Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Plasma Physics
Volume 66, Number 13
Monday–Friday, November 8–12, 2021; Pittsburgh, PA
Session CP11: Poster Session II:
Poster
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Room: Hall A |
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CP11.00001: FUNDAMENTAL PLASMA PHYSICS: PLASMA PRODUCTION AND DIAGNOSTICS; DYNAMICS, COMPLEXITY AND SELF-ORGANIZATION, STRONGLY COUPLED, DUSTY, AND INTERFACIAL PLASMAS
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CP11.00002: Wake dynamics of air filaments generated by high energy picosecond laser pulses at 1 kHz repetition rate Adam Higginson, Yong Wang, Han Chi, Andrew Goffin, Ilia Larkin, Howard M Milchberg, Jorge J Rocca An investigation outlining filamentation in air of 7 picosecond lasers pulses up to 200 mJ energy from a λ = 1.03 μm Yb:YAG laser at repetition rates up to f = 1 kHz is presented. Interferograms of the wake generated show that while pulses in a train of repetition rate f = 0.1 kHz encounter a nearly unperturbed environment, at f = 1 kHz a channel with an axial air density depression of ~ 20% is generated and maintained at all times by the cumulate effect of adjacent pulses. Measurements at f = 1 kHz show that the energy deposited decreases proportional to the molecular channel density depletion, which becomes more pronounced as the repetition rate and pulse energy increase. Numerical simulations provide insight into the dominant energy loss mechanisms. The results are of interest for the atmospheric propagation of joule-level picosecond pulses from Yb:YAG lasers, of which average powers now surpass 1 kW, and for channeling other directed energy beams. |
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CP11.00003: Kinetic analysis of the collisional layer Mantas Abazorius, Felix I Parra, Fulvio Militello To understand plasma behaviour in the scrape-off layer, we need to know the boundary conditions for the plasma and electromagnetic fields near a divertor. At the plasma-wall boundary, in the direction normal to the wall, there are four length scales of interest: the Debye length λD, the ion gyroradius ρi, the projection of the collisional mean free path in the direction normal to the wall λ⊥ and the device size L. If λD << ρi << λ⊥<< L, we can split the plasma-wall boundary into three layers, the Debye sheath, the magnetic presheath and the collisional layer of width λ⊥, this is the layer we analyse. At distances much greater than λ⊥ from the wall collisionality is high and Braginskii fluid equations are usually used to model the plasma behaviour, here the ions are approximately Mawellian. The collisional layer connects this region with the collisionless magnetic presheath. We have found that at the entrance of the collisional layer the flow of ions is supersonic. The distribution function at the entrance of the magnetic presheath is far from Maxwellian and must satisfy the Chodura condition. We use a Galerkin method to solve the drift kinetic equation in one spatial dimension with the full Fokker-Planck collision operator, the quasineutrality equation and adiabatic electrons. |
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CP11.00004: Continuum kinetic simulations of the plasma sheath for an emitting material surface Kolter Bradshaw, Petr Cagas, Bhuvana Srinivasan The dynamics of the plasma sheath, which forms near a material surface, are of great interest to applications in plasma propulsion systems and fusion devices. Electron impact against the wall results in secondary electron emission into the sheath which can influence its evolution and structure. This work applies theory describing the plasma interaction with a material boundary wall to continuum kinetic sheath simulations. The Vlasov-Maxwell equations are discretized and evolved using the discontinuous Galerkin method with the Gkeyll code. The computational implementation of the physical models is discussed, and studies illustrating the significance of different material properties and magnetic field configurations on the behavior of the sheath are presented for physically relevant cases and regimes. |
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CP11.00005: The influence of atomic collisions on the sheath profile Yuzhi Li, Bhuvana Srinivasan, Yanzeng Zhang, Xianzhu Tang
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CP11.00006: Are Electron Sheaths Common for Positive Electrodes in Higher Pressure Discharges? Brett Scheiner In low pressure discharges (P<100mTorr), the balance of global electron and ion current loss dictates that an electron-rich sheath can only occur at a positively biased electrode when the electrode area (AE) is small compared to the area of the other walls (AW) bounding the plasma. At these low pressures it is well known that the area ratio must satisfy AE/AW<(2.3me/mi)1/2, a relation that has been verified by experiments and simulations[1]. In this work simulations are used to show that electron sheaths can occur when AE/AW>(2.3me/mi)1/2 if the neutral pressure exceeds roughly 300 mTorr. The increased neutral pressure accompanies larger density gradients in the plasma approaching the positive electrode without the presence of a correspondingly large bulk electric field. These gradients reduce the electron current collection at the electrode relative to the ion current collection at the walls. This feature lessens the area ratio requirement for global balance of current loss and makes the presence of electron sheaths at positive electrodes more likely at higher pressures. Further studies will be needed to determine the limits on the range of conditions under which electron sheaths may occur at positive electrodes. |
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CP11.00007: Studying sheath profiles using the Vlasov-Poisson system with the inclusion of Z-pinch wall currents Chirag R Skolar, Bhuvana Srinivasan, James L Juno The shear flow stabilized Z-pinch has been studied as a lower cost, commercially viable, intermediate-scale thermonuclear fusion reactor. A study of sheath formation and dynamics in the presence of large Z-pinch axial currents into the wall remains an open research area. Simulations are performed using the continuum-kinetic model in the Gkeyll code. The 1X-1V Vlasov-Poisson system is used in parameter regimes of relevance to the FuZE (Fusion Z-pinch Experiment) as well as higher current regimes, through a parametric scan of various electron drift velocities. In a classical plasma sheath (with no axial current), the electrons are less dense at the wall than the ions. The electron density at the wall increases as a function of axial current; there is a critical current at which the electrons become more dense than the ions. For all cases considered, the axial current is maintained and the electrons dominate the particle and heat fluxes. |
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CP11.00008: Matching conditions for nonlinear RF sheath rectification in three-dimensional geometry Matthew J Poulos, Nicola Bertelli, Syun'ichi Shiraiwa The sheath potentials formed near the material surfaces of plasma facing components are dramatically enhanced by the application of radio frequency (RF) power in the ion-cyclotron and lower-hybrid frequency ranges. Ions accelerated in these enhanced sheath potentials that impact the material surface can lead to the undesired generation of impurities via secondary emission. A self-consistent approach that incorporates three-dimensional geometry and nonlinear sheath boundaries is introduced, allowing the complex spatial structure of the sheath potential to be calculated via analytical and iterative methods. This work generalizes previous studies (i.e., Kohno Comp. Phys. Comm. (2017)), providing robust examples of simplified situations that should serve as benchmarks for RF sheath boundary implementation in three-dimensional geometry. |
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CP11.00009: Study of the Physical Mechanism of Air Breakdown Using Picosecond Long-Wavelength Infrared Laser Pulses Eric C Welch, Sergei Tochitsky, Daniel Matteo, Gerhard Louwrens, Chandrashekhar Joshi Recent demonstration of self-guiding of terawatt-power picosecond long-wavelength infrared (LWIR) laser pulses in air has renewed interest of ionization physics in this spectral range. The measured laser intensity is around 1012 W/cm2 in the 10 µm filament, much lower than tunneling threshold for N2/O2 and therefore avalanche ionization is thought to dominate. However, dynamics of avalanche ionization for such short and intense pulses is yet to be understood. Here we report a detailed study of the avalanche breakdown process in air using space and time resolved visible light interferometry. First, we observed that air breakdown does not occur for intensities below 100 GW/cm2, i.e. the breakdown threshold is much higher than that reported for nanosecond CO2 lasers. Next, the velocity of the backward propagating “breakdown wave” was measured to be >108 cm/s, an order of magnitude faster than the optical detonation effect associated with nanosecond pulses. Finally, in a large diameter plasma channel, air impurities are seen to initially form localized, hot, high-density plasma beads that drive shock waves travelling around 105 cm/s in the air. These hot regions eventually coalesce to form a centimeter-scale, hot gas channel that persists over millisecond time scales. |
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CP11.00010: Spatial Profile of Ion Flow Accelerated by Using Electromagnetic Plasma Acceleraerion Method in Electrodeless RF Plasma Thruster Takeru Furukawa, Daisuke Kuwahara, Shunjiro Shinohara Electrodeless Radio Frequency (RF) plasma propulsion system is a promising approach to overcome the limitation of operational lifetime caused by erosion of acceleration electrodes. Rotating Magnetic Field (RMF) plasma acceleration method [1,2] is one of the electrodeless concepts and is expected to enhance the performance by means of an additional electromagnetic force [1]. The RMF method drives an azimuthal electron current owing to the non-linear Hall-term effect, and the additional force is generated in the presence of the external divergent magnetic field. To demonstrate our proposed RMF thruster concept, the spatial profile of ion velocity distribution function was clarified by using Laser-Induced Fluorescence spectroscopic measurement method [3]. The addition of the RMF method accelerates plasma flow and the axial component of the ion velocity increases under the magnetic nozzle. We have also investigated a case that the perfect magnetic field penetration of the RMF is not found and a relatively high-velocity increment has been obtained under the partial RMF penetration condition [4]. |
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CP11.00011: Characterization of a neutral calcium plasma Jacob W McLaughlin, Fred N Skiff Neutral calcium plasma is particularly interesting for studies of ion wave dynamics. Apart from the Barium Q-machine, laser induced fluorescence (LIF) measurements in most contexts rely on excitation from metastable states. However, under many circumstances involving inhomogeneous plasma or large perturbations, the metastable distribution function may be very different from the total ion distribution and may even be closer to the neutral atom velocity distribution. Achieving ground-state LIF in most plasmas (e.g. He, Ar) requires pump laser frequencies on the order of petahertz, which is currently unfeasible. In calcium, this can be achieved with a tunable diode laser operating at several hundred terahertz by pumping the 3p64s 2S1/2 to 3p64p 2P03/2 transition (397nm) and observing emission as the ion deexcites to the 3p63d 2D5/2 state (854 nm). Ionized calcium is produced by passing a neutral calcium beam through a cylindrical cavity where the TE111 mode is excited. Ionization efficiency is increased by external permanent magnets tuned for electron cyclotron resonance (ECR) at the cavity mode frequency. The plasma passes into the Iowa Multi-dipole Experiment (IME) where it is trapped by rails of dipole magnets on the walls, but left unmagnetized in the region of measurements (~2 Gauss). This novel calcium plasma source is described and characterized using ground-state LIF measurements for varying neutral beam flux, ECR magnetic field, and RF power. |
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CP11.00012: Strategy for Developing Internal Transport Barriers at Large Radius in High Poloidal Beta Plasmas on EAST Siye Ding, Xiang Jian, Andrea M. M Garofalo, Huiqian Wang Gyrokinetic modeling shows possible approaches to develop internal transport barriers (ITBs) at large minor radius in EAST high βP plasmas. EAST achieved steady state operation for 60 s with high βP (~2.0). However, no large radius ITBs (a feature of the high βP scenario on DIII-D) are observed on EAST. Analysis of large-radius ITB formation on DIII-D shows that the joint effects of low magnetic shear and high α-stabilization keep the plasma away from the kinetic ballooning mode (KBM) instability boundary and drive the plasma into a second stability region featured with low transport. Similar gyrokinetic analysis applied on EAST high βP plasmas shows that the ion temperature gradient (ITG) is the dominant micro-turbulence mode that prevents ITB formation at large radius. The analysis suggests several potential approaches for breakthrough: 1) strong off-axis current drive from external sources; 2) discharge initiation with higher magnetic shear at higher q95; 3) additional ion heating at large radius or higher pedestal ion temperature; 4) impurity injection at large radius. In addition, applying a large edge perturbation is expected to bypass the KBM instability, triggering ITB formation. Joint experiments in the ongoing EAST campaign plan to test these approaches. |
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CP11.00013: Investigation of Critical Ionization Velocity in a Rapidly Rotating Plasma Jenny R Smith, Remington Reid Rapidly rotating plasmas show promise in applications such as fusion energy and large-scale x-ray generation because of their potential to confine plasma in a magnetic mirror configuration and the tendency of the shear velocity to mitigate instabilities from forming [1]. The Maryland Centrifugal Experiment (MCX) was a magnetized rapidly rotating plasma experiment that demonstrated many of the potential benefits of centrifugal confinement, but was ultimately limited by the Critical Ionization Velocity (CIV) and the device insulators and was therefore unable to demonstrate the full potential of the confinement scheme. [2]. CIV is the velocity, in this case the rotational velocity of the plasma, when exceeded, a neutral traveling through a magnetized plasma will rapidly ionize [3]. A second generation of MCX is being developed at the AFRL to explore CIV phenomena further. There are two primary objectives of this experiment, the first being to pinpoint the neutral density needed for CIV to occur. There is no minimum density of neutral seed gas listed in the literature and this would be useful knowledge in the field of rapidly rotating plasmas. The second objective is to determine if and where, on the magnetic field lines, the plasma is CIV-limited. The experimental design has been modified from MCX to be able to explore this, most likely with spatially resolved fluorescence spectroscopy. |
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CP11.00014: Plasma Impedance Tomography for Imaging Plasma Dynamics Erik M Tejero, Ami M DuBois, George Gatling, Carl L Enloe, David D Blackwell, William E Amatucci Plasma impedance probes measuring the self-impedance of the antenna-plasma system have been shown to provide accurate measurements of electron plasma density for space and laboratory plasmas. Plasma impedance probes measuring the mutual impedance between two antennas and a plasma dielectric have been successfully flown on sounding rockets and satellites. At the US Naval Research Laboratory, we have recently developed a noninvasive method for generating real-time images of plasma density and magnetic field. The method consists of measurements of the complex mutual impedance between elements of an antenna array and an image reconstruction algorithm. The impedance spectra are collected after a short pulse has been applied to each element in sequence. These spectra provide path-independent information about the plasma dielectric that are used to reconstruct images of plasma density and magnetic field. The goal is to develop a system capable of providing tomographic reconstructions at a rate of a 1% of the peak plasma frequency of the system. Recent numerical and experimental results will be presented. |
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CP11.00015: Background gas species and pressure dependence of plasma fluorescence and RF emissions of laser driven filament plasmas Erin A Thornton, James E Wymer, Travis M Garrett, Jennifer A Elle, Andreas Schmitt-Sody, Edward L Ruden The Air Force Research Lab is investigating RF emissions generated by filaments of femtosecond lasers. We employed an 800 nm, terawatt class laser focused with a 3m focal length mirror to produce a filament inside of a quartz vacuum chamber to observe the effects of pressure and background gas on the filament. Background gases investigated at Argon, Helium, Krypton, Neon, and Nitrogen. A 4 Quik E camera was used to image 2 ns exposures of the plasma along the length of the plasma to observe the development of the plasma fluorescence from the multifilament regime into the single filament regime and to measure the width of the filament. In addition to the optical data, RF measurements of filaments will be presented. The effects of background gas and pressure on the characteristics of the filament will be detailed. This optical and RF data will aid in developing a deeper understanding of the plasma mechanisms responsible for the generation of the RF. |
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CP11.00016: Upgrading LAPD diagnostic pipelines for training generative ML models Phil Travis, Steve Vincena Machine learning may transform the way science is conducted. We seek to update the Large Plasma Device (LAPD) data acquisition system to better capture machine state information (MSI) and global diagnostics that are important for ML-based analysis pipelines. Plasma processes in LAPD are typically studied by gathering high-spatial-resolution data using probes, combining measurements over many discharges at a 1 Hz shot rate. ML can instead be used to infer behavior over larger spatial regions from a few localized probe measurements and global or spatially-averaged diagnostics. An auxiliary system was appended to the current labview data acquisition routines to record LAPD MSI and auxiliary diagnostics including interferometers, visible light diodes, a diamagnetic loop, and a fast framing camera. An implicitly-generative, neural network-based energy based model (EBM) constructed in pytorch was trained on these auxiliary diagnostics, MSI, and probe measurements. Artificial discharges are sampled from the learned model via Langevin dynamics to predict time evolution of ion saturation current profiles. The EBM is able to reproduce general trends in profile evolution for a variety of magnetic field configurations. In addition, the EBM can be conditionally sampled to find the machine state required for a desired profile. Results from this model and comments on its accuracy will be presented, and the scientific prospects of this type of generative model will be discussed. |
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CP11.00017: Plasma interactions with entangled photons Zhehui Wang, Ahmed Diallo, Karl M Krushelnick, Julia M Mikhailova, Yanhua Shih
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CP11.00018: Particle in Cell Modeling of Positively Biased Probes in Overdense ExB drifting Plasmas. Remignton R Reid, David l Cooke Supersonic, ExB flow past a positively biased probe in an overdense plasma is simulated in three dimensions using the Particle In Cell (PIC) method. These simulations are relevant to the questions of anomalously high current seen in orbital experiments as well as being of basic diagnostic interest for probes used is high desnity, rapidly flowng plasmas. The problem of anonymously high currents drawn by biased probes has been known since the tether experiments flown on the Space Shuttle. Cooke et al proposed a theoretical explanation for the anomalously high currents seen in the tether experiments. Modeling the system presents a challenge because the problem involves kinetic effects in an overdose plasma and if fundamentally 3D. We present results of directly modeling this system using a PIC simulation and compare the results to the theoretical model given by Cooke et al. |
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CP11.00019: Electron and Ion Temperature Measurements for the Terrestrial Reconnection Experiment Cameron Kuchta, Jan Egedal, Joseph R Olson The Terrestrial Reconnection EXperiment (TREX) at the Wisconsin Plasma Physics Laboratory (WiPPL) studies collisionless magnetic reconnection. Since the plasma parameters change quickly in TREX as reconnection is occuring, there is not enough time to scan the probe bias as is usually done. Instead, in order to fit an I-V curve, a 16 tip Langmuir probe array and three magnetic flux coils is inserted into the experiment. Each probe tip is biased to a different potential between -120 and 120 V allowing us to characterize the full I-V curve. The tips are made of Mo to reduce sparking of the tips when biased at the extremum. The three flux coils combined with a separate linear array of coils allows us to more accurately identify the probes placement in the layer. As the experiment is run, the reconnection layer is jogged downwards which allows us to measure across the layer in each shot. By using the evolving I-V curve, we can find the electron and ion temperature and density by assuming the plasma is Maxwellian. |
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CP11.00020: Determine Ion Composition using Alfven Wave Measurements Xiangrong Fu, Seth E Dorfman, Hui Li Natural plasmas, such as the solar wind and Earth's magnetosphere, contains multiple ion species. The ion composition provides important information about the origin of the plasma, and it may also reveal the underlying energization process of the plasma. However, direct measurement of ion composition is sometimes very difficult, especially when the plasma is cold. Alternatively, measurements of Alfven waves can be used to determine the composition because wave properties such as the dispersion relation is strongly altered by ion composition. In this work, we present results from Alfven wave experiments in a proton-helium plasma on the Large Plasma Device. Two methods are used to estimate composition. The first one involves nonlinear interactions of three waves near the helium cyclotron frequency. Two large-amplitude counter-propagating Alfven waves are launched to excite a third wave through three wave resonance. The measured properties of these waves are then used to estimate relative density of helium ions. In the second method, we scan the frequency of a small-amplitude Alfven waves launched from the antenna near helium cyclotron frequency to obtain a linear dispersion relation. The two methods show good agreement when waves are well resolved in the experiment. The outcome of this study will have implications in developing new technology to measure cold ion populations in space plasmas, which is very challenging using traditional methods. |
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CP11.00021: Development of a rotating magnetic field system to emulate a pulsar magnetosphere in BRB Steve P Oliva, Karsten J McCollam, Jeremiah Kirch, Ao Zhang, Cary B Forest
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CP11.00022: Overview of Experiments from the Wisconsin Plasma Physics Laboratory Cary B Forest, Jan Egedal, Noah C Hurst, Karsten J McCollam, Joseph R Olson, John S Sarff The Wisconsin Plasma Physics Laboratory (WiPPL) is a multi-machine, collaborative research facility directed toward fundamental topics in discovery plasma science: dynamos, reconnection, turbulence, particle acceleration, coherent structures, and plasma systems. A large fraction of the run time is for users from outside WiPPL who lead experimental projects on basic, astrophysical, and fusion plasma studies. We present an overview of WiPPL capabilities, recent and ongoing WiPPL-led projects, and key results to date. On the Big Red Ball (BRB) collisionless reconnection is studied with unprecedented spatial and temporal resolution; parallel and perpendicular shock formation are studied using pulsed power in a theta pinch and with CT injection; enormous magnetic field amplification is observed in high beta Hall dominated Couette flow, and a novel instability driven by differentially rotating electrons has been observed. New capabilities in the near term will use a rotating dipole to emulate a pulsar wind system in the lab and a planar spheromak injector is being installed to mimic the system of a galactic jet expanding into intergalactic medium. On the Madison Symmetric Torus (MST), tokamak plasmas are routinely operated for a variety of studies: the spatial structure and time dynamics of runaway electron generation are measured with; whistler frequency range waves correlated with runaway electrons have been observed, and the self-organization of low q tokamaks is measured directly using probes. RFP plasmas in MST are used for studies of plasma self-organization , with programmable power supplies expanding the Lundquist-number overlap with nonlinear MHD simulations. |
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CP11.00023: Particle-In-Cell Simulations to Study the Stability of Two-Dimensional Bernstein-Greene-Kruskal Modes James McClung, Kai Germaschewski, Chung-Sang Ng The stability of two-dimensional (2D) Bernstein-Greene-Kruskal (BGK) modes [Ng, Phys. Plasmas, 27, 022301 (2020)] in a magnetized plasma with a finite background uniform magnetic field is studied using the Plasma Simulation Code (PSC) [Germaschewski et al., J. Comp. Phys., 318, 305 (2016)], a Particle-In-Cell (PIC) code. These solutions are exact nonlinear solutions of the steady-state Vlasov equation with an electric potential localized in both spatial dimensions perpendicular to the axial magnetic field that satisfies the Poisson equation self-consistently. These solutions have cylindrical symmetry and are invariant along the axial direction, with the distribution function—ion or electron— depending on the particle energy and the axial components of the canonical linear and angular momenta. Dynamics of both ions and electrons are included in the simulations. High-resolution PIC simulations in both 2D and 3D are planned in the studies. The full investigation will take a multi-year effort due to a large parameter space. Most up-to-date preliminary results will be presented. |
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CP11.00024: Three-Dimensional Bernstein-Greene-Kruskal Modes in a Finite Magnetic Field Chung-Sang Ng We will present analytic forms and numerical solutions for three-dimensional (3D) Bernstein-Greene-Kruskal (BGK) modes in a magnetized plasma with a finite background uniform magnetic field. These are exact nonlinear solutions of the steady-state Vlasov equation with an electric potential localized in all three spatial dimensions that satisfies the Poisson equation self-consistently. Dynamics of both ions and electrons are included in the formulation. This new development is following our previous solutions for 3D BGK modes in an unmagnetized plasma [Ng & Bhattacharjee, Phys. Rev. Lett., 95, 245004 (2005)] and 2D BGK modes in a magnetized plasma with finite magnetic field [Ng, Phys. Plasmas, 27, 022301 (2020)]. These solutions have cylindrical symmetry with distribution functions of either ion or electron depending on the particle energy and the axial component of the canonical angular momentum. However, the functional forms of distribution functions for these solutions are very different from previous 3D BGK mode solutions that are based on the strong magnetic field assumption. |
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CP11.00025: Three-Dimensional Dynamics of Mutually Interacting Flux Ropes in PHASMA Peiyun Shi, Prabhakar Srivastav, Regis John, Paul Cassak, Earl Scime Interacting flux ropes are commonly observed on the Sun and in the terrestrial magnetosphere and have been recreated successfully in the PHASMA (PHAse Space MApping) facility using pulsed plasma guns. In 3D environments, the behavior of flux ropes is more complicated than simply attracting and merging. We report measurements of azimuthal writhing, radial bouncing and reconnection between two flux ropes. Unlike previous experiments, flux ropes in this study are kink-free even when created with currents above the kink threshold, eliminating the role of the kink instability in this study. Kink-free flux ropes are achieved by reducing the current pulse duration to less than the axial Alfvén time. Interestingly, two flux ropes are found to rotate around each other clockwise (in the ion gyromotion direction) or counter-clockwise as the axial current increases or decreases. The magnetic field profile and emission light intensity at different axial planes will be presented for different guide field strengths, axial currents, gas species, and collisionality. We discuss plausible explanations for the rotation of the flux ropes involving the induction from changing currents, boundary conditions, and 3D effects. |
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CP11.00026: Measuring the evolution of canonical vorticity vectors during RFP relaxation Jason Sears, Jens Von Der Linden, Karsten J McCollam, Abdulgader Almagri, Allyson M Sellner, Mikhail Reyfman, John S Sarff, Haruhiko Himura, Setthivoine You Recent theoretical work provides a picture of plasma relaxation as conversion between twisted magnetic flux tubes and twisted ion flow vorticity flux tubes, while conserving their total twist. We are developing a canonical vorticity probe to measure these fluxes simultaneously in a relaxing plasma, which has not been done previously. The probe will consist of clusters of B-dot and Mach probes, each measuring the local 3D vectors of magnetic field and ion flow. The 3D Mach vector is determined by a matrix inversion that includes the 3D angles of the tips. The clusters will be arranged in a finite difference stencil so that the curl of these vectors can be determined in the center. A prototype of a single cluster has been constructed [1] and is to be fielded on the Madison Symmetric Torus during sawtooth relaxation. Traditionally, plasma relaxation is described in terms of single fluid models as constrained by magnetic helicity alone. This work will help to generalize our understanding to a canonical-helicity-constrained relaxation explaining multi-scale dynamics. |
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CP11.00027: Evolution of an Arched Magnetized Laboratory Plasma in a Sheared Magnetic Field Kamil D Sklodowski, Shreekrishna Tripathi, Troy A Carter Arched magnetized structures are a common occurrence in space and laboratory plasmas (i.e. solar prominences, coronal loops). We present the results from a laboratory experiment on spatio-temporal evolution of an arched magnetized plasma (β ≈ 103, Lundquist number ≈ 104, plasma radius/ion gyroradius ≈ 20) in a sheared magnetic configuration. The arched plasma is produced using a hot-cathode lanthanum hexaboride (LaB6) source and it evolves in an ambient magnetized plasma produced by another LaB6 source [1, 2]. The experiment is designed to model conditions relevant to the formation and destabilization of similar structures in the solar atmosphere. The magnitude of a nearly horizontal overlying magnetic field was varied to study its effects on the writhe and twist of the arched plasma. In addition, the direction of guiding magnetic field was varied to investigate its role in formation of either forward- or reverse-S shaped plasma structures. The electrical current in the arched plasma was well below the current required to make it kink unstable. A significant increase in the writhe of the arched plasma was observed with larger magnitudes of overlying magnetic field. Forward-S shaped arched plasma was observed for a guiding magnetic field oriented nearly anti-parallel to the initial arched plasma current, while the parallel orientation yielded the reverse-S shaped arched plasma. |
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CP11.00028: Electromagnetic energy transport by tearing fluctuations during magnetic relaxation in a self-organized reversed-field pinch plasma Derek J Thuecks, Karsten J McCollam A radial Poynting flux due to tearing mode fluctuations during magnetic relaxation is measured in the Madison Symmetric Torus (MST), a reversed-field pinch plasma at the Wisconsin Plasma Physics Laboratory (WiPPL). We find that during sawtooth crash relaxation events, this flux corresponds to transient power levels larger than the input power, comparable to the global equilibrium magnetic energy transient loss rate, and sufficient to drive the fluctuation-induced dynamo EMF supporting the plasma's magnetic self-organization. The edge radial profile of the time-average flux is observed to reach a maximum at the magnetic reversal surface, where it corresponds to approximately 65% of the input power, while at the extreme edge it corresponds to approximately 20% of the input. A simple Poynting's theorem model for an incompressible, resistive MHD plasma with resistive boundary is developed, predicting that the fluctuation-induced Poynting flux out of the plasma corresponds approximately to the power lost from the equilibrium magnetic field due to the dynamo EMF. Experimental probe measurements of this flux are roughly as predicted by the model upon substitution of time-resolved equilibrium measurement data. |
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CP11.00029: Filamentation in Capacitively Coupled Magnetized Plasmas Stephen Williams Recent experiments at the Magnetized Dusty Plasma Experiment (MDPX) at Auburn University have observed the formation of filamentary structures in capacitively-coupled, rf generated plasmas at high magnetic field (B ≥ 0.5 T). These plasma filaments, when viewed from the side, appear as bright vertical columns aligned parallel to the magnetic field that can either be stable or mobile structures, depending upon the experimental conditions in the plasma. In this work, the MDPX device is used to study the threshold conditions for filamentation formation under various pressure and applied magnetic field conditions. The formation of spatial patterns of individual filaments, from spiral and circular structures, will be analyzed to determine how their physical properties (size distribution, number, etc.) vary with the plasma parameters. Recent experiments have identified and characterized the morphology of several distinct filamentary modes that are formed in low temperature argon plasmas. This presentation will focus on how those properties of the filaments compare with fundamental length scales in the plasmas (ion/electron gyroradii, collision mean free path, Debye length, etc.) and Hall parameter. |
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CP11.00030: Force-Free Equilibria and Moving Structures in a Magnetized Plasma Arcade Darren G Craig, Stephen McKay, Jodie McLennan, Elizabeth H Tan Magnetized plasma arcades and flux ropes appear in space and astrophysical contexts and have been the subject of recent laboratory, observational, and computational studies. We present results from a laboratory arcade experiment. The arcade-shaped plasma is formed between two parallel electrodes and is constrained by a magnetic coil surrounding the electrodes. ICCD cameras, photodiode cameras, magnetic probes, and plasma spectroscopy are used to characterize the plasma. A simple model for a force-free equilibrium in a thin magnetized arcade predicts the angle of the sheared arcade magnetic field lines as a function of plasma current and vacuum arcade magnetic field. The model is compared with images of tilted plasma loops within the arcade and we find that the simple model provides a good upper bound for the tilt angle observed. We also observe moving structures that form repetitively and propagate along the electrodes with a speed that is nearly independent of plasma current, magnetic field strength, ion mass (hydrogen vs. argon), and direction of propagation. The structures are present in visible emission and in magnetic field measurements, the latter of which identify these as propagating current filaments. |
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CP11.00031: Stability analysis of MHD equilibria via simulated annealing Masaru Furukawa, Philip J Morrison Simulated annealing (SA) is a method to calculate a steady state of a Hamiltonian system by solving time evolution of artificial dynamics derived from the original dynamics. The artificial dynamics is constructed so that the energy of the system changes monotonically [1], while preserving Casimir invariants. The SA leads to an energy extremum, which is an equilibrium, on a surface with constant Casimirs in the phase space. We have demonstrated that the SA succeeds to calculate low-beta reduced MHD equilibria in rectangular [2] and cylindrical domains [3], as well as high-beta reduced MHD toroidal equilibria [4]. Even if an equilibrium is spectrally stable, it is not necessarily at an energy minimum. Therefore, the SA can be utilized to analyze stability in a wider sense. In fact, we found a cylindrically symmetric, low-beta reduced MHD equilibrium that is spectrally stable against ideal MHD modes and is unstable against the SA dynamics. |
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CP11.00032: Observations and Extended MHD Modeling of Magnetized Plasma Bubbles Launched into a Magnetized Background Plasma Mark A Gilmore, Robert H Dwyer, Shakiba A Haji Sadeghi The interaction of magnetized flowing plasmas with background plasma is a topic of wide interest in astrophysics as well as high temperature laboratory plasmas. In this work, hot, dense, plasma structures are launched from a coaxial plasma gun into a background low density magnetized lab plasma. The dense gun plasma drags frozen-in magnetic flux into the chamber’s background magnetized plasma, providing a rich set of dynamics to study magnetic relaxation, boundary instabilities, magnetic and density turbulence, and magnetized shocks. Magnetic and electrostatic probe measurements and visible spectroscopy measurements, as well as fast multi-frame imaging will be presented in comparison with preliminary modeling using the Perseus extended MHD (XMHD) code. Recent experiments show a possible magnetic Rayleigh-Taylor (MRT) instability that appears asymmetrically at the interface between launched spheromaks (bubbles) and their entraining background magnetic field and plasma. Efforts to understand this instability using in situ measurements, new chamber boundary conditions, and ultra-fast camera and probe data will be presented. |
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CP11.00033: Modeling the effect of shear viscosity and diffusion in strongly coupled ionic mixtures William A Angermeier, Ioana D Dumitru, Joshua P Sauppe, Brett Scheiner Viscosity and diffusion coefficients are evaluated for strongly coupled ionic mixtures using the Green-Kubo relations through equilibrium molecular dynamics (MD) simulations. These simulations enable the calculation of these transport properties through analysis of time series correlations, and the coupling between these two transport coefficients are compared to the Stokes-Einstein relation. The parameter space for the ionic mixtures for this research is characterized by the plasma coupling parameter, which ranges from 20-100. In this highly coupled regime, the interaction between the weakly and strongly charged mixture components are assessed by taking the cross-correlation of each species’ time series. Ultimately, understanding transport properties, such as viscosity and diffusion, for such charged mixtures may be used to facilitate modification to instability growth often encountered in inertial confinement fusion and high energy density experiments. |
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CP11.00034: Expanded data and improved calibrations for measurements of electron-ion collision rates in magnetized ultracold neutral plasmas Jacob L Roberts, Puchang Jiang, Bridget O'Mara Ultracold neutral plasmas (UNPs) can have cold electron temperatures (as low as 1K) and low densities (107 cm-3) that allow the achievement of extreme degrees of electron magnetization with only moderate laboratory fields. We have conducted measurements of the AC conductivity of electrons in UNPs as a function of magnetization for high-frequency fields and found a variation in conductivity that is not explained by AC conductivity theories. Since this AC conductivity is related to electron-ion collision rates, this discrepancy indicates a difference between predicted and measured electron-ion collision rates in these magnetized systems. We report additional measurements at more magnetic fields as well as describe improvements to multiple calibrations necessary for interpreting these measurements. |
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CP11.00035: Collisional Plasma Shocks in a Warm Dense Matter Regime Brett Keenan, Chrismond D Smith, Daniel Livescu, Jeff Haack, Robert S Pavel The structure of collisional plasma shocks has been subject to an extensive, multi-decadal investigation – both in the hydrodynamic,1 hybrid kinetic ion/electron fluid,2 and fully kinetic ion/electron3 limits. Despite this, all of these studies apply exclusively to classical, weakly-coupled plasmas. Using molecular dynamics (MD) simulations, we obtain ion transport coefficients (shear viscosity and thermal conductivity) and the ion equation of state (EOS) corresponding to a subspace of the warm dense matter (WDM) regime. More specifically, we consider a plasma of fully degenerate electrons with moderate-to-strongly coupled ions. We find that the WDM ion transport coefficients and EOS differ markedly from their weak-coupling equivalents; and as a consequence, the structure of a WDM plasma shock strongly deviates from the classical picture. |
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CP11.00036: Longitudinal and Lateral Shear Flow in a Dusty Plasma Parker J Adamson, Calvin Carmichael, Jorge Carmona Reyes, Vladimir Nosenko, Truell W Hyde In this paper, the manner in which the longitudinal (axial) and lateral (radial) shear flow differ experimentally in a three-dimensional (3D) complex (dusty) plasma is examined. Data is collected employing the PlasmaKristal-4 (PK4-BU) device at Baylor University which is used to investigate this phenomenon as a preliminary step to gaining a better understanding of shear flow at a fundamental level in microgravity. The paper will consider shear flow produced in a suspended cloud of melamine formaldehyde (MF) spheres in a DC plasma within the PK-4 BU with the shear flow initially established via radiation pressure from a collimated infrared laser (i.e., manipulation laser). Specific parameter spaces are examined experimentally in order to determine the directional dependence of the shear flow under these conditions. The resulting particle trajectories will be examined for lateral shear flow given imaging constraints and the potential implications and importance of understanding the directional dependence of shear flow forces will be discussed. |
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CP11.00037: Torsion Phase Synchronization in Complex Plasma Crystals Calvin Carmichael, Parker J Adamson, Graeson Griffin, Jorge Carmona Reyes, Lorin S Matthews, Truell W Hyde Complex plasma consists of micron sized dust particles levitating in a weakly ionized gas. Given strong enough interparticle interactions between the dust particles, a plasma crystal can be formed. Torsions are an anomlaous occurence in monolayer plasma crystals where dust particles pair up and orbit outside the plane of the crystal. Their initial obeservation led to two formation mechanism possibilities, ion-wake induced mode coupling and ion drag caused by local rotations in the ion flow. As such, measuring the forces on the particles adjacent to a Torsion provide a direct look into the shape of the ion wake field. At Baylor University, a GEC RF Reference Cell is being used to experimentally study the interaction between Torsions and the surrounding particles in a monolayer lattice structure to ascertain whether multiple Torsions have the same frequency for identical phase angles with each cycle. This paper considers both the formation and the effects of Torsions on the surrounding particles experimentally, employing high speed cameras and particle tracking software. The results are then compared to previous results, with extrapolated outcomes mapped to the ion wake fields and phase angles of each individual Torsion. |
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CP11.00038: Observing Dust Response In An Inductive Plasma Generator Graeson Griffin, Eva Kostadinova, Dmitry Orlov, Jens Schmidt, Davis Crater, Roman Smirnov, Igor Bykov, Dmitry L Rudakov, Kenneth Ulibarri, Truell W Hyde In December 1995, the Galileo probe entered the Jovian atmosphere. The scientific payload of the probe survived long enough to transmit 58 minutes of data while on its entry trajectory due to its carbon-based ablative heat shield. A Frontiers in Science campaign employing the DIII-D tokamak and the IPG6 device at Baylor’s Center for Astrophysics, Space Physics, and Engineering Research (CASPER) is examining the complex processes guiding heat shield ablation under extreme conditions. In these experiments, millimeter-sized carbon pellets are launched upward into the DIII-D tokamak bulk plasma to study ablation in an environment similar to that experienced by the Galileo probe during Jovian atmospheric entry. As the carbon pellets encountered fast plasma flow, their trajectories were altered in unexpected ways. In an attempt to better understand the underlying physics involved, the impact of plasma flow on the trajectories of spherical pellets is being assessed employing data collected via a complementary set of experiments conducted at Baylor University. This presentation will provide preliminary findings from both experiments and discuss their relevance to heat shield ablation. |
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CP11.00039: Digital Representation of Fine-Grained Rim Analogs using Image Processing Jorge Martinez Ortiz, Graeson Griffin, Truell W Hyde, Lorin S Matthews, Augusto Carballido, Abbie Terrell, Izzy Thomas Fined-grained rims (FGRs) are porous structures of dust found in the bodies of carbonaceous chondrites. The attachment and compaction of dust on the surface of chondrules create a porous layer, which has been predicted to facilitate co-accretion with other chondrule compounds to happen within the solar nebula. Understanding the mechanisms of formation and evolution of FGRs would provide important information about the origin of planetesimals in the protoplanetary disk. Recent numerical modeling has simulated the initial porosity and rim structure for different turbulent gas environments. In order to obtain further insight into the dust accretion process, we are performing experiments that recreate the initial accretion and restructuring of dust grains on the surface of chondrule analogs for multiple velocity regimes. The experimentally-formed dusty layers can be compared with the FGRs found in chondrites and those generated by numerical models. In order to analyze the rim, a set-up of high-speed cameras and laser sheets is used to image the dust layer as the dust is collected. A sequence of image processing is used to analyze morphological elements of the rim such as porosity and topography. |
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CP11.00040: Using dust grains to measure plasma conditions: changing plasma density Alexandria Mendoza, Khandaker Sharmin S Ashrafi, Lorin S Matthews, Truell W Hyde Complex plasmas are of interest as they allow the self-assembly of micron sized dust particles to form both stable and unstable structures. The dynamics of these structures can be used to probe plasma parameters which are difficult to measure experimentally. The dust particle interaction with ions flowing towards a negative electrode creates an ion wake field, affecting both the charge on the dust grains as well as the electrostatic interaction between the grains. |
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CP11.00041: Waves in binary dusty plasmas Lasse Bruhn, Dietmar Block Complex plasmas containing charged dust particles are an ideal model system for research on strong coupling phenomena. In two-dimensional systems waves can be excited either thermally or by external manipulation. The dispersion of waves propagating in monodisperse complex plasmas is well understood. However, the dynamics of waves in binary mixtures, containing two differently sized particle species, are less examined, but an interesting field of research. In this contribution, a method to derive the dust charge ratio as well as the absolute charges of the two particle species from the thermal dispersion is presented. |
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CP11.00042: Filamentary and Resonant Drag Instabilities in Laboratory and Astrophysical Dusty Plasmas Ben Y Israeli, Amitava Bhattacharjee Previous work on the filamentary instability seen in laboratory dusty plasmas has demonstrated that its onset is associated with a resonance between dust and ion acoustic modes and is mediated by drag between these species.[1] Recent work on resonant drag instabilities (RDIs), which are driven by resonance between fluid waves and dust streaming through the fluid, suggests that RDIs have significant implications for planet formation.[2] Motivated by similarity between these physical pictures, we have developed a general framework for the study of both of these instabilities and the conditions for their appearance, with the objective of studying their linear and nonlinear properties more exhaustively in dusty as well as other low-temperature laboratory experiments (including microgravity conditions). We demonstrate that the filamentary instability is analogous to a zero-drift velocity-RDI. Unlike normal RDIs, we find that the addition of a nonzero streaming velocity strongly affects and eventually suppresses the filamentary instability. Further generalizations, including the consideration of plasma and dust charging effects in astrophysical systems, are considered. |
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CP11.00043: Caltech Water-Ice Dusty Plasma Experiment Upgrade and Analysis of Creation of Hot Electron Tail and its Effect on Dust Charging André Nicolov, Paul M Bellan The Caltech water-ice dusty plasma experiment has water vapor injected into a capacitively coupled RF plasma with LN2-cooled electrodes. Water ice grains form spontaneously in the weakly-ionized plasma with extremely cold background gas. The experiment is undergoing a significant upgrade wherein the RF electrodes will be cooled by a liquid helium cryocooler to enable lower and more controlled temperatures. Status of this upgrade will be reported. |
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CP11.00044: Construction of an electron beam source for dusty plasma studies Jeremiah D Williams The kinetic effects on the dust particles in a plasma crystal locally irradiated by a narrow, pulsed electron beam (EB) with energies from 10 – 15 keV have recently been presented. [C.M. Ticoş, et. al., Phys. Plasmas, Phys. Plasmas 26, 043702 (2019)., C.M. Ticoş, et. al., Plasma Phys. Control. Fusion 62, 025003 (2020)] These preliminary studies have revealed that the EB pushes the dust particles in the irradiation zone, leading to both laminar and turbulent flow. To extend these preliminary studies, we have begun constructing an electron beam source that is capable of operating over a wider parameter space. In this poster, we present work in the construction of this improved electron source and planned experiments. |
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CP11.00045: Research activities in the Magnetized Plasma Research Laboratory (MPRL) Saikat Chakraborty Thakur, Uwe Konopka, Edward E Thomas, MPRL Team The Magnetized Plasma Research Laboratory (MPRL) at Auburn University explores a wide range of plasma phenomena and complex/dusty plasmas covering the unique parameter regime of high magnetic fields (up to 4 T), but at relatively low electron (Te < 10 eV) and ion temperatures (Ti < 1 eV). The centerpiece of the laboratory is the Magnetized Dusty Plasma Experiment (MDPX) device - a highly flexible, high magnetic field (up to 4 T) research instrument with a mission to serve as an open access, multi-user facility for the dusty plasma, basic plasma, and fusion plasma research communities. Other instruments include ALEXIS, an RF driven magnetized linear plasma device capable of simulating space plasma and basic plasma experiments, and a wide variety of “tabletop” scale devices. This presentation will summarize the results from a wide variety of recent studies conducted at MPRL, including particle growth at high magnetic field, pattern formation in both background plasmas and dusty plasmas at high magnetic field, controlling dust charging and dust dynamics by externally applied UV light and the impact of charged dust on the propagation of driven low frequency, electrostatic fluctuations in magnetized plasma. |
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CP11.00046: Transmission-Type Impedance Probe Measurements for Dusty Plasmas Brandon D Doyle, Uwe Konopka Plasma impedance probes measure electron density and temperature by measuring plasma response to an applied low-power RF signal at frequencies on the order of the electron plasma frequency, ωpe. These measurements may be well-suited for use in dusty plasmas because the probes can be designed to be less perturbing to the dust particles than, for example, a Langmuir probe. Double-tipped, transmission-type impedance probes may be especially useful for dusty plasmas because they can be separately sensitive to plasma in areas near the probe tips and areas farther away. |
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CP11.00047: New Dust Charge Measurement Techniques for Dusty Plasmas Under a Strong Magnetic Field Dylan Funk, Uwe Konopka, Edward E Thomas Dusty plasmas consist of components typically found in a plasma (electrons, ions and neutral particles) as well as micrometer sized dust particles. The structural and dynamic properties of a dusty plasma system are governed by the dust particle charging state. As such the knowledge of the exact charging state of the individual dust particles is very important. Theories such as OML and ABR theories as well as modified versions of these have been used to determine dust charge value. Some recent experiments to determine particle charge indicate difference from theoretical models in the presence of a magnetic field. |
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CP11.00048: Photo-discharging: A path towards controlling dust in low-temperature plasmas Michael McKinlay, Saikat Chakraborty Thakur, Uwe Konopka, Edward E Thomas Charged microparticles (dust) suspended in low-temperature plasmas represent a tool for researchers and an unwanted contaminant for practical plasmas; the lack of independent control over the equilibrium charge of the dust particles is a significant obstacle to improving dust confinement or removal. A recent proof-of-concept test on the Auburn Dusty Plasma Experiment (DPX) combining Lanthanum Boride (LaB6) particles with a high-intensity UV source have demonstrated that photoelectric currents can significantly alter the equilibrium properties of dust; and that by tailoring the properties of the light source to the material properties of the dust and the apparatus, this control may be accomplished with minimal perturbation to the background plasma. Probe measurements of the background plasma and video analysis of the dust particles' response to the UV are presented. The potential of applying photo-discharging in research and practical plasmas is discussed. |
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CP11.00049: Excitation of Kelvin-Helmholtz instability in strongly magnetized low-pressure plasmas Mohamad Menati, Edward E Thomas, Uwe Konopka, Pintu Bandyopadhyay, Devandra Sharma The presence of strong magnetic fields in low-pressure plasmas results in the emergence of various phenomena including filamentation and Kelvin-Helmholtz (K-H) instability. It is observed in the experimental studies of magnetized electric discharges that K-H instability can appear in the bulk of the plasma as well as the sheath region near the walls of the plasma chamber. A 3D fluid model of the magnetized plasma is developed to study different aspects of this phenomenon. Numerical simulations using this model have shown that the presence of strong magnetic field results in a localized electric field in the bulk of the magnetized plasma. The localized electric field along with the gradient of density in the filamented plasma results in the drift of the ions across the magnetic field. This drift of the ions is responsible for the excitation of K-H instability in the bulk of the plasma. The electric field and gradient of the density naturally exist in the sheath region of electric discharges so, the presence of strong magnetic field is sufficient to cause the drift of the ions and electrons across the magnetic field. Experimental observation of the phenomenon along with results of the numerical simulations will be presented. |
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CP11.00050: Redistribution of Kinetic Energy in a Microgravity Complex (Dusty) Plasma Lori C Scott, Edward E Thomas, Uwe Konopka, Jeremiah D Williams, Saikat Chakraborty Thakur, Mikhail Pustylnik, Hubertus Thomas In the presence of gravity, the micron-sized charged dust particles in a complex plasma are compressed to thin layers, but under microgravity conditions, such as the Plasma Kristall-4 (PK-4) experiment on the International Space Station (ISS), the particles fill the plasma volume which allows the study of a 3D multi-particle system. When dust particles are injected into a dc glow discharge plasma they flow along an axial electric field until stopped by periodic oscillations of the electric field (polarity switching). This oscillation creates a change in the spatial ordering and thermal state of the particle system. |
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CP11.00051: Magnetized Dusty Plasma Rotation and its Sensitivity to g x B Natalija Marin, Nathan Eschbach, Carlos A Romero-Talamás Dusty plasma rotation with applied magnetic fields have been reported primarily from experiments using capacitively coupled plasmas or glow discharge, many of which cite E x B as the primary rotation driver. Observations of dust rotation using inductively coupled discharges with applied steady axial magnetic fields are presented. We investigate the different mechanisms that lead to rotation and, in some cases, vertical flow and convection cells. In particular, the sensitivity of rotation to the orientation of the magnetic field with respect to gravity is explored. The vacuum chamber and magnets are mounted on a pivoting mechanism that can be oriented with respect to the vertical plane. Preliminary data indicates that the threshold for rotation may be highly sensitive to the magnitude of g x B. Observations and measurements of dust dynamics at various angular displacements are presented. |
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CP11.00052: MFE: DIII-D TOKAMAK I
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CP11.00053: Upgrading DIII-D to Close Physics Gaps to Future Fusion Reactors Craig C Petty A proposed ten-year timeline of upgrades will allow DIII-D to reach its full potential to narrow the existing physics gaps to a Fusion Pilot Plant (FPP). Large strides in closing core-edge gaps can be made by increasing the elongation, triangularity and volume to allow higher current and density, which can be combined with a magnetic field rise to 2.5 T to achieve ITER-relevant pedestal collisionality and density simultaneously. Raising the heating power is also critical to attain high beta & current drive at the higher field & density, while new methods of FPP-relevant off-axis current drive will be investigated. A modular closed divertor will enable exploration of multiple dissipation concepts, including increased baffling for better core/edge isolation and an option for a long-legged divertor. New solid PMI materials can be rapidly prototyped using rapid tile change-out, robotics, DiMES and WITS to accelerate the theory/experiment validation cycle. Midplane 3D coils, 2D/3D power supplies, control upgrades and a disruption mitigation testbed are planned for more aggressive development of reactor transient solutions. With these new capabilities, DIII-D can address critical challenges to prepare for ITER and resolve the physics basis for a FPP. |
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CP11.00054: Highlights of Recent DIII-D Experimental Results Max E Fenstermacher Recent DIII-D experiments contributed to the ITER physics basis and to physics |
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CP11.00055: Transport and stability of the DIII-D qmin~1.5 scenario Kathreen E Thome, Deyong Liu, Craig C Petty, Christopher T Holcomb, Nikolas C Logan, Brian S Victor, Alexander F Battey, Jeremy M Hanson, Francesca Turco, Cami S Collins βN of 3.5–4 has been achieved in the DIII-D qmin~1.5 scenario using either on-axis neutral beam current drive (NBCD) or a mix of on- and off-axis NBCD. The off-axis NBCD is expected to broaden the plasma current and pressure profiles, resulting in stability and transport changes between plasmas with these different NBCD profiles. Both types of discharges are often terminated prematurely by n=1–3 tearing modes with n=1 modes being the most prevalent. However, DCON stability analysis of a discharge with off-axis NBCD finds that the n=2 ideal mode is the most unstable. These tearing modes are often triggered by bursty energetic particle modes (EPMs). Bursty n=1, 3 EPMs are often benignly present in these discharges but they are stronger and more frequent in discharges with off-axis NBCD. These discharges also have Alfvén eigenmodes (AEs) that cause fast-ion transport. However, AEs are reduced in the discharges with off-axis NBCD and the fast ion losses are closer to classical, as indicated by fluctuation diagnostics and neutron rate measurement. Local transport analysis using TRANSP and simulations using TGLF/TGYRO of these discharges will be presented. |
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CP11.00056: Comparison of n=1 and n=2 stability limits in high qmin discharges Brian Victor, Kathreen E Thome, Christopher T Holcomb As the plasma pressure profile broadens, the lowest $\beta_N$ stability limit transitions from an n=1 to n=2 mode. Recent upgrades of DIII-D have increased the off-axis neutral beam (NB) power to give a total of 8 MW of on- and 7 MW of off-axis NB power. The increased off-axis NB power broadens the pressure profile. Discharges using similar total NB power with a different ratio of on- to off-axis NB power are analyzed to determine the effect of the pressure profile on the $\beta_N$ stability limits. This analysis focuses on discharges with q$_{min}$>2, a steady-state scenario being developed on DIII-D. The pressure peaking factor, peak pressure over volume averaged pressure, is used to quantify the breadth of the pressure profile. The pressure peaking factor varies between 2-3 for these discharges. The DCON stability code is used to calculate the ideal-wall stability limits of these plasmas. The first modes that often occur in these plasmas are tearing modes, which occur when the experimental $\beta_N$ approaches the ideal-wall $\beta_N$ stability limit. Tearing modes are triggered typically within 20% of the ideal-wall $\beta_N$ stability limit. Discharges with more off-axis NB power are limited by n=2 tearing modes compared to discharges with more on-axis NB power limited by n=1 tearing modes. Thus the width of the pressure profile correlates with most unstable mode in the plasma. |
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CP11.00057: Impact of Fast-Ion Transport on Thermal Profiles in the DIII-D qmin>2 Steady-State Tokamak Scenario Cami S Collins, Eric M Bass, Christopher T Holcomb The viability of steady-state scenarios in ITER and future reactors relies on effective heating by energetic particles (EPs) to achieve high beta and bootstrap current. To clarify the impact of EP transport on thermal profiles, the TGLF-EP+Alpha critical-gradient model is used to calculate Alfvén Eigenmode (AE)-induced EP transport. Thermal ion and electron profiles are predicted using TGYRO with TGLF using the experimental density, rotation, and TRANSP power and particle sources. In the current ramp where AE-induced transport reduces core EP density by 50%, the T_i profile is overpredicted by up to 25% when input from classical TRANSP (no mode-induced transport) is used. However, the profile is accurately reproduced and the neutron rate matches measurements within 12% using TRANSP with EP diffusion calculated by TGLF-EP+Alpha. Thus, thermal profiles are accurately predicted by simply accounting for AE-induced changes to power fluxes and using thermal density consistent with transported EP density (i.e. does not include EP interaction with microturbulence). This result shows promise for applying TGLF-EP in integrated modeling for scenario development and reactor scoping studies. |
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CP11.00058: Pedestal behavior in DIII-D negative triangularity discharges Max E Austin, Alessandro Marinoni, Ruifeng Xie In DIII-D negative triangularity L-mode-edge discharges the pedestal region exhibits I-mode-like features; the Te profiles have an elevated pedestal height similar to H-mode while the ne profile is nearly the same as typical L-mode. In matched negative (NT) and positive triangularity (PT) L-mode plasmas with the same BT, Ip, and cross-sectional area, the Te pedestal at ρ=0.95 is 50% higher in NT over PT, and the NT Te gradient for 0.7 < ρ < 0.9 is 20% higher than the PT one. The resulting increase in electron pedestal pressure contributes to the higher stored energy in the NT compared to the PT discharges. Ion temperatures near ρ ~ 0.95 in the two cases appear to be about the same. No weakly coherent mode, as seen on C-MOD I-modes, has been detected in DIII-D NT discharges. In recent experiments in a lower diverted shape with upper negative triangularity a rapidly ELMing H-mode is obtained at relaxed delta (δupper = -0.2), and the pedestal height increases 60% compared to the stronger shaped (δupper = -0.4) case in L-mode. However, the global confinement remains the same in both cases, with H98(y2) = 1, indicating decreased core transport in the H-mode with relaxed negative triangularity. We explore the causes of this behavior. |
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CP11.00059: Characterization of the pedestal MHD in a negative triangularity plasma on DIII-D with Electron Cyclotron Emission Imaging Guanying Yu, Zeyu Li, Yuan Zheng, Yilun Zhu, Max E Austin, Gerrit J Kramer, Ahmed Diallo, N C Luhmann The Electron Cyclotron Emission Imaging (ECEI) system is used to characterize the edge MHD temperature fluctuations in a Negative Triangularity (NT, δ=-0.3) lower single null plasma discharge on DIII-D. The discharge exhibits small ELM-like crashes at ~ 400 Hz, a clear density pedestal with ne,ped ~ 3x1019 /m3 and a weak temperature pedestal with Te,ped ~250 - 300 eV. ECEI observed clear MHD turbulence fluctuations ( n=6 -17 at ρ~0.96) during the inter-ELMs and ELM crashes. The fluctuation amplitude is observed to increase by a factor of 2 from the inter-ELMs phase to the ELM-crash phase. The dominant mode number decreases from n~17 (25 kHz) to n~6 (9kHz) when the pedestal transitioned from the inter-ELM phase to the ELM-crash phase. The phase velocity of the mode is ~ 0.5 vion,diamagnetic /2+VExB at ρ~0.95-0.98, which is consistent with peeling-ballooning modes. Preliminary agreement of mode number and radial location is found between the linear BOUT++ simulation and the ECEI observation using the interELM profile and equilibrium. |
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CP11.00060: Comparison of Wide-pedestal Quescient H-mode Operation in Both Plasma Current Directions on DIII-D Xi Chen, Keith H Burrell, Tom H Osborne, Zeyu Li, Theresa M Wilks, Kshitish Kumar Barada, Carlos A Paz-Soldan, Wayne M Solomon, Huiqian Wang, Guanying Yu, Zheng Yan, Yilun Zhu Naturally ELM-stable, reactor relevant wide pedestal QH-mode has now been achieved with both directions of plasma current (Ip) on DIII-D. Wide-pedestal QH-mode (WPQH) was discovered in one direction of Ip using counter-Ip NBI torque ramp down in standard QH-mode plasmas. WPQH has been seen with a range of NBI torque [1] including zero net NBI torque throughout the discharge [2]. Experiments reported here with opposite sign of Ip confirm that the direction of the plasma current does not matter. Pedestal height, width, and overall confinement in both directions of Ip are equivalent. The carbon content is generally lower in WPQH plasmas using the typical Ip direction. Experiments with a slow torque scan from counter-torque (2-3Nm) to co-torque (3-4Nm) produced the transition from QH to WPQH and then to ELMy H-mode. Edge ExB shear in WPQH phase is lower than in the QH and ELMy H-mode phases, consistent with previous observations [1,2]. A low frequency (<100kHz) lab-frame IDD quasi-coherent mode is observed when the pedestal width significantly exceeds the EPED-KBM width. |
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CP11.00061: Influence of Isotope Mass on Core Turbulence and Transport Properties in Dimensionally Matched H-Mode Plasmas on DIII-D George R McKee, Kathreen E Thome, Zheng Yan, Kshitish Kumar Barada, Darin R Ernst, Nathan T Howard, Tomas Odstrcil, Tom H Osborne, Terry L Rhodes, Lothar W Schmitz Long-wavelength turbulence characteristics are found to differ significantly in dimensionally similar low-rotation hydrogen and deuterium ELM’ing H-mode plasmas, which may partially explain the large differences in transport. Spatiotemporal measurements of long-wavelength ion gyroscale density fluctuations are obtained with 2D Beam Emission Spectroscopy over ρ=0.4-0.8. The NBI and ECH-heated H and D discharges exhibited the “isotope effect” with confinement time about 80% higher in the deuterium (D) compared to hydrogen (H), though electron heating degraded D plasmas more significantly than H plasmas. H plasmas are observed to have lower integrated turbulence amplitude over 60-300 kHz: ñ/n|D~1.5*ñ/n|H. The measured turbulence radial correlation lengths, however, are larger in the H-plasma (~2.5 cm) compared to D (~1.5 cm), while the H and D poloidal correlation functions were significantly different: the D poloidal correlation function exhibited a unimodal broadband structure, while the H poloidal correlation function exhibited two distinct counter-propagating modes. Comparisons with gyrokinetic simulations will be performed to identify the underlying mechanisms for the isotope dependence of turbulence and transport. |
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CP11.00062: Investigation of Core Transport Barriers in DIII-D discharges with off-axis Te Profile peaks Ruifeng Xie, Max E Austin DIII-D discharges that transition to H-mode solely with off-axis electron cyclotron heating (ECH) often exhibit strong off-axis peaking of electron temperature profiles at the heating location. Electron heat transport properties near these off-axis temperature peaks have been studied using modulated ECH deposited at ρ ≈ 0.3. The Fourier analyzed electron temperature data has been used to infer electron diffusive and convective transport coefficients. Initial comparisons with the quasi-linear transport model TGLF find that the data is consistent with a narrow region with an electron diffusivity an order of magnitude lower than the average χe across the plasma, suggesting an electron internal transport barrier (ITB) just inside the heating location. Using precise equilibrium reconstructions created with the EFIT code with constraints from TRANSP modeling, the formation of these ITBs appear to be correlated with off-axis values of the minimum safety factor being near 1. The H-mode appears to be key for these unusual Te profiles, with the change in the current profile after the L-H transition leading to the q-profile-related barriers. |
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CP11.00063: Application of Kosuga Model to DIII-D Intrinsic Rotation Data Colin Chrystal, John E Rice A database covering 20 years of DIII-D intrinsic rotation measurements is used to test the Kosuga model[1], which was relatively successful at predicting the Alcator C-Mod intrinsic rotation[2]. This work aims to resolve differences between intrinsic rotation scaling at C-Mod and DIII-D, which has implications for predictions of ITER intrinsic rotation. There is some correlation between the predictions and the measurements, but multiple, roughly estimated O(1) factors complicate interpretation. It is shown that the basic scaling of the Kosuga model is the temperature gradient divided by the plasma current, which, within factors of O(1), is the same as predicted by T. Stoltzfus-Dueck and related to the well-established Rice scaling. Some resolution to the scaling with normalized gyroradius is shown, but uncertainties in O(1) factors, the range of agreement/ disagreement between measurements and predictions, and the generally low predicted rotations for ITER (a few krad/s) mean that ITER predictions are still a challenge. |
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CP11.00064: Edge power balance and TGLF/TGYRO-predicted quasilinear thermal fluxes in ITER-Similar-Shape DIII-D Plasmas near the L-H Transition Kyle Callahan, Lothar W Schmitz, Troy A Carter, Sterling P Smith, Shaun R Haskey, Colin Chrystal, Brian A Grierson, Max E Austin Recent experiments in ITER Similar Shape (ISS), low collisionality, low torque hydrogen plasmas near sawtooth-triggered L-H transitions have shown that the edge ion heat flux calculated by TRANSP is increased by a factor of ~3 compared to reference deuterium ISS plasmas, while edge electron heat flux is largely unchanged. This observation strengthens previous claims that the ion heat flux plays a dominant role in L-H transition physics [1]. Validation of L-mode edge thermal fluxes predicted by TGLF/TGYRO quasilinear gyrofluid simulations have identified dominant trapped electron mode (TEM) and electron temperature gradient mode (ETG) turbulence in both the deuterium and hydrogen plasmas at similar scale lengths. Additionally, TGLF has reproduced the experimental isotopic dependence on edge ion heat flux. TGYRO quasilinear simulations indicate that the predicted heat fluxes match the results from power balance analysis for both H and D plasmas using TGLF saturation rule 1 (a saturation model based on a Zonal flow mixing [2]). |
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CP11.00065: Preliminary results of density turbulence measurements using fast sweep reflectometry in DIII-D Lei Zeng, William A Peebles, Terry L Rhodes, Neal A Crocker, Rongjie Hong, George R McKee Recently, the capability of the state-of-the-art density profile reflectometer on DIII-D has been expanded to determine the radial profile of electron density fluctuations. Measurement of reflectometer phase fluctuations (f ≤ ~5 MHz) associated with O-mode launch during fast RF frequency sweep operation, has allowed both the wavenumber spectrum and radial profile of ne fluctuations to be analytically derived. The analyzed radial wavenumber range is from ~1.2 cm-1 to ~10 cm-1, which is limited by Bragg scattering condition. Preliminary results show that L-mode turbulence levels are small (δn/n ~ 1%) in the core (ρ < 0.6) when the normalized ne and Te gradients are small. However, as the gradients steadily increase with radius (ρ > 0.6), turbulence levels also steadily increase. The comparison with ne fluctuation measurements such as BES and DBS in various plasma conditions will be presented. Further investigation of this analysis will be also presented. |
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CP11.00066: Toroidal matching of DBS for higher wavenumber measurement and signal enhancement during turbulence measurement in DIII-D tokamak plasmas* Julius Damba, Rongjie Hong, Quinn Pratt, Terry L Rhodes We use experiments to demonstrate the dependence of signal strength from a Doppler backscattering (DBS) probe on its toroidal launch angle alignment, and thus optimize it for higher wavenumber (k) density turbulence measurement. With fluctuation k matching by toroidal steering of a DBS launch mirror, a spectrum of higher signal intensity is obtained. This increases the probe’s sensitivity to high-k (∼20 cm-1) fluctuations. For unmatched k, the measured DBS SNR is small, making it difficult to measure high-k turbulence. Since DBS has been used mainly for low to medium wavenumber density fluctuation and flow measurement in magnetically confined fusion plasmas, this extra toroidal matching of fluctuation k values will be critical to understanding high-k turbulent transport in fusion relevant research at DIII-D. The density fluctuation power is derived from the measured scattered wave power, and the Doppler shift of the scattered wave is used to determine the propagation velocity of turbulent structures. For the k matching, the 3-D ray tracing code GENRAY is used, which takes as input, parameters from other diagnostics such as profile reflectometry, Thomson scattering, and the EFIT plasma equilibrium code. |
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CP11.00067: Machine learning models for real-time, high bandwidth inference of ELM events and confinement regime with 2D BES at DIII-D Lakshya Malhotra, Prannav Arora, George McKee, David R Smith, Zheng Yan, Mark D Boyer, Ryan Coffee, Azarakhsh Jalalvand, Egemen Kolemen Multi-channel fluctuation diagnostics capture the spatial patterns of high-bandwidth plasma dynamics. Here, we report on an effort to develop machine learning (ML) models for the real-time identification of edge-localized-mode (ELM) events and the turbulence properties of confinement regimes using the 2D Beam Emission Spectroscopy (BES) system at DIII-D. The "edge ML" models will be deployed on a high-throughput FPGA accelerator for integration in the real-time plasma control system (PCS). The models will generate reduced signals that correspond to ELM activity and turbulence dynamics, and the real-time PCS will learn to avoid ELM regimes and to steer the plasma towards and maintain advanced confinement regimes such as the wide pedestal QH-mode. The 2D BES system captures plasma density perturbations imprinted in neutral beam emission at a 1 MHz frame rate. The edge ML models will analyze about 10 ms histories from the BES data stream to assess ELM and turbulence activity. Preliminary results for classifying active ELM events give a ROC-AUC score of about 0.98 for validation data. We also explore different neural network architectures such as autoencoders to compress spatio-temporal information in low-dimension feature space for multiple classification and prediction tasks. |
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CP11.00068: New cross-channel calibration for mm-wave scattering diagnostics at the DIII-D tokamak Quinn Pratt, Kshitish Kumar Barada, Terry L Rhodes The design and development of a new cross-channel calibration circuit for mm-wave scattering diagnostics at the DIII-D tokamak is presented. This calibration circuit is designed to measure the launch and receive power of mm-waves used by the Doppler Back Scattering (DBS) diagnostic [1]. The DBS system uses 8 simultaneous fixed-frequency channels (55-75 GHz) to make local measurements of plasma flow and density fluctuations. Reliable and frequent cross-channel calibration data simplifies comparing fluctuations at different plasma locations. The relative power between each systems' channels is recorded throughout plasma discharges using swept YIG filters and diode power-meters. The YIG filters are used to isolate the 8 channels of the DBS system. Data characterizing the performance and capabilities of the new system is presented. Plasma measurements with and without calibration are contrasted. |
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CP11.00069: Electron temperature turbulence behavior during sawtooth oscillations in DIII-D Guiding Wang, Terry L Rhodes, William A Peebles, Neal A Crocker, Rongjie Hong, Max E Austin, Michael A Van Zeeland Sawtooth oscillations are a relaxation phenomenon associated with magnetic reconnection in the central region of the plasma that are triggered by an instability near the q=1 magnetic surface. This work reports on recent experimental results of the electron temperature (Te) turbulence behavior during sawtooth oscillations in DIII-D, measured by a radial 8-channel array correlation ECE (CECE) radiometer. The radial coverage is typically ~10-15 cm (ρ~0.2-0.3). Compared to the time right before a sawtooth crash, Te turbulence amplitude decreases inside but increases outside the q=1 radius immediately after the crash. Te turbulence amplitude is at the system noise level in an inner part of the magnetic reconnection region, which has a flat electron temperature profile. At the sawtooth crash, a radially outward propagation of Te turbulence is observed. During the ramp (core temperature rising) phase of a sawtooth cycle, the Te turbulence amplitude gradually increases until the next crash, with a larger change at inner locations that scale with the inverse Te scale length. These observations combined with data from magnetic probes and linear stability turbulence simulations will be presented. |
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CP11.00070: A Transport Solution for Plasma Fluid Equations that Conserves Particles, Momentum, & Energy Earl W DeShazer, Weston M Stacey, Sterling P Smith A predictive transport computational model that solves a recently postulated form of the plasma fluidequations that conserves particles, momentum, and energy is presented. The transport equations were proposed to capture the radial profiles resulting from neutral sources thatinsert particles, momentum, and energy;ion orbit loss,which removes particles and their associated momentum and energy in a non-diffusive manner;and theequilibrium field conditions of toroidal and poloidal rotation velocities, plasma pressure, density, and temperature. The equations are first order, coupled, non-linear equations, resulting fromthe long-rangeLorentz (V×B) and electric field (??) forces.A major contribution of this research is the application of the multivariant solver,Broyden'smethod. It is optimal because it is appropriate for non-linear equations, approaches the convergence speed of a Newton-Raphson solver, and is more computationally efficient because it uses a secant method update strategy for approximating the Jacobian. The predictive performance of this approach will becompared against a variety of shots including L-, H-, and Super H-modes,some withResonant Magnetic Perturbations applied,and some with negative triangularityshape, the results of which will bepresented. |
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CP11.00071: Interpretation of Experimental Transport Coefficients Jonathan Roveto, Weston M Stacey, Richard J Groebner Inferred experimental particle diffusion and energy conduction coefficients for several DIII-D discharges in various confinement regimes are computed with a model that corrects for non-diffusive and non-conductive particle and energy loss effects. This work corrects the total ion experimental energy flux for thermal and rotation energy convection, for the work done by the flowing plasma against the pressure and viscosity, and for the ion orbit loss of particles and energy. These loss mechanisms are comparable to the diffusion and conduction losses [W.M. Stacey, Phys. Plasmas 21, 042508 (2014)]. These corrections result in an up to 50% reduction in the amount of ion heat transport interpreted as conductive heat transport in the plasma edge. The changes in the interpreted heat conduction is substantial when correcting for thermal energy convection, viscous heating, and ion orbit loss (with ion orbit loss dominating in the far edge) but are insignificant when correcting for the convection of rotational energy. A comparison with several transport theories is also provided. |
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CP11.00072: Cold Electron Effect from Neutral Beam Injection on DIII-D Bingzhe Zhao, Max E Austin Electron Cyclotron Emission (ECE) measurements show clear evidence of cooling of Te by cold electrons from neutral beam injection (NBI) and thus provide a direct experimental indicator of beam deposition, which is normally only available through Monte Carlo (MC) methods. Due to the short thermalization time (~0.1 ms) and low temperature (~10 eV) of beam electrons, cooling effect can be observed at the start of NBI pulses. Its magnitude is proportional to the beam electron source rate and becomes evident around rho=0.5 in high Te plasma, where Te drops ~1-10 keV/s for ~10-100 ms. The beam source rate calculated from the cooling rate is compared with predictions from NUBEAM. Deposition profiles from cooling and MC are both Gaussian-like curves peaked at the magnetic axis. However, the deposition from cooling rate is about 2-3 times the MC calculation at rho=0 but drops off about twice as fast, and the total electron source rate is about 20% lower. Lower total source is likely due to missing cooling measurements near the edge, and possible physics behind the discrepancy in the core region is presented. In conclusion, this method has potential to provide a direct experimental measurement for NBI core deposition. |
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CP11.00073: Isotope Impact on Alfvén Eigenmodes and Fast Ion Transport in DIII-D Michael A Van Zeeland, Colin Chrystal, Xiaodi Du, Deyong Liu, Kathreen E Thome, Genevieve H DeGrandchamp, William W Heidbrink, Eric M Bass, Cami S Collins, Neal A Crocker, Shaun R Haskey, Guanying Yu, Yilun Zhu Measurements of beam driven Alfvén eigenmode activity in matched deuterium and hydrogen DIII-D plasmas show a dramatic difference in unstable mode activity for a given injected beam power. The dependence of the unstable AE spectrum on beam and thermal species is investigated in the current ramp by varying beam power in a sequence of discharges for fixed thermal and beam species at fixed density. In general, a spectrum of reversed shear Alfvén eigenmodes (RSAEs) and toroidal Alfvén eigenmodes (TAEs) are driven unstable with sub-Alfvénic deuterium beam injection while primarily only RSAEs are driven unstable for the hydrogen beam cases investigated. Further, for a given beam power, the driven AE amplitude is always reduced with hydrogen beams relative to deuterium and for hydrogen thermal plasma relative to pure deuterium or mixed deuterium/hydrogen plasmas. Estimates of the fast ion stored energy indicate that the dominant mechanism contributing to the difference between hydrogen and deuterium beam drive is the faster classical slowing down of hydrogen beam ions relative to deuterium and the resultant lower beam ion pressure – an effect which dominates the expected increase in drive due to higher hydrogen beam ion velocities. |
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CP11.00074: Stability of sub-cyclotron Alfvén eigenmodes in mixed species plasmas Jeff B Lestz, Genevieve H DeGrandchamp, Stephen T Vincena, Neal A Crocker, Kathreen E Thome, William W Heidbrink Sub-cyclotron compressional (CAE) and global (GAE) Alfvén eigenmodes are routinely excited by fast ions in spherical tokamaks, and have more recently been observed in conventional aspect ratio tokamaks such as DIII-D and ASDEX Upgrade. Similar instabilities are also responsible for electromagnetic ion cyclotron (EMIC) waves in space plasmas, where the presence of multiple ion species explains the observed frequency bands of propagation and evanescence. In order to aid interpretation of high frequency magnetic measurements in recent DIII-D isotope experiments (H/D/3He), an expression for the local fast ion drive which accounts for multiple ion species has been derived for neutral-beam-driven CAEs and GAEs. Approximate analytic instability criteria are found which constrain the unstable mode frequencies as a function of fast ion properties (beam injection geometry and velocity). The dependence of the linear growth rate on relevant parameters is explored numerically and compared with experimentally measured trends in mode activity. The described stability analysis represents a complementary approach for experimental mode identification and characterization of the resonant subpopulation of fast ions. |
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CP11.00075: FIDA/FIHA signals from mixed deuterium/hydrogen beams in mixed species plasmas. Garrett Prechel, Genevieve H DeGrandchamp, William W Heidbrink, Luke Stagner, Michael A Van Zeeland, Shaun R Haskey Fast-ion D-alpha and H-alpha spectra are measured during experiments that assess the isotope dependence of fast-ion driven instabilities in both the Alfven and ion-cyclotron frequency bands. Hydrogen and deuterium neutral beams are injected into both hydrogen and deuterium thermal plasmas. With hydrogen injection, measured Doppler shifts are similar to deuterium injection (for a typical case). To model these plasmas, FIDASIM is upgraded to treat mixed thermal species. TRANSP calculations that utilize the measured species mix predict the fast-ion distribution. The spectra predicted by TRANSP/FIDASIM are compared with the data for cases with and without fast-ion driven instabilities. |
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CP11.00076: Novel internal measurements of ion cyclotron frequency range fast-ion driven modes Neal A Crocker, Kathreen E Thome, Amani Zalzali, Richard Dendy, William A Peebles, Kshitish Kumar Barada, Rongjie Hong, Terry L Rhodes, Guiding Wang, Lei Zeng, Genevieve H DeGrandchamp, William W Heidbrink Novel internal measurements and analysis of ion cyclotron frequency range fast-ion driven modes in DIII-D are presented. Observations, including internal density fluctuation ($\tilde_{n}$) measurements obtained via Doppler Backscattering, are presented for modes at low harmonics of localized in the edge. The measurements indicate that these waves, identified as coherent Ion Cyclotron Emission (ICE), have high wave number, consistent with the cyclotron harmonic wave branch of the magnetoacoustic cyclotron instability (MCI), excited by Doppler-shifted cyclotron resonance (DCR) with fast ions, or electrostatic instability mechanisms. Measurements show extended spatial structure (up to ~ 1/6 the minor radius). These edge ICE modes undergo amplitude modulation correlated with edge localized modes (ELM) that is qualitatively consistent with expectations for ELM-induced fast-ion transport. |
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CP11.00077: Characterization of ion cyclotron emission in L- and H-mode plasmas in the DIII-D tokamak Genevieve H DeGrandchamp, Neal A Crocker, William W Heidbrink, Jeff B Lestz, Robert I Pinsker, Kathreen E Thome, Stephen T Vincena Coherent ion cyclotron emission (ICE) has been observed on many fusion devices but requires more detailed experimental understanding before it can be leveraged as a fast ion diagnostic for future machines. A dedicated experiment on the DIII-D tokamak explored the dependence of ICE at the deuterium cyclotron frequency and its harmonics on plasma current, outer gap, and fast ion distribution in both L- and H-mode plasmas. The recently upgraded ICE diagnostic measures frequencies in the 1-100 MHz range and has expanded its measurement capabilities to include toroidal mode number and approximate polarization at the edge. Centrally-localized ICE sensitivity to fast ion distribution was explored by using seven different neutral beam injection configurations in L-mode plasmas. Changing the counter-current injecting beams from on- to off-axis, for example, drives significantly reduced levels of ICE at different harmonics. Detailed measurements in L-mode plasmas will be useful in future modeling efforts and comparison to the more commonly observed edge-localized ICE in H-mode plasmas. ICE in both L- and H-mode plasmas are sensitive to changes in the separation between the plasma and the outer wall. |
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CP11.00078: Fast Ion Losses Driven by Applied Magnetic Perturbations and Plasma Response on DIII-D Kenneth R Gage, Xi Chen, William W Heidbrink, Michael A Van Zeeland, David C Pace, Brendan C Lyons, Jeremy M Hanson, Joaquin Galdon-Quiroga, Manuel Garcia-Munoz, Gerrit J Kramer Externally applied resonant magnetic perturbations (RMPs) on DIII-D alter fast ion confinement, leading to an increase in prompt losses from beam ions born inside the last closed flux surface (LCFS). Experimentally, the light ion beam probe (LIBP) technique [1] is used to infer fast ion orbit displacement using signal from the midplane fast ion loss detector (FILD) during application of n=1 and n=2 RMPs. The applied n=1 RMPs analyzed are rigidly rotated around the vessel at a set phase offset between the upper and lower internal coils △φUL. Plasma response to RMPs with △φUL=0 increases with plasma β, and is strongest when the RMP is coupled with an internal kink. Simulations of these shots in ASCOT5, using the plasma responses calculated by M3D-C1, show high concentrations at the midplane of losses induced by RMPs. With n=2 RMPs, a continuous scan through △φUL was achieved by holding the lower coil perturbation constant while the upper coil current was rotated. Experimental losses at the midplane vary with △φUL as does the simulated response. |
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CP11.00079: Hybrid kinetic-MHD modelling of observed fast ion migration induced by Alfvénic instabilities Javier Gonzalez Martin, Xiaodi Du, Michael A Van Zeeland, Yasushi Todo, William W Heidbrink In recent experiments conducted in the DIII-D tokamak, an imaging neutral particle analyzer (INPA) [1] measures fast ion transport across a broad range of local phase space in the presence of multiple Alfvén Eigenmodes with unprecedented phase space resolution. The hybrid kinetic-MHD MEGA code [2] is used to reproduce the radial location and frequency of the modes observed in the experiment, as well as the associated fast ion transport. At the INPA-interrogated pitch angle, reversed shear Alfvén eigenmodes (RSAE) are found to create phase-space islands near the location of minimum safety factor q on the low-field side of the plasma. On the high-field side, the islands cover the boundary between passing and stagnation orbits near the axis, explaining the observed prompt pile-up of the fast ions in the plasma core, which are accelerated to higher energies than the injection energy of neutral beams by a few keV. Multi-phase simulations are performed combining sequential phases of purely kinetic with hybrid modelling, including realistic neutral beam injection and collisions. These simulations cover the entire fast ion slowing-down period, enabling a direct comparison with the time-resolved phase space transport dynamics observed by the INPA. |
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CP11.00080: Role of Beta-Induced Alfvén Eigenmode in DIII-D high βp scenario Xiang Jian, Christopher G Holland, Eric M Bass, Siye Ding, Joseph Mcclenaghan, Andrea M. M Garofalo Analysis of DIII-D high βp plasmas (A. Garofalo, Nucl. Fusion, 2015) has identified beta-induced Alfvén eigenmodes (BAEs) existing in the outer regions of the plasma, between the foot of the large radius internal transport barrier (ITB) and top of the edge pedestal. While recent analysis (X.Jian, Phys. Rev. Lett, 2019) has shown that micro-tearing modes regulate residual transport in the ITB, the processes regulating transport in other regions of the plasma remain less clear. The BAEs are observed to have a frequency of ~50 kHz (~0.3 ) in the lab frame, and toroidal mode numbers of 3-4. Their amplitude is inversely correlated with the fast ion confinement level, and thermal confinement to a lesser level. Previous studies have predicted the kinetic ballooning mode (KBM) to be unstable in the same region as the BAEs (G. Staebler, Phys. Plasmas, 2019). Gyrokinetic analysis indicates that even a small fast beam ion fraction (<1%) is able to excite the BAE, which couples strongly to KBM. Calculations of fast and thermal particle transport expected to be driven by these BAEs are underway and will be reported. Implications for further optimization of the high scenario, in terms of considerations such as first wall protection from BAE-induced fast ion transport, are discussed. |
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CP11.00081: Analysis of RSAE Radial Phase Variation in DIII-D Discharges William W Heidbrink, Erik C Hansen, Javier Gonzalez Martin, Michael A Van Zeeland, Nikolai Gorelenkov, Gerrit J Kramer, Donald A Spong, Max E Austin The radial phase variation of the eigenfunction of reversed shear Alfven eigenmodes (RSAE) depends upon the plasma shape. The eigenfunctions of RSAEs often exhibit radial variations in phase,1 an effect that can be caused by fast-ion gradients,1 symmetry breaking,2 and radial energy flows.3 The phase change at the outer peak of the eigenfunction is measured by an electron cyclotron emission radiometer diagnostic and entered into a database. The measured phase change correlates more strongly with “gapin” than with any other parameter, with a clear difference between diverted and limited discharges. (“Gapin” is the spacing between the inner wall and the last closed flux surface.) Representative DIII-D cases are analyzed by NOVA-K, MEGA, and FAR3d and compared with the data. |
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CP11.00082: Characterization of Energetic Particle-Induced Geodesic Acoustic Modes on DIII-D Daniel J Lin, William W Heidbrink, Michael A Van Zeeland, Raffi M Nazikian The energetic particle-induced geodesic acoustic mode (EGAM) is predominantly an electrostatic n=0, m=0 mode which can cause losses of injected beam ions. On DIII-D, the low frequency mode often appears with the injection of the counter-current, on-axis neutral beam between 20-40 kHz. Mode frequency and amplitude are studied using a database of DIII-D shots containing EGAM activity during the current ramp stage of the discharge. Initial observations show that the EGAM frequency scales with plasma current and density while the amplitude scales inversely with them. Additionally, a single discharge containing EGAM activity, with no other significant modes, is examined in more detail. While the EGAM density fluctuation is dominantly an m=1 mode, vertical line integrated measurements from the CO2 interferometer suggests that contributions from other poloidal harmonics are significant. Measurements from the magnetic probes show that the average period between each successive EGAM burst is found to slightly increase as the discharge evolves during the current ramp. |
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CP11.00083: MHD-kinetic hybrid modeling of low-frequency bursting modes in DIII-D high-beta high-qmin scenario Deyong Liu, Yueqiang Liu, Kathreen E Thome, Javier Gonzalez-Martin, William W Heidbrink, Christopher T Holcomb, Michael A Van Zeeland Low frequency bursting modes with frequency chirping are often observed in high-beta, beam heated DIII-D plasmas, with plasma pressure being close or even slightly exceeding the no-wall Troyon limit for the onset of n=1 ideal external kink instability. Despite the high central safety factor (qmin~1.5), bursting modes appear with many features similar to classical (1,1) fishbones. The difference is that the bursting mode often peaks near the q=2 surface. These off-axis fishbone-like modes and the tearing modes induced by them often prevent sustainment of the high-performance phase. Frequency chirping and a sudden drop in neutron rate suggest a strong interaction between the mode and fast ions. MHD-kinetic hybrid codes MARS-K and MEGA are utilized to investigate the mode stability and associated fast-ion transport. Initial analysis suggests that the bursting mode is an energetic-particle driven branch of the external ideal kink/resistive wall mode. The mode is excited by the presence of a sufficiently large fraction of trapped fast ions, with the mode frequency linked to the precessional drift frequency. The effect of the resistive wall, plasma toroidal rotation, anisotropy of the fast-ion distribution, and drift kinetic damping associated with thermal particles will be discussed. |
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CP11.00084: A heterodyne Phase Contrast Imaging diagnostic for detection of Ion Cyclotron Emission and Helicon waves Alessandro Marinoni, Charles Moeller, Chris C Rost, Miklos Porkolab, Severin S Denk, Robert I Pinsker The Phase Contrast Imaging (PCI) diagnostic on DIII-D has been upgraded with a novel optical heterodyne scheme to detect Ion Cyclotron Emission (ICE) and Helicon waves. The PCI, an absolutely calibrated interferometer that creates an image of line-integrated electron density fluctuations, is uniquely able to measure the internal structure of such waves thus extending the purely temporal measurements that are otherwise typically used. Arrays of cryogenically cooled detectors used by worldwide PCI systems operate within a 2 MHz bandwidth which, although suitable for broadband turbulence, preclude the study of faster phenomena. The laser beam power is modulated in such a way that the frequency of the wave of interest is within the PCI detector bandwidth, thus making the imaging method applicable at higher frequencies. A transverse Pockels cell made of a water cooled CdTe birefringent crystal, driven by a matched oscillator providing 2 kV pk-pk voltage, is used for the beam modulation. Bench-top tests using a fixed 10 MHz oscillator are in agreement with the expected response. Calibration data will be presented using both a variable frequency oscillator to detect ICE at various values of the confining magnetic field (18-32 MHz), and a fixed frequency oscillator locked to the 476 MHz high power Helicon antenna. |
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CP11.00085: Local helicon wave measurements in DIII-D via Doppler Backscattering Satyajit Chowdhury, Neal A Crocker, William A Peebles, Terry L Rhodes, Bart G Van Compernolle, Michael W Brookman, Robert I Pinsker, Cornwall H Lau, Larry Bradley, Roman Lantsov Experiments have been performed in DIII-D to develop high power (1 MW) 476 MHz helicon waves as a current drive tool for future fusion reactor plasmas. A prototype Doppler backscattering system to measure the helicon wave amplitude and spatial distribution for validation of helicon propagation and absorption models is being tested. It launches a fixed frequency beam (60–80 GHz range), simultaneously measuring turbulent and radio-frequency (RF) wave density fluctuations. During ~ 300 – 500 kW helicon injection, broadband fluctuations are observed in a ~ 2 MHz band around 476 MHz in L-mode and H-mode plasmas. Preliminary analysis shows the waves are localized near ~ 0.5, with < 6 cm-1. In ELMy H-modes, their amplitude is seen to diminish significantly around the time of the ELM crash. The RF frequency range is adjustable, which allowed the system sensitivity to be tested via measurement of ion cyclotron harmonic waves (20–40 MHz) spontaneously excited by neutral heating beams. The new system demonstrates significant improvements in sensitivity over previous efforts to measure these waves [N. A. Crocker, IAEA FEC 2020]. A synthetic diagnostic model is under development for interpretation measurements. |
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CP11.00086: Scenario development for the measurement of Helicon waves with Phase Contrast Imaging on DIII-D Severin S Denk, Cornwall H Lau, Alessandro Marinoni, Robert I Pinsker, Miklos Porkolab, John C Rost An optimized plasma scenario has been developed to validate the predictions for Helicon waves in DIII-D[1] by the All ORders Spectral Algorithm (AORSA) full wave code[2] against measurements from a phase contrast imaging (PCI) diagnostic. AORSA includes the hot-plasma effects needed to compute the density perturbations for the synthetic PCI diagnostic in the helicon wave frequency regime. Modeling of the PCI measurement requires prediction of the wave path in 3D as it propagates 120 degrees around the torus and inward toward the plasma center. This is accomplished by stacking 11 2D AORSA calculations for different toroidal modes. COMSOL is used to model coupling and propagation through the scrape-off layer and provide a boundary condition represented as a synthetic antenna in AORSA. Analysis with a synthetic diagnostic will be presented showing that if the intersection between the Helicon and the PCI beam occurs at the midplane of the plasma, the predicted signal level is orders of magnitude larger than the expected turbulent density fluctuation background. Optimizing the discharge for PCI measurement of the Helicon wave is consistent with previously achieved DIII-D parameters. |
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CP11.00087: A Scrape-off Layer Electron Temperature and Density Profile Diagnostic at the Helicon Antenna at DIII-D* Edward T Hinson, Barret Elward, Erik R Flom, David Su, Bart V Compernolle, Michael W Brookman, Andrea M. M Garofalo, Robert I Pinsker, Oliver Schmitz Electron density in the scrape-off layer plays an important role in coupling power from a helicon wave antenna into tokamak plasma. At DIII-D, the scrape-off layer (SOL) in front of the helicon antenna sits behind multiple limiting structures, and is likely to possess more than a single decay length. Direct measurements of profiles of electron temperature and density in this region using inverted helium atomic line emission from helium puffs will be obtained with a new helium thermal beam diagnostic installed at DIII-D. Measurements on field lines running in front of the antenna from locations above and below the antenna will enable measurements in reverse and forward toroidal field directions. Local gas feeds will enable fast duty cycling of the helium (~<10ms) and correspondingly fast measurements. Modeling of the expected signals using magnetic reconstructions of nominal target discharges, including helicon fiducial discharges, shows measurements up to the separatrix should be possible for a range of conditions. This new capability will enable research into the dynamics of the SOL conditions during helicon operations and improved knowledge of the physics of coupling of helicon power into the plasma. |
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CP11.00088: Design and Status of a High Field Side Reflectometer on DIII-D Evan Leppink, Cornwall H Lau, Yijun Lin, Stephen J Wukitch A high field side (HFS) reflectometer will provide measurements of plasma density in the HFS scrape-off-layer (SOL) of the DIII-D tokamak. This diagnostic is in support of the upcoming HFS lower hybrid current drive (LHCD) experiment on DIII-D. The coupling of LHCD power to the core of the plasma for efficient current drive is extremely sensitive to the plasma conditions in the SOL local to the LHCD launcher. Reflectometry allows for measurement of the plasma density in the region of the LHCD launcher with high temporal and spatial resolution. These results will provide key insights into the study of LHCD coupling and LHCD-SOL interaction for the novel HFS LHCD experiment scheduled for operation in FY23. The reflectometer will operate in the O-Mode configuration in the 6-27 GHz range and is planned to be installed in early FY22. To compare with experiment, full-wave simulations of the HFS SOL and LHCD launcher have been performed in Petra-M, and these results are then coupled to the GENRAY/CQL3D package for core physics. This allows for the accurate simulation of the entire LHCD process from launcher to core current drive. An overview of the HFS reflectometer's design as well as the latest simulation results and current installation progress will be presented. |
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CP11.00089: DIII-D High Field Side Lower Hybrid Current Drive Experiment: Overview and Status Stephen J Wukitch, Evan Leppink, Yijun Lin, Andrew Seltzman, Christopher T Holcomb, Robert I Pinsker A high field side lower hybrid current drive (HFS LHCD) system is scheduled for operation in FY23. A 2 MW klystron source has been installed and power testing into water loads will be utilized to verify system operation. The waveguide network is also in progress. The remaining challenges are associated with the in-vessel waveguides and coupler. The waveguides enter on the low field side and follow the vacuum vessel contour up to the center post. The coupler is a compact multijunction with a traveling wave poloidal splitter. The splitter utilizes imbedded RF elements to ensure proper power splitting and minimize reflection. These detailed RF elements were enabled by additive manufacturing techniques and the expected disruption loads and 400C bake compelled the use of a high strength copper alloy, GRCop-84. The system is designed to drive current non-inductively in $\rho\sim$ 0.6 - 0.8, with the driven current density $\sim$ 0.4 MA/m$^2$ in DIII-D AT discharges. The HFS launch position was selected to improve wave penetration and ensure off-axis, single pass absorption. From simulations, good wave penetration is achieved as a result of negligible wavenumber shift until reaching the damping region. The latest simulations, design and system status will be presented. |
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CP11.00090: Equilibrium reconstruction using Faraday-effect polarimetric measurements on DIII-D Thomas E Benedett, Ryota Yoneda, Jie Chen, David L Brower, Lang L Lao, Joseph Mcclenaghan, Wenjun Ding Measurements from the Radial Interferometer-Polarimeter (RIP) on DIII-D provide new constraints (three chords of Faraday-effect and density) into the plasma internal magnetic profile. Integration of these measurements into the EFIT Grad-Shafranov equilibrium fitting code will add further constraints on the targeted equilibrium and enable more accurate reconstruction of the plasma behavior, especially for plasmas where MSE is not available. The ongoing implementation of these constraints into EFIT is presented, as are results of the modified equilibrium fitting, demonstrating the influence of the inclusion of RIP measurements on the calculated equilibria. Comparisons between RIP and MSE constraints for equilibrium construction are presented, including on measures of q0 on-axis and on evolution during discharge, as are measures of uncertainty and explorations of scenario dependence. As Faraday-Effect polarimetry will be used on ITER, understanding its application will be important for diagnosis and control of future burning plasma reactors. |
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CP11.00091: Thomson Scattering Measurements Using In-vessel Optics in the DIII-D Small Angle Slot Divertor Fenton Glass, Thomas N Carlstrom, Adam G McLean, Rejean L Boivin Divertor Thomson scattering using in-situ optics has been extended to the DIII-D Small Angle Slot (SAS) divertor to characterize plasma dissipation phenomena, measure cold dissipative conditions with Te <1 eV near the target surface, and provide critical data for validation of boundary modeling. The SAS divertor geometry is particularly challenging for obtaining Thomson scattering measurements. The tight enclosure, situated behind a protrusion in the wall boundary, precludes typically used ex-vessel collection optics from accessing the magnetic field strike point region, a vital measurement position for evaluating the performance of this divertor configuration. In-vessel lenses and optical fiber bundles have been installed to collect Thomson-scattered light to a vacuum feedthrough, where additional ex-vessel fiber bundles transport the light to remotely located polychromators for spectral analysis. The lenses and fiber bundles are mounted beneath modified wall tiles and are designed to withstand the 375°C vessel baking process. Stray laser light is mitigated through the viewing angles of the lens-fiber system. First results from SAS divertor plasma experiments, including measurement of sub-eV electron temperatures and consistency between neutral pressures, Langmuir probes, and volumetric spectroscopy, are presented. |
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CP11.00092: Non-ideal effects and spectral response of Phase Contrast Imaging Chris C Rost, Alessandro Marinoni, Miklos Porkolab The response of Phase Contrast Imaging (PCI) [1] to long-wavelength fluctuations is enhanced by non-ideal effects, as shown in laboratory tests and measurements on the DIII-D tokamak. An optical system on a large plasma experiment is subject to misalignment caused by mechanical vibrations, j×B forces, and diffraction due to both the bulk plasma density and MHD instabilities and exacerbated by long or convoluted beam paths. PCI, which is sensitive to low-frequency deflections of 0.5 mrad, may require active beam steering to compensate [2]. Experimental tests now allow a quantitative calculation of the PCI effective response vs. wavenumber in the presence of small vibrations, showing potentially a large response to plasma fluctuations well below the PCI low-k cutoff (~1cm-1 on the DIII-D PCI). Enhanced sensitivity also results from beam diffraction by MHD modes, which causes beam deflections of order 10 μrad, as confirmed by the recently-added PCI interferometer channel. These results provide criteria for vibration compensation in the design of PCI systems for large-scale plasma devices, especially for future PCI operating with shorter laser wavelengths. |
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CP11.00093: Update on a Scanning, 2-Dimensional Divertor Thomson Scattering System on DIII-D Paul Schroeder, Fenton Glass, Adam G McLean, James Kulchar, Thomas B O'Gorman, Matthias Watkins, Douglas A Taussig, Thomas N Carlstrom, Rejean L Boivin We present recent upgrades to enable scanning, 2-dimensional divertor Thomson scattering (DTS-2D) measurements of Te and ne in DIII-D. This system expands the current capabilities of the existing lower divertor floor measurement location by introducing 7 additional laser beam paths in the poloidal plane, covering R=954 mm to R=1335 mm at 12 vertical positions from Z=-1.35 m to Z=-1.13 m. The system will redirect a ~1J, 50Hz, Nd:YAG laser pulse to a new beam path within 20 ms, continuously scanning through all 8 positions each 160ms period during a plasma shot. Up to 12 measurements are available at each beam position by dynamically refocusing the collection fiber array using a high-speed linear stage. The 8 beam paths are accessed by redirecting the laser using an ex-vessel, fast-steering mirror to 1 of 8 in-vessel prisms mounted underneath the tiles. A fixed, in-vessel mirror with a centered hole allows the laser to pass through, retaining measurements above the divertor shelf. This work focuses on system design, specifications, and initial results from the recent installation. Initial measurement work will be to investigate the differences between 2D maps made from sweeping a diverted plasma across a single Thomson scattering beam path and a stationary plasma measured with DTS-2D. |
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CP11.00094: Comparing local plasma density and temperature evolution during sawtooth crash events in DIII-D Dingyun Liu, William R Fox, Sayak Bose, Zheng Yan, George R McKee, Aaron Goodman, Hantao Ji, Valentin Igochine, Yilun Zhu, Stephen C Jardin, Nathaniel M Ferraro The sawtooth crash in fusion plasmas leads to a fast drop in core electron temperature, and several models have been proposed for the fast temperature relaxation, including fast magnetic reconnection provided by two-fluid effects or the plasmoid instability, or growth of higher-mode-number pressure driven instabilities. Experiments were conducted at DIII-D through the Frontier Science program where the Beam Emission Spectroscopy (BES) system was used for 2-D localized density measurements near the inversion layer (q=1 surface) during sawtooth. We analyzed the BES data by doing channel cross-calibration and subtraction of edge light [Bose et. al. to be submitted to Rev. Sci. Instrum. (2021)] to obtain time-domain movies of the plasma density evolution. The observations show that a (1,1) structure with high plasma density grows just before the temperature crashes and persists for a few plasma rotations afterward. We correlated the evolution of localized plasma density (BES), temperature (ECEI), and magnetic field (from external magnetics) in time to understand the physics of sawtooth crash. Initial MHD simulation results to complement the experimental finding are presented. |
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CP11.00095: Two-dimensional density evolution local to the inversion layer during sawtooth crash using Beam Emission Spectroscopy at DIII-D tokamak Sayak Bose, William R Fox, Dingyun Liu, Zheng Yan, George R McKee, Aaron Goodman, Hantao Ji We present methods for analyzing light fluctuations measured by Beam Emission Spectroscopy (BES) data to obtain the plasma density evolution in a 2-D plane associated with rapid sawtooth crash events at the DIII-D tokamak. A method is developed to remove sawtooth induced edge light pulse from the BES data, as intense Dα emission produced by edge recycling during sawtooth events makes traditional spectroscopic filtering and data analysis techniques insufficient. The cross-calibration of 64 BES channels is checked using a novel technique to ensure accurate measurements. The relationship between large-amplitude light fluctuation, δI/I0, and density variation, δni/ni0, is discussed for these intense sawtooth oscillations. 2-D images of δni/ni0 show a significant spatial variation across an 8 cm (radial) × 20 cm (poloidal) area spanning the sawtooth inversion layer. Multiple density oscillations at ~13 kHz are observed at each sawtooth event that are synchronized with magnetic field fluctuations observed with edge magnetic probes. Density has been observed to spatially vary from ∼ 3.6 to 6.3 × 1013 cm-3 about the sawtooth inversion region at the latter end of the crash [Bose et. al. to be submitted to Rev. Sci. Instrum. (2021)]. |
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CP11.00096: Internal/External Magnetic Field Decomposition and the Dynamics of Mode Locking in DIII-D Edward J Strait, Ryan Sweeney, Nikolas C Logan, Laszlo Bardoczi The magnetic field on a closed surface external to the plasma contains enough information to distinguish the field of sources internal to the surface from the field of external sources [1]. This principle has several applications to axisymmetric and non-axisymmetric fields in tokamak experiments. For example, given two- or three-axis magnetic measurements with sufficient toroidal and poloidal resolution, the field of a rotating MHD mode can be distinguished from that of the wall currents that it induces, enabling a more accurate measurement of the mode amplitude. The same analysis provides a natural framework for measurement of the electromagnetic torque exchanged between the mode and the wall. Averaged over the rotation period, the torque measured during mode locking agrees well with a simple model. The presence of a static error field introduces variation of the torque balance during each rotation, leading to more complex dynamics. If calculated in real time, the evolution of the electromagnetic torque could aid in optimizing error field compensation, and provide advance warning of a mode-locking event. |
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CP11.00097: Tearing Mode Issues in Burning Plasmas James D D Callen, Robert J La Haye This poster will highlight the key physics involved in seeding the robust growth of neoclassical tearing modes (NTMs) in burning plasmas. In seeking maximum plasma performance, physics parameters are pushed to conditions where tearing modes can grow and precipitate disruptions. In ITER baseline scenario (IBS) discharges q95 is reduced to near 3 and beta_N is increased to about 2 or more. The most problematic tearing instabilities occur at the q = 2/1 surface because they are the furthest out radially and lowest m/n modes. Their growing magnetic island widths induce resistive wall drag, mode locking to the wall and then plasma disruption. The nonlinear NTMs are seeded by MHD transients (e.g., ELMs, sawtooth crashes, or three tearing mode resonances). Recent studies of IBS-type discharges in DIII-D show the seeded, robustly growing 2/1 tearing modes that evolve into locked modes and then disruptions are pressure-gradient-driven NTMs. The classical tearing drive is negligible during seeding and as the seeded 2/1 magnetic perturbations grow algebraically (~t) in time. The 2/1 NTMs grow in accord with a novel toroidal-based modified Rutherford equation. Major experimental studies of various predicted means for preventing or controlling 2/1 NTMs at small amplitudes are urgently needed. |
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CP11.00098: Feedback control of the proximity to marginal kink mode stability in ITER-like discharges Jeremy M Hanson, Francesca Turco, Alexander F Battey, Nikolas C Logan, Cristina Rea, Edward J Strait DIII-D experiments with discharges that match key ITER normalized parameters demonstrate control of the proximity to the n=1 kink mode pressure limit. Kink mode stability is assessed in real-time by analyzing the plasma response to a small, nonaxisymmetric magnetic perturbation. This response is used as an input for neutral beam injection feedback, which modulates the plasma stored energy, and thus proximity to the pressure limit. The feedback dynamics are described by a scalar model including a single time constant and delay term. This model was used to obtain optimized feedback gains that were evaluated in experiments. In offline analysis, fitting the frequency-dependence of the response measurements to a single mode model allows the growth rate of the driven, stable mode to be determined. The growth rate increases with pressure, extrapolating to a marginal point between the predicted ideal MHD no- and ideal-wall pressure limits. This technique has the potential to enable fusion scenario optimization while avoiding stability limits. |
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CP11.00099: Form and Substance: How plasma shaping and kinetic profile modifications change the MHD stability of DIII-D plasmas William Boyes, Jeremy M Hanson, Gerald A Navratil, Francesca Turco, Alessandro Marinoni, Alan D Turnbull Plasma shape parameters such as elongation, triangularity and squareness have a strong impact on the ideal MHD stability of tokamak plasmas, due to changes in wall coupling and intrinsic MHD physics. Systematic scans of several shape parameters have been modeled on Negative Triangularity and Hybrid scenario equilibria, using the Corsica-DCON and GATO codes, to assess the variations in ideal MHD stability limits for the n=[1, 2, 3] global kink modes, and optimize upcoming DIII-D experiments. Trends in calculated pressure limits and mode structures for the marginal point identify optima attainable with DIII-D’s shaping capabilities. Scans in the core current and safety factor profiles (qmin = 1.06, 1.09, 1.12) yield differences in the n=1 external kink marginal βN of nearly 50% in some cases. Modifications of the current density and pressure profiles as well as plasma volume have been performed, some of which were matched in experiments, to assess the changes in ideal and tearing MHD limits as a function of J, p and wall stabilization. Tearing mode onset and timing is shown to correlate with the changes in the ideal MHD limits as a function of wall distance and triangularity in the DIII-D experimental database. |
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CP11.00100: Tokamak rotation profile optimization through Model Predictive Control Mitchell D Clement, Mark D Boyer Optimizing the plasma rotation profile, via neutral beam injection (NBI) and 3D magnetic field-induced neoclassical toroidal viscosity (NTV) torque, can be vital for ensuring robust pedestal performance and tearing stability in various operating scenarios. Metrics for optimizing the rotation profile include maximizing the rotation shear at various rational surfaces along the safety factor profile. A model predictive control (MPC) algorithm which controls NBI and NTV torque during TRANSP simulations, in which the rotation profile is predicted based on a fixed momentum diffusivity, is used to investigate how neutral beams and 3D coils may be actuated to tailor the plasma rotation profile as desired. Simplified models for NBI and NTV torque are provided in situ during the TRANSP simulation by the NUBEAM and Generalized Perturbed Equilibrium Code (GPEC) numerical codes, respectively. The MPC algorithm involves optimizing a simplified version of the toroidal momentum equation over a predetermined control horizon. Other objectives for optimizing the rotation profile are also discussed. |
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CP11.00101: Spatially-localized Alfvén eigenmode classification using convolutional neural networks Alan Kaptanoglu, Azarakhsh Jalalvand, Alvin V Garcia, Andrew O Nelson, Joseph A Abbate, Geert Verdoolaege, Steven L Brunton, William W Heidbrink, Egemen Kolemen We use an expert-labeled database of DIII-D discharges to classify five types of Alfvén eigenmodes (AEs) with convolutional neural networks, opening up the possibility of deep-learning-enhanced real-time control of this important class of plasma dynamics. Each DIII-D discharge in the database consists of forty radially-localized electron cyclotron emission (ECE) measurements, sampled at 500 kHz for the first 2 seconds of the discharge. The model attempts to predict when each AE type occurs in a validation dataset, and discriminates between the five types of AE activity. This strategy performs strongly at spatio-temporally localized prediction and classification of Alfven eigenmodes (approximate average true positive rates of 80% and false positive rates of 2%), indicating future promise for more sophisticated spatio-temporal models and incorporation into future real-time control strategies. |
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CP11.00102: Simultaneous Electron Temperature and Safety Factor Profile Control for DIII-D Shira Morosohk, Zibo Wang, Andres Pajares, Sai Tej Paruchuri, Tariq Rafiq, Eugenio Schuster A robust control scheme has been developed to simultaneously control the electron temperature (Te) and safety factor (q) profiles on DIII-D. Control of both kinetic and magnetic profiles is crucial to improve plasma performance and maintain magneto-hydrodynamic stability in tokamak plasmas. Because of this, techniques to simultaneously control multiple profiles need to be developed. The dynamics of these profiles can be described by coupled nonlinear parabolic partial differential equations. The plasma resistivity is modeled as an uncertainty, and robust control techniques are used to design a controller that is capable of tracking prescribed profiles for an expected range of uncertainty. The controller is then tested in closed-loop simulations by using the Control Oriented Transport Simulator (COTSIM). Preliminary predictive simulation results show that this controller is indeed capable of regulating both the electron temperature and the safety factor profiles with acceptable dynamic performance and robustness. |
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CP11.00103: Experimental Assessment of Actuator Management Strategies in DIII-D Andres Pajares, Eugenio Schuster, Kathreen E Thome, Jayson L Barr, Nicholas Eidietis, Alan W Hyatt, Anders Welander, Michael L Walker, David A Humphreys Recent experiments in DIII-D have tested the capabilities of an actuator management algorithm based on nonlinear, real-time optimization. A multitude of control tasks will need to be carried out by a finite set of shared actuators in reactor-grade tokamaks such as ITER. This motivates the development and testing of actuator managers in present devices with the ultimate goal of extrapolating these solutions to future fusion reactors. Such actuator managers must calculate, in real time, the commands of the tokamak actuators that fulfill the necessary control requirements despite changing plasma conditions and actuator availability. In this work, the actuator management problem is posed as a nonlinear optimization problem that is solved in real time in a computationally efficient manner. The proposed approach does not depend on particular control objectives or actuators, facilitating the integration of the actuator manager with independently designed controllers. Initial DIII-D results in the steady-state high-qmin scenario have demonstrated the capabilities of the scheme to perform both simultaneous-multiple-mission and repurposing actuator sharing, which will be required in ITER and future fusion reactors. |
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CP11.00104: Improved Real-time Equilibrium Reconstruction with Kinetic Constraints on DIII-D Ricardo Shousha, John R Ferron, Zichuan Xing, Oak Nelson, Keith Erickson, Egemen Kolemen Since not all quantities of interest in tokamak plasmas can be measured directly, real-time equilibrium reconstruction codes (rtEFIT [1]) are used. However, plasma internal profiles remain largely unconstrained when only data from magnetics are used. For real-time algorithms depending on profile accuracy, it is crucial to include data from measurements internal to the plasma. The inclusion of pressure profile fits as constraints has been shown to significantly improve the shape of the reconstructed pressure profile—enabling more accurate representation of the pressure pedestal. Since these first experiments, the robustness of the algorithm against data sparsity has been improved by adding data buffers for the Charge Exchange Recombination Spectroscopy, and Thomson Scattering data. Even though the reconstructed pressure profiles consistently exhibit the pedestal as expected, the reconstructed current density profile does not consistently show the bootstrap peak when expected. To address this shortcoming, real-time equilibrium reconstruction has been extended to receive current density constraints. The current density constraints are calculated using the Sauter model and used for improved reconstruction of the bootstrap current. |
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CP11.00105: Dual SPI experiments on DIII-D Jeffrey L Herfindal, Daisuke Shiraki, Larry R Baylor, Eric M Hollmann, Zana Popovic, Claudio Marini, Nicholas Eidietis, Andrey Lvovskiy, Charles J Lasnier Experiments on DIII-D utilized two shattered pellet injections (SPIs) at different toroidal locations into the same plasma simultaneously or with a slight time delay between injections. Simultaneous SPIs using small pellets of equal composition (~200 torr-L of pure Ne) result in a shorter current quench (CQ) duration, more uniform radiation, a faster plasma cooling duration, and a four-fold increase in electron density compared to a single SPI from the same experiment. Comparison of the two small simultaneous SPIs to a single, larger, pure Ne SPI (~400 torr-L) mitigating like plasmas show similar CQ durations as well as electron density increases. Despite the similar characteristics, the reduction of radiation peaking for multiple pellets is a promising result critical to the success of the massively parallel ITER SPI system. A comparison of experiments and the possible implications to the ITER DMS system will be presented. |
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CP11.00106: Experimental study of Alfvénic instabilities driven by runaway electrons during the current quench in DIII-D Andrey Lvovskiy, Carlos A Paz-Soldan, Nicholas Eidietis, Andrea Dal Molin, Genevieve H DeGrandchamp, Xiaodi Du, Eric M Hollmann, Jeff B Lestz, Chang Liu, Massimo Nocente, Daisuke Shiraki Suppressed formation of post-disruption runaway electron (RE) beams in DIII-D correlates with increased RE loss and presence of MHz-range kinetic instabilities driven by REs during the current quench. The frequency of these instabilities decreases with decreasing toroidal magnetic field while the failure rate of the RE beam formation increases. The magnetic structure of Alfvénic instabilities is accessed using an upgraded set of high-frequency magnetic antennas. Analysis of the RE energy reconstructed from hard X-ray bremsstrahlung measurements shows that the instabilities are driven by REs with energy of a few MeV. The energy of REs, thus the presence of the instabilities, can be controlled via actuation of plasma and impurity injection parameters. It is found that hot (about 10 keV) pre-disruption plasma leads to formation of RE beams with higher RE current but lower maximum RE energy (sub-MeV) and no observable kinetic instabilities. An opposite phenomena is observed for cold (about 1–2 keV) pre-disruption plasmas. Argon massive gas injection (MGI) in amounts greater than 150 torr-l is found to reduce the energy of REs and increase the rate of RE beam formation. No such effect is observed after D2 and Ne injections. Work supported by the US DOE under DE-FC02-04ER54698. |
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CP11.00107: Investigating Pedestal Changes from the Isotope Effect Ryan A Chaban, Saskia Mordijck The physical mechanism for the positive scaling of confinement with increasing isotope mass is still an active area of research and modern investigations suggest the effect primarily manifests in the edge. Hydrogen plasmas, compared to Deuterium, have much higher transport and require more power to enter and maintain H-mode, increasing the energy fluence across the separatrix. In attached SOL sheath-limited conditions this manifests as a factor 2 higher separatrix temperature and density in Hydrogen than Deuterium from preliminary results. Similarly, in AUG, plasmas with higher separatrix density have been shown to move the electron density and pressure profiles outward decreasing the peeling-ballooning stability as more bootstrap current moves onto the separatrix and the pressure gradient moves outward [1]. We study this relationship on DIII-D using a database of dimensionlessly similar H and D plasmas. We hope to show how relationships between Hydrogen edge conditions, such as increased edge neutral penetration/fueling, result in similar effects to the outward shift in density profiles observed in some AUG Deuterium plasmas. This method will allow us to quantitatively measure the physics mechanisms underlying the isotope effect on the stability limits in both the current-limited and pressure gradient-limited pedestals. |
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CP11.00108: MINI-CONFERENCE ABSTRACTS
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CP11.00109: Cross-Shock Potential Electron Heating in 2D PIC Simulations of Quasi-Perpendicular Low-Beta Shocks Aaron Tran, Lorenzo Sironi We study thermal electron heating in 1D and 2D PIC simulations of collisionless quasi-perpendicular shocks. We employ ion/electron mass ratio 200, fast Mach number 1--4, total plasma beta 0.25, and pre-shock magnetic field 55--85$^\circ$ from shock normal. We first focus on one subcritical shock with a phase-standing whistler wave precursor along the shock normal and little to no ion reflection. The heating is well described by a cross-shock parallel potential comprising both narrow peaks and a secular rise spanning the length of the shock's whistler precursor. The potential peaks coincide with magnetic troughs and host a broad spectrum of field-aligned electron acoustic waves. The electron temperature and potential jumps are lower in 2D than in 1D, which we attribute to enhanced phase-space mixing in 2D. In the parameter sweep of Mach number and pre-shock magnetic field angle, we find that the cross-shock potential is more sensitive to field angle in 1D than in 2D. |
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CP11.00110: Analysis of Instabilities in Quasi-perpendicular Magnetized Collisionless Shocks using the Field-Particle Correlation Technique Collin R Brown, James L Juno, Gregory G Howes, Colby C Haggerty, Stephen S Baek, Arunabha Batabyal Using the field-particle correlation technique, we analyze 3D-3V hybrid particle-in-cell PIC simulations of perpendicular and quasi-perpendicular magnetized collisionless shocks to study particle energization in phase space. High Mach number, oblique shock normal angles generate corrugation instabilities, also known as shock ripple. We identify the velocity-space signatures of corrugation instabilities in conjunction with the signatures of classical shock energization mechanisms such as shock-drift acceleration and shock-surfing acceleration. This diagnostic tool allows us to separate the energy transfer in phase space due to each different energization mechanism and thus study the impacts of corrugation instabilities on the energetics of the shock. We also present preliminary efforts to train a machine learning algorithm to identify the aforementioned velocity-space signatures autonomously. This algorithm can be used to process observational data and identify key plasma processes in systems such as the Earth's bow shock. |
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CP11.00111: Affiliation-Independent Mentoring at the DIII-D National Fusion Facility Kathreen E Thome, Matthias Knolker Entering a plasma physics research laboratory as a student or early career scientist can be overwhelming. There is so much to learn and understand in your research area but also about the facility, meetings, people and procedures. While many of your new colleagues are welcoming and friendly, it can be intimidating to ask them “newbie” questions. This period of integrating into your new program can be very challenging and at times frustrating, speaking from personal experiences, and has caused people to leave the program. Having more experienced colleagues that you can come to with your questions and concerns, can significantly speed up this integration time. We started mentoring a few students ourselves, grew a passion for this role and then created the DIII-D Social Connection (SoCon) program in May of 2020. Since then nine SoCon pairs have been created. Eight of the junior SoCon participants are graduate students and one a postdoctoral scientist and the senior SoCons are all early career staff scientists. Each pair share a similar topic area but are a different institution to widen the new team member’s social circle. Lessons learned from personal mentorship experiences and this inaugural program will be discussed. |
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CP11.00112: Termination of Beamed Double-Helix Structures Ejected by Gravitational Wave Emitters or Modulated Accretion Processes Bruno Coppi The ejection of double-helix beamed plasma structures from time modulated disks has been identified as one of the results of a recently formulated theory [1,2]. In particular, these structures have been shown to emerge from disks in which (GR) gravitational wave emitters such as black hole binaries can be expected to be imbedded. The considered sustaining factor [2] is the time-dependent component of the gravitational potential, characterizing these binaries, but the theory can be extended to involve other sustaining factors such as that of a single black hole with "shepherd planets" or with a modulated form of accretion. The double-helix consists of a pair of "gravitational phonons" as each of these [2] can be represented by a combination of a vertical ballooning structure and a vertically propagating mode whose phase velocity is close to the ion-sound velocity considering plasma disks with electron temperatures significantly higher than the nuclei temperatures. The collimation of the "effective beam", which the propagation of the double helix can represent, is treated as being terminated by a shock structure when the "beam" reaches rarefied plasma regions well away from the disk and it encounters higher plasma density islands. |
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