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 NP11: Poster Session V: Laser-plasma Particle Acceleration; HEDP; Turbulence and Transport; DIII-D Tokamak; Machine Learning, Data Science (9:30am-12:30pm) |
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Room: OCC Exhibit Hall A1&A |
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NP11.00001: Modeling of capillary discharge plasmas for laser- and beam-driven wakefield accelerators Nathan M Cook, Petros Tzeferacos, David L Bruhwiler, Stephen D Webb Discharge capillary plasmas have been shown to increase both the peak energy and beam quality of laser wakefield accelerators. In addition to their use as active plasma lens, this promising technology may also serve next generation beam-driven schemes, for example hollow-channel plasmas for positron acceleration. These sources will be especially sensitive to variations in the plasma density profile and temporal evolution, and thus necessitate improved modeling efforts. Careful consideration of heat transfer and magnetic field penetration at the walls of the capillary are needed to resolve the dynamics at relevant time scales. We present simulations of capillary discharge waveguides in 2D geometries using FLASH, a publicly-available multi-physics code in development at the University of Chicago. We explore parametric modifications of the radial density profile by considering variations in the relevant capillary parameters, such as radius, geometry, wall conductivity, and gas pressure. Lastly, we consider coupling effects in longitudinally varying profiles, and plans to model hollow and near-hollow channel plasmas. |
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NP11.00002: Effect of laser pulse shaping on ion acceleration via relativistic self induced transparency Shivani Choudhary We have studied the effect of the thick, low density plasma slab on the time evolution of the laser pulse profile. The dispersive nature of the plasma makes it possible to naturally chirp the laser pulse. The resultant chirped pulse is subjected to the dual layer composite target (primary+secondary), and apparently an efficient acceleration of the ions are observed from the secondary layer. For this work we have used a p-polarized, Gaussian laser pulse with intensity fwhm ≈ 10 fs and peak intensity ≈ 8.5⨯1020 W/cm2. The optimum density of the primary layer results in efficient electron heating and creating a strong longitudinal electrostatic field, which accelerates the ions from the thin, sub-critical (≈ 0.1nc) secondary layer. The effect of the pre-plasma parameters on the final energies of the ions from the secondary layer is investigated. |
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NP11.00003: Ultrafast Probing of Non-Equilibrium Plasmas Using Laser-Wakefield-Accelerated Electron Bunches Stephen DiIorio, Zhaohan He, John Nees, Bixue Hou, Archis Joglekar, Roy Clarke, Karl Michael Krushelnick, Alexander GR Thomas We explore the predominant physics behind the generation and evolution of an optical-field ionized plasma using electron bunches produced from a laser-plasma accelerator. The delay between the pump pulse and when the electron probe passes through the generated plasma has a direct effect on the measured energy of said electrons. Thus, we can use these electron bunches as a diagnostic to understand the dynamics of these non-equilibrium plasmas with picosecond temporal resolution. We employ particle-in-cell codes, including collisions and ionization, to model the plasma's electric fields that cause such an effect. We then simulate electrons passing through these electric fields at various points in its evolution and compare with experimental results. |
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NP11.00004: Wakefield manipulation via phase-matching of co-propagating laser modes in an underdense plasma channel Blagoje Zoran Djordjevic, Carlo Benedetti, Carl B Schroeder, Eric Esarey, Wim Pieter Leemans We propose the use of higher-order phase-matched laser modes to control the properties of the transverse wake generated by a laser pulse propagating in a parabolic plasma channel in the context of laser-plasma accelerators. For generating a transversely asymmetric wake in the quasi-linear regime we consider the superposition of Hermite modes H20 + H02 and Laguerre modes L10 + L02. It is shown that we can arbitrarily tune the horizontal and vertical transverse wakefields independently of the longitudinal accelerating field. This allows for the realization of flat beams in laser-plasma accelerators with matched propagation. We also consider the possibility of varying the frequency of two modes with different geometric mode numbers such that they propagate at the same group velocity. Specifically, we consider the case of L00+ L10, whereby we can fix the longitudinal accelerating wakefield but vary the strength of the transverse fields. The ability to use multiple modes of different mode numbers and colors that propagate at the same group velocity will allow for greater control of accelerated beam properties. |
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NP11.00005: Enhancing Positron Production using Front Surface Target Structures Noah Zipper, Sheng Jiang, Gerald J Williams, Anthony J. Link, Shaun Kerr, Joohwan Kim, Hui Chen, Yalamanchili Sisir, Paul Kempler, Nathan Lewis Previous studies have shown that large scale front-surface target structures can significantly increase the energy and narrow the angular distribution of hot electrons generated by laser-solid interactions compared to that for a regular flat target [1, 2]. As a consequence, Bremsstrahlung production can be greatly enhanced using a high-Z converter target [3]. The Bremsstrahlung X-rays can further interact with atomic nuclei in the converter target and create electron-positron pairs through the Bethe-Heitler process [4]. We have studied the electron and positron spectra produced by various target structures by performing 2D PIC simulations and optimized the structure shape for positron production. The simulation results are consistent with our experimental data on OMEGA EP. 1. S. Jiang, A. G. Krygier, et al., Phys. Rev. E 89, 013106 (2014) 2. S. Jiang, L. L. Ji, et al., Phys. Rev. Lett. 116, 085002 (2016) 3. S. Jiang, A. G. Krygier, et al., Eur. Phys. J. D 68, 283 (2014) 4. H. Bethe, W. Heitler, Proc. Royal Soc. of London A, 146, 83 (1934) |
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NP11.00006: Sub-Optical-Cycle Dynamics of Electron Nano-Bunches in Relativistic Laser-Plasma Interactions: Insights From Numerical Simulations Nicholas M Fasano, Matthew R Edwards, Julia Mikhailova The reflection of relativistically-intense few-cycle laser pulses from solid targets offers an efficient route toward the production of isolated attosecond pulses. A combination of theory and particle-in-cell (PIC) simulations has previously suggested that the origin of this radiation is the sub-cycle acceleration of high-density, nanometer-scale electron bunches (nanobunches). During this acceleration, the electrons follow synchrotron-like trajectories, emitting harmonics of the laser fundamental in the reflection and transmission directions. The formation and structure of the electron nanobunches are closely related to the emitted radiation. Here, multidimensional PIC simulations taking into account transverse laser forces are run and the results compared with those obtained from 1D PIC. On the short time scales considered here, we show that these nanobunches also form in 2D PIC simulations with similar structure to those found in 1D. |
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NP11.00007: Wakefield excitation via Compton scattering Fabrizio Del Gaudio, Thomas Grismayer, Ricardo Fonseca, Warren B Mori, Luis O Silva Wakefields of lasers, coherent eVs photons, or beam driver can efficiently accelerate electrons. We explore a new regime in which the wakefield is driven by high energy incoherent photons, at keV energy, via Compton scattering, similarly to what was explored numerically by Frederiksen et al. [ApJ 680, L5 (2008)]. We explore this scenario self-consistently and from first principles, using the Compton scattering module of OSIRIS 4.0, to study the interaction of high energy incoherent radiation fluxes with the plasma. Our numerical results are compared with an analytical model. |
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NP11.00008: The impact of electron heating on laser-plasma ion acceleration mechanisms Jason Chou, Anna Grassi, Rohini Mishra, Siegfried Glenzer, Frederico Fiuza Intense laser-plasma interactions offer the possibility of producing short high-energy ion beams for a wide range of applications. We have performed particle-in-cell simulations of laser-driven ion acceleration for a large set of parameters. We show that the ion acceleration mechanisms and the spectral shape of the produced beams can be controlled by the laser electron heating. In regimes of weak laser absorption, direct piston acceleration (hole boring) is dominant and no shock is formed, whereas when the electron temperature increases there is competition between shock acceleration and TNSA. We will discuss how the simulation dimensionality, target density, laser incidence angle and polarization control the electron heating and thereby the properties of the accelerated ion beams. |
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NP11.00009: Coherent ultra-broadband laser-assisted injection radiation in a laser plasma accelerator Bo Miao, Linus Feder, Jared K Wahlstrand, Andrew J Goers, George A Hine, Fatholah Salehi, Howard Michael Milchberg Electron self-injection in self-modulated laser wakefield accelerator is observed to generate an intense coherent ultra-broadband and ultrashort radiation flash, consistent with the acceleration of electrons from nearly rest to almost the speed of light in a distance ~1µm [1]. We present measurements of the flash spectra, spectral coherence, pulse duration, polarization and angular distribution. These are characteristic of laser-assisted injection of off-axis electrons, which does not induce wave-breaking and preserves wake coherence. [1] Goers, A. J., et al. "Multi-MeV electron acceleration by subterawatt laser pulses." Physical review letters 115.19 (2015): 194802. |
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NP11.00010: 3d parametric studies using reduced models for self-modulation instability Anton Helm, Jorge M Vieira, Ricardo Fonseca, Luis O Silva, Patric Muggli An overarching goal of experiments such as AWAKE [1, 2] is the production of TeV-class electron beams utilizing the self-modulation instability (SMI) [3]. In a setup where a laser pulse with ω0/ωp > 1000 is used to seed the SMI through a laser-based ionization, reduced simulations based on the ponderomotive guiding center (PGC) solver [4] are promising. In a PGC solver, the temporal scales related the plasma frequency ωp need to be resolved, but not the laser frequency ω0 as in full particle-in-cell (PIC) simulation. We present an ionization model used for PGC and its impact on the SMI. Furthermore, we studied the variation of the gas density for the ionization seeding and the stability of the SMI in terms of small parameter fluctuations. References: [1] A. Caldwell et al., Nat. Phys. 5, 363 (2009) [2] N. Kumar et al., PRL 104, 255003 (2010) [3] D. Gordon et al., PRE 64, 046404 (2001) [4] D. Gordon et. al., IEEE Trans. Plasma Sci. 28, 1135 (2000)
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NP11.00011: Characterization of energy gain and efficiency for guided and unguided laser-plasma accelerator stages Carlo Benedetti, Carl B Schroeder, Timon J Mehrling, Eric Esarey, Wim Pieter Leemans The viability of next generation plasma-based linear colliders relies on the possibility of accelerating high-charge and low-emittance bunches to high energies over short distances with high efficiency, while keeping a small relative energy spread. Laser-plasma accelerators (LPAs) can operate in different regimes, namely, linear or mildly nonlinear stages, where laser guiding is achieved by means of an external wave guide such as a plasma channel (guided LPA), or highly nonlinear stages where the laser is self-guided through the plasma (unguided LPA). For the same laser driver energy, guided and unguided LPAs are characterized by different average accelerating gradients, dephasing/depletion lengths, optimal bunch length, shape, and charge, and acceleration efficiency. In this contribution, we present a systematic investigation of the properties of guided and unguided LPA stages with fixed laser energy. Constraints imposed by using a fixed bunch emittance will also be discussed. |
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NP11.00012: Wakefield generation in near threshold and/or finite radius ionization plasmas Severin Diederichs, Timon J Mehrling, Carlo Benedetti, Carl B Schroeder, Eric Esarey, Jens Osterhoff, Wim Pieter Leemans The field ionization process in plasma wakefield accelerators (PWFAs) and the associated dynamics of the newly created plasma particles are, so far, insufficiently understood, but are important for the optimization of PWFAs. Here, we study the field ionization of a neutral gas in the vicinity of a high-current particle beam and the associated wakefield excitation in the generated plasma with finite radial extent. The wakefields focus the beam and thereby enhance the ionizing space-charge field and/ or can itself trigger additional ionization during the beam propagation through the gas. We show by means of theory and with 3D particle-in-cell simulations with HiPACE that the near-threshold ionization leads to the generation of exotic wakefields with decoupled longitudinal and transverse field magnitude suitable for the quality- preserving acceleration of witness beams with reduced focusing. We also investigate the possibility to use this highly nonlinear process as a beam diagnostic, in order to probe the charge density and the beam slice emittance. |
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NP11.00013: Compact Kilohertz Laser Wakefield Acceleration Sahel Hakimi, Hunter Allison, Nick Beier, Deano Farinella, Tam Nguyen, Matthew Stanfield, Toshiki Tajima, Franklin J Dollar Laser wakefield acceleration (LWFA) utilizes short pulse lasers focused down to relativistic intensities which drive large acceleration gradients suitable for electron acceleration. The relativistic electron beams are useful for wide ranging applications such as radiography, radiochemistry, x-ray generation, and positron generation. Preliminary work is shown for experiments performed on the UCI laser system, in which a compact, single stage laser system generates electrons at kilohertz repetition rates. Experimental measurements of wavefront, density, electrons, and x-rays will be presented, along with Particle-in-Cell simulations. |
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NP11.00014: Relativistic laser-solid interaction: from near-infrared to mid-infrared Jinpu Lin, John Nees, Xuai Xiao, Igor Jovanovic, Karl Michael Krushelnick There has been growing interest in the physics and application of mid-infrared laser-plasma interactions. The ponderomotive potential in laser-plasma interactions increases with longer laser wavelength, so there may be significant differences in the physics of x-ray and electron generation and other phenomena driven by mid-infrared lasers. In this work, we report the results of experiments performed with millijoule, 40 fs, 2 µm laser pulses generated by an optical parametric amplifier (OPA) which are focused onto metallic and silicon targets. We characterize the hard x-ray source and electron source produced via both 2 µm and 800 nm driving laser in the same setup. Hard x-ray source size, energy efficiency, emission of k-alpha relative to bremsstrahlung, electron energy and beam divergence are studied and compared from near-infrared to mid-infrared. |
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NP11.00015: Multiple species laser-driven ion-shock acceleration Brandon K Russell, Peter R Kordell, Alexander GR Thomas, Louise Willingale Using 2-D OSIRIS particle-in-cell simulations, laser-driven ion-shock acceleration of a multi-species plasma is explored. We perform simulations consisting of over-dense plasmas of electrons, protons, and various ion species at several densities, to obtain a generalized picture of shock propagation in a system consisting of multiple particle species. These results are relevant to high-intensity laser-driven shocks using gas jets with multiple ion species such as methane, or exploded plastic targets. |
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NP11.00016: High-intensity laser-driven electron beam and radiation generation from an underdense plasma in an axial magnetic field Anatoly M Maksimchuk, Brandon Russell, Peter R Kordell, Gennady Fiksel, Alexander GR Thomas, Karl Michael Krushelnick, Louise Willingale The experiments to study electron beam and betatron radiation generation were performed using the T-cubed laser at the University of Michigan. 15 TW (6J, 400 fs) laser pulses were focused with an f/4 off-axis parabolic mirror to the intensity of 1019 W/cm2 onto a supersonic He gas jet with plasma densities ne<1020 cm-3. The experiments were performed with a static magnetic field B=0.5 T or with a pulsed solenoidal magnetic field Bmax=20 T, both were applied in the axial (laser) direction. The characterization of the generated electron beam divergence, charge and spectrum were performed for experiments with and without an axial magnetic field. A comparison of the divergence of the produced betatron radiation and its source size for both cases and different plasma densities will be presented. |
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NP11.00017: Laser system development towards a compact Thomson photon source Tobias M Ostermayr, Hai-En Tsai, Cameron GR Geddes, Jorge G Otero Munoz, Theo Larrieu, Jeroen Van Tilborg, Sam K Barber, Fumika Isono, Remi Lehe, Jean-Luc Vay, Anthony J Gonsalves, Kei Nakamura, Csaba Toth, Carl B Schroeder, Eric Esarey, Wim P Leemans Thomson scattering of laser pulses from relativistic electron beams can provide MeV photons, relevant to numerous applications including nuclear nonproliferation. We present the concept and construction of such a source. Here we focus on a 100 TW class laser system dedicated to this development, and its operation. We present stability data and alignment techniques to enable the multi-beam experiment. Experiments to produce controllable laser-plasma-accelerated (LPA) electron-beams with narrow energy- and spatial distributions are described. Initial experiments towards Thomson scattering using this new setup have started and will be presented. |
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NP11.00018: High-order harmonic generation in an electron-positron-ion plasma Wenlong Zhang, Thomas Grismayer, Ricardo Fonseca, Luis O Silva The high-order harmonics generation (HHG) in an electron-positron-ion plasma with varying proportions is investigated. We find that the amplitude of HHG spectrum is enhanced in mixed targets compared with a pure electron-ion target. Several prominently amplified spectral features are observed in the spectrum, i.e., harmonics of the local plasma frequency in the inertial length region. This is due to the strong excited plasma oscillation driven by an energetic positron flow. The spectral features provide detailed information of the pair plasma properties in the target, thus serving as an in situ diagnostic of the pair plasmas generated in the experiments using intense lasers. |
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NP11.00019: Fast electrons from plasma channels for e-e+ pair production Martin Jirka, Marija Vranic, Thomas Grismayer, Luis O Silva A long laser interacting with an under-dense plasma can form a plasma channel. When an intense laser propagates through such a channel, under certain conditions it directly accelerates electrons to higher energies than allowed in vacuum interaction [1]. One of the mechanisms at play is the parametric resonance, that can lead to an increase of the maximum energy compared to vacuum [2]. At very high laser intensities, the radiation reaction changes the onset of the parametric resonance. |
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NP11.00020: High intensity laser interactions with near critical density target for shock ion acceleration. Peter R Kordell, Paul T Campbell, Brandon Russell, Anatoly M Maksimchuk, Karl Michael Krushelnick, Louise Willingale We will present ion acceleration measurements from the interaction a near-infrared (λ = 1.053μm), relativistic intensity (3 × 1019 W/cm2) laser pulse with a near-critical density plasma. By controlling the front side density gradient a forward accelerated, narrow divergence (< 0.8o) proton beam was generated with energies (> 400 keV). We also assess the effect of peak plasma density and density length scale on electron heating and beam transmission. Particle-in-cell simulations are used to investigate the ion acceleration mechanisms and to provide insight into the experimental measurements.
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NP11.00021: Electron Beam Probing of Plasma Structures in CO2 Laser Driven Wakefields Lígia Diana Amorim, Pietro Iapozzuto, Navid Vafaei-Najafabadi, Jiayang Yan, Michael C Downer, Rafal Zgadzaj, James R Welch, Mikhail Fedurin, Mikhail Polyanskiy, Mark A Palmer, Marcus Babzien, Christina Swinson, Igor Pogorelsky, Karl Kusche, Wei Lu, Chan Joshi Laser wakefield acceleration (LWFA) is a technique being studied for generating compact particle and radiation sources. Electrons are accelerated through high electric fields associated with plasma waves which are excited by a high energy laser. The accelerating forces associated with this scheme can be hundreds of times higher than in conventional accelerators. At Brookhaven National Laboratory’s Accelerator testing facility (ATF), we are exploring LWFA driven by a long CO2 laser pulse. The long-pulse assist to investigate the physics of long-pulse laser and plasma interactions in the mid-IR regime. The plasma structure is excited by a CO2 laser pulse and focused by an off-axis parabolic mirror onto a hydrogen gas jet. The spectral signatures, are detected by observing Forward Collective Thomson Scattering (FCTS) of a separate probe laser pulse. FCTS measures the plasma density variations, while the electron probes will measure the internal fields. In this poster, we present preparations for the experiment with an integrated electron beam probe. This research is important for development of CO2 and electron beam diagnostic techniques, which will be used to conduct future short-pulse CO2 LWFA experiments in the blowout regime at the future AFT II upgraded facility. |
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NP11.00022: Narrow bandwidth Laser-Plasma Accelerator driven Thomson photon source development C. G. R. Geddes, H.-E. Tsai, T.M. Ostermayr, G. Otero Munoz, T. Larrieu, J. Van Tilborg, Cs. Toth, J.-L. Vay, R. Lehe, C.B. Schroeder, A. Friedman, D.P. Grote, E. Esarey, W.P. Leemans Compact, high-quality photon sources at MeV energies for nuclear nonproliferation and other applications can be provided by Thomson scattering of a laser from the electron beam of a Laser-Plasma Accelerator (LPA). Simulations indicate that high flux with narrow energy spread can be achieved via control of the LPA along with the scattering laser pulse shape and laser guiding, and that undesired background bremsstrahlung can be mitigated by plasma based deceleration of the electron beam after photon production. Progress of experiments to combine these elements will be presented, along with initial operations. The path from these experiments towards a compact high performance photon source will be discussed. |
(Author Not Attending)
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NP11.00023: Picosecond laser driven x-ray radiation enhanced by target normal sheath Wei Wang, Jun Xiong, Zhiheng Fang, Honghai An, Zhiyong Xie, Anle Lei, Chen Wang, Ruirong Wang, Zhao Liu, Xiuguang Huang, Wenbing Pei, Sizu Fu Experimental results of x-ray radiation driven by picosecond laser on SGII-UP laser facility are presented. The PW laser beam which delivering 400J in 1 picosecond pulse was focused onto the foil target. The laser intensity on target is about 5-7×1018 W/cm2. Different materials, from high atomic number Z material Tantalum, to middle Z material Copper, and low Z material plastic, were used as foil targets. The target thickness is about 20 microns. The experimental results show that, x-ray radiation intensity of low Z plastic is a little lower than that of high Z Copper or Tantalum, but not that much as regular Bremsstrahlung expected. Also, x-ray radiation tends to be isotropic rather than along the laser direction. In which we believe target normal sheath field plays an important role. |
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NP11.00024: Proton acceleration from nano-engineered targets using high-intensity short pulse lasers George M Petrov, Maylis Dozieres, Pierre Forestier-Colleoni, Paul C Campbell, Karl Michael Krushelnick, Anatoly M Maksimchuk, Jorge Juan Rocca, Christopher S McGuffey, Farhat N Beg A theoretical and experimental study of proton acceleration from planar gold targets covered with gold nanowires is presented. The nano-engineered foils composed of a flat substrate with thickness ranging from 7 to 20 μm and protruding gold nanowires having length equal to the foil thickness were irradiated by a short pulse high intensity laser (Hercules: 3 J, 1.5x1021 W/cm2, 30 fs, 1.5 μm focal spot). The nanowires enhance significantly the laser energy coupled to the plasma and maximum proton energies. Different nanowire and substrate parameters were tested showing that the most important parameter is the number of nanowires per focal spot, N. Theoretical analysis and experimental data indicate that optimum is reached for nanowire density of N=1. Proton spectra are compared to simulations using a 2D-3V Particle-In-Cell (PIC) code which reproduces the experimental data with a good agreement. |
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NP11.00025: Planar Laser-Induced Fluorescence For Custom Laser Plasma Accelerator Targets Liona Fan-Chiang, Hann-Shin Mao, Wim Pieter Leemans The ability to precisely shape gas jets for controlled injection of electrons in laser plasma accelerators (LPAs) is crucial for developing high quality electron beams. Verifying tailored density profiles has called for more detailed gas density diagnostic than those traditionally used. Most diagnostics give line-of-sight measurements, integrating over and blurring sharp or asymmetric features. In this study, planar laser-induced fluorescence (PLIF) has been prototyped for characterizing laser plasma accelerator gas jet targets. PLIF has the distinct advantage of isolating a 2-D slice of the jet plume using a laser sheet providing more direct density information at regions of interest. This sheet can be scanned across the jet to map out the entire 3-D gas plume profile. It was shown that PLIF is able to resolve critical features such as gas density shocks. Blade-in jets under low vacuum were characterized with PLIF and found to change drastically in character with blade position. Changes included change in the axis of the entire gas plume as well as its effective Mach number. |
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NP11.00026: Investigation of CO2 Long Intense Laser Pulse Plasma Instabilities in LWFAs Jiayang Yan, Lígia Diana Amorim, Pietro Iapozzuto, Mael Flament, Yichao Jing, Vladimir N Litvinenko, Navid Vafaei-Najafabadi, Roman V. Samulyak, Prabhat Kumar, Igor Pogorelsky, Christina Swinson, Marcus Babzien, Karl Kusche, Mikhail Polyanskiy, Mikhail Fedurin, Mark A Palmer, Rafal Zgadzaj, James R Welch, Michael C Downer, Chan Joshi, Warren B Mori, Wei Lu Laser Wakefield Acceleration (LWFA) is a promising method for future medical applications and light sources. Previous studies on particle-driven plasma Wakefield acceleration showed that beam hosing instability precluded stable acceleration [J. Vieira, et al., PRL, 112(20), 205001, 2014]. A process is observed in LWFA similar to hosing instability in particle beams, when a long (~ps), intense (~TW) laser pulse propagates into a plasma, that is still not well understood. We model the long laser LWFA setup of the AE71 experiment at Brookhaven National Laboratory with PIC OSIRIS simulations [R. A. Fonseca et al., LNCS (2331) 342, 2002]. Focusing on interactions of the front part of the laser, located before the section that hoses, with the plasma, which are governed by the well documented self-modulation instability. In this poster we characterize that instability and compare its growth rate with theoretical findings for the AE71 laser and plasma explored range of configurations. Finally, we discuss the origin of the hosing instability for long intense laser pulses. |
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NP11.00027: Quasi-monoenergetic Laser Plasma Positron Accelerator using Particle-Shower Plasma-Wave Interactions Aakash Sahai An all-optical centimeter-scale laser-plasma positron accelerator is modeled to produce quasi-monoenergetic beams with tunable ultra-relativistic energies. A new principle elucidated here describes the trapping of divergent positrons that are part of a laser-driven electromagnetic shower with a large energy spread and their acceleration into a quasi-monoenergetic positron beam in a laser-driven plasma wave. Proof of this principle using analysis and Particle-In-Cell simulations demonstrates that, under limits defined here, existing lasers can accelerate hundreds of MeV pC quasi-monoenergetic positron bunches. By providing an affordable alternative to kilometer-scale radio-frequency accelerators, this compact positron accelerator opens up new avenues of research such as in Channeling undulators, Gamma Induced Positron Spectroscopy etc apart from being a viable positron source for future colliders. |
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NP11.00028: Effects of Nonlinear Plasma Waves on the Initiation and Evolution of the Weibel Instability and Associated Anisotropic Velocity Distribution Function Phenomena in High Energy Density Plasmas Bedros Afeyan, Archis Joglekar, Richard Dwayne Sydora, Brad Shadwick We show theoretical and computational results related to the dynamics of the Weibel instability coexisting with nonlinear plasma waves. Kinetic modeling is used to distinguish between new regimes of behavior relating these seemingly unrelated phenomena. Using external drivers to build such waves via optical mixing, we interweave their drivers in 3-space and time to effectuate new regimes of plasma behavior that constitute parts of a series of tools and techniques we refer to as femtosecond phase-space plasma photonics, or FP^3. The aim is to initiate nonlinear, coherent kinetic high energy density plasma responses by way of electron plasma waves, KEEN waves and KEEPN waves to steer and direct light and particle streams on the femtosecond time scale. Controlling plasma waves and instabilities via the interplay between electrostatic and electromagnetic instabilities is the long term goal with potential applications throughout high energy density science and nonlinear plasma physics. The states we create may be recognized as examples of self-organization far from equilibrium, through the action of intense coherent ultrashort pulse fields. |
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NP11.00029: Kinetic simulations of particle energization by magnetized collisionless shocks in expanding laboratory plasmas Kirill Lezhnin, William Fox, Jackson Matteucci, Derek Schaeffer, Kai Germaschewski, Amitava Bhattacharjee Collisionless shocks are common features in space and astrophysical systems where supersonic plasma flows interact, such as the solar wind, the heliopause, and supernovae remnants. Recently experimental capabilities and diagnostics evolved sufficiently to allow detailed laboratory investigations of high-Mach number shocks [1]. Using 2D and 3D PIC simulations, we investigate mechanisms which may contribute to the generation of energetic particle populations in the laboratory high-Mach number collisionless shocks. We consider two geometries, (1) two colliding quasi-1-D slabs, which can be cross-validated with previous numerical studies, and (2) an ablation model which mimics plasma profiles observed in the expanding plasma experiments. We perform a parametric scan study to determine the accelerated particle distributions as a function of the plasma parameters of the shock, to predict experimental signatures of collisionless shock acceleration, such as the accelerated particle spectrum and angular distribution, that can be compared against other known laser-plasma processes that energize particles. [1] Schaeffer et al., Phys. Rev. Lett. 119, 025001 (2017) |
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NP11.00030: The effect of advected magnetic fields in jet propagation experiments Daniel R Russell, Sergey V Lebedev, Guy C Burdiak, Jack W D Halliday, Jack D Hare, Lee G Suttle, Francisco Suzuki-Vidal, Eleanor R Tubman
Pulsed power-driven ablation of conical wire arrays has been used to produce supersonic, radiatively cooled jets which are scalable to astrophysical systems [1]. In these experiments, the advected magnetic field can become dynamically significant, especially in the jet-ambient interaction. To enable the full range of jet-ambient interactions to be studied, it is desirable to be able to modify jet magnetisation within the same experimental setup. We present experimental results from a new conical wire array jet-launching platform, in which the magnetic field advected by the jet is significantly reduced in comparison with previous experiments. This is achieved by preconditioning the wires with a current pre-pulse, allowing the formation of a supersonic jet driven by a fast plasma implosion. These experiments are carried out on the MAGPIE pulsed power generator (∼1 MA, 250 ns), using a suite of spatially and temporally resolved diagnostics, including laser interferometry, Faraday rotation polarimetry and high speed self-emission imaging. [1] D. Ampleford et al. PRL 2008 |
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NP11.00031: Temporally Resolved Ion Fluorescence Measurement of the Interaction of a Field-Parallel Laser Produced Plasma and an Ambient Magnetized Plasma Robert Dorst, Peter V Heuer, Martin S. Weidl, Derek Schaeffer, Carmen G Constantin, Stephen T Vincena, Shreekrishna Tripathi, Walter N Gekelman, Dan Winske, David Jeffrey Larson, Christoph Niemann We present measurements of a super-Alfvenic debris velocity distribution of a laser-produced plasma (LPP) using a high temporal and spectral resolution monochromator. The LPP was created by focusing one of two high energy lasers onto a high density polyethylene (HDPE) target embedded in the ambient magnetized plasma of the Large Plasma Device (LAPD) at the University of California, Los Angeles. The resulting ablated debris ions interact with the background magnetic field (300 G) through electromagnetic instabilities over 12 m, equivalent to 80 ion inertial lengths (δi ), of the LAPD. The monochromator measures fluorescence from debris and ambient ions to determine the debris velocity distribution by charge state. Monochromator time traces were integrated post-experiment to conduct a low resolution spectroscopic survey from 185-680 nm. This is compared to NIST spectral data in order to evaluate the relative populations of elements and charge states in the ambient plasma and LPP. Results are discussed in the context of laboratory laser-driven electromagnetic instability growth. |
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NP11.00032: Long duration x-ray source for laboratory photoionized plasmas R. P. Schoenfeld, R. C. Mancini, D. C. Mayes, R. Heeter, D. Martinez, P. Celliers, S. P. Regan Long duration, i.e. tens of ns, broadband x-ray sources are important to perform laboratory photoionized plasma experiments relevant to astrophysics. In a series of experiments performed at the OMEGA EP laser facility we have used the Gatling-Gun source to produce an x-ray drive that lasts for 30ns and has a radiation temperature of approximately 90eV. The Gatling-Gun source is comprised of a three Cu hohlraum arrangement. Each holhraum is filled with TPX foam and is driven by a 10ns-square pulsed laser beam with 4.4kJ of UV laser energy1. The total source duration of 30ns is achieved by driving the three hohlraums sequentially in time. The radiation temperature is monitored with a VISAR diagnostic to check the source’s performance consistency. We present systematic measurements of the radiation temperature and x-ray source performance obtained from VISAR data recorded over two series of experiments performed OMEGA EP. Also, we have used the 4ω probe laser available at OMEGA EP to observe the effect of the Cu/TPX plasma blow off from the hohlraum. 1D. Martinez, 2017 Annual OLUG Workshop. |
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NP11.00033: Complex Hydrodynamic Instabilities on Oblique Interfaces Codie Fiedler Kawaguchi, Alexander M Rasmus, Carlos Di Stefano, Forrest W Doss, Jonathon S Zingale, Kirk A Flippo Hydrodynamic instabilities are important phenomena that occur in high energy-density systems in which pressure, density and velocity gradients exist. These instabilities lead to mixing across interfaces which can greatly impact the performance of the systems they are in. This is seen in the evolution of super-novae as well as inertial confinement fusion (ICF) implosions. When a shock is incident normal to an interface, the Richtmyer-Meshkov (RM) process is driven, which grows impulsively at early times. When an oblique shock interacts with an interface a shear flow is driven in addition to the impulsive acceleration. This shear flow drives Kelvin-Helmholtz (KH) in addition to RM. Recent work has demonstrated that an oblique shock incident on a perturbed interface can result in growth which is dominated by RM early in time, but asymptotes to a KH-like vorticity distribution. |
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NP11.00034: Code comparison of cylindrical implosion experiments for deceleration phase Rayleigh-Taylor Nomita Vazirani, John L Kline, Sasikumar Palaniyappan, Joshua P Sauppe, Bhuvana Srinivasan The Rayleigh-Taylor (RT) hydrodynamic instability occurs when a lower density fluid pushes on a higher density fluid. This occurs in inertial confinement fusion (ICF) implosions at each of the capsule interfaces during the initial acceleration and the deceleration as it stagnates. The RT instabilities mix capsule material into the fusion fuel degrading the DT reactivity and ultimately play a key role in limiting target performance. Significant effort has recently been focused on understanding deceleration phase RT using codes such as xRAGE1 to design experiments for OMEGA and NIF. Here we present the design of a cylindrical implosion system using the code FLASH2 for comparison to xRAGE results. We also use FLASH to explore the influence of magnetic fields on RT growth in the implosions. 1 Gittings, Michael, et al. "The RAGE radiation-hydrodynamic code." Computational Science & Discovery 1.1 (2008): 015005. 2 Fryxell, Bruce, et al. "FLASH: An adaptive mesh hydrodynamics code for modeling astrophysical thermonuclear flashes." The Astrophysical Journal Supplement Series 131.1 (2000): 273. |
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NP11.00035: 2D and 3D modeling of bright X-ray sources on LMJ Michel Primout, Laurent Maurice Jacquet, Laurent Videau Multi-keV bright X-ray sources are needed for applications like plasma diagnostics for inertial confinement fusion and for that purpose, X-ray sources are designed using gas-filled cans and metallic foils. These targets are illuminated by 12 quads of the LaserMegaJoule facility (LMJ). We focus on the simulations of xenon-filled cans and silver foils. The radiative-hydrodynamics simulations are carried out with the 2D code FCI2 [1] and the 3D code Troll [2]. The two codes use the same physics package to simulate radiation transfer, non-local-thermal-equilibrium atomic physics, thermal conduction and laser absorption. For several target configurations, the energy emitted in multi-keV X-rays is computed with FCI2 and Troll. The comparison of 2D and 3D results enables to quantify the effect of the asymmetric laser irradiation on the X-ray emission.
[1] E. Buresi et al., Laser Part. Beams 4, 531 (1986). [2] E. Lefebvre et al, Nuclear Fusion, accepted for publication. |
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NP11.00036: New Prism EOS and Opacity Tables with NLTE Atomic Kinetics. Igor Golovkin, Joseph J MacFarlane We present new features of PROPACEOS, a code that generates equation-of-state (EOS) and opacity tables for radiation-hydrodynamics and spectroscopic simulations. In addition to existing capabilities to produce tables for LTE and optically thin NLTE plasmas, these new features allow PROPACEOS to perform calculations that include other effect of NLTE atomic kinetics. The primary purpose of this development is to facilitate efficient spectroscopic simulations for short-pulse laser experiments. The simulations are based on post-processing of PIC calculations and focus on the analysis of K-alpha/K-beta emission signatures. PROPACEOS can now produce emissivity and opacity databases on a grid with up to six independent parameters, for example: plasma temperature, plasma density, and hot electron parameters. Hot electron distributions are specified in terms of analytic functions. We will also discuss new capabilities that allow for computing opacities for optically thick NLTE plasmas. We will present simulation results relevant to ongoing experiments on Omega EP laser facility. |
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NP11.00037: Solenoid for Laser Induced Plasma Experiments at Janus Sallee R. Klein, Heath Joseph LeFevre, G. Elijah Kemp, Derek Mariscal, Gerald J Williams, Mario J.-E. Manuel, Carolyn C Kuranz, Paul A Keiter, R. Paul Drake Creating invariant magnetic fields for experiments involving laser induced plasmas is particularly challenging due to the high voltages at which the solenoid must be pulsed. Here we present a solenoid that is robust through 40 us pulses at a ~13 kV potential. This solenoid is a vast improvement over our previously fielded designs in peak magnetic field capabilities and robustness. Designed to be operated at small-scale laser facilities, the solenoid housing allows for versatility of experimental set-ups among diagnostic and target positions. Within the perpendicular field axis at the center there is 300 degrees of clearance which can be easily modified to meet the needs of a specific experiment, as well as an ~f/3 cone for transmitted or backscattered light. After initial design efforts, these solenoids are relatively inexpensive to manufacture. |
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NP11.00038: Updated Simulations of a Molecular Cloud experiment using CRASH Matthew Trantham, Robert Vandervort, Paul A Keiter, Carolyn C Kuranz, R. Paul Drake Laboratory experiments explore molecular cloud radiation hydrodynamics. Our experiment irradiates a gold foil with a laser producing x-rays to drive the implosion of a foam ball. We show simulation results of the CRASH code, a Eulerian code with block-adaptive mesh refinement, multigroup diffusive radiation transport, and electron heat conduction developed at the U. of Michigan to design and analyze HED experiments. We explore how experimental conditions, such as a dynamic x-ray spot size, the ablated foam ball’s interaction with the expansion of the irradiated gold foil, and how the interaction between the stalk and the foam ball will affect the experiment. We also show simulated radiographs designed for direct comparison to experimental ones. The results of this study are used to recommend improvements in future experimental designs. |
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NP11.00039: A High Repetition Rate Target and Plasma Mirror Inserter Based on Liquid Crystal Films Nicholas Czapla, Anthony Zingale, Ginevra Cochran, Jordan Purcell, Douglass W Schumacher Multiple >1 Hz PW class lasers are now on-line with more under construction. There is an immediate need for provision of targets and plasma mirrors at these high rates so these machines can perform high energy density (HED) experiments on solid density targets. We have shown previously that films made of the liquid crystal (LC) 8CB make quality thin (10 nm to >1 μm) targets for ion acceleration (Poole, et al., Applied Physics Letters 109, 151109 (2016)) and plasma mirrors for pulse contrast enhancement (Poole et al., Scientific Reports 6, 32041 (2016)) at ~1 shot/minute repetition rates. Here we describe a novel device that can deliver 10-30 nm thick LC films for use as plasma mirrors at repetition rates up to 3 Hz with good film flatness, uniformity, and pointing at a very low cost per film. We discuss the significant challenges met with and overcome for this work. Additionally, we discuss different challenges involved in developing a similar device for targets at high repetition rates as well as other novel approaches being considered. |
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NP11.00040: Plasma mirror reflectivity and focal spot quality for glass and aluminum mirrors for laser pulses up to 30 picoseconds Brandon Christian Edghill, Pierre Forestier-Colleoni, Jaebum Park, Sasha Rubenchik, Farhat N Beg, Tammy Yee Wing Ma The Advanced Radiographic Capability (ARC) short pulse laser at the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory is capable of producing a high energy,∼4kJ , laser pulse of 30 ps in a ∼100μm focal spot, leading to an intensity of 1018 W/cm2. For many applications, including radiography and warm dense matter creation, higher laser intensities are desirable to produce the necessary x-ray and particle fluxes. Due to the complexity of the NIF target chamber, changing the location of the final focusing parabola in order to improve the focal spot size is not an option. This leads to the possible use of disposable ellipsoidal plasma mirrors placed within the chamber, close to the target in an attempt to focus the four ARC beams. The behavior of plasma mirrors at these relatively long pulse durations (>10 ps) is not well characterized. The results from the COMET laser at the Jupiter Laser Facility (JLF) carried out at similar pulse durations show the deterioration of glass plasma mirrors’ focal spots at longer pulse durations and less degradation when using aluminum plasma mirrors. |
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NP11.00041: Resistive Filamentation and Collimation of Relativistic Electron Beams in Collisional PIC Joshua J May, Christopher S McGuffey, Toshinori Yabuuchi, Mingsheng Wei, Farhat N Beg, Warren B Mori Experiments on OMEGA EP using one beam in high-intensity mode (eA/mec > 1, duration ~ 10ps) to accelerate electrons, and optionally a second (earlier) beam with nanosecond duration to create a plasma transport layer, find significantly different electron transport through CH plasma as compared to cold CH foam. We use the particle-in-cell code OSIRIS, together with a Monte Carlo Coulomb collision calculation, to model the experiment in 2D. We find the transport is greatly affected by the self-generated magnetic field, in particular the field whose growth is governed by the collisional resistivity of the return current; transport of the field with the material current of the return flow is also a significant effect. We find the cause of the differing evolution of the field is the different density in the plasma and foam case, which affects the heat capacity. In the simulations, we find the collimation to be sensitive to the target size, with reduced-scale targets showing more divergent beams. |
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NP11.00042: Photon kinetics and collective plasma dynamics in the presence of two populations of photons Luis Silva, Thomas Grismayer, Fabrizio Del Gaudio, Robert Bingham We use a photon fluid description and a generalized photon kinetic theory, based on the Wigner-Moyal description, to capture capture collective plasma dynamics in the relativistic regime driven by broadband incoherent or partially coherent sources. We consider a population of soft photons, coupled with the plasma via the ponderomotive force, and a population of hard photons, whose interaction with the plasma can include single scattering mechanisms. We explore several plasma instabilities/modifications to the collective plasma dynamics present in this scenario due to the presence of hard photons. Comparisons with numerical simulations are also presented. |
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NP11.00043: Effective Potential Theory in Hybrid Fluid/Kinetic Modeling Of Magnetized High Energy Density Plasmas David B Hansen, Eric Held, Jacob R King, Peter Stoltz, Robert L Masti, Bhuvana Srinivasan The magnetized liner inertial fusion (MagLIF) concept is negatively impacted by the magneto-Rayleigh-Taylor (MRT) instability, which is seeded early in time by the electro-thermal (ET) instability. One of the project goals is to solve the electron drift kinetic equation in conjunction with the evolving fluid model in Tech-X's USim to correctly assess the ET instability’s dependence on resistivity, which is a sensitive function of density and temperature. Large thermodynamic drives associated with gradients at the interface between the liner and the coronal regions distort distribution functions and likely lead to non-local transport effects in a plasma which varies from weakly to strongly coupled. Effective potential theory (EPT) [1] has been shown to extend into the strongly coupled regime. The hypernetted chain approximation may be used to produce a coupled system of equations for the radial distribution function, g(r), giving a potential of mean force. In this poster, this potential of mean force is compared with two other potentials (cutoff and screened Coulomb) in predicting scattering cross sections and collisional transport coefficients. 1. S. Baalrud and J. Daligault, Phys Plasmas 21, 055707 (2014). |
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NP11.00044: An HED-regime diagnostic to obtain supersonic radiation wavefront profiles using Ti-doped foams on OMEGA Todd Urbatsch, Heather M Johns, Nicholas Lanier, Chris Fryer, Christopher J Fontes, Colin RD Brown, John Morton Los Alamos' COAX experimental campaign is developing a Marshak wave diagnostic for constraining HED radflow experiments. COAX uses absorption spectroscopy of backlit Ti-doped foams to infer temperature profiles of the supersonic radiation wavefront propagating down the doped foam tube from a laser-lit halfraum on the OMEGA facility. Radiography shows the blastwave as the radiation goes subsonic. The titanium 1s-2p and 1s-3p absorption lines occur around a frequency of 4400-5400 eV, well above the temperature of the Marshak wave at 50-150eV. Los Alamos custom opacity tables in the OPLIB format with dTe = 2eV provide the ability to infer the temperature profile of the wavefront in space at multiple snapshots in time. The uncertainty of that final step is a few eV, but the inferred experimental temperatures differ by about 15 eV from Eulerian AMR radiation-hydrodynamics simulations that model the integral setup including laser-ray-tracing into the hohlraum and that are calibrated to the radiographed blastwave. We continue to develop the diagnostic and its overall uncertainty, refine our simulations and their calibration, and seek other physics explanations for the current experiment-simulation inconsistencies, such as the dilute Planckian resulting from one-way radflow. |
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NP11.00045: Validation of Synthetic Proton Radiography for HED Experiments Kwyntero Van Kelso, Hui Li, Yingchao Lu, Shengtai Li, Andy Liao, Codie Y Fiedler Kawaguchi, Jordon T. Laune, Colin Wilburn, Kevin Meaney We study self-generated magnetic fields in laser-driven hydrodynamic experiments, using the OMEGA laser, with applications to inertial confinement fusion (ICF), high energy density (HED) plasma physics, and astrophysical phenomena. These experiments aim to replicate the compression of a cold spherical shell driven by a laser beam which produces intense shock waves through the target and potential instabilities at the many interfaces in the capsule, or other extreme boundaries. I will discuss our use of proton radiography and gamma ray spectroscopy as two methods to diagnose the plasma and laser-matter interactions. The experiments are pre- and post-modeled using the radiation hydrodynamic code FLASH which uses Adaptive Mesh Refinement (AMR) on an Eulerian grid. We have developed a new synthetic proton radiography package for the FLASH output and have benchmarked it against trusted Monte Carlo stopping power models. We present these validated synthetic radiographs. |
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NP11.00046: HYDRA Simulations of the Effect of External Magnetic Fields on Plasma Temperature and X-Ray Emission in Titanium-Doped Foam Targets Kurt Tummel, Gregory E Kemp, David Jerome Strozzi, Derek Mariscal The application of external magnetic fields to laser driven targets reduces electron thermal transport which can increase electron temperatures and improve high energy photon emission, particularly for multi-keV photons. Simulations and experiments report up to 50% increases in the electron temperature in magnetized targets, consistent with unrestrained thermal transport along the external field lines. However, the strength and orientation of the magnetic field can vary significantly over the laser pulse due to MHD effects including the Nernst effect and the Biermann battery, which are often neglected in the literature. The simulations presented here use the radiation-hydrodynamics code HYDRA to predict high-energy X-ray emission, electron temperatures, and magnetic field evolution in Titanium doped cylindrical foam targets at 3mg/cm^3 with a direct laser drive in external magnetic fields ranging from 0 to 100 Tesla. The simulations investigate the influence of the Nernst effect and Biermann battery on electron temperatures and X-ray emission, and the results are compared with experiments performed with the Janus laser. |
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NP11.00047: Laser Backscatter and Propagation in Magnetized Low-Density SiO2 Foams Derek Mariscal, Gerald J Williams, Siddharth Patankar, Kurt Kenneth Tummel, Heath Joseph LeFevre, Joseph Levesque, Sallee Klein, Ted Roswitha Baumann, John D Moody, Pierre A Michel, Gregory E Kemp Self-generated and externally imposed magnetic fields can have significant impact on the dynamics of laser-driven plasma systems. We report on experiments that examine the effects of an externally applied external magnetic field on laser heat front propagation through sub-critical-density aerogels. Both pulsed-power-driven and laser-driven coils were used to impose external magnetic fields up to ~25 T. The plasma conditions were diagnosed using an imaging x-ray spectrometer, self-emission x-ray imaging, and backscatter diagnostics. Significantly more uniform laser energy deposition and differences in the observed backscattered Stimulated Brillioun Scattering spectrum are observed in the presence of external magnetic fields. These dynamics are consistent between either method of imposing an external magnetic field suggesting that laser-driven coils are a good candidate for use in generating magnetic fields for magnetized plasma studies. The results will be presented with a comparison to HYDRA simulations that explore the importance of various MHD terms in replicating the experimental results. |
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NP11.00048: Modeling Kα emission with Spect3D for Short-Pulse Laser Experiments Timothy Walton, Igor E Golovkin, Joseph J MacFarlane Cold Kα and Kβ emission provides a diagnostic for hot electron distributions produced in short pulse laser experiments. Spect3D is able to simulate high-resolution spectra for plasmas containing arbitrary distributions of hot electrons, however the calculations can become prohibitively expensive for plasmas requiring complex atomic data and variable temperatures and densities. To significantly increase calculation speed, it is desirable to use pre-configured emissivity/opacity tables for hot electron conditions, rather than calculating on the fly. However, to tabulate results for arbitrary hot electron energy distributions, a general method must be found to describe arbitrary energy distributions with an analytic function of just a few parameters. We will present a set of PrismSPECT and Spect3D results validating the replacement of hot electron distributions with a Gaussian function determined by each hot electron distribution. Tests are performed on particle in cell data sets produced by the LSP software. |
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NP11.00049: Spectroscopic Diagnostics Using Line-Radiation in Laser Driven Non-equilibrium Plasmas in a Ti-doped Silica Aerogel Foam Target Arati Dasgupta, Nicholas David Ouart, Gregory Elijah Kemp, Heath Joseph LeFevre, John L Giuliani, Emil E. Petkov Experiments were performed at the Jupiter Laser Facility at LLNL, where x-ray spectroscopic measurements among other data were acquired from sub-critical-density, Ti-doped silica aerogel foams driven by a 2ω laser at ~ 5x1014 W/cm2. The main objective is to study the effect of external B-field in thermally insulating the hot plasma and investigating line-radiation in multi-keV, non-equilibrium plasmas. The near term goal is to infer a time-integrated temperature at several positions along the laser propagation axis for several B-field cases and observe any sensitivity to density for estimated plasma conditions of ne/nc ~ 0.2 and Te ~ 1 keV with 4.5% of Ti by atomic fraction in SiO2 foam target. We present time-integrated synthetic Ti spectra employing a recently developed non-LTE collisional-radiative spectroscopic model with detailed multi-frequency radiation transport scheme to diagnose the spectroscopic data. This approach was successfully used to replicate observed x-ray line spectra and diagnose plasma conditions from various wire-array and gas puff implosions on the Z machine at Sandia National Laboratories. |
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NP11.00050: Strong-flow gyrokinetic simulations with a unified treatment of all length scales. Amil Yograj Sharma, Ben Fynney McMillan, Julien Dominski Tokamak turbulence exhibits interaction on all length scales, but standard gyrokinetic treatments consider global scale flows and gyroscale flows separately, and assume a separation between these length scales. However, the use of a small-vorticity ordering (Dimits, 2010) allows for the presence of large, time-varying flows on large length scales, whilst providing a unified treatment including shorter length scales near and below the gyroradius. We present the numerical implementation of these gyrokinetic equations, benchmarking of the resulting code, and differences in representation compared to the weak-flow theory. Our Euler-Lagrange and Poisson equations contain an implicit dependence that appears as a partial time derivative of the E × B flow. This is analogous to the v||-formulation of gyrokinetics. However, as these implicit terms are small, we use an iterative scheme to resolve this. Additionally, we have developed a stand-alone Poisson solver based on that from the ORB5 code, and use this to simulate certain flow and density gradient driven instabilities in cylindrical geometry. We also explain the differences in representation of trajectories and distribution functions between the weak- and
strong-flow theories. |
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NP11.00051: Gyrokinetic particle simulation of electrostatic drift wave turbulence in W7-X and LHD stellarators Hongyu Wang, Zhihong Lin, Ihor Holod, Jian Bao, Lei Shi, Sam Taimourzadeh We report gyrokinetic particle simulation of ion temperature gradient (ITG) instabilities in Wendelstein7-X (W7-X) and Large Helical Device (LHD) stellarators by using Gyrokinetic Toroidal Code (GTC). The ITG instabilities have been studied in both full torus and partial torus geometries. The linear toroidal coupling effects due to the 3D magnetic fields are kept in partial torus simulation of toroidal domain with minimal length by taking advantage of the toroidal periodicity of stellarators. The mode structure in W7-X is more localized in the toroidal direction, and LHD is more extended in the toroidal direction and tokamak-like. GTC simulation results of linear frequency, growth, and mode structures are in good agreement with EUTERPE simulation results. Initial nonlinear simulation results of ITG instability saturation and turbulent transports in stellarators are presented, which support the view that turbulence-driven fluctuating zonal flows can suppress turbulent transport. |
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NP11.00052: GEM development for the spatial core-edge coupling of the PIC gyrokinetic codes GEM and XGC Junyi Cheng, Julien Dominski, Scott Edward Parker, Yang Chen, Choong Seock Chang Within the Exascale Computing Program, the High-Fidelity Whole Device Modeling project aims at delivering a first-principles-based computational tool that simulates the plasma neoclassical and turbulent dynamics from the core to the edge of the tokamak. To permit such simulations, the different gyrokinetic codes need to be coupled. My work focuses on the coupling of two particle-in-cell (PIC) gyrokinetic codes, GEM and XGC. The coupling scheme initially implemented for XGC-XGC coupled simulations has been applied to GENE-GENE and GEM-GEM runs. These simplified core-couplings allowed us to learn about the behavior of the coupled system. The coupling of the continuum code GENE and the PIC code XGC has already started yielding results. Here, the first results obtained with the coupled PIC gyrokinetic codes, GEM and XGC, will be presented. For the coupling of GEM and XGC, besides the interpolation on different grids, exchange of particle information between the two will be attempted in order to maintain accurate particle dynamics. In addition, for a more efficient coupling, the GPU optimization for GEM is ongoing and the first results will be also presented. |
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NP11.00053: Fully implicit particle-in-cell simulation of gyrokinetic electromagnetic modes in XGC1 without the cancellation issue Seung-Hoe Ku, Benjamin J Sturdevant, Robert Hager, Choong-Seock Chang, Luis Chacon, Guangye Chen Electromagnetic gyrokinetic simulation of tokamak plasma has been suffering from the so-called “cancellation problem,” in which the long wavelength modes make the simulation blow up. Recently, a fully implicit electromagnetic Vlasov-Darwin particle-in-cell algorithm using fluid preconditioner is developed [1]. We adopt this scheme to XGC1, a 5D total-f gyrokinetic code. For the parallel velocity, v|| is used instead of the canonical momentum p||. Picard iteration is used with fluid pre-conditioner. Subcycling of particle motions and fixed-point accelerator raise the simulation efficiency. The cancellation problem does not appear even in the longest wavelength Alfven wave range (n=0, m=1) at tokamak plasma βe values. Kinetic ballooning and micro-tearing modes will be discussed for NSTX and other tokamak plasmas. [1] G. Chen, L. Chacon, Comput. Phy. Comm. 185, 2391 (2014) |
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NP11.00054: Towards global electrostatic ion-scale turbulence simulations in stellarators with XGC Michael Cole, Robert Hager, Toseo Moritaka, Seung Hoe Ku, C-S Chang In this presentation, we detail progress towards simulations of electrostatic ion-scale turbulence in stellarator geometries with the whole volume, total-f, gyrokinetic particle-in-cell code XGC. In recent work, XGC has been adapted for stellarator/heliotron geometries. XGC has been successfully benchmarked against the kinetic codes EUTERPE and FORTEC-3D for energetic particle confinement and GAM physics in such devices. So far, no fully global electrostatic turbulence simulation has been performed for a stellarator with any gyrokinetic code. A goal of this project is to perform such simulations with the XGC code. The stellarator developments in XGC are first benchmarked with a standard circular tokamak linear ITG case. An unstructured field-aligned mesh is then constructed for Wendelstein 7-X geometry. Preliminary electrostatic results with XGC will be presented for linear ion-scale calculations in Wendelstein 7-X, global to the last closed flux surface. |
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NP11.00055: Gyrokinetic full-f study on the role of the X-point height on pedestal structure and L-H transition in tokamak Jugal Chowdhury, Robert Hager, Seung-Hoe Ku, Randy Michael Churchill, C-S Chang The change in the X-point height can alter the location where the outer strike point hits the divertor. It may even change the strike point from the horizontal to the vertical divertor plates. Such an event may influence the neutral particle penetration into the pedestal region which in turn can change the pedestal physics in H-mode. We investigate the X-point height effect on EXB flow profile in the presence of neutral recycling, without however modeling the vacuum pump. For this purpose we use the total-f global gyrokinetic neoclassical code XGCa. Results show that the neoclassical Er profile is sensitive to the vertical X-point location, which implies that the ExB profile could cause a difference in the pedestal physics such as the ELM stability and turbulent/neoclassical transport. |
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NP11.00056: Global gyrokinetic PIC simulation of Alfven wave, ITG and KBM in GTS Chenhao Ma, Weixing Wang, Edward A. Startsev, Stephane Ethier We report the recent development in the electromagnetic simulations for general toroidal geometry based on the global particle-in-cell gyrokinetic code GTS. Because of the cancellation problem, the EM gyrokinetic simulation has numerical difficulties in the MHD limit where kperp ρi≈0 and/or β>me/mi. Several approaches have been implemented in the GTS framework to circumvent this problem: (1) p|| formulation with analytical skin depth term iteratively approximated by simulation particles (Yang Chen), (2) A modified p|| formulation with ∫E|| dt used in place of A|| (Mishichenko). Our simulation shows that both of these approaches successfully mitigate the cancellation problem in the simulation of shear Alfven wave for large aspect ratio geometry. The simulation of electromagnetic ion temperature gradient mode (ITG) and kinetic ballooning mode (KBM) will be presented based on the Cyclone Base Case parameters. |
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NP11.00057: Continuum Gyrokinetic Simulations of Turbulence in Model Tokamak Scrape-Off Layer Geometry Ammar Hakim, Eric Shi, Gregory W Hammett, Noah Mandell, Tess Bernard, Manaure Francisquez We describe results obtained from Gkeyll, a full-F continuum gyrokinetic code, designed to study turbulence in the edge region of fusion devices. The edge region is computationally challenging, requiring robust algorithms that can handle large amplitude fluctuations and stable interactions with material walls. We have designed an energy conserving high-order discontinuous Galerkin scheme that solves the gyrokinetic equations in Hamiltonian form. Efficiency is improved by a careful choice of basis functions and automatically generated computation kernels. Plasma interaction with material walls is handled by model sheath boundary conditions that allow current to flow into/out of the wall, while maintaining overall quasineutrality. Verification tests have been performed in the straight field-line LAPD machine[1, 2]. Results for turbulence in a scrape-off layer (SOL) for NSTX-type parameters with a model magnetic geometry have been obtained [2]. A version of the code for the full Vlasov-Maxwell equations with applications to solar wind and Hall thrusters will be briefly described. |
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NP11.00058: An energy- and momentum-conserving (gyro)kinetic Lenard-Bernstein collision operator in helical, open-field line continuum simulations Tess Bernard, Ammar Hakim, Manaure Francisquez, Gregory W Hammett, Eric Shi, Noah R Mandell, James L. Juno The Lenard-Bernstein collision operator (LBO) has been implemented in Gkeyll for use in a Vlasov-Maxwell system and in a gyrokinetic system. Gkeyll is a continuum plasma physics code that uses a modal discontinuous Galerkin (DG) computational method. A novel calculation employing a DG weak form of division is used for the fluid velocity and thermal velocity moments that appear in the LBO, in order to conserve particles, momentum and kinetic energy. Simple 1x-2v and 1x-3v tests demonstrate an evolution to the correct steady state. Furthermore, the gyrokinetic LBO in Gkeyll has been tested in up to 3x-2v dimensions. Building upon work by E. Shi [1,2], simulations with open, helical field lines are performed in different parameter regimes, including the Texas Helimak experiment and the scrape-off layer of NSTX. These results are compared with turbulence measurements from the Helimak. [1] Shi, E. L., et al. (2017). J. Plasma Phys., http://doi.org/10.1017/S002237781700037X [2] Shi, E.L., Princeton Ph.D. (2017), arXiv:1708.07283v1 |
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NP11.00059: Electromagnetic Continuum Gyrokinetic Turbulence Simulations in the Tokamak Edge Noah Mandell, Gregory W Hammett, Ammar Hakim
Gkeyll, a full-F continuum gyrokinetic code, is being developed to study turbulence in the edge region of fusion devices. The edge region involves large-amplitude fluctuations, electromagnetic effects, and plasma interactions with material walls, making it more computationally challenging than the core region. Gkeyll models the turbulence by solving the 5-D full-F gyrokinetic system in Hamiltonian form using an energy conserving high-order discontinuous Galerkin scheme. We present an extension of the scheme to include self-consistent electromagnetic perturbations that maintains the energy conservation properties of the scheme. We describe our choice of the symplectic (v||) formulation of electromagnetic gyrokinetics instead of the Hamiltonian (p||) formulation. We discuss how our scheme avoids the Ampere cancellation problem. We present some preliminary electromagnetic turbulence results. |
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NP11.00060: Exponential reconstructions and positivity for discontinuous Galerkin algorithms, and linear benchmarks of the Gkeyll gyrokinetic code Gregory W Hammett, Ammar Hakim, Tess Bernard, Henry Burns, Noah R Mandell, Eric Shi One of the challenges of simulating turbulence in the edge regions of tokamaks is handling the large amplitude fluctuations that can occur and avoiding negative density overshoots, which might cause various problems. One of the attractive features of discontinuous Galerkin (DG) algorithms is that they can conserve particles and energy exactly for Hamiltonian systems even if limiters are used for fluxes at cell boundaries. To preserve the realizability of the solution within a cell, there are situations where it is actually necessary to enhance the boundary flux, not limit it. We show a new method that accomplishes this with exponential reconstructions, while still preserving exact particle and energy conservation. Exponential reconstructions are also helpful for handling the infinite velocity domain. We demonstrate the algorithm in 1D with a series of tests, show that this gives a systematic local method of ensuring positivity and realizability of the solution, and show some tests in multiple dimensions. Finally, we demonstrate some linear benchmark tests of the full 3D+2v Gkeyll gyrokinetic code, comparing with the toroidal ITG linear dispersion relation over a range of parameters. |
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NP11.00061: Regimes of weak ITG/TEM modes to enable strong transport barriers for fusion Michael T. Kotschenreuther, Xing Liu, David R Hatch, Swadesh Mitter Mahajan Recent gyrokinetic calculations find that low velocity shear in burning tokamak H-modes can enable the reemergence of electrostatic modes in the pedestal, that are suppressed in present tokamaks, resulting in excessive transport1,2. Here we explicate regimes that hugely weaken these modes to enable the low transport needed for burning plasmas. We employ GENE and analytic theories. This result is unexpected, given the diversity of destabilization mechanisms, and the expectation of strong driving from steep gradients. Very different properties are found for Ion Temperature Gradient Modes coupled to Trapped Electron Modes (ITG/TEM), compared to the core. Simulations are deciphered using a Simplified KInetic Model (SKIM), which includes the panoply of destabilizing effects: passing resonances for ions and electrons, trapped electron effects, and curvature effects. SKIM includes critical physical effects with exceptional transparency, agrees well with GENE, and substantiates the robustness of the regimes. 1 M Kotschenreuther, D R Hatch, S Mahajan, et. al. , Nucl. Fusion 57 (2017) 064001 2 D R Hatch , R D Hazeltine, M Kotschenreuther, S Mahajan PPCF 60 (2018) 084003 |
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NP11.00062: Analysis of Finite-β Ion Temperature Gradient Turbulence Saturation Paul Willis Terry, Garth Whelan, Moritz J Pueschel, Ping-Yu Li, Taweesak Jitsuk, Benjamin Faber Gyrokinetic simulations of finite-β ion temperature gradient (ITG) turbulence for Cyclone-base-case conditions show strong dependence of the ion heat flux on the triplet correlation time of the interaction of unstable modes, zonal flows and stable modes1. Analysis of a simpler fluid model dominated by the same saturation channel shows that the β scaling of the correlation time arises when the detuning of the three-wave resonance by finite Larmor radius induced dispersion uncovers the β dependence of the complex mode frequency. Consequently, the triplet correlation time has strong dependence on both β and inverse perpendicular and parallel wavenumbers. A five-field finite-β ITG model is derived and studied in low β and strong ballooning limits to further investigate the triplet correlation time and specifically, the role of parallel mode structure, whether the ion parallel flow can decouple as it does at zero β, and whether flutter nonlinearities can compete with electrostatic zonal flow catalyzed transfer from unstable to stable modes. The role of marginally stable modes, particularly in lower β regimes, is investigated, and parameter space is searched for regions with low fluxes. 1. G.G. Whelan et al., PRL 120, 175002 (2018). |
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NP11.00063: Progress with the 5D continuum gyrokinetic code COGENT: moving toward an edge turbulence code M. Dorf, M. Dorr, A. Dimits, D. Martin, P. Colella The presence of a strong magnetic shear in the edge of a diverted tokamak provides a significant challenge for continuum numerical modelling of edge microturbulence, which generates highly-anisotropic perturbations aligned with the magnetic field. In this work we propose a finite-volume mapped multiblock approach to the discretization of a gyrokinetic system of equations for plasma species and electrostatic fields that can accommodate large values of a magnetic shear and can handle an X-point geometry. In this approach, the toroidal direction is divided into blocks, such that within each block cells are field-aligned and a non-matching (non-conformal) grid interface is allowed at block boundaries. The toroidal angle is playing the role of the “coarse” field-aligned coordinate, whereas the poloidal cross-section, comprised of the radial and poloidal directions, is finely gridded to resolve short-scale perpendicular turbulence structures and to support accurate re-mapping (interpolation) at block boundaries. The method is implemented in the gyrokinetic code COGENT and the results of initial simulations in simplified geometries are presented. |
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NP11.00064: Characterizing near-edge DIII-D L-mode plasmas with gyrokinetic GENE simulations Tom F Neiser, Frank Jenko, Troy A Carter, Lothar Schmitz, Paul Crandall, Gabriele Merlo, Daniel Told, Alejandro Banon Navarro, George R McKee, Zheng Yan Studying the L-mode edge is an important prerequisite for understanding the L-H Transition. Moreover, correctly predicting L-mode profiles is important for vertical stabilization of the plasma during current ramp-up and ramp-down phases of ITER. Here we present nonlinear gyrokinetic simulations with GENE of a DIII-D L-mode plasma in the near-edge, which we loosely define as 0.80 ≤ ρ ≤ 0.95. We have previously reported on flux-matched single-scale simulations at ρ=0.80. These have been followed up by multi-scale simulations, which indicate that a widely used heuristic rule on the importance of multi-scale effects is on the cautious side: Preliminary results show that single-scale simulations are sufficient when multi-scale effects were expected. At ρ=0.90, single-scale flux-matched simulations indicate that the E×B shearing rate plays an important role already in the near-edge and is predicted to become increasingly important in the edge-region of L-mode plasmas. Linear and non-linear simulations at ρ=0.95 are currently underway and will be presented. |
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NP11.00065: Gyrokinetic Simulations of JET Pedestals David R Hatch, Michael T. Kotschenreuther, Swadesh Mitter Mahajan, Xing Liu, Samuli Saarelma, Jon C Hillesheim, Costanza Maggi, Carine Giroud, Anthony Field Gyrokinetic simulations using the GENE code target a fundamental understanding of JET pedestal transport and, in particular, its modification after installation of an ITER like wall (ILW). In a representative carbon wall discharge, magnetic diagnostics identify washboard modes, which preferentially affect the temperature pedestal. Linear gyrokinetic simulations identify microtearing modes with scale lengths and frequencies comparable to magnetic fluctuation data. A similar ILW discharge is examined, which recovers a similar value of H98, albeit at reduced pedestal temperature. This discharge is distinguished by a much higher value of η, which produces strong ion temperature gradient (ITG) and electron temperature gradient driven instabilities. Global nonlinear simulations indicate that ITG turbulence produces substantial heat flux and very little particle flux. Sensitivity tests varying the density gradient demonstrate an ITG particle pinch that balances diffusion, suggesting that ITG may mediate the density profile. The ITG heat flux exhibits a scaling with ExB shear rate that agrees with basic theory [Hatch et al PPCF 2018] and suggests high sensitivity to ρ*. |
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NP11.00066: Conversion from Gyrokinetic to Full-Orbit Particles for Plasma Sheath Boundary Conditions Rishi Pandit, Davide Curreli, Jong Youl Choi, Robert Hager, Michael Churchill, C-S Chang A general issue of gyrokinetic models of magnetized plasmas is handling boundary conditions which can properly account for the full-orbit physics of the particles in the plasma sheath and magnetic presheath region at the plasma-wall transition. In this work we address the problem of matching a gyrokinetic model to a full-orbit model by enforcing conservation laws at the interface between the two models. We explore a two-region model consisting of a bulk plasma described using gyrokinetics, plus a boundary layer described using a full-orbit model. We enforce the matching conditions by guaranteeing continuity on the moments of the particle distributions up to a given order (density, particle fluxes, energy fluxes, etc.). The approach is general enough that moments up to an arbitrary order can be enforced at the interface. We present preliminary numerical tests implemented using the two codes XGCa (gyrokinetic) and hPIC (full-orbit) as a test bench.
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NP11.00067: Understanding the RMP and density pump-out physics from a coupled gyrokinetic-MHD simulation Robert Hager, Nathaniel Mandrachia Ferraro, C-S Chang, Raffi Nazikian Density pump-out caused by external resonant magnetic perturbations (RMP) is studied in a model DIII-D discharge with advanced coupled gyrokinetic-MHD simulation. RMP has been accepted into the ITER design as the primary control tool to suppress edge localized modes (ELMs). Strong RMPs, however, often reduce edge particle confinement (pump-out), which degrades fusion efficiency. In our advanced model, the perturbed plasma equilibrium is calculated with the M3D-C1 code. This equilibrium (including the RMP) is coupled into the edge gyrokinetic XGC suite of codes, which calculate neoclassical and turbulent transport, background plasma and ExB profile evolution including neutral particle physics and X-point orbit loss. In the zero-turbulence limit, we find a significant increase of (neoclassical) radial particle fluxes to levels similar to experimental observations. Edge turbulence intensity rises up to 50% due to the RMPs, mostly from the weakening of the ExB shearing rate, which is not enough to explain the density pump-out without the increase of neoclassical fluxes. |
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NP11.00068: Effects of RMP-Induced Changes of Radial Electric Fields on Microturbulence in DIII-D Pedestal Top Sam Taimourzadeh, Lei Shi, Zhihong Lin, Raffi Nazikian, Ihor Holod, Donald Spong Gyrokinetic simulations of the DIII-D tokamak with axisymmetric equilibrium show that the reduction in the radial electric field shear at the top of the pedestal during edge localized mode (ELM) suppression with the $n=2$ resonant magnetic perturbations (RMPs) leads to enhanced drift-wave turbulence and extended turbulence spreading to the top of the pedestal relative to ELMing plasmas with similar pedestal parameters. The simulated turbulent transport at the top of the pedestal in ELM suppressed conditions is consistent with experimental observations of enhanced turbulence at the top of the pedestal during ELM suppression by the RMPs. These results suggest that enhanced drift-wave turbulence can contribute to the additional transport required to prevent the pedestal growing to an unstable width. |
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NP11.00069: Eulerian variational formulations and momentum balance equations for kinetic plasma systems Hideo Sugama, Masanori Nunami, Shinsuke Satake, Tomo-Hiko Watanabe Eulerian variational formulations are presented to derive governing equations for kinetic plasma systems. As examples, the Vlasov-Poisson-Amp`ere system and the drift kinetic systems are investigated. For the drift kinetic system, the additional case is also considered in which the quasineutrality condition and Amp`ere’s law are included as supplementary governing equations to describe the self-consistent fields. For all cases treated here, general spatial coordinates are used to represent the action integrals and the derived governing equations which take the forms being invariant under an arbitrary transformation of spatial coordinates. Furthermore, the invariance of the action integral under the spatial coordinate transformation is made use of to derive the momentum conservation laws and/or the momentum balance in which the functional derivatives of the Lagrangians with respect to the metric tensor components yield the proper symmetric pressure tensors more directly than conventional techniques using translational and rotational symmetries or taking the moments of the kinetic equations. |
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NP11.00070: Physics of Scale Selection for Mesoscale Patterns in Drift Wave Turbulence Guo Weixin Recently, a spatially quasi-periodic and temporally long-lived ExB pattern was reported in numerical and experimental works. A theoretical model was then developed and found that a positive feedback process drives the staircase formation via a Rhines scale dependent mixing length. The main questions are: What is the principal feedback loop physics? In this work, we study several shearing feedback mechanisms and compare them to the Rhines scale loop. The preliminary results show that nonlinear density gradient dependence of mixing length is key loop. Surprisingly, shearing feedback is in-effective. Moreover, the emergence of staircases as well as the sustainment of staircase structure, are very sensitive to the driving and damping parameters. Furthermore, we also find that the initial condition has significant effects on staircase structure. Increasing the initial mean vorticity (mean shear) weakens the staircase by suppressing drift waves, and inputting a spatially oscillating (zonal) initial shear pattern produces a persistent footprint on the staircase pattern. |
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NP11.00071: Phase Dynamics and Energetics in Drift Wave-Zonal Flow Turbulence Hoony Kang, Patrick Henry Diamond The roles of wave phase structure and evolution in zonal flow dynamics are investigated. Previous work (Guo, Diamond; 2016) has noted that wave phase curvature is sufficient to produce a gradient in the Reynolds force and thus to drive zonal flow formation — even for turbulence of homogeneous fluctuation intensity. In this study, we investigate the energetics and nonlinear evolution of the process. For energetics, refraction (i.e. changes in phase gradient due to Z.F. shear) allows wave energy — and thus the ZF — to evolve at fixed wave amplitude. Using the phase evolution equation derived for the Hasegawa-Mima model, we prove energy conservation between waves and ZF, for fixed wave potential intensity. By separating the mesoscale phase into mean and fluctuation, we show that phase evolution induces ZF formation due to phase gradient shocks. Then, we investigate how linear and nonlinear feedback of the gradient affect the synchronization of the shock structure. Finally, we consider dynamic amplitude to explore the full predator-prey relationship between ZF (predator) and drift waves (prey). |
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NP11.00072: Zonal Flow Dynamics and Shear Layer Collapse in High-Density regime Mikhail A Malkov, Reema Hajjar, Patrick Henry Diamond, Zhibin Guo We report on a numerical study of the Hasegawa-Wakatani drift-wave model that we have recently adopted for describing the zonal flow drift wave (ZF-DW) interactions. A quasi-linear derivation of the system is provided in detail elsewhere (R.J. Hajjar et al. Physics of Plasmas 25, 062306 (2018); doi: 10.1063/1.5030345). It consists of the three equations for averaged plasma density, and zonal flow speed, along with the fluctuating fields accounted for by the enstrophy. The latter is introduced as an averaged square of potential vorticity fluctuations, by analogy to the quantity oft-used in the studies of geostrophic flows. We investigate the generation of zonal flow and collapse of an edge shear layer depending on the adiabaticity parameter. Critical issues to be addressed here include the radial scale over which ZFs collapse and depth of penetration of the resulting inward turbulence spreading. The work is ongoing, and we will add a particle source at the edge along with simple treatment of heat transport to investigate the depth of cooling resulting from the ZF collapse. |
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NP11.00073: A closer look at turbulence spreading: how bistability admits intermittent, propagating turbulence fronts Robin Heinonen, Patrick Henry Diamond In magnetic fusion plasmas, the observation of hysteresis in the turbulence intensity level (Inagaki et al. 2013) strongly suggests bistability. Commonly employed reduced models for turbulence spreading are unistable. In this work, a minimal phenomenological model for \emph{bistable} turbulence spreading is introduced and analyzed. The model is related to the FitzHugh-Nagumo system. In addition to to being able to account for hysteresis, we show that, in contrast to previous models of turbulence spreading, this model predicts stronger penetration of turbulence into the stable core as well as the existence of avalanche-like, intermittent turbulence pulses. We also derive a novel spatial size threshold for the spreading of a localized slug of turbulence. This lays a groundwork for future models, which should include couplings to zonal flow and profiles. |
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NP11.00074: What is the role of intermittency in determining the plasma profiles in the SOL? Edward Taylor, William L. Rowan, Wendell Horton The exhaust of particles and heat in magnetic fusion devices determines the level of interaction between the plasma and material surfaces. A previous study observed cross-field transport increasing with distance into the SOL. In addition, experiments have reported flattening of density and temperature profiles a certain distance from the separatrix. These suggest increased interaction with the first wall than generally thought and motivate a focus on the interaction relative to the divertor. The SOL is observed to be very turbulent with blobs erupting from the core and accounting for much of the transport in the far-SOL. A framework describing how blob dynamics generate SOL profiles and fluctuation statistics is used to interpret experimental profile and fluctuation data in the Helimak, a device that models the SOL of a tokamak. Langmuir probes are used for determination of fluctuation statistics and equilibrium profiles. The inputs to the Militello framework are experimentally or theoretically derived models for draining, radial blob velocity, and amplitude and width probability distributions. |
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NP11.00075: Transition from weak to strong turbulence in magnetized plasmas Vasil Bratanov, Swadesh Mitter Mahajan, David R Hatch The scaling of turbulent heat flux with respect to electrostatic potential is examined in the framework of a reduced (4D) kinetic system describing electrostatic turbulence in magnetized plasmas excited by the ion temperature gradient instability. Numerical simulations were instigated by, and tested the predictions of generic renormalized turbulence models like the 2D fluid model for electrostatic turbulence [Y. Z. Zhang and S. M. Mahajan, Phys. Fluids B 5 (7), pp. 2000 (1993)]. A fundamental, perhaps, universal result of this theory-simulation combination is the demonstration that there exist two distinct asymptotic states (that can be classified as Weak turbulence (WT) and Strong turbulence (ST) states) where the turbulent diffusivity Q scales quite differently with the strength of turbulence measured by the electrostatic energy ||φ||2. In the case of WT Q is proportional to ||φ||2, while in ST Q has a weaker dependence on the electrostatic energy and scales as ||φ||2. |
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NP11.00076: Turbulence and Transport in Strong Interchange-Type Turbulence Kenneth Gentle, Mark E Koepke, Samuel H Nogami The Helimak is an approximation to the infinite cylindrical slab with a size large compared with turbulence transverse scale lengths, but with open field lines of finite length. A pressure gradient in unfavorable magnetic curvature is unstable to interchange-type modes, leading to large amplitude nonlinear fluctuations similar to those in a tokamak SOL. A novel magnetically-baffled probe cluster permits full characterization of the turbulence, including density, temperature and plasma potential fluctuations as well as particle and thermal radial transport rates across the full plasma profile. Turbulence varies in a complex way with plasma parameters, but it can be most strongly modified by the application of bias to alter the transverse (poloidal, orthogonal to B and R) flow patterns. Despite the short coherence lengths, the level of saturated turbulence cannot be inferred from local parameters. The transport is mediated by two, often independent, mechanisms. The flows change the amplitudes of the fluctuating fields responsible for the transport. The flows also change the coherence between the fields (seen in either time or frequency domains), leading to changed net transport. Work partially supported by the Department of Energy OFES DE-FG02-04ER54766. |
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NP11.00077: Effects of imposed electric fields on turbulence-induced fluxes associated with strong interchange instabilities Samuel H Nogami, Mark E Koepke, Vladimir I. Demidov, Kenneth W Gentle The Texas Helimak, an experimental approximation of the infinite cylindrical slab with dimensionless parameters similar to those of a tokamak scrape-off layer [Gentle and Huang, Plasma Sci. Technol. 10, 284, (2008)], is dominated by strong interchange turbulence. Magnetically-insulated baffled-probe compact-cluster enables real-time measurement of the fields, flows, and fluctuations associated with equilibrium discharges in the Helimak [Rev. Sci. Instrum., in prep.] and the relative phases between the oscillations. Measurement confirms that thermal and particle transport rates are reduced when external bias is applied to introduce a radially varying radial electric field and, thus, enhance plasma poloidal flow. We present results from recent experiments that were designed to compare transport-related measurements from the probes within the compact-cluster as the cluster is radially translated from the low-field-side of the plasma density peak to the high-field-side. |
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NP11.00078: Approaches to decomposing turbulence during an L-to-H mode transition in DIII-D tokamak Serdar A Bilgili, Samuel H Nogami, Mark E Koepke, Kenneth W Gentle Previous works analyzing the Low-to-High confinement-mode transition in tokamaks depict a transient event involving nonlinear interactions between flows, turbulence, and inhomogeneous velocity shear [Phys.Fluids B2, 1 (1990)]. The modal ansatz adopted in neoclassical theory describes only the time-asymptotic behavior of instabilities, neglecting local finite-time effects and other transient growth mechanisms. Simulations are consistent with important experimental observations, but fall short in explaining the finite-time behavior of the transition dynamics [Phys. Plasmas 4, 1736 (1997)], pointing to the necessity to refine the theory to incorporate the interactions of evolving non-orthogonal (non-normal) modes. We treat the transient growth of the perturbation as being physically equivalent to the development of an instability. We have identified transients in the DIII-D shot data, explained transients using non-modal decomposition of turbulence [Annu. Rev. Fluid Mech. 39:129–62 (2007)] in the presence of velocity shear [Phys. Rev. E 66, 066409 (2002)], and distinguished stationary from nonstationary dynamics. Examples of the above, in theory, in experiment, and in conceptual illustrations, will be presented. |
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NP11.00079: Phase dynamics of edge transport bifurcation in plasma interchange turbulence Xueyun Wang, Yong Lang, Bo Li The mechanism of turbulence suppression of transport bifurcation is investigated using a flux-driven nonlinear fluid model. Both spatial structures and frequency spectrums of potential during different states are shown, revealing the process of turbulence decorrelation by the mean E × B shear flow. Along with the amplitude of fluctuations, the phase angle between pressure and radial E × B velocity fluctuations is also modified dramatically by the self-generated mean E × B shear flow, which contributes to turbulence suppression as well. |
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NP11.00080: Analytical and numerical studies of electron-temperature-gradient-driven inverse cascade of energy Lucio M Milanese, Nuno F Loureiro, Alexander A Schekochihin, William Dorland Recent numerical results demonstrated that the interaction between ion-scale and electron-scale turbulence in tokamaks plays an important role in setting the overall level of energy transport (Howard et. al, 2015). One of the principal cross-scale interaction mechanisms is the inverse cascade of energy from electron scales to ion scales driven by unstable electron-temperature-gradient (ETG) modes. This inverse cascade mechanism is not understood. We report on a set of novel fluid-kinetic simulations and analytical results aimed at elucidating the physical mechanisms underpinning the ETG inverse cascade in slab geometry, using a reduced gyrokinetic model (Zocco and Schekochihin, 2011). |
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NP11.00081: Tornado-like transport in a heated magnetized plasma Matthew J Poulos, Suying Jin, Bart G.P. Van Compernolle, George J Morales A transient, large-scale, spiraling flow-pattern, akin to a tornado, is observed to spontaneously form in a transport experiment performed in the Large Plasma Device (LAPD) at UCLA in which a magnetized, ring-shaped region of elevated temperature is created within a large, pre-existing, cold plasma. The structure is generated by applying a voltage pulse (~15V) to a thermionic-emitting LaB6 ring cathode during the afterglow phase of the main discharge. Significant density changes (~30%) are produced far from the heated region by the spiral arms. The presence of sheared flow introduces azimuthal phase-mixing that relaxes the tornado to an organized state of marginal stability, priming the plasma for future avalanche-like events. Detailed measurements of the spatio-temporal evolution of the tornado and its impact on cross-field transport are compared to the predictions of a Braginskii transport model that incorporates the self-consistent evolution of vorticity sourced by emissive-sheath boundaries. |
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NP11.00082: Follow the Power – Pathways to Steady State Reactors Craig Petty Diagramming pathways of normalized power is a potent method for extrapolating between operating points in current tokamaks and future burning plasma devices. The current drive power, loss power from transport and L-H threshold power can be expressed in dimensionally correct form using the normalized power Pa3/4 (where a is the minor radius), with the relative gyroradius (ρ*) dependence ranging from gyroBohm-like for transport (Pa3/4~ρ*-1.5), Bohm-like for current drive (Pa3/4~ρ*-2.5) and worse than Bohm-like for L-H transition (Pa3/4~ρ*-3). The D-T fusion power cannot be normalized in the same fashion since it is governed by nuclear physics, but at fixed BT it scales like Pa3/4~ρ*-4.5. Diagramming these normalized powers vs. ρ* for fully non-inductive scenarios shows a specific pathway corridor between DIII-D and ITER, capped off at small ρ* by the Greenwald density limit, with the more favorable pathways residing at high β and BT. Using a steady-state hybrid with βN<4 from DIII-D as the starting point, reactors with ITER’s BT=5.3 T achieve high fusion gain (Q~20) only at small ρ*, whereas reactors with BT=10 T have a pathway to Q~20 even at large ρ* (e.g., a JET-sized device). |
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NP11.00083: Modeling Expansion of the DIII-D Steady-State Operating Regimes John R Ferron, Jin Myung Park Equilibrium and transport-code-based modeling are being used to evaluate the benefits for DIII-D fully noninductive operation (fNI=1) at high βN and either high qmin or high ℓi of increasing the triangularity (δ), the plasma density, and/or the toroidal field strength. The δβN increase (to above 6) of the ideal n=1 stability limit enabled by a δ increase from 0.65 to 0.85 would ensure the required stability margin at fNI=1. However, increasing δ at fixed q95 results in lower bootstrap current fraction (fBS) as the total bootstrap current increases more slowly than Ip. Core-edge coupling physics at the ITER pedestal pressure would be possible at BT>3 T and ne>0.85fG. Achieving the ITER collision rate, though, requires the high BT but lower ne. Although the ne gradient should most efficiently generate bootstrap current, at fixed βN simply increasing ne does not increase fBS because the collision rate increases. fNI=1 operation at high ℓi is examined in detail using transport-code-based modeling, initially by validating the TGLF transport model against experimental discharges. |
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NP11.00084: Extrapolation of DIII-D Steady-State Scenarios to ITER Jin Myung Park, John Roderick Ferron, Craig Petty, Christopher T Holcomb, Francesca Turco DIII-D is exploring a range of potential steady-state scenarios including high qmin, steady-state hybrid, and high li in the scaled ITER shape. The FASTRAN modeling self-consistent with core transport (TGLF), edge pedestal (EPED1), external heating/current drive (NUBEAM, TORAY), and low-n ideal/resistive stabilities (DCON, PEST3) has been tested extensively against these experiments, reproducing the measured density, temperature, rotation and current profiles reasonably well without any significant free parameters other than anomalous current diffusion in steady-state hybrid and uncertainty of the MHD-induced beam ion loss. Extrapolations to ITER from DIII-D using the same theory-based models show that the fusion performance Q increases rapidly with the density at the fully non-inductive condition, fNI=1 in conjunction with the increased bootstrap current fraction and improved energy confinement. All three scenarios achieve Q>3 using the “Day-1” heating/current drive if the normalized density, ne,ped/nGW approaches to 1. An optimum choice of the H/CD upgrades for each scenario is investigated to achieve the fNI=1 and Q=5 ITER mission. |
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NP11.00085: Transport Variations in High qmin>1.5 Plasmas with Heating and Current Drive Actuators K.E. Thome, J.R. Ferron, C.C. Petty, B.S. Victor, C.T. Holcomb, E. Schuster, W Wehner, J.M. Park, B.A. Grierson, F.M. Poli ‘Predict-first’ methodology was employed to develop an experimental plan on DIII-D to study the effect of heating and current drive actuators on the current and pressure profiles of high-qmin>1.5, βN>3 plasmas, with the goal of achieving stable unchanging Vloop profiles as early in the discharge as possible to avoid the appearance of deleterious tearing modes. Predictive transport modeling conducted with TGLF in TRANSP indicated how varying the Ip ramp rate, ECCD timing, and NBI timing would change these profiles. During the experiment, qmin>2 discharges were sustained and their tearing instability, qmin, and performance was altered by these actuators. Changing the Ip ramp rate had little effect on the discharge but applying ECCD and/or NBI earlier increased qmin and improved n=2 tearing stability. TGLF predicted the effect of ECCD but not the other results. Increased fast-ion transport was also observed. Comparison of pre-experiment modeling, experimental results including turbulence data, and post-experiment simulations will be discussed in detail. These results will be used to validate transport models in this regime and to prepare for the upcoming 2019 campaign when increased off-axis co-Ip NBI power will be available. |
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NP11.00086: Evolution of the current density profile in advanced tokamak scenario plasmas Brian S Victor, Christopher T Holcomb, Steven L Allen, K.E. Thome, John Roderick Ferron, C. Craig Petty, Jin Myung Park, Eugenio Schuster, Will Wehner, Brian A Grierson, Francesca M Poli Recent experiments on DIII-D have been performed to study the evolution of the current density profile in high qmin plasmas (qmin > 2). Modifying the plasma current (Ip) ramp rate and the timing and power of the neutral beam (NBI) and electron-cyclotron current drive (ECCD) changes the distribution of Ohmic current and the shape of the current density profile. NBI drives current in the inner-half radius of the plasma and increases the pressure gradient, which increases the bootstrap current. Applying ECCD earlier in the discharge increases mid-radius current leading to a broader current density profile and higher qmin. Changing the Ip ramp rate has the largest effect on the edge loop voltage. This knowledge is then applied to predict effects of the ongoing DIII-D upgrade, which will increase the off-axis NBI current drive and power in the co-current direction. Increased off-axis NBI current drive will broaden the current density profile and raise qmin. Increased co-current NBI power will benefit the high qmin scenario through higher plasma rotation. The research goal is to achieve fully non-inductive operations with βn > 4, and zero Ohmic current density across the profile after the DIII-D upgrade. |
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NP11.00087: Prospects for ITER Q=10 operation without harmful core MHD and without harmful ELMs Andrea M Garofalo, Carlos A Paz-Soldan, David B Weisberg, Jeremy M Hanson, Francesca Turco, Alessandro Bortolon, Raffi Nazikian, Theresa M Wilks The DIII-D program is assessing options for Q=10 equivalent operation without harmful core MHD or harmful ELMs, integrated with ITER-relevant low collisionality and input torque (or rotation). Experiments have yielded robust Q~10 performance either at zero torque in high collisionality ELMing plasmas, or in low collisionality ELM-stable plasmas with finite NBI torque. Two main lines of research are being pursued, distinguished by the ELM control approach: RMP ELM suppression, and QH-mode edge. Various challenges prevent simultaneous achievement of Q=10 performance without ELMs at low collisionality and low rotation. At high collisonality, the core is stable even at low rotation, but RMP suppression appears precluded, and RMP application tends to cause core instabilities in the experiments so far. At low collisionality, both QH-mode and RMP suppression are possible. However, RMP suppression is lost when the torque is reduced from the co-Ip side, while n=1 core modes are destabilized in QH-mode when the torque is reduced from the counter-Ip side. Plans for further progress will be discussed. |
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NP11.00088: Development and extension of the non-inductive high beta poloidal regime to ITER relevant dimensionless parameters on DIII-D David B Weisberg, Andrea M Garofalo, Xianzu Gong, Juan Huang, Joseph McClenaghan, Siye Ding The high βp scenario on DIII-D has been extended to higher plasma current and higher βN in the pursuit of ITER relevant dimensionless parameters. This pursuit is motivated by the fusion gain metric G=βNH89p/q952, which is required to be ~0.27 in the ITER Q=5 steady state mission. High βp experiments on DIII-D maximize this metric in steady-state relevant scenarios through the formation of an internal transport barrier (ITB) and the resulting high confinement and bootstrap fraction. Progress in this ITER-relevant metric will be presented, including details of scenario development and increased understanding of the underlying physics. Inductively-driven DIII-D plasmas with ITBs have achieved high confinement and fusion gain (H89p=2.8, G=0.21) at either higher current or higher βN, but increasing both parameters simultaneously has proven difficult. Obstacles such as the fast external kink mode and wall-interactions of counter-streaming ions will be discussed, as will the varying efficacy of RMP-ELM suppression and resistive wall mode feedback. A correlation between core impurities and ITB collapses will also be presented, suggesting that cyclic drops in confinement occur in the absence of adequate particle control. |
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NP11.00089: Linear Gyrokinetic Analysis of High Poloidal Beta Discharge on DIII-D Siye Ding, Joseph McClenaghan, Andrea MV Garofalo, Gary Staebler, George McKee, Zheng Yan, Wenfeng Guo, Jinping Qian, Xianzu Gong, Chengkang Pan Linear gyrokinetic simulations performed on kinetic equilibrium reconstructions of high poloidal beta DIII-D discharges show the features of the dominant micro-turbulent instabilities. In low-k range, ITG-like and KBM-like modes co-exist outside the large radius internal transport barrier (ITB) in the low pressure gradient region (rho=0.73) as dominant modes for different k_thetas. In these simulations, the ITG-like mode is very marginal to many parameters, e.g. beta, collisionality and ion temperature scale length. Some increase in these parameters will bring a conversion of this mode to KBM-like mode, which has out-of-phase parity in the parallel part of the vector potential. The linear growth rate of the KBM-like mode is proportional to plasma beta. Ion temperature gradient is the major driving force of this instability. In the steep gradient region of ITB, the KBM-like mode can still be seen unstable and the ITG-like mode changes to an electron mode. The comparison with turbulence measurement in experiment will also be presented. |
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NP11.00090: Confinement in DIII-D negative triangularity discharges M.E. Austin, M. Marinoni, G.R. McKee, C.C. Petty, S.P. Smith, K.E. Thome, C. Sung DIII-D discharges with negative triangularity (-δ) shape and an L-mode edge have been shown to have the same stored energy and global confinement as matching positive triangularity discharges (+δ) that are in H mode and have an edge pressure pedestal. Detailed profile analysis has been carried out to determine how the transport compares in the two shapes. It is seen that while the -δ shape has lower edge confinement, it has higher confinement in the region 0.3 < r/a < 0.8 which makes up the difference. The improved core confinement is attributed to reduced turbulent transport (consistent with measured turbulence reduction), due to effect of shape on the dominant TEM modes and also on the increased Shafranov shift and higher plasma rotation. Profile studies also show that the negative triangularity has significantly higher bootstrap current than the comparison positive triangularity discharge. TRANSP runs indicate as much as 30% higher bootstrap current in negative over positive discharges consistent with loop voltage analysis of these shots. |
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NP11.00091: L-H Transition Dynamics in ITER-Similar Plasmas with Applied n=3 Resonant Magnetic Perturbations L. Schmitz, T.L. Rhodes, L. Zeng, M. Kriete, Z. Yan, G. R. McKee, R. Wilcox, C. Paz-Soldan, P. Gohil, C. C. Petty, A. Marinoni In ITER-similar plasmas in DIII-D (<ne>=1.5-5x1019m-3, Bt=1.9-2T, Ip=1.5MA, q95~3.6), the L-H threshold power PLH with n=3 Resonant Magnetic Perturbations (RMP) is found to increase with decreasing collisionality [PLH~(nu*)-0.3]. This is a concern for H-mode access in primarily ECH-heated ITER plasmas since RMP may be applied before the L-H transition to safely suppress the first ELM. Non-axisymmetric modifications of the L-mode shear layer with RMP include a substantial local reduction of the Er well and ExB shear (including Er reversal at high RMP field), and spatially modulated low-wavenumber turbulence amplitude, increasing substantially on field lines mapped to high RMP field. Two-fluid M3D-C1 modeling [1,2] shows that the normalized gradients a/L_n, a/L_Te are toroidally modulated and periodically increased on the outboard midplane with RMP. The increase in PLH is atttributed to the combined effects of reduced (and non-axisymmetric) ExB flow / flow shear, and locally enhanced instability drive (via increased kinetic gradients). [1] N.M. Ferraro,Phys. Plasmas 19 056105 (2012); [2] R.S. Wilcox et al. Phys. Plasmas 25 056108 (2018). |
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NP11.00092: Turbulence in the Er well and diagnosing fast-timescale changes with Phase Contrast Imaging on DIII-D Jon Christian Rost, Alessandro Marinoni, Evan Davis, Miklos Porkolab, Keith Burrell Measurements with the Phase Contrast Imaging diagnostic on DIII-D have identified density turbulence situated near the center of the Er well in H-mode plasmas. These fluctuations are seen prominently in ELM-free H-mode and wide-pedestal QH-mode plasmas. The turbulent mode forms on sub-ms times scales after L-H transitions and ELMs, consistent with the time scale for changes in the Er well. The lab-frame phase velocity is approximately equal to the ExB velocity at the center of the Er well and varies with the Er well depth. Observations of this turbulence during limit-cycle oscillations at the L-H transition show that the Er well collapses and reforms on ms timescales, progressively deeper with each cycle. Initial linear simulations with CGYRO have found a high phase velocity, ion scale instability near the center of the Er well in wide-pedestal QH-mode plasmas in general agreement with these experimental observations. These observations provide information on the Er well evolution on a time scale of 10s of μs, much faster than is available by direct observation. |
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NP11.00093: Pedestal-to-SOL Transport by Intermittent Fluctuations During RMP ELM-Suppressed Plasmas George McKee, Zheng Yan, Carlos Paz-Soldan, Max E Austin, Terry L Rhodes Low-wavenumber fluctuation measurements in the outer core (~0.75<rho<0.92) and pedestal (0.95<rho<1) demonstrate that different processes take place in these regions in response to application of Resonant Magnetic Perturbations (RMPs). Inboard of the steep-gradient pedestal region, turbulence is significantly and rapidly (few ms times scale) increased in response to initial application of RMPs, and then further changes (can increase or decrease) when ELMs are fully suppressed. In the pedestal and SOL region, a different phenomenon is observed: “Bursty Wiggles” begin as soon as ELMs are fully suppressed. These intermittent but large amplitude (ñ/n~10%) fluctuation events have a <100 microsecond lifetime and occur at approximately 1 kHz; their frequency band extends to 200 kHz. These events peak in amplitude in the steep gradient pedestal region, and have a poloidal wavenumber k_perp*rho_s=0.05-0.1. These events correlate with bursts of increased density in the near SOL region and have positive skewness. The mechanisms behind this increased particle transport during RMP ELM-suppressed plasmas need to be characterized to accurately predict their impact on confinement.
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NP11.00094: Multiscale fluctuation measurements with a combined phase contrast imaging & interferometer diagnostic on DIII-D Evan M Davis, Jon Christian Rost, Miklos Porkolab, Alessandro Marinoni, Nathan T Howard A novel, combined phase contrast imaging (PCI) and interferometer diagnostic capable of measuring line-integrated electron density fluctuations that extend from magnetohydrodynamic (MHD) scales to the lower range of the electron temperature gradient (ETG) mode has detected evidence of cross-scale coupling in the turbulent density spectrum in DIII-D. In an attempt to elicit a multiscale response, the ETG drive a/LTe was increased locally relative to the ion temperature gradient (ITG) drive a/LTi (while maintaining βN=1.9), and the PCI measured distinct "spectral flattening'" around kρs∼1, which has been found by gyrokinetic simulations to be a tell-tale signature of increased coupling between ITG and ETG scales. The measured spectral flattening is in qualitative agreement with predictions from the reduced model TGLF, but quantitative comparisons await completion of the corresponding multiscale, nonlinear gyrokinetic simulations. Additionally, the interferometer detects a low-k, electromagnetic mode driven unstable by collisionality and ηe, features reminiscent of the micro-tearing mode (MTM); linear gyrokinetic modeling is used to further constrain and identify this mode. |
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NP11.00095: Comparison of Spatiotemporal Turbulence Characteristics in HL-2A and DIII-D Plasmas Xijie Qin, George R McKee, Zheng Yan, Raymond John Fonck, Matt Kriete, Rui Ke, Ting Wu, Min Xu The 2D spatiotemporal characteristics of long-wavelength density fluctuations arising from turbulence are measured and compared on the HL-2A and DIII-D tokamaks with beam emission spectroscopy (BES) to better understand the influence of tokamak geometry, magnetic configuration, local and global plasma parameters, and heating methods on turbulence characteristics and the resulting transport properties. The toroidal magnetic fields and major radii are comparable on HL-2A and DIII-D, while HL-2A has a smaller minor radius and typically one-third the plasma current of DIII-D. Turbulence characteristics are compared with dominant ion and electron heating with NBI, ECH, and LHCD (on HL-2A). The impact of varying ion gyroradius, Te/Ti ratio, and q95 on radial and poloidal correlation lengths, decorrelation times, and fluctuation amplitude profiles and resulting transport behavior will be examined and compared. Analysis of angular momentum transport will be emphasized with an aim to understanding the role of turbulence in intrinsic torque drive and cross-field transport. |
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NP11.00096: Ion-scale turbulence modified by applied magnetic islands in DIII-D Lucas Morton, Morgan W. Shafer, David M Kriete, Todd E Evans, Max E Austin, Daniel J Den Hartog, George R McKee The interaction of ion-scale turbulence with 3D magnetic fields including applied magnetic islands is an emerging area of interest for both tokamaks and stellarators. We present 2D maps of density turbulence characteristics in the presence of large quasi-static magnetic islands using beam emission spectroscopy (BES) at DIII-D. A slowly-rotating (f ~ 1 Hz) n=1 resonant magnetic perturbation (RMP) generated m=2 islands in limited L-mode plasmas. The island structure from vacuum calculations is comparable with measured electron temperature contours. Density fluctuations are enhanced several-fold near the island X-point and reduced near the O-point, compared to the unperturbed equilibrium, in qualitative agreement with predictions from slab models. The turbulence phase velocity and radial correlation length are also modified by the island. The temporal dynamics of the turbulence during the RMP application and island formation are also analyzed. When higher applied torque prevents island formation, the shielded RMP induces a weaker, more radially-uniform response. |
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NP11.00097: Ion Thermal Turbulence Measurements with UF-CHERS in Plasmas with Electron Cyclotron Heating and Impurity Injection Dinh Truong, George R McKee, Zheng Yan, Raymond John Fonck, Choongki Sung, Terry L Rhodes In plasmas with electron cyclotron heating (ECH) confinement degradation is observed as Te/Ti increases. Simultaneous measurements of electron density (via BES, kperp<3 cm-1; reflectometer, kperp<6 cm-1; DBS, kperp~3-6 cm-1; & mm-wave backscattering, kr~35 cm-1) indicate excitation of high-k fluctuations (kperp≥10 cm-1) & low-k electron temperature fluctuations (CECE, kperp<1 cm-1) that propagate in the electron diamagnetic direction. However, these measurements do not explain increased ion energy transport (ion modes wavenumber: kperp≤3 cm-1). In a recent L-mode experiment, ion fluctuations between 0.55≤ρ≤0.85 at near-unity Te/Ti was obtained with the Ultra Fast Charge Exchange Recombination Spectroscopy (UF-CHERS) diagnostic on DIII-D. Initial analysis showed increased Ti & vφ fluctuations when ECH was added to neutral beam injection (NBI) plasmas, compared to NBI-only plasmas. Impurities were injected during ECH to mitigate confinement deterioration & to study the mechanisms behind the reduced turbulence in radiative improved (RI-mode) plasmas. Impurity injection led to partial confinement recovery & reproduced RI-mode results (i.e. increased τE). Further analysis will be presented. |
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NP11.00098: First step towards a synthetic diagnostic for cross-polarization scattering measurements of magnetic turbulence on DIII-D Guiding Wang, Terry L Rhodes, Neal A Crocker, William A Peebles, Kshitish Barada A cross-polarization scattering (CPS) diagnostic measures localized magnetic fluctuations in fusion plasmas. CPS on DIII-D utilizes the probe beam of a Doppler backscattering (DBS) diagnostic combined with a cross-view CPS receiver system, which allows simultaneous measurement of magnetic and density fluctuations with good spatial resolution and wavenumber coverage. A synthetic CPS diagnostic is essential to interpret data and perform detailed validation tests of non-linear turbulence simulations due to the complex plasma propagation of the probe and receive beams. This work reports the first step towards a synthetic CPS diagnostic utilizing GENRAY, a 3-D ray tracing code, to simulate propagation of the DBS probe and CPS scattered rays within the plasma. The factors that determine measured wavenumbers and spatial locations are discussed and initial comparison with experimental data is presented. Future development steps for the synthetic diagnostic include 2 and 3D full wave simulations. |
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NP11.00099: Impact of electron heating on a turbulent transport of middle and high-Z impurities in DIII-D tokamak Tomas Odstrcil, Nathan T Howard, Francesco Sciortino, Pablo Rodriguez Fernandez, Kathreen Thome, Eric Matthias Hollmann Central electron cyclotron heating (ECH) is an effective tool for preventing the accumulation of impurities in the plasma core. It is believed that impurities are removed by a combination of reduced neoclassical pinch and destabilization of turbulent modes. To clarify a role of turbulence, trace Al and W ions were introduced in DIII-D ELM-y H-mode discharges via new laser blow-off system. In these experiments, the balance of ECH and neutral beam injection (NBI) was varied during each shot. Temporal behavior of soft X-ray radiation of these impurities was analyzed with the STRAHL code to obtain radial profiles of impurity transport coefficients. The diffusion of both impurities is enhanced outside of ECH resonance, while inside the transport is neoclassical. Comparison of the measured transport coefficients for Al and W impurity transport with the TGLF and CGYRO modeling will be presented for various heating levels and deposition locations, spanning a wide range of electron/ion heat fluxes and mixes of turbulent modes. |
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NP11.00100: Development of compact charge-exchange detectors and initial testing on DIII-D Dean Buchenauer, Joseph L Barton, Jonathan Watkins, Peter C Stangeby, Dmitry Rudakov, Alec Talin, Dan M Thomas, Ezekial Unterberg The erosion and non-local redeposition of first wall materials due to charge exchange sputtering is predicted to result in 102 - 105 of kg/yr material migration in future tokamaks. To validate these predictions, benchmarking of the coupled edge plasma / neutral codes requires measurements of the energy and spatial distribution of charge exchange fluxes in existing experiments. We are working to develop rugged, compact solid-state detectors that detect atomic hydrogen by sensing the barrier height change of a metal-oxide-semiconductor (MOS) junction. Initial work to develop and calibrate the sensors, and evaluation of sensor response using the Divertor Material Evaluation System (DiMES) on DIII-D are described. Dosemetric measurements made during DIII-D operations have demonstrated sensitivity to the atomic hydrogen flux not only during plasma discharges, but also due to the hydrogen liberated during helium glow discharge conditioning used between shots. Changes in detector response of a few % of the saturation level of the sensors (nominally 2×1015 H /cm2) were observed for NBI discharges. Use of new materials to improve the fabrication of more rugged sensors is also presented. |
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NP11.00101: Role of ExB drift in impurity transport in radiative divertors Rongjie Hong, Thomas Petrie, Brent M Covele, Houyang Guo, Dan M Thomas In DIII-D puff-and-pump experiments, argon impurities are injected into the private flux region (PFR) in the upper single-null configuration for heat flux mitigation. Previous results show that the argon core accumulation is reduced, when the ion grad-B drift is directed away from the upper divertor, with the ExB drift across the PFR towards the outer strike point. This indicates that the ExB drift across the PFR plays an important role in preventing impurity leakage from the outer divertor. Further experiments show that, as the impurity injection rate increases, the electron temperature gradient in the PFR decreases near the target, thereby weakening Er as well as the Er x B drift. Such a decrease in grad-Te is also associated with enhanced argon accumulation in core plasmas. These findings suggest that the effect of ExB drift on seeded impurity trapping may be diminished at larger argon impurity injection rates due to the reduced grad-Te in the PFR. Modeling with the SOLPS-ITER code has been employed to investigate the role of the ExB drift in confining argon seed impurity in divertors. |
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NP11.00102: Theory based prediction of tokamak energy confinement scaling Joseph T McClenaghan, Orso Meneghini, Andrea MV Garofalo, Sterling P Smith, Gary M Staebler, Lang Li Lao, Philip B Snyder A new tool is developed for quick estimation of the energy confinement in a tokamak based on flux matching TGLF at a single mid-radius point (r/a=0.6) and EPED neural network. This model addresses one of the present limitations of systems studies, which currently rely on the commonly used experimental scaling law τ98,y2 to project plasma energy confinement. A benchmark of this model is performed against full transport simulations, and good agreement is found under reactor conditions and when the core scale length does not change significantly across the core plasma. A web-app is developed using this model for easy exploration of energy confinement. The model is compared to the τ98 dataset, where good agreement is seen. By systematically perturbing the global parameters of various existing and future tokamaks, an approximate theory based scaling law is developed. A linear regression of the simulation data shows significant differences in several global parameter scalings compared to τ98,y2 including major radius, aspect ratio, plasma current and elongation. The difference between the two scaling is currently being investigated. |
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NP11.00103: Development of 1-D Pinhole Lyman-alpha Cameras for Neutral Fueling and Particle Transport Studies on DIII-D Aaron Rosenthal, Alessandro Bortolon, Jerry W Hughes, Rui F Vieira, Rick Leccacorvi, Shaun R Haskey We present the design and implementation of a pinhole camera for the measurement of edge Lyman-alpha emission profiles on the DIII-D tokamak. The system consists of two cameras, each equipped with a narrowband Lyman-alpha reflective mirror, optical filter and AXUV photodiode detector array. The two cameras will provide a toroidal fan of views at 0.75 m below the midplane covering the scrape-off layer and pedestal region on both the high field and low field side. The targeted spatial resolution is ~8 mm with coverage area of ~180 mm. The Lyman-alpha camera is intended to provide an improved characterization of neutrals for DIII-D by measuring the Lyman-alpha brightness and using an Abel inversion to determine Lyman-alpha emissivity. The emissivity is used to calculate neutral density and ionization profiles in the pedestal and scrape off layer. The camera is designed to investigate divertor leakage, main chamber fueling and radial particle transport. Recent work on design, and component calibration procedures are detailed. |
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NP11.00104: Magnetic shear effect on plasma transport in ECH injected DIII-D plasma Maiko Yoshida, George R McKee, Andrea MV Garofalo, C Craig Petty, Motoki Nakata, B.A. Grierson, Darin R Ernst, Terry L Rhodes, Christopher T Holcomb, Makoto Ono The effect of magnetic shear on plasma transport for Te/Ti near unity has been explored in DIII-D during ECH. Previous reports showed that significant confinement degradation occurs at Te/Ti~1 in positive shear (PS) plasmas in DIII-D and JT-60U [1,2], whereas confinement degradation is reduced at Te/Ti~1 in negative central shear (NCS) plasmas [1]. In this study, plasma transport in weak magnetic shear (WS) plasmas with ECH is investigated and compared with that in NCS and PS plasmas. Weak magnetic shear is found to be effective in minimizing degradation of ion thermal confinement as Te/Ti increases through ECH application, and an improved confinement factor of H98y2~1.2 is maintained, similar to NCS plasmas. During ECH, the ion thermal diffusivity in the core region becomes smaller with decreasing magnetic shear and the density and toroidal rotation in the core region tend to be steeper in the negative shear region. The variation in plasma transport will be investigated using gyro-kinetic simulations. [1]M. Yoshida et al Nucl. Fusion 57 (2017) 056027 [2]G.R. McKee et al 25th IAEA FEC (St Petersburg, Russia, 2014) EX/2-2 |
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NP11.00105: IMAS Compatible Neural-Network Accelerated Core-Pedestal Simulations with Self-Consistent Transport of Impurities Orso Meneghini, Chieko Sarah Imai, Garud Snoep, Arsene Stephane Tema Biwole, Brendan C Lyons, Joseph McClenaghan, Sterling P Smith, Emily A Belli, Philip B Snyder, Gary M Staebler, Jeff Candy, Lang Li Lao STEP (Stability, Transport, Equilibrium, and Pedestal) is a new predictive workflow developed within the OMFIT framework [https://gafusion.github.io/OMFIT-source/] to find stationary plasma scenarios with self-consistent core transport, pedestal structure, current profile, and plasma equilibrium. Key features of the workflow are: (1) Self-consistent modeling of impurity transport reduces the number of free parameters and assumptions that are used in the simulations; (2) Fast yet accurate simulations by leveraging neural network based models for the pedestal structure, neoclassical bootstrap current, and turbulent and neoclassical transport; (3) Full compatibility with the ITER Integrated Modeling and Analysis Suite (IMAS) achieved by transferring information among the different physics components with the newly developed OMAS library [https://gafusion.github.io/omas]. Simulation results and comparison with experimental DIII-D measurements will be presented. |
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NP11.00106: Machine Learning-Based Real-time Plasma Control on DIII-D Yichen Fu, Egemen Kolemen, Mark Boyer, Qiming Hu Future tokamaks such as ITER will require high-reliability systems for prediction and avoidance of plasma disruptions. Two machine learning algorithms (MLA) have been developed for disruption prediction and avoidance and were tested in real-time. The first MLA is designed to predict the onset of the 2/1 neoclassical tearing mode (NTM), generates a ‘tearability’ that quantifies the likelihood that an NTM will occur. This algorithm was used to command the neutral beam (NB) and electron cyclotron heating (ECH) in real-time on DIII-D in two scenarios: In the first scenario, a ‘tearability’ threshold was chosen above which an NTM was expected to occur. The algorithm was then used to modulate the NB power with the aim of preventing the ‘tearability’ from exceeding its threshold. In the second scenario, the ‘tearability’ was monitored and used to trigger ECH for preemptive NTM suppression when the ‘tearability’ exceeded the threshold. The second MLA generates a ‘disruptivity’ to predict the occurrence of disruptions. During several experiments, this MLA was used to trigger a controlled ramp-down of the plasm current when the ‘disruptivity’ exceeded a user-defined threshold. |
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NP11.00107: Recent Developments in the DIII-D PCS for Integrated Plasma Control and Actuator Sharing Andres Pajares, William P. Wehner, Eugenio Schuster, Nicholas Eidietis, Jayson Barr, Anders Welander, Robert La Haye, Alan Hyatt, John Ferron, Michael Walker, David Humphreys DIII-D experiments have tested new modifications to the DIII-D Plasma Control System to assess the capability of simultaneously regulating different aspects of the plasma dynamics under the supervision of exception handling algorithms. A first experiment sought combined central safety-factor (q0) and normalized beta (βN) control with zero neutral-beam injection (NBI) torque. Whereas NBI was employed to regulate βN, electron-cyclotron heating & current drive (ECH&CD) was used to control q0. A second experiment showed the Off-Normal & Fault Response system’s capability to monitor the plasma state and assign the authority over the ECH&CD system either to the Profile Control category (for current profile + βN control) or to the Gyrotrons category (for NTM suppression). Actuator sharing strategies were utilized. Finally, control-oriented models for the q0, q95, toroidal rotation Wf, βN and line-averaged electron density evolutions have been developed. These models have been validated using DIII-D experimental data to support future control experiments on integrated plasma-scalar regulation. Based on such models, a robust, Lyapunov-based controller has been designed and tested in simulation for a DIII-D scenario. |
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NP11.00108: Imaging Neutral Particle Analyzer Measurements of Alfvén Eigenmode (AE) Induced Fast Ion Transport in DIII-D Michael Van Zeeland, Xiaodi D Du, Cami S Collins, William Walter Heidbrink A new imaging neutral particle analyzer (INPA), which provides unprecedented energy-resolved radial profiles of confined fast ions, has been fielded on the DIII-D tokamak. The INPA measures charge-exchanged energetic neutrals by viewing an “active” neutral beam through a 1D pinhole camera with a rear collimating slit that defines the neutral particle collection sightlines. The incident neutrals are ionized by stripping foils and the local tokamak magnetic field acts as a magnetic spectrometer to disperse ions on the scintillator. A fast camera provides 2D images of the escaping neutrals mapped to energy and radial position in the plasma. The INPA clearly shows fast ion transport in localized regions of phase space due to AEs. Measurements in reversed magnetic shear current ramp experiments have revealed for the first time a large preferential transport of stagnation orbits over passing orbits at nearby radii. The data also show a large net outflow across the plasma midplane during strong AE activity while at lower amplitude, redistribution occurs from the core to larger radii. |
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NP11.00109: Validation of the “kick model” of Alfvén eigenmode induced fast-ion transport via Orbit Tomography Luke Stagner, William Walter Heidbrink, Cami S Collins, Mario Podesta A new data analysis method, Orbit Tomography, that can reconstruct the entire fast-ion distribution from experimental measurements is used to validate models of beam-ion transport. In an ideal scenario increasing heating power should correlate with increased fusion performance. However, in the presence of many overlapping Alfvén eigenmodes (AEs) the fast-ion density profile becomes resistant to increased heating. This is caused by the AEs redistributing the fast ions, degrading confinement. The "kick model", which models the "kicks" the fast ions experience due to the AEs, has been shown to be effective at reproducing experimental results. However, direct comparison of the predicted fast-ion distribution with experiment has been out of reach until recently. With Orbit Tomography, the kick model's local phase-space predictions can be validated. In this work fast-ion phase-space effects predicted by the kick model will be compared with Orbit Tomography reconstructed fast-ion distributions form experiment. |
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NP11.00110: Measurements of CAE mode structure in DIII-D and identification of intermediate frequency AEs Neal A Crocker, Kshitish K Barada, Shawn X Tang, Kathreen Thome, David C Pace, Robert I Pinsker, William Walter Heidbrink, Terry L Rhodes, Robert J La Haye, the DIII-D Team New measurements are reported of fast-ion driven compressional Alfvén eigenmodes (CAE, ω ≲ ωci) and intermediate frequency AEs (vA/R ≪ ω ≪ ωci). These modes are of interest because they potentially can cause electron energy transport; additionally, these measurements advance the development of AE spectroscopy as a tool for non-invasive diagnosis of fast-ions in DIII-D and burning plasmas. Measurements of ñ mode structure for the CAEs are obtained with an array of eight fixed-frequency reflectometers (55–75 GHz) offering full coverage of the plasma minor radius under the experimental conditions (BT=1.3 T, neL=2.8 x 1019 m-3). Preliminary analysis of some CAEs (with f≈5.5 MHz=0.55 fci) shows them to be core localized. Intermediate frequency modes were observed in beam-heated ELMing H-mode plasmas (BT=2.1 T, neL=5 x 1019 m-3) with f∼1–2 MHz∼2–5 vA/2πR (where vA/R∼0.03 ωci). The mode frequencies sweep downward as plasma density rises during the inter-ELM period by an amount Δf/f ∼ −ΔneL/2neL, consistent with their identification as AEs. However, the frequencies are too low for CAEs and too high for toroidicity-induced or reverse shear AEs. Two-point toroidal mode number measurements yield |n|=3–6. |
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NP11.00111: Analysis of compressional Alfvén eigenmode stability, frequency, and toroidal mode number on DIII-D Shawn X Tang, Neal A Crocker, Troy Carter, Kathreen Thome, Robert I Pinsker, David Carl Pace, William Walter Heidbrink Beam-driven Doppler-shifted cyclotron resonant compressional Alfvén eigenmodes (CAEs), with frequencies and toroidal mode numbers sensitive to the fast-ion phase space distribution, are studied in an experiment to validate and advance theoretical understanding of these modes. This experiment explores a wide range of plasma parameters and exploits the flexibility and variety of neutral beams on DIII-D, allowing for analysis of mode characteristics and its separate dependence on beam pitch angle, density, and power. These modes, observed with frequencies around f ≈ 0.57fc, are measured with a pair of toroidally separated magnetic field sensing loops, permitting novel analysis of CAE amplitude, toroidal mode number, and frequency. The on-axis co-beams, which inject in the same direction as plasma current, were found to excite co-propagating modes, with 4 ≤ n ≤ 10, while the off-axis co-beam and counter-beam were found to excite counter-propagating modes, with -10 ≤ n ≤ -1. The possibility of mode numbers aliasing is under evaluation. A beam current threshold at constant voltage was also observed, suggesting a beam density threshold for the instability. |
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NP11.00112: Quantitative Modeling of Fast Ion Transport by NTMs in Integrated TRANSP Simulations Laszlo Bardoczi, Mario L. Podesta, William Walter Heidbrink, Michael A Van Zeeland A new analysis tool for determining the experimental island width (W) [1] was integrated with the TRANSP-”Kick” reduced transport model [2] to study Neoclassical Tearing Mode (NTM) driven fast ion (FI) transport with no free parameters for the first time. Initial tests are encouraging with the model quantitatively predicting measured neutron rates over a broad range of DIII-D plasmas. The model retains all TRANSP functionality and self-consistently predicts the NTM impact on beam torque, current drive and heating. These effects depend on the location and width of FI phase space resonances which is not accounted for by the ad-hoc anomalous beam diffusivity model of TRANSP. FI transport was found to be significant when resonances overlap, resulting in a transport threshold at W~5cm. This model also shows that 3/2 (2/1) NTMs broaden (peak) the beam driven core current profile, which may contribute to flux pumping in hybrid plasmas. [1] L. Bardóczi et al, PoP 23, 052507 (2016) [2] M. Podestà, PPCF 56, 055003 (2014) |
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NP11.00113: Characterization of fast ion instabilities in DIII-D hybrid discharges D. Liu, W. W. Heidbrink, C. C. Petty, M. A. Van Zeeland, F. Turco Significant variations in MHD activity and fast-ion transport are observed in high-beta, steady-state hybrid discharges with a mixture of Electron Cyclotron (EC) waves and Neutral Beam Injection (NBI). For neutral-beam-only heating, many Alfven Eigenmodes (AE), which are likely TAEs/EAEs, are observed at frequencies of 100-250 kHz and caused a ~35% degradation in the neutron rate. With both NBI and EC, the AE activity is usually suppressed and replaced by bursts of fishbones. The inferred beam-ion transport coefficient during EC is much smaller than the case with NBI only. A leading explanation for the MHD activity change is because qmin dropped close to unity during EC current drive, which is confirmed by kinetic/MHD hybrid simulations for a pair of hybrid shots. However, experimental cases exist where the MHD activity changes significantly but the q profile apparently does not. A database is being built to find the exact cause(s) for the observed impact of EC on fast-ion instabilities with the goal of utilizing EC to control AE activity in a predictable way. Correlations of fast-ion instabilities with the q profile, the thermal plasma pressure, the fast-ion distribution, and the fast-ion beta will be presented. |
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NP11.00114: Verification of the Imaging Neutral Particle Analyzer via Pitch Angle Scattering of Injected Beam Ions Daniel Jarway Lin, Xiaodi Du, William Walter Heidbrink, Michael A Van Zeeland The pitch angle scattering rates of deuterium beam ions in low density, nearly MHD-quiescent plasmas are measured using an imaging neutral particle analyzer (INPA) in DIII-D. The INPA is a scintillator-based diagnostic that provides energy and radially resolved measurements of confined fast ions [1]. The main purpose of this study is to validate this novel diagnostic system during classical fast ion behavior. The pitch angle scattering rate is manipulated by varying the electron temperature using electron cyclotron heating (ECH) and by changing Zeff through Neon gas puffing. To compare with the experimental data, a series of synthetic INPA images are simulated through the following steps: (1) the time evolution of the fast ion distribution is obtained by the NUBEAM module of the TRANSP code; (2) the neutral flux towards the INPA is estimated by the FIDASIM code; (3) the INPASIM code simulates the neutral-foil interaction and traces the ions to the strike position of the INPA phosphor. In addition, the charge exchange process between confined fast ions and edge cold neutrals is modelled and compared with the measurement. [1] Xiaodi Du et al Nucl. Fusion 58 (2018) accepted. |
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NP11.00115: Thermal load characterization during disruptions mitigated by shattered pellet injection on DIII-D Daisuke Shiraki, Jeffrey L Herfindal, Larry Robert Baylor, Charles J Lasnier, Eric Matthias Hollmann, Ryan M Sweeney Conducted and radiated heat loads during disruptions are characterized under a variety of shattered pellet injection (SPI) scenarios on the DIII-D tokamak. Disruption heat loads are inferred from infrared (IR) thermography, and can detect both localized conduction as well as the more diffuse radiated heat loads on the first wall. Injecting the pellet from different toroidal locations allows the IR camera to compare near- and far- heat loads, relative to injection location. These comparisons support the existence of localized radiative heat loads in the vicinity of the injection location, of roughly 15% above the average value. This variation is relatively small compared to that resulting from changes in the radiating impurity injection quantity, which can change the radiative heat loads by up to 50%. The heat loads detected by IR thermography and by bolometry are compared with the available energy being dissipated in the plasma during the disruption, which is shown to be reduced by magnetic coupling to external conductors. The overall energy balance in these SPI shutdowns is compared with those during unmitigated disruptions. |
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NP11.00116: Comparison of initial poloidal impurity distributions for MGI, SPI, and dual SPI plasma shutdowns. J. L. Herfindal, D. Shiraki, L. R. Baylor, E. M. Hollmann, R. A. Moyer, N. W. Eidietis, T. Odstrcil A comparison of the pre-thermal quench radiation profiles due to high-Z impurity injection by massive gas injection (MGI), shattered pellet injection (SPI), and dual SPI from different toroidal locations into the same plasma is presented. Two different injection locations on the low-field side either above or below the midplane for MGI mitigated plasmas confirm previously found results that injection above the midplane flows inboard towards the high-field side prior to the thermal quench while MGI injection below the midplane remains trapped and even flows in the opposite direction towards the X-point.[1] Two SPI systems on DIII-D allow for comparison of impurity flows along field lines pitched towards the high or low-field side. These comparisons show that unlike MGI, the impurities are deposited inside the pedestal and are more constrained to the field lines resulting in less poloidal spreading. Tomography reconstructions of radiated power measurements for dual SPIs fired at different toroidal locations show cooling of multiple flux tubes which could explain the observed difference in mitigation metrics compared to single SPI experiments. [1] Eidietis, N. W, et al., PoP 24, 102504 (2017) |
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NP11.00117: COMSOL Modeling of Plasma Disruption Induced Effects on the DIII-D Tokamak Systems Humberto Torreblanca, Melissa Medrano, Ben Fishler, Randy Nguyen, Terry Rhodes The DIII-D tokamak can produce plasma currents of 2.5 MA with magnetic fields up to 2.1 T for up to 10 seconds. The collapse of this current (‘disruption’) in a few milliseconds induces a current on the vessel walls which interacts with the toroidal magnetic field producing forces and torques on attached metallic structures. COMSOL Multiphysics, a commercial finite element analysis software package, was used to calculate the plasma current varying magnetic flux and its interaction with the vessel and attached 3D structures. The magnitude and direction of these calculated magnetic fields are compared with measurements of the tokamak magnetic probes. The induced current distribution is plotted on the vessel wall and attached structures to better understand its paths. In addition, the effect of the structure’s shape as well as different materials are studied in order to understand their role on the induced forces and torques and how to minimize them. This analysis is presented for a number of recent DIII-D projects and hardware additions, including the Helicon Antenna, TZM Molybdenum plates for protecting Neutral Beam injection ports and Cross Polarization System diagnostic. |
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NP11.00118: Whistler Waves Driven by Runaway Electrons Kenneth Gage, Xiaodi Du, William Walter Heidbrink, Carlos Alberto Paz-Soldan, Kathreen Thome, Michael A Van Zeeland, Donald Spong, Andrey Lvovskiy, Richard A Moyer, Max E Austin In quiescent runaway electron plasmas, whistler waves with frequencies between 90-190 MHz are driven unstable in plasmas with appreciable hard x-ray and non-thermal electron cyclotron emission (ECE). Narrow (δf < 50 kHz) discrete modes are observed at erratically spaced frequencies, likely due to the bounding of the plasma [1]. The dependency of the frequency on field and density implies a wavenumber k ≈ 140 m-1 with kparallel much less than k. Reducing the gap between the plasma and the wall increases the number of detected modes. The high intensity gamma ray bremsstrahlung and synchrotron emission measurements suggest the waves are driven by electrons of several MeV through the anomalous Doppler resonance. The ECE signals often jump at whistler bursts, suggesting that the modes pitch-angle scatter the runaways via nonlinear predator-prey dynamics, implying that whistler waves can potentially be used to mitigate reactor damage from runaways. [1] D.A. Spong et at., Phys. Rev. Lett. 120 (2018) 155002. |
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NP11.00119: Current Profile Optimization for Tearing Mode Avoidance in DIII-D Steady-State Scenarios Kyungjin Kim, J.M. Park, David L Green, John Canik, John Roderick Ferron, Christopher T Holcomb, the DIII-D team DIII-D discharges with a relatively high value of internal inductance (li) or with a high minimum value of q (qmin) have improved confinement and stability that is beneficial for steady-state operation at high normalized pressure βN. In these high βN discharges, the duration of the high-performance phase can be limited by the occurrence of tearing modes (TMs). Tearing mode onset is addressed with theory-based integrated modeling using IPS-FASTRAN. The model is validated against the experimental profiles from the magnetic axis to divertor/wall for DIII-D high βN discharges by iterating core transport, edge pedestal, equilibrium, stability, heating, and current drive self-consistently. Resistive MHD simulations of TMs performed with accurate equilibrium reconstruction and well-measured plasma profiles determine the onset dependency on the tearing stability index Δ′ for the purpose of predicting TM onset and evolution in the experiment. Based on IPS-FASTRAN coupled with the TM predictor, the current and pressure profiles are optimized simultaneously for high-performance steady state operation without deleterious TMs. |
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NP11.00120: Species mix and distribution function dependence of instabilities near ωci Genevieve H DeGrandchamp, William Walter Heidbrink, Xiaodi D Du, Kathreen Thome, Laszlo Bardoczi, Michael A Van Zeeland, Cami S Collins, Stephen T Vincena, Shawn Tang, Neal A Crocker, Sergei Sharapov, Mark E Koepke, Samuel H Nogami, Shreekrishna Tripathi The instabilities known as compressional Alfven eigenmodes (CAE) and ion cyclotron emission (ICE) in tokamaks resemble the electromagnetic ion cyclotron (EMIC) and “equatorial noise” instabilities in the Van Allen radiation belts. In this Frontier Science experiment, the dependence of CAE and ICE on the hydrogen, deuterium and 3He concentration was measured for ten different fast-ion distributions created by changing the neutral beam parameters: hydrogen/deuterium, co-current/counter-current, tangential/perpendicular, and on-axis/off-axis. Below ωci, CAE modes are more unstable at 1.25 T than at 2.1 T and the amplitude increases with increasing hydrogen concentration. Above ωci, the ICE is stronger at 2.1 T and hydrogen has little effect. |
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NP11.00121: Real-time Optimization of the q Profile Under Constraints for Avoidance of MHD Instability and Achievement of Stationary Conditions W.P. Wehner, E. Schuster, J.R. Ferron, C.T. Holcomb, D.A. Humphreys, A.W. Hyatt, K.E. Thome, B.S. Victor A novel q profile control approach and recent DIII-D experimental results aimed at reaching stationary plasmas characterized by a flat loop voltage profile are presented. The control approach combines feedforward and feedback control commands. Both command components are computed via numerical optimal control techniques. The key advantage of the numerical computation approach is that it allows explicit incorporation of state and input constraints to prevent the controller from driving the plasma outside of stability limits and obtain, as closely as possible, stationary conditions characterized by a flat loop voltage profile. Using a suitable control-oriented model, the simulated plasma evolution in response to the actuators is embedded into a nonlinear optimization problem that provides a feedforward control policy (set of actuator waveforms) that under ideal conditions guides the plasma evolution to the desired state. The feedback controller computes updates to the feedforward control law to account for variability in plasma conditions; optimizing in real-time the plasma response to the available actuator set over a finite horizon (number of future control time-steps) while satisfying input and state constraints. |
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NP11.00122: Rotating RMP stabilization of ``Neoclassical Tearing Modes'' Linda Sugiyama, Michio Okabayashi, Liqing Xu In DIII-D, a slowly rotating nonaxisymmetric magnetic field (RMP) with toroidal harmonic n=1 can, using feedback, lock onto the rotation of an unstable m=2/n=1 neoclassical tearing mode (NTM) and prevent it from growing to a disruption[1]. Nonlinear simulation with the extended MHD code M3D shows a complex picture. The MHD evolves much faster than the RMP rotation. The background RMP-stabilized plasma has relatively high central beta. Without the RMP, it is unstable to a new type of instability over q=1, a nonlinear ideal-MHD unstable mode with coupled n=1 and 2 harmonics, that grows to an internal-kink-like sawtooth crash. A perturbation with n=1 and m=2/n=2 remains over q=1+ and couples to other harmonics across the entire plasma radius, consistent with observations of δTi and δTe. No large 2/1 magnetic island is seen in simulation or the experiment. The vacuum rotating field from the I-coil with 2.5 kA current is larger than the MHD mode on the large-R side of the plasma and comparable over q<1, suggesting that the RMP couples to, and influences, the full n=1 perturbation. These processes will be further studied. |
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NP11.00123: Tearing Mode Locking Avoidance by Applying Rotating External 3D Field Michio Okabayashi, Shizuo Inoue, Nathaniel Ferraro, Steve Jardin, Nikolas C Logan, Edward Strait, Zane Taylor Recent cylindrical non-linear reduced MHD simulation [S. Inoue (PPCF 2018)] has demonstrated a possibility of “shielding-out” an error field by applying a rotating 3D field, weakening significantly a single-helicity 2/1 locked mode (LM). DIII-D experiments support this proposal. The applied n=1 fields include several poloidal components, even for m/n=2/1 LM avoidance and produce a shielding-like response at q≥3 and weak response inside of q<3. This suggests the 1/1 and 2/1 resonant components are shielded-out. 3D feedback for LM avoidance produces an edge tearing resonant magnetic perturbation (RMP) in such a way that the edge RMP appearance coincides in time with disappearance of the interior RMP, again suggesting a shielding effect. The relative dependence of the tearing and shielded response on the frequency of the applied field and other physical parameters will be discussed in a qualitative manner with the M3DC1 code, using both one- and two-fluid models. |
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NP11.00124: Experimental Studies on low temperature Helium plasmas for investigations of Arc chamber failure on DIII-D Neutral Beam System Marielena Velasco Enriquez, Jasper P Beckers, Brendan J Crowley, Joe M Rauch, J Timothy Scoville, Tijs A Wijkamp The Neutral Beam Injection system of the DIII-D tokamak consists of eight ion sources based on the US Common Long Pulse Source (CLPS), with a total output power of 20 MW. During helium operation, desired for research regarding the ITER pre-nuclear phase, it has been observed that the ion source arc chamber performance steadily deteriorates. Also, the device eventually fails due to electrical breakdown in the insulation material that electrically separates the energized arc and filament plates from the body of the ion source. To address the cause of this failure, a table-top ion source analog with similar parameters was constructed. The Miniature Arc Chamber Experiment (MACE) is equipped with Langmuir probes, spectroscopes, thermocouples and infrared imaging. This work presents an overview of the MACE system, a comprehensive characterization of the plasma parameters within the operating space of the ion source and results of recent studies on alternate insulator designs. |
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NP11.00125: Top Launch for Higher Off-axis Electron Cyclotron Current Drive Xi Chen, Ron Prater, C Craig Petty, John M Lohr, David Su, Matt Smiley, Lang Li Lao, Vincent S Chan Efficient off-axis current drive is crucial for economic, steady-state tokamak fusion power plants. ’Top Launch’ ECCD is a promising off-axis CD method to achieve the desired broad current profile. By launching the waves downwards (or upwards) parallel to the resonance plane with a large toroidal steering, high CD efficiency can be obtained at large radii owing to the large Doppler shift, wave-particle interactions on HFS of the plasma, and the long absorption path. Previous modeling for FNSF predicts Top Launch can provide significantly higher (~50%) off-axis ECCD efficiency with broader profile peaked off-axis than standard LFS launch. In this work, a systematic study of Top Launch for CFETR, using the applied EC frequency and launch location as free parameters, finds a >35% improvement in off-axis ECCD over LFS launch. In addition, its physics potential in various DIII-D scenarios is investigated along with the sensitivity to launch location, magnetic field, density, beam aiming and divergence, etc. Enhancement in off-axis ECCD efficiency of up to a factor-of-2 seems possible. A prototype fix-injection Top launch system is being designed and planned to be installed on D3D to characterize and evaluate effects of this injection scheme. |
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NP11.00126: Extension of the Density Range for Electron Cyclotron Heating (ECH) on DIII-D With 2nd Harmonic O-mode R.I. Pinsker, C.C. Petty, X. Chen, J.M. Lohr, M. Cengher, P.B. Snyder, M. Porkolab, M.E. Austin Studies of high-density plasma regimes that use second harmonic X-mode (X2) ECH are hampered by excessive wave refraction and loss of accessibility to the absorption layer. The upper density limit can be extended by a factor of two by using the O-mode polarization, with the price being < 100% single-pass absorption for O2. Investigation of this option both experimentally and theoretically is underway. An initial experiment in the Super H-mode regime was performed in which 110 GHz EC power was injected in O-mode from the launchers above the outboard midplane in which single-pass absorption of 80-90% was achieved, yielding a slight measured increase in local Te. In this configuration power not absorbed on the first pass through the resonance layer is expected to reflect from the center post and be dissipated in the lower part of the vacuum vessel, which does not contain microwave-sensitive equipment or windows. Comparison of predicted damping with the TORAY and GENRAY codes, as well as with analytic estimates, is being carried out. The dependence of the damping on aiming and on plasma parameters is under study, with the goal of optimizing the scenario so O2 can be applied to core "impurity-chasing" in high-density regimes. |
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NP11.00127: Helicon Antenna Diagnostic and Testing Program for DIII-D Tokamak Michael W Brookman, Robert I Pinsker, Alex Nagy, Charles Moeller, Humberto Torreblanca, Raymond O'Neill, Melissa Medrano A test program has characterized the rf response of a prototype high power helicon antenna module. A quarter length module made out of uncoated copper was installed in a vacuum chamber and fed via resonant circuit with commercial television amplifier providing 1 ms-long 7 kW pulses at 0.1% duty cycle. The high power helicon system will apply 1 MW to 30 full-length antenna modules [Nagy FST 2017], so the resonant circuit is designed to couple power into the quarter module to reach an electric field of 18 keV/cm, comparable to the full module. Multipactor is significant only at modest powers, and can be conditioned to a minimal level. The effects of a titanium nitride coating added to the copper were found to be minimal, no more effective in terms of standoff than well-cleaned copper, simplifying a material choice for the final product. Quarter module testing is occurring alongside instrumentation and diagnostic development. Arc detection, rf probe measurements, and potential optical and piggyback signal-mixing techniques now being developed for use with the high power system are discussed.
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NP11.00128: COMSOL Modeling for the DIII-D Helicon Antenna Test Stand Experiment M. Medrano, M.W. Brookman, H. Torreblanca, C.P. Moeller, R. I. Pinsker A program integrating COMSOL modeling of the 1 MW helicon antenna has explored design parameters such as RF coupling between modules, multipactor analysis, heat dissipation and mitigation of disruption forces. A 30-module comb-line antenna is being designed for driving off-axis non-inductive current in the DIII-D tokamak using electromagnetic waves at 476 MHz and 2.1 T of magnetic field. In order to validate design criteria, an experimental module is built using a segmented version of an antenna module. This test module is fed with 7 kW of RF power (450-525 MHz), with up to 0.1 T of background magnetic field. In addition the planned 1MW stripline power feed is explored using a high-Q RF resonator designed to replicate the electromagnetic field values of the stripline and stripline-to-module connection interface using the available 7 kW of power. COMSOL was used to simulate the RF characteristics of the segmented module and to design and optimize the RF performance of the resonator. Data on the scaled module: magnetic field, electric field, temperatures and RF resonator performance are presented. |
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NP11.00129: High Field Side Lower Hybrid Current Drive for Efficient, Off-axis Current Drive in DIII-D Stephen James Wukitch, Andrew Seltzman, Syun'ichi Shiraiwa, Gregory Marriner Wallace, Paul Thaddeus Bonoli, John Christopher Wright, Christopher T Holcomb, Aaro E Jarvinen, Brian S Victor, Adam Mclean, Robert I Pinsker For DIII-D, efficient off axis current at r/a~ 0.6-0.8 with peak current density ~0.4 MA/m$^2$ is sought for advanced tokamak plasmas. With high field side launch, lower hybrid waves can drive current peaked off-axis, r/a~0.6-0.8, and driven current up to 0.39 MA/MW couple with single pass absorption. Utilizing GENRAY+CQL3D, wave penetration was found to be strongly influenced by local q value. In AT discharges, the wave penetration is dominated by the poloidal upshift resulting in good wave penetration. Practically, a small inner gap and a metallic launcher at the plasma edge were thought to be potential issues. Experiments found that small inner gap had a negligible impact on plasma performance. Using a mock-up coupler, inner wall limited discharges with up to 10 MW injected power were investigated and the plasma mock-up coupler interaction was restricted to the carbon protection tiles. No significant Mo I source was observed in any discharge and the core Mo contamination was subtle to non-existent. These experimental and simulation results indicate that HFS LHCD is compatible DIII-D discharges and has potential to provide efficient off-axis current drive in DIII-D. |
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NP11.00130: Coupling simulations for high field side lower hybrid antenna on DIII-D Gregory Wallace, Andrew Seltzman, Syun'ichi Shiraiwa, Stephen James Wukitch, Paul Thaddeus Bonoli, Walid Helou, Julien Hillairet, Christopher T Holcomb, Brian S Victor, John Roderick Ferron, Robert I Pinsker Calculations show that high field side (HFS) launch of lower hybrid (LH) power represents an integrated current drive solution that both improves core physics (higher efficiency/proper location) and mitigates plasma material interaction/coupling issues. An HFS LHRF system has been developed for DIII-D to demonstrate these benefits, which represents the first design for an operating tokamak. The RF performance of the antenna was simulated by COMSOL coupled to ALOHA for the plasma response. Simulations predict low reflected power for a range of anticipated plasma conditions in the HFS scrape off layer. A vacuum gap of 0.5 mm at the antenna mouth results in a slight increase of power reflection coefficients (from <1% to <5%) and decrease of directivity (from 70% to 60%) as compared to cases without the vacuum gap. Experimental results from the final weeks of the 2018 DIII-D run campaign indicate that a 0.5-1.0 mm recess depth provides sufficient protection of the molybdenum structures from the plasma. This result motivates recessing the radiating molybdenum waveguides a similar distance behind the local graphite protection limiters. |
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NP11.00131: Simulation of a High Field Side Lower Hybrid Current Drive (LHCD) Coupler for DIII-D Andrew Seltzman, Syun'ichi Shiraiwa, Gregory Marriner Wallace, Stephen James Wukitch, Paul Thaddeus Bonoli In DIII-D, an high field side LHCD launch scenario with r/a~0.6-0.8 deposition and high efficiency has been proposed for current profile control. A multi-junction (MJ) coupler is simulated in COMSOL and MFEM to optimize spectrum, directivity, and reflection coefficient for a range of plasma conditions. Coupler directivity and n|| spectrum depend critically on correct phasing and power splitting between MJ arms; characteristics that vary with plasma vacuum gap at the antenna aperture, local density and density gradient. The COMSOL model utilizes a lossy dielectric benchmarked against ALOHA [1] simulations. COMSOL analysis allows for rapid optimization of the coupler geometry and verification of proper n|| launch spectrum for given plasma edge conditions. The MFEM model allows more complex coupler features including curvature and a warm plasma wave solver. The latest coupler modelling results including COMSOL with a cold plasma model will be presented. [1]J. Hillairet et al, Nucl. Fusion 50, 125010 (2010). |
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NP11.00132: Highlights from the community white paper “Enhancing US fusion science with data-centric technologies” David R Smith, Robert S Granetz, Martin J Greenwald, Julian Kates-Harbeck, Egemen Kolemen, Orso Meneghini, Lucas A Morton, Nicholas A Murphy, Cristina Rea, Steven A Sabbagh, Sterling P Smith, Joshua Stillerman, William M Tang, Kevin L Tritz, John C Wright The continued growth of data size and complexity from fusion experiments brings new opportunities and challenges for fusion science. Modern analysis activities like machine learning, automated analysis, and real time control can transform large, complex datasets into new scientific insight. At the same time, data-intensive applications depend on a foundation of enabling technologies like workflow managers, metadata tools, and data frameworks that promote access, collaboration, comprehension, and productivity. Analysis techniques that leverage large data volumes and enabling technologies that facilitate scientific productivity are complementary aspects of modern data science. Here, we present highlights from the community white paper “Enhancing US fusion science with data-centric technologies” which assesses the challenges and opportunities for data science in experimental fusion research. Investments in data science efforts can boost the scientific productivity of experimental data, and transforming data into actionable scientific insight in a timely and economical manner is a priority for all stakeholders. |
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NP11.00133: Right answer for the wrong reasons: a cautionary tale from mix experimental modeling Alex Zylstra, Nelson M Hoffman, Hans Herrmann, Mark Jude Schmitt, Yongho Kim Mix of high-Z material into the fuel is a significant degradation mechanism for fusion experiments, yet is often observed via indirect means. For inertial fusion, the ‘separated reactant’ technique is a popular experimental platform. We fit a radiation-hydrodynamics model including a turbulent mix model to previous data, obtaining an excellent match to observables, to predict a new experimental series. Unfortunately this model missed every experimental point by at least a factor of three due to missing physics in the model, clearly indicating that the training data was insufficient and the pre-shot model got the right answer for the wrong reasons. Adding a second mechanism (diffusion) to the model plus additional data gives a statistically significant discrimination between the two mechanisms [A.B. Zylstra et al., Phys. Rev. E 97, 061201(R) (2018)]. This is a cautionary tale about the underlying bias often present in models from our preconceptions, which will be a challenge for applying ML methods to physical science. |
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NP11.00134: Machine Learning-Based Real-time Plasma Control of DIII-D Yichen Fu, Egemen Kolemen, Mark Boyer, Qiming Hu Future tokamaks such as ITER will require high-reliability systems for prediction and avoidance of plasma disruptions. Two machine learning algorithms (MLA) have been developed for disruption prediction and avoidance and were tested in real-time. The first MLA is designed to predict the onset of the 2/1 neoclassical tearing mode (NTM), generates a ‘tearability’ that quantifies the likelihood that an NTM will occur. This algorithm was used to command the neutral beam (NB) and electron cyclotron heating (ECH) in real-time on DIII-D in two scenarios: In the first scenario, a ‘tearability’ threshold was chosen above which an NTM was expected to occur. The algorithm was then used to modulate the NB power with the aim of preventing the ‘tearability’ from exceeding its threshold. In the second scenario, the ‘tearability’ was monitored and used to trigger ECH for preemptive NTM suppression when the ‘tearability’ exceeded the threshold. The second MLA generates a ‘disruptivity’ to predict the occurrence of disruptions. During several experiments, this MLA was used to trigger a controlled ramp-down of the plasm current when the ‘disruptivity’ exceeded a user-defined threshold. |
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NP11.00135: Inferring time resolved electron temperature of imploded capsules using Convolutional Neural Networks Ji Hoon Kang, Shahab Khan, John E Field, Jayson Dean Lucius Peterson, Ryan Nora, Pravesh K Patel Here, we illustrate the application of a deep neural network structure to aid in understanding the results from fusion experiments at the National Ignition Facility. X-ray data generated from capsule implosion experiments can be used to infer the temperature of the hot core within the implosion, which can reach several millions of degrees! In order to get the temperature, the measured x-ray data is used in a forward fit algorithm that compares the measurement to synthetic signal based on several models. Since the resulting temperature depends heavily on the model used, there is some uncertainty in this technique. As an alternative, a deep neural network is developed using thousands of 2-D and 3-D hydrodynamic simulations. Several experiments with known electron temperatures will be used as a bridge from simulations to data. This presentation will describe the deep learning technique employed, as well as the parameters and strategy used to match the simulations. The results from this approach will be compared with that obtained with analytical models. |
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NP11.00136: A support vector regression method to efficiently determine neutral profiles from metastable-pumped laser induced fluorescence data Dustin M Fisher, Ralph F Kelly, Mark Gilmore A support vector regression (SVR) method is presented that utilizes a collisional radiative (CR) model of helicon plasmas in the Helicon-Cathode (HelCat) linear plasma device to determine Ar I profiles based on metastable-pumped laser induced fluorescence (LIF) measurements. A machine learning approach to the CR model allows for an efficient exploration of the input parameter space and can inherently incorporate probe and LIF measurement errors in profile inputs to which a CR model would normally be sensitive. A training set is created for mapping CR model outputs to Ar I input profiles using radial points as SVR input features and parameters of a sigmoidal-type function as output features. This SVR method may be easily adapted to other LIF pumping schemes and may even be used in conjunction with a CR model to validate electron temperature and density plasma profiles if neutral or ion profiles are already known. |
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NP11.00137: Bayesian Parameter Estimation for Data Integration in ICF Experiments Patrick F Knapp, Michael E Glinsky, Matthew Evans, Stephanie Hansen, Christopher Jennings, Eric Harding, Matthew Weis, Stephen A Slutz, Matt Gomez, Kelly D Hahn, Matthew R Martin, Matthias Geissel, Ian C. Smith, Pierre-Alexandre Gourdain, Kyle J Peterson, Brent M Jones, Jens Schwarz, Gregory A. Rochau, Daniel B Sinars Bayesian parameter estimation is a powerful tool for the interpretation of experimental data and discriminating between models. It is particularly powerful when applied to data integration, the task of simultaneously integrating multiple disparate diagnostic data sets to constrain a model. This technique is demonstrated on data obtained from MagLIF experiments where imaging, spectroscopic, x-ray, and neutron data are all used to simultaneously constrain the set of parameters that best describe the observables. Our algorithm also gives confidence intervals and correlations directly from the analysis, as well as the ability to estimate the value of information for each of the diagnostic inputs. |
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NP11.00138: Using Machine Learning Based Moment Closures to Capture Kinetic Turbulence Akash Shukla, David R Hatch, Vasil Bratanov Gyrofluid models are attractive because they provide a computationally efficient alternative to gyrokinetic models. They rely on a moment closure, which approximates the highest order fluid moment as a function of the lower order moments. Conventional gyrofluid models use linear moment closures designed to match the plasma dispersion function and can produce linear physics that closely matches gyrokinetics in many parameter regimes. However, these linear closures can break down in the presence of turbulence, where the nonlinearity can strongly modify the kinetic physics. We apply a machine learning approach to developing moment closures that correctly capture kinetic effects in a relatively simple kinetic turbulent system produced by the DNA code. The DNA code solves a set of reduced gyrokinetic equations in a Hermite representation, which lends itself naturally to a moment closure. The algorithms are trained on kinetic simulation data (i.e. using dozens of Hermite moments) and are designed to predict a closure for a four moment system of equations. |
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