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 YP11: Poster Session IX: The crossover between high-energy-density plasmas and ultracold neutral plasmas ; Supplemental; Post-Deadline Abstracts (9:30am-12:30pm) |
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Room: OCC Exhibit Hall A1&A |
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YP11.00001: The Scientific Prototype, the only reasonable next step for the American MFE program* Wallace Manheimer After a tremendous triumph in about 1999, where TFTR generated ~ 15 MW of neutrons for a gain of about 0.5, the American MFE program has been floundering, the rest of the world leaving us far behind. Instead, there were multiple proposals for ignition experiments, new stellarator programs, and ST’s. The former two have been rejected, and for good reason; the latter almost certainly has a fatal flaw in the center post. With the closing of Alcator, and the long repair of NSTX, leaving only D3-D to carry the flag, it is fair to say that the American MFE program is in crisis, and possibly will not be around much longer if things continue at the present rate. There is just a single cure, and that is to unify the entire base program around “The Scientific Prototype”. This is a tokamak the size of TFTR, but run steady state in DT with a breeding blanket and breeding its own tritium. If not now, when? True steady state and tritium breeding are things ITER will not do. If both ITER and The Scientific Prototype are successful, we will have gone a long way toward fusion. |
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YP11.00002: Modeling of sources and transport of impurity ions and their interaction with material surfaces in powder-dropping experiments M. V. Umansky, A. Bortolon, D. Curreli, J. T. Drobny, R. Maingi, T. D. Rognlien, R. D. Smirnov In powder-dropping experiments, a stream of dust particles containing boron nitride, pure boron, or other materials is continuously injected into tokamak edge plasma [1]. This is potentially a powerful tool for real-time wall conditioning and edge plasma radiation control. We are undertaking modeling of impurity particles dynamics and their interaction with plasma facing material surfaces in powder-dropping experiments as a sequence of several processes. First, to capture dynamics of injected dust particles, the DUSTT code is used, which allows modeling of physical processes for individual dust grains, including ablation. As dust particles are ablated, they produce a localized impurity ion source; impurity ion transport is modeled with the UEDGE code solving fluid equations for impurity ions and the background plasma. Interaction of plasma and impurity ions with material surfaces is modeled with the F-TRIDYN code which follows evolution of surface morphology. For boron powder injection, of particular interest is the thickness of the steady state layer of boron atoms on the surface, which is the result of dynamic balancing between deposition and sputtering. [1] Bortolon et al., to be submitted to Nucl. Mater. & Energy. |
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YP11.00003: Acceleration of L-shell gold ions from sub-micrometer structured foil irradiated by a high-contrast and high-intensity picosecond laser. Mathieu Bailly-Grandvaux, Brandon C Edghill, Christopher S McGuffey, Maylis m Dozieres, Mingsheng Wei, Neil B Alexander, Alex Haid, Christian Brabetz, Vincent Bagnoud, Paul Neumayer, Farhat N Beg Laser-driven heavy ion beams are ideally suited for experiments requiring fast isochoric heating due to the exceptionally high current density of the beams and the high energy deposition rate of the ions. We report here the acceleration of highly charged gold ions (51+ to 61+) up to ~ 400 MeV by laser irradiating sub-micrometer-thick foils as is typical in studies of proton and lighter ion acceleration, without removing the contaminant layer. Microstructures with variable array geometry were 3D-printed on the front surface of the foils to tune the spectral shape and relative number of accelerated ions. The experiment was performed with the PHELIX laser at GSI (Darmstadt, Germany) and the pulses (~150 J, 0.5 ps, 1.054 μm) were focused at ~15º incidence angle on the targets to intensities of ~6×1020 W/cm2, with a prepulse contrast ratio of ~10-12. The high charges and accelerated energies of such low q/m ions (<0.3) were enabled by the very high contrast and sustained pulse duration of the laser, inducing very strong volumetric heating. X-ray line emissions from different target components help elucidate the interaction. |
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YP11.00004: Characterization of Laser-Wakefield Produced Positron Beams Gerald J Williams, Felicie Albert, Nuno Lemos, Jaebum Park, Bradley Pollock, Hui Chen A laboratory-based source of positron-electron pair jets is necessary for scaled relativistic astrophysics studies. Most prominently, such a platform could test the microphysics of particle acceleration from Weibel-mediated relativistic shocks, mimicking environments found in gamma-ray bursts, active galactic nuclei, or pulsar wind nebulae. Recent attention has been paid to LWFA-generated pairs due to claims of charge neutrality [1], incredibly high densities (>1016/cm3), and near-collimation (5 mrad) [2]. These results have not been reproduced using similar LWFA electron sources [3] where simulations suggested the divergences are an order of magnitude larger than estimated in Ref. [1,2] due to Coulomb scattering in mm-to-cm-scale targets. We explore these discrepancies, present new measurements of positron beam emittance, and examine the fundamental limitations to laser-produced pair plasmas for scaled astrophysical studies. [1] G. Sarri, et al. Nat. Commun., 6, 04 2015 [3] G. Sarri, et al. Phys. Rev. Lett., 110:255002, Jun 2013 [4] G. J. Williams, et. al. Phys. Plasmas, 22(9):093115, 2015 |
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YP11.00005: Sparse Grid Methods for Solving Vlasov-Poisson and Vlasov-Maxwell Systems Lin Mu, David L Green, E. D'Azevedo, Tianyang Wang, Wael Elwasif, Tyler McDaniel, Matthew Lopez The Vlasov-Poisson and Vlasov-Maxwell models are fundamental to the kinetic treatment of plasma physics. Here we consider the fully-kinetic Vlasov equation which describes the time evolution of a distribution function in a six dimensional phase space (3x-3v), plus time, and is coupled either with the Poisson’s or Maxwell’s equations in three dimensions (3x). For continuum methods, the challenging issue is how to design an efficient and stable numerical scheme for handling such high dimensionality. In this work, we present a numerical scheme for approximating Vlasov-Poisson and Vlasov-Maxwell systems in a generalized dimensionality. Our approach is based on the multiwavelet basis and sparse grids, yielding significant reductions in the number of unknowns required for high dimensional approximation. We will present several standard benchmark tests, reporting on how their degree of freedoms and accuracy compare with those of the standard full grid approach. |
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YP11.00006: Characterization of electron density in laser-induced plasma with terahertz wave Zhigang Zheng, Xiaowei Wang, Jiayu Dai, Jianmin Yuan, Zengxiu Zhao Terahertz (THz) wave is of fundamental importance in fusion plasmas diagnostics . In this work, we demonstrate the experimental results of the laser induced plasma diagnostics at the THz frequency range. Resonance absorption has been observed and identified through THz spectroscopy, which can be understood by a Lorentz-modified Drude model. The absorption is studied by varying the polarization and energy of the plasma-generated pulse. The results suggest no significant polarization dependence on the absorption features. As the laser intensity of the plasma-generated pulse grows, the central frequency of the resonance absorption increases, as well as the electron density. This indicates the intrinsic relationship between the plasma frequency and the resonance absorption. Therefore, the resonance absorption at the THz frequency range provides an alternative to characterize plasma properties, especially for the plasma with typical frequency at the THz range. Furthermore, our results are supplements to the existing theories on THz generation from the filament. When using the filament as a source to generate the THz wave, the absorptions and the phase variations, as determined by the electron density, should be considered, both in calculations and experiments. |
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YP11.00007: Investigating the pressure wave generated across a solid target as the result of the impact of a pulsed multi-MeV electron beam Thierry D'Almeida, Maxime Ribiere, Gael Le Blanc, Rémi Maisonny ASTERIX is a Marx generator operated at the CEA Gramat that excites an electron diode with a maximum voltage of 6.4 MV, and a current of about 100 kA with a 45 ns full width at half maximum. The electron beam emitted from the cathode interacts with an aluminum-backed tantalum anode target, resulting in high x-ray pulse generation efficiency. In order to complement the set of diagnostics used for characterizing the plasma generated on the target surface upon the incidence of this high flux multi-Mev electron beam, a Doppler laser interferometer was used to measure the velocity of the target back surface. A series of experiments were performed with a 230 mm tantalum target backed with various thicknesses of Aluminum layer. The experimental surface velocity profiles were compared to simulated velocity profiles obtained using a Lagrangian hydrodynamic code. The latter used tabulated energy deposition source term inferred from calculated electron fluence on the anode, as an input. We show that back surface velocity measurements can be related to spectroscopic measurements obtained previously in order to characterize the source term. |
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YP11.00008: Investigation of Parasitic Plasma Formation in EUV Lithography Sources using a Z-pinch Gianluca Panici, David N Ruzic The formation of parasitic or secondary plasmas in EUV lithography sources is investigated in detail. As dense, high temperature plasma travels through medium-pressure background hydrogen, energy transfer and ionization events between the laser-produced Sn plasma and the hydrogen create a cold plasma that lingers after the dissipation of the initial plasma. In the case of a pulsed device this cold plasma has a lifetime that is orders of magnitude greater than the initial hot plasma. This parasitic plasma can affect critical functions in the source ranging from debris mitigation to metrology. A XTS 13-35 DPP EUV source was utilized to investigate the secondary plasma formation. The z-pinch creates a hot, dense plasma that is similar to the laser produced plasmas used in commercial EUV sources. An analytical model was created to estimate parasitic plasma densities. Experiments were performed in the XTS 13-35 source. Time-resolved measurements of electron densities allow for accurate evaluation of dominant particle interaction mechanisms between initial plasma particles and the ambient gas. Axial and pressure profiles for these time-resolved measurements are presented and compared to analytical results. |
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YP11.00009: Dynamic structure factor using electron force field in lithium Tianyu Gao, Xiaowei Wang, Jiayu Dai, Yongjun Choi, Michael Sean Murillo Accurate x-ray scattering techniques to measure the physical properties of dense plasmas have been developed for applications in high energy density physics. We notice that structure factors determine the physical properties of matter. We prepare to present molecular dynamics simulations based on electron force field (eFF) that determine self-consistently the dynamic ion structure factor and dynamic electronic structure factor in lithium, Our comprehensive data set allows for the calculation of the dispersion relation for collective excitations, and the calculation of the sound velocity. Due to the use of Gaussian wave function in electron force field method, it can improve the traditional electronic structure factor calculation method without the Chihara approximation. The results will be compared with available experimental x-ray and DFT results. We will check for both the liquid metal and warm dense matter domain. |
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YP11.00010: Ultrafast nonequilibrium electron dynamics in a solid-density aluminium interacting with an ultra-intense ultrafast x-ray pulse Cheng Gao, Jiaolong Zeng, Jianmin Yuan Ultrafast nonequilibrium dynamics of free electrons in a solid-density aluminium produced by an ultra-intense ultrafast x-ray pulse is investigated by solving Fokker-Planck equation. Electron energy distribution function (EEDF) contains two parts: a lower energy part at nearly equilibrium and a higher energy part at evident nonequilibrium. The former part accounts for the most population of the total electron number. X-ray transmission and bound-bound emissivity show little difference between the results with EEDF obtained by solving Fokker-Planck equation and by Maxwellian distribution assumption. Yet the bremsstrahlung emissivity shows great difference. |
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YP11.00011: The Effect of Gas Dynamics in Plasma Gun Performance for Simulating Fusion Wall Response to Disruptions William Riedel, Thomas C Underwood, Vivek Subramaniam, Mark A Cappelli In this work, the suitability of a pulsed coaxial plasma accelerator to simulate the interaction of edge-localized modes with plasma first-wall materials is investigated. Experimental measurements are presented that focus on both the properties of the plasma flow and the manner in which such jets couple with material interfaces. Specific emphasis is placed on quantifying the variation in these properties using tungsten tokens exposed to the plasma plume. Time-resolved Schlieren visualization of the density gradient within the flow indicates the existence of two distinct modes with vastly different characteristic timescales, spatial evolution, and plasma properties. Time-resolved quantification of the associated plasma heat flux for both modes, including a range spanning 150 MW/m2 - 10 GW/m2, is presented using a fast thermocouple gauge, an IR camera, and a high-frequency two-color IR pyrometer. These diagnostics, in conjunction with a DSMC simulation of the expansion of neutral gas within the volume, are used to resolve the energy transfer dynamics of the heating process and provide an accurate description of the operational characteristics of plasma guns. |
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YP11.00012: Integration of Photoionization Sensor CES into Oil and Gas Industry and Medicine Aleksandr Mustafaev, Fatima Arslanova, Iuliia Rastvorova The main advantages of the CES detectors are low price, small size, high sensitivity (1 ppm and better), a wide dynamic range of measured concentrations, the ability to determine a wide range of simultaneously analyzed substances including complex organic molecules. This is owing to the ionization of all the present molecules that have a corresponding ionization potential without fragmentation and their classification according to the photoelectron energy spectrum when the CES cell of the VUV is irradiated with photons. The features of the method, which have no analogues, made it possible to create a portable analyzer that is capable to become a personal device of the people health monitoring. Owing to its advantages, this device can be effectively used both at hazardous production facilities and for medical purposes. In the oil and gas industry, the CES microplasma sensor can monitor workers' health (a silicone bracelet with a built-in sensor). It is possible to conduct an air analysis with an express method to detect exceedances of maximum permissible concentrations, control drilling and blasting operations and create a reference point for building aeroecological map. It can analyze the main biomarkers in outpatient clinics removed from equipped medical centers. |
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YP11.00013: Stuctures and transport properties of warm dense hydrogen Jiayu Dai, Zengxiu Zhao, Xiaowei Wang The structural, thermodynamic and transport properties of warm dense matter (WDM) are crucial to the fields of astrophysical physics, planet science, and inertial confinement fusion. In the warm dense matter regime, matter exhibits moderately or strongly coupled, partially degenerate. Therefore, the methods which used to deal with condensed matter and isolated atom should be validated for WDM properly. It is therefore a big challenge to understand WDM within a unified theoretical description with reliable accuracy. Here we study the liquid-liquid phase transition of dense hydrogen with first principles molecular dynamics including van der Waals interactions and non-local interactions. The results are in good agreement with the quantum monte carlo simulations and we give the dynamical properties such as diffusion coefficients. Also, by using electron force field molecular dynamics, we study the dynamics of electron-ion energy exchange in warm dense hydrogen, giving the temperature relaxation time after strong laser deposition. The results show that interplay between quantum electrons and coupled ions are crucial for the scattering processes, which result in much lower relaxation rate comparing with the results from traditional kinetic models and classical molecular dynamics. |
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YP11.00014: Tomographic Reconstruction of Magnetic Field structures from Proton Radiography Data Ben Spiers, Ramy Aboushelbaya, Marko Mayr, James Sadler, Alexander Savin, Robin Hsiao-Wu Wang, Peter Andrew Norreys We present recent progress in our development of techniques and algorithms used to recover spatially-resolved magnetic field structures from measurements made using the proton radiography diagnostic. The nature of proton radiography places severe limitations on the angular resolution achievable by tomographic measurements, due to space constraints in the target chamber as well as the difficulty of simultaneously producing many suitable proton beams; we investigate the effectiveness of real-space regularisation and Fourier-domain inpainting techniques to avoid the severe artefacts usually associated with reconstruction of sparse-view tomographic data. Finally, we demonstrate the full reconstruction process, from particle-in-cell derived magnetic fields to synthetic proton radiographs, then to line-integrated magnetic fields recovered from raw radiographs using the Monge-Ampère formalism and finally to a single spatially-resolved magnetic field component. |
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YP11.00015: Low Pressure High Density Plasma Diagnostics Development of on the Helicon Plasma Experiment (HPX) Royce W James, Richard N. Paolino, Tooran Emami, Anita J. Green, Maylis J. Yepez, Trenton E. Robledo-Thompson, Jeremy L. Turk The small Helicon Plasma Experiment (HPX) at the Coast Guard Academy Plasma Lab (CGAPL), continues to progress toward utilizing the reputed high densities (1013 cm-3 and higher) at low pressure (.01 T) of helicons, for eventual high temperature and density diagnostic development in future laboratory investigations. HPX has installed an Impedans Langmuir probe and constructed an RF-shielded triple probe diagnostic to compare the plasma perameters and behavior during experiments. Our 2.5 J YAG laser Thomson Scattering system operates at its first and second harmonics, 532 and 1064 nm respectively. It utilizes a volume-phase-holographic (VPH) grating spectrometer, a CCD camera with a range of 380-1090 nm for second harmonic (532 nm) photon emissions. At 1064 nm, a new polychromator has been procured from General Atomics optimized for TS measurements of 5 eV < Te < 2000 eV over a 109-degree scattering angle. HPX diagnostics are backed by our Data Acquisition (DAQ) system is capable 12 bits of sampling precision at 2 MS/s. Progress on the construction of the Helicon Mode development, Thomson Scattering, plus particle and electromagnetic diagnostic observations will be reported. 1 K. Toki, et al., Thin Solid Films 506-507 (2005)
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YP11.00016: Plasma structure deformation due to injection of a strong plasma pulse in a detached plasma Seiji Ishiguro, Theerasarn Pianpanit, Hiroki Hasegawa, Toseo Moritaka, Yuki Hayashi, Noriyasu Ohno, Hirohiko Tanaka, Shin Kajita Linear machine experiments have been performed to investigate the physical condition for the detached plasma formation[1,2]. A recent experiment reveals collapse of the detached plasma due to injection of a strong plasma pulse and subsequent regeneration. We have developed PIC simulation code with Monte Carlo collision to investigate microscopic physical processes in the detached plasma formation and observed the formation of a large temperature gradient[3]. We extended the above code to include plasma pulse and observed time evolution of the plasma structure. It is found that a strong plasma pulse creates a potential hill in front of the target and, as a result, ions are decelerated. [1] W. L. Hsu et al., Phys. Rev. Lett. 49, 1001 (1982). [2] N. Ohno et al., Nuclear Fusion 41, 1055 (2001). [3] T. Pianpanit et al., Plasma and Fusion Res. 11, 2403040 (2016). |
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YP11.00017: A portable field emission based x-ray generator Ming-Chieh Lin, Po-Yu Chang A portable field emission based x-ray generator powered by a mini Marx generator has been proposed. Field emission is generated from the cold cathode employing either ultrananocrystalline diamond (UNCD) films or field emission arrays under the pulsed high voltage from the mini Marx generator. Numerical simulations using a conformal finite-difference time-domain particle-in-cell code along with SPICE have been performed for validating the concept. A prototype is under development. To date, a pulsed voltage up to 30 kV was measured from the mini Marx. Details of simulation results and experimental progress will be presented. |
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YP11.00018: Gyrokinetic and ideal MHD ballooning at high beta Rahul Gaur, Dylan Langone, William Dorland, Pierre-Alexandre Gourdain High beta tokamaks are attractive concepts for magnetic confinement fusion, but their stability properties can be challenging to assess. In this poster, we present high mode number (ballooning mode) stability analyses for high beta tokamak equilibria, using GS2. To benchmark the code, we compare the gyrokinetic results with the ideal MHD energy principle, in the MHD limit. For a specific, high-beta ITER configuration, we have computed the region of the second stability for arbitrary ballooning angle theta_0, using both models. We compare the results in the appropriate small gyroradius limit.
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YP11.00019: First Experimental Observation of Electron Acoustic Wave Propagation in Laboratory Plasma Satyajit Chowdhury, Subir Biswas, Nikhil Chakrabarti, Rabindranath Pal The electron acoustic wave (EAW) is one of the basic electrostatic waves in an unmagnetized collisionless plasma. EAW is an acoustic-like mode that requires a non-Maxwellian electron distribution. It is also known to be heavily Landau damped and never seen in a laboratory device directly. Our theoretical analysis in the fluid description, predicts that in presence of a drifting, cold electron component, this mode can be destabilized. Incidentally, our Magnetize Plasma Linear Experimental (MaPLE) device provided such favorable platform and we have succeeded for the first time in observing the EAW propagation. Detailed experimental studies verified a wave indeed was excited and it propagated axially with a phase velocity of∼ 1.8 times the electron thermal velocity. Analytic treatment predicted the cold drifting electrons in the plasma can ease the stringent condition of hot-to-cold electron temperature ratio, Teh >> Tec and destabilizes EAW, which is also established experimentally. The observed dispersion relation matches well with the analytical outcome. A critical drift velocity, dependent on density and temperature ratios, is observed above which the mode ceases to exist. Experimental observations also support this aspect.
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YP11.00020: The Talbot-Lau x-ray deflectometer: a refraction-based electron density diagnostic for High Energy Density experiments Maria Pia Valdivia, Felipe Veloso, Dan Stutman, Dan Stutman, Christian Stoeckl, Chad Mileham, Wolfgang R. Theobald, Ildar Begishev, Sean P Regan, Milenko Vescovi, Wilmer Useche Talbot-Lau X-ray Deflectometry (TXD) has been developed as an electron density diagnostic for High Energy Density (HED) plasmas. The diagnostic delivers refraction, attenuation, elemental composition, and scatter information from a single-shot Moiré image. A Talbot-Lau interferometer has been benchmarked using laser-target and X-pinch x-ray backlighters. Grating survival and electron density mapping were demonstrated for: a) 25–29 J, 8–30 ps laser pulses using Cu targets and b) a 4 x 25 µm copper X-pinch driven by a ~350kA/350ns generator. X-ray backlighter quality was assessed in order to optimize areal electron density gradient retrieval and electron density mapping. TXD enabled accurate areal electron density detection with high contrast (>25%) and spatial resolution of ~50 µm in the high-power laser experiments, while lower contrast (<15%) and a higher spatial resolution of <27 µm were found in pulsed power experiments, demonstrating the potential of TXD as an electron density diagnostic for HED plasmas. |
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YP11.00021: Tracking inner-hole-state dynamics of complex atoms via double-color x-ray laster beams Yongqiang Li, Cheng Gao, Xiaowei Wang, Zengxiu Zhao, jianmin Yuan Both coherent pumping and energy relaxation play important roles in understanding physical processes of ultra-intense coherent light-matter interactions. Here, using a large-scale quantum master equation approach, we describe dynamical processes of practical open quantum systems driven by both coherent and stochastic interactions. As examples, two typical cases of light-matter interactions are studied. First, we investigate coherent dynamics of inner-shell electrons of a neon gas irradiated by a high intensity X-ray laser along with vast number of decaying channels. In these single-photon dominated processes, we find that, due to coherence-induced Rabi oscillations and power broadening effects, the photon absorptions of a neon gas can be suppressed resulting in differences in ionization processes and final ion-stage distributions. Second, we demonstrate a new scheme for the investigation of hole dynamics of complex atoms based on two-color ultrashort X-ray pulses. |
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YP11.00022: Local Gyrokinetic Study of Electrostatic Microinstabilities in a Field-Reversed Configuration Shu-ying Sun, Hua-sheng Xie, Yang Li Field-reversed configuration (FRC) has been increasingly studied in recent years, such as the C-2U in Tri. Alpha Energy Co. and KMAX in USTC. The results on C-2U experiments and GTC simulations [1] showed that, under higher parameters, turbulence caused by electrostatic micro-instabilities have significant impact on transport in a FRC. However, the theoretical illustration on these micro-instabilities behavior in FRC under a wider range of parameters has not been reported up to now. Here, the electrostatic micro-instabilities in the core region and scrape-off-layer of an equilibrium FRC are investigated, based on the gyro-kinetic dispersion relationship and 1D particle-in-cell simulation. The effects of a wide range of parameters, such as temperature gradients, density gradients, vertical wave-vector, electron-to-ion temperature ratio and mass ratio, on the growth-rate and mode structure of the instabilities were given. This work provides more theoretical support for understanding turbulent transport in a FRC. |
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YP11.00023: MeV electron acceleration at 1 kHz using mJ-scale few-cycle laser pulses Fatholah Salehi, Sina Zahedpour Anaraki, Scott W Hancock, Howard Michael Milchberg We demonstrate acceleration of quasi monoenergetic electron bunches to MeV-scale energies at 1 kHz repetition rate using laser pulses with ~7fs duration and ~2.2mJ energy focused on a near-critical density gas target. Our previous experiments using 30fs laser pulses showed that using a near-critical density gas jet lowers the critical power for relativistic self-focusing sufficiently to enable MeV-scale electron acceleration using a high repetition rate laser system with mJ-scale pulse energy [1]. However, those electron bunches, accelerated in the self-modulated wakefield regime, had a large energy spread and divergence angle. |
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YP11.00024: Characterization of Plasma Properties in a Hollow Cathode Discharge Nathaniel Wirgau, John E Foster, Hani Kamhawi An important life limiter in conventional gridded ion and Hall thrusters is lifetime of the hollow cathode assembly. The cathode’s lifetime is a function of the barium supply within the insert. While barium can be lost from the cathode via gas phase diffusion, models predict that significant barium recirculation within the insert actually occurs. The recirculation and transport of barium is dependent on plasma conditions prevailing in the hollow cathode. Presented here are plasma measurements made within the hollow cathode under conditions of spot and plume mode operation. Ion acoustic wave spectra is also examined in an attempt to assess recirculating barium concentration within the insert region using krypton as a propellant. |
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YP11.00025: Discharge and ion beam optimization of an advanced annular ion engine John E Foster, Michael J Patterson The annular ion engine is a new type of gridded ion thruster featuring a central ion optics support. The central support allows the engine to accommodate larger ion optics and thus operate at higher power, thereby circumventing the span to gap limitations of conventional ion engines. The architecture is inherently scalable in power. The geometry also provides additional magnetic collection area for electrons thereby increasing the engine’s capacity to accommodate much larger discharge currents before the onset of convective instabilities. In contrast to conventional ion engines, the discharge cathode is not located on axis and thus discharge uniformity relies on drifts defined by the magnetic circuit and local electric field. Presented here is the use of a 3-D magnetosolver code in conjunction with a plasma 0-D model aimed at optimizing discharge performance to minimize discharge losses. The design and implementation of an advanced graphite ion optics set is also discussed. |
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YP11.00026: Electromagnetic shocks in quantum vacuum Hedvika Kadlecová, Georg Korn, Sergei Bulanov The interaction of two counter-propagating electromagnetic waves in vacuum is analyzed within the framework of the Heisenberg-Euler formalism in quantum electrodynamics. The nonlinear electromagnetic wave in quantum vacuum is characterized by the wave steepening, subsequent generation of higher-order-harmonics, electromagnetic shock wave formation with electron-positron pair generation at the shock wave front. |
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YP11.00027: NIMROD simulations of reconnection during LHCD in C-Mod Eric D Emdee, Fatima Ebrahimi, Amitava Bhattacharjee We examine the role of reconnection during LHCD experiments in C-Mod. Some of these experiments exhibit strong MHD activity after application of LH current drive. Here we employ the resisitive magnetohydrodynamic code NIMROD to examine growth time and severity of both core and edge modes during LHCD experiments with MHD activity in C-Mod. To examine the role of reconnection, we first use high aspect ratio C-Mod equilibrium profiles from EFIT from LHCD experiments. We find that the core n=1 reconnecting mode, as well as edge resonant resistive modes localized near the edge, are unstable. We perform a Lundquist number scan, and find that growth rates of both core and edge modes scale with a negative power of the Lundquist number. The nature of edge peeling-like modes, created when the edge peak current density is sufficiently peaked [F. Ebrahimi, Phys. Plasmas 24, 056119 (2017)], as well as preliminary nonlinear results will be presented.
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YP11.00028: Exploring the effects of magnetic helicity in reconnecting plasma loops Landry Horimbere, Francisco Matos Ortiz, Timothy W. Koeth, Daniel Perry Lathrop Magnetic helicity is believed to play an important structuring role in coronal reconnection events, such as solar flares and coronal mass ejections. Reconnection is responsible for the topological changes necessary for plasma loops to erupt into the solar wind and, in the process, redistributes energy from magnetic fields to particles, accelerating them to relativistic speeds. Experiments exploring the interaction of magnetized plasma loops have, in the counter-helical configuration, yielded unexpectedly high X-ray fluxes.1 These results suggest that the energy spectrum of solar cosmic rays is dependent on the relative helicity configurations of the reconnecting loops. This experiment aims to directly measure the particle energy distribution over a full 180-degree range of relative helicity configurations. Here, we present progress in the construction of the pulse plasma discharge system. Our preliminary discharges were supplied by a 2 µF capacitor at 3 kV and reached peak currents of 4 kA. We are re-configuring the device to simultaneously produce two plasma loops as well as expanding the power supply to reach our design current of 20 kA. |
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YP11.00029: Construction of the BLUE Linear Transformer Driver System at the University of Michigan Ryan McBride, Brendan Sporer, Nick Jordan The University of Michigan has received four linear transformer driver (LTD) cavities, which were previously a part of the Ursa Minor experiment at Sandia. As such, the stack has been somewhat facetiously deemed the Bestowed LTD from the Ursa Minor Experiment, or BLUE. BLUE will be capable of delivering up to 8kJ of energy in a ~200ns, ~200kA pulse for high-power microwave and Z-pinch experiments. Currently, the cavities are fully disassembled, and located within a newly-built concrete wall. Based on our existing LTD system, MAIZE, we have begun to consider the charging and triggering sub-systems for BLUE. Our ambitious goal is to rep-rate the entire system as fast as 10Hz, though this may only prove possible with lower voltage and/or lower total capacitance. We have considered transforming BLUE into a relatively new driver architecture known as an impedance-matched Marx generator, or IMG. An IMG is very similar to an LTD but does not require the ferrite cores, which can represent a significant fraction of an LTD cavity’s weight. We are carefully evaluating other possible issues to determine the feasibility of an LTD-IMG transition with BLUE. |
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YP11.00030: ELM elimination via lithium pellet gravitational injection into H-mode plasmas using the ITER-like tungsten divertor on EAST Zhen Sun, Rajesh Maingi, Jiansheng Hu, Guizhong Zuo, John Canik, Erik P Gilson, Alex Nagy, Yuzhong Qian, Ming Huang, Wei Xu, Xiancai Meng, Dennis K. Mansfield, Robert A. Lunsford, Xianzu Gong A reproducible, fully non-inductive, low q95, H-mode regime devoid of large ELMs has been achieved by Li granule gravitational injection in EAST with tungsten divertor, extending previous results with Li powder [J.S. Hu et al., PRL 114 (2015) 055001, R. Maingi et al. Nucl. Fusion 58 (2018) 024003]. Spherical Li pellets with 0.7mm diameter were gravitationally delivered by a new impurity powder dropper above the top X-point. Li pellets injected into the scrape-off layer of low q95(3.8), low natural ELM frequency(70Hz) and high auxiliary heating (8MW) plasmas successfully eliminated ELMs for 2.5s. These discharges did not suffer from density or impurity accumulation and constant core radiated power was maintained. The observed ELM elimination is correlated with a decrease in particle recycling with significantly reduced Dα baseline emission, with a stronger effect on wall conditioning than Li powder injection. Because granules can penetrate farther into the scape-off layer than powder, their extrapolability to future devices is more promising than pure powder injection. |
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YP11.00031: Laser-driven magnetic reconnection and particle acceleration by snail-shaped target irradiation King Fai Farley Law, Yuki Abe, Philipp Korneev, Joao J Santos, Shinsuke Fujioka Magnetic reconnection is a process of magnetic field topology rearrangement which converts magnetic field energy into energy of charged particles through the magnetic reconnection outflow. In this work, we report a laboratory experiment scheme of magnetic reconnection study by using high intensity laser irradiation of a snail-shaped target. In our experiment, a magnetized plasmoid with magnetic field magnitude of about several kilo-teslas and with anti-parallel geometry was produced inside the target volume. The magnetic field magnitude and structure was characterized by proton radiography measurements. As the consequence of magnetic reconnection, ions are accelerated along the outflow direction and current sheet direction were separately observed in the experiment. In both directions, we observed proton beams with conversion efficiency comparable to the Target Normal Sheath Acceleration(TNSA) scheme. This also showed the potential of the presented setup as an alternative laser-driven ion acceleration scheme. 3-D PIC simulations are performed for further understanding of above phenomenon. Simulation results would be discussed in detail, especially about the details of particle acceleration mechanism. |
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YP11.00032: Studies of inelastic collision effects on the alpha particle flux with the FOCUS code Cesar Fernando Clauser, Ricardo Farengo, Hugo Emilio Ferrari, Jose Armando Boedo The effect of inelastic collisions on the alpha particle flux to the divertor is studied using the FOCUS code [1]. FOCUS calculates the exact orbits of the alpha particles including elastic and inelastic collisions. It is written in CUDA and runs on GPUs. FOCUS can read equilibria reconstructed by EFIT and use the information provided by SOL codes (i. e. SOLPS-ITER [2]). The results of simulations performed with FOCUS for an ITER equilibrium are presented. We find that for low energy alpha particles (E< 150 keV), the addition of inelastic collisions produces a dramatic change in distribution of the flux, reversing the asymmetry between the inner and outer targets and increasing the flux to the dome region. At intermediate energies (150 keV< E<1 MeV ) the addition of inelastic collisions spreads the flux over a larger area, including the dome and its supporting structure, but does not reverse the asymmetry. Finally, at high energies (E > 1 MeV), were the reaction rates for the inelastic processes decrease rapidly, both cases give approximately the same result. [1] C. F. Clauser, R. Farengo, H. E. Ferrari. Comp. Phys. Comm., in press (2018). [2] H. Pacher et al., J. Nucl. Mater. 463 (2015) 591. |
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YP11.00033: Core Confinement Optimisation Using Nonlinear Simulations Edmund Highcock, Noah Mandell, Michael Barnes, William D Dorland The heat confinement in a toroidal axisymmetric plasma is optimised using the CoRFU framework. This incorporates Trinity transport calculations which include full nonlinear turbulent transport simulations. These are carried out using GRYFX, a GPU-based hybrid-gyrofluid code which includes a gyrokinetic treatment of the zonal flows, and novel closures to model perpendicular phase mixing. Starting from a simple oval equilibrium, the elongation and triangularity of the outer flux surface are varied by the algorithm to optimise the fusion power per unit volume. This performance metric is increased by 91% during the simulation. |
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YP11.00034: Stacked-pulse driven laser-plasma electron accelerator for compact gamma-ray sources. Serge Youri Kalmykov, Xavier Davoine, Bradley A. Shadwick Driving a laser-plasma accelerator (LPA) with an incoherent superposition (stack) of multi-TW optical blocks of different colors opens the way to GeV-scale electron acceleration in a mm-length, dense plasma (n0 ∼ 1019 cm-3), without resorting to single-shot PW facilities. Stack-driven LPA affords production of electron beams (e-beams) at a kHz repetition rate, at a manageable average laser power. Immunity of the stack to self-compression (i) avoids deformation (elongation) of the accelerating bucket (a ``bubble'' of electron density), keeping the e-beam background-free through dephasing, and (ii) delays dephasing, thereby doubling electron energy against the predictions of standard scaling. In our PIC simulations, quasi-monoenergetic e-beams (or trains of such beams, in a single shot) have a 5-D brightness 1016 - 1017 A/m2 and mean energy tunable up to 1 GeV without changing the target. These unconventional beams, inaccessible with standard acceleration techniques, emit highly collimated, quasi-monochromatic, gigawatt γ-ray pulses via Thomson scattering process, in the range 3 - 17 MeV, each pulse corresponding to a distinct energy band (with ∼ 106 photons per band) [S. Y. Kalmykov et al., New J. Phys. 20, 023047 (2018).] |
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YP11.00035: Investigation of the standoff distance of the bow shock in a laboratory space science experiment I-Lin Yeh, Chih-Ruei Hsieh, Po-Yu Chang The interaction between the solar wind and unmagnetized planet will be studied in laboratory conditions using a 6 kJ pulsed-power system in Institute of Space and Plasma Sciences, National Cheng Kung University, Taiwan. The experiment will be implemented by generating a supersonic plasma flow with Mach number up to 20 using a conical wire array flowing through an obstacle. We use the semi-empirical model to determine the standoff distance of the bow shock produced by the supersonic plasma jet around a spherical obstacle. Factors that influence the standoff distance will be analyzed. This result will be used for designing future experiments. |
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YP11.00036: Study on beam wave interaction and mode competition in a fusion gyrotron using 3-D electromagnetic particle-in-cell simulation Ming-Chieh Lin, David N Smithe 3-D time-domain modeling of beam wave interaction in a fusion gyrotron is very challenging due to the open-end features of cavity structure and higher-order mode excitation employed to achieve high efficiency. In a previous project, namely GyroPIC, funded by DOE, several required capabilities or algorithms had been developed in a 3-D particle-in-cell (PIC) simulation code in order to simulate the beam wave interaction in a high-order mode gyrotron. In past decades, it has been considered as an impossible mission to simulate a fusion gyrotron in 3-D using a PIC based method. The preliminary results showed that the 3-D electromagnetic PIC method could provide an alternative modeling tool for studying the gyrotron cavity interaction and mode competition for gaining more physics insight and further improving the efficiency. Upon finishing the code development, a research effort has been continued in recent years to study the beam wave interaction and mode competition using the well-developed 3-D electromagnetic PIC simulation. |
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YP11.00037: Development of 320CH 2D ion Doppler tomography for the investigation of fine structure measurement of high guide field reconnection Haruaki Tanaka, Hiroshi Tanabe, Qinghong Cao, Moe Akimitsu, Asuka Sawada, Yasushi Ono The fine structure of high guide field reconnection has been investigated using a new 96CH/320CH sub-cm 2D ion Doppler tomography diagnostics in TS-6 tokamak merging experiment. In addition to the previously reported ion heating characteristics of double peak profile formation, the new system successfully detects two types of fine structure of high guide field reconnection: (i) localized ion heating in the diffusion region and (ii) field-aligned fine structure formation in the downstream. The former case is affected by Hall effect and forms more tilted profile with higher mass ratio; while in the latter case, high Ti area starts propagating along the closed flux surface formed by reconnected field lines because the ratio of parallel/perpendicular heat diffusivity c⊥/c∥~ 2(wcitii)2 is higher than 10 even in low field side for tokamak merging. The improvement of transport coefficient with high guide field strongly suppresses the perpendicular heat conduction and the reconnection heating transport finally forms a double-ring-like structure after the end of merging. |
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YP11.00038: A fast spectral solver for the nonlinear Boltzmann equation George J Wilkie Recent advances in applied mathematics have brought direct solution of the Boltzmann equation within reach of practical applications. The Galerkin-Petrov approach developed by Gamba and Rjasanow [1] is shown to be particularly efficient. LightningBoltz is a newly developed implementation of this algorithm, generalized for non-standard Gaussian quadrature rules and inelastic collisions. Collision matrices are pre-computed and stored in an online database. Such a tool will be useful for studying many kinetic problems in neutral gases and weakly-ionized plasmas, particularly when atomic and/or molecular excitation is important. Such applications include discharges, lightning, reentry communications blackout, divertor detachment, and more. |
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YP11.00039: A Laser-Wakefield-Acceleration based Thomson Source for Medical Imaging Jan-Patrick Schwinkendorf, Simon Bohlen, Theresa Karoline Bruemmer, Martin Meisel, Kristjan Poder, Paul Pourmoussavi, Jan-Hendrik Roeckemann, Lucas Schaper, Theresa Staufer, Florian Gruener, Jens Osterhoff The combination of laser wakefield acceleration and intense laser pulses is capable of providing a very compact and versatile source of X-rays. X-rays in the 50-100 keV energy range are of particular interest, by having high enough energy to penetrate through soft tissue while being absorbed by gold nanoparticles. Experiments and developments towards such a source for X-ray fluorescence imaging (XFI) are presented, including the calibration of non-invasive, cavity based charge diagnostics. Further, different techniques to determine the emittance as well as the design of the gas target infrastructure and feedback-based optimization of the pulse shape are presented. |
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YP11.00040: A Methodology for Evaluation of Charged Particle Trajectory Integrators Larry D Ludeking Simulation codes for the design of advanced charged particle (CP) devices are necessary because of the nonlinear nature of the combined Newton-Lorentz/Maxwell equations. Numerical techniques for the integration of charged particle trajectories are core features of these codes. The range of parameters spanned by CP devices requires a variety of techniques to meet accuracy, speed and stability requirements. This paper examines the accuracy and performance of several leap-frog charged-particle pushers used in both electro-static and electromagnetic simulations. Accuracy was evaluated by comparison to relativistically correct analytic solutions for a set of six standard field configurations. The relativistic modified-Boris push and several variants developed to address specific shortcomings were examined. As with nearly all numerical methods, results depend on the type of problem and the parameter space o interest. We do not settle on a "best" algorithm, but rather suggest a general methodology for rapidly assessing the performance of individual algorithms for specific performance and problem constraints. |
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YP11.00041: Active control for high repetition-rate laser wakefield accelerators Jon Murphy, Zhaohan He, Peter R Kordell, Bixue Hou, Yong Ma, Mark Mathis, John Nees, Alexander GR Thomas, Karl Michael Krushelnick Many potential applications of laser accelerator sources require operation at high repetition rate. Here, few-milliJoule pulses are generated at kilohertz repetition rate for wakefield acceleration. A genetic algorithm is implemented using a deformable mirror with the wakefield electron beam signal optimized onto several different masks used as feedback. This procedure allows a heuristic search for the optimal wavefront under laser-plasma conditions that is not known a priori. We are able to improve the electron beam charge and angular distribution by an order of magnitude. These improvements do not simply correspond to finding the ‘best’ focal spot, as the highest quality vacuum focal spot produces an inferior electron beam. The saved deformable mirror configurations coupled with the use of a Shack-Hartmann wavefront sensor promise to characterize the laser phase front that induces a plasma wave which provides optimal accelerating fields. In progress is the addition of a cold jacket that insulates the gas delivery system to achieve a gas target with higher density for complete characterization. |
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YP11.00042: Direct laser acceleration of electrons in a plasma bubble by a tightly confined laser pulse. Vladimir Khudik, Tianhong Wang, Gennady Shvets Direct laser acceleration (DLA) in a plasma bubble presents a unique opportunity to manipulate the transverse and longitudinal phase spaces of the electron beam propagating inside a plasma bubble. We present an analytical theory and supporting PIC simulations that illustrate several overlooked effects in DLA. A small but finite longitudinal electric field of the laser pulse will be shown to produce a significant deceleration effect that partially offsets the acceleration by its transverse component. It is also shown that the DLA effect is not confined to moderate-energy electrons, but can be extended to GeV-scale beams. Finally, we show that DLA exists even in the decelerating portion of the plasma bubble. Applications of the DLA to X-ray generation and to the enhancement of the transformer ratio of a plasma wakefield accelerator will be discussed, and the results of the full PIC and a novel quasi-static code will be presented. |
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YP11.00043: The design and preliminary test of a portable unit for generating both neutrons and X-rays based on a high-intensity pulsed discharge plasma Zhen Yang, Jidong long, Jie Li, Tao Wang, Pan Dong, Yuhang Chen The conventional imaging technologies are not easily visualized to detect and identity various of materials. X-ray radiography can highlight heavier chemical elements well but not good at revealing lighter elements; By contrast, neutron radiography is good at visualizing lighter elements. A concept of combining the complementary benefits of X-ray and neutron radiography in one portable unit was introduced as the new imaging tool. A compact high-intensity pulsed discharge plasma source was applied to produce deuterons to generate neutrons by the fusion reaction D-D/D-T. Meanwhile, the secondary electrons were produced near the target by ions bombarding. The high voltage was used to accelerate electrons towards the ion source where their bremsstrahlung produces X-rays. The axes of X-ray emission and neutron emission can be parallel to create the images of the object one after the other. Now, the ion beam current was about hundreds milliampere and electron beam current was about tens of ampere with tens of Hz repetition rate. Some detailed technical issues and future works are also introduced and discussed in the text. |
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YP11.00044: A benchmark of the eigenfunction method and the 5D simulation on the fast ion slowing down velocity distribution in the NBI-heated helical plasmas Shin Nishimura, Hiroyuki Yamaguchi Recently, the eigenfunction method for the fast ion slowing down velocity distribution in NBI-heated and/or burning plasmas [1,2] was generalized to general stellarator/heliotron configurations. It is already applied to calculations of momentum input [3] and the fast ions’ anisotropic pressure [4] in NBI-heated stellarator/heliotron plasmas. However, complicated guiding center motions in the perpendicular direction are basically neglected in this semi-analytical method. Another tool for investing the NB-produced fast ions that has been used for the LHD (Large Helical Device) experiment is a 5D simulation code GNET. [5] In this presentation, a benchmark of these contrasting methods for the LHD configuration is shown. [1] C.T.Hsu, P.J.Catto, and D.J.Sigmar, Phys.Fluids B 2, 280 (1990)[2] M.Taguchi, Nucl.Fusion 32, 143 (1992) [3] S.Nishimura, et al., Phys.Plasmas 22, 092505 (2015) [4] S.Nishimura, Phys.Plasmas 25,042509 (2018) [5] H.Yamaguchi and S.Murakami, Plasma Fusion Res. 9, 3403127 (2014) |
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YP11.00045: Heat transport and core turbulence measurements on the optimized stellarator Wendelstein 7-X G. M. Weir, J. H. E. Proll, T. Windisch, A. v. Stechow, S. Bozhenkov, A. Dinklage, E. Edlund, T. Estrada, G. Fuchert, O. Grulke, M. Hirsch, U. Hoefel, S. Kwak, A. Langenberg, N. Pablant, E. Pasch, M. Porkolab, E. Scott, J. Smoniewski, T. Stange, F. Warmer, P. Xanthopoulos, D. Zhang, Z. Huang, T. Klinger The optimized stellarator Wendelstein 7-X (W7-X) is designed to have an approximately quasi-isodynamic (QI) magnetic configuration with reduced neoclassical transport compared to a classical stellarator, and turbulent transport is expected to be a significant source of heat transport across the plasma minor radius. The Ion Temperature Gradient (ITG) and Trapped Electron Mode (TEM) are the dominant turbulence channels in low beta W7-X plasmas, and gyrokinetic modelling indicates two major differences between ITG/TEMs in W7-X and previous tokamak and stellarator experiments. The ITG mode is predicted to be highly localized along a helical band in W7-X, causing a reduction of the associated turbulence, and the nearly QI configuration of W7-X is predicted to be resilient to collisionless TEMs. In this contribution, the electron heat transport is compared to neoclassical predictions, and the stiffness in the electron heat transport is compared to TEM/ITG mode driven turbulence in two magnetic configurations of W7-X: one nearly QI configuration as well as a configuration with deliberately reduced QI. Additionally, electron temperature and plasma density fluctuations are compared to linear and nonlinear gyrokinetic calculations to assess the stabilization of TEM turbulence in W7-X. |
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YP11.00046: Radial dependence of parametric instability due to magnetic perturbation: a scenario for ELM control Salil Das, Raghvendra Singh, Hogun Jhang The stability of the tokamak edge pedestal to the ballooning-kink mode is examined in the presence of the applied stochastic field lines using the four-wave parametric process. It is shown that the H-mode (high confinement mode) edge pedestal is unstable to the ideal ballooning mode in the absence of the applied stochastic fields. It is also shown that in the presence of the kink effect, dominated by the bootstrap current, the ballooning mode is more virulent. The modes are suppressed if the applied magnetic field perturbation amplitude is larger than. The instability driven due to small dissipation, which may be a possible candidate for transport in the pedestal region, is unaffected by these applied fields. Hence we expect no change in the edge pedestal confinement. We have opposing results in the outer edge region where the density and the temperature profiles are flattened. These results are consistent with the observations by Evans et al1. 1. T. E. Evans, R. A. Moyer, P. R. Thomas et al., Phys. Rev. Lett. 92, 235003 (2004) |
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YP11.00047: Advances on the High Field Ultra-Low Aspect Ratio Tokamak (HF-ULART) Celso Ribeiro The High Field Ultra-Low Aspect Ratio Tokamak (HF-ULART) is a medium-size device proposal[1]. The major objective is to explore the high beta and plasma pressure under broad range of toroidal field and operational modes, yet using present day technology. This might be one of pathway scenarios for a potential ultra-compact pulsed neutron source (UCP-NS) based on the spherical tokamak (ST) concept, which may lead to more steady-state NS or even to a fusion reactor, via realistic design scaling. The HF-ULART pulsed mode operation is created by applying quasi-simultaneous adiabatic compression (AC) in both minor and major radius of a very high beta plasma, possibly with further help of passive-wall stabilization, as envisaged in the RULART concept[2]. This may help the revival of the studies of the AC technique in tokamaks. In addition, by similarities, studies in HF-ULART as a UCP-NS may also help to test the feasibility of the compact NS via the spheromak concept, which also uses the AC technique[3]. Simulations of AC and in plasma equilibrium and stability in HF-ULART plasmas will be presented. [1] C. Ribeiro IAEA-RUSFD 2017 [2] C. Ribeiro Proc. 26th SOFE 2015 [3] S. Woodruff et al, Fus. Sci. Tech.-2017 |
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YP11.00048: WEST first plasma operation with all tungsten plasma facing components Clarisse Bourdelle, Jérôme Bucalossi, Nicolas Fedorczak, Thierry Loarer, Philippe Moreau, Emmanuelle Tsitrone The transformation of Tore Supra into WEST, from limited to diverted magnetic configuration and from carbon to tungsten plasma facing components was completed at the end of 2016. Plasma breakdown was readily obtained. Plasma current ramp up to 800 kA was achieved after the alteration of in-vessel copper passive structures. The operational window was narrow: at lower prefill pressure, no breakdown occurred while at higher prefill pressure, transition to electron slideaway discharges happened. After a first boronization, performed in July 2018, plasma startup window opened up. Slideaway electron discharges disappeared and higher plasma density was reached. Dedicated PFC diagnostics allow for first power deposition studies. So far, up to 2.8 MW of LHCD and 0.6 MW of ICRH have been injected into the plasma with a fraction of radiative power around 60%. The radial electric field (measured by Doppler reflectometer) deepens inside the separatrix with additional power, although H mode has not be reached yet. The ongoing phase of operation is dedicated to H mode access and test of the ITER-like plasma facing unit prototypes. A full ITER like actively cooled lower divertor will be installed in 2019. In this 2nd phase, the pulse length will be extended to 1000s. |
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YP11.00049: Performance impacts on direct drive ICF due to beam width William Richard Magrogan, Brett Scheiner, Mark Jude Schmitt In direct drive ICF, beams incident on a spherical target impose a variety of incidence angles across the illumination surface which may be characterized by the beam radius (rb) to target radius (rt). Previous studies [1] examine the effects of the rb/rt ratio on implosion velocity, scattered light fraction, and neutron yield in the rb/rt ~ 1 regime. In this presentation, the impact of the beam radius to target radius on the ablation pressure and the mass ablation rate due to incidence angle spread is explored using radiation hydrodynamic simulations. An analytic expression is derived for the average incidence angle of a beam as a function of the rb/rt ratio. Comparisons of the ablation pressure, mass ablation rate, and scattered light fraction are made with respect to this average incidence angle parameter. Analysis of the effects of beam width may indicate benefits of using narrower beams such as in the Revolver design [2]. [1] D. Froula, et al. Phy. Rev. Lett. 108 125003 (2012) [2] K. Molvig, et al. Phy. Rev. Lett. 116, 255003 (2016) |
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YP11.00050: Laser-Ion Lens and Accelerator (LILAC): Towards Point-Like Sources of Relativistic Ions Vladimir Khudik, Tianhong Wang, Gennady Shvets We report the focusing of a shaped thin target by a circular-polarized laser pulse at 1022 W/cm2 intensity, to a low-emittance, quasi-monoenergetic proton beam. The target shape is designed to be simultaneously deformed and focused into a cubic micron spot by the radiation pressure during its acceleration. A simple model reminiscent of geometric optics of the ions will be described. The model predicts the self-consistent dynamics of the nanostructured thin target, as well as the target’s shape that is necessary for focusing without aberrations. Three-dimensional particle-in-cell (PIC) simulations show that the focal length and the final energy are in good agreement with the scaling laws obtained from the geometric optics model. Extensive scans of the laser and target parameters identify the stable propagation regime where the Rayleigh-Taylor-like instability is suppressed. Several simulations for different laser powers (from under a petawatt to multi-petawatt) are presented. The possibility of obtaining proton beams with densities of order 1023/cm3 and energy density up to 2 × 1013 J/cm3 at the focal point is discussed.
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YP11.00051: Development of Burst-Mode Laser based High-speed Thomson Scattering Instruments for Fusion Plasma Devices Cary D. Smith, Zhili Zhang, Theodore Mathias Biewer, Naibo jiang, Sukesh Roy Practical fusion plasma devices at various DOE and international facilities need the state-of-the-art diagnostic tools to understand, predict, and control fusion plasmas. Thomson scattering (TS), as a popular diagnostics technique, has successfully measured electron temperature and density inside the plasma system. However, the traditional TS technique operates only at 10Hz. Since the devices operate at on the order of 10- 100ms, it means only one data point can be obtained for each cycle. We develop high repetition rate Thomson scattering instruments based on high-speed pulsed burst laser system. Pulse burst laser was originally developed for supersonic or hypersonic flow diagnostics. In a typical pulse burst laser, a burst (or train) of high-energy laser pulses is generated at high repetition rates over a period of time (~20 ms). The pulse sequence can reach energies of ~100 mJ per individual pulse up to MHz rates or 1 Joule per individual pulse up to 10 kHz, while maintaining low average power. Thomson scattering system is being used to obtain electron number density and electron temperature in the plasma. |
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YP11.00052: Simulation of channel formation through laser gas interaction Jarrod Leddy, Aaron Bernstein, Jason Brooks, Michael C Downer, Isabella M Pagano, John Robert Cary Lasers of sufficient intensity passing through a neutral gas will ionize the gas creating a plasma channel in their wake. A shock can propagate from this locally heated region through the created plasma and background gas, however the density of the plasma will determine the dynamics of the plasma. For collisional (high density) plasmas this can be modelled with a fluid code, however a kinetic simulation is required for low collisionality. The Vorpal code [1] allows for self-consistent modelling of the laser pulse, plasma formation via field ionization, laser-plasma interaction, and subsequent plasma dynamics via particle-in-cell and EM simulation. It also includes a new reaction framework that enables the simulation of collisional dynamics such as elastic collisions, impact ionizations, and charge exchange reactions. We will show results for these laser-plasma interactions with varying gas/plasma densities spanning from highly collisional to collisionless regimes. [1] C. Nieter and J. Cary. 2004. VORPAL: a versatile plasma simulation code. J. Comput. Phys. 196, 2, 448-473. |
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YP11.00053: Fourier-Fourier numerical method for Vlasov-Poisson system and comparison of different Fourier filters for suppression of the filamentation in the velocity space. Denis Silantyev We compare numerical methods (Fourier-Fourier method with various Fourier filters in the velocity space vs. finite difference and finite volume methods) for evolution of nonlinear periodic Langmuir waves via solving 1D+1V Vlasov-Poisson system. The focus is made on the solution accuracy during a long term evolution for which some kind of numerical filtering in velocity space is inevitable due to increasing filamentation issue in the velocity space (formation of the sharp features in the solution) and impossibility of resolving such solutions accurately in the long run. Explicit filters are used for Fourier-Fourier method while similar effects come in implicitly for finite difference and finite volume methods due to their discretizations. |
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YP11.00054: Numerical Simulation of an Experimental Turbulent Dynamo on the OMEGA-EP Laser Andy Sha Liao, Shengtai Li, Kirk Flippo, Hui Li, Chikang Li Turbulent dynamos that exponentially amplify initially small, seed magnetic fields are crucial in magnetizing the universe. Until now, the ideal environment for turbulent dynamos to grow has proved difficult to recreate. In a new approach, we leverage the long pulse capability of the OMEGA-EP laser to recreate the highly conductive and inviscid (Rem ~ 3000, Prm ~ 1) growth environment of the turbulent dynamo within the magnetized plasma jet ablated from a simple cone target of CH plastic. In 3-D FLASH simulations of our scheme, we find that the dynamo environment is a typically ~ 0.5 mm3, ~ 1 keV hot spot where the laser beams intersect to produce maximum direct heating of the jet plasma. The dynamo environment is maintained from the onset of steady flows through the ~ 10 ns length of the laser pulse. For a plasma vorticity of 0.3-1.5 ns-1 and a dynamo active over 2-4 ns, the magnetic energy detectably increases on an exponential trajectory by a factor of a few. By demonstrating the tight dependence of dynamo activity on an easily-controlled variable, i.e. the laser beams, we show that our scheme can readily be extended to study even more complex behaviors. |
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YP11.00055: Bad Foils in Simulated Opacity Experiments Andy S Liao, Manolo Sherrill, Igor Olegovich Usov, Douglas Vodnik, Christopher J Fontes, Todd J Urbatsch Discrepancies in Solar interior models have ignited ongoing efforts to directly measure opacities of metals in Solar interior conditions recreated in high energy density facilities. Occasionally, alloy foils made for these experiments arrive defective with cracks. However, whether these cracks can actually bias the results of experiments is unquantified. To close this gap, Cassio simulations are used to predict the dynamics of defective foils in numerical experiments on a representative iron-magnesium alloy. We find that, across a range of preheat and radiation source models, the cracks self-heal. However, for foils with visible cracks wider than 10 μm, the healing is too slow to seal the cracks by the time of the opacity measurement. Consequently, a significant amount of unattenuated backlight radiation can still reach through the cracks to the spectrograph, leading to underestimating the attenuation of the transmitted light. This bias is unrecoverable. If the foils’ density away from cracks and the nominal opacity were known a priori, the area covered by cracks at measurement time can still be estimated using partial covering models. |
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YP11.00056: Gyrokinetic Dynamic Fidelity Refinement William Dorland, Michael Nastac, Anna Grafov, Joe Taylor Most gyrokinetic codes are designed to run on high-end supercomputers, including exascale devices. Key problems that such codes are used for include microstability, turbulence, transport, and concept optimization. As part of "whole device modeling" in particular, we need accurate calculations of these phenomena on teraflop devices (or even gigaflop) when the gyrokinetic phenomena comprise only part of a complex problem. Doubling the resolution of each dimension of a 5D + time problem requires an approximately 64X increase in resources at a minimum (for linear scaling of resources with problem size). Tripling the resolution in all dimensions with typical application performance available today (eg, to enable coupled ITG/ETG turbulence calculations) typically requires well over 1000X increase in resources. There is a major opportunity in these grim assessments, for algorithms that can dynamically choose the mesh discretization levels and even the physics model automatically, as demanded at any instance within any particular calculation, particularly to meet user-supplied uncertainty targets. We present key advances toward this goal of "dynamic fidelity refinement" (DFR). Our prototype electromagnetic, gyrokinetic DFR algorithm runs on a desktop. |
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YP11.00057: Measure of dissipation region in collisionless magnetic reconnection Obioma Ohia, Peter Montag A new measure to identify dissipation regions in collisionless magnetic reconnection is proposed. This measure builds upon the measure proposed by Zenitani (PRL 2011) and is also a Lorentz invariant scalar related to nonideal energy conversion. However, this measure uses a reference frame with vanishing perpendicular electric field. The significance of this frame is discussed and the measure is applied to two-dimensional particle-in-cell simulations. |
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YP11.00058: Closures for moment-based gyrokinetic models Michael Nastac, William Dorland, Noah Mandell Moment-based models bridge the gap between fluid and kinetic descriptions of plasmas. While high collisionality naturally produces the fluid limit from the infinite moment hierarchy, systems that are weakly collisional do not have this advantage, leaving the question of how the resolved moments are affected by their coupling to the unresolved moments. This is the classic moment closure problem. What is the “right” closure? How many moments (how much velocity space resolution) are required to resolve the key physics in any given calculation? Existing gyrokinetic algorithms generally rely upon high resolution to sidestep these questions. This approach can be very expensive. Closures, on the other hand, must address complications such as finite Larmor radius effects and nonlinearities, which introduce complicated couplings to unresolved moments. We present recent work that builds upon the parallel phase-mixing closures of Hammett and Perkins [PRL, 1990], in the context of gyrokinetics. |
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YP11.00059: The Bayesian Inference Engine (BIE): a computational statistical inference framework for deceleration-phase Rayleigh-Taylor instability studies Benjamin J Tobias, Sasikumar Palaniyappan, Joshua Paul Sauppe, Codie Yoshiko Fiedler Kawaguchi, Kirk Flippo, John L Kline The Bayesian Inference Enging (BIE) has been utilized for the analysis of radiographic images capturing the dynamic evolution of deceleration-phase R.-T. modes during laser-driven implosions of cylindrical targets at Omega. Within the BIE, an analyst may construct a parameterized physical model representing the object being imaged, produce synthetic data, and optimize model parameters to obtain a maximum a posteriori solution that considers both weighted statistical likelihood and prior information. 2D implosions are modeled so as to infer the growth rate and evolution of cylindrical modes in a driven Al marker layer, comprehensively accounting for blur, alignment and illumination effects to achieve unprecedented accuracy for comparison to hydrodynamic simulation. The BIE also allows uncertainties to be quantified in a rigorous manner through response surface methodologies, establishing sensible error bars and guiding the refinement of experimental techniques. |
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YP11.00060: Inclusion of higher-order and uncertainty heuristics for accelerated Jacobian-free inexact Newton convergence with the PIES code Daniel Raburn, Allan Reiman We have investigated a novel Jacobian-free inexact Newton method for use with the Princeton Iterative Equilibrium Solver (PIES code). This enhanced method makes use of heuristics for uncertainties and higher-order terms in the non-linear objective function in order to optimize between (conventional) "compounding" and "reorthnormalization" adaptive preconditioning; equivalently, this method can be thought of as an enhanced Broyden's method, one which aims to produce a more accurate approximation of the Jacobian of the objective function based on the available numerical information. A speed-up over conventional methods is expected. |
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YP11.00061: Ion orbital behavior in diverted tokamak geometries Y. Nishimura, J.C. Lyu, L.J. Zheng, F.L. Waelbroeck Ion particle transport in diverted tokamak geometries is investigated employing analytical models for the equilibrium magnetic field. In the presence of magnetic stochasticity, magnetic field lines inside the separatrix can be connected to the open magnetic field line regions. After crossing the separatrix, passing particles can easily reach the divertor plates within finite toroidal evolutions while trapped particles persist due to conservation of second adiabatic invariant. Interestingly, a steady state H-mode is sustained with RMPs [T.E.Evans, R.A.Moyer et al., Nature Phys. 2, 419 (2006).] even in the presence of an enhanced ion particle loss. Global plasma profiles and distribution function at the steady state with the RMPs is computed. The computational model is also applied to study ion transport in negative triangularity tokamaks. While they are expected to reduce the heat flux because of the larger major radius (at the radius where the separatrix crosses the divertor plates), radial extent of the particle deposition can be localized due to small magnetic drifts at the outboard side which has nearly a vertical shape. Computation of heat loads on the divertor plates, with an optimized magnetic configuration is discussed. |
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YP11.00062: Development of a Poisson solver in a global field aligned mesh in a tokamak edge geometry Yasutaro Nishimura A Poisson solver is developed to study macroscopic and microscopic instabilities in a diverted tokamak geometry. A global field aligned mesh [B.D.Scott, Phys. Plasmas 5, 2334 (1998).] provides us with computational efficiency to describe structures of toroidal eigenmodes, which follow the helically twisted magnetic field lines. The field aligned mesh can be applied to the open-field-line regions as well. However, on the separatrix, the magnetic field line is purely toroidal at the X-point (an infinite number of toroidal transits is required for the magnetic field to reach there). Inevitably, in the very vicinity of the separatrix, one needs to revert back to a mesh in a cylindrical coordinate. We employ a finite element elliptic solver (with the field aligned mesh) and analyze realistically how close we can approach to the separatrix. |
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YP11.00063: Radial dependence of parametric instability due to magnetic perturbation: a scenario for ELM control. Salil Das, Raghvendra Singh, Hogun Jhang The stability of the tokamak edge pedestal to the ballooning-kink mode is examined in the presence of the applied stochastic field lines using the four-wave parametric process. It is shown that the H-mode (high confinement mode) edge pedestal is unstable to the ideal ballooning mode in the absence of the applied stochastic fields. It is also shown that in the presence of the kink effect, dominated by the bootstrap current, the ballooning mode is more virulent. The modes are suppressed if the applied magnetic field perturbation amplitude is larger than 0.18%. The instability driven due to small dissipation, which may be a possible candidate for transport in the pedestal region, is unaffected by these applied fields. Hence we expect no change in the edge pedestal confinement. We have opposing results in the outer edge region where the density and the temperature profiles are flattened. These results are consistent with the observations by Evans et al1. |
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YP11.00064: The Scarlet Laser Facility – Part of Laser Network US – Now available for external users Rebecca L. Daskalova, Ginevra Cochran, Nicholas Czapla, Derek Nasir, Anthony Zingale, Zachary Cotman, Jordan Purcell, Enam Chowdhury, Douglass Schumacher, Linn D Van Woerkom The Scarlet laser facility at The Ohio State University is based on a 300 TW, 30 fs, 815 nm pulsed laser system capable of firing up to one shot per minute. Using an off axis parabola, pulses can be focused to under a spot size of 3 μm resulting in an on-target intensity above 5x1021 W/cm2. The facility includes a large 76 inch diameter, 85 inch tall experimental chamber with 38 large aperture diagnostic ports and 7 large doors for each access. A range of diagnostics are available including electron and ion spectrometers. Scarlet is now available for external users as part of the new DOE funded Laser Network US. This network, in addition to Scarlet, is based on lasers at Colorado State University, Lawrence Berkeley Laboratory, University of Michigan, University of Nebraska – Lincoln, University of Texas – Austin, and the Stanford Linear Accelerator. |
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YP11.00065: Systematic behavior of spherically converging shocks in the gigabar regime Damian Swift, Natalie Kostinski, Brian Maddox, Andrea Kritcher, Amy Lazicki, Heather D Whitley, Joseph Nilsen Spherically-converging shocks at the National Ignition Facility can generate states in the gigabar range, which can be probed by x-ray radiography to give absolute equation of state (EOS) measurements. We have previously used profile-matching to reconstruct Hugoniot states without having to assume the functional form of the EOS, although this approach does not exclude solutions which are not consistent with hydrodynamics and thus the uncertainties on deduced Hugoniot states may be larger than necessary. In principle, profile-matching can be used to deduce isentropes through each Hugoniot point in the converging-shock radiograph, though the experimental data have been too noisy to do this without hydrodynamic constraints. Previous studies dating back to Guderley (1942) have used self-similar solutions of converging shocks in an ideal gas to relate the acceleration to the heat capacity ratio. Hydrodynamic simulations show that flow is only approximately self-similar in the gigabar regime, but the acceleration can be used to constrain and discriminate between EOS. Such solutions test off-Hugoniot states as well as those on the Hugoniot. |
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YP11.00066: Numerical Investigation of Radiative Heat Fronts in Optically Thick Materials Griffin Cearley, Robert Vandervort, Matthew Trantham, Paul A Keiter, R. Paul Drake, Eric Johnsen When an optically thick body is exposed to an intense radiation field, its temperature equilibrates with the radiation field by the propagation of a nonlinear heat front. This causes the surface of the body to ablate and a shock to pass into the interior. Radiative heat fronts of this type have been created in recent experiments performed on the OMEGA laser system in the context of studying radiation-driven implosion of molecular gas clouds. We provide results of simulations performed with the CRASH radiation hydrodynamics code to be used for the design of future experiments. These simulations incorporate details of the system created in these experiments (spot size, time-dependent drive). We provide details of shock structure and strength from simulations for comparison with experimental radiographs, and we interpret an apparent bow-shock feature observed in radiographs. Furthermore, we evaluate the potential use of self-similar solutions, which may allow estimation of quantities for experimental design without further simulations. |
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YP11.00067: Wakefields in a Cluster Plasma Marko W Mayr, Muhammad F Kasim, Luke Ceurvorst, James Sadler, Kevin Glize, Alexander Savin, Ben Spiers, Robin Hsiao-Wu Wang, Ricardo Fonseca, Luis O Silva, Peter Andrew Norreys We report the first study of large amplitude Langmuir waves in a plasma of nanometer-scale clusters. The shape of these wakefields is captured by an oblique-angle frequency-domain holography diagnostic for the first time. The wavefronts are observed to curve backwards, in contrast to the forwards curvature of wakefields in uniform plasma. The first wakefield period is longer than those trailing it. Next, the experimental data is compared to fully relativistic 2D particle-in-cell simulations and a new injection mechanism is found. Our simulations indicate that this mechanism leads to a greater maximum momentum and energy spread compared to electrons trapped in the uniform case. |
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YP11.00068: Time Dependent Gamma Measurement of Ablator Areal Density Velocity Near Bangtime on Inertial Confinement Capsules Kevin Meaney, Yongho Kim, Hans Herrmann, Hermann Geppert-Kleinrath, Nelson M Hoffman The Gamma Reaction History (GRH) diagnostic at the National Ignition Facility (NIF) measures the neutron induced inelastic scattered gammas as a function of time. By taking different gamma thresholds, the carbon signal from the ablator of the capsule can be isolated from the gammas created by the surrounding hohlraum and thermal mechanical package. The areal density of the carbon near bangtime gives insight into trajectory of the shell as well its compression and stagnation due to the hot spot outward pressure. Measured values of the major NIF campaigns are compared with expected shell areal density scaling from simple hydrodynamic simulation (Helios) as well as an analytical toy rocket model. Furthermore, the time difference of the carbon gamma peak compared to the fusion burn gives information about the movement of the shell throughout the burn time. Shell velocity at bangtime reveals insights to shot performance and gives a lower limit for residual kinetic energy. The measured carbon rhoR for the different major NIF capsule campaigns will be reported, contrasted and discussed. |
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YP11.00069: 3-D electromagnetic particle-in-cell simulation study on low-frequency oscillation in a fusion gyrotron Ming-Chieh Lin, David N Smithe In a previous study, it was found that not only magnetic compression profile but initial thermal velocities of electrons play an important role in causing a low-frequency oscillation (LFO) in the operation of a magnetron injection gun (MIG) employed in an MIT fusion gyrotron. An unphysical particle boundary condition, i.e., large initial thermal velocities of electrons, had to be assumed to produce the LFO in the 3-D electromagnetic (EM) particle-in-cell (PIC) simulation. In this work, we have included the gyrotron cavity along with the MIG as well as a large vacuum envelope representing the vacuum chamber similar to that employed in the MIT experiments. In the EM PIC simulation, it is found that the momentum of electrons is suppressed by the space charge due to the vacuum envelope. For the first time, without unphysical approximations, the LFO could be reproduced in the 3D time domain EM PIC simulations. The initial velocity spread at the cathode temperature is assumed in this simulation in contrast to an unphysical larger velocity spread formerly used. |
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YP11.00070: Noise and error minimization in particle-based plasma simulation methods Evstati G Evstatiev, John Finn, Brad Shadwick We describe a novel noise-error minimizing method applicable to particle-based plasma simulation algorithms; e.g., PIC, (variational) energy-conserving, delta-f, hybrid, etc.
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YP11.00071: Atom-in-jellium calculations of equations of state for warm dense matter Thomas Lockard, Mandy Bethkenhagen, Sebastien Hamel, Alfredo Correa, Lorin Benedict, Philip A Sterne, Charles Starrett, Carl W Greeff, Richard Gordon Kraus, Bard Bennett, Damian Swift Recent path-integral Monte Carlo and quantum molecular dynamics simulations have shown that average-atom models can predict thermodynamic states in warm dense matter to within a few percent. Atom-in-jellium models have typically been used to predict the electron-thermal behavior only, although they can calculate the Einstein frequency of ion oscillations and hence to predict the entire equation of state (EOS). We have extended this approach to calculate the mean displacement of an ion over a wide range of compression and temperature. Expressed as a fraction of the Wigner-Seitz radius, the displacement is a measure of the asymptotic freedom of the ion at high temperature, and thus of the change in heat capacity from 6 to 3 modes per atom. A functional form for free energy was proposed with a single free parameter representing the effective number of potential modes to be saturated, and investigated using molecular dynamics simulations. The ion-thermal contribution was thus estimatedwithout requiring a large number of molecular dynamics simulations. |
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