Bulletin of the American Physical Society
60th Annual Meeting of the APS Division of Plasma Physics
Volume 63, Number 11
Monday–Friday, November 5–9, 2018; Portland, Oregon
Session TP11: Poster Session VII: Basic Plasma Physics: Pure Electron Plasma, Strongly Coupled Plasmas, Self-Organization, Elementary Processes, Dusty Plasmas, Sheaths, Shocks, and Sources; Mini-conference on Nonlinear Waves and Processes in Space Plasmas - Posters; MHD and Stability, Transients (2), Runaway Electrons; NSTX-U; Spherical Tokamaks; Analytical and Computational Techniques; Diagnostics (9:30am-12:30pm) |
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Room: OCC Exhibit Hall A1&A |
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TP11.00001: Exploring MHD-driven plasmas created using different amounts of magnetic helicity injection and relevance to MHD jets and spheromaks Byonghoon Seo, Magnus A Haw, Ryan Marshall, Paul M Bellan MHD-driven plasmas generated by a disk electrode surrounded by a coplanar annulus electrode form various topological configurations such as a collimated straight jet or a spheromak depending on the amount of helicity injection. Morphology is tracked using the combination of visible light movies and 3-D volumetric magnetic probe measurements. In the regime where the plasma detaches from the electrodes, a magnetic island is observed in the poloidal direction indicating formation of a closed surface flux and thus formation of a spheromak. This detachment is accompanied by X-ray bursts and magnetic oscillations in the range of whistler frequencies. Since spheromak formation requires magnetic reconnection, the observed X-rays and high-frequency magnetic oscillations are presumed to be evidence of magnetic reconnection by which the plasma dissipates energy and evolves into a spheromak. |
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TP11.00002: Three-dimensional Evolution and Formation of Multiple Current-filaments in a Laboratory Arched Magnetized Plasma Kamil Krynski, Shreekrishna Tripathi, Troy Carter The study and understanding of fundamental processes taking place in arched magnetized plasmas is relevant to space and magnetic fusion plasmas. We present new results on measurements conducted on laboratory arched magnetized plasmas produced using a hot-cathode lanthanum hexaboride (LaB6) source. The arched plasma evolves in an ambient magnetic field (0-80 Gauss). Typical plasma parameters are: β≈103, Lundquist number ≈102-105, plasma radius/ion gyroradius ≈20, B≈1000 Gauss at footpoints, and 0.5 Hz repetition rate. Evolution of the arched magnetized plasma has been recorded under different ambient magnetic fields. Experimental results demonstrate that the application of a stronger ambient magnetic field results in outward or inward motion of the leading edge of the arched plasma (depending on the direction of the net JxB force). In addition, this motion is accompanied by generation of multiple-current filaments that form a complex three-dimensional structure. We present results on experimental measurements of plasma density, electron temperature, and magnetic-field using dual-tip Langmuir probes, a three-axis magnetic-loop probe, and a recently developed 3D probe drive system. |
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TP11.00003: Measurement and Modeling of Global Dynamics and Instabilities in Magnetized Plasma Bubbles and Jets in Background Plasma* Robert H Dwyer, Dustin M Fisher, Mark Gilmore, Daniel F Puentes The PBEX (Plasma Bubble Expansion EXperiment) utilizes a compact coaxial plasma gun and a linear plasma device to study magnetic relaxation and other dynamics of dense plasma jets and bubbles propagating in background magnetized plasma. In previous experiments, high-speed multiframe imaging suggested the presence of magneto-Rayleigh-Taylor (MRT) instabilities of cm- and microsecond-scales. Both B-dot and multi-tip Langmuir probe in-situ measurements of the magnetic field, ion density and temperature are being used to characterize the instabilities, as well as the global dynamics. These data are also being compared with ongoing MHD Modeling of flux-rope expansion using the University of Michigan’s BATS-R-US code. Measurement, simulations results, and comparisons, will be presented. |
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TP11.00004: Symmetry of magnetic field and the existence of global Clebsch coordinates Hong Qin, Joshua W Burby, Alexander S Glasser Darboux's theorem guarantees that a magnetic field B(x) in R3 can always be written as B(x) =∇α×∇β for two locally-defined scalar functions α(x) and β(x), which are known as the Clebsch coordinates. The local Clebsch coordinates exist even when the magnetic field is chaotic. However, global Clebsch coordinates do not exist in general. We prove that a necessary and sufficient condition for the existence of global Clebsch coordinates over a contractible domain is that the magnetic field admits a global symmetry, i.e., there exists a vector field η such that the Lie derivative of the 1-form potential A(x) along η is exact. When the symmetry exists, let Lη dA=dS and we can construct the global Clebsch coordinates for B(x) over contractible subsets of the level sets of the function A·η-S. It turns out that this result is implied by the Poincaré lemma on 2D contractible surfaces. |
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TP11.00005: Phase switching dynamics between three inductively coupled glowdischarge plasma sources Neeraj Chaubey, Pankaj Kumar Shaw, Subroto Mukherjee, Abhijit Sen The phase switching dynamics is studied between three inductively coupled glow discharge plasma sources. For this, DC glow discharge plasma is produced in three separate glass chambers of same configuration. Due to the asymmetry in the sizes of the electrode areas, anode glow is formed in each of the systems which in turn produces the oscillations in the plasma. The plasma parameters in three systems are adjusted such that the oscillation frequencies of these systems are 242, 216 and 268 kHz respectively. Firstly, inductive coupling is provided between the systems with oscillation frequencies of 242 and 268 kHz. It has been observed that the oscillation frequencies of two systems goes to a anti-phase synchronized state with a frequency of 199 kHz. After that, third system with oscillation frequency of 216 kHz is inductively coupled with these two systems. It has been observed that with this coupling oscillations of previously coupled systems switches from anti-phase state to in phase state with a coupling frequency of 185 kHz. Also, these experimental observations are numerically modelled using three environmentally coupled Van der Pol oscillators and the obtained results are found to be in good agreement. |
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TP11.00006: Magnetic suppression of zonal flows on a beta plane Navid Constantinou, Jeffrey Parker Zonal flows in rotating systems have been previously shown to be suppressed by the imposition of a background magnetic field aligned with the direction of rotation. Understanding the physics behind the suppression may be important in systems found in astrophysical fluid dynamics, such as stellar interiors. However, the mechanism of suppression is not yet explained. We provide a theoretical explanation that shows how magnetic fluctuations directly counteract the growth of weak zonal flows. Using the quasilinear, statistical CE2 framework we predict a self-consistent growth rate of the zonal flow. We find that a background magnetic field suppresses zonal flow if the bare Alfvén frequency is comparable to or larger than the bare Rossby frequency. However, suppression occurs for even weaker magnetic fields if the resistivity η is small enough to allow sizable magnetic fluctuations. Our calculations reproduce the η/B02 = const. scaling that describes the boundary of zonation, as found in previous work, and we explicitly link this scaling to the amplitude of magnetic fluctuations. |
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TP11.00007: Observation of enhancement in water window laser plasma X-ray under nitrogen atmospheres. Yasuhiro Matsumoto, Christian John, Noboru Kakunaka, Jang Yu-Jin, Takehiro Morishita, Maki Kishimoto, Shinichi Namba X-ray microscope using water window (WW) X-rays (2.3 to 4.4 nm) is expected as one of the tools to observe living cells and biomolecules. In order to obtain their images with a higher spatial resolution, X-ray photons number has to be increased significantly. Recently, we found that the WW intensity was enormously enhanced when a gold target was irradiated with a laser pulse (1064 nm, ~7 ns, 1 J) under low-pressure N2 atmosphere. In order to clarify the mechanism of the phenomenon, emission spectroscopy was carried out under various gas species (N2, O2, Ne, He and Ar) and their pressures up to 1 kPa. For X-ray measurement, a grazing incidence spectrometer (flat field grating: 2400 grooves/mm) with a toroidal mirror was used. In addition, a pinhole camera with an aperture of 10 mm was installed to measure two dimensional X-ray images. The mechanism of the WW X-ray enhancement will be described in terms of atomic physics, especially, X-ray innershell ionization and subsequent Auger electron process. |
(Author Not Attending)
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TP11.00008: Dynamics of positrons during relativistic electron runaway Ola Embréus, Linnea Hesslow, Mathias Hoppe, Gergely Papp, Katya Richards, Tünde Fülöp In plasmas, sufficiently strong electric fields can accelerate charged particles to relativistic energies via the runaway mechanism. In this contribution we describe the dynamics of positrons that are created during a runaway avalanche. |
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TP11.00009: Fully coherent plasma x-ray laser by injection of a parametrically amplified high-order harmonic beam Shinichi Namba, Dinh Thanch Hung, Noboru Hasegawa, Maki Kishimoto, Masaharu Nishikino, Jozsef Seres, Carles Serrat Conventional plasma x-ray lasers have the characteristics of spatial coherence, while less temporal one. In order to realize fully coherent plasma x-ray lasers, we have employed a seed-amplifier scheme. In this setup, a high-order harmonics as a seeder is injected into an amplifier plasma medium. As a result, the harmonics is amplified while maintaining its original coherencies. For generation of a narrow beam divergence and intense high-order harmonics, we focus on the x-ray parametric amplification. The high-order harmonics generated in two He gas jets are amplified due to parametric interaction, by which the quality of the harmonics are even improved. On the other hand, plasma amplifier is created by using the same Ti:Sa laser by a grazing incident pumping. To meet the requirements on the spatial and spectral coupling is essential to achieve the efficient amplification. Thus, the central wavelength of the laser is tuned at 792.3 nm, by which the lasing transition (13.9 nm) matches the 57th order harmonics. The preliminary results of the parametric amplification of the harmonics will be presented. |
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TP11.00010: Dust chain formation in microgravity complex plasma Evdokiya Kostadinova, Kyle Busse, Lorin S Matthews, Truell W Hyde The one-dimensional (1D) dusty chain is the simplest stable complex plasma structure that exhibits a variety of fundamental interactions and at the same time allows for easy experimental tracking and elegant theoretical modeling. We investigate string formation utilizing video data from the Plasmakristal-4 (PK-4) facility on the International Space Station. In the absence of gravity, a variety of 1D dust structures form in the bulk of the plasma, where phenomena specific to self-ordering become increasingly important. The possible mechanisms guiding such 1D self-ordering can be organized into three groups: 1) ion wake-mediated interaction (symmetric and asymmetric), 2) dust particle effects (grain polarization, temperature/charge fluctuations, and demixing), and 3) system-induced interactions (discharge striations, shear flow, and thermal gradient effects). We first present a theoretical formulation of each mechanism and then identify the system parameter space where each effect is observable under microgravity conditions. Finally, the theoretical predictions are tested against the experimental data from the PK-4 laboratory. |
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TP11.00011: Non-invasive impedance measurements of electron density and temperature and implications for complex dusty plasmas Eric D Gillman, Erik M Tejero, William E Amatucci A non-invasive method of probing the complex impedance of a plasma discharge has been developed where a discharge electrode is used to sustain the discharge as well as measure the discharge impedance. This diagnostic models the discharge as a collection of circuit elements and measures the resonant frequencies of the discharge. The resonant frequencies elucidate several discharge characteristics that in turn are used to calculate electron density and temperature. More refined analysis techniques and further experimental measurements have been performed in both non-dusty and dusty discharges. Comparisons between this impedance diagnostic method and Langmuir probe measurements will be discussed, as well as implications and advantages of this probing technique for use in dusty plasma applications. |
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TP11.00012: Using Laser Induced Fluorescence to Measure Temperature in the Caltech Water-Ice Dusty Plasma Ryan S Marshall, Paul M Bellan A Laser Induced Fluorescence (LIF) diagnostic has been built to measure temperatures and flows in the Caltech Water-Ice Dusty Plasma Experiment. This diagnostic uses a tunable, ultra-narrow-band diode laser, a photomultiplier, a mechanical chopper, and a lock-in amplifier. Two separate laser heads operate one at a time to perform LIF on either neutral or singly ionized argon. Neutral argon LIF measurements yield signal-to-noise ratios greater than 100. Remarkably, the LIF signal can be clearly seen without the use of signal averaging. As a consequence, substitution of the photomultiplier, mechanical chopper, and lock-in amplifier by a fast CMOS camera still reliably detects LIF emission. Lamb Dip has been observed using the photomultiplier which allows for absolute calibration of the laser wavelength and a way to measure flows [1]. Work is underway to program a motorized 5-axis stage to allow for automated temperature measurement as a function of position. Work is also underway to focus light from the plasma through lenses onto the camera chip to create images of the plasma using the LIF photons. To date, argon ion LIF has been unsuccessful and further investigation will begin shortly. [1] Kohei Ogiwara et al 2011 Jpn. J. Appl. Phys. 50 036101 |
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TP11.00013: Modeling Volcanic Shocktube Lightning Jens Von Der Linden, Jason Sears, Allen L Kuhl, Dave Grote, Mark Converse, Christopher Kueny, Brian Poole Lightning discharges occur in ash plumes of volcanic eruptions. Identifying their radiofrequency emission may aid in forecasting volcanic ash along aviation routes and characterizing planetary volcanic activity. We propose a model for volcanic lightning in which dust particles accelerate in shocks, separate by their inertia, charge triboelectrically, and spark through streamer breakdown. These processes are strongly affected by the explosive shocks of the volcano eruption. For example, charge magnitude and polarity are sensitive to the shocked drift velocity of small and large particle species. In addition, charge clustering in turbulent eddies creates rapid variations in the Townsend ionization coefficient leading to intermittent breakdown. To validate the model breakdown statistics are compared to measurements from a volcanic shocktube experiment [1] with a well-controlled "dust" consisting of a bimodal distribution of glass beads. [1] Cimarelli, C. et al. (2014). Experimental generation of volcanic lightning. Geology, 42(1), 79–82 |
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TP11.00014: Scientific basis for future experiments using the Magnetized Dusty Plasma Experiment (MDPX) Edward Thomas, Spencer P LeBlanc, Taylor H Hall, Surabhi Jaiswal, Mohamad Menati, Michael McKinlay, Lori Scott, Stephen Williams, Brandon Dolye, Dylan Funk, Uwe Konopka, Robert L Merlino, Marlene Rosenberg For over a decade, it has been postulated that the addition of a magnetic field can have a profound influence on the properties of a complex/dusty plasma. The Magnetized Dusty Plasma Experiment (MDPX) device at Auburn University is the most recent facility dedicated to studying the properties of dusty plasmas at high magnetic field (B ≥ 1 T). The MDPX device 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. Experiments on the MDPX device have given us new insights into particle charging, self- and imposed-ordering, particle transport, particle growth, and plasma and dust instabilities in strongly magnetized plasmas. This presentation will use these observations to motivate the next generation of experiments on the MDPX device. |
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TP11.00015: RF Resonance Measurement of Electron Depletion in a Dusty Plasma Brandon Doyle, Uwe Konopka, Edward E Thomas Active plasma resonance spectroscopy (APRS) is a class of plasma diagnostics which utilizes plasma resonances at or near the electron plasma frequency, ωpe. Adding dust to a plasma affects ωpe as dust particles become charged. This dust charge is typically highly negative because of the higher mobility of electrons as compared to ions. The negative dust charging leads to a reduction in the electron density in the surrounding plasma and a reduction in ωpe related to this electron density depletion. APRS diagnostics are useful for measuring this phenomenon. Such a measurement is difficult with many common plasma diagnostic techniques because many of these techniques are undesirable for use with dusty plasmas. For example, a Langmuir probe can perturb the dust, or it can become unreliable due to dust contamination. In this poster, we present experimental results from APRS measurements showing ωpe reduction in a plasma due to the injection of dust. We also present preliminary results from similar experiments conducted in the presence of a magnetic field. |
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TP11.00016: Magnetic field influence on a steady dusty plasma flow Dylan Funk, Uwe Konopka, Edward E Thomas Dusty plasmas consist of the standard plasma components (electrons, ions and neutrals) as well as micrometer sized particles. The dust particles become highly charged as a result of their interaction with the other plasma components. The charge of these dust particles is a difficult quantity to estimate precisely, especially when influenced by a magnetic field. Because of this difficulty, a method for the experimental determination of the dust particle charge under the influence of a magnetic field is required. In our chosen approach we attempt to utilize the dynamics of a driven dust particle flow perpendicular to a static magnetic field. A dust particle density gradient will build up across the flow due to the Lorentz force. We will demonstrate, using an MD simulation, that studying the particle distribution in the flow, depending on magnetic field and flow velocity, can be used to estimate an average dust particle charge. We plan to apply our method on MDPX at Auburn University. |
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TP11.00017: Electrostatic potential measurements of imposed, ordered structures in MDPX Taylor Hall, Edward Thomas Imposed, ordered structures are a recently discovered phenomenon in the Magnetized Dusty Plasma Experiment (MDPX). These structures are the alignment of microparticles, or dust, within a low temperature plasma to the spatial pattern of a conducting wire mesh present in the experiment, and which are observed at high magnetic fields (B ≥ 1 T) and low neutral pressures (P ≤ 100 mTorr). Currently, much of the work investigating these imposed, ordered structures has examined the individual particle dynamics, and recently, developed methods to parameterize the onset of these structures. However, the potential profile within the plasma which gives rise to the confinement of the dust particles is still not well understood. This work will examine these potential structures experimentally with a large grid-like electrode and an in-situ probe to measure the potential structure under the grid-like electrode. The results of these experiments and a discussion of possible models will be given. |
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TP11.00018: Effects of externally applied magnetic fields on dust voids within a dusty plasma Spencer LeBlanc, Edward E Thomas “Voids,” or dust-free regions, within the body of complex plasmas have been observed and studied in several configurations and parameter spaces, both in microgravity and earth-based environments. The boundaries of these voids are formed at the equilibrium position of a Coulomb force on the (typically negatively charged) dust and the opposing drag force due to a net ion drift. While the properties and creation of these voids has been understood for some time, there has not been an accepted model that includes the effects of an externally applied magnetic field. Using the Magnetized Dusty Plasma Experiment (MDPX) device at Auburn University, dust voids and their properties are studied under the influence of a large magnetic field, and a theoretical model of this dust voids within strong uniform magnetic fields is presented. Results are also presented from a simulation code developed at Auburn University to examine the collective effects of ions on the void structure in both low- and high-magnetization regimes. |
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TP11.00019: Controlling equilibrium dust charge in a complex plasma with AC signals Michael McKinlay, Edward E Thomas, Uwe Konopka Reproducible displacements in the equilibrium position of clouds of charged microparticles (dust) suspended in an argon, DC glow discharge have been observed in response to AC signals applied to a confining electrode. The applied signals are low power (mW), unbiased, and at frequencies approaching the ion-neutral collision rate. Probe measurements indicate that surfaces in the plasma experience a sizable increase (50% or greater in some cases) in incident ion flux, reducing the floating potential of the surfaces relative to the plasma. A theoretical model is presented which demonstrates how an unbiased AC field may produce a net increase in ion flux, and the effect of these signals on background plasma parameters, fields, and instabilities is examined in order to determine whether these displacements are the result of a controlled change in dust charge. |
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TP11.00020: Numerical simulation of low pressure plasma filamentation Mohamad Menati, Edward E Thomas, Uwe Konopka In a high magnetic field regime, filamentary structures can develop in low-pressure and low-temperature plasma discharges – particularly in plasmas that are bounded by parallel plate electrodes. These filamentary structures appear as bright columns parallel to the external magnetic field and can be in circular or spiral patterns when viewed from the top. Filametation has been observed and reported by different groups but the phenomenon has not yet been fully investigated theoretically. In this work, a Finite Difference (FD) method is used to numerically solve diffusion and continuity equations of plasma in a self-consistent algorithm to attempt to describe the filamentation process. Perturbations in electron and Ion densities are introduced to the background plasma in presence of high magnetic field to initiate the filamentary structures. Results from two- and three-dimensional simulations are presented. |
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TP11.00021: Transitions between reciprocal and non-reciprocal interactions in complex plasmas Lori Scott, Edward E Thomas, Uwe Konopka, Jeremiah D Williams, Mikhail Y Pustylnik, Hubertus Thomas Complex plasmas are composed of electrons, ions, neutrals, and micron-sized, charged particles (dust). Under microgravity conditions, the dust particles can fill the plasma volume, which enables the study of small-scale inter-dust forces. In this work, we seek to determine how the thermodynamic properties of a complex plasma evolve when the inter-particle potential is modified from a non-reciprocal to a reciprocal structure. In the Plasma Kristall-4 (PK-4) microgravity laboratory on the International Space Station (ISS), particles are injected into a dc glow discharge plasma and flow along an axial electric field. Upon the application of a periodic oscillation of the electric field ("polarity switching"), there is change in the spatial ordering of the particles. This presentation discusses initial results from molecular dynamics simulations that are motivated by these experiments. It is shown that modifying the interaction potential affects the spatial ordering and thermal properties of a dusty plasma. This presentation will also present supporting experimental results on these processes.
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TP11.00022: Measurements of Thermal Effects in the Dispersion Relation of the Dust Acoustic Wave Jeremiah D. 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. Over the past twenty years, the dust acoustic wave has been a subject of intense study and recent studies have shown that thermal effects can, in some cases, have a significant role in the measured dispersion relation. In this talk, we report on the results of an experimental study of the dispersion relation of this wave mode over a range of neutral gas pressures in a weakly-coupled dusty plasma system in an rf discharge plasma. The experimental dispersion relations are modeled using a theoretical dispersion relation that includes thermal effects and the effects of dust correlations that arise fr om the confinement of dusty plasmas in the plasma volume. (K. Avinash, Phys. Plasmas, 22, 033701, 2015). |
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TP11.00023: BU Ground PlasmaKristal-4 (PK-4) Striation Studies and their Correlation to Dust Cloud Morphology Jorge Carmona Reyes, Peter Hartmann, Michael Cook, Kenneth Ulibarri, Lorin S Matthews, Truell W Hyde On board the International Space Station (ISS) data from the PlasmaKristal-4 (PK-4) routinely exhibits formations of long chains and dust acoustic waves. Both are of interest due to their application to other areas in physics and engineering. The fundamental physics of these phenomena is still being investigated where one possible mechanism is the so-called striation which has been observed in all PK-4 ground models. This work will discuss observed striations at different operating conditions and its correlation to dusty plasma cloud morphologies. PIC computer model results will also be examined to determine the role each particle species plays in the process. Both experimental and theoretical data will be presented and compared against ISS PK-4 data to first determine calibration of the ground unit and produce a hypothesis yielding an explanation to the observed PK-4 phenomena.
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TP11.00024: A particle-in-cell approach to the charging of dust particle inclusions immersed in plasma Alexander Klepinger, Robert A Mitchell, Douglass W Schumacher, David V Rose We describe a new model of dusty plasmas based on the particle-in-cell (PIC) code LSP. Dusty plasmas are important for understanding a wide range of applications and phenomena such as plasma machining and planet formation. These plasmas are typically highly coupled with inclusions of nanometer to millimeter particles. A fundamental question is how dust charges via absorption of plasma particles and this is investigated using kinetic simulations. In our model, a dust particle can be of arbitrary shape and is represented by a collection of PIC particles which can absorb electrons and ions. Simulations can be run in 1D3V, 2D3V, and 3D, explicitly or implicitly, with a variety of field solvers and particles pushers. An algorithm is also implemented to mitigate the unphysical depletion of plasma due to finite grid size. Comparison to existing results and theory is presented. Given this platform, the model can be extended to include photoionization, charge migration, and other effects in a straightforward manner. |
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TP11.00025: Particle growth in highly magnetized Ar/C2H2 capacitively-coupled radio-frequency discharge Lenaic Couedel, Edward E Thomas, Darrick Artis, Cedric Pardanaud, Min P Khanal, Spencer P LeBlanc, Stephane Coussan, Cecile Arnas, Taylor H Hall, Uwe Konopka, Minseo Park The growth of nanoparticles was explored in the Magnetized Dusty Plasma Experiment (MDPX) device. Radio-frequency (rf) plasmas were produced in a reactive gases (acetylene) mixed with argon. The growth of nanoparticles was followed by monitoring discharge parameters such as the powered electrode self bias and neutral gas pressure. The dynamics of the growing dust particle cloud was 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 were studied ex-situ using scanning and transmission electron microscopes. Raman spectroscopy was also performed. Complementary particle-in-cell simulations were also carried out. The influence of the strength of the magnetic field was explored and the changes in NP growth dynamics and transport were discussed. |
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TP11.00026: High Rotation Rates and Stability in Magnetized Dusty Plasmas C. A. Romero-Talamas, J. Stefancik, G. Suarez, W. J. Birmingham Molecular dynamics (MD) simulations of magnetized dusty plasmas are used to explore the role of competing forces in making 50 µm hollow silica spheres stably rotate, at rates up to an order of magnitude higher than previously reported with much smaller particles. Experiments at Auburn University’s MDPX [E. Thomas, et al. Plasma Phys. 81, 345810206 (2015)] using UMBC’s 15 cm cylindrical glass chamber, motivated this research. In the experiments, argon plasmas were produced using an inductively coupled, 22 – 30 kHz RF source with antennae external to the glass chamber, creating density and temperature gradients that are perpendicular to B and become steeper with increasing B-field strength. We conjecture that such gradients are in turn responsible for dust rotation through ion momentum transfer in the 𝛁P x B direction, as well as for balancing the centrifugal force that appears on the heavy particles during rotation. An experimental setup being built at UMBC to further explore fast rotation and stability, and to compare with the simulations, is also discussed. |
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TP11.00027: High frequency electric-field-driven instability in the electron presheath. Lucas Paul Beving, Scott D Baalrud, Frederick N Skiff, Ryan T Hood Recent work has shown that the electron sheath near a probe biased above the plasma potential is accompanied by an electron presheath. The electron presheath extends several times further into the plasma than its ion rich counterpart and provides a large scale equilibrium electric field. Linear analysis of the Vlasov equation predicts that such an equilibrium electric field can excite an instability near the electron plasma frequency. Furthermore, using a BGK collision operator for electron-neutral collisions shows that the instability is excited over wavelengths on the order of 1-10 electron Debye lengths. Higher neutral pressure narrows this range, while higher electric field strength increases the maximum growth rate. Properties of this predicted electric field driven instability have been tested in a low-temperature laboratory plasma experiment, where the electron sheath is provided by a positively biased electrode immersed in an argon plasma. Measurements of wave correlations have been done in an attempt to distinguish the electric-field driven instability from a sheath-plasma resonance instability. |
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TP11.00028: Studies of plasma sheaths and plasma-wall interaction using continuum kinetic simulations Petr Cagas, Ammar Hakim, Bhuvana Srinivasan A continuum kinetic plasma model is used to study plasma sheaths by directly evolving the ion and electron distribution functions using the Vlasov equation along with Maxwell's equations. With the inclusion of an emitting boundary, the plasma sheath simulations become relevant for a wide range of applications, for example, space propulsion or fusion. However, such boundary conditions can significantly alter the results; theoretically up to point when the electric field potential is reversed. Therefore, these boundary conditions need to be physics-based and self-consistent. In this work, a novel method is presented which allows for both self-consistent and efficient implementation of the emitting boundary conditions and its application to magnetized plasma sheath simulations. |
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TP11.00029: Investigation of centimeter-scale shock structures observed during plasma jet collisions Maximilian Schneider, Michael Douglas Sherburne, Brandon Christensen, Patrick Crandall, Colin Adams Bow shock structures are observed in collisions of high velocity (∼10 km/s), low density (∼1022 m-3) plasma jets. Plasma jets are accelerated by a small gas-fed linear railgun featuring a 0.50 x 0.32 cm rectangular bore and 10.2 cm long rails powered by an LC pulse-forming network delivering ∼100 kA pulses of 15 microsecond duration. Chord-integrated electron number densities of ∼1020 m-2 were estimated in several-centimeter diameter jets 10 cm downstream of the gun muzzle by a Mach-Zehnder heterodyne interferometer, jet velocities of ∼10 km/s were estimated with a photodiode array, a 750 mm imaging spectrograph and scanning monochromator was used to estimate plasma temperature, Rogowski coils were used to estimate current pulse width and amplitude delivered to the accelerator, and short-exposure images of shock phenomena were captured with an image-intensified CCD camera. A campaign is underway to study the spatial distribution of multiple ion species within the shock front and to infer the role of diffusive phenomena in the observed shock structures. |
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TP11.00030: Construction of a He cascade arc plasma window device for an alternative differential pumping system Yuto Asano, Yuki Iwamoto, Kohei Fukuyama, Naoki Tamura, Shinichi Namba A cascade arc plasma window device with a channel diameter of 8 mm has been fabricated for a differential pumping system in He gas cell ion beam stripper facility. The required pressure difference is 7 kPa to 1 Pa through 20-cm plasma window channel without large vacuum pumps. Stationary He plasma up to a current of 100 A was generated as the plasma window, and the performance of the pressure difference was investigated by measuring the pressures of the upstream and downstream sections. The pressure ratio through the plasma window was ~230 at a 100-A discharge of 0.05 L/min. Our cascade arc source demonstrated the pressure difference from 7 kPa to 27 Pa, and has a high potential for alternative differential pumping to separate the gas cell from the downstream chamber. To characterize the He plasma at the anode electrode, emission from vacuum UV to visible wavelengths were observed. Spectral analysis of He I continuum spectra and H-b Stark broadening showed that the plasma temperature was ~0.9 eV and density was 3.4×1013 cm-3 at a 100-A discharge of 0.05 L/min. |
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TP11.00031: Extension of the ray tracing for mode mixed, splitting, and dissipative wave beams in inhomogeneous magnetized plasmas Kota Yanagihara, Shin Kubo, Ilya Y Dodin, Toru Tsujimura, Hiroaki Nakamura Geometrical-optics (GO) ray tracing has been widely used for modeling electron-cyclotron waves in inhomogeneous magnetized fusion plasmas. However, this reduced approach cannot calculate the wave-beam structure near the focus, and it is also inapplicable in low-density plasmas with a sheared magnetic field, where mode coupling between the two electromagnetic cold-plasma modes can occur [Phys. Plasmas 24, 122116 (2017)]. Here, we present a new beam-tracing code based on the second-order extended geometrical optics (XGO), which captures refraction, diffraction, mode coupling, and dissipation simultaneously without assuming any specific beam structure. The power splitting between the O and X modes is simulated using this new code and is found to be in good agreement with one-dimensional full-wave simulations. The focused- and defocused-beam structure is compared with that predicted by other model. Also, the profiles of the split O- and X-wave beams are discussed. |
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TP11.00032: Formation of Hollow Density Profile in Magnetically Expanding Plasma Produced by Helicon Antenna Sonu Yadav, Soumen Ghosh, Sayak Bose, Kshitish Kumar Barada, Prabal Kumar Chattopadhyay The radial density profile in the magnetic nozzle of the Helicon eXperimental (HeX) device is found to be modified from being centrally peaked to that of hollow nature as external applied magnetic field is increased. It occurs above a characteristic magnetic field value for which the ions become magnetized in the expansion chamber. The hollow density formation is independent of the location of geometric expansion and also on the presence of a radial electric field. The density profile in the source chamber behind the nozzle, however, remains peaked on-axis irrespective of the magnetic field. The electron temperature there, is observed to be hollow and this characteristics continues to the expansion chamber along the field lines. Rotation of the tail electrons in the azimuthal direction due to the gradient-B drift in the expansion chamber leads to an additional off-axis ionization and forms the hollow density profile there. It is shown that, if the ions are not magnetized, then off-axially produced additional plasma is not confined and the density profile retains the on-axis peak nature. The present work demonstrates how the knowledge of the ion magnetization together with tail electrons significantly contributes to the design of an efficient helicon plasma based thruster. |
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TP11.00033: Pulsed High Power Helicon Plasma Source Paul Melnik, James R Prager, Timothy Ziemba, Kenneth E Miller, Connor Liston, Alex Henson Eagle Harbor Technologies, Inc. (EHT) has developed a high power helicon plasma source for testing a repetitively pulsed, high power RF system for plasma heating. This system is built on the work of multiple SBIR programs including the development of a SiC MOSFET full-bridge and electrodeless plasma source. This full bridge drives a series resonant load, which allows for a low-cost way to produce large antenna currents in excess of 2.5 kA at 1 MHz. The resulting plasma has been characterized over a wide range of magnetic field strengths and input power levels. We will present Langmuir probe data showing plasma density and electron temperature over a range of plasma parameters. |
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TP11.00034: Microwave Assisted Helicon Plasmas John McKee, Derek Thompson, Ivan Arnold, David D Caron, Earl E Scime The use of two (or more) rf sources at different frequencies is a common technique in the plasma processing industry to control ion energy characteristics separately from plasma generation. A similar approach is presented here with the focus on modifying the electron population in argon and helium plasmas. The plasma is generated by a helicon source at a frequency f0 = 13.56 MHz. Microwaves of frequency f1 = 2.45 GHz are then injected into the helicon source chamber perpendicular to the background magnetic field. The microwaves damp on the electrons via Electron Cyclotron Heating (ECH), providing additional energy input into the electrons. The effects of this secondary-source heating on electron density, temperature, and energy distribution function are examined and compared to helicon-only single source plasmas as well as comparison to dispersion relations for the damping waves. Optical Emission Spectroscopy (OES) is used to examine the impact of the energetic tail of the electron distribution on ion and neutral species via collisional excitation. |
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TP11.00035: A microscopic model for charge transfer between dielectric surfaces in dielectric barrier discharge plasmas Purnima Ghale, Harley T. Johnson In field emission plasmas, electrons that initiate plasma come from the surface of a metallic electrode, or wall; emission is controlled by the workfunction of the wall, and computed via the Fowler-Nordheim formula. Impinging ions modify the rate of electron emission, which is accounted via the coefficient of secondary electron emission. However, in the case of dielectric surfaces, the microscopic mechanism by which electrons are emitted is not as well understood. Here we consider electron emission from dielectric surfaces in the context of dielectric barrier discharges. The configuration of interest consists of two parallel-plate metallic electrodes, each covered by a dielectric layer. We present a quantum mechanics based computation of the rates of charge transfer between dielectric surfaces, under moderate strength AC fields with frequencies in the kilohertz range. We compute the rate of charge transfer between surfaces, which is necessary condition, but not sufficient, for plasma formation. The microscopic model of electron transfer described here has potential applications in the design of micro and nano-scale plasma generators. |
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TP11.00036: Collisional damping of plasma waves by H- in room-temperature electron plasmas Andrey Kabantsev, C. Fred Driscoll Collisional effects play an important role in the dynamics of plasma waves, by setting a minimal damping rate and by disrupting the wave-particle (Landau) resonant damping. For room-temperature electron plasmas accumulating H- ions, this damping results from the long-range e/H- frictional drag, and it is relatively independent of the spatial structure (mr, mθ, mz) of the wave. Here, the collisional damping rate γcl(H-) is one-half of the electron-H- scattering rate νei(nH,Te), i.e., γcl(H-) = νei /2, but with an unusual Coulomb logarithm [1] for the repulsive (like-sign) particle collisions. Operating with room-temperature electron plasmas (ne ≈ 107/cm3) accumulating a 10% fraction of H- ions, the observed damping rates γcl(H-) show three-fold increase over their background level (γbg ≈ 103/sec) for a variety of plasma waves with low wave numbers (mr, mθ, mz). This increase is somewhat less than the theory estimate νei /2 ≈ 4.6×103/sec, probably due to ongoing centrifugal mass separation of electrons and heavy ions. The “background” damping rate is sensitive to the plasma injection geometry, so it may be caused by an unnoticed few percent fraction of negative ions produced during the electron injection. [1] D.H.E. Dubin, Phys. Plasmas 21, 052108 (2014) |
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TP11.00037: Measurements of Cross-Field Thermal Conductivity in a Pure Electron Plasma Kurt Thompson, Andrey Kabantsev, C. Fred Driscoll Cross-field thermal conductivity in a magnetized pure electron plasma is measured and found to be 105 times larger than the value predicted using classical short-range collisions. The cylindrical plasma column of density n0~107 cm-3 and temperature T~1 eV is confined in a Penning-Malmberg trap at Bz=12 kG giving rc<<λD. Oscillating the column through a θ-symmetric electrostatic squeeze causes centrally-peaked separatrix dissipation, giving a centrally peaked T(r). The temperature profile is diagnosed by analyzing the number of particles that escape from the trap when the end confinement potential is lowered. The thermal diffusivity is calculated from the radial heat flux, Γ(r,t), which is derived from the time evolution of T(r,t). For plasmas with rc<λD, heat transport is dominated by long-range collisions with impact parameter rc<ρ<λD, rather than by classical short-range collisions with ρ<rc. The measured thermal diffusivity is in close agreement with long-range collisional theory, as χL=0.49νcλD2, and this is more than five orders of magnitude larger than classical diffusivity χc=(16π1/2/15)νcrc2ln(rc/b). |
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TP11.00038: Temperature and Length Dependence of Finite Length Diocotron Modes Daniel K Walsh, Daniel H E Dubin Diocotron modes are surface waves that propagate azimuthally on a nonneutral plasma column, via ExB drifts. Their azimuthal dependence is exp(i l θ). For an infinite length column, the mode frequency ω is independent of temperature in the large B drift limit where ω ~ 1/B. For finite length plasma columns, and for mode number l = 1, there is an approximate theory in the drift limit[1] for the (typically weak) effect of temperature on ω. This temperature dependence is a useful thermometer in experiments [2]. This poster discusses an extension of the Fine-Driscoll theory to mode numbers l > 1, and compares the theory for both l = 1 and l = 2 to numerical solutions of the finite-length bounce-averaged Vlasov equation. Surprisingly, the radial dependence of finite-length mode eigenfunctions is nearly identical to infinite-length theory, even near the plasma ends. The finite-length theory shows that the temperature–dependent frequency shift for l = 2 has the opposite sign to that for l = 1, in agreement with experiments. [1] K.S. Fine and C. F. Driscoll, Phys. Plasmas 8, 407 (2001). [2] K. Thompson and A. Kabantsev, adjacent poster |
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TP11.00039: Proposed Experiment to Measure Test Particle Diffusion in a Correlated Nonneutral Plasma Daniel H E Dubin, Francois Anderegg, Charles Fred Driscoll We have measured cross-magnetic-field test particle diffusion D due to long range collisions in plasmas with rc< λD and 0.05 eV<T<3 eV. We propose to extend these measurements to lower temperature, where ions become correlated. Magnesium ion plasmas are routinely cooled down to T= 5x10-6eV resulting in a correlation parameter Γ=(e2/aT)~10. We have measured collisional equipartition rates in this regime, and observe a Salpeter enhancement of 109x over the rate neglecting correlations [1]. For a diffusion measurement, laser cooling with a standard cyclic transition is used to optically pump all ions into one electronic spin state. The inner radial part of the plasma can then be tagged by reversing the spins using direct optical pumping. These “spin reversed” test particles can be followed as they diffuse across the magnetic field, using a weak probe tuned to a cyclic transition. We intend to test theory [2] for Γ>1 predicting decreasing D as T decreases, and a transition from D~ B-2 to D~B-1 . [1] F. Anderegg et al., PRL, 102, 185001 (2009). [2] T. Ott and M. Bonitz, PRL, 107, 135003 (2011); S.D. Baalrud and J. Daligaut, Phys. Rev. E 96, 043202 (2017). |
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TP11.00040: Measurement of Correlation-Enhanced Collision Rates in the Mildly Correlated Regime (Γ≈1) Francois Anderegg, Daniel H.E. Dubin, C. Fred Driscoll We have measured correlation-enhanced perpendicular-to-parallel collision rates νperp-par in cryogenic, strongly magnetized mildly correlated ion plasmas. The enhancement of νperp-par is directly analogous [1] to the correlation-enhancement of fusion collisions in hot dense stellar plasma, as first analyzed by Salpeter [2]. The enhancement occurs because plasma screening reduces the repulsive Coulomb potential between charges, allowing closer collisions for a given relative energy.Correlations are parameterized by Γ=e2/aT which is the ratio of the nearest neighbor potential energy to the ion thermal energy. Our prior measurements [3] over the range 0 < Γ < 15 observed enhancement up to 107x, in broad agreement with Salpeter “equilibrium screening” theory. However recent “dynamical screening” theories [4] predict negligible enhancement for Γ≈1. |
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TP11.00041: Observation of a large banana orbit due to a background asymmetry in a coaxial Malmberg-Penning trap D.L. Eggleston In our coaxial Malmberg-Penning trap, electrons are injected off-axis from a small electron gun.2 We expect the zeroth order azimuthal drift of the resulting electron column to be set by the center wire potential, the induced image charges, and the end potentials. However, when the wire potential is adjusted to minimize the drift, the phosphor screen images of the dumped column trace out (with increasing dump time) a large banana-shaped orbit in the r-θ plane, apparently in response to a background construction asymmetry. Wall probe signals are also consistent with such an orbit. We can directly measure the orbit period (T≈300μs) and the radial thickness of the banana (Δr/Rwall≈0.25) and then deduce from Δr≈vr/ωT the size and radial dependence of the asymmetry. Assuming an electrostatic asymmetry of the form φ1(r)cos[m(θ-θ0)], we find φ1(V)≈0.3r/Rwall with m≈1 and θ0≈60°. The source of the asymmetry has not yet been positively identified, but we suspect a small offset in the center wire position. The characterization of this background asymmetry may help resolve discrepancies with theory in our experiments with applied asymmetries.3 2. See, for example, D.L. Eggleston, Phys. Plasmas 1, 3850 (1994). 3. D.L. Eggleston and B. Carrillo, Phys. Plasmas, 10, 1308, (2003). |
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TP11.00042: Superimposition of Li+ ion plasma on e- plasma for producing two-fluid plasma state Haruhiko Himura A two-fluid plasma model is one of extended magnetohydrodynamics (MHD) models and widely used for explaining macroscopic plasma phenomena that cannot be explained by the conventional one-fluid MHD. In the two-fluid plasma model, velocity fields of ion and electron (e-) fluids are determined by corresponding each fluid equation of motion. However, such a two-fluid plasma state has not ever been verified in laboratory experiments. To investigate the two-fluid plasma state experimentally, we have experimented using lithium ion (Li+) and e- fluids. These fluids can be confined not only independently but also simultaneously in the BX-U linear trap. In the BX-U, they are trapped in the corresponding positive and negative potential wells of the BX-U, respectively. In experiments, changes in two-dimensional shape of both Li+ and e- fluids are measured after the elapse of two-fluid plasma state. Currently, we attempt to adjust the diameter of Li+ plasmas and the diameter of e- plasmas by changing each confinement time before the superimposition. In this conference, we will present the method in detail and compare the data obtained by use of it with the previous data. |
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TP11.00043: Electron Cyclotron Resonance Magnetometry with a Plasma Reservoir Andrew J Christensen, Eric D Hunter, Jonathan S Wurtele, Joel Fajans Measuring the magnetic field strength in a Malmberg-Penning trap is of direct interest to non-neutral plasma experiments. The magnetic field strength determines the cyclotron frequency of constituent particles, how the number density relates to the plasma rotation frequency, and how quickly the plasma cools. To measure the magnetic field in the Berkeley electron plasma experiment, we expose plasma to microwaves to heat electrons and study the resulting plasma temperature as function of the microwave frequency. The observed heating curves have a resonant structure which depends on the axial bounce frequency and the rotation frequency of the plasma as well as the magnetic field. By understanding this dependence we identify the peak corresponding to electron cyclotron resonance, providing a measurement of the magnetic field to an accuracy of 30 parts per billion. To scan microwave frequencies, we develop a method of quickly preparing low density electron plasmas for destructive temperature measurement. This method involves drawing low density target plasmas from a reservoir. This technique was developed for use in the ALPHA experiment at CERN, where it will be used to improve gravity and hyperfine structure measurements of antihydrogen. |
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TP11.00044: Cavity Cooling of the Pure Electron Plasma Eric D Hunter, Nathan A Evetts, Joel Fajans, Walter N Hardy, Huws Landsberger, Ethan Ward, Jonathan S Wurtele Magnetized electrons in free space emit cyclotron radiation, cooling at the Larmor rate -1/T dT/dt = 0.24 s-1 * B[T] 2 This rate can be enhanced by confining the plasma in a microwave cavity, where the density of EM oscillator states is concentrated at the cavity resonances [A. P. Povilus, PhD thesis: Berkeley 2015]. Here we present data for the cooling of plasmas with 2*106 electrons, held in three different cavities, over a range of magnetic field from 0.15-1.50 T. We explore in detail the cooling enhancement for a variety of magnetic gradient and field-plasma overlap geometries. Optimizing these parameters, we measure the cooling rate and final temperatures achieved for the range 100 < N < 108 plasma electrons. |
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TP11.00045: Electron Counting and Plasma Temperature Measurement with a Silicon Photomultiplier Dana Zimmer, Eric D Hunter, Joel Fajans, Nicole A Lewis, Alexander P Povilus, Carlos E Sierra The Berkeley nonneutral plasma trap uses a microchannel plate-phosphor screen assembly to detect plasma particles as they emerge from the strong magnetic field of a Penning-Malmberg trap. Until recently, charge was measured with a pickup capacitively coupled to a high voltage line. This method suffered from a sensitivity limit set by microphonic noise. We have employed a silicon photomultiplier (SiPM) to measure the charge via the light emitted by a fast (P47) phosphor, thus completely decoupling from measurable noise sources. The design and performance studies of the new SiPM diagnostic will be reported, including the achievement of single electron resolution. |
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TP11.00046: Theoretical and Numerical Study of Cavity Cooling of Lepton Plasmas Francis Joseph Robicheaux, Andrew J Christensen, Nathan A Evetts, Joel Fajans, Walter N Hardy, Eric D Hunter, Jonathan S Wurtele, Zachary T Schroeder Electron plasmas confined in Malmberg-Penning traps cool via cyclotron emission. Recently experiments [A. Povilus, et al., PRL 117, 175001 (2016); E. D. Hunter, et al., Phys. Plas. 25, 011602 (2018)] demonstrated that the cooling rate is significantly increased over its free space value by resonant interaction with a mode of a microwave cavity. T. M. O’neil [Phys. Fluids 23 (1980)], using Vlasov theory, predicted plasma cooling is enhanced in a cavity, and emphasized the importance of cyclotron detuning and frequency spread. Here, a simple model of the system based on interacting oscillators is derived and studied. We explore how the cooling rate depends on the spatial dependence of the microwave mode, the number of electrons, resonance detuning, spatial dependence of the magnetic field, and other system parameters. |
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TP11.00047: A dual-species hybrid MOT/Paul trap Sarah Hill, Tyler Bennett, Robert T Sprenkle, Scott Bergeson We report on progress to create a hybrid dual-species calcium and ytterbium magneto-optical trap (MOT) superimposed onto a linear Paul trap. This configuration will allow us to trap neutral atoms in the MOT, ionize them using ns-duration pulsed lasers, and then trap the resulting plasma in the Paul trap. By driving the trap at two frequencies we will eliminate centrifugal separation inherent in simultaneous trapping of different mass ions. The primary goal of this experiment is to measure collisional momentum transfer between the Yb+ and Ca+ ions as a means of determining the Coulomb logarithm in a strongly coupled plasma environment. Using carefully aligned probe laser beams and by spatially imaging ion fluorescence, we anticipate being able to distinguish between the coherent ion micromotion and the thermal ion motion in the plasma. |
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TP11.00048: Simulation of Ions Axial and Planar Motion in an Ultra-Cold Penning Trap with a Rotating Wall Potential Chen Tang, Dominic Meiser, Scott Edward Parker, John Jacob Bollinger Ultra-cold ions trapped in a rotating-wall Penning trap form a two-dimensional crystal with approximately triangular symmetry. A rotating wall torque balances the cooling laser torque, allowing ions to be cooled to very low temperatures. The spectrum of normal modes can easily be measured from the motion of the ions both in the directions parallel and perpendicular to external magnetic field. Spectral analysis shows many discrete peaks thereby identifying the vibrational modes and associated energies. Assuming thermal equilibrium, all modes would share the same energy or temperature. We use the “Cold-Atom” code to directly simulate the N-body classical system and study the ion dynamics, temperature of the normal modes and possible non-equilibrium excitation.
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TP11.00049: Density effects on the detectability of axisymmetric Bernstein modes in a finite-length non-neutral plasma. Grant W Hart, Bryan G Peterson We use a 2-D (r-z) PIC code to model high-frequency axisymmetric oscillations in a finite-length pure-ion plasma. These modes are not detectable in the wall surface charge of infinite-length plasmas because of axisymmetry and lack of z-dependence. This is not true in a finite-length plasma, because the perturbed density has to have nodes a short distance beyond the ends of the plasma. This gives the modes a sinusoidal dependence, with a kz such that an integral number (approximately) of half-wavelengths fit into the plasma. This z-dependence makes the mode detectable in the wall surface charge. Near a density of 6/7 of the Brillioun limit modes with different kz are well separated in frequency. The higher the kz in these modes, the lower the frequency. When the density is lowered to values which are more easily accessed experimentally, however, the modes are much more closely spaced and therefore overlap and are no longer distinguishable. Because the modes with higher kz tend to be damped, this causes moderately heavy damping of all the modes at these lower densities, making them very difficult to observe. We will quantify what size of density perturbation is necessary to produce experimentally measurable signals as a function of plasma density. |
(Author Not Attending)
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TP11.00050: Viscosity Control Experimental Design Heidi Morris, Paul A Bradley, Nelson M Hoffman Turbulent mix has been invoked to explain many results in Inertial Confinement Fusion (ICF) and High Energy Density (HED) physics, such as reduced yield in capsule implosions. Many simulations use fluid turbulent mix models to help match simulation results to data, but this is not appropriate if fully developed turbulence is not present. Mix, turbulent or not, can turn a plasma of a single material into a multi-component plasma, causing the viscosity to change by orders of magnitude depending on the fraction of high-Z material mixed in. Unlike fluids, mixed plasmas can have a change in viscosity that is high enough to ward off further turbulent growth. This effect is most pronounced for plasmas that are strongly coupled, where increases in high-Z dopant fraction can increase the viscosity by orders of magnitude. In contrast, the viscosity for ideal plasmas decreases with increasing Z as Z-4. We discuss the development of experimental designs to measure the effect of viscosity on instability growth. Our ultimate goal is to conduct future experiments where instability growth or turbulent mixing could be turned on or off by changes in high-Z dopant concentration. This could lead to ways to mitigate turbulence in Omega and NIF implosions as well as other HED experiments. |
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TP11.00051: Molecular Dynamics and Hydrodynamics Hybrid Method for Moderately Coupled Plasmas Lucas Stanek, Andrew Christlieb, Michael Sean Murillo Starting from a multispecies Klimontovich description, we derive a hybrid method that models dense, moderately coupled plasmas. The model consists of coupled molecular dynamic (MD) and hydrodynamic equations of motion to govern the dynamics of the ion species and electrons respectively. With this model, we keep correlation information and is exact at describing the system until closures are added. Different closures allow for the modeling of phenomena such as heat conduction, or energy transfer between particles that would not be possible if simpler methods like Yukawa particles were employed. Unlike a pure MD model which requires a small timescale to resolve the dynamics of the fast moving electrons relative to the ion species, this model avoids small timescales making computation more tractable while retaining physics of interest. As a first test case, we aim to reproduce the dispersion relation for ultra-cold plasmas as presented in [1]. As our first application, we will explore interface mixing where the usual pair potential is known to fail. |
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TP11.00052: Effective Potential Theory for Magnetized Plasmas Scott D Baalrud The effective potential theory (EPT) is a recent proposal for extending traditional transport theory into the strongly coupled regime [1]. By comparing with ultracold plasma experiments and classical molecular dynamics simulations, it has proven accurate for modeling a variety of transport coefficients (including diffusion, viscosity and temperature relaxation) for Coulomb coupling parameters less than 20. This work extends EPT to magnetized plasmas. A Chapman-Enskog type method is applied based on a collision cross section determined by EPT. The generalization of resistive MHD to the strongly coupled regime is thus obtained, where explicit expressions for all single fluid transport processes are computed, including diffusion, electrical conductivity, thermal conductivity and viscosity. Results for diffusion, as well as the relaxation of a temperature anisotropy, are compared with recent molecular dynamics simulations of the one-component plasma [2]. Applications to both ultracold neutral plasma experiments, and magnetized high energy density plasmas will also be discussed. [1] Baalrud and Daligault, PRL 110, 235001 (2013). [2] Baalrud and Daligault, PRE 96, 043202 (2017). |
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TP11.00053: Functional form of distribution and correlation functions of strongly coupled plasmas. Vikram Dharodi, Abdourahmane Diaw, Michael Sean Murillo The advent of high energy-density experimental facilities, such as the National Ignition Facility and the Linac Coherent Light Source, reinforces efforts to explore matter under extreme states produced in laboratories and stellar objects. Understanding the properties of non-ideal matter through computational modeling is critical for designing and analyzing high energy-density experiments. A central element to our understanding of numerous properties such as transport properties and equation of state are the radial distribution function (RDF). The RDF is often computed using molecular dynamics, Monte Carlo or integral equations; embedding these methods into large-scale hydrodynamics codes is prohibitively expensive; thus, a simplified and general functional form of the RDF is needed. Here, we investigate a theoretical approach to get the functional form of the RDF for the strongly coupled and multicomponent plasmas. The results are validated with MD and HNC simulations. |
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TP11.00054: Magnetized electrons in a low-density ultracold plasma John Guthrie, Puchang Jiang, Jacob Roberts For sufficiently strong magnetic fields, it is possible to create plasmas where the electrons' length scale associated with the magnetic field (Larmor radius) is smaller than any screening lengths, interparticle spacings, or Coulomb collision parameters. Through using low-density ultracold plasmas, such conditions can be obtained at modest laboratory fields. We describe the basic properties of ultracold plasmas created under these conditions. |
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TP11.00055: Electric Field Heating during Ultracold Plasma Formation Puchang Jiang, Jacob Roberts Ultracold plasmas are interesting in part because they are cold enough that strong coupling effects can be made manifest in these systems at their typical densities. In order to study strong coupling effects, sufficiently low temperatures need to be obtained. While DC electric fields are useful in manipulating and detecting electrons from ultracold plasmas, a DC electric field applied during ultracold plasma formation can result in significant heating of the electron component. We used molecular dynamics simulations to study this heating process and to determine its impact as a function of ultracold plasma initial electron temperature, density, and spatial size. We find that this heating can have a significant impact on the lowest achievable temperatures for lower-density ultracold plasmas in particular. |
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TP11.00056: Excitation and Transport of solitons and Their effects on Ion Beam Neutralization Chaohui Lan, Igor D Kaganovich The excitation and transportation of BGK mode electron acoustic (EA) solitons were first observed in two-dimensional particle-in-cell simulations of ion beam neutralization by electron injection. The EA solitons, which are excited by two-stream instability of trapped electrons, can reach more than tens of Debye length in longitudinal size and last more than ten microseconds. They periodically move back and forth in a pulsed ion beam, causing a circular movement of electron phase-space-density hole. The velocity of solitons is close to the thermal velocity of injected electron. The decreased electron density results in a local maximum in electric potential and the potential peak can be several times higher than that of other places without solitons. With the attenuation of solitons, the electrons captured by the pulsed ion beam gain energy from solitions and become totally thermalized. In the absence of additional electron supplement, some of hot electrons are escaped from the ion beam, leading to the increase of beam potential and the decline of neutralization degree. |
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TP11.00057: A magnetized plasma experiment to study non-linear microwave interactions K Ronald, A D R Phelps, R A Cairns, R Bingham, B Eliasson, M E Koepke, C W Robertson, A W Cross, D C Speirs, C G Whyte Following a series of laboratory experiments1,2 simulating natural plasma instabilities3, a larger apparatus is being constructed for experiments on nonlinear coupling between microwaves in plasma, relevant to laser-plasma, ionospheric, and magnetically confined fusion plasma environments. Normalized intensities approaching those used in some recent laser plasma interactions can be generated using flexible microwave amplifiers, whilst the relatively accessible plasma relevant to coupling of microwave frequency signals will enable the use of insertion diagnostics in addition to stand-off analysis of the EM signals. The linear plasma experiment will be magnetized at up to 0.08T, an RF helicon source will be used to generate a dense, large, cool plasma with high ionization fraction (ne up to 1019m-3 has been reported in other helicon experiments). The paper will present the proposed apparatus and consider its application to space relevant phenomena. [1] S.L. McConville, et al., Plasma Phys. Control. Fusion, 50, 2008, 074010, [2] K. Ronald, et al., Phys. Plasmas, 15, 2008, 056503, [3] D.C. Speirs, et al., Phys. Rev. Lett., 113, 2014, 155002. |
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TP11.00058: Beam-Generated Waves and Fluctuations in the Heliosphere C. Fred Driscoll Waves and fluctuations in heliospheric plasmas are primarily generated by the coherent outward-flowing photon and particle beams, which are poorly described by MHD equations with "frozen-in" magnetic fields. In the solar chromosphere and corona, low inter-particle collision rates preclude hydro models. Rather, plasma sheath kinetics is required to describe the development of the (60.W/m2) solar wind beams by the (60.MW/m2) outward photon flow[1], with resultant coronal "heating". In contrast, the many contradictions of the "frozen-in" moving magnetic spiral model are readily apparent [2], and the model provides no valid equilibrium basis for waves and fluctuations. Within planetary magnetospheres, modern models properly describe magnetic distortions and waves driven by the solar wind. Elsewhere, the rapidly fluctuating magnetic fields observed by spacecraft are apparently caused by local fluctuating currents from statistical charge fluctuations, current filamentation, and neutral gas interactions with the solar wind. Here, the images of a spiral IMF "rooted" in the solar surface are less than helplful. |
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TP11.00059: Dissipative Dynamics of Nonlinear Alfven Waves in the Solar Wind Mikhail Medvedev Nonlinear Alfven waves, sometimes observed in the Solar Wind, represent the ponderomotive coupling of Alfvenic magnetic energy to ion-acoustic quasi-modes, which modifies the phase velocity v_A and caused their wave-front to get steeper. In the warm, collisionless Solar Wind plasma the resonant particle-wave interactions result in relatively rapid formation of quasi-stationary Rotational Discontinuities, which have been the subject of intense satellite observations and theoretical investigations, and whose emergence and dynamics has not been understood for a long time. We have shown that these discontinuities are quasi-stationary wave-form remnants of nonlinearly evolved coherent Alfven waves. In the long-term asymptotic, the resonant particles are trapped in the quasi-stationary Alfvenic discontinuities by mirroring forces giving rise to the nonlinear Landau damping and to a formation of a plateau on the distribution function, so that the linear collisionless damping vanishes. Using virial theorem for trapped particles, we analytically compute their effect on the nonlinear dynamics which is highly non-trivial and forces a significant departure of the theory from the conventional paradigm. |
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TP11.00060: Interpretation of bi-coherence in space and lab plasma-wave dynamics G Riggs, S Nogami, M Koepke, N Crocker, G Howes, S Savin, V Budaev, L Zelenyi Responsible bi-coherence analysis and hypothesis-testing requires adequate sampling and resolution. This investigation focuses on the assessment of temporal dynamics associated with the bispectrum cross-phase and amplitude. We detect, document and interpret wave-wave coupling evidenced in time series from interacting toroidal Alfven eigenmodes arising in DIII-D tokamak plasma, from super-low-frequency (0.02 - 10 millihertz) resonances near magnetospheric boundaries as measured by the Cluster satellite instruments, from a forced van der Pol oscillator, and from various audio signals, both recorded and contrived. We monitor the adjustable plasma parameters in the tokamak, natural time-varying conditions in the solar wind, driving-force frequency and amplitude in the van der Pol oscillator, and spectral content in the audio synthesizer. Examples of wave-wave interaction, daughter-wave generation, driven-oscillator phenomena, sideband production, and beat-wave coupling will be presented. The generality of the method is an advantage for interpreting nonlinear waves and processes across a variety of experimental platforms. |
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TP11.00061: Experimental investigation of the nonlinear spatio-temporally stationary inertial Alfven wave Samuel Nogami, Mark E Koepke, Vladimir I. Demidov, Kenneth W Gentle The stationary inertial Alfven wave is a non-fluctuating, non-traveling, spatially periodic pattern in electromagnetic field and fluid quantities [Knudsen, J. Geophys. Res. 101, 10761, (1996)]. This nonlinear Alfvenic perturbation is supported by the simultaneous presence of magnetic-field-aligned current and cross-magnetic-field plasma flow. In the upgraded Large Plasma Device (LAPD) at UCLA [Gekelman et al., Rev. Sci. Instrum. 87, 025105 (2016)], experiments were performed in helium plasma to verify the theoretically predicted [Finnegan et al., Nonlin. Processes Geophys., 15, 957 (2008)] patterns in ion density and electron energy. A variety of electrodes (e.g., multi-disk, mesh, emissive, etc.) provided experimental control of the azimuthal plasma flow across an off-axis localized current channel having electron drift directed either parallel or antiparallel to the background axial magnetic field. In this poster, theoretical predictions and experimental measurements are compared for various cases. |
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TP11.00062: Laboratory Investigation of Nonlinear Subcyclotron Damping Erik M Tejero, Chris E Crabtree, David Blackwell, Carl L Enloe, William E Amatucci, Gurudas Ganguli Triggered emissions experiments conducted in the Space Physics Simulation Chamber at the Naval Research Laboratory (NRL) have shown electron beam driven whistler turbulence that exhibits cutouts in the observed spectrum at approximately 1/2 and 1/3 of the electron cyclotron frequency. Similar gaps are observed in whistler chorus from satellite measurements. It has been suggested that nonlinear subcyclotron resonances could be a mechanism for the formation of these gaps, where nonlinear resonances at subharmonics can form due to the perturbed particle motion in the fields from the wave. A series of laboratory experiments are being conducted to test whether these observed gaps could be formed by this mechanism. Recent results from these experiments will be presented. |
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TP11.00063: Measurements of Canonical Helicity and Transport in a Gyrating Kink Jens Von Der Linden, Jason Sears, Thomas Intrator, Setthivoine You Canonical flux tubes provide a topological perspective on reconnection and dynamo problems as the flux tubes do not break, even under non-ideal conditions, and the conservation of their helicity constrains the energy transfer between magnetic field and particles. We present measurements of canonical flux tubes, their helicity, and their helicity transport in a gyrating plasma kink. The helicity gauge is removed with general techniques valid even if only a limited section of the plasma is measured. Taylor diagrams evaluate these gauge removal techniques in terms of their effect on the normalized helicity density, and root mean square difference and lengths of the vector fields. Temporal asymmetries in the helicities confirm irreducible 3D fields in the kink. The transport of canonical helicity includes static injection through an applied voltage, a resistive sink term, and the change in reference fields corresponding to the change in helicity due to the motion of the flux rope in and out of the measurement volume. |
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TP11.00064: Signatures of Energy Dissipation and Phase-Space Dynamics in Eulerian Vlasov-Maxwell Turbulence and Reconnection Jason TenBarge, James L. Juno, Gregory Gershom Howes, Ammar Hakim Turbulence and magnetic reconnection are the two primary mechanisms responsible for the conversion of stored magnetic energy into particle energy in space and astrophysical plasmas. The magnetospheric multiscale mission (MMS) has given us unprecedented access to high cadence particle and field data of turbulence and magnetic reconnection at earth's magnetopause, including there first detailed view of the particle distribution function in and around the magnetic x-line and in the turbulent magnetosheath. Motivated by these observations, we present a studies of zero guide field reconnection and 2x-3v turbulence using the fully kinetic Eulerian Vlasov-Maxwell component of the Gkeyll simulation framework. In addition to studying the configuration space dynamics, we leverage the recently developed field-particle correlations to identify and diagnose the dominant sources of dissipation and compare the results of the field-particle correlation to guiding center energy dissipation measures and measures of energy transfer through the pressure tensor. We also compare the distribution function generated in the Gkeyll simulation to MMS magnetopause data. |
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TP11.00065: Plasma Structures Associated with Binaries of Collapsed Objects, Magneto-gravitational Modes and Ballooning Mode-particle Interactions Bruno Coppi Binaries involving collapsed objects such as black holes when approaching their final merger produce an oscillatory component of the relevant gravitational potential with (orbiting) frequencies that are relatively high. Assuming that these binaries are immersed in a coherent magnetized plasma structure, magneto-gravitational modes of the ballooning type [1] are shown to be sustainable in it when their frequencies equal the (orbiting) frequencies of the gravitational potential. These ballooning modes are treated as a superposition of oppositely travelling waves along the magnetic field, with relativistic phase velocities. Thus high energy electron populations can be produced by the relevant mode-particle interactions and a consistent theoretical frame to create high energy particles capable of generating detectable electromagnetic radiation can be formulated. On this basis the prediction [2] that the final merger of two collapsed object should be preceded by precursors [3] of high energy radiation emission can be expected to be verifiable in future detected events. |
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TP11.00066: Charged particle acceleration in astrophysical plasmas. Naveen Kumar Jaiman Highly turbulent plasmas are extensively found in the space. Magnetohydrodynamics (MHD) is the main analytical tool for studying the magnetic properties of such ionised fluids and nonlinear phenomena, e.g., galaxies, star formation, interstellar winds and clouds etc. A large amplitude wave produces a strong turbulence. The turbulence is still to be well understood. This characteristic can be exploited for many applications. Charged particle acceleration is one of the important ones. There are many possible mechanisms of particle acceleration in astrophysical plasmas. The stochastic mechanisms and aggravated wave activity is one of them. The modified two stream instability (MTSI), being one of the examples of this mechanism, is driven by the ion motion in the crossed magnetic field leading to electron acceleration. The energy distribution of particles along with the mechanism of instability saturation is one of the main issues in the study. |
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TP11.00067: Overview of current research and future plan for Tokamak Disruption Simulation (TDS) SciDAC Center Xianzhu Tang The program goal of the Tokamak Disruption Simulation (TDS) SciDAC Center is to develop the fundamental physics understanding and the predictive simulation tools for tokamak disruption mitigation design. The three kinds of damage that a major disruption can bring to a tokamak reactor are: (1) rapid erosion of wall surface through melting, vaporization, and sublimation due to the orders of magnitude increase in the power exhaust in the thermal quench phase; (2) breaking and shifting of vacuum vessel and blanket modules due to the extreme electromagnetic loading by eddy and halo currents in the current quench phase; (3) deep damage of surface and substrate in the plasma facing components by runaway electrons that can induce costly secondary damages. While (2) has the flavor of 3D MHD, (1) and (3) are distinctly transport problems. Further complicating the disruption transport physics are the passive and active actuators, which include various gas and pellet injection schemes of impurities, externally applied 3D magnetic field perturbations, and fast electromagnetic wave injection. Here we give an overview of recent physics findings and future plan of TDS SciDAC center, with a focus on (1) and (3) aspect of the disruption mitigation problem. |
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TP11.00068: Toroidal studies of fast crash with Hall effects in tokamak Wei Zhang, Zhiwei Ma Both linear and nonlinear evolution of m/n=1/1 resistive kink mode is performed by a three-dimensional, fully toroidal, non-reduced and Hall-MHD code CLT. It is found that the hall terms can increase the linear growth rate of resistive kink mode when the thermal conductivity is small. However, they can greatly decrease the linear growth rate when the thermal conductivity exceeds the threshold. A transition from a ‘Y-type’ reconnection layer to a ‘X-type’ layer and a nonlinear enhanced growth rate are observed in Hall-MHD simulations, which are consistent with previous cylindrical simulations and theoretical analysis. This can be a good candidate to explain the fast crashes observed in tokamaks. |
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TP11.00069: Modeling Disruptions with M3D-C1 Nathaniel Ferraro, Isabel Krebs, Stephen Charles Jardin, Brendan C Lyons, Lang Li Lao Disruptions in tokamaks involve large electromagnetic perturbations and displacement of the plasma, rapid mixing, ionization, and radiation from impurities, global changes in magnetic topology, and strong interaction between the plasma current and surrounding conducting structures. We describe significant progress in implementing integrated models to simulate these processes self-consistently in M3D-C1, with the goal of developing validated models for predicting the consequences of disruptions and optimizing disruption mitigation. New capabilities include the inclusion of impurity transport, ionization, recombination, and radiation using the KPRAD coronal model, and the ability to model complex, non-axisymmetric conducting structures outside the plasma. M3D-C1 implements a fully compressible extended-magnetohydrodynamic model of the plasma and open-field line region. Simulations of vertical displacement events with and without disruption mitigation from impurity injection are demonstrated. Prospects for the validation of these models are considered using synthetic flux loop diagnostics and halo current measurements. |
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TP11.00070: Disruption Mitigation and Impurity Radiation Modeling with M3D-C1 Brendan Carrick Lyons, Nathaniel Mandrachia Ferraro, Stephen Charles Jardin, Charlson ChiSun Kim, Yueqiang Liu, Paul B Parks, Lang Li Lao The risk of damage to future tokamaks requires robust disruption mitigation techniques, the most promising of which uses pellet injection of impurities to radiate stored energy. In order to simulate pellet mitigation for projection to ITER and beyond, we have coupled the M3D-C1 extended-magnetohydrodynamics code to the KPRAD ionization/radiation code. Four different numerical and physical models of the coupling have been implemented. The model with a single temperature-evolution equation has been verified in a 2D, nonlinear benchmark with coupled NIMROD-KPRAD simulations, showing near-perfect agreement between the two codes. Simulations with two temperature equations have been performed, demonstrating longer thermal quench times due to the necessary thermal equilibration between ions and electrons. Initial results from a 3D benchmark between M3D-C1 and NIMROD will be presented, as will progress on 2D and 3D, nonlinear M3D-C1 simulations of pellet injection using ablation models of various levels of sophistication. Plans for validation against DIII-D and JET shattered-pellet injection (SPI) experiments, along with predictive simulations of ITER will also be discussed. |
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TP11.00071: The influence of the pre-existing islands on thermal quench and radiation asymmetries during massive gas injection on J-TEXT Ruihai Tong, Zhongyong Chen, Zhonghe Jiang, Zhifang Lin, Jing Huang, Wei Li, Yunong Wei Locked modes are one of the major causes of disruptions in tokamak. The disruption mitigation system, like massive gas injection (MGI), is expected to be deployed with a non-rotating modes. A series of plasmas were penetrated with different phases m/n=2/1 resonant magnetic perturbation (RMP) were terminated by massive gas injection of helium in J-TEXT tokamak. The results show that the phase of the 2/1 mode relative to the MGI valve can influence the gas penetration progress. Thus, the thermal quench (TQ) onset and duration will be influenced: the core TQ duration can be delayed when the o-point is aligned with the MGI valve and reduced when x-point is aligned with the MGI valve. The radiation asymmetries have also been analyzed with AXUV arrays. The experimental results have been compared with the 3D extended MHD code NIMROD. The simulation results indicate that the pre-existing 2/1 mode can influence the parallel impurity spreading and the energy loss. |
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TP11.00072: Electromagnetic Particle Injector (EPI) as a Fast Time Response Disruption Mitigation Concept Roger Raman, Robert A. Lunsford, W-S. Lay, Thomas R Jarboe, Raffi Nazikian, Jonathan Edward Menard, Masayuki Ono The Electromagnetic Particle Injector (EPI) has the potential for delivering the radiative payload composed of micro spheres of B, BN or Be, inside the q = 2 surface on a 3-4 ms time scale, much faster, and deeper, than what can be achieved using present methods. Experimental tests on a proto-type system have been able to verify the primary advantages of the EPI concept over other disruption mitigation concepts for a tokamak. These are the rapid response time and the capability to attain the projected speeds on this fast time scale. In preparation for a tokamak test of the concept, a much-improved compact system has been built. Compared to the initial test version, this system employs 2T magnetic field augmenting coils to substantially reduce the current through the system to attain the required velocity. Capability for radiative payload deposition in the core provides a means to suppress the formation of the runaway current. With EPI, one can precisely calculate the injection parameters needed for deep penetration into any plasma, including the ITER plasma, giving confidence that simulation capabilities validated with present tokamak experiments can be used to reliably project to ITER. |
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TP11.00073: A force-free motion of cold plasma during the current quench Boris Breizman, Dmitrii Kiramov Cold disruptive plasma tends to move during the current quench. Its motion is essentially force-free since the current quench timescale is resistive rather than Alfvenic. In contrast with the hot vertical displacement events, the frozen-in condition is violated in the cold plasma case, and the plasma motion is not governed by magnetic flux conservation but rather by its dissipation. We present a numerical model of the cold plasma dynamics. This model predicts electromagnetic loads on the vacuum vessel, the plasma flow and density evolution, and the plasma centroids evolution. Our calculations include poloidal wall currents. We demonstrate their significan contribution to the force acting on the vacuum vessel. |
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TP11.00074: Onset and nonlinear relaxation of coherent edge current-carrying filaments during VDEs Fatima Ebrahimi The onset and nonlinear evolution of coherent current-carrying filaments are examined using global nonlinear three-dimensional resistive MHD NIMROD simulations in a spherical tokamak (ST). We show that time-evolving current sheets/layers develop near the tokamak edge under different circumstances during transient events. In particular, during induced vertical displacement event (VDE) simulations by driving large current in the open field region, we investigate the stability of halo current and the formation of reconnecting edge peeling-driven filaments. We show that as the plasma is vertically displaced, the edge halo current sheet becomes MHD peeling-tearing unstable and form non-axisymmetric coherent edge current filament structures. Similar to fast reconnection due to axisymmetric plasmoids, we find that the growth rate of these edge filamentary structures becomes independent of Lundquist number. As well as edge reconnection physics in tokamaks, the 3-D coherent current-carrying filament structures and their nonlinear dynamics due to the dynamo effect presented here are also relevant to flares, which also exhibit ejection of field-aligned filamentary structures into surrounding space. Supported by DOE grants DE-SC0010565, DE-AC02-09CH11466. |
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TP11.00075: Transient-MHD-Induced Transitions In Metastable Tokamak Plasmas James D Callen, Matthew T Beidler, Chris C Hegna When small resonant magnetic perturbations (RMPs) are present in flowing plasmas, they can produce metastable states. Namely, tokamak plasmas can be in benign high-slip, flow-screened states or in low-slip, mode-penetrated states that produce magnetic islands which can be problematic and growing. It has been demonstrated recently in slab model calculations with NIMROD [1] that MHD-type perturbations of large enough magnitude and duration can induce transitions into low-slip states. In tokamaks, while static RMPs are typically produced by field errors and applied fields, sawteeth and ELMs cause nonlinear resonant field responses. MHD-transient-induced transitions in tokamaks include RMP ELM suppression and excitation of neoclassical tearing modes (NTMs). We are developing reduced fluid-based models of these processes in tokamak plasmas that include near-axisymmetric geometry, low collisionality effects, and poloidal and toroidal flow (radial electric field) responses to MHD transients. Applications to RMP ELM suppression and NTMs will be discussed. [1] M.T. Beidler, J.D. Callen, C.C. Hegna, and C.R. Sovinec, “Mode Penetration Induced By Transient Magnetic Perturbations,” report UW-CPTC 18-3, June 25, 2018, available via cptc.wisc.edu. |
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TP11.00076: Study of full impurity Shattered Pellet Injection by non-linear 3D JOREK simulation in JET plasma Di Hu, Daniele Bonfiglio, Eric Nardon, Guido T. A. Huijsmans, Michael Lehnen, Daan van Vugt The effect of Shattered Pellet Injection (SPI) with full impurity pellets on the radiative energy loss and MHD modes is studied by the reduced MHD model of the JOREK code with a JET target equilibrium and injection configurations resembling that of the JET SPI system. |
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TP11.00077: Plasma Response to Perturbed Nonaxisymmetric Fields in Double Null Configurations Alan Douglas Turnbull, Brendan C Lyons, Todd E Evans, Yueqiang Liu, Carlos Alberto Paz-Soldan, Edward J Strait, David Weisberg, Morgan Shafer, Andreas Wingen, Allan Reiman The plasma response to external nonaxisymmetric perturbations is a key ingredient in the observed ELM suppression from external fields. Dedicated experiments in DIII-D have confirmed a prior observation that the magnetic response on the inboard high field side in balanced double null (DN) plasmas is greatly reduced compared to the signal in similar single null (SN) discharges, as well as to the outboard signals. This is a key part of the puzzle in explaining ELM suppression: full suppression has not been observed in DN in any experiments despite the large density pumpout that typically occurs in those conditions. Analysis of sequences of equilibria with scans in the magnetic balance and edge safety factor q95 indicates that there is a relatively narrow window in magnetic balance over which the inboard signal is suppressed by a factor of five. In this range, the density pumpout is enhanced and the ELMs are mitigated but not suppressed. Comparisons of various code predictions for the response with external magnetic data and internal SXR data confirm the observed inboard magnetic field suppression. The results are discussed in the context of the key competing models for ELM suppression and mitigation. |
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TP11.00078: Interpreting ion cyclotron emission associated with transient events in Large Helical Device deuterium plasmas Richard Dendy, Bernard Reman, Tsuyoshi Akiyama, Sandra Chapman, Hiroe Igami, Kenji Saito, Gunsu Yun Ion cyclotron emission (ICE), driven by fast collective relaxation of energetic ion populations, has been detected from most large magnetic confinement fusion plasmas. ICE is suprathermal, with narrow spectral peaks that can be mapped to the cyclotron harmonics of energetic ion species in the emitting region. Advances in the RF diagnostics in the LHD heliotron-stellarator enable time-resolved ICE measurements during transient, bursty, plasma events that evolve on microsecond timescales. Recent ICE measurements from LHD deuterium plasmas suggest that ions born in DD fusion reactions, in addition to neutral beam injected ions, may generate ICE during bursty events. A distinctive feature is the role of apparent large Doppler frequency shifts comparable to the separation between ICE peaks. Using a particle-in-cell hybrid code to simulate the relaxation of one or more energetic ion species within deuterium plasma under the self-consistent Maxwell-Lorentz system of equations, we find that the magnetoacoustic cyclotron instability drives the ICE. The transient event can cause rapid evolution of the energetic ion distribution in real and velocity space, and of the ambient plasma. We describe how the foregoing physics can be integrated into a model for ICE from transient events in LHD. |
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TP11.00079: Dependence of Perpendicular Viscosity on Magnetic Fluctuations in a Stochastic Magnetic Field B.E. Chapman, R. Fridstrom, A.F. Almagri, L. Frassinetti, P.R. Brunsell, T. Nishizawa, J.S. Sarff In a magnetically confined plasma with a stochastic magnetic field, the dependence of the perpendicular viscosity on magnetic fluctuation amplitude has been measured for the first time [PRL 120, 225002 (2018)]. With a controlled, ∼ 10-fold variation in the fluctuation amplitude, the viscosity increases ∼100-fold, exhibiting the same (b/B)^2 dependence as the predicted rate of stochastic field line diffusion. The absolute value of the viscosity is well predicted by a model [Finn et al., 1992] based on momentum transport in a stochastic field, the first in-depth test of this model. Derived for the tokamak, we tested this model in MST RFP plasmas. The viscosity in MST plasmas reaches about 55 m^2/s and can exceed the Braginskii prediction by several-hundred-fold. Measurements of the viscosity in a stochastic topology are rare but can be of critical importance. Viscosity is a key parameter in visco-resistive nonlinear MHD modeling, which is being applied to stochastic scenarios like tokamak disruptions. Viscosity, like the resistivity, is likely to increase substantially during a disruption. And while the modeled resistivity, or at least the electron temperature, is sometimes based on experimental data, the viscosity is often assumed. |
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TP11.00080: Measurements of internal MHD activity and seed runaway electrons during MST tokamak disruptions Mihir D Pandya, Brett Edward Chapman, Abdulgader F. Almagri, Karsten J McCollam Disruptions in MST tokamak plasmas can be effected by rapid ramp down of the toroidal field, causing a temporary increase in the plasma current and magnetic fluctuations followed by a current quench. Fast Thomson scattering reveals an initial slow drop in the core Te followed by a final rapid drop, resulting in a flat Te profile. The equilibrium and fluctuating magnetic field components have been diagnosed with a probe inserted up to r/a = 0.45 and consisting of 20 triplets of coils separated by 1 cm. The q profile, measured before the disruption for q(a) = 2.5, Ip = 50 kA, and Te < 100 eV plasmas, exhibits a q = 1 surface inside mid-radius. With the disruption, the q profile flattens more broadly to a value just above unity. High-energy electrons are generated and rapidly lost during these disruptions, both potentially due to MHD activity. The x-ray flux from these electrons shows a strong inverse dependence on density just before start of Bt ramp down. Runaway electron and other data will also be shown from newly produced plasmas with q(a) = 1.5 - 3.5 and improved control of Ip. Work supported by US DOE. |
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TP11.00081: Control of runaway electron energy in tokamak Zehua Guo, Christopher McDevitt, Xianzhu Tang One way of mitigating runaway electron (RE) damage of the plasma-facing components is by limiting the RE energy under a few MeV, while not reducing the runaway current appreciably. Here we report a physics mechanism by which such momentum-space engineering can be facilitated by externally injected whistler (helicon) waves. By introducing a wave that resonantly interacts with runaways at a chosen range of energy, the enhanced scattering would reshape the vortex by cutting off the highly relativistic part. The power requirement from external wave injection is estimated to be practical. The investigation has been extended to the tokamak geometry using a bounce-averaged formulation. It is found that the magnetic trapping effect reduces the volume of runaway vortex as the momentum-space fluxes are strongly modified in the trapped-region. As a result, the avalanche growth rate is reduced at off-axis locations. The reduction can be effectively enhanced by injected whistler waves. The induced pitch-angle scattering pushes the runaway vortex into trap-region where electrons are no longer accelerated. So the wave injection scheme becomes more efficient in large tokamaks and can even be designed to substantially increase the avalanche threshold electric field. |
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TP11.00082: Production rate of runaway electrons in dynamic scenarios: a probabilistic backward Monte-Carlo method Diego del-Castillo-Negrete, Guannan Zhang The computation of the production rate of runaway electrons (RE) is important because, if not avoided or mitigated, RE can severely damage the plasma facing components. Recently, we proposed a novel approach to solve this problem using the backward Monte-Carlo (BMC) method [1]. The BMC is based on the Feynman-Kac formula that establishes a link between the adjoint Fokker-Planck problem (which gives the probability of runaway) and the stochastic differential equations describing the trajectories of RE in the presence of collisions. Computationally, the BMC is a deterministic algorithm that reduces the problem to the evaluation of Gaussian integrals using Gauss-Hermite quadrature rules. Following a description of the method, we present results on the computation of the time evolution of the probability of runaway, the expected runaway time, the expected loss time, and the production rate. Going beyond the results presented in [1] we use a more detailed collision operator and extend the BMC to dynamic scenarios where the electric field and the plasma temperature exhibit time dependence. In particular, we compute the RE production rate due to hot-tail generation during a rapid drop in plasma temperature. [1] Zhang and del-Castillo-Negrete, Phys. of Plasmas 24, 092511 (2017). |
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TP11.00083: Kinetic Radial Transport and RF Quasilinear Diffusion of Runaway Electrons Calculated with CQL3D R.W. (Bob) Harvey, Yu.V. Petrov, P.B. Parks, L.L. Lao, Charlson C. Kim, C.B. Forest Runaway electron (RE) distributions which are driven by large toroidal electric fields induced by rapid Te-drops in a tokamak, e.g., due to plasma disruption or pellet injection, are comprehensively simulated by the toroidal 3D bounce-average Fokker-Planck solver CQL3D [1], including the Ampere-Faraday equations. CQL3D has been extensively applied to calculate plasma kinetic distributions f(vpar,vperp,rho,t) resulting from RF induced quasilinear and collisional diffusion. For runaways, we present results of the combined effects of (1) radial diffusion and pinch terms, which can strongly reduce the RE population [2]; and (2) injection and/or instability of RF which can pitch angle scatter the RE distribution thus increasing their synchrotron radiation, thereby reducing their energetic potency. The background time-dependent plasma profiles, including radial diffusion, are obtained from the NIMROD code [4]. [1] R.W. Harvey and M. McCoy, “The CQL3D Fokker Planck Code,” www.compxco.com/cql3d.html. [2] R.W. Harvey, V.S. Chan, S.C. Chiu et al., Phys. Plasmas 7, 4590 (2000). [3] Pavel Aleynikov and Boris Breizman, Nucl. Fusion 55, 043014 (2015). [4] V.A. Izzo and P.B. Parks, Phys. Plasmas 24, 060705 (2017).
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TP11.00084: Spatial Transport of Runaway Electrons in Axisymmetric Tokamak Plasmas Chris McDevitt, Zehua Guo, Xianzhu Tang An implicit assumption made in the vast majority of studies of runaway electrons is that they are well confined to a given magnetic flux surface, thus allowing the use of slab or bounce averaged formulations. While such an assumption is known to break down in the presence of strong 3D magnetic field perturbations, here we show that it can be violated even for an axisymmetric magnetic field under conditions representative of an actively mitigated disrupting plasma. Specifically, the low temperature and large impurity content typical of a post thermal quench plasma are shown to provide a drastic enhancement of the diffusive and convective transport of runaways electrons, where the convective component is found to be dominated by the Ware pinch. This inward convective flux allows runaway electrons to be displaced toward the plasma center, where they are eventually detrapped and reaccelerated, thus focusing populations of runaway electrons in the inner portion of the plasma. The resulting runaway electron distribution is shown to settle into a well-defined spatial eigenmode, whose form is often insensitive to the spatial distribution of the original seed population. |
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TP11.00085: Progress in the Development of Nanoparticle Plasma Jet for Runaway Electron Suppression I. N. Bogatu, S. A. Galkin Successful suppression of runaway electrons (REs) by impurity injection requires 1 - 2 ms overall response, few to several km/s speed, enough mass with hundreds of m2/g specific surface area able to penetrate 2 - 5 T tokamak toroidal B-field over 1 - 2 m distance, with large assimilation fraction in post-TQ residual core plasma. A nanoparticle plasma jet (NPPJ) prototype demonstrated C60 fullerene NPPJ (compatible w. CFC tokamaks) on test bed. Work on ITER-compatible boron nitride (BN) NPPJ is in progress. Once injected into plasma, C60 and BN NP undergo fragmentation and ablative sublimation, respectively. C (from C60) or B and N (from BN) ions with charge q = +1 to +6 produced in the core lead to rapid increase of Zeff(r,t), with the effect of primary REs suppression. We report on our recent progress in theoretical modeling and numerical simulation of C60 and BN NPPJ penetration and gradual mass delivery into post-TQ plasma with the following Zeff effect on RE dynamics. The conceptual design for NPPJ specific installations to a tokamak (e.g., DIII-D) will also be presented. |
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TP11.00086: Asymmetric wall force reduction in ITER and JET including Runaway Electrons Henry Strauss It has been shown that a fast current quench (CQ) in JET [1] and ITER [2] AVDE disruptions greatly reduces the asymmetric wall force. This was shown in simulations with the M3D 3D MHD code [3] and in JET experiments [4] in which the current was quenched with massive gas injection (MGI). If the current quench (CQ) time is less than the resistive wall penetration time the asymmetric wall force is reduced. A fast CQ may cause production of runaway electrons (REs), which tend to have a long quench time. JET data shows that they produce a small asymmetric wall force. Simulations using a modified version of M3D with a fluid RE model [5] show a small asymmetric wall force, even when the runaways are in a regime with CQ time much longer than the wall time. Evidently the REs cause stabilization of MHD activity. Details of the simulations of REs in ITER and JET will be reported. [1] H. Strauss, E. Joffrin, V. Riccardo, teal, Phys. Plasmas 24, 102512 (2017). [2] H. Strauss, Physics of Plasmas 25, 020702 (2018). [3] W. Park, E. Belova, G. Y. Fu, etal, Phys. Plasmas 6, 1796 (1999). [4] S. Jachmich, P. Drewelow, etal, 43rd EPS Conf. Plasma Physics (2016) [5] Huishan Cai and Guoyong Fu, Nucl. Fusion 55, 022001 (2015). |
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TP11.00087: Runaway electrons suppression by the resonant magnetic perturbations during disruptions on J-TEXT Zhifang Lin, Zhongyong Chen, Ruihai Tong, Yunong Wei, Wei Yan, Li Da The runaway electrons (REs) generated during disruptions pose a serious problem for the safe operation of tokamaks. One optional method for REs suppression is external applied resonant magnetic perturbations (RMP). The combined effect of different modes of RMP on the REs suppression has been investigated on the J-TEXT tokamak. It was found that m/n=3/1 mode RMP with an appropriate amplitude was beneficial for the REs suppression. The runaway current can be significantly suppressed by 2/1 mode RMP with high strength. With the combination of 2/1 mode and 3/1 mode RMP, the performance of REs mitigation was different. By applying a high strength of 2/1 RMP combined with 3/1 mode RMP, remarkable enhancement of runaways was found compared to the case of 2/1 mode RMP only. This findings implied that the 3/1 component of RMP might play an important role in the REs suppression during disruption. |
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TP11.00088: Validation of runaway electron models using synchrotron radiation measurements and full-orbit simulations Leopoldo Carbajal, Diego del-Castillo-Negrete, Carlos Paz-Soldan We present recent progress on the validation of pitch angle distribution (PAD) models of runaway electrons (RE). The study is based on the comparison of synchrotron radiation (SR) measurements in DIII-D and synthetic signals computed using the Kinetic Orbit Runaway electrons Code that incorporates full-orbit dynamics as well as the geometry and spectral resolution of the camera. We focus on DIII-D quiescent plasma #165826 for which spatial, temporal, and energetically resolved RE distribution functions are available. The PAD models of interest are based on Fokker-Planck (FP) descriptions that neglect orbit effects and balance collisional scattering with pinching due to electric field. We use Proper Orthogonal Decomposition data-mining methods to guide the comparison of experimental and computational diagnostic signals. It is concluded that spatial effects play an important role and that phase-space models overestimate the decay rate of the PDA which affects its width. We also show that the FP PAD model ceases to be an equilibrium distribution when spatial degrees of freedom are taken into account. At high energies the computed PAD significantly deviates from the model PAD, a result consistent with observed discrepancies between Fokker-Planck models and DIII-D measurements. |
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TP11.00089: Oscillatory behaviors in wave-particle interactions of runaway electrons Chang Liu, Dylan P. Brennan, Amitava Bhattacharjee, Guo-Yong Fu Runaway electrons generated in tokamaks can excite whistler waves in a wide frequency range. The excited whistler waves can cause energy diffusion and pitch angle scattering of resonant electrons, and alter the distribution function. In this study, we find that the interaction of unstable whistler modes and resonant runaway electrons can be described using a predator-prey model. The modes driven by the inhomogeneity of the runaway electron distribution function can cause flattening of the distribution through quasilinear diffusion, which can stabilized the mode, and cause it to begin to decay. After some time, the inhomogeneous distribution function is recovered, and the mode can begin growing again. This periodic process can cause several oscillatory phenomena associated with runaway electrons, including the oscillations of low-frequency whistler waves and high-frequency extraordinary electron waves, and sawtooth-like behavior of ECE signals. In addition, we also study the oscillatory behavior of the synchrotron radiation pattern that has been found in experiments, which is also connected to the excitation and damping of whistler modes. |
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TP11.00090: Status of the NSTX-U Recovery Project Plasma Facing Components Michael Jaworski Plasma-Facing Components play a critical role in the operation of magnetic fusion experiments. These components intercept plasma heat and particle flux preventing damaging fluxes from impinging vessel components and other structures. In addition, these components can present a uniform material composition that optimizes the experiment toward certain conditions. The NSTX-U Recovery Project is deploying upgrade plasma-facing components composed of isotropic graphites in order to minimize intrinsic impurity radiation and simultaneously present a surface that is immune to melting damage. Upgrades and redesign of the PFCs will maximize performance of these materials in order to enable the greatest possible flexibility in experimental operations. The impact of finite tolerances on component shaping designs, in particular, is considered and the resulting, subtle variations in design will be discussed. |
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TP11.00091: Heat Flux Testing of Prototype NSTX-U Plasma Facing Components Travis Gray, Nicole Allen, Robert Graham Ellis, Michael Jaworski, Andrei Khodak, Matthew L Reinke, Gustav Smalley, Dennis L Youchison, Dennis E Wolfe The upgrade to the National Spherical Torus eXperiment (NSTX-U) doubles the neutral beam power and enables plasmas to be sustained for up to 5 seconds. The graphite plasma facing components (PFCs) have been re-designed to handle greater heat and energy fluxes than were seen in NSTX using a castellated design. These new PFCs were designed to be able to withstand uniform, perpendicular heat fluxes of up to 8 MW/m2 for up to 5 seconds before reaching a surfaced averaged temperature limit of 1600 C. We present experimental testing of prototype, castellated graphite PFCs in an electron beam (EB) facility which provides temporally non-uniform heat fluxes on target. Due to the temporally non-uniform nature of the heat fluxes the EB facility provides, the thermal analysis used to design the prototype PFC tiles will be benchmarked against in-situIR thermography and calorimetric measurements with self-consistent boundary conditions to the prototype testing. |
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TP11.00092: Heat Flux Model Validation Utilizing Machine Learning and Sub-surface Thermocouples for NSTX-U Plasma Facing Components Tom Looby, Matthew L Reinke, David C Donovan, Travis Gray A proof of concept convolutional neural network (CNN) has been developed to assist in operating tokamaks outside of existing empirical scalings for the heat flux width, lq. NSTX-U has designed new plasma facing components (PFCs) to withstand increased halo current forces as well as elevated heat fluxes driven by increased Bp and PNBI compared to NSTX. Larger graphite tiles are castellated to ~2.5 cm x 2.5 cm to reduce bending stresses. Maintaining PFCs below engineering limits will be an important consideration for operation of NSTX-U. Sub-surface temperature transducers (thermocouples) will be utilized to demonstrate validation of the heat load model, using the castellated designs to quantify the shot-integrated energy deposited in the NSTX-U divertor. A CNN has been trained using ANSYS simulations of PFC response to a variety of time-varying heat flux profiles. In practice the CNN will accept time evolving thermocouple data and various 0-D engineering parameters and output the heat flux model parameters, such as the Bp scaling of lq. The CNN enables satisfactory validation of the heat flux model, despite a limited number of simulated NSTX-U shots, expected noise, and systematic errors in the thermocouple data. |
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TP11.00093: Solenoid-free Start-up Modeling of 2ndHarmonic Electron Cyclotron Heating and Current Drive Masayuki Ono, Nicola Bertelli, Hiroshi Idei, Kazuaki Hanada The QUEST ECH solenoid-free start-up experiment utilizing the 28 GHz gyrotron at 2ndharmonic frequency has demonstrated remarkable efficiency and record start-up current values. An ST/tokamak start-up modeling is a highly-coupled non-linear problem as the magnetic field topology evolves dramatically from an open to a closed configuration, and the plasma temperature evolves from very cold collisional to very hot collision-less regime. For this task, we developed a grid-based modeling code where the plasma parameters, generated plasma currents, and resulting changing poloidal magnetic fields are evolved from the vacuum fields. Initially at low temperature, weak 2ndharmonic ECH takes place requiring multi-pass absorption. The current generated in this phase is purely pressure driven. The grad-B drift driven current together with processional currents can then create a closed flux surface configuration where the bootstrap current can further enhance the plasma current. Once plasma temperature is sufficient, a single-pass absorption can rise sufficiently for ECCD to become dominant. This entire start-up process is a self-amplifying “explosive” non-linear problem, where a very rapid plasma current rise can be expected. |
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TP11.00094: Characterization of deuterium neutral density profiles in the National Spherical Torus Experiment Upgrade Filippo Scotti, Ronald E Bell, Benoit Paul LeBlanc, Steven Anthony Sabbagh, Vlad Soukhanovskii, Daren P Stotler, Stewart J Zweben Neutral density profiles are measured on the outboard midplane of the National Spherical Torus Experiment Upgrade (NSTX-U) using a two dimensional camera looking at the D-α emission. In magnetically-confined fusion devices, assessing the distribution of neutrals is critical to estimate particle sources and energy loss mechanisms in the edge plasmas, which play an important role in defining the structure of the H-mode density and temperature pedestals. Neutral densities are calculated by inverting the line-integrated D-α brightness and using local measurements of electron density and temperature to infer atomic rate coefficients. Neutral distributions on the low-field-side midplane are compared for L-mode and H-mode discharges in NSTX-U, including the evolution during sawtooth and ELM cycles. Neutral density profiles with boronized wall conditions in NSTX-U are compared with those measured in lithium-conditioned discharges in NSTX. The particle source profiles will be inferred from simulations with the Monte Carlo neutral transport code DEGAS2 and the multi-fluid edge transport code UEDGE, constrained by the experimental D-α emissivities. |
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TP11.00095: High harmonic fast wave propagation in the scrape-off layer of NSTX and NSTX-U Eun-Hwa Kim, Nicola Bertelli, Joel C. Hosea, Ernest Valeo, Rory J. Perkins, Syun'ichi Shiraiwa, John Wright We perform simulations using a 2D full-wave (FW2D) code for high harmonic fast wave (HHFW) in the scrape-off layer (SOL), the region of the plasma between the last closed flux surface (LCFS) and the tokamak vacuum vessel, of NSTX and NSTX-U. The SOL is important for HHFW heating because up to 60% of the coupled power can be lost in the SOL. In this presentation, we examine the collisional power losses in the SOL (Pabs) by adopting various values of the electron density in front of antenna (Nant), the magnetic field strength (B0) as well as the distance between the LCFS and the antenna (Δ). The results clearly show that Pabs increases with increasing Nant, and/or weaker B0, showing consistency with NSTX experiments. We also found that vessel shape strongly affects the power lost to the SOL, hence Pabs increases when Δ increases. We also examine the HHFW in the NSTX/NSTX-U using recently developed Petra-M code and compare with the results from FW2D. |
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TP11.00096: Multi-beam effects on compressional Alfven eigenmode stability Jeff Lestz, Elena Belova Beam-driven sub-cyclotron compressional (CAE) and global (GAE) Alfven eigenmodes are routinely excited in spherical tokamaks such as NSTX and MAST. Early NSTX-U operations demonstrated robust suppression of GAEs with the new, more tangentially injecting neutral beam [1]. This phenomenon motivates the theoretical investigation of similar multi-beam effects on CAE stability in characteristic NSTX-U plasmas. Self-consistent hybrid simulations of CAEs excited by two-beam distributions are performed and compared with an analytic calculation for the perturbative fast ion drive. A stabilization criterion depending on the difference in the central pitch λ1 - λ2 and relative beam power fractions P1 / P2 is explored. Predictions for multi-beam (de)stabilization of CAEs for NSTX-U are presented and compared with extrapolations to parameter regimes for MAST-U and DIII-D. [1] E.D. Fredrickson et al. Phys. Rev. Lett. 118, 265001 (2017) |
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TP11.00097: Development of a reduced energetic particle transport model by low-frequency MHD for time-dependent integrated tokamak simulations Mario L. Podesta Low frequency instabilities such as kinks and fishbones are well know causes of enhanced energetic particle (EP) transport in tokamaks. However, physics-based models to account quantitatively for their effects in integrated, time-dependent integrated tokamak simulations are still missing. Development of such models is needed for more reliable projections of operating scenarios from today’s devices to future burning plasmas, e.g. ITER and FNSF. This work will report on recent developments of a reduced EP transport model by fishbones and kinks, based on the existing “kick model” infrastructure already implemented in the TRANSP code. For example, the rapid (~1-5 ms) sweep in frequency characteristic of a fishbone burst implies that different regions of energetic particle phase space are affected as time evolves. The kick model can account for such changes by representing the instability as superposition of multiple, fixed-frequency instabilities with time-dependent amplitudes. Based on the initial results from the kick model, perspectives for the development of a self-consistent model for low-frequency MHD in TRANSP will be discussed. |
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TP11.00098: Introduction of passive signals and 3D geometry to FIDASIM Alvin Villagrana Garcia, Luke Stagner, William Walter Heidbrink FIDASIM is a synthetic diagnostic code that simulates fast ion D-alpha (FIDA) and neutral particle analyzer (NPA) signals. The code accepts a theoretical fast-ion distribution function as input and predicts the FIDA and NPA spectra. Originally, FIDASIM assumed axisymmetry and included active signals produced only by charge exchange (CX) with injected neutrals. However, passive signals produced by CX with cold neutrals can also be important. For example, passive-FIDA signals of comparable magnitude with active signals were experimentally measured on NSTX-U [1]. Therefore, FIDASIM has been improved to predict passive signals for a given edge-cold neutral population. The time series passive spectra output by FIDASIM have been successfully cross checked with 2D passive-FIDA modeling done on NSTX-U. The effect of 3D fields on fast ion confinement is an ongoing field of study, e.g. toroidal field ripple, stellerators, etc. Thus, the functionality of FIDASIM has been enhanced to simulate signals produced in 3D geometry. |
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TP11.00099: Nonlocal transport in toroidal plasma devices Roscoe White Several heuristic models for nonlocal transport in plasmas have been developed, but there has been little possibility of detailed comparision with experimental data. Collisional particle transport is examined in several toroidal plasma devices in the presence of perturbations typical of modes leading up to a disruption, of saturated tearing modes, or of unstable Alfv ́en modes. The existence of subdiffusive transport for electrons is found to occur in some cases at very low mode amplitudes and to also exist even for ions of high energy.The existence and nature of subdiffusive transport is found to depend on the nature of the mode spectrum and frequency as well as the mode amplitudes. 1. M.B. Isichenko, Plasma Phys. Controlled Fusion, 33, 795 (1991) |
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TP11.00100: Velocimetry and the aperture problem for 2D incompressible flows Timothy Stoltzfus-Dueck, Ahmed Diallo, Stewart J Zweben The inference of velocity fields from 2D movies evolving conserved scalars (optical flow) is fundamentally ambiguous due to the well-known “aperture problem”: velocities along isocontours of the scalar are not visible. This may even corrupt the inference of velocity fields averaged at scales longer than the typical length scale of features in the scalar field, as in the barber-pole effect. However, for divergence-free flows, a stream-function formulation allows us to show that the "invisible velocity" vanishes in the surface average over any closed scalar isocontour. This error-free averaged velocity may be used as an “anchor” for a more reliable inference of the larger-scale velocity field, or to test model-based optical-flow schemes. We have also used the stream-function formulation to derive a new method of optical flow for divergence-free flows. We discuss the new algorithm, including details of discretization, boundary conditions, and image preprocessing that can significantly affect its performance. A simple implementation of the new method is shown to work well for a number of synthetic movies, and is also applied to a GPI movie of edge turbulence in NSTX. |
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TP11.00101: Simultaneous High-k Scattering and Microwave Imaging Reflectometry on NSTX-U N.C. Luhmann, Jr., Robert Barchfeld, Yang Ren, Robert Ellis, Nicole Allen, Robert Kaita, Brent Stratton, Jon Dannenberg, Yilun Zhu, Calvin Domier An 8-channel 693 GHz poloidal high-k scattering system is under development, replacing a 5-channel 280 GHz toroidal scattering system, to study high-k density fluctuations on NSTX-U. The far-infrared probe beam is launched from Bay G towards Bay L, where large aperture optics collect radiation at 8 simultaneous scattering angles ranging from 2 to 15°. This yields measurement of poloidal wavenumbers from 7 cm-1 to >40 cm-1, while translatable optics allow placement of the scattering volume from r/a = 0.1 out to the pedestal region (r/a ~ 0.99). A microwave imaging reflectometry (MIR) system will co-exist with the High-k Scattering system on Bay L, with MIR optics positioned above that of the scattering system, to monitor low-kθ (< 3 cm-1) density fluctuations. Details of the 5×4 channel (5 poloidal, 4 radial) MIR system, spanning frequencies of 51 to 75 GHz, will be presented together with that of the High-k Scattering system. |
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TP11.00102: NubeamNet: Accelerated predictive modeling of NSTX-U beam deposition for optimization and control Dan Boyer, Keith Erickson, Stanley Kaye, Vaish Gajaraj, Justin Kunimune, Michael Zarnstorff Model-based control and scenario development will be critical for safely and efficiently reaching optimal performance of present-day and future fusion devices. The model-based approach relies on a hierarchy of models of varying fidelity and speed, tailored to different roles in the design process. To enable high-fidelity beam deposition calculations for real-time optimization and control applications on NSTX-U, a neural network model, NubeamNet, has been developed based on NUBEAM calculations. The model evaluates heating, torque, and current drive profiles from equilibrium parameters and measured profiles. The database was generated from interpretive TRANSP analysis of shots from the 2016 NSTX-U campaign. Predictions made for the testing data demonstrate the ability of the model to generalize and accurately reproduce profiles and scalar quantities. Hardware-in-the-loop simulations of the model in the NSTX-U plasma control system demonstrate the suitability of the model for real-time applications. Applications of the model, including estimation of Zeff and anomalous fast ion diffusivity to match measured neutron rates, will be presented. |
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TP11.00103: TRANSP: status and plans Francesca M Poli, Joshua Breslau, Johan Carlsson, Marina Gorelenkova, Jai Sachdev, Xingqiu yuan, Stanley Martin Kaye TRANSP is a time-dependent, 1.5D equilibrium and transport solver, used for modeling of tokamak plasma discharges and for experimental planning. TRANSP incorporates state of the art heating/current drive sources and turbulence transport models. Recent additions include extension of the pedestal pressure predictions based on a neural-network based on peeling-ballooning stability calculations, a model for pellet ablation, impurity transport predictions, extension of the Sauter model for bootstrap current to higher collisionality. This poster describes present uses of the code worldwide, as well as ongoing work and plans for upgrading the physics modules and the code framework, including extension of the transport to the wall and development of a fusion flight simulator.
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TP11.00104: Analysis of the Updated ITPA Global H-Mode Confinement Database Stanley Martin Kaye, Clemente Angioni, Otto Kardaun, Mikhail Maslov, Michele Romanelli, Francois Ryter, Knud Thomsen, Geert Verdoolaege, ASDEX Upgrade Team, MST1 Team, JET Contributors We report on the initial analysis of the ITPA global H-mode database, which has been updated with data closer to ITER baseline and hybrid conditions, including discharges from high-Z wall devices, specifically from ASDEX Upgrade W-wall and JET ILW operation. New and revised data has led to a reduction in the linear correlations among predictor variables relative to those that existed in developing the IPB98y2 scaling. Using standard constraints, individual device scalings suggest that the relatively strong density dependence observed in IPB98y2 arises primarily from the older, more circular devices and JET carbon-wall discharges. The data from higher- shaped and high-Z wall discharges exhibit a more modest density dependence. High-Z wall data also exhibit power degradations less severe than IPB98y,2, although on average IPB98y,2 slightly overpredicts their confinement by 5 to 10%. Regression analysis of the collection of this standard set confirm the more modest density and power degradation dependences. Results of this analysis, along with those of subsets of the data that reside close to the ITER operating range in q95, a/R, elongation, and Zeff will be presented.
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TP11.00105: Deuterium irradiation studies of lithium and boron coatings on graphite samples from NSTX-U Hanna Schamis, Heather Sandefur, Jean-Paul Allain, Felipe Bedoya, Robert Kaita During the 2015-2016 NSTX-U experimental campaign, a plasma facing component (PFC) diagnostic, the Material Analysis and Particle Probe (MAPP), was installed. MAPP has the capability studying materials without exposing them to atmospheric conditions. The first MAPP results have shown what occurs to boron layers on graphite and molybdenum on a day-to-day basis. The next NSTX-U campaign will continue to feature a full graphite wall, and both boron and lithium will be used as PFC conditioning techniques. While the surface dynamics of Li-C-O complexes are fairly well understood, the dynamics of Li-B-C-O surfaces are not. Experiments were conducted at the IGNIS facility at UIUC. Cored graphite tiles from the last campaign were used. These tiles contain on average 15-30% B, according to XPS analysis. The samples were dosed with Li and then irradiated with D. XPS was performed on the samples for various steps of deposition and irradiation. In addition to providing insight into the surface processes of deuterium retention, offline experiments of these systems will provide a baseline for MAPP measurements during the next NSTX-U campaign. |
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TP11.00106: Establishing Low-Field-Side to High-Field-Side Local Helicity Injection Startup Scenarios A. T. Rhodes, G. M. Bodner, M. W. Bongard, R. J. Fonck, C. Pierren, J. A. Reusch, N. J. Richner, C. Rodriguez Sanchez, C. E. Schaefer, J. D. Weberski Local Helicity Injection (LHI) is a non-solenoidal startup technique that utilizes electron current injectors to initiate a tokamak-like plasma. LHI startup in Pegasus employs a low-field-side (LFS) injector set on the outboard midplane, and/or a high-field side (HFS) injector set in the lower divertor region. HFS injection is of interest due to the dominance of helicity drive in sustaining Ip. This drive term increases with decreased injector radius (Rinj). To evaluate scalability of LHI to larger machines, operation at full field (BT,0 = 0.15 T) is necessary. Previous HFS-only operation at full BT (Rinj = 27 cm) was restricted due to stream pitch angle constraints impeding relaxation. Additionally, increased susceptibility to cathode spots that markedly reduce LHI drive was observed. These issues are mitigated by first using the LFS injectors to initialize the plasma and then handing off to the HFS system for Ip growth and sustainment. Ip ~ 0.2 MA is achieved at full BT with this new scenario. Thomson scattering measurements in these plasmas show centrally-peaked pressure, with Te ~ 125 eV and ne ~ 1×1019 m-3. This LFS to HFS handoff scenario enables HFS injection at lower Rinj, and thus increased current drive potential. |
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TP11.00107: Advancing US Non-Solenoidal Tokamak Startup Studies with a Proposed Upgraded Pegasus Experiment M. W. Bongard, G. M. Bodner, R. J. Fonck, C. Pierren, J. A. Reusch, A. T. Rhodes, N. J. Richner, C. Rodriguez Sanchez, C. E. Schaefer, J. D. Weberski Developing attractive means of initiating plasma current without using magnetic induction from a central solenoid benefits both the ST and AT concepts. Research on the Pegasus ST is developing the physics and technology basis for non-solenoidal startup using Local Helicity Injection (LHI). LHI employs strong edge current sources that can produce high-Ip tokamak plasmas (0.2 MA to date). An expansion of the non-solenoidal research on Pegasus to compare most startup candidates in a single experiment will provide guidance on scalable methods for future, larger experiments. These include: DC helicity injection, with LHI, sustained and transient Coaxial Helicity Injection (CHI); RF electron heating/current drive; and poloidal field induction. Future additions may include removable iron core induction and/or neutral beam current drive. Plans and status of the physics mission, facility, and diagnostic upgrades are reported, including: a new, solenoid-free centerstack, increasing BT 4× to 0.6 T; new divertor coils; improved magnet power systems; LHI injectors with advanced geometry and active control of the helicity input rate; a CHI system; EBW auxiliary heating; diagnostic neutral beam and VUV spectroscopy; and radiation diagnostics. |
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TP11.00108: Development of Fully Digital Control of Pegasus Power Systems C. Pierren, M. W. Bongard, R. J. Fonck, B. T. Lewicki Non-solenoidal startup research on Pegasus relies on programmable control of a ~250 MVA modular power supply system. Digital control systems using real-time and Field Programmable Gate Array (FPGA) technology are in development to support power supply upgrades. These will replace analog PWM feedback controllers and the optical interface between individual power supply switches and the control system. Digital systems will allow for reconfigurable control algorithms, are easily expandable with off-the-shelf hardware, strongly reduce susceptibility to EMI, and eliminate analog drift. Added protections provided by FPGA control will restore Ohmic operations, allowing for experiments to study the Taylor limit and coupling efficiency of Local Helicity Injection (LHI) plasmas. The FPGA-based systems will enable expansion of the PF and TF coil sets. The PF expansion will improve shape and position control. Increased TF is necessary for future non-solenoidal startup experiments that examine: higher-power LHI; EBW heating and current drive; and coaxial helicity injection. Digital control will also drive a new 32 MVA Cuk topology LHI power supply to enable time-varying LHI voltage control and improve energy storage utilization. |
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TP11.00109: Impurity Characterization in LHI-Driven Discharges on the Pegasus Spherical Tokamak C. Rodriguez Sanchez, G. M. Bodner, M. W. Bongard, R. J. Fonck, J. A. Reusch, A. T. Rhodes, J. D. Weberski Small, high power current sources are employed in the Pegasus ST to inject helicity at the plasma edge and create high Ip (~0.2 MA) tokamak plasmas without a central solenoid. The characterization of plasma impurities and radiated power losses is particularly important in these Local Helicity Injection (LHI) discharges because helicity input is balanced by the resistive dissipation, and the current injectors lie in the scrapeoff layer ~1 cm from the plasma boundary. Three diagnostics are being deployed for use in Pegasus. Two 16-channel AXUV photodiodes estimate the radiated power across the midplane, spanning the whole plasma volume. Visible Bremsstrahlung (VB) spectroscopy and Thomson scattering profiles are used to obtain <Zeff>. Impurity species are identified by a SPRED VUV spectrometer with a temporal resolution of ~0.75 ms. Low-Z impurities are observed to dominate the spectrum when the injector arc plasma sources are turned on. When helicity injection is terminated, initial VB estimates find <Zeff> ˂ 2.5, with O and N the dominant impurity species. Planned upgrades to the diagnostics include: a new thermistor bolometer array; an imaging VB array with higher spatial resolution; and a high-resolution grating for SPRED. |
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TP11.00110: Investigating the Role of High-Frequency Magnetic Activity in Local Helicity Injection Dynamics N. J. Richner, M. W. Bongard, R. J. Fonck, J. A. Reusch, C. E. Schaefer Local Helicity Injection (LHI) uses biased plasma arc sources at the plasma edge for non-solenoidal tokamak startup. Understanding the magnetic activity present in LHI and its scaling could prove crucial for applying this technique to future devices. Internal magnetic measurements on the Pegasus ST show three main features are present in LHI: a ~20–40 kHz peak from n = 1 line-tied kink motion of the injector current streams; an intermediate region near 0.6 MHz with higher fluctuation power; and broadband turbulence for f < 3 MHz. A novel LHI regime is found at low BT ≤ 0.075 T where the n = 1 activity is suppressed, power at frequencies f > 0.1 MHz increases, and current drive efficiency is improved. This suggests that high-frequency activity could play a critical role in the current drive process. To investigate this, experiments to characterize and identify the observed activity are underway. Discharges with only the LHI current streams isolate the ~0.6 MHz feature to the injector arc and show sensitivities to injector voltage and/or current and magnetic field strength, suggestive of arc and/or kinetic instabilities. Experiments to determine the characteristic length and time scales of the broadband turbulence are underway. |
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TP11.00111: Predictive Modelling and Helicity Dissipation Scaling Studies for Local Helicity Injection Non-Solenoidal ST Startup J. D. Weberski, G. M. Bodner, M. W. Bongard, R. J. Fonck, J. A. Reusch A 0D power balance model is being tested on the Pegasus ST to develop predictive capability, interpret experiments, and inform future system design for Local Helicity Injection (LHI). The model calculates Ip(t) by balancing LHI effective drive (VLHI), helicity dissipation, and inductive effects while enforcing the Taylor relaxation current limit. Experimentally constrained drive inputs (plasma geometry, ℓi, βp, injector parameters) allow for prediction of upper bounds on Ip. Namely, predictive modeling suggests nonlinear increases in achievable Ip are possible by higher BT and/or Iinj to increase the early-phase Taylor limit. This motivates a new injector design and facility enhancements to further test LHI scalability. However, proper treatment of the helicity dissipation term is still a model uncertainty. Thus far, helicity dissipation has been attributed to neoclassical resistivity. This has been challenged by experiments showing Ip scales linearly with VLHI while Thomson scattering indicates a variety of Te profiles (from hollow to peaked, 40<Te,0 <150 eV) depending on BT, ne, and injector parameters. Systematic scaling studies of Te with discharge parameters are underway to resolve this model uncertainty. |
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TP11.00112: Characterization of Low-Frequency MHD Activity in Local Helicity Injection C. E. Schaefer, M. W. Bongard, R. J. Fonck, J. A. Reusch, N. J. Richner Strong, low-frequency (~20–40 kHz) n = 1 activity is generally observed on the low field side (LFS) during Local Helicity Injection (LHI) non-solenoidal tokamak startup. NIMROD simulations of LHI suggest this n = 1 mode is associated with reconnection of the edge current streams via island coalescence instabilities. Prior work has characterized the LHI n = 1 mode in Pegasus as a singly line-tied kink instability of the injected current streams. A new operational regime was found at low BT above a threshold Ip where this n = 1 mode is abruptly stabilized, leading to improved performance. However, access to this regime is restricted at higher BT with Ip,crit ∝ Ip/ITF ~ 1, contrary to expectations from kink stability theory. A new 3D Hall sensor probe is used to measure internal B(R,t) on equilibrium timescales (< 50 kHz). Initial measurements during LHI indicate the probe can directly locate the plasma boundary and, in conjunction with external magnetics, give insight into the spatial structure of LFS MHD. Planned experiments will measure the n = 1 activity and edge B(R,t) using the 3D probe in discharges that feature the MHD transition. |
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TP11.00113: Microtearing Instabilities, ∇B Reversal, and Magnetic Drifts in the Pegasus Local Minimum |B| Regime D. R. Smith, M. W. Bongard, R. J. Fonck, G. R. McKee, M. J. Pueschel, J. A. Reusch, P. W. Terry, Z. R. Williams A local minimum |B| “magnetic well” region is readily accessed in high-𝛽 plasmas driven by local helicity injection in the A ~ 1 Pegasus ST. This magnetic topology may afford novel, favorable characteristics affecting turbulent transport. ∇B reversal on the low-field-side is stabilizing for drift waves, reduces the trapped particle fraction, and expands the parameter space for fast ion trapping. The magnetic configuration, however, remains net-paramagnetic with near omnigeneity (|B| ≈ |B|(ψ)) in the bad curvature region. Small banana orbit widths in an omnigeneous region reduce neoclassical transport. Here, we report on the gyrokinetic stability of microtearing modes in the Pegasus minimum |B| regime. Multiple classes of microtearing instabilities arise at ky𝜌s ~ 0.1-1 in the magnetic well region at ψN ~ 0.3-0.9 on the outboard midplane. Modes at ky𝜌s ~ 0.2 are insensitive to the ∇p component in magnetic drifts which is common in lower 𝛽 scenarios. Modes at ky𝜌s ~ 0.8, however, are highly sensitive to the ∇p component. Also, exclusion of the ion species is destabilizing for ky𝜌s ~ 0.8 modes in contrast to typical microtearing modes. Preliminary nonlinear simulations will also be presented. |
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TP11.00114: Impurity concentrations and transport in LTX plasmas fully surrounded by liquified lithium surfaces Dennis P Boyle, Ronald E Bell, Paul E Hughes, Matthew J Lucia, Dick Majeski, E. Merino, John C Schmitt, Filippo Scotti, Christopher Hansen, R. Kaita, Shigeyuki Kubota, Theodore Mathias Biewer, Drew B Elliott, Travis Gray The first successful operation of a tokamak almost fully surrounded by liquified lithium surfaces was achieved in the Lithium Tokamak Experiment (LTX), prior to its upgrade to LTX-β. While early attempts at operating with lithium coatings above the melting temperature suffered poor performance due to excessive impurities, improved techniques for lithium evaporation and wall/vacuum-conditioning allowed for operation at 260 ℃. Here we present new analysis of lithium, carbon, and oxygen impurity profiles in the experiments with liquified lithium coatings, and compare them to previous measurements with solid coatings. Preliminary analysis shows similar, but modestly higher impurity concentrations with liquified Li. Enhanced diagnostics in LTX-β, including improved spectroscopy and Thomson scattering systems, will enable detailed measurements in a wider parameter space of plasma and surface conditions. Analysis and comparison of impurity profiles and transport will be presented. |
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TP11.00115: First Operation of LTX-β R. Majeski, R. E. Bell, D. P. Boyle, P. E. Hughes, T. Kozub, E. Merino, X. Zhang, T. Biewer, J. M. Canik, D. B. Elliott, M. L. Reinke, C. Hansen, T. Jarboe, S. Kubota, T. Rhodes, P. Beiersdorfer, F. Scotti, V. Soukhanovskii, B. E. Koel, D. Donovan, R. Kaita, A. Maan, J. K. Anderson, J. Goetz, L. Zakharov LTX-β, the upgrade to the Lithium Tokamak Experiment, approximately doubles the maximum toroidal field (to 3.4 kG), plasma current (to 150 – 175 kA), and discharge duration (to ~100 msec) while retaining the same plasma geometry, and the heated high-Z liner featured in LTX. Neutral beam injection (NBI) at 20 kV, 35 A provides auxiliary heating, core fueling, and momentum injection, with an injector provided by Tri-Alpha Energy. New lithium evaporation sources allow between-shots recoating of the walls. Upgrades to the diagnostic set strengthen the research program in the critical areas of equilibrium, core transport, scrape-off layer physics, and plasma-material interactions. Here we will discuss first results from LTX-β, as well as the research goals. |
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TP11.00116: Data acquisition and control systems for the Lithium Tokamak eXperiment-Beta (LTX-β) Enrique Merino, Tom Kozub, Dick Majeski, Dennis P. Boyle, Paul E Hughes, Greg Tchilinguirian, Gretchen Zimmer, Drew B Elliott, Anurag Maan The recent upgrade of the Lithium Tokamak eXperiment (LTX), dubbed LTX-β, has entailed the development of new control systems for the safe and efficient operation of the experiment and its diagnostics, as well as the development and upgrade of many data acquisition systems. Among these, the development of a new control logic for the vacuum systems, the programming of a new user interface to control the operation of the experiment, the setup of an enterprise level data server with an improved experimental database structure (MDSplus), addition of a neutral beam and its control system, upgrade to the ohmic heating system, addition of a toroidal field power supply and surface temperature diagnostics. An overview of these and other improvements on LTX-β will be presented. |
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TP11.00117: Initial Results from the LTX-β Magnetics Upgrade Paul E. Hughes, Christopher Hansen, Dennis Patrick Boyle, Dick Majeski The newly upgraded Lithium Tokamak eXperiment—Beta (LTX-β) has begun plasma operation to study the low-recycling, high-performance regime previously observed in LTX [D.P. Boyle et al. PRL July 2017]. The addition of neutral beam core-fueling and heating for longer-lasting low-recycling plasmas with more steady-state plasma parameters is expected also to increase plasma β and MHD instability drive. We describe the first measurements of the new toroidal array (TA), shell eddy sensor array (SESA), and upgraded re-entrant poloidal array (REPA) of Mirnov coils, and summarize the previously existing magnetic diagnostics such as poloidal flux loops and Rogowski coils. The TA and REPA are analyzed for preliminary signs of n ≤ 5 MHD activity, and the SESA is studied for comparison to the eigenmode modeling of predicted shell eddy currents. Initial reconstructions in the PSI-Tri equilibrium code [C. Hansen et al. PoP Apr. 2017], employing the full upgraded magnetic diagnostics suite, are compared against prior LTX reconstructions. |
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TP11.00118: Charge exchange spectroscopy on the Lithium Tokamak eXperiment-β: calibration and initial results Drew Elliott, Theodore Mathias Biewer, Ronald E Bell, Dennis Patrick Boyle, Dick Majeski The Lithium Tokamak eXperiment has recently undergone an upgrade to LTX-β which includes increased field strength and the addition of a neutral beam. Neutral beam injection will allow for active charge exchange spectroscopy for measuring ion temperature, impurity density, and plasma rotation. For this reason, a new light collection system was designed, fabricated, installed, and calibrated on and in LTX-β. Spatial calibration ensured the tangency radii of the views pointing towards the beam matched those pointing away from the beam, for background subtraction and shot-to-shot spectral calibration. The new system has tangency radii which extend from outside of the last closed flux surface to within the magnetic axis (25-59 cm) during a majority of plasma conditions. For spectral and throughput calibration, an in-vessel, positionable integrating sphere was built and utilized, which was then calibrated to a standard light source. Prior to beam operation, the system will be used for passive spectroscopy of impurity species. The Thomson scattering system has also been upgraded which will increase the reliability of the charge exchange measurements. |
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TP11.00119: Hardware Upgrades for Microwave/Millimeter-Wave Diagnostics on LTX-β Shigeyuki Kubota, Richard Majeski, Roman Lantsov, Xuan V. Nguyen, William A. Peebles, Terry L Rhodes, Robert Kaita Fluctuation measurements and their relation to transport will be of key interest in the LTX-β device, which will have higher BT and IP, and neutral beam heating. The microwave/millimeter-wave diagnostics on the device have been upgraded to enhance fluctuation measurements on the new machine. Faster data acquisition hardware with additional channels for the FM-CW (frequency-modulated continuous-wave) reflectometer (13.5-33 GHz) will increase the repetition rates to 200 kHz. New data acquisition hardware and a new detector for the 288 GHz interferometer will increase the S/N ratio as well as allow higher intermediate frequencies for robust tracking of the line density. New diagnostics include the two-channel tunable fixed-frequency quadrature reflectometer (13.5-20.5 and 27-40 GHz) and far-forward scattering (FFS) using the interferometer hardware. Optimization for FFS will require modifications to the interferometer beam transmission line. Future upgrade plans, as well as detailed descriptions and experimental data from the above diagnostics, will be presented. |
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TP11.00120: Preparations for liquid tin plasma-surface interaction and transport experiments in LTX-beta Vlad Soukhanovskii, Peter Beiersdorfer, Mark Joseph May, Howard A Scott, Filippo Scotti, Dennis Patrick Boyle, Dick Majeski, Stephanie Hansen Liquid tin is considered a candidate for the divertor and first wall of fusion plasma devices, thanks to its low melting temperature and low evaporation rate. Experiments with a liquid tin probe are planned in the small spherical tokamak LTX-beta with expected central electron temperatures up to 200-300 eV with auxiliary NBI heating. Experiments are aimed at 1) Assessing physical sputtering and evaporative fluxes using Sn I - Sn III ultraviolet and visible emission line intensities, and 2) Assessing core impurity transport with well characterized tin source using Sn V – Sn XX spectra from an extreme ultraviolet spectrometer. Spectra modeling is based on the new atomic data generated by HULLAC and FAC atomic codes. Impurity transport code STRAHL with new ionization and recombination rates generated by FLYCHK code is used to look for significant spectral emission differences in plasma scenarios with LTX-like transport, flat and peaked kinetic plasma profiles and various fractions of lithium and tin impurities. |
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TP11.00121: Initial LTX-β Plasma Facing Component and Scrape-Off Layer Characterization A Maan, R Kaita, D B Elliott, D P Boyle, R Majeski, D C Donovan, R A Ellis, B E Koel, T M Biewer, X Zhang Lithium coatings on high-Z PFCs in the Lithium Tokamak eXperiment (LTX) led to flat temperature profiles. The flat temperature profiles were observed along with a broad collisionless Scrape-Off Layer (SOL). Additionally, an in-vacuo X-Ray Photoelectron Spectroscopy (XPS) system established that evaporatively deposited lithium coatings oxidized, while retaining the ability to achieve flat temperature profiles. LTX continued to observe performance comparable to freshly deposited lithium coatings with partially oxidized lithium plasma facing components (PFCs). Theory attributes flat temperature profiles to low recycling walls. The presence of oxidized lithium, however, raises questions regarding the mechanism of hydrogen retention in LTX. To investigate these questions, LTX-β upgrade will be equipped with a Sample Exposure Probe (SEP) for in-vacuo XPS analysis with higher resolution, along with new SOL particle density and energy diagnostics. We will present on the status of development of these diagnostics, and include some preliminary results characterizing the SOL and the LTX-β PFCs using them. |
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TP11.00122: A Theoretical Study of Electrostatic Potential in the SOL plasma and a Computational Model for NBI in LTX-β Xin Zhang, Leonid Zakharov, Richard Majeski The Lithium Tokamak eXperiment (LTX) is a spherical tokamak device designed to study the effects of low recycling lithium plasma PFCs on tokamak confinement & equilibria. The lithium coated wall of LTX has been demonstrated to allow for a low density, high temperature, and hence low collisionality plasma edge. The low collisionality region extends into the scrape-off layer (SOL). With a high mirror ratio near the LCFS, the majority of particles in the SOL will be mirror-trapped, and will modify the physics of the SOL plasma. Here we present a theoretical study of the formation of ambipolar potential in the collisionless SOL via differential loss of the electrons and ions. Progress towards an analytical model and preliminary numerical results will be presented. The recent upgrade to LTX-β includes a 17 keV NBI system, which provides further fueling and heating of the plasma. A 3D computational model is implemented to study particle deposition and first orbit losses. Beam-particle penetration is simulated via a Monte Carlo model, with hot ion trajectories integrated with both full orbit and guiding center equations. |
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TP11.00123: Experimental studies of plasmoid reconnection by using CHI start-up plasma on HIST Masayoshi Nagata, A. Fujita, Y. Ibaragi, Y. Kikuchi, N. Fukumoto Plasmoid reconnection for the flux closure has been for the first time demonstrated by transient-coaxial helicity injection (T-CHI) in the Helicity Injected Spherical Torus (HIST) device. The intensive measurement of internal magnetic structures indicates that two or three plasmoids are generated after the tearing instability of an elongated Sweet-Parker current sheet during the T-CHI. Here, we report that in the T-CHI start-up plasmas (H, D and He), (i) regular oscillations of reconnecting magnetic field, electron density and ion flow observed in the T-CHI process indicates repetitive generation of small-size plasmoids due to the magnetic reconnection, (ii) one of reconnecting plasmoids grows up a large-size plasmoid, results in the formation of doublet-type ST configurations. Consequently, the plasmoid reconnection could be the leading mechanism for the formation of multiple X-point, i.e., the fast flux closure in the T-CHI discharge. |
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TP11.00124: Development of a microwave polarimeter for the measurement of the lower-hybrid driven current profile on the TST-2 spherical tokamak Naoto Tsujii, Yuichi Takase, Akira Ejiri, Osamu Watanabe, Satoru Yajima, Yusuke Yoshida, Hibiki Yamazaki, Yusuke Iida, Kotaro Iwasaki, Yuya Kawamata, Sho Sakamoto Non-inductive plasma startup is considered to be essential for a spherical tokamak fusion reactor. On the TST-2 spherical tokamak at the University of Tokyo, plasma current ramp-up up to a quarter of the Ohmically driven plasma current has been achieved using only the lower-hybrid current drive. Equilibrium analysis suggests a formation of a very broad, possibly hollow, current profile, but the analysis has a large uncertainty due to lack of internal magnetic field measurements. A microwave polarimeter is being developed to constrain the equilibrium reconstruction and determine the current profile experimentally. Due to the relatively low plasma current, the Cotton-Mouton effect is significant and needs to be taken into account in the fitting process. Current profile estimation of a fully non-inductive lower-hybrid driven discharge will be presented if available. |
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TP11.00125: A High Frequency Ćuk Converter for Fusion Science Applications (Phase I) Alex Henson, Timothy Ziemba, Kenneth E Miller, James R Prager The Pegasus Toroidal Experiment at the University of Wisconsin will upgrade their solid-state switching power systems for Local Helicity Injection (LHI) and for magnet driving/control. A new modular system that addresses the output ripple, efficient capacitor bank utilization, and electromagnetic interference is required. Eagle Harbor Technologies (EHT), Inc. is developing a Ćuk converter for fusion science applications. The Ćuk converter has low output ripple; high efficiency; voltage gain greater than one, allowing for deeper energy storage utilization; continuous power flow that lowers output EMI reducing noise generation; continuous input and output current – energy flow from the series capacitor allows for greater control of the injector currents; series arrangements can be utilized that isolates individual switch modules so a failure does not potentially damage all solid-state switches. EHT will utilize previously developed precision gate drive technology that allows for high frequency switching, which reduces the capacitor and inductor values significantly, making the design more compact and lower cost. We will present the Phase I project plan and overview of progress to date. |
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TP11.00126: Progress in coupling a transport solver to global gyrokinetic simulations Jeffrey Parker, Lynda L LoDestro, Lee Ricketson, Alejandro Campos, Jeffrey A Hittinger, Daniel Told, Gabriele Merlo, Frank Jenko Predictive modeling with transport solvers has tended to use surrogate models, such as quasilinear transport models, to represent turbulent fluxes. One route to improved predictive modeling is instead to couple with direct numerical simulations (DNS) of gyrokinetic turbulence. We have coupled the transport code Tango to the gyrokinetic code GENE. The first results used global gyrokinetic simulations with adiabatic electrons in a circular magnetic geometry, and the transport solver evolved ion pressure. Here, we report on progress in using a DIII-D-like geometry. We also report on progress using kinetic electrons and evolving ion pressure, electron pressure, and density. An additional complication of coupling a transport solver to turbulence simulations rather than a surrogate model is that the fluctuations hinder convergence, and we analyze this issue. |
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TP11.00127: Performance Optimization of the XGC code on KNL architecture Brian MacKie-Mason, Paulius Velesko, Robert Hager, C-S Chang, Timothy J. Williams Gyrokinetic particle-in-cell (PIC) simulations have played an important role to the fundamental understanding of plasma edge turbulence, which is one of the barriers to a successful tokamak operation. In order to fully resolve the Vlasov transport equation the ion and electron species must be treated separately due to their different velocities. The electron push for kinetic electrons occurs many times per ion time step, becoming a dominant consumer of computer time. This makes the electron push a prime candidate for vectorization using modern computer architectures, such as Intel Knights Landing architecture used in many high performance computing settings. Such improvements include the proper selection of Array of Structures or Structure of Arrays datatypes, the optimal combination of nested openMP and vector processing, and the optimization of the memory access pattern within the electron push kernel. Results demonstrating the performance improvement of the electron push kernel on such architectures will be presented. |
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TP11.00128: Recent Progress in EM-GTS code Edward Startsev, Weixing Wang, Peter Porazik, Wei-li Lee In this presentation I will give the overview of the recent progress in the development of the electromagnetic capabilities in the gyro-kinetic PIC code GTS. I will describe the Startsev-Lee EM scheme that is capable of simulating the micro-tearing (MTM) modes and which recently has been extended to the general tokamak geometry and implemented into GTS. Initial attempts to simulate the MTM modes in the core of the NSTX will be presented. We have also implemented the modified Mishichenko’s general geometry EM scheme into GTS for cross-scheme-verification. The EM-GTS is currently being used to study the electromagnetic modes in the pedestal region of the DIIID. The benchmarking of the code in the Cyclone and pedestal DIIID geometry against published results is ongoing to better understand the resolution requirements of the implemented EM schemes. |
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TP11.00129: A Multi-Scale Time Integration Method for Kinetic Simulations Benjamin Sturdevant, Scott Edward Parker, Robert Hager, C-S Chang, Julien Dominski, Seung-Hoe Ku We report progress developing a kinetic multi-scale time integration method based on equation-free projective integration [1]. Here, a fully resolved kinetic simulation is performed over a short time interval to produce a history of fluid moments. The moments are then extrapolated over a large time step and used in initializing a subsequent kinetic simulation to repeat the process. This enables long timescale simulations without the need for coupling to transport equations. A method for “lifting” fluid moments to a distribution function has been developed based on transforming a previous time step distribution function using polynomials in the velocity space variables. This method is implemented in XGCa and is demonstrated to eliminate spurious transients, which were present for previous lifting operators. When the fluid moments are extrapolated over too large of a time step however, inaccuracies may excite fast time scale modes. This imposes a constraint on the extrapolation time step size, limiting the computational gains that can be achieved. We explore advanced extrapolation methods and constraints on the fluid moments to mitigate this effect. [1] I. G. Kevrekidis et. al., Comm Math Sci, 1(4), 715, (2003). |
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TP11.00130: Electromagnetic Field Solver for Fully Kinetic Simulation of Ion-Temperature-Gradient Instabilities in Tokamaks Youjun Hu, Matthew T Miecnikowski, Yang Chen, Scott Edward Parker The feasibility of using full ion kinetics, rather than gyrokinetics, in simulating toroidal Ion-Temperature-Gradient (ITG) instabilities has recently been demonstrated in the electrostatic adiabatic electron case. The present work extends this model to the fully electromagnetic case with drift-kinetic electrons. This model provides an important validation tool for gyrokinetics in applications where higher order terms may be important. We have developed an electromagnetic field solver for this model. The field equations are discretized in the field-line-following coordinates. The solver uses the Fourier spectral method in the toroidal direction and finite differencing in both the radial and field-line following directions. The matrix resulting from the toroidal and radial discretization is directly inverted whereas the variation along the field-line direction is solved by an iterative method. The plasma response enters the field equation via the ion and electron currents and these are calculated by an implicit delta-f method. Numerical results of ITG instabilities obtained from this model will be presented and compared with the gyrokinetic results for the Cyclone base case. |
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TP11.00131: A quasineutral full-kinetic-ion drift-fluid-electron model forsimulation of low-frequency plasma turbulence Matthew T Miecnikowski, Scott Edward Parker, Yang Chen Fully kinetic ion models have recently garnered attention as a means to verify and extend the ubiquitous gyrokinetic models applied to studies of low-frequency plasma turbulence in tokamaks. Previous studies have had success in reproducing gyrokinetic results, particularly the growth and saturation of the ion-temperature-gradient instability, but a robust alternative to gyrokinetics in this context remains elusive due to numerical challenges associated with the quasineutral limit and the inherent multiscale nature of the problem. We present a new approach, in which the assumption of quasineutrality, along with the first moments of the kinetic equation for ions and the drift-kinetic equation for electrons, provides a soluble field equation without undesirable high-frequency solutions. As a proof of concept, we present linear and nonlinear results under the electrostatic approximation with the assumption of isothermal electrons. We favorably compare the measured linear dispersion to the full-kinetic-ion drift-kinetic electron theory and study nonlinear couplings to ion Bernstein waves and electrostatic ion cyclotron waves. |
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TP11.00132: Core Infrastructure Upgrades to the NIMROD Codebase Brian S Cornille, Jacob R King, Eric C Howell, Carl R Sovinec In order to increase functionality of the NIMROD code, we are refactoring data structures and interpolation routines using object-oriented features introduced in Fortran 2003/2008. This change is motivated by physics-oriented objectives. First, the element representation needs another dimension for the continuum kinetic model to take advantage of sparsity in the pitch-angle coordinate. Second, we are interested in flexibility with regard to the mesh representation and the finite-element function spaces. We have created new abstract interfaces for the concepts of mesh block, finite-element field, and for the linear algebra data structures. Currently, mesh blocks are in practice limited to regions of structured quadrilaterals. Greater use of object-oriented programming will facilitate implementation of unstructured triangle and quadrilateral meshes. We currently represent our quadrilateral finite element fields as C0 functions. In order to investigate novel methods for solving Hall-MHD, we are also interested in being able to approximate other function spaces such as H(curl). Here we present the current status of these updates and provide example code changes. |
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TP11.00133: A comparison of NIMROD's continuum and delta-f PIC approaches to energetic particle physics Trevor Taylor, J. Andrew Spencer, Scott E Kruger, Eric Held Small number of energetic ions can have effect on long-wavelength modes in magnetized fusion [1]. PIC models have been used to model interaction in the context of initial value codes. The extended MHD code NIMROD has both continuum and delta-f PIC [2] drift kinetic (DK) capabilities that incorporate the energetic particle (EP), anisotropic stress tensor into its flow evolution equation. Two advantages of the continuum EP implementation are 1) the advance of the DK and flow evolution equations can be made fully implicit using Picard iterations within a single time step, and 2) a finite-element approach in pitch-angle resolves trapped/passing physics in velocity space. The continuum and delta-f methods are compared using model of giant sawtooth cycles in DIII-D shot \#96043. In this discharge, energetic ions are driven by RF and modifies the sawtooth stability. We present results from linear calculations that compare the accuracy and efficiency of the continuum and delta-f PIC methods using the slowing-down part of the EPs. [1] W. Park, S. Parker, et al., Physics of Fluids B: Plasma Physics 4, 2033 (1992). [2] C. C. Kim, C. R. Sovinec, et.al., Comp. Physics Communications 164, 448 (2004).
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TP11.00134: Parallel closures and transport for toroidal plasmas in the collisionless limit Jeong-Young Ji, J. Andrew Spencer, Eric Held Although the general moment method [1] provides an accurate description of Coulomb collision effects in solving the kinetic equation, it requires a large number of moments to obtain convergent solutions at low collisionality. Therefore, we directly solve a reduced drift kinetic equation with a Krook-type model collision operator to obtain parallel closures in the collisionless limit. The distribution function is expressed by velocity-dependent kernel weighted integrals of closure drives. Taking closure moments and solving a coupled system for Fourier coefficients, we express the closure coefficients in terms of drive coefficients. Using the closures we solve the momentum balance equation to calculate the parallel flows and the bootstrap current. The formalism is valid for arbitrary aspect ratios of toroidal geometry. |
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TP11.00135: Continuum drift kinetics applied to parallel heat transport J. Andrew Spencer, Eric Held, Brett Adair, Jeong-Young Ji The Chapman-Enskog like electron drift kinetic equation1 provides kinetic closure of fluid equations and extends to the long mean free path regime of magnetized plasmas. In this work we discuss the application of a continuum numerical solution to this equation to provide closure for parallel heat flux in NIMROD. Accuracy is improved by expressing the equation in velocity coordinates using pitch-angle and speed normalized by the thermal speed. This leads to a tight coupling of temperature, T, to kinetic distortion, F, and demands a careful semi-implicit time advance for large time steps. Results are obtained from two integration schemes applied to a simultaneous advance of T and F: 1) Picard iteration, and 2) Newton's method. We compare the computational efficiency of both approaches. Additional parallelism was recently developed parallelizing the preconditioning step in the linear solver over speed collocation points in the velocity domain. We present the parallel scaling performance of this development. Using NIMROD we explore the effects of particle trapping on thermal transport in toroidal geometry in the presence of magnetic islands.
1J. J. Ramos, Phys Plasmas 17, 082502 (2010). |
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TP11.00136: An implicit, scalable, relativistic nonlinear Fokker-Planck solver for runaway electrons Don Daniel, William Taitano, Luis Chacon, Eero Hirvijoki, Zehua Guo, Christopher Joseph McDevitt, Xianzhu Tang On the application of a sufficiently strong electric field, electrons break away from thermal equilibrium and approach relativistic speeds. These highly energetic `runaway' electrons (∼MeV) play a crucial role in understanding tokamak disruption events, and therefore their accurate simulation is essential to develop reliable mitigation technologies. For this purpose, we have developed a fully implicit, scalable relativistic Fokker-Planck kinetic electron solver. Energy and momentum conservation is ensured for the electron-electron relativistic collisional interactions. Electron-ion interactions are modeled using the Lorentz operator, and synchrotron damping using the Abraham-Lorentz-Dirac reaction term. We use positivity preserving finite-difference schemes for both advection1 and tensor diffusion2 terms. This numerical treatment, combined with suitable preconditioning and multigrid strategies, allows us to accurately investigate phenomena that span a wide range of temporal scales. We demonstrate the scheme with numerical results ranging from small electric field electrical conductivity measurements, to accurate reproduction of runaway tail dynamics when strong electric fields are applied. 1) P Gaskell, A Lau Int J Num Meth Fluids 1988 2) W Hundsdorfer et al. J Comp Phys 1995 |
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TP11.00137: Effects of Nanoparticle Plasma Jet Injection on Runaway Electron Dynamics in Tokamaks S. A. Galkin, I. N. Bogatu Numerical simulations indicate that runaway electron (RE) current generation can be suppressed by injecting a few km/s velocity nanoparticle plasma jet (NPPJ) from a plasma accelerator [1,2] into tokamak plasma at the beginning of current quench. RE dynamics is based on the model [3], which includes Dreicer and “avalanche” mechanisms and has a strong non-linear dependence on Zeff. The model was extended with self-consistent Zeff calculations using the atomic code FLYCHK [4]. Injection of carbon, boron nitride (BN) or other impurities into plasma can increase Zeff to 4~5 and even higher. Essential influence of larger than one Zeff on the RE dynamics was shown. Penetration of C60 and BN NPPJ through increasing B-field was studied with the HEM-2D code [5], which was equipped with two new models/modules for the fragmentation (C60) and the sublimation (BN). Simulations were made with both DIII-D and ITER parameters. [1-2] I.N. Bogatu, S.A. Galkin, J.S. Kim: [1] J. Fusion Energy 27, 6, 2008, [2] J. Fusion Energy 28, 144, 2009; [3] H. Smith et al., Phys. Plasmas 13, 102502, 2006; [4] H.-K. Chung, et al. High Energy Density Physics 1, 3, (2005); [5] S.A. Galkin, Bull. APS DPP 2008, BO5.005, https://www.researchgate.net/publication/252593039 |
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TP11.00138: Hybridized Discontinuous Galerkin method for extremely anisotropic diffusion problems François Waelbroeck, Ali Samii, Craig Michoski In hot plasma, the ratio of the thermal conductivities parallel and perpendicular to the magnetic field can exceed 10 orders of magnitude. Numerical errors associated with the discretization of the parallel heat flux can cause it to pollute the perpendicular heat flux, leading to the underestimation of the confinement properties of a 3D configuration. A widespread approach to this problem has been to choose a coordinate system such that one of the coordinates is aligned with the flux surfaces. This method is becomes impractical in the presence of even a single island chain. Here, we show that the hybridized discontinuous Galerkin method provides a parallel and adaptive framework for solving extremely anisotropic problems. In particular, we demonstrate that it can model correctly, on a rectangular grid, the temperature flattening in neighboring magnetic islands before the onset of chaos.
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TP11.00139: Machine learning applications for plasma diagnostics Kevin L Tritz Machine learning (ML) has become an increasingly popular tool in the plasma physics community, especially for plasma control applications such as disruption identification. In addition, ML can play an import role in advanced diagnostic analysis. We will present different applications of neural networks (NN) that highlight some of the potential of these ML applications. First, we will demonstrate a technique using Thomson Scattering to train a NN with multi-energy Soft X-ray measurements, along with other diagnostics, to enable fast measurements of the electron temperature profile. Also, we will present a NN based technique to combine slow, yet accurate, foil bolometry measurements of radiated power with faster diode-based measurements to provide both fast and accurate measurements of Prad. These are examples of using machine learning to learn the complex relationship between various diagnostics and the desired measurement value. Finally, we will use NN-based classification to identify impurities using spectroscopic measurements. One weakness of NN-based techniques is the difficulty in applying a trained network to other input systems. We will explore the portability of a trained NN by adding a ‘translation’ layer to the input. |
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TP11.00140: Detector Design and Analysis Technique for Local Electric Field Fluctuation Diagnostic in High Temperature Plasmas Using Spatial Heterodyne Spectroscopy Marcus Galen Burke, Raymond J Fonck, George R McKee, Greg Winz A novel diagnostic for measuring plasma electric field fluctuations is being developed. It employs high-speed measurements of the spectral separation of the Motional Stark Effect (MSE) split neutral beam emission, where fluctuations in the MSE component separation is proportional to local magnetic and electric field fluctuations. A spatial heterodyne spectrometer (SHS) with high etendue (~5 mm2sr) and resolution (~0.14 nm) has been developed to spectrally resolve the Stark multiplet while collecting all available light to minimize photon noise. An analysis technique based on a least-squares fitting to linear perturbations of the SHS interferogram is used to perform sensitivity studies and to guide the design of new volume phase holographic gratings to maximize the sensitivity of the diagnostic. In preliminary studies, the uncertainty in electric field fluctuations scale with the relative photon noise applied to the interferogram. A low readout noise, high sampling speed (1 MHz), high etendue detector system with modest spatial resolution is in development to record the time-varying interferogram. Both a high-speed, low readout noise CMOS sensor and a multianode microchannel plate photomultiplier are under evaluation. |
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TP11.00141: Experimental test of the prototype system of Laser-driven Ion-beam Trace Probe (LITP) Xiaoyi Yang, Chijie Xiao, Tianchao Xu, YiHang Chen, Shuai Huang, Renchuan He, Yi Yu, Min Xu, Long Wang, Chen Lin, Xiaogang Wang The LITP method, first proposed in 2014, is suggested to diagnose both the poloidal magnetic field and radial electric field in tokamaks [1,2]. Recently a prototype system of LITP was setup to validate the reconstructing method in the PKU Plasma Test (PPT) device. The prototype system includes two parts: the laser accelerator, and the scintillator detector. In the experiments, some engineering problems were fixed, such as the docking of laser-accelerator to plasma device, the protecting and shielding of the detector, error analysis, and calibration of the system. This is a key step for LITP to be applied on the tokamak plasma diagnostics. [1] Yang et al. Rev. Sci. Instrum. 85(11), 11E429 (2014). [2] Yang et al. Rev. Sci. Instrum. 87(11), 11D610 (2016). |
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TP11.00142: Local, High Resolution, Electron Density and Magnetic Field Measurements via Doppler-free Saturation Spectroscopy Abdullah Zafar, Elijah Henry Martin, Steve Shannon The extreme environment encountered in fusion relevant devices provide a challenging obstacle for most diagnostics. Traditional approaches are either (1) unable to endure the harsh conditions, (2) are constrained in areas they can access, or (3) suffer from poor measurement resolution. At Oak Ridge National Laboratory, we have developed a diagnostic method that allows us to overcome these limitations. The diagnostic is based on measuring the spectral line profile using Doppler-free saturation spectroscopy (DFSS). DFSS is a laser-based absorption technique that greatly reduces Doppler broadening and essentially eliminates instrument broadening by crossing two counter-propagating beams. This results in high resolution spectroscopic measurements that can be localized along the line of sight. Plasma parameters are then extracted by fitting the measured spectrum to a quantum mechanical model using the Explicit Zeeman Stark Spectral Simulator (EZSSS) code. The effects of Zeeman-splitting and Stark broadening on the spectrum allow for diagnosing the magnetic field and electron density, respectively. DFSS has been successfully employed to diagnose a magnetized (500-800 G), low temperature (~5 eV), low density (1016-1018 m-3), helium plasma. The results and accuracy are presented here. |
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TP11.00143: Additive plasma pushing resulting from the propagation of a magnetosonic soliton in a finite size plasma Renaud Gueroult, Amnon Fruchtman, Nathaniel J Fisch Solitons are solitary localized wave solutions to the Korteweg-de-Vries (KdV) equation which describes waves in weakly dispersive media for which dispersion balances out nonlinear effects. Soliton solutions exist both for ion-acoustic and magnetosonic (MS) waves in homogeneous plasmas. Besides their remarkable stability with respect to interactions, an interesting property of MS solitons is that they lead to plasma displacement. Indeed, owing to the odd longitudinal electric field associated with the pulse, compressive pulses push plasma along the direction of propagation while rarefaction pulses push plasma in the direction opposite to propagation. However, the form self-preserving nature of solitons breaks down in the presence of inhomogeneities. An interesting example illustrated here is the behavior of a MS soliton incident on a plasma-vacuum interface. We show that a compressive MS pulse turns into a rarefaction pulse upon reflection at the interface, and vice-versa. The nature of a MS pulse thus alternates from compressive to rarefactive at each reflection when bouncing in a plasma slab, and the displacement induced by each of the pulse passages adds constructively. This interesting theoretical finding is illustrated and validated using particle-in-cell simulations. |
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TP11.00144: Development of a Ross Filter-Based Aluminum Line Radiation Detector (NICKAL2) for the MST Toroidal Plasma Experiment Nicholas J. Lauersdorf, Lisa M Reusch, Abdulgader F. Almagri, Paolo Franz, John Goetz, Patrick VanMeter, Daniel J Den Hartog Modeling and data from the new NICKAL2 Ross filter-based x-ray diagnostic, installed on the MST, are presented. NICKAL2 has three filtered detectors sensitive to soft x-ray (SXR) radiation. Because MST has an Al plasma-facing surface, Al line radiation is present in the SXR spectrum, complicating the spectrum. The filters are designed to isolate Al line radiation and quantify their absolute intensities. Simulations show that the NICKAL2 passbands are dominanted by this Al line radiation. This results in a strong constraint on Al density at electron temperatures (Te) as low as 250 eV. NICKAL2 will aid the two-color SXR tomography (SXT) diagnostic, which can not distinguish between continuum and line radiation. Line radiation is typically filtered out of the SXT signal using thick Be filters to ensure only continuum radiation contributes, thus restricting the SXT measurement range to Te ≥ 1 keV. Using NICKAL2 to constrain Al line radiation independently permits the use of thinner filters on SXT diagnostic, enabling better inference of Te and extending measurement cababilities to lower Te. NICKAL2 data also provide important information to infer Zeff profiles, particularly when combined with charge exchange recombination spectroscopy data. |
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TP11.00145: Radiated power diagnostic capabilities on MAST Upgrade Jack Lovell, Matthew Reinke, Anthony Field, Matthew Carr, Bruce Lipschultz, Fabio Federici Capabilities to measure radiated power on MAST Upgrade are discussed. A new 32-channel resistive bolometer (RB) system is used in the device’s unique Super-X divertor (SXD) chamber, complemented by the 32-channel system originally used on MAST, viewing the core plasma. Both utilise modern FPGA-based electronics, and the resulting improvements in signal-to-noise ratio and signal processing capabilities are summarized. |
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TP11.00146: Stereoscopic Image Analysis of SOL Filaments in MAST Ryan A Chaban, James R Harrison, Fulvio Militello, Saskia Mordijck, Tom Farley, Nick Walkden We are developing a standardized method to analyze stereoscopic fast camera data from MAST using the ELZAR framework to calculate 3D images of filaments as they cross the separatrix and travel into the scrape-off layer. The MAST fast camera setup uses two cameras adjacent to each other and separated by a baseline of ~3cm to capture the same filaments during a shot. Currently, in the 2D camera data, the analysis assumes that the filament structures are aligned with the magnetic field and relies on the EFIT++ reconstruction of the field to project the filaments into a 3D geometry. This poster concerns the software implementation to take the established ELZAR method of filament identification and quantification, and improve it by cross-correlating the data with the second camera. We will also compare the modified ELZAR measurement with a magnetic field free approach to asses the assumption that filaments follow field lines. Furthermore, we will use synthetic data to assess the validity of the method for current data and future use on MAST-Upgrade. |
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TP11.00147: Development of a passive visible spectroscopic diagnostic on HL-2A Liu Liang A passive visible spectroscopic diagnostic which may measure the profiles of line emission and continuum radiation simultaneously is first installed on HL-2A tokamak. The line-integrated brightness profiles covering the whole plasma cross section are measured with the spatial resolution of 2.7 cm and temporal resolution of 10 ms. Graphite plates to face the objective are installed on the inner vessel wall, which reduces the influence of wall reflection on the optical measurements. A local relatively-low temperature region, located at the plasma edge above the mid-plane and caused by plasma-baffle interaction, is confirmed by this diagnostic. The obvious increase of the line-averaged Zeff during auxiliary heating discharge is mainly due to the inward impurity diffusion. Assumed Zeff with flat profiles in Ohmic and NBI discharges, the line-integrated electron density profiles measured by the visible bremsstrahlung are in good agreement with the ones measured by the FIR interferometer. |
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TP11.00148: Gas Puff Imaging Diagnostic on HL-2A Tokamak Jinbang Yuan, Boda Yuan, Min Xu, Lin Nie, Shaobo Gong A new Gas Puff Imaging (GPI) diagnostic has been developed and put into experimental research on HL-2A tokamak to study plasma turbulence and transport dynamics in the edge and Scrape-Off Layer (SOL). Here we will present some results from the recent 2018 Experimental Campaign. The statistic characteristics of blob w/wo Resonant Magnetic Perturbation (RMP), including burst rate, mean lifetime and velocities in different areas, are compared. During the application of RMP, the shear layer of poloidal velocity (Vθ) was found shifting inwards and the Vθ increasing in SOL and breaking down inside LCFS. Also, Electron Cyclotron Resonance Heating (ECRH) was found having a similar effect on Vθ in SOL and the edge area. Besides, we also observed two different kinds of turbulence with different frequencies moving inwards and outwards the LCFS.
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TP11.00149: Visualization of ion dynamics during gas puffing in the RT-1 magnetospheric plasmas using a coherence imaging spectroscopy Kaori Nakamura, Masaki Nishiura, Zensho Yoshida, Naoki Kenmochi, Shotaro Katsura, John Howard Particle transport in laboratory magnetospherere is studied by using the Ring trap 1 (RT-1) device. The RT-1 imitates a self-organized plasma in a planetary magnetosphere. The plasma achieved the stable confinement of high beta plasma in a dipole magnetic field. A steep peaked electron density profile and a spontaneous toroidal rotation of ions are observed under the self-organization phenomenon. Neutral Helium gas is puffed during 5 ms at t = 0.1 s into a plasma to perturb the electron density. Just after the gas puffing, the recovery of ion temperature and flow velocity is measured by using a coherence imaging spectroscopy to elucidate the ion dynamics in self-organized plasmas. The ion temperature in a poloidal plane with the gas puffing increases at t = 0.2 s, and the increased area forms a belt structure at the outer confinement region. The acceleration and deceleration of the flow velocity is also divided into the inner and the outer areas. Near the boundary area the electron density fluctuation at the frequency of 600 Hz is observed concomitant with a magnetic fluctuation. Based on these results, the self-organization mechanism will be discussed. |
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TP11.00150: Electron Cyclotron Emission Imaging (ECEI) System for the J-TEXT Tokamak Calvin W Domier, Jinhua Cao, Guanying Yu, Yilun Zhu, Xiaoming Pan, Zhoujun Yang, N.C. Luhmann, Jr. A dual array Electron Cyclotron Emission Imaging (ECEI) system is being developed for the J-TEXT tokamak in Wuhan, China. Each 16-element array (expandable to 20 element) provides 2-D 16x8 time-resolved images of Te profiles and fluctuations over extended regions of the J-TEXT plasma. Unique to the J-TEXT ECEI system is an electronics architecture that takes full advantage of the capabilities of PXI multifunction I/O modules which not only provide simultaneous 16-bit 2 MS/s sampling of the ECEI data, but also provide digital I/O signals that are used to remotely control individual RF channel attenuations. These are coupled with Arduino-based controllers which provide remote control of RF channel spacings (selectable between 0.8, 1.0 and 1.3 GHz) and video bandwidths. Details of this ECEI system are presented. |
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TP11.00151: Characterization of plasma temperatures and heat fluxes during HIDRA operation Matthew Parsons, Rabel Rizkallah, Andrew Shone, Nathan Bartlett, Daniel Andruczyk HIDRA is a medium-sized stellarator/tokamak hybrid operated at the University of Illinois at Urbana-Champaign. With a primary research objective of exposing novel plasma-facing component (PFC) technologies to steady-state plasma conditions, this device is typically used in its configuration as an l = 2, m = 5 stellarator. In order to assess the performance of PFCs in HIDRA, it is necessary to characterize the plasma temperature, as well as heat fluxes to limiting surfaces, during normal operation. These measurements are made by employing a set of optical spectrometers and a pair of infrared cameras, which have all been equipped with mountings that allow placement at multiple locations around the machine. Measurements will be presented here of the operating temperatures and heat fluxes achieved in HIDRA with up to 26 kW of microwave heating power. |
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TP11.00152: Advanced System-on-Chip (SoC) Electron Cyclotron Emission Imaging (ECEI) Arrays for DIII-D Yu Ye, Jo-Han Yu, Calvin W Domier, Yilun Zhu, Ahmed Diallo, Yang Ren, Gerrit J Kramer, N C Luhmann The Electron Cyclotron Emission Imaging (ECEI) System on the DIII-D tokamak has been upgraded with a 20-element System-on-Chip (SoC) E-band (72-80 GHz) imaging receiver array for 2-D Te measurements. The SoC technique integrates different functional components together into a single chip, including a low noise preamplifier, a balanced mixer, IF amplifier and a local oscillator multiplier circuit. Mounted into a shielded receiver module with a machined horn antenna, system noise temperatures were reduced from ~55,000 K to ~5,000 K while simultaneously increasing isolation and reducing inter-element crosstalk. This E-band array of SoC-based receiver modules will be joined in late 2018 by a companion 20-element SoC-based W-band (75-110 GHz) array being developed for wider coverage on DIII-D, spanning from the core region (rho ~ [-0.2, 0.4]) to the pedestal region (rho ~ [0.85,0.99]). |
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TP11.00153: Evaluating retention and erosion properties of SiC via high-flux plasma exposures Gregory Sinclair, Tyler W Abrams, Stefan A Bringuier, Dan Thomas, Leo Holland, Sean Gonderman, Russell Chakraborty Doerner Excellent thermal strength and low fuel permeability may make silicon carbide (SiC) a viable candidate for future plasma facing components (PFCs). While tungsten (W) exhibits favorable thermal and mechanical properties, high line radiation from eroded material may induce major disruptions. SiC is a promising low-Z alternative, but concerns regarding tritium accumulation due to retention and chemical sputtering under hydrogenic bombardment necessitate further experimentation. Fuel retention and surface erosion by high-flux, low-energy deuterium plasma implantation were investigated on the PISCES linear plasma device to complement concurrent DiMES exposures in DIII-D. Irradiations were performed at different surface temperatures, ion impact energies, and fluences on C, SiC, and W surfaces. Differences in methane production between C and SiC samples were tracked via in situ quadrupole mass spectrometry. Trap energies and desorption fluxes determined from post-mortem thermal desorption spectroscopy were used to assess the viability of SiC PFCs. Future exposures on functionally-graded W-SiC surfaces will explore potential benefits of mixed-material components. |
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TP11.00154: The statistical properties of solar wind temperature parameters near 1 AU observed by Wind Lynn Wilson III, Michael Stevens, Justin C Kasper, Kristopher Klein, Bennett A. Maruca, Stuart D Bale, Trevor A. Bowen, Marc P. Pulupa, Chadi S. Salem We present a statistical analysis of the temperatures, plasma betas, and temperature ratios for the electron, proton, and alpha-particle populations observed by the Wind spacecraft near 1 AU for ~10 years of data. The mean(median) temperatures are Te,tot = 12.2(11.9) eV, Tp,tot = 12.7(8.6) eV, and Tα,tot = 23.9(10.8) eV; mean(median) plasma betas are βe,tot = 2.31(1.09), βp,tot = 1.79(1.05), and βα,tot = 0.17(0.05); and mean(median) temperature ratios are (Te/Tp)tot = 1.64(1.27), (Te/Tα)tot = 1.24(0.82), and (Tα/Tp)tot = 2.50(1.94). We compared during and excluding interplanetary (IP) shocks, magnetic obstacles (MOs), and slow/fast solar wind. There are differences for slow vs fast wind and during MOs, but with or without IP shocks did not seem to matter. We also compute particle-particle and wave-particle collision rates finding the latter dominates for conservative assumptions of wave amplitude and occurrence rates. Thus, wave-particle interactions should be included when modeling the solar wind. |
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TP11.00155: Exploratory Analysis of Fast Rotating Mode Signals as Precursors of Locked Mode-driven Disruptions at DIII-D Michael Bergmann, Cristina Rea, Erik Olofsson, Robert S Granetz, Hartmut Zohm This work investigates the information content of fast rotating modes (FRM) as precursors of locked mode-driven disruptions, in DIII-D ITER Baseline Discharges (IBS). Rotating precursors, when existing, are not easily identifiable; their evolution in time can strongly vary between 20 and 200ms. We present an exploratory analysis on signals coming from a specific combination of the Mirnov coils, capable of detecting FRMs while discriminating between even and odd poloidal mode number. The analysis focuses on even m modes, these being likely precursors of m/n=2/1 locked modes at DIII-D. We use amplitude and frequency of even m FRM signals in Machine Learning-based models, exploring their correlation features with Brn=1, proxy for nonrotating modes. We additionally include 12 plasma parameters, e.g. the normalized internal inductance and the pedestal information, for ∼200 discharges. Different Machine Learning algorithms are implemented, using both regression and classification schemes. While it is possible to see a correlation between the FRM amplitude and Brn=1 when inspecting the shots, FRM precursors information seem to play a small role in the predictive algorithms. |
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TP11.00156: Particle- in- cell simulations of plasma heating inside a density scale length for the interaction of ultra-short laser pulse with under-dense plasma Seyed Abolfazl Ghasemi In this paper, nonlinear mechanisms of electron heating during the earlier stage of the interaction of an ultra-intense short laser pulse () with an under-dense plasma () inside a preformed density scale length has been investigated for exponential density profile. The fully kinetic particle in cell simulation code (1D3V) has been made to study the plasma heating at onset time of interaction. Our simulations indicate that, for ultra-intense short laser pulse, the nonlinear pulse scatterings are found to be very fast at the earlier time of interaction and cause plasma heating. As a key point in our discussion, our parametric simulations show that at the very earlier stage of interaction, two main electrostatic mechanisms, vacuum-plasma wave break together with longitudinal plasma oscillations, are the dominant acceleration mechanisms which lead to electron chaotic pattern and plasma heating. Meanwhile, studying the effect of density scale lengths on plasma heating revealed that, for exponential density profile, main acceleration mechanisms mentioned above are sufficiently developed during the propagation in typical short density scale lengths about for normalized vector potential . |
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TP11.00157: Plasmoid instability in the semi-collisional regime Pallavi Bhat, Nuno F Loureiro Theoretical and numerical investigations were performed to validate the existence of the semi-collisional regime, which is unstable to plasmoids, in the magnetic reconnection phase space; governed by Lundquist number S and system size L in units of ion (sound) Larmor radius, ρs/L. The semi-collisional regime of the plasmoid instability is defined by the inequality δSP >> ρs >> δin, where δSP is the width of a Sweet-Parker current sheet, and δin is the width of the boundary layer that arises in the plasmoid instability analysis. This inequality translates to theoretically predicted bounds, given by (L/ρs)14/9 < S < (L/ρs)2 (for a sinusoidal-like magnetic configuration; for a Harris-type sheet the lower bound is replaced with (L/ρs)8/5). |
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TP11.00158: Discharge Development on EAST to Produce Sustained Reverse Magnetic Shear for High-Beta Steady State Operation Christopher T Holcomb, Jinping Qian, Xianzu Gong, Juan Huang, Andrea MV Garofalo A goal of the EAST tokamak experiment is to develop fully non-inductive, high beta operating scenarios for future burning plasmas like ITER and CFETR. One possible scenario relies on reversed magnetic shear and elevated qmin to access improved confinement, high-beta stability, and high bootstrap fraction. We report on EAST experiments designed to create sustained shear reversal in H-mode. 2.45 GHz and 4.6 GHz lower hybrid (LH) systems were the primary heating and current drive methods, augmented by electron cyclotron heating and in some cases neutral beam injection. 4.6 GHz LH is the key system capable of driving current off-axis to sustain reverse shear, but control of the damping location is not trivial. Previous results indicated off-axis damping is maximized in high-density L-mode. New experiments used n=1 RMP to maintain L-mode while applying high power to achieve the target q-profile. Then, RMP was switched off and power increased to trigger H-mode. Preliminary polarimetry-constrained equilibrium reconstructions indicate reverse shear was sustained in H-mode. |
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