Bulletin of the American Physical Society
59th Annual Meeting of the APS Division of Plasma Physics
Volume 62, Number 12
Monday–Friday, October 23–27, 2017; Milwaukee, Wisconsin
Session CP11: Poster Session II: Dusty Plasmas and Sheaths; Z-Pinch, X-Pinch, Dense Plasma Focus, and HED; Stellarator, Disruptions, and MHD |
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Room: Exhibit Hall D |
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CP11.00001: DUSTY PLASMAS AND SHEATHS |
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CP11.00002: Study of ND$_{\mathrm{3}}$-enhanced MAR processes in D$_{\mathrm{2}}$-N$_{\mathrm{2}}$ plasmas to induce plasma detachment Shota Abe, Saikat Chakraborty Thakur, Russ Doerner, George Tynan The Molecular Assisted Recombination (MAR) process is thought to be a main channel of volumetric recombination to induce the plasma detachment operation. Authors have focused on a new plasma recombination process supported by ammonia molecules, which will be formed by impurity seeding of N$_{\mathrm{2}}$ for controlling divertor plasma temperature and heat loads in ITER. This ammonia-enhanced MAR process would occur throughout two steps. In this study, the first step of the new MAR process is investigated in low density plasmas (N$_{\mathrm{e}} \quad \approx $ 10$^{\mathrm{16}}$ m$^{\mathrm{-3}}$, T$_{\mathrm{e}} \quad \approx $ 4 eV) fueled by D$_{\mathrm{2}}$ and N$_{\mathrm{2}}$. Ion and neutral densities are measured by a calibrated Electrostatic Quadrupole Plasma (EQP) analyzer, combination of an ion energy analyzer and mass spectrometer. The EQP shows formation of ND$_{\mathrm{3}}$ during discharges. Ion densities calculated by a rate equation model are compared with experimental results. We find that the model can reproduce the observed ion densities in the plasma. The model calculation shows that the dominant neutralization channel of D$_{\mathrm{x}}^{\mathrm{+}}$ (x$=$1-3) ions in the volume is the formation of ND$_{\mathrm{y}}^{\mathrm{+}}$ (y$=$3 or 4) throughout charge/D$^{\mathrm{+}}$ exchange reactions with ND$_{\mathrm{3}}$. Furthermore, high density plasmas (N$_{\mathrm{e}} \quad \approx $ 10$^{\mathrm{16}}$ m$^{\mathrm{-3}})$ have been achieved to investigate electron-impact dissociative recombination processes of formed ND$_{\mathrm{y}}^{\mathrm{+}}$, which is the second step of this MAR process. [Preview Abstract] |
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CP11.00003: Dressing effects on the occurrence scattering time retardation and advance in a dusty plasma Myoung-Jae Lee, Young-Dae Jung The dressing effects on the occurrence scattering time for the dust-dust interaction are investigated in a complex plasma. The first-order eikonal analysis is applied to obtain the scattering amplitude and the occurrence scattering time for the dust-dust interaction. The result shows that dressing effect enhances the retardation phenomena of the occurrence scattering time in the forward scattering domain. It is shown that the oscillatory behavior of the scaled occurrence scattering time is getting more significant with an increase of the Debye length. It is also found that the retardation domain of the occurrence scattering time increases with a decrease of the Debye length. The variation of the occurrence scattering time retardation and advance due to the dressing effect is also discussed. [Preview Abstract] |
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CP11.00004: Research progress and status of the Magnetized Dusty Plasma Experiment (MDPX) Edward Thomas, Uwe Konopka, Robert Merlino, Marlene Rosenberg The addition of a magnetic field has a profound influence on the properties of a complex/dusty plasma. The Magnetized Dusty Plasma Experiment (MDPX) device at Auburn University is a flexible, high magnetic field research instrument with a mission to serve as an open access, multi-user facility for the dusty plasma and basic plasma research communities. In the last year, the MDPX device has performed a broad range of experimental studies at magnetic fields B ≥ 3 T; these are conditions where the electron gyro-radius is comparable to the diameter of the microparticles and the ion gyro-radius is comparable to the spacing between the microparticles. A variety of emergent phenomena are observed including a new type of imposed spatial ordering, significantly modified particle charging, coupling between ion and microparticle/nanoparticle transport, and new regimes of nanoparticle behavior. [Preview Abstract] |
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CP11.00005: Microparticle Deflection in a Plasmas as a Result of a Strong Magnetic Field Brian Lynch, Uwe Konopka, Dylan Funk, Edward Thomas In recent years the influence of a high magnetic field on the physics of complex plasma has been a topic of great interest. In particular, the dynamics may be significantly modified by the presence of large external magnetic fields. In a recent experiment using the Magnetized Dusty Plasma Experiment, the g x B deflection of charged dust grains falling through the bulk region of a magnetized plasma was observed. Using the deflection angle of the dust grains, it is possible to derive the particle charge. It is found that the charge on the grains is significantly lower than the estimates obtained from the OML model. This presentation will describe the experiments, the determination of the particle charge, and will discuss possible mechanisms that may be responsible for reducing the particle charge in a magnetized plasma. [Preview Abstract] |
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CP11.00006: Effects of external magnetization on probe-induced voids in complex plasmas Spencer LeBlanc, Edward Thomas Voids, or dust-free regions within a dust cloud in a complex plasma, have been observed and characterized for some time, both in microgravity settings [1] and in earth-based environments [2]. Created by a concentration of charge in the center of the void structure, the void boundary is formed at the point of equilibrium between the Coulomb force and ion-drag force on the dust grains. While there exists an extensive theoretical framework for understanding the dynamics of these voids [3,4], many mechanisms are still not completely understood. Of particular interest is the effect on void structure resulting from an externally implied magnetic field. Recently developed apparati for studying complex plasmas within magnetic fields have allowed experimental observation of magnetized voids [5]. Recent results and analysis are presented from such experiments performed on the Magnetized Dusty Plasma Experiment (MDPX). [1] M. Klindworth, et al., Phys. Rev. Lett., 93, 195002 (2004). [2] G. Praburam and J. Goree, Phys. Plasmas, 3, 1212 (1996). [3] G. E. Morfill, et al., Phys. Rev. Lett., 83, 1598 (1999). [4] K. Avinash, et. al., Phys. Rev. Lett., 90, 075001 (2003). [5] B. Tadsen, et. al., Phys. Plasmas, 21, 103704 (2014). [Preview Abstract] |
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CP11.00007: Methods for the characterization of imposed, ordered structures in MDPX Taylor Hall, Edward Thomas It is well understood that the microparticles in complex, or dusty, plasmas will form self-consistent crystalline patterns (plasma crystals) under the proper plasma parameters. In the Magnetized Dusty Plasma Experiment (MDPX), studies have been made of an imposed ordering of the dust particles to a two-dimensional grid where the dust particles are shown to become spatially oriented to the structure of a wire mesh embedded in the upper electrode~[1]. At high magnetic fields (B \textgreater 1.5 T), the particles become more confined to this structure with their motion limited to ``hopping'' through the grid pattern, or being confined to a single grid point~[2]. A reliable and meaningful method of describing the degree to which the dust particles are restricted to this grid pattern is needed and a several potential methods for doing so are presented. The application of these techniques to characterize the background plasma parameters at which these imposed, ordered structures appear will be shown. [1] E. Thomas, et al., Phys. Plasmas \textbf{22}, 30701 (2015). [2] E. Thomas, et al., Phys. Plasmas \textbf{22}, 113708 (2015). This work is supported by the U. S. Dept. of Energy and the NSF. [Preview Abstract] |
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CP11.00008: DC response of dust to low frequency AC signals Michael McKinlay, Uwe Konopka, Edward Thomas Macroscopic changes in the shape and equilibrium position of clouds of charged microparticles suspended in a plasma have been observed in response to low frequency AC signals. In these experiments, dusty plasmas consisting of 2-micron diameter silica microspheres suspended between an anode and cathode in an argon, DC glow discharge plasma are produced in a grounded, 6-way cross vacuum chamber. An AC signal, produced by a function generator and amplified by a bipolar op-amp, is superimposed onto the potential from the cathode. The frequencies of the applied AC signals, ranging from tens to hundreds of kHz, are comparable to the ion-neutral collision frequency; well below the ion/electron plasma frequencies, but also considerably higher than the dust plasma frequency. This presentation will detail the experimental setup, present documentation and categorization of observations of the dust response, and present an initial model of the response. [Preview Abstract] |
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CP11.00009: Pattern formation and filamentation in low temperature, magnetized plasmas – a numerical approach Mohamad Menati, Uwe Konopka, Edward Thomas In low-temperature discharges under the influence of high magnetic field, pattern and filament formation in the plasma has been reported by different groups. The phenomena present themselves as bright plasma columns (filaments) oriented parallel to the magnetic field lines at high magnetic field regime. The plasma structure can filament into different shapes from single columns to spiral and bright rings when viewed from the top. In spite of the extensive experimental observations, the observed effects lack a detailed theoretical and numerical description. In an attempt to numerically explain the plasma filamentation, we present a simplified model for the plasma discharge and power deposition into the plasma. Based on the model, 2-D and 3-D codes are being developed that solve Poisson’s equation along with the fluid equations to obtain a self-consistent description of the plasma. The model and preliminary results applied to the specific plasma conditions will be presented. [Preview Abstract] |
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CP11.00010: Growth of nanoparticles in a strongly magnetized plasma Lenaic Couedel, Spencer LeBlanc, Taylor Hall, Uwe Konopka, Edward Thomas This presentation reports on the growth of nanoparticles in the Magnetized Dusty Plasma Experiment (MDPX) device. Two methods of production are investigated: (i) radio-frequency (rf) plasmas are produced in reactive gases (methane and acetylene) mixed with argon or hydrogen and (ii) nanoparticles are grown by sputtering the rf electrode (made of carbon, aluminium, copper, etc). The growth of nanoparticles is followed by monitoring discharge parameters such as the powered electrode self bias and the rf current harmonic content. The dynamics of the growing dust particle cloud is investigated by recording the scattered light of a laser sheath with a high speed video camera. The size distribution and the internal structure of the produced nanoparticles are studied ex-situ using scanning and transmission electron microscopes. The influence of the strength of the magnetic field is explored and the changes in NP growth dynamics and transport are discussed. [Preview Abstract] |
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CP11.00011: Effect of magnetic field on the phase transition in dusty plasma. Surabhi Jaiswal, Edward Thomas, Rupak Mukherjee The formation of self-consistent crystalline structure is a well-known phenomenon in complex plasmas. In most experiments the pressure and rf power are the main controlling parameter in determining the phase of the system. We have studied the effect of externally applied magnetic field on the configuration of plasma crystals, suspended in the sheath of a radio-frequency discharge using the Magnetized Dusty Plasma Experiment (MDPX) device. Experiments are performed at a fixed pressure and rf power where a crystalline structure formed within the confining ring, but ramping the magnetic field up to 1.28 T. We report on the breakdown of the crystalline structure with increasing magnetic field. The magnetic field affects the dynamics of the plasma particles and first leads to a rotation of the crystal. At higher magnetic field, there is a radial variation (shear) in the angular velocity of the moving particles which we believe leads to the melting of the crystal. This melting is confirmed by evaluating the variation of the pair correlation function as a function of magnetic field. References: \\wati Baruah and nilakshi Das, Physics of Plasmas 17, 073702 (2010). [Preview Abstract] |
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CP11.00012: Molecular dynamic simulation of weakly magnetized complex plasmas Dylan Funk, Uwe Konopka, Edward Thomas A complex plasma consists of the usual plasma components (electrons, ions and neutrals), as well as a heavier component made of solid, micrometer-sized particles. The particles are in general highly charged as a result of the interaction with the other plasma components. The static and dynamic properties of a complex plasma such as its crystal structure or wave properties are influenced by many forces acting on the individual particles such as the dust particle interaction (a screened Coulomb interaction), neutral (Epstein) drag, the particle inertia and various plasma drag or thermophoretic forces. To study the behavior of complex plasmas we setup an experiment accompanying molecular dynamic simulation. We will present the approach taken in our simulation and give an overview of experimental situations that we want to cover with our simulation such as the particle charge under microgravity condition as performed on the PK-4 space experiment, or to study the detailed influences of high magnetic fields. This work was supported by the US Dept. of Energy (DE-SC0016330), NSF (PHY-1613087) and JPL/NASA (JPL-RSA 1571699). [Preview Abstract] |
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CP11.00013: RF attenuation as a dusty plasma diagnostic Brandon Doyle, Uwe Konopka, Edward Thomas When a dusty plasma is formed by adding dust to a plasma environment, the electron density of the background plasma is depleted as the dust particles acquire their negative charge. The magnitude of the electron depletion depends on the dust particle charge, and thus its properties, as well as the dust number density. A direct measurement of the electron density in a dusty plasma therefore contains information about the charging state of the dust particles. This measurement is difficult to obtain without influencing the system. For example, Langmuir probes influence the system by creating voids, or they become unreliable due to their potential contamination with dust. A less invasive diagnostic tool might be realized using plasma chamber electrodes for a plasma impedance measurement as it depends on the excitation frequency: the spatially averaged electron density is derived from the electron plasma frequency, which is related to the radio frequency attenuation characteristic. We present preliminary experiments using two impedance probe designs: probes immersed in a plasma and electrodes located at the edge of the plasma. We evaluate the potential application of this method for ground-based laboratory experiments and future microgravity experiment facilities aboard the ISS. [Preview Abstract] |
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CP11.00014: Complex Plasmas under free fall conditions aboard the International Space Station Uwe Konopka, Edward Thomas, Jr., Dylan Funk, Brandon Doyle, Jeremiah Williams, Christina Knapek, Hubertus Thomas Complex Plasmas are dynamically dominated by massive, highly negatively charged, micron-sized particles. They are usually strongly coupled and as a result can show fluid-like behavior or undergo phase transitions to form crystalline structures. The dynamical time scale of these systems is easily accessible in experiments because of the relatively high mass/inertia of the particles. However, the high mass also leads to sedimentation effects and as a result prevents the conduction of large scale, fully three dimensional experiments that are necessary to utilize complex plasmas as model systems in the transition to continuous media. To reduce sedimentation influences it becomes necessary to perform experiments in a free-fall (“microgravity”) environment, such as the ISS based experiment facility “Plasma-Kristall-4” (“PK-4”). In our paper we will present our recently started research activities to investigate the basic properties of complex plasmas by utilizing the PK-4 experiment facility aboard the ISS. We further give an overview of developments towards the next generation experiment facility “Ekoplasma” (formerly named “PlasmaLab”) and discuss potential additional small-scale space-based experiment scenarios. [Preview Abstract] |
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CP11.00015: Volumetric nature of synchronization of the dust acoustic wave with an external modulation Jeremiah Williams The dust acoustic wave (also known as the dust density wave) is low-frequency, longitudinal mode that propagates through the dust component of the dusty plasma system and is self-excited by the free energy from the ion streaming through the dust component. In the laboratory setting, the majority of the self excited dust acoustic waves that are observed are nonlinear, which allows for detailed studies of the nonlinear properties of this wave mode at the kinetic level. One such nonlinear process is synchronization, which is observed when the self-excited dust acoustic wave mode couples with and adjusts to an externally applied modulation. In this poster, we will present volumetric measurements of naturally occurring dust acoustic waves in an rf discharge as it becomes synchronous with an externally applied modulation in the spatial and temporal domains by applying a time-resolved Hilbert Transform to high-speed video imaging of the wave mode over a range of experimental conditions. [Preview Abstract] |
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CP11.00016: ALPHA (Adjustable Long Pulse High-Field Apparatus) W. J. Birmingham, E. M. Bates, C. A. Romero-Talamas, W. F. Rivera The Dusty Plasma Laboratory (DPL) at the University of Maryland, Baltimore County (UMBC) is now finalizing the design and fabricating components for ALPHA, a high-field Bitter-type electromagnet to be used for magnetized dusty plasma experiments. When the system is complete, ALPHA will be programmable to dynamically increase or decrease fields of up to 10 T for nominally 10 seconds and up to several minutes. The magnet dimensions as well as power and cooling requirements were optimized according to a genetic algorithm developed in the DPL [1]. The cooling channel pattern design was obtained using an analytic methodology also developed in the DPL [2]. The final design parameters as well as the predicted performance characteristics of the magnetic core, the water cooling shell, and the DC power source are presented. [1] E. M. Bates, W. J. Birmingham, and C. A. Romero-Talamas. IEEE Trans. Magn. 53, 7200310 (2017): [2] W. J. Birmingham, E. M. Bates, and C. A. Romero-Talamas, J. Thermal Sci. Engr. Appl. 8, 021008 (2015) [Preview Abstract] |
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CP11.00017: Excitation of an acoustic pulse by an impulsive shear flow in a dusty plasma Bin Liu, John Goree A dusty plasma is a strongly-coupled plasma that contains micron-sized particles. These particles, also called dust particles, are highly charged by ambient plasma; they interact with each other, sustaining collective wave motion. Both longitudinal and transverse waves can in general be excited. Here we use an electrostatic three-dimensional (3D) simulation to reveal a wave excitation mechanism that is due to viscous heating. In the simulation, an impulsive force was applied to drive a shear flow motion with a sudden onset. After a delay, a longitudinal acoustic pulse wave was observed, propagating outwards from the edge of the flow. We found that the viscous heating due to shear motion can result in a brief localized rarefaction in the dust cloud, leading to the excitation of a longitudinal acoustic wave. The simulation parameters were motivated by the PK-4 instrument on the International Space Station (ISS). [Preview Abstract] |
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CP11.00018: BETA (Bitter Electromagnet Testing Apparatus) Evan M. Bates, William J. Birmingham, William F. Rivera, Carlos A. Romero-Talamas The Bitter Electromagnet Testing Apparatus (BETA) is a 1-Tesla (T) prototype of the 10-T Adjustable Long Pulse High-Field Apparatus (ALPHA). These water-cooled resistive magnets use high DC currents to produce strong uniform magnetic fields. Presented here is the successful completion of the BETA project and experimental results validating analytical magnet designing methods developed at the Dusty Plasma Laboratory (DPL). BETA's final design specifications will be highlighted which include electromagnetic, thermal and stress analyses. The magnet core design will be explained which include: Bitter Arcs, helix starters, and clamping annuli. The final version of the magnet's vessel and cooling system are also presented, as well as the electrical system of BETA, which is composed of a unique solid-state breaker circuit. Experimental results presented will show the operation of BETA at 1 T. The results are compared to both analytical design methods and finite element analysis calculations. We also explore the steady state maximums and theoretical limits of BETA's design. The completion of BETA validates the design and manufacturing techniques that will be used in the succeeding magnet, ALPHA. [Preview Abstract] |
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CP11.00019: Potential profile near the virtual cathode in a dusty plasma device. Dinesh Rathod, Saroj Kumar Dash, Arun Sarma Existence of a virtual cathode in presence of dusty plasma has been studied by theoretical and numerical analysis. Using basic equations of charge dust, ions and electrons, the behavior of the potential in presence of dust has been calculated and plotted as a function of dust density. It was found that there is a change in potential difference between cathode and sheath potential which changes the threshold wall temperature compared to normal plasma condition. The threshold wall temperature has been increased due to the ability of micro-particles acquiring some electron charge and hence, reducing potential at the wall. Further with different values of $\alpha $(depends on dust density), threshold temperature remained same for an observed virtual cathode. Hence, behavior of potential was plotted for different $\alpha $with increasing wall temperatures. It has been observed that, at lower dust density, double layer like structure is formed near the virtual cathode. Occurrence of two virtual cathodes is observed, one before threshold temperature and one after it. However, irrespective of variation of potential difference near the wall and existence of two virtual cathodes, threshold temperature remained same. [Preview Abstract] |
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CP11.00020: Shock-like pulse experiment in a strongly coupled dusty plasma Anton Kananovich, J. Goree Compressional pulses are excited in a dusty plasma using a wire moved at a supersonic speed. The dusty plasma consists of a 2D monolayer of polymer microspheres electrically levitated in a low-temperature argon RF plasma. The microspheres gained a large negative charge so that they interacted with each other as a strongly coupled component, partly shielded by the electrons and ions. The wire, which had a negative potential that repelled microspheres, was moved at a constant speed, causing a compressional pulse to propagate. This pulse had shock-like properties because the wire was moved faster than the longitudinal sound speed in the microspheres. The experiment was repeated for the dusty plasma both in liquid and solid states, all of the controlled parameters except for the dust kinetic temperature being equal. The laser rastering method was used to change the kinetic temperature. Several experimental runs were done with different wire speeds for the both cases. An increase in the wire propagation speed increased the propagation speed of the compressional pulse. High pulse propagation speeds were obtained with Mach numbers up to 5. For high pulse propagation speeds crystal buckling was observed. Video microscopy was the main diagnostic. [Preview Abstract] |
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CP11.00021: Features of Gaussian Beam Traversing in Complex Plasma Ruchi Sharma, Suresh C. Sharma A theoretical model describing the effect of dust on the self-focusing of the amplitude modulated laser beam propagating in a complex plasma has been proposed. For non-linear irradiance of the beam, electrons carry the non-uniform temperature results in more accretion of electrons on dust particles close to the axis. This leads to distorted electron density passage near the axis even after considering the ambipolar diffusion and direct the electromagnetic beam. In this analysis, while considering non-linear ohmic heating, dust charge balance, elastic and inelastic collision of constituent particles, momentum and energy balance equations have been solved simultaneously to govern the relation between the beam width parameter and distance of propagation. The dependence of beam width parameter with dimensionless propagation distance have been evaluated for different values of laser beam spot size, modulation index and different amplitude of the beam. Numerical calculations have been done to calculate the self -focusing length of the amplitude modulated beam propagating in the presence of dust particles. It is found that the self-focusing effect increases with the amplitude and modulation index of the beam but behave inversely with laser beam spot size. [Preview Abstract] |
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CP11.00022: Studies of magnetized plasma sheaths with secondary electron emissions using continuum kinetic simulations Petr Cagas, Ammar Hakim, Bhuvana Srinivasan A continuum kinetic plasma model is used to study magnetized plasma sheaths by directly evolving the ion and electron distribution functions along with Maxwell's equations. Appropriate boundary conditions are included to account for secondary electron emissions at the walls. Secondary electron emission (SEE) from a solid surface can drastically influence the plasma behavior -- some recent works suggest that SEE can even reverse the gradient of the electrostatic potential in the plasma sheath. Therefore, a self-consistent SEE model based on real material parameters needs to be included in numerical models. Currently, SEE is commonly implemented using Monte-Carlo algorithms. However, this work presents a novel approach where the full velocity distribution function of SEE is directly constructed using the incident electron population and phenomenological material fits. This distribution function is then used as the boundary condition in the continuum kinetic simulation. [Preview Abstract] |
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CP11.00023: Measurement of sheath potential by three emissive-probe methods in DC filament plasmas near a biased grid In-Je Kang, In-Sun Park, Eugene Wackerbarth, Min-Keun Bae, Noah Hershkowitz, Greg Severn, Kyu-Sun Chung Plasma potential structures are measured with an emissive probe near a negatively biased grid ($\approx -100V$, 80mm diam., 40 lines/cm) immersed in a hot filament DC discharge in Kr. Three different methods of analysis are compared: inflection point (IP), floating potential (FP) and separation point (SE) methods. The plasma device at the University of San Diego (length = 64 cm, diameter = 32 cm, source = filament DC discharge) was operated with $5 \times 10^8 < n_e <5 \times 10^9 cm^{-3}$, $1 |
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CP11.00024: Investigation of Ion Acoustic Wave Instabilities Near Positive Electrodes Ryan Hood, Feng Chu, Scott Baalrud, Robert Merlino, Fred Skiff Electron sheaths occur when an electrode is biased above the plasma potential, most often during the electron saturation portion of a Langmuir probe trace. Through the presheath, electrons are accelerated to velocities exceeding the electron thermal speed at the sheath edge, while ions do not develop any appreciable flow. PIC simulations have shown that ion acoustic instabilities are excited by the differential flow between ions and electrons in the presheath region of a low temperature plasma. We present the first experimental measurements investigating these instabilities using Laser-Induced Fluorescence diagnostics in a multidipole argon plasma. The plasma dispersion relation is measured from the power spectra of the imaged LIF signal and compared to the simulation results. In addition, optical pumping is measured using time-resolved LIF measurements and fit to a model in order to determine the diffusion rate, which may be enhanced due to the instability. [Preview Abstract] |
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CP11.00025: Progress on the Development of the hPIC Particle-in-Cell Code Cameron Dart, Alyssa Hayes, Rinat Khaziev, Stephen Marcinko, Davide Curreli Advancements were made in the development of the kinetic-kinetic electrostatic Particle-in-Cell code, hPIC, designed for large-scale simulation of the Plasma-Material Interface. hPIC achieved a weak scaling efficiency of 87\% using the Algebraic Multigrid Solver BoomerAMG from the PETSc library on more than 64,000 cores of the Blue Waters supercomputer at the University of Illinois at Urbana-Champaign. The code successfully simulates two-stream instability and a volume of plasma over several square centimeters of surface extending out to the presheath in kinetic-kinetic mode. Results from a parametric study of the plasma sheath in strongly magnetized conditions will be presented, as well as a detailed analysis of the plasma sheath structure at grazing magnetic angles. The distribution function and its moments will be reported for plasma species in the simulation domain and at the material surface for plasma sheath simulations. [Preview Abstract] |
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CP11.00026: Three-dimensional and Sheath Boundary Effects on the Instabilities and transport in ExB Plasma Discharges Vincent Morin, Oleksandr Koshkarov, Andrei Smolyakov, Yevgeny Raitses, Igor Kaganovich Plasma devices based on the ExB drift are used for a variety of applications. However, transverse electron current due to ExB drift and diamagnetic flows are the sources of gradient-drift type instabilities which results in turbulence and anomalous transport. In the simplest case, the instabilities and resulting plasma dynamics are considered in neglect of the electron motion along the magnetic field. However, parallel electron dynamics may significantly affect the instability criteria for gradient-drift modes and result in new instabilities. In bounded systems, where the magnetic field lines are intercepted by material walls, the sheath becomes important. The sheath boundary conditions constrain the parallel electron dynamics and thus, via current closure (due to charge neutrality), modify instabilities. We consider sheath boundary conditions for dielectric walls and investigate their effect on the Simon-Hoh and lower-hybrid instabilities. The 3D eigenmode structure is investigated and the nonlinear evolution is studied with fluid simulations. The effects of boundary conditions open the way to control the instabilities via the ``smart wall'' boundaries which use segmented electrodes and active circuit elements to suppress the instabilities. [Preview Abstract] |
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CP11.00027: Effects of discrete stochastic charging of dust grains in protoplanetary disks K. S. Ashrafi, S. Esparza, C. Xiang, L. Matthews, A. Carballido, T. Hyde, B. Shotorban The stochastic nature of grain charging can play a significant role in the development of dust aggregate structure when the grains have a small charge. In this work, we use a model of discrete stochastic charging to calculate time-dependent electric charging of dust aggregates. We compare the electron and ion currents to micron and submicron aggregate grains, which consist of spherical monomers, to the currents to spherical grains of equivalent mass. The average charge and charge distribution are compared for aggregates composed of different monomer sizes. The aggregate morphology (whether the grain is compact or porous) affects the amount of charge collected and the available surface area for recombination on dust grains. Thus, the aggregate morphology as well as the dust fraction can affect the overall ionization balance in a plasma. The implications of our results for non-ideal magnetohydrodynamics in protoplanetary disks are briefly discussed in terms of the effect of disk ionization fraction and chemical networks. [Preview Abstract] |
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CP11.00028: Z-PINCH, X-PINCH, DENSE PLASMA FOCUS AND HED |
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CP11.00029: An university-scale pulsed-power system using a bipolar Marx generator Po-Yu Chang, Sheng-Hua Yang, Mei-Feng Huang A bipolar Marx generator is being built for x-ray sources or laboratory astrophysics and space research for university-scale laboratory. The system consists of ten stages. In each stage, two $1\;{\rm \mu F}$ capacitors connected in series are charged to $\pm30\;{\rm kV}$ storing $9\;{\rm kJ}$ of total energy. It delivers a current of $\thicksim200\;{\rm kA}$ to the load with a $\thicksim200\;{\rm ns}$ rise time during the discharge. It will be used for following three purposes: (1) gas-puff z pinches generating soft x-ray for bio-medical research in the future; (2) generating plasma jets to study interactions between plasma flows and unmagnetized/magnetized obstacles analogous to the interactions between solar winds and planetary magnetic fields or unmagnetized planets; and (3) studying the pinch in a dense plasma focus device. The results of current measurements and circuit characteristics are shown. [Preview Abstract] |
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CP11.00030: Development of a Self-Clearing MITL Topology Matthew Martin, Andrew Porwitzky, Daniel Dolan Recent experiments on the Z facility at Sandia National Labs have demonstrated significant plasma outflows onto the surface of our fusion targets when the load current exceeds 10MA. These plasmas are believed to be sourced at the inner MITL from both hydrocarbon contamination and the metal of the MITL itself. We present simulations and the initial experimental results of a new MITL topology that attempts to minimize the production and transport of shorting plasma in the inner MITL, leading to improved current delivery and compression in magneto inertial fusion experiments at the Z facility. [Preview Abstract] |
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CP11.00031: Spectral Dependence of Stratified Electrothermal Instability in Tamped Aluminum 6061 with Current in a Skin Layer Bruno Bauer, Trevor Hutchinson, Thomas Awe The stratified electrothermal instability (ETI) was recently observed on the surface of thick aluminum 6061 pulsed with rapidly rising lineal current density ($3\times 10^{15}$\,A\,m$^{-1}$s$^{-1}$) for 70\,ns.\footnote{T.M. Hutchinson, T.J. Awe, B.S. Bauer, K.C. Yates, E.P. Yu, W.G. Yelton, and S. Fuelling, `Experimental Observation of the Stratified Electrothermal Instability on Aluminum with Thickness Greater than a Skin Depth,’ submitted (2017) to Phys. Rev. Lett. Also see Invited Talk by T.J. Awe at this APS-DPP meeting.} A transparent 70-$\mu $m-thick Parylene-N coating tamped the aluminum expansion and suppressed surface plasma. The evolution of the aluminum surface emission pattern was recorded with time-resolved microscopy (3-$\mu $m resolution). The images were converted into a series of blackbody surface-temperature maps. Analysis of these temperature maps provides information on the evolution of temperature fluctuations, as a function of axial wavelength and azimuthal width. Perturbations with axial wavelength longer than 20\,$\mu $m grow, while those with axial wavelength shorter than 10\,$\mu $m decay. Comparing the spectral dependence of growth/decay rates with MHD simulations could test the modeling of ETI positive feedback and of damping by thermal conduction. [Preview Abstract] |
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CP11.00032: Experimental Investigation of Micrometer Scale Areal Density Variations in Metal Liners Driven by the 1 MA COBRA Pulsed Power Generator Levon Atoyan, Sergei Pikuz, Tania Shelkovenko, David Hammer, Tom Byvank On the 20 MA Z machine, the seed for the MRT instability was mitigated in the Magnetized Liner Inertial Fusion experiment using a thick dielectric coating.$^{\mathrm{1\thinspace }}$We have used high-resolution radiography to study the development of small-scale (\textasciitilde 10-30 $\mu $m) features in thin foils on the 1 MA, 100-200 ns COBRA pulsed power generator$^{\mathrm{2}}$. We examined those features quantitatively in a 16 \textmu m thick cylindrical Al liner, where we show areal density variation of up to 40-50{\%}. We then show how the features' wavelength decreases when the material is changed from Al to Ni, Cu, and Ti, going from 21 \textpm 4 \textmu m for Al to 11 \textpm 2 \textmu m for Ti. Moreover, we show that expansion inhibition on both sides by dielectric material reduces small-scale feature size and density, and we show how pattern seeding can affect those parameters. 1. T. J. Awe \textit{et al.}, ``Experimental Demonstration of the Stabilizing Effect of Dielectric Coatings on Magnetically Accelerated Imploding Metallic Liners'', Phys. Rev. Lett., 2016, \textbf{116} 065001. 2. J. D. Douglass \textit{et al.}, ``Capabilities of the Reconfigured COBRA Accelerator'', Proceedings of the 15th IEEE Pulsed Power Conference, June 13-17, 2005, pp. 273-276. [Preview Abstract] |
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CP11.00033: Time-Resolved Thomson Scattering On Gas-Puff Z-Pinch Plasmas At Pinch Time Sophia Rocco, Jacob Banasek, William Potter, Bruce Kusse, David Hammer The conditions and dynamics of neon gas puff z-pinch plasmas at pinch time are studied on COBRA, Cornell's pulsed power generator (current rise time of \textasciitilde 240ns and approximately 0.9MA peak current). Radial tailoring of the gas puff mass-density profile using a triple-nozzle coaxial valve (two annular gas puffs and a central jet) enables production of both more stable and less stable (in regards to the magneto-Rayleigh Taylor instability) z-pinch implosions. A 526.5nm, 10J Thomson scattering diagnostic laser enables probing of the flow dynamics and plasma conditions of these implosions with both spatial and temporal resolution. The 3ns laser pulse is split in half, one of the beams delayed by up to 10ns relative to the other.~ This allows observation of streaked spectra for a total consecutive time of 6ns, providing sub-nanosecond resolution of the evolution of the pinch through stagnation. A gated spectrometer provides spatially-resolved spectra at the same time for comparison. Extreme ultraviolet imaging and laser schlieren imaging at multiple times enable monitoring of the implosion morphology as a function of time. [Preview Abstract] |
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CP11.00034: Axial Magnetic Field Compression in Laboratory Plasma Jets Tom Byvank, William Potter, John Greenly, Charles Seyler, Bruce Kusse Compression of an axial magnetic field correlates with density hollowing and azimuthal rotation of a plasma jet generated by the COBRA pulsed power machine (1 MA peak current in 100 ns rise time) in a radial foil (thin disk of 15 $\mu $m Al or Ti) configuration. The plasma jet compresses an initially uniform \textasciitilde 1 T axial magnetic field (Bz) as it collimates along the central z-axis. Experimental measurements use a Bdot magnetic probe placed in the center of the hollow plasma jet. Experimental results show compression of an applied 1.0$+$/-0.1 T Bz to 2.4$+$/-0.3 T with aluminum jets and to 2.2$+$/-0.2 T with titanium jets. Predictions made by the extended magnetohydrodynamics (XMHD) code, PERSEUS, show compression to a 3.4 T Bz at the probe location for aluminum plasmas. For titanium plasmas, implementing radiation into the code is in progress. Additionally using the XMHD simulation, we explore the effects of changing current directions and how the magnetic field being tied to the electrons in Hall MHD (rather than being frozen to the ions in ideal MHD) influences the magnetic field advection. We overview physical reasons for the discrepancy between the experimental and simulation magnetic field compression measurements, including: surface plasma on Bdot probes, 2D and 3D simulation effects, and differences between ablation of a solid foil compared to a foil initialized as a plasma. [Preview Abstract] |
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CP11.00035: Investigations of advected magnetic fields in experiments with supersonic plasma flows. Eleanor Tubman, Sergey Lebedev, Guy Burdiak, Lee Suttle, Meriame Berboucha, Daniel Russell, Thomas Clayson, Jack Hare, Simon Bland, Jack Halliday, Francisco Suzuki-Vidal Plasma flows created from the ablation of wires in wire array z-pinches can transport frozen-in magnetic fields. The presence of this advected magnetic field can significantly affect the structure of bow shocks that are formed when obstacles are placed in the plasma flow [1]. This is in contrast to the bow shocks that are present in many astrophysical scenarios where the fields do not dominate the conditions. We will present results of experiments performed using the MAGPIE (1 MA, 250 ns) pulsed power facility at Imperial College, London to better understand and control the magnetic fields transported within the plasma. Plasma flows used in these experiments were created using inverse wire arrays [1] and radial foil [2] configurations. Methods of putting obstacles or grids into the plasma flow to change the magnetic field structures are being tested and various diagnostics including Thomson scattering, laser interferometry, shadowgraphy and magnetic probes were used to characterise the resulting flow and magnetic field. [1] G. C. Burdiak et al., The structure of bow shocks formed by the interaction of pulsed power driven magnetised plasma flows with conducting obstacles. PoP., 2017 (in print). [2] F. Suzuki-Vidal et al., Astrophys. J. 815, 2 (2015) [Preview Abstract] |
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CP11.00036: Diagnostics for Magnetically Driven Implosions on the 1-MA MAIZE Facility Paul Campbell, David Yager-Elorriaga, Stephanie Miller, Jeff Woolstrum, Michael Jones, Nicholas Jordan, Y.Y. Lau, Ronald Gilgenbach, Ryan McBride The Michigan Accelerator for Inductive Z-pinch Experiments (MAIZE) is a 3-m-diameter Linear Transformer Driver (LTD) at the University of Michigan which supplies a fast electrical pulse (0--1 MA in 100 ns, for matched loads) to various experimental configurations. In order to better investigate these loads, new diagnostics are being developed. First, an EUV/XUV micro-channel plate pinhole camera and a UV laser imaging system are being implemented to better observe the instability structures that form during implosions. Second, an x-pinch radiography diagnostic is being developed to probe deeper into the plasma loads. Third, Rogowski coils are being developed for enhanced load current measurements. Finally, a bolometry system and photo-conducting diamond (PCD) detectors will be implemented to measure x-ray power and energy. These new systems, combined with the existing twelve-frame laser shadowgraphy, and b-dot current monitors, will be powerful tools for the investigation of imploding z-pinch experiments. [Preview Abstract] |
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CP11.00037: Study of the effect of collisionality and cooling on the interactions of counter-streaming plasma flows as a function of wire material Gilbert Collins, Julio Valenzuela, Nicholas Aybar, Fabio Conti, Farhat Beg We report on the effects wire material on collisionality and radiative cooling on the interactions of counter-streaming plasma jets produced by conical wire arrays on the $\sim$ 200 kA GenASIS driver. In these interactions, mean free path ($\lambda _{mfp}$) scales with jet velocity (v$_{jet} ^4$), atomic mass (A$^2$), and ionization (Z* $^{-4}$), while cooling scales with atomic mass. By changing the material of the jets one can create slowly cooling, weakly collisional regimes using C, Al, or Cu, or strongly cooled, effectively collisionless plasmas using Mo or W. The former produced smooth shocks soon after the jets collide (near the peak current of 150 ns) that grew in size over time. Interactions of the latter produced multiple structures of a different shape, at a later time ($\sim$ 300 ns) that dissipated rapidly compared to the lower Z materials. We will report on the scaleability of these different materials to astrophysical phenomena. [Preview Abstract] |
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CP11.00038: Simulations Of Xenon and Krypton Doped Gas Puff Z-Pinch Implosions* Varun Tangri, J.L. Giuliani, A. L. Velikovich, N. D. Ouart, A. Dasgupta, J. P. Apruzese, A. J. Harvey-Thompson, B. Jones, C.A. Jennings The intriguing result that the presence of a small fraction of Xe (0.8{\%} by number in the center jet) in an Ar gas puff shot can have a significant impact on the emitted K-shell radiation is examined. Experimental indications of this result were recently obtained from a pair of experiments [1] on Sandia National Laboratories' Z machine. These shots were Z2603 (with Xe) and Z2605 (without Xe). The pair had similar initial density profile (outer shell, inner shell, and center jet) but one had the small aforementioned percentage of Xe dopant in the center jet. In addition, both shots had 1{\%} Krypton in the middle shell. The resultant Ar K-shell yield considerably reduced from 373$\pm $9{\%} to 129$\pm $9{\%} kJ without an analogous change in the total radiation yield or emitted power. Simulations of this pair of shots will be presented using the using the Mach2-TCRE code. Detailed examination of the implosion dynamics and emitted radiation and its time- and space resolved K-shell synthetic spectra will be reported and compared with previous analysis [2]. The effect of varying the Xenon fraction as well as Krypton fraction on K-shell radiation and yield will also be examined. [1] Harvey-Thompson et. al., Phys. Plasmas \textbf{23}, 101203 (2016). [2] Apruzese at. al. Phys. Plasmas \textbf{23}, 123303 (2016) [Preview Abstract] |
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CP11.00039: Simulations of an Argon Z-pinch Implosion with time-dependent non-LTE kinetics N. Ouart, A. Dasgupta, J. Giuliani, B. Jones, D. Ampleford, A. Harvey-Thompson, C. Jennings, V. Tangri, R. Clark Three argon gas-puff implosions were performed on the Z-machine at SNL. These three loads had the same density profile from an 8cm dia. nozzle, a 1mg/cm mass, and a 2.5cm length. The experiments produced similar radiative powers and yields (B. Jones et al. PoP 22,020706(2015)). Simulations with the 2D MHD code Mach2-TCRE reproduced the experimental K-shell powers, yields, and emission region. It was also shown that the ratio of the Ly$\alpha $ to He$\alpha +$IC lines from the simulation had good agreement to measurements after peak K-power; however, the simulation's line ratio was higher prior to the peak power. The authors attribute the difference to 3D effects or on the implicit assumption of steady-state population kinetics (J. Thornhill et al. IEEE TPS 43,2480(2015)). This presentation will illustrate the effect of time-dependent level populations on the radiation from simulations using the NRL DZAPP code. DZAPP is a coupled 1D MHD, detailed non-LTE atomic physics with radiation transport, incorporating a transmission line circuit. The line ratios and K-powers from the steady-state and time-dependent populations will be presented and compared with experiment. This work supported by DOE/NNSA. SNL is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the US DOE/NNSA under contract DE-NA-0003525. [Preview Abstract] |
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CP11.00040: Optimizing Dense Plasma Focus Neutron Yields With Fast Gas Jets Matthew McMahon, Elizabeth Stein, Drew Higginson, Christopher Kueny, Anthony Link, Andrea Schmidt We report a study using the particle-in-cell code LSP to perform fully kinetic simulations modeling dense plasma focus (DPF) devices with high density gas jets on axis. The high-density jets are modeled in the large-eddy Navier-Stokes code CharlesX, which is suitable for modeling both sub-sonic and supersonic gas flow. The gas pattern, which is essentially static on z-pinch time scales, is imported from CharlesX to LSP for neutron yield predictions. Fast gas puffs allow for more mass on axis while maintaining the optimal pressure for the DPF. As the density of a subsonic jet increases relative to the background fill, we find the neutron yield increases, as does the variability in the neutron yield. Introducing perturbations in the jet density via super-sonic flow (also known as Mach diamonds) allow for consistent seeding of the m$=$0 instability leading to more consistent ion acceleration and higher neutron yields with less variability. Jets with higher on axis density are found to have the greatest yield. The optimal jet configuration and the necessary jet conditions for increasing neutron yield and reducing yield variability are explored. Simulations of realistic jet profiles are performed and compared to the ideal scenario. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and supported by the Laboratory Directed Research and Development Program (15-ERD-034) at LLNL. [Preview Abstract] |
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CP11.00041: Development of Diagnostics for the Livermore DPF Devices James Mitrani, Rahul R Prasad, Yuri A Podpaly, Christopher M Cooper, Steven F Chapman, Brian H Shaw, Alexander P Povilus, Andrea Schmidt LLNL is commissioning several new diagnostics to understand and optimize ion and neutron production in their dense plasma focus (DPF) systems. Gas fills used in DPF devices at LLNL are Deuterium (D$_{\mathrm{2}})$ and He accelerated onto a Be target, for production of neutrons. Neutron yields are currently measured with Helium-3 tubes, and development of yttrium-based activation detectors is currently underway. Neutron time-of-flight (nTOF) signals from prompt neutrons will be measured with gadolinium-doped liquid scintillators. An ion energy analyzer will be used to diagnose energy distribution of D$+$ and He$+$2 ions. Additionally, a fast frame ICCD camera has been applied to image the plasma sheath during the rundown and pinch phases. Sheath velocity will be measured with an array of discrete photodiodes with ns time responses. A discussion of our results will be presented. [Preview Abstract] |
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CP11.00042: Varying Radii of On-Axis Anode Hollows For kJ-Class Dense Plasma Focus Brian Shaw, Steven Chapman, Steven Falabella, Alexei Pankin, Jason Liu, Anthony Link, Andréa Schmidt A dense plasma focus (DPF) is a compact plasma gun that produces high energy ion beams, up to several MeV, through strong potential gradients. Motivated by particle-in-cell simulations, we have tried a series of hollow anodes on our kJ-class DPF. Each anode has varying hollow sizes, and has been studied to optimize ion beam production in Helium, reduce anode sputter, and increase neutron yields in deuterium. We diagnose the rate at which electrode material is ablated and deposited onto nearby surfaces. This is of interest in the case of solid targets, which perform poorly in the presence of sputter. We have found that the larger the hollow radius produces more energetic ion beams, higher neutron yield, and sputter less than a flat top anode. A complete comparison is presented. This work was prepared by LLNL under Contract DE-AC52-07NA27344 and supported by Office of Defense Nuclear Nonproliferation Research and Development within U.S. Department of Energy's National Nuclear Security Administration [Preview Abstract] |
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CP11.00043: Deploying Solid Targets in Dense Plasma Focus Devices for Improved Neutron Yields Y.A. Podpaly, S. Chapman, A. Povilus, S. Falabella, A. Link, B.H. Shaw, C.M. Cooper, D. Higginson, I. Holod, N. Sipe, B. Gall, A.E. Schmidt We report on recent progress in using solid targets in dense plasma focus (DPF) devices. DPFs have been observed to generate energetic ion beams during the pinch phase; these beams interact with the dense plasma in the pinch region as well as the background gas and are believed to be the primary neutron generation mechanism for a D$_{2}$ gas fill. Targets can be placed in the beam path to enhance neutron yield and to shorten the neutron pulse if desired. In this work, we measure yields from placing titanium deuteride foils, deuterated polyethylene, and non-deuterated control targets in deuterium filled DPFs at both megajoule and kilojoule scales. Furthermore, we have deployed beryllium targets in a helium gas-filled, kilojoule scale DPF for use as a potential AmBe radiological source replacement. Neutron yield, neutron time of flight, and optical images are used to diagnose the effectiveness of target deployments relative to particle-in-cell simulation predictions. A discussion of target holder engineering for material compatibility and damage control will be shown as well. [Preview Abstract] |
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CP11.00044: Load Designs For MJ Dense Plasma Foci A. Link, A. Povlius, R. Anaya, M. G. Anderson, J. R. Angus, C. M. Cooper, S. Falabella, D. Goerz, D. Higginson, I. Holod, M. McMahon, J. Mitrani, E.S. Koh, A. Pearson, Y. A. Podpaly, R. Prasad, D. Van Lue, J. Watson, A. E. Schmidt Dense plasma focus (DPF) Z-pinches are compact pulse power driven devices with coaxial electrodes. The discharge of DPF consists of three distinct phases: first generation of a plasma sheath, plasma rail gun phase where the sheath is accelerated down the electrodes and finally an implosion phase where the plasma stagnates into a z-pinch geometry. During the z-pinch phase, DPFs can produce MeV ion beams, x-rays and neutrons. Megaampere class DPFs with deuterium fills have demonstrated neutron yields in the 10$^{\mathrm{12}}$ neutrons/shot range with pulse durations of 10-100 ns. Kinetic simulations using the code Chicago are being used to evaluate various load configurations from initial sheath formation to the final z-pinch phase for DPFs with up to 5 MA and 1 MJ coupled to the load. Results will be presented from the preliminary design simulations. LLNL-ABS-734785 [Preview Abstract] |
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CP11.00045: The Effect of Interchanging the Polarity of the Dense Plasma Focus on Neutron Yield Sheng Jiang, Drew Higginson, Anthony Link, Andrea Schmidt The dense plasma focus (DPF) Z-pinch devices can serve as portable neutron sources when deuterium is used as the filling gas. DPF devices are normally operated with the inner electrode as the anode. It has been found that interchanging the polarity of the electrodes can cause orders of magnitude decrease in the neutron yield$^{\mathrm{1}}$. Here we use the particle-in-cell (PIC) code LSP$^{\mathrm{2,3}}$ to model a DPF with both polarities. We have found the difference in the shape of the sheath, the voltage and current traces, and the electric and magnetic fields in the pinch region due to different polarities. A detailed comparison will be presented. 1. G. Decker, W. Kies and G. Pross, Phys. Lett. 89A, 393 (1982) 2. D. R. Welch, D. V. Rose, R. E. Clark, T. C. Genoni, and T. P. Hughes, Comput. Phys. Commun. 164, 183 (2004) 3. A. Schmidt, V. Tang, D. Welch, Phys. Rev. Lett. 109, 205003 (2012) [Preview Abstract] |
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CP11.00046: Ultra-Compact Electrostatic Confinement Fusion Device Garrett Young A unique, linear dual-beam configuration with an internal volume of 144 cc was simulated and operated. Deuteron ion paths were simulated using \textit{Mathematica} and the electric field distribution was optimized relative to convergence density, potential well efficiency, and confinement time. The resulting cathode design is a departure from conventional systems, with gradual conical surfaces. The simulated trajectories correlated well to the observed operation, evidenced by two principle factors. First, the high transparency of the cathode due to the focused beams allowed for \textgreater 1 kW operation without duration-limiting temperature rise. Second, when compared to inertial electrostatic configurations, the constructed device achieved record steady-state D-D fusion rates per internal volume including 3.7E$+$4 fusions/sec/cc at 52 kV applied potential and 28 mTorr operating pressure. [Preview Abstract] |
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CP11.00047: Progress towards experimental realization of extreme-velocity flow-dominated magnetized plasmas T.E. Weber, C.S. Adams, D.R. Welch, G. Kagan, I.A. Bean, B.R. Henderson, A.J. Klim Interactions of flow-dominated plasmas with other plasmas, neutral gases, magnetic fields, solids etc., take place with sufficient velocity that kinetic energy dominates the dynamics of the interaction (as opposed to magnetic or thermal energy, which dominates in most laboratory plasma experiments). Building upon progress made by the Magnetized Shock Experiment (MSX) at LANL, we are developing the experimental and modeling capability to increase our ultimate attainable plasma velocities well in excess of 1000 km/s. Ongoing work includes designing new pulsed power switches, triggering, and inductive adder topologies; development of novel high-speed optical diagnostics; and exploration of new numerical techniques to specifically model the unique physics of translating/stagnating flow-dominated plasmas. Furthering our understanding of the physical mechanisms of energy conversion from kinetic to other forms, such as thermal energy, non-thermal tails/accelerated populations, enhanced magnetic fields, and radiation (both continuum and line), has wide-ranging significance in basic plasma science, astrophysics, and plasma technology applications such as inertial confinement fusion and intense radiation sources. [Preview Abstract] |
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CP11.00048: STELLARATOR |
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CP11.00049: Overview of physics goals for OP1.2a on Wendelstein 7-X Thomas Sunn Pedersen Wendelstein 7-X achieved, and in many cases exceeded, the pre-defined goals for its first operation phase, OP1.1. Results include core values of T$_{\mathrm{e}} \quad =$ 8 keV, T$_{\mathrm{i}}=$2 keV and n$_{\mathrm{e}}$\textgreater 3*10$^{\mathrm{19}}$ m$^{\mathrm{-3}}$ and confinement times of 100-150 ms [1,2]. The next operation phase, OP1.2a, scheduled to start in fall 2017, features a much more elaborate set of plasma-facing components. 10 inertially cooled graphite test divertor units (TDU) have been installed, as have graphite tiles on all the heat shields and baffles. Upgrades have also been made to heating systems, diagnostics, and particle fueling systems. This will allow for significantly increased pulse lengths, heating power and plasma performance, in particular, higher plasma density, and higher ion temperatures, thereby enabling a much more detailed investigation of the W7-X optimization and significantly higher triple products than achieved in OP1.1. The robustness of the TDU allows for an aggressive exploration of divertor operation scenarios in this phase. The main goals and plans, and, if available, first results of OP1.2a will be presented. [1] T. Sunn Pedersen, et al., Physics of Plasmas \textbf{24}, 055503 (2017) [2] R. Wolf, et al., Nuclear Fusion, at press (2017) [Preview Abstract] |
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CP11.00050: Initial observations on core transport in W7-X island divertor plasmas Novimir Pablant The current campaign of the Wendelstein 7-X (W7-X) stellarator, specified as OP1.2a, features the first operation with an island divertor and a completed carbon first wall. With the completion of the divertor, and recent upgrades to the ECRH heating system, higher temperatures and densities are expected than previously available during the first campaign (OP1.1), which featured a limiter plasma. After completion of wall conditioning, plasmas with $T_e \sim T_i$ are expected to become accessible, allowing the investigation of plasma performance in the ion-root regime. Initial investigations of core transport in the W7-X island divertor are reported, along with measurements of the radial electric field. Measurements of temperature, density and radial electric field are compared at similar ECRH input powers between the island divertor plasmas from OP1.2a and the limiter plasmas from OP1.1. [Preview Abstract] |
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CP11.00051: Energetic Particle Loss Estimates in W7-X Samuel Lazerson, Simppa Akaslompolo, Micheal Drevlak, Robert Wolf, Douglass Darrow, David Gates The collisionless loss of high energy H$^+$ and D$^+$ ions in the W7-X device are examined using the BEAMS3D code \footnote{Matthew McMillan and Samuel A Lazerson 2014 Plasma Phys. Control. Fusion {\bf 56} 095019}. Simulations of collisionless losses are performed for a large ensemble of particles distributed over various flux surfaces. A clear loss cone of particles is present in the distribution for all particles. These simulations are compared against slowing down simulations in which electron impact, ion impact, and pitch angle scattering are considered. Full device simulations allow tracing of particle trajectories to the first wall components. These simulations provide estimates for placement of a novel set of energetic particle detectors \footnote{G. Szalkowski, Douglas Darrow and F. Cecil 2013 PPPL-4956}. Recent performance upgrades to the code are allowing simulations with > 1000 processors providing high fidelity simulations. Speedup and future works are discussed. [Preview Abstract] |
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CP11.00052: 3D Numerical Analysis of Radiative Edge Cooling in Wendelstein 7-X Island Divertor Scenarios Florian Effenberg, Y. Feng, H. Frerichs, O. Schmitz, T. Barbui, J. Geiger, M. Jakubowski, R. König, M. Krychowiak, H. Niemann, T. Sunn Pedersen, Y. Suzuki, G.A. Wurden Radiative edge cooling is a promising method for mitigation of high heat and particle fluxes in the 3D field geometry of Wendelstein 7-X. A new high mirror island configuration is investigated featuring a more uniform distribution of heat and particle fluxes on horizontal and vertical divertor targets. For an upstream density of $n_{up}=2\times$10$^{19}$m$^{-3}$ at $P_{ECRH}$=8MW maximum heat loads up to $q_{max}\approx$7.2MWm$^{-2}$ are calculated with the 3D fluid and kinetic edge transport Monte Carlo Code EMC3-EIRENE. Carbon eroded from the divertor targets is predicted to serve as effective intrinsic radiator enabling detached operational regimes at higher densities ($n_{up}>4\times$10$^{19}$m$^{-3}$). The feasibility of active control of heat and particle flux levels by impurity seeding (C$_{x}$H$_{y}$, N$_{2}$, Ne) will be discussed for the new island geometry. Impurity line radiation tends to concentrate in the islands for lower densities and causes a drop of flux levels correlated to the power loss fraction, $\Delta q\propto \frac{P_{rad}}{P_{SOL}}$. $\beta$-effects are taken into account based on the 3D MHD-equilibrium code HINT. [Preview Abstract] |
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CP11.00053: First results from the Wendelstein 7-X Phase Contrast Imaging Diagnostic Eric Edlund, Miklos Porkolab, Olaf Grulke, Adrian von Stechow, Lukas-Georg B{\"o}ttger Experiments in the first W7-X campaign achieved conditions of $T_e>8$ keV, $T_i>2$ keV and line-integrated densities of $\approx 3\times 10^{19}$ m$^{-2}$, with energy confinement times close to the ISS$_{04}$ scaling at about 100 ms [1]. The addition of an island divertor for the OP1.2 campaign is expected to lead to improved plasma performance. Experiments from this campaign will investigate the relative balance of neoclassical and turbulent transport of energy and particles and further test the ISS04 scaling. Gyrokinetic modeling indicates that there may be measurable differences in turbulence amplitude and quality as the mirror ratio and rotational transform of the magnetic geometry are changed. Among the new W7-X diagnostics is a Phase Contrast Imaging (PCI) system, a joint effort between MIT and IPP. The PCI diagnostic measures electron density fluctuations that may arise from turbulence or coherent modes. The system is capable of detecting fluctuations spanning a frequency range of about 1 kHz to 2 MHz, and wavenumbers of about 0.5 cm$^{-1}$ to 30 cm$^{-1}$, depending on the optical configuration. We present initial PCI measurements of turbulent fluctuations in relation to global system parameters.\newline \newline[1] Wolf et al., 2017, Nucl. Fusion, at press. [Preview Abstract] |
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CP11.00054: Fast-camera imaging on the W7-X stellarator S.B. Ballinger, J.L. Terry, S.G. Baek, K. Tang, O. Grulke Fast cameras recording in the visible range have been used to study filamentary (``blob'') edge turbulence in tokamak plasmas, revealing that emissive filaments aligned with the magnetic field can propagate perpendicular to it at speeds on the order of 1 km/s in the SOL or private flux region. The motion of these filaments has been studied in several tokamaks, including MAST, NSTX, and Alcator C-Mod. Filaments were also observed in the W7-X Stellarator using fast cameras during its initial run campaign [1]. For W7-X's upcoming 2017--18 run campaign, we have installed a Phantom V710 fast camera with a view of the the machine cross section and part of a divertor module in order to continue studying edge and divertor filaments. The view is coupled to the camera via a coherent fiber bundle. The Phantom camera is able to record at up to 400,000 frames per second and has a spatial resolution of roughly 2 cm in the view. A beam-splitter is used to share the view with a slower machine-protection camera. Stepping-motor actuators tilt the beam-splitter about two orthogonal axes, making it possible to frame user-defined sub-regions anywhere within the view. The diagnostic has been prepared to be remotely controlled via MDSplus. [1] G. Kocsis et al., 44th EPS Conf. (2017). [Preview Abstract] |
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CP11.00055: Feasibility of a Heavy Ion Beam Probe for W7-X T.P. Crowley, D.R. Demers, P.J. Fimognari, O. Grulke, R. Laube A feasibility study of a Heavy Ion Beam Probe (HIBP) diagnostic for the Wendelstein 7-X (W7-X) superconducting stellarator, incorporating the accelerator and energy analyzer (currently in Greifswald) from the 2 MeV TEXT-U HIBP, is being carried out. The study's results are positive: beam trajectory simulations in the W7-X standard magnetic configuration, with central densities up to 10$^{\mathrm{20}}$ m$^{\mathrm{-3}}$, predict that it will be possible to measure the equilibrium plasma potential and E$_{\mathrm{r}}$ at all radii, and simultaneously measure temporally and spatially resolved fluctuations of n$_{\mathrm{e}}$ and potential for $r/a$ \textgreater 0.5. This will provide a unique capability to advance understanding of neoclassical and turbulent particle and energy transport in W7-X. Within this feasibility study, the beam is injected and detected through the K11 and N11 ports respectively, and the toroidal magnetic field is in the '$+\varphi $' direction. Additional beam simulations reveal that most radii can be accessed in 7 other paradigm magnetic configurations. It's anticipated that electrostatic beam steering suitable for studying all these configurations is plausible; it will have plate dimensions comparable to TEXT-U's with smaller electric fields and higher voltages. Initial estimates of anticipated heat load from the W7-X plasma on the steering systems indicate it will be significant, but tractable. Our conclusion from these studies is that an HIBP diagnostic for W7-X is feasible. This work is supported by US DoE Award DE-SC0013918. [Preview Abstract] |
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CP11.00056: 3D Equilibrium Reconstruction with Islands M. Cianciosa, S.P. Hirshman, S.K. Seal, M.W. Shafer Up until now, equilibrium reconstruction for studying 3D effects in fusion plasmas, has been limited to plasmas with nested magnetic topologies. To reconstruct plasmas with more general topologies, such as the island diverter of W7-X, it is necessary to use an equilibrium solver allowing for non-nested or stochastic magnetic fields. The SIESTA code tears the nested magnetic surfaces by applying resonant magnetic perturbations. These perturbations control the size of islands in the equilibrium solution and add another unknown parameter (the perturbation strength) to the equilibrium solution. Experiments show that measured temperature and density profiles flatten inside magnetic islands. Using this signal information, the size of the SIESTA island perturbation can be reconstructed by matching temperature and density profiles to flattened regions in experimental measurements. Recent work has coupled SIESTA into the V3FIT 3D equilibrium reconstruction code. From the SIESTA solution, V3FIT computes synthetic signals in the presence of magnetic islands. The unknown parameters of the model are then adjusted to minimize the mismatch between the observed and synthetic signal. Using this capability, initial results of reconstructed islands in a tokamak equilibrium will be presented. [Preview Abstract] |
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CP11.00057: High-rep-rate Thomson scattering for LHD D. J. Den Hartog, M. T. Borchardt, D. J. Holly, O. Schmitz, R. Yasuhara, I. Yamada, H. Funaba, M. Osakabe, T. Morisaki A high-rep-rate pulse-burst laser system is being built for the LHD Thomson scattering (TS) diagnostic. This laser will have two operating scenarios, a fast-burst sequence of 15 kHz rep rate for at least 15 ms, and a slow-burst sequence of 1 kHz for at least 50 ms. There will be substantial flexibility in burst sequences for tailoring to experimental requirements. This new laser system will operate alongside the existing lasers in the LHD TS diagnostic, and will use the same beamline. This increase in temporal resolution capability complements the high spatial resolution (144 points) of the LHD TS diagnostic, providing unique measurement capability unmatched on any other fusion experiment. The new pulse-burst laser is a straightforward application of technology developed at UW-Madison, consisting of a Nd:YAG laser head with modular flashlamp drive units and a customized control system. Variable pulse-width drive of the flashlamps is accomplished by IGBT (insulated gate bipolar transistor) switching of electrolytic capacitor banks. Direct control of the laser Pockels cell drive enables optimal pulse energy extraction, producing \textgreater 1.5 J $q$-switched pulses with \textasciitilde 20 ns FWHM. Burst operation of this laser system will be used to capture fast time evolution of the electron temperature and density profiles during events such as ELMs, RMP perturbations, and various MHD modes. [Preview Abstract] |
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CP11.00058: Simulation studies of neutron production and triton burn-up rates in the deuterium plasma of LHD S Murakami, Y Saito, M Homma, H Yamaguchi, M Isobe, K Ogawa, T Nishitani The deuterium plasma experiment has been started from 2017 campaign in LHD. The study of the energetic particle is one of the important issues in the deuterium plasma experiment of LHD. We investigate the D-D fusion reaction rates in the deuterium plasma to compare with the experimental results in LHD. The NBI blip experiment was performed and the time behaviour of the neutron production rate was measured. We evaluate the neutron production rate by GNET-TD assuming the experimentally observed density and temperatures of the NBI blip experiment. We see a relatively good agreement in the time behavior of the neutron production rate. Also, we compare the simulation and experimental results in the stational plasma. Next, we perform the triton burn-up simulation of the deuterium experiment of LHD and evaluate the D-T fusion reaction rates to compare with the experimental results of the 14 MeV neutron diagnostic system. [Preview Abstract] |
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CP11.00059: Overview, Progress, and Plans for the Compact Toroidal Hybrid Experiment G.J. Hartwell, N.R. Allen, D.A Ennis, J.D. Hanson, E.C. Howell, C.A Johnson, S.F. Knowlton, J.D. Kring, X. Ma, D.A. Maurer, K.G. Ross, J.C. Schmitt, P.J. Traverso, E.N. Williamson The Compact Toroidal Hybrid (CTH) is an $\ell=2, m=5$ torsatron/tokamak hybrid ($R_0=0.75$\,m, $a_p\sim 0.2$\,m, and $|B|\leq 0.7$\,T) which generates highly configurable confining magnetic fields solely with external coils but typically uses up to 80\,kA of plasma current for heating and disruption studies. The main goals of the CTH experiment are to study disruptive behavior as a function of applied 3D magnetic shaping, and to test and advance the V3FIT reconstruction code and NIMROD modeling of CTH. The disruptive density limit is observed to exceed the Greenwald limit as the vacuum transform is increased with no observed threshold for avoidance. Low-q operations ($1.1 < q(a) < 2.0$) are routine, with disruptions ceasing if the vacuum transform is raised above 0.07. Sawteeth are observed in CTH and have a similar phenomenology to tokamak sawteeth despite employing a 3D confining field. Application of vacuum transform has been demonstrated to reduce and eliminate the vertical drift of elongated discharges. Internal SXR diagnostics, in conjunction with external magnetics, extend the range of reconstruction accuracy into the plasma core [Preview Abstract] |
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CP11.00060: Thomson scattering diagnostic on the Compact Toroidal Hybrid Experiment P.J. Traverso, D.A. Ennis, G.J. Hartwell, J.D. Kring, D.A. Maurer A Thomson scattering system is being commissioned for the non-axisymmetric plasmas of the Compact Toroidal Hybrid (CTH), a five-field period current-carrying torsatron. The system takes a single point measurement at the magnetic axis to both calibrate the two-color soft x-ray $T_e$ system and serve as an additional diagnostic for the V3FIT 3D equilibrium reconstruction code. A single point measurement will reduce the uncertainty in the reconstructed peak pressure by an order of magnitude for both current-carrying plasmas and future gyrotron-heated stellarator plasmas. The beam, generated by a frequency doubled Continuum 2 J, Nd:YAG laser, is passed vertically through an entrance Brewster window and a two-aperture optical baffle system to minimize stray light. Thomson scattered light is collected by two adjacent f/2 plano-convex condenser lenses and routed via a fiber bundle through a Holospec f/1.8 spectrograph. The red-shifted scattered light from 533-563 nm will be collected by an array of Hamamatsu H11706-40 PMTs. The system has been designed to measure plasmas with core $T_e$ of 100 to 200 eV and densities of $5\times10^{18}$ to $5\times10^{19}$ $m^{-3}$. Stray light and calibration data for a single wavelength channel will be presented. [Preview Abstract] |
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CP11.00061: Coherence Imaging Measurements of Impurity Flow in the CTH and W7-X Experiments D.A. Ennis, N.R. Allen, G.J. Hartwell, C.A. Johnson, D.A. Maurer, S.L. Allen, C.M. Samuell, D. Gradic, R. Konig, V. Perseo, W7-X Team Measurements of impurity ion emissivity and velocity in the Compact Toroidal Hybrid (CTH) experiment are achieved with a new optical coherence imaging diagnostic. The Coherence Imaging Spectroscopy (CIS) technique uses an imaging interferometer of fixed delay to provide 2D spectral images, making it ideal for investigating the non-axisymmetric geometry of CTH plasmas. Preliminary analysis of C III interferograms indicate a net toroidal flow on the order of 10 km/s during the time of peak current. Bench tests using Zn and Cd light sources reveal that the temperature of the interferometer optics must be controlled to within 0.01\textdegree C to limit phase drift resulting in artificially measured flow. A new collaboration between Auburn University and the Max-Planck-Institute for Plasma Physics is underway to develop two new coherence imaging instruments for ion impurity flow measurements in orthogonal directions to investigate the 3D physics of the W7-X island divertor during OP1.2. A continuous wave laser tunable over most of the visible region will be incorporated to provide immediate and accurate calibrations of both CIS systems during plasma operations. [Preview Abstract] |
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CP11.00062: Non-axisymmetric equilibrium reconstruction and suppression of density limit disruptions in a current-carrying stellarator Xinxing Ma, D. A. Ennis, J. D. Hanson, G. J. Hartwell, S. F. Knowlton, D. A. Maurer Non-axisymmetric equilibrium reconstructions have been routinely performed with the V3FIT code in the Compact Toroidal Hybrid (CTH), a stellarator/tokamak hybrid. In addition to $50$ external magnetic measurements, $160$ SXR emissivity measurements are incorporated into V3FIT to reconstruct the magnetic flux surface geometry and infer the current distribution within the plasma. Improved reconstructions of current and $q$ profiles provide insight into understanding the physics of density limit disruptions observed in current-carrying discharges in CTH. It is confirmed that the final scenario of the density limit of CTH plasmas is consistent with classic observations in tokamaks: current profile shrinkage leads to growing MHD instabilities (tearing modes) followed by a loss of MHD equilibrium. It is also observed that the density limit at a given current linearly increases with increasing amounts of 3D shaping fields. Consequently, plasmas with densities up to two times the Greenwald limit are attained. Equilibrium reconstructions show that addition of 3D fields effectively moves resonance surfaces towards the edge of the plasma where the current profile gradient is less, providing a stabilizing effect. [Preview Abstract] |
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CP11.00063: Simulations of Low-q Disruptions in the Compact Toroidal Hybrid Experiment E.C. Howell, J.D. Hanson, D.A. Ennis, G.J. Hartwell, D.A. Maurer Resistive MHD simulations of low-q disruptions in the Compact Toroidal Hybrid Device (CTH) are performed using the NIMROD code. CTH is a current-carrying stellarator used to study the effects of 3D shaping on MHD stability. Experimentally, it is observed that the application of 3D vacuum fields allows CTH to operate with edge safety factor less than 2.0. However, these low-q discharges often disrupt after peak current if the applied 3D fields are too weak. Nonlinear simulations are initialized using model VMEC equilibria representative of low-q discharges with weak vacuum transform. Initially a series of symmetry preserving island chains are excited at the q=6/5, 7/5, 8/5, and 9/5 rational surfaces. These island chains act as transport barriers preventing stochastic magnetic fields in the edge from penetrating into the core. As the simulation progresses, predominately m/n=3/2 and 4/3 instabilities are destabilized. As these instabilities grow to large amplitude they destroy the symmetry preserving islands leading to large regions of stochastic fields. A current spike and loss of core thermal confinement occurs when the innermost island chain (6/5) is destroyed. [Preview Abstract] |
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CP11.00064: Benchmarking calculations of the magnetic field near the last closed flux surface with finite plasma pressure and current J.C. Schmitt, A. Bader, M.R. Cianciosa, M. Drevlak, H. Frerichs, S.A. Lazerson, J.D. Lore, D. Maurer The calculation of the magnetic field near the last closed flux surface in the presence of finite plasma pressure and current is important because of its impact on subsequent calculations. Subtle differences in the calculation of the magnetic field results in different magnetic field line trajectories, which are subsequently used to generate a `field-aligned' grids for edge transport and divertor modeling with the EMC3-EIRENE code. Two methods of field calculations are discussed. One uses the virtual casing (VC) theorem and the other uses the magnetic vector potential. The VC theorem is implemented in the DIAGNO and EXTENDER codes, while the BMW code uses the magnetic vector potential. For both methods, a plasma equilibrium provided by VMEC is provided. Differences in the magnetic field and subsequent calculations for W7-X configurations will be presented and discussed. This work will help determine the strategy on how best to pass reconstructed equilibrium information from V3FIT to EMC3/EIRENE. [Preview Abstract] |
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CP11.00065: Non-resonant divertors for stellarators Allen Boozer, Alkesh Punjabi The outermost confining magnetic surface in optimized stellarators has sharp edges, which resemble tokamak X-points. The plasma cross section has an even number of edges at the beginning but an odd number half way through the period. Magnetic field lines cannot cross sharp edges, but stellarator edges have a finite length and do not determine the rotational transform on the outermost confining surface. Just outside the last confining surface, surfaces formed by magnetic field lines have splits containing two adjacent magnetic flux tubes: one with entering and the other with an equal existing flux to the walls. The splits become wider with distance outside the outermost confining surface. These flux tubes form natural non-resonant stellarator divertors, which we are studying using maps. [Preview Abstract] |
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CP11.00066: Effect of Magnetic Islands on Divertors in Tokamaks and Stellarators Alkesh Punjabi, Allen Boozer Divertors are required for handling the plasma particle and heat exhausts on the walls in fusion plasmas. Relatively simple methods, models, and maps from field line Hamiltonian are developed to better understand the interaction of strong plasma shaping and magnetic islands on the size and behavior of the magnetic flux tubes that go from the plasma edge to the wall in non-axisymmetric system. This approach is applicable not only in tokamaks but also in stellarators. Stellarator diverters in which magnetic islands are dominant are called resonant and when shaping is dominant are called non-resonant. Optimized stellarators generally have sharp edges on their surface, but unlike the case for tokamaks these edges do not encircle the entire plasma, so they do not define an edge value for the rotational transform. The approach is used in the DIII-D tokamak. Computation results are consistent with the predictions of the models. Further simulations are being done to understand why the transition from an effective cubic to a linear increase in loss time and area of footprint occurs and whether this increase is discontinuous or not. This work is supported by the US DOE grants DE-FG02-01ER54624 and DE-FG02-04ER54793 to Hampton University and DE-FG02-95ER54333 to Columbia University. This research used resources of the NERSC, supported by the Office of Science, US DOE, under Contract No. DE-AC02-05CH11231. [Preview Abstract] |
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CP11.00067: Nonsymmetric 3D MHD equilibrium and radial localization of trapped particles Wrick Sengupta, Harold Weitzner Quasisymmetry and omnigeneity are key ideas proposed to ensure radial confinement of trapped particles in a stellarator. These constraints have stringent restrictions on magnetic geometry, some aspects of which are yet to be fully explored. In this work we obtain a local 3D MHD equilibrium expansion by analytically solving the MHD equilibrium equations. This expansion, although local, is sufficient to explore the deeply trapped particle physics, since it is carried out around a region of local minima of the magnitude of the magnetic field. Based on this analytical 3D equilibrium solution, we obtain the aforementioned constraints. We then extend this local analysis to a global one by expanding around the magnetic axis of the stellarator. Effects of curvature and torsion of the axis are treated self-consistently. We demonstrate that due to toroidal mode coupling, the expansion in flux coordinate near the axis is logarithmic and not purely algebraic. These non analytic terms can not be in general neglected. Our results show that it is far easier to satisfy the omnigeneity condition than the quasisymmetry requirement. This implies, that there exists a large class of equilibrium close to quasisymmetry, which are still omnigeneous and allow inclusion of symmetry breaking error fields. [Preview Abstract] |
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CP11.00068: Relaxed MHD equilibria inside 3D shaped conducting surfaces A. Hassam, J. Tenbarge, W. Dorland, M. Landreman, W. Sengupta A 3D nonlinear dissipative MHD code is developed to allow relaxation to low-beta MHD equilibrium inside a shaped 3D conducting boundary with prescribed conserved axial magnetic flux and no external current. Formation of magnetic islands is allowed. Heat sources would be eventually introduced to allow possible non-stationary convection depending on the MHD stability properties. The initial development is done using UMHD (Guzdar et al, PF, 1993). A primary objective is to minimize numerical boundary noise. In particular, codes which specify the normal magnetic field \textbf{B.n} on bounding surfaces are prone to boundary noise generation. We shape the boundary to conform to the desired field shape so that \textbf{B.n} is zero on the boundary, employing curvilinear coordinates. Significant noise reduction has been achieved by this approach. Boundary noise is strongly suppressed if the boundary is modeled as a sharp ramp-down in resistivity, allowing relaxation to equilibrium but no penetration into the low resistivity region. Initial results have been verified w.r.t. analytic calculation in the weak shaping limit. A rotational transform is observed in helical shaping. Relaxed equilibria inside helically symmetric conducting boundaries will be presented. [Preview Abstract] |
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CP11.00069: Influence of Thermal Anisotropy on Equilibrium Stellarator Beta Limits T. A. Bechtel, C. C. Hegna, C. R. Sovinec The effect of anisotropic heat conduction on the upper beta limit of stellarator plasmas is studied using the nonlinear, extended MHD code NIMROD. The configuration under investigation is an l=2, M=10 torsatron with vacuum rotational transform near unity. Finite-beta plasmas are created using a volumetric heating source and temperature dependent resistivity; modeled with 22 stellarator symmetric (integer multiples of M) toroidal modes. Extended MHD simulations are then performed to generate steady state solutions that represent 3D equilibria. With increased heating, Shafranov shifts occur, and the associated break up of edge magnetic surfaces limits the achievable beta. Due to the presence of finite parallel heat conduction, pressure profiles can exist in regions of magnetic stochasticity. Here, we present results of independently varying the parallel and perpendicular thermal anisotropy. In particular, simulations show that the attained stored energy is a function of the magnitude of parallel and perpendicular thermal conduction for a given heat source, indicating that equilibrium beta limits are sensitive to anisotropic transport properties. Preliminary studies of MHD stability with non-stellarator symmetric modes, near the highest achievable beta, are also presented. [Preview Abstract] |
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CP11.00070: Extension of the XGC code for global gyrokinetic simulations in stellarator geometry Michael Cole, Toseo Moritaka, Roscoe White, Robert Hager, Seung-Hoe Ku, Choong-Seock Chang In this work, the total-f, gyrokinetic particle-in-cell code XGC is extended to treat stellarator geometries. Improvements to meshing tools and the code itself have enabled the first physics studies, including single particle tracing and flux surface mapping in the magnetic geometry of the heliotron LHD and quasi-isodynamic stellarator Wendelstein 7-X. These have provided the first successful test cases for our approach. XGC is uniquely placed to model the complex edge physics of stellarators. A roadmap to such a global confinement modeling capability will be presented. Single particle studies will include the physics of energetic particles’ global stochastic motions and their effect on confinement. Good confinement of energetic particles is vital for a successful stellarator reactor design. These results can be compared in the core region with those of other codes, such as ORBIT3d. In subsequent work, neoclassical transport and turbulence can then be considered and compared to results from codes such as EUTERPE and GENE. After sufficient verification in the core region, XGC will move into the stellarator edge region including the material wall and neutral particle recycling. [Preview Abstract] |
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CP11.00071: GTC simulations of ion temperature gradient driven instabilities in W7-X and LHD stellarators Hongyu Wang We report GTC linear simulations of ion temperature gradient (ITG) instabilities in Wendelstein7-X (W7-X) and Large Helical Device (LHD) stellarators. GTC has recently been updated to treat 3D equilibria by interfacing with MHD equilibrium code VMEC. GTC simulations of ITG have been carried out in both full torus and partial torus taking into account the toroidal periodicity of the stellarators. The effects of toroidal mode coupling on linear dispersions and mode structures in W7-X and LHD are studied. The mode structure in W7-X is more localized in the toroidal direction, and LHD is more extended in the toroidal direction and tokamak-like. Linear growth rates, real frequencies, and mode structures agree reasonably with results of EUTERPE simulations. In collaboration with I. Holod, J. Riemann, Z. Lin, J. Bao, L. Shi, S. Taimourzadeh, R. Kleiber, and M. Borchardt. [Preview Abstract] |
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CP11.00072: Theory of turbulent saturation in stellarators: identifying mechanisms to reduce turbulent transport C. C. Hegna, P. W. Terry, B. J. Faber A theory for ion temperature gradient (ITG) turbulent saturation in stellarators is developed using a three field fluid model that allows for general 3D geometry. The model relies on the paradigm of nonlinear energy transfer from unstable to damped eigenmodes at comparable wavelength as the dominant saturation process. This mechanism is enabled by a three-wave interaction where the third mode primarily regulates the nonlinear energy transfer rate and depends upon the properties of the magnetic geometry. In particular, this work suggests that quasi-helically symmetric configurations may have an intrinsic advantage with regard to turbulent saturation physics relative to other configurations as multiple energy transfer channels can be exploited. Nonlinear energy transfer physics is quantified by the product of a turbulent correlation lifetime as computed from a three-wave frequency mismatch and a geometric coupling coefficient with larger turbulent correlation times denoting larger levels of nonlinear energy transfer and hence smaller turbulent transport. The theory provides an analytic prediction for how 3D shaping can be tuned to lower turbulent transport through saturation processes that can by used in optimization schemes for improved stellarator design. [Preview Abstract] |
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CP11.00073: A multi-institutional Stellarator Configuration Study David Gates A multi-institutional study aimed at mapping the space of quasi-axisymmetric stellarators has begun. The goal is to gain improved understanding of the dependence of important physics and engineering parameters (e.g. bootstrap current, stability, coil complexity, etc.) on plasma shape (average elongation, aspect ratio, number of periods). In addition, the stellarator optimization code STELLOPT will be upgraded with new capabilities such as improved coil design algorithms such as COILOPT$++$ and REGCOIL, divertor optimization options, equilibria with islands using the SPEC code, and improved bootstrap current calculations with the SFINCS code. An effort is underway to develop metrics for divertor optimization. STELLOPT has also had numerous improvements to numerical algorithms and parallelization capabilities. Simultaneously, we also are pursuing the optimization of turbulent transport according to the method of proxy functions. Progress made to date includes an elongation scan on quasi-axisymmetric equilibria and an initial comparison between the SFINCS code and the BOOTSJ calculation of bootstrap current currently available in STELLOPT. Further progress on shape scans and subsequent physics analysis will be reported. The status of the STELLOPT upgrades will be described. The eventual goal of this exercise is to identify attractive configurations for future US experimental facilities.. [Preview Abstract] |
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CP11.00074: Recent Results from HSX and New Directions David Anderson, HSX Team HSX has demonstrated many improvements resulting from quasi-helical symmetry in the magnetic field. Work has continued on measuring the plasma radial electric field and bootstrap current utilizing the Pfrisch-Schluter flows, with a new MSE diagnostic under development. With suppressed neoclassical transport, turbulent transport is dominant in HSX. Experimental measurements of turbulence have been undertaken with probes in the edge and interferometry in the core. GENE has shown TEM to be dominant mode with heat flux comparable to measurements. No profile stiffness is observed under present conditions. These studies are being extended to look at comparisons between heat flux and density fluctuations as a function of gradient scale length. A new microwave scattering diagnostic and CECE are being implemented. Laser blow-off impurity studies are underway. Energetic ion confinement, a key issue for stellarators, will be studied using NBI of energetic deuterons into a deuterium plasma as done in MST and CHS. Improved coil design is being examined using the REGCOIL and FOCUS codes, with a goal of reduced ripple and coil complexity for either an upgrade to HSX or a new device to extend quasisymmetry studies into hot ion physics [Preview Abstract] |
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CP11.00075: Measurements and modeling of radial electric field and bootstrap flows in HSX S. T. A. Kumar, T. J. Dobbins, W. Goodman, J. N. Talmadge, F. S. B. Anderson, K. M. Likin, D. T. Anderson, M. Landreman Counter-streaming Pfirsch-Schl\"{u}ter (PS) parallel ion flows have been measured in HSX using charge exchange recombination spectroscopy. This method has provided an improved measurement of the radial electric field and ion bootstrap flows in the core of the plasma. The magnitudes of the experimentally measured radial electric field and ion bootstrap flows do not demonstrate the neoclassical features calculated with the PENTA code; the measured electric field values agrees with ion-root solution and the measured ion bootstrap flows agree with the electron-root solution. Several approaches have been undertaken recently to understand this discrepancy, for example, using a biased electrode to look for the helical ion resonance and improvements in the neoclassical calculation. Recent advances in measurements and modeling are presented. [Preview Abstract] |
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CP11.00076: Evolution of intrinsic flows and radial electric field in HSX stellarator T. J. Dobbins, S. T. A. Kumar, J. N. Talmadge, K. M. Likin, F. S. B. Anderson, D. T. Anderson The inboard/outboard asymmetry in impurity ion flow on the HSX stellarator is measured by the Charge Exchange Recombination Spectroscopy (CHERS) diagnostic. This allows the calculation of the bootstrap and Pfirsch-Schluter flows on a flux surface. The evolution of bootstrap flows has been measured in the HSX stellarator and compared with the evolution of bootstrap current. The flow evolution was found to be dependent on magnetic configuration and plasma parameters, while the electric field was found to be constant in time. With 50 kW ECRH heating, the bootstrap flow decreased from 13 to 8 km/s from the beginning to the end of the discharge in the core of the plasma (r/a of .2) while the electric field remained constant at 2.5 kV/m. The Pfirsch-Schluter flow measurements are used to find the radial electric field. These measurements of radial electric field are compared the neoclassical values calculated by the PENTA[1]. [1] J. Lore at al., Phys. Plasmas 17 (2010) 056101 [Preview Abstract] |
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CP11.00077: Carbon impurity measurements in the HSX stellarator J. M. Mohoney, S. T. A. Kumar, K. M. Likin, D. T. Anderson Impurity behavior in stellarators is not fully understood despite important implications on device performance, in particular, an accumulation of core impurities can lead to degradation of plasma energy due to radiative losses. Experiments are being conducted at HSX to measure the radial profiles and the time history of carbon impurity density using the charge exchange recombination spectroscopy (CXRS) diagnostic. Measurements of fully ionized carbon have been performed on various magnetic configurations, showing a peaked profile at the core in the standard configuration. An inversion technique was also developed to calculate localized C +5 profiles. Comparisons of impurity behavior between the standard and broken-symmetry configurations are presented. [Preview Abstract] |
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CP11.00078: Turbulence Measurements by Interferometry and Far-forward Scattering on the HSX Stellarator C.B. Deng, D.L. Brower, D.T. Anderson, F.S.B. Anderson, K.M. Likin, J.N. Talmadge After neo-classical transport was reduced by restoring symmetry along the helical axis, a primary physics goal for HSX is to study how 3-D shaping can reduce turbulence thereby requiring measurement of turbulence with $k_{y}\rho_{s} $up to 1. For characteristic HSX parameters (Te \textasciitilde 200 eV at r/a \textasciitilde 0.5 where the density gradient peaks), this condition corresponds to $k_{y} $up to 7 cm$^{\mathrm{-1}}$. To accommodate this goal, a new 9-chord HSX interferometer/far-forward scattering system has been designed to measure density turbulence at higher k. The new system employing two high-power (30 mW each, 320 GHz), solid-state sources with frequency offset up to 6 MHz. This will permit true heterodyne detection, thereby realizing faster measurement time response, increased bandwidth and reduced noise. High power sources and high sensitivity planar-diode mixers will allow us to reduce the aperture of the receiver optics to a few mm thereby increasing the maximum wavenumber to k\textasciitilde 15 cm$^{\mathrm{-1}}$. Reconfiguring the interferometer system into a finite-angle collective scattering arrangement is also planned as it will increase the measured k-spectrum up to 18 cm$^{\mathrm{-1}}$ with some spatial resolution (core or edge). [Preview Abstract] |
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CP11.00079: Experimental study of Nonlinear, Multi-Scale Turbulence in the HSX stellarator S. Ohshima, C.B. Deng, R.S. Wilcox, T. Nishizawa, A.F. Almagri, K.M. Likin, J.N. Talmadge, D.T. Anderson, F.S.B Anderson, S.R. Sarff Micro scale turbulence depends on parameters such as local magnetic shear and curvature, and also excitation and damping mechanisms of zonal flows relate to the topology of the configuration. In the HSX stellarator, the Langmuir probe measurements indicate that a nonlinear interaction exists among a zonal flow like mode in the frequency range up to 5 kHz, a coherent mode at 20 kHz, and broadband turbulence. These two coherent modes are interacting with broadband fluctuation, and moreover these modes couples with each other. Interestingly, the nonlinear interaction appears differently depending on the location on the flux surface, which demonstrates a toroidal asymmetry, attributed to three dimensional configurations, exists on the multi-scale interactions. The detail of the analysis results and a new dedicated experiment for zonal flow physics in HSX using a newly designed capacitive probe will be discussed in this poster. [Preview Abstract] |
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CP11.00080: TEM heat transport and fluctuations in the HSX stellarator: experiments and comparison with gyrokinetic simulation J. Smoniewski, B.J. Faber, E. S\'anchez, I. Calvo, M.J. Pueschel, K.M. Likin, C.B. Deng, J.N. Talmadge The Helically Symmetric eXperiment (HSX) has demonstrated reduced neoclassical transport in the plasma core with quasi-symmetry [Lore Thesis 2010], while outside this region the electron thermal diffusivity is well above the neoclassical level, likely due to the Trapped Electron Mode (TEM) [Weir PoP 2015, Faber PoP 2015]. We compare gyrokinetic simulations of the TEM to experimental heat flux and density fluctuation measurements for two configurations: Quasi-Helical Symmetry (QHS) and broken symmetry (Mirror). Both experiment and simulation show that the heat flux for Mirror is larger than for QHS by about a factor of two. Initial interferometer measurements provide evidence that density-gradient-driven TEMs are driving turbulence. Calculations of the collisionless damping of zonal flows provide another perspective into the difference between geometries. Similar to other stellarators [Monreal PPCF 2016], the zonal flow residual goes to zero at long wavelengths in both configurations. Additionally, the very short time decay of the zonal flow due to neoclassical polarization is constant between configurations. However, the collisionless damping time is longer and the zonal flow oscillation frequency is smaller in QHS than Mirror, consistent with reduced radial particle drifts. [Preview Abstract] |
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CP11.00081: Comparing Turbulent Transport in Quasi-Helically Symmetric and Quasi-Axisymmetric Stellarators I.J. McKinney, M.J. Pueschel, J.N. Talmadge, D.T. Anderson, B.J. Faber, H.E. Mynick Stellarator optimization of turbulent transport requires designing a magnetic geometry unfavorable to excitation of microinstabilities and turbulence. This work focuses on a comprehensive comparison of two neoclassically optimized configurations, quasi-axisymmetry (NCSX) and quasi-helical symmetry (HSX), using a hierarchy of gyrokinetic models from adiabatic to kinetic electrons to fully electromagnetic physics. Linear simulations of the ion-temperature-gradient-driven mode with gyrokinetic code \textsc{Gene} reveal distinct differences between geometries. There are a significantly greater number of unstable eigenmodes in quasi-helical symmetry, with the long wavelength eigenmodes being more slab-like for quasi-helical symmetry as opposed to toroidal-like for quasi-axisymmetry. Additionally, each configuration has unique finite-$\beta$ and kinetic ballooning characteristics. Nonlinear simulations show key differences in transport levels and scaling between flux tubes in each geometry. These findings inform the next step: changing magnetic geometry to affect microturbulence. [Preview Abstract] |
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CP11.00082: Laser Blow-Off Impurity Injection Experiments at the HSX Stellarator J.F. Castillo, A. Bader, K.M. Likin, D.T. Anderson, F.S.B. Anderson, S.T.A. Kumar, J.N. Talmadge Results from the HSX laser blow-off experiment are presented and compared to a synthetic diagnostic implemented in the STRAHL impurity transport modeling code in order to measure the impurity transport diffusivity and convective velocity. A laser blow-off impurity injection system is used to rapidly deposit a small, controlled quantity of aluminum into the confinement volume. Five AXUV photodiode arrays are used to take time-resolved measurements of the impurity radiation. The spatially one-dimensional impurity transport code STRAHL is used to calculate a time-dependent plasma emissivity profile. Modeled intensity signals calculated from a synthetic diagnostic code provide direct comparison between plasma simulation and experimental results. An optimization algorithm with impurity transport coefficients acting as free parameters is used to fit the model to experimental data. [Preview Abstract] |
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CP11.00083: First steps in investigating fast ion confinement on the HSX stellarator E.M. Schilling, K.M. Likin, F.S.B. Anderson, D.T. Anderson The Helically Symmetric eXperiment (HSX) is a Quasi-Helically Symmetric (QHS) stellarator that has been successfully optimized for improved neoclassical confinement, but fast ion confinement has not yet been investigated. Fast ion studies have been performed on similar experiments, such as the Madison Symmetric Torus (MST)\footnote{G. Fiksel et al., PRL \textbf{95}, 125001 (2005).} and the Compact Helical System (CHS)\footnote{M. Isobe et al., RSI \textbf{68}, 532 (1997).}, but not yet for a QHS geometry. A $20$ kV, $0.5$ MW, $1.2$ ms beam system has been adapted for use on HSX to perform such a study. By calculating the charge exchange and electron/proton impact cross sections for an approximated HSX plasma, a beam attenuation of at least $15\%$ has been predicted. The density of beam ions has then been calculated together with a target ion density assuming some fast ion confinement, and a resulting D-D fusion rate has been predicted to produce no less than $1x10^{6}$ neutrons/sec overall. Once the beam system is mounted onto HSX, this neutron flux will be measured by a neutron detector and a fast ion confinement time will be inferred. Currently, a test vacuum chamber with basic diagnostics is being constructed to verify the beam's published performance characteristics. [Preview Abstract] |
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CP11.00084: Development of a limiter imaging system at the Helically Symmetric Experiment C. Buelo, L. Stephey, A. Bader, F.S.B. Anderson, D. Eisert, D.T. Anderson A visible camera diagnostic has been developed to study the HSX limiter plasma interaction. A straight line view from the camera location to the limiter was not possible due to the complex 3D stellerator geometry of HSX, so a mirror/lens system was inserted into the plasma edge. A custom support structure for this optical system tailored to the HSX geometry was designed and installed which allows the system to be inserted and retracted as needed. The camera system has been absolutely calibrated and using H$_{\mathrm{\alpha }}$ and C-III filters, can provide hydrogen and carbon photon fluxes, which through an S/XB coefficient, can be converted into particle fluxes. The resulting measurements have been used to obtain the characteristic penetration length of these species and will in the future be compared to magnetic field line following calculations and plasma edge simulations using EMC3-EIRENE to better understand the physics in the HSX edge. [Preview Abstract] |
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CP11.00085: Improving Coil Designs for the HSX Stellarator with FOCUS Thomas Kruger, Caoxiang Zhu, Aaron Bader, Luquant Singh, David Anderson We use the FOCUS code to generate improved coil sets for the HSX stellarator. FOCUS produces curves in 3D space to best reproduce a target plasma equilibrium. Unlike similar codes, the curves in FOCUS are not constrained to lie on a user-defined 2D surface. Therefore FOCUS can inherently solve problems such as determining the optimum coil-plasma distance for a given equilibrium. By adjusting the relative weights between a) the match to the plasma boundary, and b) the average coil length. We present the results from FOCUS where we attempt to improve the coil set by moving coils further away to reduce coil ripple, decreasing the number of coils to improve accessibility, and better matching the target plasma surface. We also present results of alternative coil designs with helical and saddle coils. [Preview Abstract] |
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CP11.00086: Optimizing stellarator coil winding surfaces with Regcoil Aaron Bader, Matt Landreman, David Anderson, Chris Hegna We show initial attempts at optimizing a coil winding surface using the Regcoil code [1] for selected quasi helically symmetric equilibria. We implement a generic optimization scheme which allows for variation of the winding surface to allow for improved diagnostic access and allow for flexible divertor solutions. Regcoil and similar coil-solving algorithms require a user-input winding surface, on which the coils lie. Simple winding surfaces created by uniformly expanding the plasma boundary may not be ideal. Engineering constraints on reactor design require a coil-plasma separation sufficient for the introduction of neutron shielding and a tritium generating blanket. This distance can be the limiting factor in determining reactor size. Furthermore, expanding coils in other regions, where possible, can be useful for diagnostic and maintenance access along with providing sufficient room for a divertor. We minimize a target function that includes as constraints, the minimum coil-plasma distance, the winding surface volume, and the normal magnetic field on the plasma boundary. Results are presented for two quasi-symmetric equilibria at different aspect ratios. [Preview Abstract] |
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CP11.00087: Application of adjoint methods for stellarator geometry sensitivity using REGCOIL Elizabeth Paul, Matt Landreman, William Dorland A significant challenge to the feasibility of the stellarator is the design of simple coils which allow for diagnostic access and optimal physics properties. REGCOIL [Landreman, Nucl. Fusion 57 (2017) 046003] employs a Tikhonov regularization approach to compute the current potential on a specified coil winding surface given a desired plasma surface. An objective function, which includes the normal magnetic field on the plasma surface and the squared current density, is minimized by solving a single linear system. This method achieves lower surface-integrated and maximum current potential and normal magnetic field and allows for greater control over the level of regularization than the NESCOIL method. We extend the REGCOIL approach by computing sensitivity of the objective function with respect to coil geometry parameters. We apply an adjoint method, a common technique for shape optimization problems in aerodynamics, allowing the gradients with respect to a large number of control parameters to be computed rapidly. We compute the sensitivity by analytically differentiating the objective function. This extended REGCOIL approach can be applied within an optimization iteration to obtain coil surfaces which better reproduce the desired plasma shape and maximize coil-coil separation. [Preview Abstract] |
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CP11.00088: DISRUPTIONS AND MHD |
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CP11.00089: Comparison of Cryogenic Pellet Shatter Theory to Experiments using Disruption Mitigation Pellets T. E. Gebhart, S. K. Combs, S. J. Meitner, L. R. Baylor, P. B. Parks, T. Ha Mitigating disruptions is essential for high energy density tokamaks such as ITER. The technique of injecting large shattered cryogenic pellets is presently the best option. To better understand the mitigation of a disruption using shattered pellet injection (SPI), we must better understand the process behind the shattering and subsequent flight of the shattered pellet material. The main questions that are being addressed are 1) what criteria must be met for a pellet to break upon impact with an angled surface? and 2) what is the resulting particle size distribution after shattering? These questions are addressed using theoretical shattering models and comparison with experimental measurements. Solid deuterium, neon, and argon are used in the various phases of disruption mitigation (DM) and thus, an overall model must accommodate the shattering of all mixtures of these gasses. Designs of SPI disruption mitigation systems are heavily influenced by the strategic shattering of pellets just before the entering the plasma. Experimental apparatuses that include pellet shatter tubes for JET and DIII-D were tested along with a large pellet, small angle, impact test that has implications for the ITER DM system. Comparison of the shattering measurements with the theoretical models will be shown. [Preview Abstract] |
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CP11.00090: Simulations of Neon Pellets for Plasma Disruption Mitigation in Tokamaks Nicolas Bosviel, Roman Samulyak, Paul Parks Numerical studies of the ablation of neon pellets in tokamaks in the plasma disruption mitigation parameter space have been performed using a time-dependent pellet ablation model based on the front tracking code FronTier-MHD. The main features of the model include the explicit tracking of the solid pellet/ablated gas interface, a self-consistent evolving potential distribution in the ablation cloud, JxB forces, atomic processes, and an improved electrical conductivity model. The equation of state model accounts for atomic processes in the ablation cloud as well as deviations from the ideal gas law in the dense, cold layers of neon gas near the pellet surface. Simulations predict processes in the ablation cloud and pellet ablation rates and address the sensitivity of pellet ablation processes to details of physics models, in particular the equation of state. [Preview Abstract] |
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CP11.00091: Interpreting Disruption Prediction Models to Improve Plasma Control Matthew Parsons In order for the tokamak to be a feasible design for a fusion reactor, it is necessary to minimize damage to the machine caused by plasma disruptions. Accurately predicting disruptions is a critical capability for triggering any mitigative actions, and a modest amount of attention has been given to efforts that employ machine learning techniques to make these predictions. By monitoring diagnostic signals during a discharge, such predictive models look for signs that the plasma is about to disrupt. Typically these predictive models are interpreted simply to give a `yes' or `no' response as to whether a disruption is approaching. However, it is possible to extract further information from these models to indicate which input signals are more strongly correlated with the plasma approaching a disruption. If highly accurate predictive models can be developed, this information could be used in plasma control schemes to make better decisions about disruption avoidance. [Preview Abstract] |
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CP11.00092: Real-time plasma event monitoring on TCV Thomas Cornelis Blanken, Federico Felici, Cristian Galperti A tokamak reactor plasma control system (PCS) supervisor must take decisions about discharge segment scheduling, exception handling (e.g. emergency shutdown), and prioritized control tasks [D. Humphreys et al, 2015 Physics of Plasmas 22 021806], based on chains of plasma events, such as stability limit violations, deviations from targets and expected/predicted behavior, and actuator failure [P.C. de Vries et al, 2011 Nuclear Fusion 51 053018]. We present first results of a real-time plasma event monitor for NTMs and locked modes on the TCV tokamak. The event monitor contains finite-state automata, with events based on user-defined thresholds on MHD amplitude signals [C. Galperti et al, 2017 IEEE Transactions on Nuclear Science 64]. This work supports the integration of high-level plasma supervision, control and actuator management for disruption avoidance experiments. [Preview Abstract] |
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CP11.00093: Disruption Event Characterization and Forecasting in Tokamaks* J.W. Berkery, S.A. Sabbagh, Y.S. Park, J.H. Ahn, Y. Jiang, J.D. Riquezes, S.P. Gerhardt, C.E. Myers The Disruption Event Characterization and Forecasting (DECAF) code, being developed to meet the challenging goal of high reliability disruption prediction in tokamaks, automates data analysis to determine chains of events that lead to disruptions and to forecast their evolution. The relative timing of magnetohydrodynamic modes and other events including plasma vertical displacement, loss of boundary control, proximity to density limits, reduction of safety factor, and mismatch of the measured and desired plasma current are considered. NSTX/-U databases are examined with analysis expanding to DIII-D, KSTAR, and TCV. Characterization of tearing modes has determined mode bifurcation frequency and locking points. In an NSTX database exhibiting unstable resistive wall modes (RWM), the RWM event and loss of boundary control event were found in 100{\%}, and the vertical displacement event in over 90{\%} of cases. A reduced kinetic RWM stability physics model [1] is evaluated to determine the proximity of discharges to marginal stability. The model shows high success as a disruption predictor (greater than 85{\%}) with relatively low false positive rate. [1] J.W. Berkery, et al., Phys. Plasmas \textbf{24} (2017) 506103. $^{\mathrm{\ast }}$Supported by US DOE Contracts DE-FG02-99ER54524, DE-AC02-09CH11466, and DE-SC0016614. [Preview Abstract] |
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CP11.00094: Studies of the DIII-D disruption database using Machine Learning algorithms Cristina Rea, Robert Granetz, Orso Meneghini A Random Forests Machine Learning algorithm, trained on a large database of both disruptive and non-disruptive DIII-D discharges, predicts disruptive behavior in DIII-D with about 90\% of accuracy. Several algorithms have been tested and Random Forests was found superior in performances for this particular task. Over 40 plasma parameters are included in the database, with data for each of the parameters taken from $\sim$500k time slices. We focused on a subset of non-dimensional plasma parameters, deemed to be good predictors based on physics considerations. Both binary (disruptive/non-disruptive) and multi-label (label based on the elapsed time before disruption) classification problems are investigated. The Random Forests algorithm provides insight on the available dataset by ranking the relative importance of the input features. It is found that q$_{95}$ and Greenwald density fraction (n/n$_{G}$) are the most relevant parameters for discriminating between DIII-D disruptive and non-disruptive discharges. A comparison with the Gradient Boosted Trees algorithm is shown and the first results coming from the application of regression algorithms are presented. [Preview Abstract] |
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CP11.00095: Disruption Warning Database Development and Exploratory Machine Learning Studies on Alcator C-Mod Kevin Montes, Cristina Rea, Robert Granetz A database of about 1800 shots from the 2015 campaign on the Alcator C-Mod tokamak is assembled, including disruptive and non-disruptive discharges. The database consists of $\sim$40 relevant plasma parameters with data taken from $\sim$160k time slices. In order to investigate the possibility of developing a robust disruption prediction algorithm that is tokamak-independent, we focused machine learning studies on a subset of dimensionless parameters such as $\beta_{p}$, $n/n_{G}$, etc. The Random Forests machine learning algorithm provides insight on the available data set by ranking the relative importance of the input features. Its application on the C-Mod database, however, reveals that virtually no one parameter has more importance than any other, and that its classification algorithm has a low rate of successfully predicted samples, as well as poor false positive and false negative rates. Comparing the analysis of this algorithm on the C-Mod database with its application to a similar database on DIII-D, we conclude that disruption prediction may not be feasible on C-Mod. This conclusion is supported by empirical observations that most C-Mod disruptions are caused by radiative collapse due to molybdenum from the first wall, which happens on just a 1-2ms timescale. [Preview Abstract] |
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CP11.00096: Development of a 1.5D plasma transport code for coupling to full orbit runaway electron simulations J.D. Lore, D. del Castillo-Negrete, L. Baylor, L. Carbajal A 1.5D (1D radial transport $+$ 2D equilibrium geometry) plasma transport code is being developed to simulate runaway electron generation, mitigation, and avoidance by coupling to the full-orbit kinetic electron transport code KORC [1]. The 1.5D code solves the time-dependent 1D flux surface averaged transport equations with sources for plasma density, pressure, and poloidal magnetic flux, along with the Grad-Shafranov equilibrium equation for the 2D flux surface geometry. Disruption mitigation is simulated by introducing an impurity neutral gas `pellet', with impurity densities and electron cooling calculated from ionization, recombination, and line emission rate coefficients. Rapid cooling of the electrons increases the resistivity, inducing an electric field which can be used as an input to KORC. The runaway electron current is then included in the parallel Ohm's law in the transport equations. The 1.5D solver will act as a driver for coupled simulations to model effects such as timescales for thermal quench, runaway electron generation, and pellet impurity mixtures for runaway avoidance. Current progress on the code and details of the numerical algorithms will be presented. [1] L. Carbajal, et al, Phys. Plasmas 24, 042512 (2017). [Preview Abstract] |
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CP11.00097: Lifetime and Universal Distribution of the Seed Runaway Electrons Adrian Fontanilla, Boris Breizman The lifetime (LT) of pre-existing runaway electrons (RE) determines how likely the RE will undergo avalanche multiplication. We calculate the LT of RE via the kinetic equation (KE). We show that the rate of thermalization of RE depends on the value of the parameter $\alpha \equiv (Z+1)/\sqrt{\tau_{rad}}$ (where $\tau_{rad}$ is the synchrotron time scale normalized to the collisional and $Z$ is the ion charge) compared to the electric field. We identify two cases where the rate is slow enough to enable a transformation of the KE into an eigenequation; the eigenfunction typifies the shape of the distribution function and the eigenvalue is the LT. In one case, $\alpha^2 \ll 1$: the field required to sustain the pre-existing runaways is barely larger than the Connor-Hastie field, $E_C$. In the same manner as Aleynikov and Breizman\footnote{P.~Aleynikov and B.N.~Breizman \textit{PRL 114}, 155001 (2015)} we solve the KE perturbatively but extend the work to demonstrate that the LT grows exponentially with the field at a rate that depends on $\alpha$. In the second case, $\alpha^2 \gg 1$: the requisite field is much greater than $E_C$. The largeness of the field in this case enables us to universalize the KE via rescaling procedure. [Preview Abstract] |
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CP11.00098: Topological Depedendence of Runaway Avalanche Threshold in Momentum Space Christopher McDevitt, Zehua Guo, Xian-Zhu Tang A detailed study of the physics responsible for the formation of an avalanche instability of runaway electrons is carried out. A set of large-angle collision operators of varying complexity, ranging from a simple source term to a novel energy-momentum conserving form, are developed and implemented. The use of a conservative form allows for the back reaction of the secondary electrons onto the primary electrons to be accounted for. The incorporation of this feedback process requires the modification of the Coulomb logarithm in order to avoid double counting collisions. A systematic procedure for delineating small and large angle collisions, and thus avoiding the double counting of collisions, is developed. It is found that the avalanche threshold is tightly linked to the merger of an O and X point in the momentum space of the primary electrons. Such a close correlation is shown to be largely independent of the details of the large-angle collision operator employed, and thus provides a robust indicator of an avalanche instability. [Preview Abstract] |
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CP11.00099: Suppression of high-energy electrons generated in both disrupting and sustained MST tokamak plasmas M.D. Pandya, B.E. Chapman, S. Munaretto, B.S. Cornille, K.J. McCollam, C.R. Sovinec, A.M. DuBois, A.F. Almagri, J.A. Goetz High-energy electrons appearing during MST tokamak plasma disruptions are rapidly lost from the plasma due apparently to internal MHD activity. Work has just recently begun on generating and diagnosing disruptions in MST tokamak plasmas. Initial measurements show the characteristic drop in central temperature and density preceding a quench of the plasma current. This corresponds to a burst of dominantly n=1 MHD activity, which is accompanied by a short-lived burst of high-energy electrons. The short-lived nature of these electrons is suspected to be due to stochastic transport associated with the increased MHD. Earlier work shows that runaway electrons generated in low density, sustained plasmas are suppressed by a sufficiently large m=3 RMP in plasmas with q(a)$<$3. RMPs of various poloidal mode number can be generated with an array of saddle coils wound around the vertical insulated gap in MST's thick conducting shell. With an m=3 RMP, the degree of runaway suppression increases with RMP amplitude, while an m=1 RMP has little effect on the runaways[1]. Nonlinear MHD modeling with NIMROD of these MST plasmas indicates increased stochasticity with an m=3 RMP, while no such increase in stochasticity is observed with an m=1 RMP. [1] S. Munaretto et al., PoP in preparation. [Preview Abstract] |
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CP11.00100: Nonlinear Fluid Model Of 3-D Field Effects In Tokamak Plasmas J D Callen, C C Hegna, M T Beidler Extended MHD codes (e.g., NIMROD, M3D-C1) are beginning to explore nonlinear effects of small 3-D magnetic fields on tokamak plasmas. To facilitate development of analogous physically understandable reduced models, a fluid-based dynamic nonlinear model of these added 3-D field effects in the base axisymmetric tokamak magnetic field geometry is being developed. The model incorporates kinetic-based closures within an extended MHD framework. Key 3-D field effects models that have been developed include: 1) a comprehensive modified Rutherford equation for the growth of a magnetic island that includes the classical tearing and NTM perturbed bootstrap current drives, externally applied magnetic field and current drives, and classical and neoclassical polarization current effects, and 2) dynamic nonlinear evolution of the plasma toroidal flow (radial electric field) in response to the 3-D fields. An application of this model to RMP ELM suppression precipitated by an ELM crash [1] will be discussed. [1] J D Callen, R Nazikian, C Paz-Soldan, N M Ferraro, M T Beidler, C C Hegna and R J La Haye, “Model of $n=2$ RMP ELM suppression in DIII-D,” report UW-CPTC 16-4 (to be published). [Preview Abstract] |
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CP11.00101: Nonlinear Modeling of Forced Magnetic Reconnection with Transient Perturbations Matthew T Beidler, James D Callen, Chris C Hegna, Carl R Sovinec Externally applied 3D magnetic fields in tokamaks can penetrate into the plasma and lead to forced magnetic reconnection, and hence magnetic islands, on resonant surfaces. Analytic theory has been reasonably successful in describing many aspects of this paradigm with regard to describing the time asymptotic-steady state [1]. However, understanding the nonlinear evolution into a low-slip, field-penetrated state, especially how MHD events such as sawteeth and ELMs precipitate this transition, is in its early development. We present nonlinear computations employing the extended-MHD code NIMROD, building on previous work [2] by incorporating a temporally varying external perturbation as a simple model for an MHD event that produces resonant magnetic signals. A parametric series of proof-of-principle computations and accompanying analytical theory characterize the transition into a mode-locked state with an emphasis on detailing the temporal evolution properties. [1] R. Fitzpatrick, Nucl. Fusion 33, 1049 (1993). [2] M.T. Beidler, J.D. Callen, C.C. Hegna, and C.R. Sovinec, Phys. Plasmas 24, 052508 (2017). [Preview Abstract] |
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CP11.00102: Algebraic motion of vertically displacing plasmas Amitava Bhattacharjee, David Pfefferle, Eero Hirvijoki The vertical displacement of tokamak plasmas is modelled during the non-linear phase by a free-moving current-carrying rod coupled to a set of fixed conducting wires and a cylindrical conducting shell. The models capture the leading term in a Taylor expansion of the Green's function for the interaction between the plasma column and the vacuum vessel. The plasma is assumed not to vary during the VDE such that it behaves as a rigid body. In the limit of perfectly conducting structures, the plasma is prevented from coming in contact with the wall due to steep effective potential barriers by the eddy currents, and will hence oscillate at Alfvénic frequencies about a given force-free position. In addition to damping oscillations, resistivity allows for the column to drift towards the vessel on slow flux penetration timescales. The initial exponential motion of the plasma, i.e. the resistive vertical instability, is succeeded by a non-linear sinking behaviour, that is shown analytically to be algebraic and decelerative. The acceleration of the plasma column often observed in experiments is thus conjectured to originate from an early sharing of toroidal current between the core, the halo plasma and the wall or from the thermal quench dynamics precipitating loss of plasma current [Preview Abstract] |
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CP11.00103: Model of vertical plasma motion during the current quench Boris Breizman, Dmitrii Kiramov Tokamak disruptions impair plasma position control, which allows the plasma column to move and hit the wall. These detrimental events enhance thermal and mechanical loads due to halo currents and runaway electron losses. Their fundamental understanding and prevention is one of the high-priority items for ITER. As commonly observed in experiments, the disruptive plasma tends to move vertically, and the timescale of this motion is rather resistive than Alfvenic. These observations suggest that the plasma column is nearly force-free during its vertical motion. In fact, the force-free constraint is already used in disruption simulators. In this work, we consider a geometrically simple system that mimics the tokamak plasma surrounded by the conducting structures. Using this model, we highlight the underlying mechanism of the vertical displacement events during the current quench phase of plasma disruption. We also address a question of ideal MHD stability of the plasma during its resistive motion. [Preview Abstract] |
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CP11.00104: Model Development for VDE Computations in NIMROD K. J. Bunkers, C. R. Sovinec Vertical displacement events (VDEs) and the disruptions associated with them have potential for causing considerable physical damage to ITER and other tokamak experiments. We report on simulations of generic axisymmetric VDEs and a vertically unstable case from Alcator C-MOD using the NIMROD code [Sovinec, $\textit{et. al.}$, JCP $\textbf{195}$, 355 (2004)]. Previous calculations have been done with closures for heat flux and viscous stress. Initial calculations show that halo current width is dependent on temperature boundary conditions, and so transport together with plasma-surface interaction may play a role in determining halo currents in experiments. The behavior of VDEs with Braginskii thermal conductivity and viscosity closures and Spitzer-like resistivity are investigated for both the generic axisymmetric VDE case and the C-MOD case. [Preview Abstract] |
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CP11.00105: Computations of Vertical Displacement Events with Toroidal Asymmetry C. R. Sovinec, K. J. Bunkers Nonlinear numerical MHD modeling with the NIMROD code [https://nimrodteam.org] is being developed to investigate asymmetry during vertical displacement events. We start from idealized up/down symmetric tokamak equilibria with small levels of imposed toroidally asymmetric field errors. Vertical displacement results when removing current from one of the two divertor coils. The Eulerian reference-frame modeling uses temperature-dependent resistivity and anisotropic thermal conduction to distinguish the hot plasma region from surrounding cold, low-density conditions. Diffusion through a resistive wall is slow relative to Alfvenic scales but much faster than resistive plasma diffusion. Loss of the initial edge pressure and current distributions leads to a narrow layer of parallel current, which drives low-n modes that may be related to peeling-dominated ELMs. These modes induce toroidal asymmetry in the conduction current, which connects the simulated plasma to the wall. [Preview Abstract] |
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CP11.00106: Mechanism for destabilization of ideal ballooning modes in a 3D tokamak. T B Cote, C C Hegna, M Willensdorfer, E Strumberger, W Suttrop, H Zohm Recent observations on ASDEX-Upgrade have shown toroidally localized MHD activity in the presence of applied 3D fields[1]. In this study, we investigate the physical mechanisms that determine this result with an emphasis on 3D shaping. Experimentally relevant 3D VMEC equilibria are analyzed to determine stability in the edge pedestal region, and the ballooning mode is found to localized at specific field-lines corresponding to minima in the local magnetic shear. 3D distortion of the flux surfaces cause significant change in the normal torsion, a key component of the local shear, and act as the primary mechanism for ballooning destabilization on certain field-lines. The degree of localized ballooning instability is shown to scale with the amplitude of the 3D displacement through its effect on the local shear. [1] M. Willlensdorfer et al., submitted to Physics Review Letters, 2017. [Preview Abstract] |
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CP11.00107: Nonlinear Plasma Response to Resonant Magnetic Perturbation in Rutherford Regime Ping Zhu, Xingting Yan, Wenlong Huang Recently a common analytic relation for both the locked mode and the nonlinear plasma response in the Rutherford regime has been developed based on the steady-state solution to the coupled dynamic system of magnetic island evolution and torque balance equations. The analytic relation predicts the threshold and the island size for the full penetration of resonant magnetic perturbation (RMP). It also rigorously proves a screening effect of the equilibrium toroidal flow. In this work, we test the theory by solving for the nonlinear plasma response to a single-helicity RMP of a circular-shaped limiter tokamak equilibrium with a constant toroidal flow, using the initial-value, full MHD simulation code NIMROD. Time evolution of the parallel flow or ``slip frequency'' profile and its asymptotic approach to steady state obtained from the NIMROD simulations qualitatively agree with the theory predictions. Further comparisons are carried out for the saturated island size, the threshold for full mode penetration, as well as the screening effects of equilibrium toroidal flow in order to understand the physics of nonlinear plasma response in the Rutherford regime. [Preview Abstract] |
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CP11.00108: Effects of Toroidal Rotation on Neoclassical Toroidal Viscosity Torque in Tokamak Edge Pedestal Induced by Resonant Magnetic Perturbation Xingting Yan, Ping Zhu Previous analysis for static tokamak equilibria indicates that the neoclassical toroidal viscosity (NTV) torque in edge pedestal region induced by external resonant magnetic perturbation (RMP) can reach the same order of magnitude as other momentum sources such as neutral beam injections [1]. However, toroidal rotation often persists in tokamak experiments, especially in the edge pedestal region. How the edge rotations may affect the NTV torque remains an open question. In this work, we evaluate the influence of toroidal rotation on NTV torque in the edge pedestal region, using the method developed in previous work [1]. We find that toroidal rotation can modify not only the magnitude, but more importantly, also the profile of NTV torque significantly. Even for a rigid toroidal rotation, as its magnitude increases, the peak value of NTV torque decreases, whereas its peak location moves towards the core region. The detailed comparisons of NTV torque for different toroidal rotation magnitudes and profiles, in terms of its significance in the edge pedestal region, will be reported and discussed. [1] X.-T. Yan, P. Zhu, and Y.-W. Sun, submitted to Phys. Plasmas (2017). [Preview Abstract] |
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CP11.00109: A Model of Energetic Ion Effects on Pressure Driven Tearing Modes in Tokamaks M.R. Halfmoon, D.P. Brennan, A.J. Cole, J.M. Finn An analytic, reduced cylindrical model of linear resistive tearing modes, taking into account the effect of a high-energy, non-Maxwellian ion population as a perturbation in pressure, is applied to study the stability of high aspect ratio tokamak equilibria. The model captures the essential physics driving or damping the modes through variations in the magnetic shear. We focus on the stability of the $m/n=2/1$ tearing mode. The drift-kinetic motion of high-energy ions is modeled after a method discussed by Hu and Betti (B.~Hu and R.~Betti, Phys. \ Rev.\ Lett. {\bf 93}, 105002 (2004)), and entered into an asymptotic matching formalism for the resistive MHD dispersion relation. Toroidal magnetic field line curvature is included to model trapping in the particle distribution, in an otherwise cylindrical model. The results show that the energetic ions damp and stabilize the mode when orbiting in significant positive shear, and drive the mode unstable in reversed shear regions (M.R.~Halfmoon and D.P.~Brennan, Phys.~of Plasmas {\bf 24}, 062501 (2017)). These result explain $\delta f$ - MHD simulations of tokamak experiments with varying shear, and are also found to be consistent with related experimental observations of the $2/1$ stability limit. [Preview Abstract] |
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CP11.00110: Identifying the interaction mechanisms between the tearing mode and drift-wave turbulence S.D. James, D.P. Brennan, C. Holland We present nonlinear simulations of a three-field model evolving density, vorticity, and magnetic flux in a slab geometry. Drift wave turbulence is driven by an equilibrium density gradient, extending throughout the domain while a magnetic island can be driven unstable at a rational surface in the center of the domain. We utilize an equilibrium with prescribed tearing stability properties and turbulent drives. The results show the stability of the tearing mode is affected by the presence of the turbulence and the energy transport between them is discussed in the context of a turbulent resistivity. Nonlinear island widths are presented as a function of the tearing mode stability parameter, $\Delta^{\prime}$, and the equilibrium density gradient. The threshold for significant nonlinear growth of the tearing mode is modified by the background flow and turbulence, and the nonlinear saturated states of the island become oscillatory in the turbulent fields. [Preview Abstract] |
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CP11.00111: A Novel Kinetic Electron Model for Tearing Mode in Magnetic Confined Plasmas Dongjian Liu, Jian Bao, Zhihong Lin, Tao Han Compared with the fluid simulation, kinetic simulation of the collisionless tearing mode has been performed via a novel kinetic electron model. Based on the Gyrokinetic Toroidal Code simulation, the new electron model can not only recover the linear behavior of collisionless tearing mode, but also show great computational advantages in the kinetic simulation of long wavelength magnetohydrodynamic (MHD) and short wavelength drift-Alfvenic instabilities. [Preview Abstract] |
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CP11.00112: Magnetic flux pumping mechanism prevents sawtoothing in 3D nonlinear MHD simulations of tokamak plasmas Isabel Krebs, Stephen C. Jardin, Sibylle Guenter, Karl Lackner, Matthias Hoelzl, Erika Strumberger, Nate Ferraro 3D nonlinear MHD simulations of tokamak plasmas have been performed in toroidal geometry by means of the high-order finite element code M3D-C$^1$ [Jardin et al., Comput. Sci. Disc. 5 (2012)]. The simulations are set up such that the safety factor on axis ($q_0$) is driven towards values below unity. As reported in [Jardin et al., PRL 115 (2015)] and [Krebs et al., subm. to Phys. Plasmas] the resulting asymptotic states either exhibit sawtooth-like reconnection cycling or they are sawtooth-free. In the latter cases, a self-regulating magnetic flux pumping mechanism, mainly provided by a saturated quasi-interchange instability via a dynamo effect, redistributes the central current density so that the central safety factor profile is flat and $q_0\approx 1$. Sawtoothing is prevented if $\beta$ is sufficiently high to allow for the necessary amount of flux pumping to counterbalance the tendency of the current density profile to centrally peak. We present the results of 3D nonlinear simulations based on specific types of experimental discharges and analyze their asymptotic behavior. A set of cases is presented where aspects of the current ramp-up phase of Hybrid ASDEX Upgrade discharges are mimicked. Another set of simulations is based on low-$q_{\mathrm{edge}}$ discharges in DIII-D. [Preview Abstract] |
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CP11.00113: 3D nonlinear modeling of the coupling and phase locking of magnetic Islands in tokamaks Stephen Jardin, Nathaniel Ferraro, Jin Chen, David Pfefferle Many tokamak discharges develop multiple tearing modes possessing different mode numbers. These modes are observed to phase lock to one another, resulting in a flattening of the core toroidal plasma rotation profile, which can have deleterious effects on transport and MHD stability. In order to study these phenomena with minimum assumptions, we use the M3D-C1 3D nonlinear MHD code to perform initial value simulations of the evolution of equilibria unstable to both the 2/1 and 3/2 modes, but having sheared toroidal rotation. Initial attempts to perform these simulations led to numerical instabilities developing once the islands got to a certain size. In order to study the cause of this instability, we developed a small model code that solves a pure convection equation in 1D. We find that an implicit Crank-Nicholson method in time and Hermite Cubic finite elements (as are used in the toroidal direction in the M3D-C1 code) is not a convergent algorithm. Adding a small second order diffusion term, proportional to the velocity, improves the numerical stability properties but is not convergent in the first-derivative of the solution. Instead, adding a much smaller forth-order spatial derivative term proportional to the velocity leads to an algorithm in which both the solution and the first derivative converge as 1/N$^{\mathrm{2,}}$. Adding similar toroidal forth derivative terms to the M3D-C1 code eliminated the numerical instability. This work was supported by US DOE Contract DE-AC02-09-CH11466. [Preview Abstract] |
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CP11.00114: Modeling Error Fields and Disruptions in NSTX-U N.M. Ferraro, C.E. Myers, J.-K. Park, D. Pfefferle, S.C. Jardin, M.T. Beidler, M.L. Reinke Error field penetration and mode locking are among the most common sources of disruptions in tokamaks, and may also play an important role in the suppression of edge-localized modes (ELMs). Results from NSTX-U operations suggest that error fields may have had a considerable impact on plasma stability and transport, with locked modes commonly observed in L-mode discharges. We present models of error fields due to imperfections in the toroidal and poloidal field coils in NSTX-U, and consider the impact of these fields on the plasma equilibrium, including the impact on the magnetic pitch angle and heat flux at the divertor targets in high-performance NSTX-U model equilibria. Furthermore, we report on progress on modeling the nonlinear processes of error field penetration and disruptions with the extended-MHD code M3D-C1, with the goal developing predictive models of the processes by which error field penetration leads to disruptions or ELM suppression. [Preview Abstract] |
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CP11.00115: Poloidal structure of the plasma response to $n=1$ Resonant Magnetic Perturbations in ASDEX Upgrade L Marrelli, P Bettini, P Piovesan, D Terranova, L Giannone, V Igochine, M Maraschek, W Suttrop, M Teschke, Y.Q. Liu, D Ryan The hybrid scenario, a candidate for high-beta steady-state tokamak operations, becomes highly sensitive to 3D magnetic field near the no-wall limit. A predictive understanding of the plasma response to 3D fields near ideal MHD limits in terms of validated MHD stability codes is therefore important in order to safely operate future devices. Slowly rotating ($5-10Hz$) $n=1$ external magnetic fields have been applied in hybrid discharges in ASDEX Upgrade for an experimental characterization: the global $n=1$ kink response has been measured by means of SXR and complete poloidal arrays of $b_{\theta}$ probes located at different toroidal angles and compared to predictions of MHD codes such as MARS-F and V3FIT-VMEC. A Least-Squares Spectral Analysis approach has been developed together with a Monte Carlo technique to extract the small plasma response and its confidence interval from the noisy magnetic signals. MARS-F correctly reproduces the poloidal structure of the $n=1$ measurements: for example, the dependence of the dominant poloidal mode number at the plasma edge from $q_{95}$ is the same as in the experiment. Similar comparisons with V3FIT-VMEC and will be presented. [Preview Abstract] |
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CP11.00116: Plasma response model to resonant magnetic perturbations Marisa Roberto, Andre Carlos Fraile Junior, Ibere Luiz Caldas Magnetically confined plasmas in tokamaks usually do not operate under equilibrium conditions and high temperature particles are transported to the reactor wall, causing its erosion. External resonant windings or coils can be used to generate a perturbative magnetic field that modifies particle transport near the plasma border. A pair of helical wires placed at the reactor external wall is used to create a resonant magnetic perturbation (RMP) capable of reducing the amount of particles colliding with the wall. However, the plasma is also affected by RMPs and it modifies the magnetic field lines. In this work, a semi-analytical model in polar toroidal coordinates was developed to study the plasma response, which has been modeled as a helical current sheet at the resonant surface with the condition that the total radial magnetic field vanishes at this surface. This condition is associated to the mitigation of magnetic islands around the resonant surface. Poincar\'{e} plots have shown that the plasma response regularizes the magnetic field lines around the resonant surface. [Preview Abstract] |
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CP11.00117: Tearing mode dynamics and sawtooth oscillation in Hall-MHD Zhiwei Ma, Wei Zhang, Sheng Wang Tearing mode instability is one of the most important dynamic processes in space and laboratory plasmas. Hall effects, resulted from the decoupling of electron and ion motions, could cause the fast development and perturbation structure rotation of the tearing mode and become non-negligible. We independently developed high accuracy nonlinear MHD code (CLT) to study Hall effects on the dynamic evolution of tearing modes with Tokamak geometries. It is found that the rotation frequency of the mode in the electron diamagnetic direction is in a good agreement with analytical prediction. The linear growth rate increases with increase of the ion inertial length, which is contradictory to analytical solution in the slab geometry. We further find that the self-consistently generated rotation largely alters the dynamic behavior of the double tearing mode and the sawtooth oscillation. [Preview Abstract] |
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CP11.00118: Pfirsch-Schluter Current in and near a Magnetic Island: Singular Behavior and Symmetry Effects. Allan Reiman, Dhanush Radhakrishnan The current along magnetic field lines that enforces quasi- neutrality is called the ``Pfirsch-Schluter current''. We show that the Pfirsch-Schluter current has, in general, a logarithmic singularity at the X-line of a magnetic island separatrix if $\nabla \cdot {\rm {\bf j}}_{\bot } $ is nonzero there. The singular component of the Pfirsch-Schluter current vanishes if the configuration is stellarator symmetric about a point on the X-line. (Symmetric with respect to simultaneous reflection in the poloidal and toroidal angles.) We consider, in particular, the case where ${\rm {\bf j}}_{\bot } $ is determined by the MHD equilibrium force-balance equation and the pressure gradient is determined by a diffusion equation. There is a critical scale length, xc, determined by the ratio of the perpendicular and parallel diffusion coefficients, such that the pressure is not flattened on flux surfaces within a distance of order xc about the X-line. The variation of pressure on flux surfaces in that region leads to a nonzero $\nabla \cdot {\rm {\bf j}}_{\bot } $ at the X-line, and a large Pfirsch-Schluter current near the X-line. This is a significant piece of physics that is absent in analytical calculations for perturbed cylindrical models having a single resonant Fourier component, and in 3D codes that have no variation in pressure within their flux surfaces. [Preview Abstract] |
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CP11.00119: Influence of toroidal rotation on tearing modes Huishan Cai, Jintao Cao, Ding Li Tearing modes stability analysis including toroidal rotation is studied. It is found that rotation affects the stability of tearing modes mainly through the interaction with resistive inner region of tearing mode. The coupling of magnetic curvature with centrifugal force and Coriolis force provides a perturbed perpendicular current, and a return parallel current is induced to affect the stability of tearing modes. Toroidal rotation plays a stable role, which depends on the magnitude of Mach number and adiabatic index $\Gamma $, and is independent on the direction of toroidal rotation. For $\Gamma $ \textit{\textgreater }1, the scaling of growth rate is changed for typical Mach number in present tokamaks. For $\Gamma \quad =$ 1, the scaling keeps unchanged, and the effect of toroidal rotation is much less significant, compared with that for $\Gamma $ \textit{\textgreater }1. Reference: [1] Huishan Cai, Jintao Cao, Ding Li, influence of toroidal rotation on tearing modes, Nuclear Fusion 57, 056006(2017) [Preview Abstract] |
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CP11.00120: Tearing mode dynamics in the RFX-mod tokamak Luigi Cordaro, Paolo Zanca, Matteo Zuin, Fulvio Auriemma, Emilio Martines, Barbara Zaniol, Gianluca Pucella, Roberto Cavazzana, Gianluca De Masi, Alessandro Fassina, Gustavo Grenfell, Barbara Momo, Silvia Spagnolo, Monica Spolaore, Nicola Vianello The study of the physical mechanisms that influence the tearing mode (TM) rotation is of interest because, while in present day devices, a significant TM rotation can be induced by Neutral Beam Injection, future reactors, ITER included, are not expected to provide enough induced momentum. We present a study of tearing mode dynamics in the RFX-mod device, a Reserved Field Pinch in Padua (Italy) that can be run as low-current, circular tokamak. Magnetic, flow and kinetic measurements are integrated to characterize the (2,1) and (3,2) TMs fast rotation. We are especially interested to study the role played by the diamagnetic electron drift on the TM rotation, including the slowing down and the wall-locking phases. When the latter occurs, the radial magnetic field penetrates the shell and the TM amplitude increases at a rate given by the wall resistive time constant. This phenomenon can lead to a rapid discharge termination via a disruption. A comparison of experimental data with a two-fluid MHD cylindrical model [Nucl. Fusion 54 (2014) 122001] has been used to interpret the observed TM fast rotation frequencies [Preview Abstract] |
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CP11.00121: Effect of Gyroviscosity on Tearing Modes in Tokamak Plasmas Ryan White, Alan Glasser We present an extension of the Glasser-Greene-Johnson equations, incorporating the Braginskii gyroviscosity. It is found that the dominant terms from the gyroviscous stress are all due to poloidal variation of the equilibrium profile, implying that these physical effects are not captured in a large-aspect-ratio (cylindrical) model. Because these purely toroidal contributions dominate, we conclude that thewell-known``gyroviscous cancellation'' is a higher-order effect in toroidal confinement systems. We also present preliminary numerical results showing the effect of gyroviscosity on tearing mode stability. [Preview Abstract] |
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CP11.00122: Verification of kinetic/MHD hybrid simulation of neoclassical tearing mode in fusion plasmas Kaijie Wang, Chijie Xiao, Wenlu Zhang, Zhihong Lin, Xiaoquan Ji Simulations and predictions of the excitation threshold of neoclassical tearing mode (NTM) are challenges, since the excitation threshold depends on large-scale MHD instabilities, collisional transport, microturbulence, and energetic particle effects, etc. A hybrid simulation of NTM with gyrokinetic ions and fluid electrons has been developed and verified in the gyrokinetic toroidal code (GTC) to study this problem. An extra pressure transport equation is included to cover the pressure fattening effect. The bootstrap current is included with a simple model, $\mathrm{j}_{\mathrm{bs}}\mathrm{=-1.46}\frac{\sqrt \epsilon }{B_{\theta }}\frac{\partial p}{\partial r}$. In the fluid limit, it is verified that the linear growth rate of NTM is proportional to the poloidal beta $\mathrm{\beta }_{\mathrm{p}}$ when the equilibrium profiles are fixed, and the linear and nonlinear simulation results of NTM are compared with theory and quantitatively agree with the modified Rutherford equation. The kinetic effects of thermal ions are found to reduce the linear growth rates of NTM and the finite Larmor radius effects of thermal ions have little impacts on NTM. [Preview Abstract] |
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CP11.00123: Kinetic Effects on Resistive Tearing Mode and Drift Tearing mode Hao Shi, Wenlu Zhang The kinetic effects on stability of resistive tearing mode are investigated by global simulations in cylindrical geometry using Gyrokinetic Toroidal Code(GTC). The fluid simulation of resistive tearing mode agrees well with theory prediction. Kinetic effects are found to reduce the growth rate of the tearing mode and the radial width of mode structure. The drift-tearing mode is obtained when considering density gradient, which has the frequency of the diamagnetic drift frequency. The decrease of growth rate due to the diamagnetic drift motion is observed, which agrees well with the derivation of theory. Besides, the radial mode width of the drift tearing mode is wider. [Preview Abstract] |
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CP11.00124: Two-fluid modifications on the Kelvin-Helmholtz instability in a Tokamak Plasma Omar Lopez Ortiz, Luca Guazzotto In a two-fluid magnetohydrodynamical description of axisymmetric equilibria stream surfaces do not exactly overlap with flux surfaces; instead, there is a relative shift which is proportional to the toroidal flow. In a reduced massless electrons scenario it is the ion's poloidal velocity which possesses a finite component perpendicular to magnetic surfaces. Starting from a self-consistent, single-fluid analytical equilibrium developed for a high-beta, high-aspect ratio configuration with sheared flows, we obtain the two-fluid normal component of the velocity perturbatively and benchmark it against the code FLOW2 [1]. We explore the modifications that the normal component of the velocity causes for the development of a Kelvin-Helmholtz instability driven by a sheared toroidal flow.\\ \\$[1]$ L. Guazzotto \textit{et. al. Phys. Plasmas}, 22:032501, 2015 [Preview Abstract] |
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CP11.00125: Nonlinear GAM and lower kinetic TAE generation by TAE Zhiyong QIU, Liu Chen, Fulvio Zonca The decay of toroidal Alfv\'en eigenmode (TAE) [1] into a geodesic acoustic mode (GAM) [2] and a heavily damped lower kinetic TAE (LKTAE) [3], is investigated as possible mechanism for TAE saturation using nonlinear gyrokinetic theory. The equations describing the nonlinear interactions among TAE, GAM and LKTAE are derived analytically, and exibit an interesting analogy to those describing convective cells generation by kinetic Alfv\'en waves [4]. It is shown that the decay is induced by the anti-Hermitian part of the LKTAE dispersion function due to its strong radiative damping, analogous to the scattering due to heavily Landau damped ion quasi-modes. Control parameters regulating the nonlinear decay are discussed. Possible diagnostics are also suggested for the experimental verification of the nonlinear process analyzed here. \newline --------------------------------- \newline {\small \noindent [1] C. Z. Cheng, L. Chen and M. Chance, Ann. Phys. {\bf161}, (1985) 21.\newline \noindent [2] N. Winsor, J. L. Johnson, and J. M. Dawson, Phys. Fluids {\bf11}, (1968)2448. \newline \noindent [3] F. Zonca and L. Chen, Phys. Plasmas {\bf 3}, (1996) 1. \newline \noindent [4] F. Zonca, Y. Lin and L. Chen, Europhys. Lett. {\bf 112}, (2015) 65001. \newline } [Preview Abstract] |
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CP11.00126: A generalized Ginzburg-Landau model for nonlinear relaxation oscillation of magnetized plasma boundary with shear flow Gunsu Yun, Youngmin Oh, Jieun Lee, H.J. Hwang, Jaehyun Lee, Michael Leconte The boundary of high-temperature plasma confined by a toroidal magnetic field structure often undergoes quasi-periodic relaxation oscillations between high and low energy states. On the KSTAR tokamak, the oscillation cycle consists of a long quasi-steady state characterized by eigenmode-like filamentary modes, an abrupt transition into non-modal filamentary structure [Lee JE, {\it Sci. Rep.} 7, 45075], and its rapid burst (via magnetic reconnection) leading to the boundary collapse. A phenomenological model including the effects of time-varying perpendicular flow shear, turbulent transport, and external heating has been developed to understand the nonlinear oscillation. The model, which has the form of a generalized complex Ginzburg-Landau equation, shows that the flow shear amplitude and the shear layer width determine the nonlinear oscillation. Numerical solutions revealed that there exists a critical flow shear level below which steady states can exist. This result suggests that the abrupt transition to the non-modal unstable state is due to the flow shear increasing above the critical level. The model predicts that high wavenumber ($k$) modes can coexist with low-$k$ modes at sufficiently low level of flow shear [Lee J, {\it Phys. Rev. Lett.} 117, 075001]. [Preview Abstract] |
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CP11.00127: Surface currents on the plasma-vacuum interface in MHD equilibria James Hanson The VMEC non-axisymmetric MHD equilibrium code can compute free-boundary equilibria$^{\mathrm{1}}$. Since VMEC assumes that magnetic fields within the plasma form closed and nested flux surfaces, the plasma-vacuum interface is a flux surface, and the total magnetic field there has no normal component. VMEC imposes this condition of zero normal field using the potential formulation of Merkel$^{\mathrm{2}}$, and solves a Neumann problem for the magnetic potential in the exterior region. This boundary condition necessarily admits the possibility of a surface current on the interface. While this surface current may be small in MHD equilibrium, it is readily computed in terms of the magnetic potentials in both the interior and exterior regions, evaluated on the surface. If only the external magnetic potential is known (as in VMEC), then the surface current can be computed from the discontinuity of the tangential field across the interface. Examples of the surface current for VMEC equilibria will be shown for a zero-pressure stellarator equilibrium. Field-line following of the vacuum magnetic field shows magnetic islands within the plasma region. $^{\mathrm{1}}$ Hirshman S P, Van Rij W I and Merkel P, Comp. Phys. Comm. \textbf{43} 143--55 (1986) $^{\mathrm{2}}$ Merkel P, J. Comp. Phys. \textbf{66} 83--98 (1986) [Preview Abstract] |
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CP11.00128: Exploring the limits of analytical solutions to the Grad-Shafranov equation with the Solov'ev profile J. Julio E. Herrera-Velazquez, Kassandra Salguero-Martinez, Miguel Angel Segura-Ramirez Solutions to the Grad-Shafranov equation for the Solov'ev profiles, which are the simplest ones, can reproduce several features of the experiments, such as the average $\beta $poloidal, the average safety factor \textit{q*}, the Shafranov shift, etc., when the free parameters are appropriately chosen. This provides a flexible instrument to understand the role of the aspect ratio, elongation and triangularity on the physics of tokamaks. This work starts from the solutions proposed by Cerfon and Freidberg for the equatorially symmetric case [1], and stretches them to their limits. The starting point is the set of parameters for the spherical tokamak START, and then the consequences of varying the inverse aspect ratio $\varepsilon$. Similar solutions have also been proposed by Zheng et al. [2], with a different particular solutions for the homogeneous Grad-Shafranov equation $ \triangle ^{ \ast }\psi _{p}\left (R ,Z\right ) =0$. We show there is a more general family of particular solutions, but the satisfaction of the boundary conditions leads to an adjustment of the coefficients in the general solution that produce the same results. [1] A.J. Cerfon y J.P. Freidberg, Physics of Plasmas 17 (2010) 032502 [2] S.B. Zheng, A.J. Wootton and E. Solano, Physics of Plasmas 3 (1996) 1176 [Preview Abstract] |
(Author Not Attending)
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CP11.00129: Self-consistent and robust estimation of the MHD equilibrium suitable for control oriented models of the q-profile evolution P. Garcia-Martinez, P. Montes, E. Schuster The feasibility of controlling the q-profile using closed-loop controllers designed from first-principles-driven control-oriented models has been demonstrated in tokamaks like DIII-D. These control-oriented models typically use the magnetic diffusion equation for the poloidal magnetic flux profile evolution, combined with simplified models for other plasma quantities such as the electron density, the electron temperature, and the noninductive current-drives. The magnetic diffusion equation is expressed in flux coordinates thus requiring several geometric profiles that depend on the underlying MHD equilibrium of the plasma. In this work, a self-consistent method to improve the estimation of the MHD equilibrium and the required geometric profiles is proposed. The method combines a two-dimensional linear model, that takes into account the geometry of the flux surfaces, with a one-dimensional non-linear model that incorporates the evolution of the magnetic profiles resulting in a robust and fast strategy for MHD equilibrium estimation. [Preview Abstract] |
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CP11.00130: Fast-Time-Scale Equilibria for Rotating Nonaxisymmetric Toroidal Plasmas Linda Sugiyama Toroidal fusion plasmas are strongly driven systems sustained by complicated and incompletely characterized sources and sinks. For tokamaks, basic axisymmetric ideal MHD equilibria based on the poloidal magnetic flux are a useful first approximation to the plasma and magnetic geometry for experimental data analysis and physics studies. The large MHD terms in the plasma force balance rapidly establish a quasi-steady state, on time scales much faster than other plasma processes. Widely used equilibrium reconstruction tools, such as EFIT, usually ignore additional effects such as plasma rotation, applied nonaxisymmetry, and non-MHD processes, except for the edge bootstrap current. Good general ideal MHD configurations exist for nonaxisymmetry or rotation separately, but for the combination of the two, the single-fluid nature of MHD requires that the toroidal dependences of the plasma density, rotation, and magnetic field be related by simple expressions. These are unlikely to be satisfied over the wide range of applied heating, torque, current drive, and nonaxisymmetric fields; the mismatch is experimentally testable. This work investigates the minimal additions to basic axisymmetric ideal MHD needed to provide more consistent fast-time-scale steady states. [Preview Abstract] |
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CP11.00131: Abstract Withdrawn
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CP11.00132: Physical processes in high field insulating liquid conduction. Michael Mazarakis, Mark Kiefer, Joshua Leckbee, Delmar Anderson, Frank Wilkins, Robert Obregon In the power grid transmission where a large amount of energy is transmitted to long distances, High Voltage DC (HVDC) transmission of up to 1MV becomes more attractive since is more efficient than the counterpart AC. However, two of the most difficult problems to solve are the cable connections to the high voltage power sources and their insulation from the ground. The insulating systems are usually composed of transformer oil and solid insulators. The oil behavior under HVDC is similar to that of a weak electrolyte. Its behavior under HVDC is dominated more by conductivity than dielectric constant. Space charge effects in the oil bulk near high voltage electrodes and impeded plastic insulators affect the voltage oil hold-off. We have constructed an experimental facility where we study the oil and plastic insulator behavior in an actual HVDC System. Experimental results will be presented and compared with the present understanding of the physics governing the oil behavior under very high electrical stresses. [Preview Abstract] |
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