double shell experiments are being performed on the National Ignition Facility (NIF) to measure symmetry of Al outer shells driven by x-ray radiation. It is important to observe the symmetry of the capsule, as we want the implosion to be as round as possible to maximize the yield. The initial conditions of the capsule may have a significant impact on the evolution of the capsule. After the shells are assembled we take CT scans of them. We have been using MATLAB routines to analyze the target CT data. Specifically, we have utilized spherical harmonics to fit inner and outer surface measurements, in order to determine how asymmetries in the capsule present initially. This data will be used in conjunction with simulations to explore the impact the asymmetries have on the experiment.   

Reel-To-Reel Contact Critical Current Measurement System for Superconductor Tape Testing

The Plasma Science and Fusion Center at MIT is developing high-field superconducting magnets for the compact tokamak SPARC, for which the rapid qualification at large volumes of high temperature superconductors (HTS) is vital. To determine the critical current, or the maximum current the HTS is able to conduct without resistance, either small sections of HTS can be removed from larger reels and contact-tested with an input current, or entire reels can be run through an external field, inducing an internal current. The former process is time-consuming and ignores inconsistencies throughout the reel, but the latter does not directly pass current into the HTS. My project unites these methods with a novel reel-to-reel contact critical current testing system to rapidly conduct essential large-scale measurements. Three primary designs were explored for a testing rig. The first system utilizes a screw linear actuator to clamp each subsection of HTS onto two copper current leads, between which voltage is measured. The second system instead harnesses a pneumatic air cylinder and piston to apply a clamping force. The third system, in contrast, tensions the tape across the current leads. All three systems are designed to function in temperatures from 55-80K and ambient fields of 0-3T.   

Impact of Coil Design on Alpha Particle Confinement

Enhanced confinement of fusion alpha particles is an essential requirement of a fusion reactor. Previous studies of reactor scale stellarator configurations indicate that alpha particle losses would be higher than acceptable in the analyzed configurations [F.~Najmabadi,~et al.,~Fusion Sci. Technol. \textbf{54},~655~(2008)]. More recent work indicates quasi-helically symmetric configurations can achieve enhanced alpha confinement [A. Bader, et al., J. Plasma Physics \textbf{85}, 905850508 (2019)]. The demonstration that reactor-scale coil configurations can be developed with sufficient alpha confinement is necessary to provide a pathway for stellarators as a fusion reactor concept. In this work, the FOCUS code [C. Zhu, et al., Plasma Phys. Contr. Fusion \textbf{60}, 065008 (2018)] will be used to develop coil configurations for a four-field period, reactor-scale quasi-helically symmetric stellarator. The SIMPLE code [C. Albert, et al., J. Plasma Physics~\textbf{86}, 815860201 (2020)] will then be used to analyze the confinement of fusion alpha particles in these configurations. Alpha particle confinement will be examined for the fixed-boundary equilibrium and for the free-boundary equilibrium. The dependence of alpha confinement on the number of modular field coils will be examined.   

Impact of varying neutral densities on turbulence and transport in LAPD

Theoretical predictions and simulations of turbulence in plasmas are often derived for fully ionized plasmas and the impact of neutrals is neglected. However, most plasmas are not fully ionized and the effects of neutrals might have an impact on the turbulence characteristics and the resulting transport. LAPD provides a unique environment in which we can increase the ionization of the plasma by increasing the voltage on the cathode and we can increase the neutral density by increasing the pressure in the vessel. We compare the profiles and turbulence characteristics in a set of experiments at 3 levels of neutral gas pressure and various levels of ionization. The density and radial velocity perturbations peak at the same radius and this location shifts inward with increasing gas pressure. As the neutral density and thus collisions increase and the voltage on the cathode decreases we observe that the parallel flow slows down and even changes sign. Using this dataset we will investigate how and whether changes in ionization affect turbulence.   

Studying the Validity of 1D Models in Analyzing Transport in LAPD

Understanding how drift-wave driven plasma turbulence affects heat transport in magnetic confinement devices has improved considerably using high performance models and improved turbulence diagnostics. In fully ionized plasmas, most drift-wave driven turbulence should lead to the flattening of the electron density gradient, but it is unclear in partially ionized plasmas how the direct influence of a neutral source affects the profile as well as the underlying turbulence. Experiments on LAPD spanning a range of neutral pressures and cathode voltages allow us to explore how turbulence and turbulent radial particle transport is affected. In small linear devices, the electron density and temperature are determined by 1D transport along the magnetic field lines. To assess how much the electron density profile diverges in LAPD from these calculations, we will compare our experimental profiles to this 1D theory and add measured radial losses using a flux probe.   

Laser-pulse propagation and interaction with low-temperature electron-hole plasmas in nanowire arrays

We present results from finite-difference time-domain simulations solving for the electric and magnetic fields of ultrashort laser pulses. These laser fields propagate through arrays of semiconductor quantum wires, in which they excite and interact with a low-temperature electron-hole plasma. These low-temperature plasmas are driven by ultrashort laser pulses, electron-hole scattering between all energy bands, and resistive forces for momentum relaxation due to Coulomb scattering and collisions with the lattice. The simulations allow us to study the correlation between the localized plasma response of quantum wires and the spatial-temporal features and phases of the scattered laser pulses. Presented results showing the influence of these effects on the quantum wire polarization and current density may inform studies of optical high-harmonic generation, while the results on plasma mobility can further our understanding of ultrafast electronics.   

Magnetic Field Measurement Corrections for Double Probe and B-dot Design~~

Pulsed power and turbulence experiments conducted in the Big Red Ball at the Wisconsin Plasma Physics Laboratory present unique measurement challenges. When the double probe tips collect current, that current induces a magnetic field. With large fluctuations in plasma density, the fluctuation currents interfere with the B-dot coils. To properly adjust for this interference, the mutual inductance between the probe tips and the B-dot coils is calculated theoretically and measured experimentally. The induced EMF due to the additional magnetic field for a given current can then be subtracted from the total signal collected. The theoretical reasoning, methods, and results will be presented.~~   

Verification and Study of Soliton Behavior of Trivelpiece-Gould Modes of a Non-Neutral Plasma

The density and z-component of the velocity of a non-neutral plasma were obtained from a 2-D PIC simulation of Trivelpiece-Gould oscillations. Animations of the calculated density and velocity time evolution indicate strong evidence for the existence of two distinct solitons in the plasma. Solitons are described by the counteracting principles of dispersion and nonlinear wave effects which create a single propagating wave envelope. Solitons have several specific defining properties such as stability over large distances and a height dependent propagation velocity. We are currently verifying the existence of soliton-like behavior in the plasma. We will then study the behavior of the solitons during interactions with the boundaries and with each other. These results will be presented.   

Magnetic moment (non-)conservation for a pair plasma trap

Confining pair plasma is key to experimental investigations of its properties. In order to optimize traps, it is essential to understand single-particle orbits by which electrons/positrons escape, or in the case of a levitated dipole, pass the floating coil center. This project will explore the influence of ``$\mu$-breaking'' on those orbits. ``$\mu$-breaking'' refers to the non-conservation of the magnetic moment $\mu\equiv m v_\perp^2/(2B)$ when its adiabatic invariance does not fully hold, and some particles that would be expected to magnetically mirror do not and vice versa. At any position in a trap, a ``loss cone'' can be defined containing all perpendicular-parallel velocity combinations for which particles stream along the field instead of mirroring. When the magnetic field relative to the particle motion is adiabatically invariant, the loss cone is determined by the pitch angle at which the particle is launched. Without adiabatic invariance, a shift of the loss cone can be observed: an additional dependence on the initial phase angle appears. The magnitude of this effect in a dipole trap will be investigated using particle trajectory simulations at conditions relevant for pair plasma. The results will help with the considerations to design effective future plasma traps.   

Measurements and Simulations of Bow Shocks in Laboratory Plasma Jets

Recent Experiments on the University of New Mexico's HelCat (Helicon-Cathode) Dual Source Plasma Device have focused on the study of bow shocks formed by the collision of Argon plasma jets launched from a coaxial plasma gun with a cylindrical Pyrex rod placed into the vacuum chamber. The HelCat chamber allows for shock formation in a variety of regimes including gas backfills, background plasmas and background magnetic fields for a magnetized shock case. Several diagnostics have been deployed including high speed imaging, spectroscopy, and Langmuir Probes to measure the shock structure in various regimes. Recent high-speed images, using filters to separate the Ar-I and Ar-II lines, have yielded measurements of the spatial and temporal distribution of ions and neutrals across the shock for each regime. These measurements have also shown evidence of instabilities downstream of the shock front when the jet is launched into an argon gas backfill. These data at each regime will be presented along with initial 2-D hydrodynamic simulations using the University of Chicago's FLASH$^{\mathrm{\thinspace }}$code.   

The On-Axis Magnetic Well for Arbitrary Stellarator Geometries

A simplified analytical form of the on-axis magnetic well in Mercier's criterion for pressure-driven instabilities is derived for stellarators in both vacuum and MHD equilibrium. An analytical result allows for a better understanding and more efficient search of the parameter space for stellarator design. The derivation uses a direct coordinate approach [1] that expresses the toroidal flux surface function in terms of radial distance near the magnetic axis. The flux is expanded in powers of the inverse aspect ratio to fourth order, and the resulting integral is solved over the poloidal angle. This reduces to a one-dimensional integral with respect to the axis arc length parameter that is a function of only the axis shape, as well as the elliptical and triangularity components of the stellarator's cross sections. Finally, this integral is numerically calculated using the Stellarator Equilibrium Near-Axis Code (SENAC) [1], and validated against the magnetic wells for several current stellarator configurations including Wendelstein 7-X. [1] R. Jorge, W. Sengupta, M. Landreman, Journal of Plasma Physics \textbf{86} 1, 905860106 (2020)   

Hybrid Stellarator Divertor

Recently an efficient method for simulation of stellarator divertor was developed by Boozer and Punjabi [A. H. Boozer and A. Punjabi, Phys. Plasmas \textbf{25}, 092505 (2018)]. This method was used to simulate nonresonant stellarator divertor [A. Punjabi and A. H. Boozer, Phys. Plasmas \textbf{27}, 012503 (2020)]. The three parameters denoted by $\varepsilon _{\mathrm{0}}$, $\varepsilon_{t}$, and $\varepsilon_{x}$ in the Hamiltonian for the trajectories of magnetic field lines control the shape of the outermost confining surface in nonresonant stellarator divertor. These parameters are called shape parameters. They control the elongation, triangularity, and the sharp edges on the outermost confining surface, respectively. In the 2020 Punjabi and Boozer paper on simulation of nonresonant stellarator divertor in PoP, the shape parameters had the values $\varepsilon_{\mathrm{0}}=\varepsilon_{t}=$0.5, and $\varepsilon_{x}=$-0.31. It is found that in for $\varepsilon _{\mathrm{0}}=\varepsilon_{t}=$0.5, when $\varepsilon_{x}$ is varied in the range -0.25 to -0.1, a new kind of divertor is formed. We call this hybrid divertor. Hybrid divertor has features of both the nonresonant divertor as well as the resonant divertor. The size of the resonant islands varies as $\varepsilon_{x}$ is varied in this range; islands become smaller as $\varepsilon_{x}\to $-1/4. In this paper, we give the rotational transforms and sizes of the islands for different values of the parameter $\varepsilon_{x}$ in this range. Work supported by the DOE OFES grants DE-SC0020107 and DE-FG02-07ER54937 to Hampton University, and DE-FG02-03ER54696 to Columbia University. Research used resources of the NERSC, supported by the Office of Science, U.S. DOE, under Contract No. DE-AC02-05CH11231.   

Quasi-Helically Symmetric Stellarator Optimization with Poloidal Field Coils

Quasi-symmetry provides a pathway for improved neoclassical confinement in stellarators. Recent work has indicated that combining optimization of quasi-helical symmetry and minimization of the radial drift velocity can lead to configurations with improved neoclassical confinement and improved confinement of energetic particles [A. Bader, et al., J. Plasma Physics \textbf{85}, 905850508 (2019)]. An effort is underway to develop coil configurations that both optimize the physics and meet engineering constraints. The FOCUS code was developed to flexibly explore the configuration space in coil optimization [C. Zhu, et al., Plasma Phys. Contr. Fusion \textbf{60}, 065008 (2018)]. In this work, the FOCUS code will be used to develop and analyze coil configurations for five-field period configurations with quasi-symmetry and enhanced energetic particle confinement. We will examine modular coil configurations with and without a set of poloidal field coils and analyze the effects of different coil configurations on Ideal MHD stability, neoclassical transport and energetic particle confinement. This work will include optimization of a coil set for a midscale stellarator experiment and the development of a coil set for a reactor scale device.   

Energetic Particle Confinement in Optimized Stellarators

Testing and analysis of the confinement of fusion alpha particles and energetic ions in optimized stellarator configurations is presented. The SIMPLE code has been developed to speed up the calculation of fusion alpha particle losses in stellarator configurations [A C. Albert, S. Kasilov, and W. Kernbichler, J. Plasma Physics~\textbf{86}, 815860201 (2020)]. The primary focus of this work will be using the SIMPLE code to analyze alpha confinement in quasi-helically symmetric stellarator configurations. First, the results of the SIMPLE code will be tested against an alpha confinement code developed by Nemov, et al. [V.V. Nemov, \textit{et al}., Phys. Plasmas \textbf{21}, 062501 (2014)]. Alpha confinement will be calculated for an optimized five-field period reactor configuration as a function of flux surface. This will be coupled with estimates of alpha production to calculate the total alpha confinement. Finally, the confinement of energetic ions in a midscale stellarator experiment is analyzed. This will include analysis of the impact of optimizing with both a modular coil set and a poloidal field coil set.   

Neutral Density Profiles in the HSX Stellarator

Neutral density distribution in HSX plasma is calculated using the 3D Monte-Carlo neutral particle code, DEGAS. Calculated neutral density is then scaled with poloidal and toroidal H-alpha measurements to obtain a radial profile of averaged neutral densities in the plasma. Calculations are done for the quasi-helically symmetric and symmetry degraded configurations of HSX, for various plasma and wall conditions. The scaling factor obtained from present and previous experiments (with core electron temperature \textasciitilde 1.5keV and plasma density \textasciitilde 3x10$^{\mathrm{18}}$/m$^{\mathrm{3}})$ is used to scale neutral density profiles from simulations of the HSX upgrade, where higher plasma density (\textasciitilde 2x10$^{\mathrm{19}}$/m$^{\mathrm{3}})$ is expected. Initial results show a significant reduction (about a factor of 4) in the neutral hydrogen density for the HSX upgrade parameters.   

Creating Testing Procedures and Determining Location of 70 GHz Gyrotron in HSX Upgrade

A 70 GHz gyrotron is currently being installed at the Helically Symmetric eXperiment (HSX) in Madison. This new electron cyclotron resonance heating (ECRH) source, donated by IPP-Greifswald, will provide up to 300 kW of power and allows plasma operation with densities up to 3x10$^{\mathrm{18}}$ m$^{\mathrm{-3}}$. This will permit access to a completely new operational range for HSX. However, before installation, the gyrotron must be tested, including tests of the gyrotron’s 2.7T field  superconducting magnet and the necessary liquid helium cooled cryostat. Moreover, efforts are needed to evaluate the overall safety hazards, cost effectiveness, and overall lab improvements and changes needed to operate the gyrotron. Multiple possibilities were evaluated to decide on the final location of the new gyrotron and routing of the transmission line.   

Computing the Shape Gradient of Coil Complexity with Respect to the Plasma Boundary.

Coil complexity is a critical factor in stellarator design. Complex coils are difficult and expensive to engineer and tight coil-coil spacing makes maintenance difficult. The traditional two-step approach to stellarator design can produce plasma shapes which require excessively complex coils. Coil complexity metrics can be computed with REGCOIL (Landreman 2017), which optimizes coil shapes using a linear least squares method based on a current potential approximation. We extend REGCOIL to compute analytic derivatives of these metrics with respect to parameters describing the plasma boundary using an adjoint method. It is then only necessary to solve the linear system used in REGCOIL twice, rather that for every surface parameter. This provides a great computational advantage over finite-difference differentiation. Shape gradients of the REGCOIL metrics are computed from these derivatives; they tell us how normal perturbations of the plasma surface alter these metrics. We also compute these shape gradients while fixing coil complexity. This reveals how to alter the plasma surface to be better reproduced by coils with a desired complexity. We present a new representation of the plasma surface which uses a single Fourier series to describe the radial distance from an axis. This representation is advantageous over the VMEC (Hirshman {\&} Whitson 1983) representation, as it only requires a series for a single variable rather than for two separate variables. It also uses a uniquely defined poloidal angle, which eliminates a null space in the optimization over the plasma surface.   

Numerical study of neutral beam injection heating in the HSX stellarator

Neutral beam injection (NBI) is a key component to achieve high ion temperatures in small scale fusion experiments since this allows direct ion heating. The installation of a new NBI source is therefore foreseen at the Helically Symmetric Experiment (HSX) in UW Madison. Here we present a numerical study using the FIDASIM code and a newly developed orbit following code to determine suitable NBI injection geometries. FIDASIM is used to calculate fast-ion birth positions for various NBI geometries and fast ions are then followed through the 3D magnetic field structure of HSX using the orbit following code. This allows determining the radial distribution of the injected fast ions such that analytic formulas can be used to compute electron and ion heating profiles. Through the use of these computational tools, suitable NBI geometries will be presented that minimize fast-ion losses and promote efficient heating.   

Corrections to Energy Conservation When Including Density Variation in 2-Fluid Models

Conservation of energy in 2-fluid turbulence models is important for capturing the correct balance of sources and sinks, and improves numerical stability. Most 2-fluid drift-reduced models rely on the Boussinesq approximation for the plasma density, effectively neglecting any perpendicular density gradients in the divergence of polarization current. This approximation is violated especially at the plasma edge in magnetically confinement devices where very large density gradients can be observed. In this poster we will study the impact of introducing a perpendicular dependence of the plasma density on energy conservation, and energy transfer channels . We will assess the impact of these new terms and include them in the Hermes 2-fluid model, a 3D hot-ion turbulence code based on BOUT$++$.   

A Multi-Staged Biorthogonal Decomposition to Identify Spatial Mode Structures

Mode feedback control and disruption prediction of tokamaks rely on predefined spatial mode structures to rapidly process data. Here, a new algorithm is introduced for finding experimentally defined modes as an extension of biorthogonal decomposition (BD). The first stage uses a BD to find spatial modes in multiple individual data sets. These spatial modes are used to form a mode-shot matrix, where each column is a spatial mode found in the first stage. The second stage uses a BD on the mode-shot matrix to find the orthonormal vectors which best represent these spatial modes. This process finds the best representation for the spatial mode structures of many shots. This algorithm is tested using data from magnetic sensors and extreme ultraviolet diode arrays on the HBT-EP tokamak to study disruptions. All considered shots are selected using criteria that they disrupt in similar ways; they are well centered in the tokamak, have a high loop voltage, are relatively long-lived, and do not have minor disruptions. The results confirm the current model of disruptions using a data-driven method, which made no assumptions about the underlying physics. Other tokamaks would benefit from using this algorithm to define mode structures used for feedback control and disruption prediction.   

An Ultrahigh Energy-Range Thomson Parabola Spectrometer to Investigate Laser-Driven Proton-Ion Acceleration

Here, we report the development of a Thomson Parabola Spectrometer capable of simultaneously resolving protons and low-Z ions across an energy range of 1-200 MeV. This new design utilizes a dual image plate and dual electrode-magnet pairs to achieve a 200 MeV energy range and a maximum relative error of $\Delta$E/E $<$ 2$\%$. The first image plate collects the low energy protons and ions, but is offset from the neutral beam line to allow the higher energy particle to move on to a second electrode-magnet pair and image plate. This scheme decreases the size of the spectrometer's laboratory footprint. This new Thomson Parabola Spectrometer will be used to detect and resolve ion energy spectras generated from Texas Petawatt laser-driven ion acceleration experiments at high and ultra-high intensity regimes.   

Exploring physics-based models for a multi-machine L-H power threshold database

The threshold power required to access H-mode is modeled using statistical techniques informed by past experimental results. Using the international multi-machine H-mode threshold power database (Martin et al., 2008), an empirical expression for the density at the minimum threshold power is determined and compared with a predictive expression from Ryter et al. (2014). This minimum is then used to separate the high- and low- density branches in a piecewise expression for the threshold power in terms of density, magnetic field, and plasma surface area. A single-machine database from Alcator C-Mod, with substantially more data points, has been assembled, allowing investigation of the effects of additional parameters, such as those defining divertor configuration and plasma shape. Updated values for the expected threshold power and density associated with its minimization are presented for SPARC.   

Turbulent density, Reynolds stress and vorticity measurements in the COMPASS tokamak edge plasma

A novel probe array that combines Langmuir probes with floating ball-pen probes is described. The array measures the radial profiles of density fluctuations, poloidal and parallel Reynolds stresses, and turbulent vorticity, together with time-averaged density, electron temperature and plasma potential. The array was used to make these measurements in the COMPASS tokamak device in Ohmically heated limiter and diverted discharges across a range of plasma densities and a limited scan of plasma currents. The linkage and correlation between density and vorticity fluctuations are compared with the theoretical predictions of potential vorticity mixing across the edge plasma ExB shear layer. In addition, the variation of the poloidal Reynolds stress and the ExB shear layer with plasma conditions is examined. Preliminary observations show a systematic degradation of the edge shear layer during a density ramp. Research supported by U.S. National Science Foundation Grant 1928843.   

Electron Density Measurement using a Partially Covered Hairpin Resonator in an Inductively Coupled Plasma

Hairpin probes are used to determine electron density via measuring the shift of the resonant frequency of the probe structure when immersed in a plasma. We present new developments in hairpin probe hardware and theory that have enabled measurements in high electron density plasma, up to approximately $10^{12}$ cm$^{-3}$, corresponding to a plasma frequency of about 9 GHz. Hardware developments include use of both quarter-wavelength and three-quarter-wavelength partially covered hairpin probes in transmission mode, together with an easily reproducible implementation of the associated microwave electronics using commercial off-the-shelf components. The three-quarter-wavelength structure is operated at its 2nd harmonic with the purpose of measuring higher electron densities. New theory developments for interpreting the probe measurements include the use of a transmission-line model to find an accurate relationship between the resonant frequency of the probe and the electron density, including effects of partially covering the probes with epoxy. Measurements are taken in an inductively coupled plasma (ICP) sustained in Argon at pressures below 50 mTorr. Results are compared with Langmuir probe and interferometry measurements.   

Simulated Validation of the ITER XRCS Core using Ray-Tracing Algorithm

We present a simulated experimental validation of the ITER X-Ray Crystal Spectroscopy Core (XRCS Core), a novel spectrometer design that incorporates graphite pre-reflectors to distance the detectors away from the reactor plasma's neutron and thermal fluxes. This validation was accomplished through a virtual model of the spectrometer, consisting of a Highly Oriented Pyrolytic Graphite (HOPG) pre-reflector, a germanium spherical crystal, and a Pilatus detector, as well as a model of an x-ray emitting ITER plasma with realistic geometry, temperature profile, and emissivity profile. The model took into account Bragg reflections with finite rocking curves, the mosaic crystallite structure of pyrolytic graphite, and the x-ray emissions of Xe44$+$ and Xe51$+$. We used a new python-based ray-tracing algorithm of our own design, dubbed XICSRT, to simulate the x-ray optics of the spectrometer. XICSRT can model the distribution of x-ray impact points on the detector and the individual optical elements, outputting expected photon counts in real units. As part of the validation, we investigated how the spectrometer would tolerate optics misalignment. We intend to release XICSRT as a tool to aid in the development of x-ray spectrometers for future nuclear fusion reactors.   

Investigation of Variable Radii Crystal Spectrometers for HEDP Applications Through X-Ray Raytracing

X-Ray raytracing results will be presented for a family of new crystal geometries that have been proposed for spectrometers to study High Energy Density Physics (HEDP) experiments. These geometries, which feature in common variable radii of curvature across the crystal, are expected to dramatically improve achievable wavelength resolution and total throughput. The most important feature of these geometries is that they allow crystals of arbitrary size to be utilized with little or no degradation in focusing and wavelength resolution. To evaluate the performance of these designs, a new x-ray raytracing code, XICSRT, has been developed. Raytracing and performance metrics will be presented for three crystal geometries: the multi-cone, modified toroid, and sinusoidal spiral. The Ray-Tracing results will be compared to analytical calculations, and in the case of the multi-cone crystal also to experimental measurements made with visible light. Finally a study of optic size and surface quality will be presented.   

Coordinate System Invariant Space Charge Limited Current for General Initial Velocity

Originally derived exactly for planar diodes [1], more recent analysis of space-charge-limited current (SCLC) has applied variational calculus (VC) to derive a coordinate system invariant solution for any general rectilinear geometry and obtain exact, closed form solutions for planar, spherical, and cylindrical geometries [2]. We extend this VC approach to first derive a coordinate system invariant solution for any generalized emission velocity and then derive closed form solutions for planar, cylindrical, and spherical diodes. The resulting functions show that the position of the virtual cathode approaches the anode with cylindrical and spherical geometries doing so more slowly than planar. Extensions of this VC approach to incorporate relativistic behavior and, ultimately, both relativistic behavior and initial electron velocity will be discussed. [1] P. Zhang, A. Valfells, L. K. Ang, J. W. Luginsland, and Y. Y. Lau, Appl. Phys. Rev. 4, 011304 (2017). [2] A. M. Darr, A. M. Loveless, and A. L. Garner, Appl. Phys. Lett. 114, 014103 (2019). [3] A. D. Greenwood, J. F. Hammond, P. Zhang, and Y. Y. Lau, Phys. Plasmas 23, 072101 (2016).   

Collisionless Phase Space Equilibria

According to Boltzmann's H-theorem, all collisional processes eventually relax to a Maxwellian particle distribution function, as that particular form maximizes the entropy. However, despite the lack of collisions, collisionless plasmas are often observed to be in a quasi-Maxwellian state. This work will determine the universality of equilibrium states in collisionless plasmas by examining the relaxation of the two-stream and Weibel instabilities, through the continuum Vlasov-Maxwell model implemented in the Gkeyll simulation framework. The timescale and final state of the collisionless equilibration will be analyzed.   

Optimal laser focusing for positron production in laser-electron scattering

Laser-electron beam collisions that aim to generate electron-positron pairs require laser intensities $I\sim10^{21}$ W/cm$^2$, which can be obtained by focusing a 1-PW optical laser to a spot smaller than 10 $\mu m$. Spatial synchronization is a challenge, because of the Poynting instability that can be a concern both for the interacting electron beam (if laser-generated) and the scattering laser. One strategy to overcome this problem is to use an electron beam coming from an accelerator (e.g. the planned E-320 experiment at FACET-II). However, this configuration brings other challenges - the electron beam is long and wide and there is a trade-off between using a short focal length to obtain the highest conceivable laser intensity, and having a wider interaction volume where more seed electrons participate in the interaction. This work extends analytical scaling laws for pair production in laser-electron beam scattering, previously derived for the case of a plane wave and a short electron beam. We investigate positron yield in a focused laser beam, considering an electron beam with an arbitrary distribution function. We take the spatial and temporal synchronization of the interaction into account and prescribe the optimization strategies for near-future experiments.   

Implementing the Advanced Annular Couette Centrifuge Method to Optimize Liquid Centrifugation

We focus on research and examination of various types of centrifuges used in industries around the world to determine the effectiveness of implementing the Advanced Annular Couette Centrifuge (AACC) technology developed by researchers at PPPL. The AACC method was developed to enhance the separation efficiency of liquid centrifuges by modifying a Taylor-Couette (T-C) device. T-C flow is the flow of a fluid between two co-axial differentially rotating cylinders. The PPPL method limits secondary flows and turbulence by having end-cap rings spinning at intermediate speeds so that the inner cylinder spinning at higher speeds can create higher effective gravity. The modified T-C device can promote mixing or separation by keeping the fluid in one container and adjusting the two rings' speed. An engineering study examines the current applications of traditional centrifuges in agricultural and chemical engineering processes, energy applications, and the separation of sub-micron-sized particles in liquids. The research conducted allows the conclusion that AACC technology can reduce process times and increase cost-efficiency. This new method can be applied to optimize the current technological properties and various applications of liquid centrifuges used in industries around the world.   

Are Two Laser Pulses (With Half Energy) Better Than One for Ion Acceleration?

Ultra intense lasers are a promising source of ions for various industrial and biomedical applications. An interesting paper is Ferri et. al. 2019 which argues from PIC Simulations that using two beams of half energy is more effective at accelerating ions than one beam at full energy. We explore this possibility with a wider range of laser intensities to better understand the physics that is at work in this phenomenon. In this way, we extend the parameter space that Ferri et. al. 2019 investigated. We explore this phenomenon with parameters which resemble the Scarlet Laser Facility at OSU and the Extreme Light Laser System at Wright-Patterson Air Force Base.   

Particle-in-Cell Simulation Code Comparison for 2D Laser Propagation

There are a variety of Particle-in-Cell (PIC) codes used today to simulate laser propagation. In this paper, we perform a verification test of three popular codes, namely EPOCH, LSP, and WARPX, testing against the analytic formula for 2D Gaussian laser propagation in a test with no particles. We compare the simulation with the expected behavior to investigate the following effects: peak intensity, beam focusing, spot size, and numerical effects of phase speed on the laser. These simulations were run on the same node on The Ohio State University's Unity Computer cluster for a valid performance comparison of CPU time and memory usage.