Session JP11: Poster Session IV: Education and Outreach; Undergraduate or High School Research; Plasma technology, Fusion reactor Nuclear and Materials Science; Propulsion; Materials Interfaces (2:00pm-5:00pm)

Vocational Educational Institutions as a Recruitment Tool for PPPL’s High School and Community College Internship Programs

The Princeton Plasma Physics Lab, runs yearlong internship programs for high school, community college, and undergraduate students. PPPL’s longitudinal study of our undergraduate interns has found that our pipeline is “sticky” and students that begin their career in the STEM fields tend to stay. With the technical workforce aging and retiring at rate faster than they can be replaced, recruiting from a highly qualified diverse applicant pool is essential. Recently, we began collaborating with vocational high schools in NJ due to a rapid expansion of that population to meet the needs of the state’s manufacturing sector. By recruiting these students into our internship programs, we hope to recreate the “sticky” nature of what we see with undergraduates into our technical workforce. Preliminary results from our recruitment efforts will be presented.   

Physics of two-ion-species plasmas in a linear plasma device

Understanding of turbulence and transport in multi-ion-species plasmas is important for establishing predictive capability for burning tokamak plasmas with comparable densities of D and T. Fundamental modifications to drift-wave instabilities and resulting turbulence are expected from theoretical studies, including new instabilities driven by dissimilar ion density gradients. In addition, information on the ratio of ion densities is essential in the context of tokamak fueling, although a method for reliably measuring this ratio is currently lacking. Recently, experiments have been performed on a small linear plasma device at UCLA in which mixed Hydrogen-Helium plasmas were created and the relative concentration was varied systematically. Properties of pressure-gradient driven instabilities were documented, and initial results will be presented. Additionally, schemes for directly exciting fluctuations at the ion-ion hybrid resonance were explored, and their viability as a diagnostic for local measurements of the ion density ratio will be discussed as well. 
  

Production of Deuterated Polymer Thin Films for Ion-Beam Fusion Experiments

The SUNY Geneseo Accelerator Laboratory consists of a 1.7 MV Tandem Pelletron Accelerator from the National Electrostatic Corporation (NEC). The Alphatross RF Alkali Charge-Exchange ion source is capable of producing a 30 nano-amp deuterium beam with a maximum energy of 3.4 MeV. In this experiment, a 3 MeV deuteron beam is incident on an 40Ar gas cell. The subsequent 40Ar(d,p) reaction produces 41Ar which beta decays to the first excited state of 41K 99.1% of the time. The excited 41K promptly decays and emits a 1.293 MeV gamma ray. The emitted gamma is detected with a high purity germanium gamma ray detector positioned alongside the Argon gas cell. The radioactive 41Ar gas will be used as an electron calibration source for a phoswitch detector in ICF and HEDP nuclear reaction studies.   

Langmuir Probe Measurements in Low Frequency Inductively Coupled RF Plasmas

Experiments at the University of Maryland, Baltimore County’s Dusty Plasma Laboratory are being carried out with Langmuir probes used to measure electron temperature and radial density in inductively coupled plasmas with antennae frequencies ranging between 22 - 30 kHz, used to sustain dusty plasmas. The aim is to measure unmagnetized plasma density and temperature gradients, which we conjecture are amplified and responsible for ion momentum transfer to dust in the ∇P × B direction when a magnetic field is turned on. Our setup consists of a cylindrical glass chamber with external RF antennae, a glass shelf with ring electrodes to levitate and hold dust, and a Langmuir probe that is moveable in the axial and radial directions. Signals from the probe are routed into a digitizer with sample capability of up to 1 megasamples per second. The probe can be configured as a single or triple Langmuir probe that can measure argon plasma temperature and density changes as fast as 50 kHz, using 400 kHz probe sweeps.   

Using Capon's method to measure the Turbulent Frequency-Wavenumber Power Spectrum in a Laboratory Magnetosphere

Capon's ``maximum likelihood method'' computes a high-resolution frequency-wavenumber power spectrum from simultaneous multi-point measurements\footnote{Capon, \textit{Proc. IEEE},  \textbf{57}, 1408 (1969).}. We apply this analysis to an array of floating potential probes in a laboratory magnetosphere. %exhibiting turbulence dominated by long wavelength modes that have chaotic amplitudes and phases\footnote  {Grierson, \textit{et al.}, \textit{Phys Plasmas}, \textbf{16}, 055902 (2009).}. A levitated dipole magnet confines plasma and enables the study of interchange and entropy mode turbulence in near steady-state conditions\footnote{Garnier, \textit{et al.}, \textit{Phys Plasmas}, \textbf{24}, 012506  (2017).}. We report a high-resolution measurement of the frequency-wavenumber power spectrum using Capon's analysis. An error analysis to numerically evaluate optimal parameters such as probe number and distribution through synthetic test data is included, and we present a geometrically intuitive derivation with a view to wider application of this technique for other plasma experiments.

Maxwellianization of Hot Electrons Observed in a Pure Electron Plasma

The non-neutral electron plasma trap at UC Berkeley is used to study lepton plasma dynamics and optimize routines used at CERN in producing trappable antihydrogen. Some of the parameters explored at Berkeley include electrostatic shaping, confinement time, and magnetic field profile. This experiment also features high sensitivity, at the level of single plasma electron detection. Maxwellianization is the tendency for the distribution function to evolve towards the maximum entropy state f(v)~exp(-mv^2/2kT). In the context of this experiment, Maxwellian (or thermal) electrons form this characteristic distribution when the plasma is held long enough in its confinement well. Under some conditions a fraction of the population may be excited to a higher energy. When such a plasma is released from confinement into a parallel energy analyzer, we may observe the "Non-Maxwellian" electrons. These electrons relax collisionally with the "bulk", or the part of the plasma which is thermalized at a lower temperature. By observing the distribution at various points during this relaxation, we can test the rules for test particle relaxation in a cold stationary background outlined in the plasma formulary.    

Simulations of Hydrogen Interactions with Boron Nitride Nanostructure Synthesis and Growth

Boron nitride (BN) nanostructures have long been regarded as one of the most promising inorganic nanosystems due to their stiffness, thermal conductivity, and stability. These structures have been successfully and efficiently synthesized using plasma arc discharge. Although the structure of these nanosystems have long been studied, the process through which they form is still not completely understood. Using the latest version of the DFTB+ (Density Functional based Tight Binding) code, we were able to study the interaction between hydrogen and BN nanotubules and the effects of hydrogen on BN nanostructure synthesis and growth.   

Development of a Triple Langmuir Probe System

Electric probes are among the most basic and widely used plasma diagnostics.  The most commonly used electric probes is a single Langmuir probe.  However, Langmuir probes require relative stable plasmas, complicated circuits to measure the I-V characteristics and provided limited temporal resolution of the plasma parameters.  To address these limitations, we have built a triple Langmuir probe system.  This poster presents the design of a triple Langmuir probe system and the corresponding data acquisition system.  Preliminary results will also be presented.   

Development of a Time-Resolved Stereoscopic PIV System

Over the past twenty years, a variety of particle image velocimetry (PIV) techniques have been used to characterize the particle transport and thermal state of dusty plasma systems. While the majority of these techniques required the use of a dedicated PIV system, recent advances in imaging technology have led to the development of a time-resolved two-dimensional (planar) version of this diagnostic technique which allows this diagnostic technique to be applied without the need for a dedicated PIV system. This poster will present recent efforts in developing a time-resolved stereoscopic PIV system that can measure the full three-dimensional transport in a dusty plasma, including upgrades that have been made to the Schiempflug adapter, illumination system and timing system, to improve the performance of the system.  Preliminary results will be presented.

Clarifying the Role of Aspect Ratio in 0D Tokamak Design Studies

Reducing tokamak aspect ratio leads to improved stability limits (beta, elongation) and potentially larger bootstrap fraction, offering the promise of achieving 100% non-inductive reactor designs in a relatively compact configuration. However, these potential benefits will be constrained by the reduction in achievable on-axis magnetic field, the need for inner wall shielding and a central solenoid, and potentially larger mechanical stresses. As such, this study uses 0D design methods to clarify and illustrate the interplay between these constraints when optimizing aspect ratio for various tokamak design targets, such as a pilot plant, DEMO, or FNSF. We further consider the sensitivity of the optimization to variation in assumed technologies and design choices (HTS vs. LTS, NBI heating vs. RF heating, non-inductive vs. inductive current drive, etc.) and confinement and stability assumptions. Simple cost metrics are also considered to aid illustration of potential optimal designs.   

Exploration of shape parameters for nonresonant stellarator divertor

Recently an optimized Hamiltonian for field line trajectories in nonresonant nonaxisymmetric stellarator divertor is calculated and a symplectic method for efficient and accurate integration of field line trajectories is developed [A. H. Boozer and A. Punjabi, submitted for publication]. It is also found that a nonresonant island divertor for stellarator can be designed by appropriate choice of the parameters in the field line Hamiltonian. We call these shape parameters. The shape parameters control the magnetic topology. We are exploring the shape parameter space to determine which choices of shape parameters will give correct rotational transform for the stellarators. We use the symplectic map for the field line trajectories to calculate the rotational transform. Results of this exploration will be presented.
  

Accelerating CR-39 Track Detector Processing by Utilizing UV

As demonstrated in prior work, utilizing CR-39 as a Solid State Nuclear Track Detector provides a useful source of data in high-energy density experiments such as inertial confinement fusion. Chemically etching CR-39 in 6M NaOH at 80°C, after irradiation, forms micron-scale pits at the location of each nuclear track. Prior work has shown that CR-39 exposed to UV light before etching increases bulk and track etch rates. Effects of UV exposure are more apparent when temperatures elevate up to 85°C. Increased temperatures are a side-effect of broadband UV sources, due to infrared emissions. By analyzing spectroscopic data, we find the most effective UV wavelengths are shorter than 350nm, consistent with theoretical models of absorption in amorphous solids. We evaluate the potential of UV exposure to shorten the etching process. Results show how aging CR-39 reduces the effect of UV, independent of changes to the underlying etch rate. Noise can develop from UV exposure. Both noise and increased etch rates are characterized for different exposure times and temperatures to mitigate noise and maximizing particle sensitivity.   

Development of a kJ-class dense plasma focus

A kJ-class dense plasma focus (DPF) is in development at Virginia Tech for the purposes of studying the generation of axial electric fields in the focus and associated neutron and x-ray production. Design and construction of the experiment was done as part of a capstone senior design project. A custom fiber optic control system is used to operate the DPF from a remote computer. A Rogowski coil was constructed and calibrated to measure the current from the 1.3 uF, 40 kV RLC network. Preliminary results indicating a peak current of 92 kA when operating at 35 kV are presented along with photographs of initial helium plasmas and estimates of neutron production in deuterium plasmas. Additional diagnostics are needed to characterize neutron and x-ray production, and the device is intended to be operated using either helium or deuterium.   

ECH Ray Propagation and Deposition in Under- and Over-Dense Plasmas on DIII-D

The electron cyclotron heating (ECH) deposition profile and location has been examined by modulating the power from a single gyrotron on the DIII-D tokamak. These measurements are compared to theoretical calculations from the TORAY-GA ray tracing code to check the deposition width in low density cases and refraction of EC waves out of the plasma in high density cases. In under-dense plasmas, the modulations in electron temperature measured by electron cyclotron emission (ECE) are fitted to the linearized energy conservation equation to find the best-fit transport coefficients and ECH source profile. If an anomalously wide ECH source profile is found, the reasons for this will be investigated. As the density is increased and the right-hand cutoff is approached, the ECE analysis will look for the expected shift in deposition location and a sudden drop in absorbed ECH power. In a novel experiment, an IR camera was used to image the carbon tiles in the outer wall to observe any heat deposition from unabsorbed EC waves after their first pass through the plasma. Modulation analysis of the IR camera data will compare the measured heat flux on the outer wall to TORAY-GA predictions.   

Enhancement of ion thermal anisotropy in helicon plasmas

Temperature anisotropies provide a source of free energy for instability growth in space and laboratory plasmas. The growth of electromagnetic ion temperature anisotropy driven instabilities is dependent on the anisotropy parameter T_⟘/T_|| and the local plasma β. These instabilities reduce the ion temperature anisotropy by inducing velocity-space diffusion, resulting in a limit on the ion temperature anisotropy that scales with plasma β. Here we report measurements of the ion temperature anisotropy in a helicon plasma source where the RF antenna responsible for plasma creation is amplitude modulated at the ion cyclotron frequency. The ion temperatures parallel and perpendicular to the magnetic field are measured with laser induced fluorescence in an argon plasma and the ion thermal anisotropy is characterized as a function of modulation frequency, modulation amplitude, and helicon source plasma parameters.
  

High Field Side Scrape-off-Layer Reflectometry

Microwave reflectometry is commonly used in plasmas to measure density profiles and fluctuations. Using reflectometry to measure scrape-off-layer density profiles and fluctuations on the high field side of a tokamak is desired to address important boundary physics issues as well as possible heating and current drive applications. This application of reflectometry, however, has not been done before due to the challenging spatial constraints, large frequency bandwidth ratio, and low frequencies necessary. While the frequency sources and transmission line are challenging, the most challenging aspect is the design of the antenna in a highly spatially constrained and high magnetic field region. Broadband antennas with coaxial inputs are available commercially but will need to be modified for the nuclear fusion environment. This poster outlines preliminary results for microwave modeling and bench-top testing of a commercial 5-27 GHz antenna for this reflectometry application. Plans for possible modifications of the antenna to measure scrape-off-layer densities on the high field side of a tokamak are discussed.    

Electromagnetic instabilities driven by ion thermal anisotropy in helicon plasmas

Temperature anisotropies provide a source of free energy for instability growth in space and laboratory plasmas. The growth of electromagnetic ion temperature anisotropy driven instabilities is dependent on the anisotropy parameter T_⟘/T_|| and the local plasma β. These instabilities reduce the ion temperature anisotropy by inducing velocity-space diffusion, resulting in a limit on the ion temperature anisotropy that scales with plasma β. Here we report measurements of the spectra and parameter dependence of magnetic fluctuations in a helicon plasma source where the RF antenna responsible for plasma creation is amplitude modulated at the ion cyclotron frequency to increase the perpendicular ion temperature and thereby increase the ion temperature anisotropy. Magnetic fluctuation spectra up to 500 kHz, parallel and perpendicular to the magnetic field, are reported as a function of ion temperature anisotropy. Ion velocity distribution functions in the same region, measured by laser induced fluorescence, provide direct measurements of the ion temperatures parallel and perpendicular to the magnetic field.   

Using Measurements of the Trivelpiece-Gould Mode Dispersion Relation to Determine Temperature in Toroidal Pure-Electron Plasma

In the Lawrence Non-Neutral Torus II (LNT II), electron plasma is confined using a purely toroidal magnetic field (R₀ = 18 cm, B < 1 kG) for confinement times exceeding 1 second. The LNT II can be operated as a partial torus in which plasma is confined in C-shaped toroidal sectors or as a fully toroidal, closed trap. In the past, a plasma expansion method has been used to measure the temperatures of the electron plasma, which are approximately 2 eV. We attempt to measure the temperature a second way. We use the Trivelpiece-Gould wave dispersion relation. These waves propagate toroidally and are launched and detected from opposite ends of the plasma. Toroidally propagating bounded plasma waves exist below the plasma frequency due to the boundary conditions of the torus. The Trivelpiece-Gould mode has higher frequencies than the diocotron modes of plasma waves. Using this method, for a plasma frequency of 20 MHz, we expect to be able to measure differences in dispersion relation at different temperatures of frequencies in the range of 10 MHz.   

Is Bohm's criterion satisfied near a negatively biased grid in a single ion species Krypton Plasma?

We performed the first test of Bohm's Criterion for sheath formation in a single species Kr⁺ ion plasma in the vicinity of a negatively biased grid (-100V, with respect to chamber ground),  in a hot filament, DC discharge, multi-dipole device (length = 64 cm, diameter = 32 cm) at the University of San Diego,  in which a Kr discharge was operated with a neutral pressure of 0.1mTorr,  n_e~ 3 x 10⁹ cm^-3 and T_e  ~ 3.5eV. We found that the first moment of the ion velocity distribution function was 0.8 √(T_e/M)  ± 6% at the sheath edge, as determined by diode laser based laser-induced fluorescence (LIF) together with emissive probe measurements analyzed by the inflection point measurement in the limit of zero emission. This result raises questions about assumptions. It appears that counter ion flow through the grid complicates the results.  We also present results of ion flow to a solid, negatively biased boundary plate to test Bohm's Criterion in a low temperature weakly collisional pure Kr plasma, free of counter streaming ion flow for comparison.   

Development of a Motorized Telescope for the Pulse-Burst Laser System

The Pulse-Burst Laser System (PBLS) has been installed and commissioned on NSTX-U to measure the temperature and density of particles in the plasma. This system is part of the NSTX-U Thomson scattering system and will be operated at 30 Hz continuously, and 1 kHz and 10 kHz in burst mode. To control the beam divergence, a telescope is used after the oscillator in the laser cavity. Here, we develop and model a motorized telescope to enable the control of the beam divergence between plasma discharges. The design constraints are minimal footprint (12 cm by 8 cm), reduction of friction on the collar where the lens translates, and the accuracy of the positioning of lens (sub-millimeter over 25-30 mm travel) by the motorized system. In this presentation we will discuss the design parameters, the methodology used in developing the motorized system and the results from testing the prototype.   

External micro-controller adjustment of the radial position of in-vessel walls in HBT-EP

Tokamak’s internal walls act as passively-stabilizing conductors that suppress plasma instabilities through induced eddy currents. HBT-EP’s in-vessel walls are divided toroidally into 10 sections and poloidally into 2 sections; for a total of 20 upper and lower wall segments. Each wall segment has an approximate 4 cm radial range of motion and each segment’s radial position can be changed with an external DC motor without breaking vacuum. Using an Arduino Feedback and Control System (AFCS), HBT-EP operators can configure the position of all wall segments within minutes between HBT-EP operational periods. The AFCS’s user-interface allows HBT-EP operators to view the real time positions of each wall segment and set their positions manually or into pre-defined configurations. We use the AFCS to study the effects of variable wall geometry boundary conditions on plasma modes and scrape-off layer currents.   

Neural-Network accelerated TGLF model for ITER scenario modeling

Preparation for ITER operation relies on our ability to efficiently predict the plasma confinement with high physics fidelity. High-fidelity turbulent transport models such as TGLF remain one of the major bottlenecks in this process. To accelerate prediction of the turbulent transport, the neural network (NN) approach of TGLF (TGLF-NN) described in [Meneghini NF 2017] has been extended for ITER. A large database of ITER TGLF simulations was assembled for both the commissioning phase (H only) and the nuclear phase (D+T and He ash). The original NN implementation has been re-written to leverage state-of-the-art machine learning libraries (e.g. Tensorflow). Tuning of the NN model hyper-parameters (topology and training parameters) was carried out on GPU enabled clusters, both using Gaussian process based optimization, and random sampling of the configuration space. Dimensional reduction with auto-encoders, and training on a latent-space data-set was also investigated. Results of coupled core-pedestal ITER simulations leveraging the latest EPED1-NN and TGLF-NN models will be presented.
  

Dispersion calculation for lower hybrid waves in the current sheet of reconnection with guide field

Lower hybrid waves have been observed in the current sheet during reconnection with guide field in both the Magnetic Reconnection Experiment (MRX) and Magnetospheric Multiscale (MMS) mission. The observed waves produce density fluctuations correlated with electric field, generating anomalous resistivity. To understand the excitation mechanism and propagation of the lower hybrid waves, the dispersion relation and growth rate will be calculated by a linear calculation [Ji et al. 2005]. In this local calculation, both parallel and perpendicular electron flows exist in the ion rest frame. Based on the measured value of the phase velocity, which is comparable to the ion thermal speed, it is important to include ion thermal effects in the calculation. We use the J-pole expansion of the plasma dispersion function to capture ion thermal effects in the dispersion relation. Results with ion thermal effects will be compared with those from the cold ion limit.   

Electrostatic potential map in a Penning-Malmberg trap using numerical simulations

In order to produce antihydrogen, antiprotons and positrons must be mixed in a Penning-Malmberg trap. Before mixing, antiproton and positron plasmas are first confined at cryogenic temperatures. The positrons, for instance, originate in a trap consisting of a stack of eight hollow cylindrical electrodes. A potential is produced by applying a voltage to each electrode. Detailed modeling of the potential in each trap is crucial for tailoring the plasmas. We have found the potential well by solving the Laplace equation. We begin by developing a mechanical model of the trap in a computer-aided design (CAD) program, then inserting the model into a multiphysics modeling software, retrieving and converting the solution to the suitable form for the program that controls the experiment.   

Continuum kinetic simulations of stochastic heating in low collisionality plasmas

The effect of stochastic heating in plasma is important in understanding the heating of the solar corona and the solar wind. With the coming launch of the Parker Solar Probe Plus that will gather unprecedented data of the solar wind and corona, further theoretical investigation of stochastic heating processes is desired to compare with experimental data. In this study we use the continuum code Gkeyll to solve the Vlasov-Maxwell equations for the evolution of a plasma distribution function. Using a continuum solver avoids numerical issues such as particle noise characteristic of traditional particle-in-cell methods. We investigate the evolution of a distribution function in different electromagnetic (EM) field configurations such as those observed in solar wind conditions. In addition to test particle simulations, we will compare to distributions evolved self-consistently, such that the particle motion feeds back to the seed EM field, and including particle collision effects. Energy transfer from fields to particles will be studied to determine the extent to which stochastic motion of particles leads to particle heating.   

An investigation of polarisation properties of Thomson scattering in relativistic plasma

Thomson scattering is employed to measure the temperature of plasma. Traditional methodology involves measurement of the spectrum of scattered radiation from a pulsed laser in a small scattering volume. New theoretical investigations involving the use of Mueller matrices suggest an approach of determination of the temperature using the polarisation properties of the scattered radiation in the region where relativistic effects are significant. These methods are investigated here numerically, with the intention of proposing an application at the W7-X stellarator.   

Optimization and application of neural network models for accelerated predictive modelling of NSTX-U

A critical component of advancing modern fusion devices to cost-effective energy sources will be model-based control and scenario development. Specifically, a hierarchy of models of varying complexity, speed, and precision is needed to meet all the needs of the design process. Neural networks have proven capable of producing fast and reliable models of fusion plasmas, and their levels of complexity can be easily tuned. However, each network has many hyperparameters, the tuning of which is time-consuming and does not always result in the best possible model. An algorithm has been developed to systematically generate and optimize neural networks of varying complexities by tuning hyperparameters for the purposes of accelerated predictive models of NSTX-U. It uses a genetic algorithm to rapidly arrive at optimal model parameters that could previously be found only through exhaustive grid searches. This algorithm was used to tune and train neural networks of neutral beam deposition based on the NUBEAM code. This new approach yields more optimal neural networks and enables tuning the trade-off of model fidelity and computation time based on the requirements of any application.   

Effects of Magnetic Surface Shaping Parameters on Ion Temperature Gradient Turbulence in Tokamaks

Transport driven by ion temperature gradient (ITG) turbulence is believed to be responsible for most observed energy and particle losses in tokamaks. The impact of magnetic surface shaping in tokamaks on ITGs and ITG-driven turbulence are studied using local, axisymmetric equilibria. This is accomplished by computing equilibria with varying magnetic shear, pressure gradient, triangularity, and elongation. Linear ITG stability analysis is performed using the gyrokinetic code GENE. The effects of shaping on nonlinear turbulent transport physics is performed using a recently developed theory for turbulent saturation that relies on nonlinear energy transfer from unstable to damped eigenmodes [C.C. Hegna, P.W. Terry, and B.J. Faber, Phys. Plasmas 25, 022511 (2018)].   

Neutron Time-of-Flight Measurements Initiated with 25-keV Deuterons

Neutron time-of-flight (nTOF) measurements have been completed using d-d fusion reactions produced by 25 keV deuterons.  Deuterons produced using the SUNY Geneseo 30 kV Peabody Scientific Duoplasmatron ion source are focussed onto thick deuterated polyethylene films producing fusion products.  In the laboratory coordinate system, 2.566 MeV neutrons detected by a BC-412 plastic scintillator at 39° correspond to 0.699 MeV ³He ions detected by a surface barrier detector (SBD) placed at 135°. At these low energies, the charged particle spectra have little to no background. Timing signals from the SBD preamplifier initiate a start signal and the associated timing signal from the photomultiplier tube initiates the stop signal.  By placing the scintillator at various distances from the target, the time-of-flight spectra can be used to determine an experimental value for the neutron energy, and this provides confirmation of the method. Using this nTOF technique, the neutron response for different scintillation detectors can be determined.   

UHV testing of 3d printed components for antimatter studies

In support of the ALPHA and ALPHA-g antihydrogen experiments, we have tested 3-d printed components for UHV compatibility.  Using a vacuum chamber capable of reaching UHV pressures at Marquette university, we have tested a variety of commercially available 3d printed metals and ceramics.  Results of these studies are directly applicable to ongoing efforts to construct ALPHA-g, and are also highly relevant to a variety of plasma physics experiments.       

Inertial-Electrostatic Confinement (IEC) Fusion Neutron Sources for Highly Enriched Uranium Detection

Detecting shielded highly enriched uranium (HEU) and other special nuclear materials (SNM) remains elusive.  The research reported here combines multi-dimensional neutron sources, computational modeling of neutron transport, and principal component analysis of detector signals. The project's objective is to provide a high sensitivity technique for detecting HEU and SNM.  An overview of the project will be presented, and progress will be described in: (1) developing optimized, adaptive-geometry, inertial-electrostatic confinement neutron source configurations with neutron pulses distributed in space and/or phased in time, (2) performing related Monte Carlo calculations of neutron transport through idealized cargo container models, and (3) applying sparse data algorithms, such as principal component analysis (PCA), to enhance detection fidelity.   

A compressed sensing approach to 3D spectrometry

We present the design of a spectrometer that can retrieve a 3D spectral profile with a single measurement. The design of the spectrometer is built around a compressed sensing algorithm - which allows the spectrometer to retrieve 3D information from an otherwise 2D sensor. Numerical trials has demonstrated confidence in the accuracy of retrieved 3D spectral profiles, even in the presence of Gaussian noise. Experimental trials have proved functional. The spectrometer may be implemented to accurately analyse ultrashort pulses such as femtosecond chirped pulses in diagnosing plasmas.   

Blended Isogeoemtric Discontinuous Galerkin (BIDG) Methods for Plasma Physics

The Blended  Isogeometric Discontinuous Galerkin (BIDG) method was developed to facilitate the creation of computational frameworks capable of simulating large integrated systems. In plasma applications, BIDG enables the modeling of processes that involve interfaces between highly anisotropic plasma and solid structures/materials such as vessel walls, protective tiles, ports, nozzles, substrate holders, antennae, etc.  BIDG utilizes tools from IsoGeometric Analysis (IGA) to automate the construction of analysis-suitable finite element meshes capable of exactly preserving: (1) boundary domain geometries (e.g. exact reactor geometries, etc.), (2) internal geometric entities (e.g. contact surfaces, PMI, PFC, etc.), and (3) prominent physical features of the system (e.g. magnetic flux surfaces, etc.).  Offering direct and lossless coupling to computer aided design (CAD) and computer aided manufacturing (CAM), BIDG closes the design-to-analysis bottleneck, making it possible to automate the design, manufacturing, and analysis of devices (e.g. fusion reactors) and systems in a correct and fully integrated manner.   

Passive Detection of High Energy Particle Loss using Rippled Tiles

A novel detection approach for energetic particle loss is being implemented on DIII-D. Lost energetic ions from neutral beam injections can produce a measurable temperature change on the DIII-D outer wall. The lost ion heat profiles are however challenging to distinguish from that of the scrape-off layer heat flux. The new detection technique relies on modified tile geometries composed of short barriers that prevent small gyroradius particles from impacting the downstream wall surface and consequently create unique heat patterns on the modified tile surface. The barrier geometry sets the energetic particle energy and pitch angle sensitivity, and both are modeled to inform the tile design. Three rippled tiles are in use on DIII-D. Temperature measurements from a wide-angle, high-speed infrared camera indicate that the rippled tile shapes can affect downstream ion impacts. This passive detection technique is potentially applicable to ITER-class fusion devices.    

Permeation rates of H/He through a Pd foil for isotope separation applications in the exhaust stream of a fusion reactor

The Princeton Field-Reversed Configuration fusion experiment is a type of magnetic confinement device that utilizes odd-parity rotating magnetic fields to induce closed field lines. The D-³He fuel is aneutronic, however, deuterium (D) atoms in the plasma can fuse with each other to produce either tritium (T) or ³He. The T must be extracted in order to have a low radioactivity plasma, and in the future, it can then be stored, and breed more ³He fuel through its decay.  One way of separating hydrogen (H) and helium (He) isotopes is by introducing a high Z material – permeation barrier – high Z material configuration, where the permeation barrier, such as Al₂O₃, will prevent high energy ions from diffusing back into the plasma. High Z, large surface area materials, such as Palladium (Pd), have high permeability of H and He and low sputtering yields. Pd has a high H sorption rate and high permeability through conversion to a metallic hydride when heated to high temperatures, which increases H diffusion. Under these conditions, surface oxides and carbides of Pd may dissolve, decreasing surface impurities. We will report data on H/He permeability in Pd foil at a temperature range of 300-800 K, focusing on the effects of pressure and temperature.   

Observation of melting tungsten surface under disruption-like thermal load

In the plasma wall interaction, there is a concern about surface melting of tungsten(W) by transient thermal loads such as ELM(pulse width :0.1~1.0ms, heat fluxes :0.1~10GW/m²) and disruption(~several ms, 1.0～10GW/m²). If the molten layer becomes unstable, droplet ejection occurs, which generates high-Z dusts and accelerates erosions. W has a high melting point (3695K), and there is almost no situation other than nuclear fusion assuming melting. Therefore it is necessary to study molten layer behavior of W. In this study, surface stability conditions of melting W under the thermal loads equivalent to the reactor transient events have been investigated. We simulated disruption-like thermal loads using a Nd:YAG laser and evaluated the threshold over which droplets are generated with a square-shaped and a triangular-shaped pulsed heat fluxes. Since in actual disruptions, various shapes of pulsed influxes influence the surface stabilization condition, we have tried to correctly simulate these shapes by changing the pulse shapes. Surface melting of W, instability of molten layer, and ejection of droplets were observed by a high-speed camera and a long-distance microscope. These thresholds are summarized as a function of the average values of the pulsed heat flux and the time profile.   

Deuterium Retention in Tungsten-rhenium Alloy

Tungsten (W) is a candidate plasma facing material for fusion reactors. Irradiation by fusion neutrons will result in defect formation and transmutation to rhenium (Re). The presence of Re in W can change the solubility and diffusion properties of hydrogen isotopes, affecting tritium inventory in a fusion reactor. In this study, we focus on clarifying how Re affects the hydrogen trapping behavior by investigating the temperature dependence (300~700 K) of deuterium (D) retention behavior in undamaged W-Re samples. For W-Re (3% wt.) samples at T < 550 K, D-retention in W-Re alloy was one order of magnitude smaller than pure-W samples. At T > 550 K, D-retention was similar for both samples. Thermal desorption spectra suggest such differences arise from different penetration depths (i.e. diffusivity). We discuss whether such differences can be directly attributed to Re or arise due to microstructural differences.   

Ultrashort pulsed laser filament-produced plasma electrical conductivity measurement by guided microwave attenuation

The electrical conductivity of USPL filament-produced plasma is measured by guided microwave attenuation at 3 GHz and 40 GHz. A waveguide (WR284 for 3 GHz and WR22 for 40 GHz) is driven and signal received via 50 Ω adapters on opposite ends. A filament cluster is produced by focusing the beam of a Ti:Sapphire 600 nm, 400 fs laser in air using a 50 mm diameter, 3 m focal length mirror. The cluster passes through a hole in the waveguide to attenuate the signal. 3-D transient time-resolved COMSOL Multiphysics™ simulations, where the cluster is represented by a 1 mm diameter cylinder of time-varying conductivity, is used to design the diagnostic and interpret data. Conductivity is inferred from S-band measurements to be approximately 600 mho m⁻¹ from a 26 mJ shot. This is based on attenuation matching that of a simulation of the same conductance, after taking into account the estimated actual filament diameter (100 μm) and number (13). Available Q-band results will be presented.   

Overview of the latest results from HIDRA

The Hybrid Illinois Device for Research and Applications (HIDRA) is a toroidal magnetic confinement device located at the University of Illinois. The primary purpose of HIDRA is to do plasma material interaction (PMI) experiments, development of plasma facing (PFC) technologies and the education of future generations of plasma and fusion scientists and engineers. The last year has seen much activity on HIDRA with magnetic flux line measurements for different rotational transforms (ι) and magnetron plasma operation with plasma characterization. Measurement of the plasma shape and heat fluxes, up to 1 MWm^-2, were performed as part of an overall strategy for testing LiMIT and FLiLi limiters that eventually will go into EAST. A material analysis test stand (HIDRA-MAT) has also been developed to study in situ new and innovative materials. This is based on the successful MAPP diagnostic which has been used on NSTX-U and LTX. This talk will give an overview of the results of the last year of operation and experiments on HIDRA and what future experiments are planned.   

Simulation of the antenna-plasma coupling for the Low-field side ITER reflectometer system

High resolution edge density profile and fluctuation information is important for ITER operations and a Low-Field Side Reflectometer (LFSR) system will be installed to measure these quantities. For a reliable operation it is necessary that a significant amount of the transmitted microwave power is reflected back to the receiver antenna. The High-gain antennas used in the LFSR design give a good antenna-plasma coupling only when they are aligned with the plasma mid-plane. In ITER the plasma mid-plane can vary over vertically 0.5 m and in order to cover this range a vertical antenna array was proposed. Reflectometer simulations were performed to optimize the antenna geometry for optimal performance. The LFSR performance was studied with a 3D full-wave reflectometer code that include field-line aligned density fluctuations and relativistic effects. Parameter scans were done for various frequencies of the LFSR bandwidth (30-165 GHz), L- and H-mode profiles, density gradients, and fluctuation characteristics. The simulations show that a lay-out of 4 vertical antennas can cover the mid-plane variation and although density fluctuations reduce the coupling, sufficiently strong signals are reflected back to the receiver for reconstructing edge density profiles.

  

Determination of time-resolved electron mobility in a Hall Effect Thruster via laser-induced fluorescence

Time-resolved measurements of the electron mobility in a Hall Effect Thruster are made non-invasively using laser-induced fluorescence. The ion velocity distribution function is determined in the ionization and acceleration regions of a Hall thruster throughout a cycle of a ubiquitous low frequency oscillation known as the breathing mode. Moments of the one-dimensional Boltzmann equation form a system of partial differential equations that are solved explicitly for the axial electric field, ionization rate, and density, relying on the measured distributions and a minimally-perturbative in situ probe measurement. Electron velocity is determined by simultaneously acquiring the thruster discharge current. Cumulatively, this information is used to compute the electron mobility over the course of a breathing oscillation. The calculated results show a time-averaged profile similar to that measured in other Hall thrusters. The mobility profile varies significantly over a breathing cycle, with exceptionally low mobilities emerging downstream of the acceleration region when the discharge current reaches a minimum in the breathing cycle.
  

Multi-View, Simultaneous Measurements of Helicon Plasma Emission Lights using High-Speed Camera

Most electric thrusters have a short lifetime because of their electrode damages caused by a direct contact between electrodes and plasmas. One of the ways to solve this problem is to develop a completely electrodeless plasma thruster [1]. To promote this scheme, it is necessary to measure detailed plasma parameters with spatial distributions, e.g., electron density. Here, we have proposed spectral measurement method using a high-speed camera to observe emission lights from the plasma with three fiberscopes, spectral filters and optical lenses, which can be located on any cross sections outside the plasma of the discharge tube. We utilized an Algebraic Reconstruction Technique (ART) [2] and Fourier-Bessel expansions method (F-B method) [3], using three fan-views to reconstruct a local intensity distribution from measured line-integrated values. This report shows estimations of measurements on Large Mirror Device (LMD) using these methods along with an Abel inversion technique. [1] S. Shinohara et al., IEEE Trans. on Plasma Sci., 42 (2014) 1245. [2] R. Gordon and G. T. Herman, Int. Rev. Cytol., 38 (2014) 111. [3] Y. Nagayama, J. Appl. Phys., 62 (1987) 2702.   

Global characteristics of plasma acceleration across the magnetic field: sonic point regularization and global profiles

We consider the one-dimensional problem of acceleration of quasineutral partially magnetized plasma across the magnetic field. The ions assumed to be unmagnetized, while magnetized electrons are considered in the drift diffusion approximation. Ionization processes  are also included allowing finite velocity of neutrals as is typical for Hall thrusters.  The electric field is created by the potential difference applied across the discharge. We show that resolving the sonic point singularity impose strong constraints on stationary solutions and plasma parameters profiles. We find a simple condition defining the range of plasma paramaters where the stationary solutions are possible identifying two separate branches.  The stationary profiles for all  plasma parameters are build starting from the sonic point. These solutions show that plasma profiles are very stiff (rigid)  for a fixed potential difference  and the whole problem should be considered as an eigen-value problem for the electron temperature (or equivalently for the ionization rate).  The implications for the matching  of the quasineutral plasma with the sheath region as well as for the breathing mode oscillations are discussed.      
           

Reciprocal Coupling of Surface Plasmon Polariton Modes in Plasma-Functionalized Metamaterials

When interfaced with a metamaterial, plasmas behave as a dielectric medium that can be adjusted to control of the metamaterial’s intrinsic properties.  An experiment investigating the response of a corrugated-microstrip microwave filter coupled to a plasma discharge has been developed in the UCLA Plasma & Space Propulsion Laboratory.  The microstrip behaves as a simple 1-D metamaterial device, capable of exciting plasmon-like surface waves along its axis. Results from this experiment suggest that these “spoof” surface plasmons can excite Surface Plasmon Polaritrons (SPPs) within the plasma discharge.  Observed absorption peaks in the frequency response of the plasmonic device show strong coupling of the microstrip to these SPPs at resonance frequencies that correspond to the measured plasma electron density.   

Chemical and depth functionality in deuterium irradiated porous tungsten/liquid Li PFC system

One challenge to plasma-burning fusion reactors is discovery and development of a material system, that balances bulk damage and surface response, that is not only resistant to the heat and particle fluxes but can adapt and evolve under the extreme conditions. A porous tungsten-liquid metal hybrid system is a potential balance between favorable bulk W properties while scaffolding for a liquid metal (LM) protecting from high-Z emission.  Overall design combines LM self-healing properties to delay the mechanical failure and to protect the structure via radiative vapor shielding. With advances in additive manufacturing, hierarchical designs can be achieved with different materials/properties emphasized based on relevant mechanisms occurring within the material and providing optimal functionality through the structure. Porous W-substrates have been fabricated, with PMI and micro-hydraulic testing, such as enhanced Li wetting. Micron-scale Li percolation during exposure to D plasma has been observed using ERD.  The synergistic behavior of the D relative to the Li during this exposure is characterized with in-operando NRA and post-mortem SIMS. Differing chemical complexes of D/Li/O/C due to W surface structure is observed with in-situ XPS.