Bulletin of the American Physical Society
58th Annual Meeting of the APS Division of Plasma Physics
Volume 61, Number 18
Monday–Friday, October 31–November 4 2016; San Jose, California
Session GP10: Poster Session III (Particle Beams and Radiation; MHD, Transients and Simulation; Mini-Conference: Gyrokinetic Algorithms; Magneto-inertial Fusion; Basic Plasma Physics)Poster
|
Hide Abstracts |
Room: Exhibit Hall 1 |
|
GP10.00001: PARTICLE BEAMS AND RADIATION |
|
GP10.00002: Wakefield-acceleration of relativistic electrons with few-cycle laser pulses at kHz-repetition-rate Diego Guenot, Dominykas Gustas, Aline Vernier, Frederik Boehle, Benoit Beaurepaire, Rodrigo Lopez-Martens, Jerome Faure The generation of relativistic electron beams using laser wakefield acceleration has become a standard technique, providing low emittance electron bunches with femtosecond durations. However, this technique usually requires multi-ten-terawatt lasers and is thus limited to low repetition-rate (typically 10 Hz or less). We have recently demonstrated the generation of few MeV electrons using 2.5-mJ, 4-fs, 1-kHz repetition-rate laser pulses, focused to relativistic intensity onto a gas jet with electron density $\approx $10$^{\mathrm{20}}$ cm$^{\mathrm{-3}}$. We have investigated the influence of the pulse duration, the gas density. We demonstrated that an electron beam with a charge in the range of 10-fC/shot, with a divergence of 20-mrad and a peaked spectrum with energies between 2 and 4 MeV can be generated at kHz repetition-rate. These results confirm the possibility of using few-cycle laser pulses with very low energy for exciting wakefields in the bubble regime and for trapping electrons, as predicted by PIC simulations. This kHz electron source is ideally suited for performing electron diffraction experiments with very high temporal resolution. Our results also open the way to other applications, such as the generation of a kHz ultrafast X-ray source. [Preview Abstract] |
|
GP10.00003: Improving the Performance and Portability of VPIC Robert Bird, Evan Peters, David Nystrom, Brian Albright VPIC is a Particle-in-Cell (PIC) code which is able to deliver novel science at unprecedened scale. Most notably, this includes the attainment of petaflop performance during the simulation of trillions of particles. Such high levels of performance have historically been achieved though the use of vectorization, and explicit compiler intrinsics in VPIC. An approach which can offer good performance, at the cost of the code needing to be re-written for each new vector-width or hardware architecture. In this work we present an investigation of how modern coding techniques and auto-vectorization can be used to enable VPIC to automatically scale to new vector-widths and hardware platforms, using a single codebase which no longer needs to be manually adapted for new vector-widths. To achieve this, we express the core PIC algorithm in such a way that the compiler is able to generate auto-vectorized instructions, including an adaptation of the algorithm so that it no longer contains data dependencies. We also present a performance study for this new code variant, showing that it is able to achieve vector performance comparable to that of historic hand coded intrinsics on both Intel Knights Landing and traditional Intel Xeon platforms. [Preview Abstract] |
|
GP10.00004: Applying Boundary Conditions Using a Time-Dependent Lagrangian for Modeling Laser-Plasma Interactions Jonathan Reyes, B. A. Shadwick Modeling the evolution of a short, intense laser pulse propagating through an underdense plasma is of particular interest in the physics of laser-plasma interactions. Numerical models are typically created by first discretizing the equations of motion and then imposing boundary conditions. Using the variational principle of Chen and Sudan, we spatially discretize the Lagrangian density to obtain discrete equations of motion and a discrete energy conservation law which is exactly satisfied regardless of the spatial grid resolution. Modifying the derived equations of motion (e.g., enforcing boundary conditions) generally ruins energy conservation. However, time-dependent terms can be added to the Lagrangian which force the equations of motion to have the desired boundary conditions. Although some foresight is needed to choose these time-dependent terms, this approach provides a mechanism for energy to exit the closed system while allowing the conservation law to account for the loss. An appropriate time discretization scheme is selected based on stability analysis and resolution requirements. We present results using this variational approach in a co-moving coordinate system and compare such results to those using traditional second-order methods. [Preview Abstract] |
|
GP10.00005: 3D hybrid simulations of the plasma penetration across a magnetic field Yuri Omelchenko The expansion of hot dense plasmas across ambient magnetic fields in physical systems with spatial scales comparable to the ion gyro and inertial lengths is of great interest to space physics and fusion. This work presents results from recent three-dimensional hybrid simulations (kinetic ions, fluid electrons) of experiments at the LAPD and Nevada Terawatt Facility where short-pulse lasers are used to ablate solid targets to produce plasmas that expand across external magnetic fields. The first simulation recreates flutelike density striations observed at the leading edge of the carbon plasma and predicts an early destruction of the magnetic cavity in agreement with experimental evidence. In the second simulation the plasma contains protons and carbon ions produced during the ablation of a polyethylene target. A mechanism is demonstrated that allows protons to penetrate the magnetic field in the form of a collimated flow while the carbon ion component forms a supporting magnetic structure. The role of ion kinetic and Hall effects in creating an electric field responsible for plasma transport is discussed and results are compared to experimental data. The hybrid simulations are performed with a massively parallel hybrid code, HYPERS that advances fields and particles asynchronously on time scales determined by local physical and geometric properties. [Preview Abstract] |
|
GP10.00006: SMILEI: A collaborative, open-source, multi-purpose PIC code for the next generation of super-computers Mickael Grech, J Derouillat, A Beck, M Chiaramello, A Grassi, F Niel, F Perez, T Vinci, M Fle, N Aunai, J Dargent, I Plotnikov, G Bouchard, P Savoini, C Riconda Over the last decades, Particle-In-Cell (PIC) codes have been central tools for plasma simulations. Today, new trends in High-Performance Computing (HPC) are emerging, dramatically changing HPC-relevant software design and putting some - if not most - legacy codes far beyond the level of performance expected on the new and future massively-parallel super computers. \underline {SMILEI} is a new open-source PIC code co-developed by both plasma physicists and HPC specialists, and applied to a wide range of physics-related studies: from laser-plasma interaction to astrophysical plasmas. It benefits from an innovative parallelization strategy that relies on a super-domain-decomposition allowing for enhanced cache-use and efficient dynamic load balancing. Beyond these HPC-related developments, SMILEI also benefits from additional physics modules allowing to deal with binary collisions, field and collisional ionization and radiation back-reaction. This poster presents the SMILEI project, its HPC capabilities and illustrates some of the physics problems tackled with SMILEI. [Preview Abstract] |
|
GP10.00007: Approaches to Simulating the Prompt Electromagnetic Pulse Alex Friedman, Bruce I. Cohen, Chester D. Eng, William A. Farmer, David P. Grote, Hans W. Kruger, David J. Larson LLNL is developing a suite of modern tools for simulating the generation and propagation of the prompt (E1) electromagnetic pulse (EMP). These include the 3-D EMPulse code, based on PIC methods with a Cartesian grid in the laboratory frame [1], and a companion 3-D approach which builds on the methods used in Longmire's fast-running CHAP code [2]. In CHAP, and in our own CHAP-lite [3], 1-D spherical symmetry is assumed, and the calculation takes advantage of a separation of scales. The independent coordinates are (r,$\tau )$, where r is the distance from the source and $\tau \quad =$ t-r/c; the pulse varies slowly with r at fixed $\tau $, so a coarse radial grid can be used. We seek similar efficiencies in 3-D, incorporating non-spherically-symmetric physics via a vector spherical harmonic decomposition. For each (l,m) harmonic, the radial equation is similar to that in CHAP-lite. We describe this approach, along with other aspects of our project. [1] B. I. Cohen, et. al., \textit{this Conference}. [2] C. L. Longmire, \textit{IEEE Trans. Electromagnetic Compatibility} \textbf{20} no. 1, 3 (1978). [3] W. A. Farmer, et al., \textit{IEEE Trans. Nuclear Science} \textbf{63}, 1259 (2016). [Preview Abstract] |
|
GP10.00008: EMPulse, a new 3-D simulation code for electromagnetic pulse studies Bruce Cohen, Chester Eng, William Farmer, Alex Friedman, David Grote, Hans Kruger, David Larson EMPulse is a comprehensive and modern 3-D simulation code for electro-magnetic pulse (EMP) formation and propagation studies, being developed at LLNL as part of a suite of codes to study E1 EMP originating from prompt gamma rays [1]. EMPulse builds upon the open-source Warp particle-in-cell code framework$^{\mathrm{\thinspace }}$developed by members of this team and collaborators at other institutions. The goal of this endeavor is a new tool enabling the detailed and self-consistent study of multi-dimensional effects in geometries that have typically been treated only approximately. Here we present an overview of the project, the models and methods that have been developed and incorporated into EMPulse, tests of these models, comparisons to simulations undertaken in CHAP-lite [2] (derived from the legacy code CHAP due to C. Longmire and co-workers [3]), and some approaches to increased computational efficiency being studied within our project. [1] A. Friedman, et. al., \textit{this Conference.} [2] W. A. Farmer, et al., \textit{IEEE Trans. Nuclear Science }\textbf{63}, 1259 (2016). [3] C. L. Longmire, IEEE Trans. Electromagnetic Compatibility \textbf{20 no. 1}, 3 (1978). [Preview Abstract] |
|
GP10.00009: Electron Beam Focusing and Spreading due to interactions With Copropagating Plasma Waves and Lasers: Explanation of Simulation Results A. Bowman, R. L. Williams Numerical simulation results suggest that a low energy electron beam, injected perpendicularly across co-propagating plasma waves and laser beams, can be compressed to a line focus under certain conditions, but under different conditions can be spread out into two main lobes on which bunching patterns are impressed. We report several explanations for these observations, and also discuss the similarity of these results to other research results previously reported in the literature. The prospects for testing these results in a laboratory will be discussed, as well as the use of these phenomena as diagnostics. Supported by the Department of Energy. [Preview Abstract] |
|
GP10.00010: OSIRIS 4.0: new version of the OSIRIS framework Ricardo Fonseca, Adam Tableman, Jorge Vieira, Viktor Decyk, Warren Mori, Luís Silva OSIRIS [1] is a state of the art, fully relativistic massively parallel particle in cell code, that is widely used in kinetic plasma modeling for many astrophysical and laboratory scenarios. Over the years the code has been continuously improved, adding new features and algorithms, resulting in a large and complex code base with the inherent difficulties on maintenance and development. We report on the new version of the OSIRIS framework, focusing on the new structure of the code that leverages on the object oriented features of Fortran 2003, that are now widely supported by available compilers. Details on the new object-oriented structure, that allows for the encapsulation of specific features, and better collaboration between the development team, are given. We also focus on the new strategy for run-time selection of simulation mode, that allows for a single binary to be used with all code features, and report on the template based code generation for multiple interpolation levels. Finally, we report on our experience on implementing these features with multiple compilers, and the code changes required to ensure a wide compiler support. [1]~R. A. Fonseca et al., Lecture Notes in Computer Science \textbf{2331}, 342-351 (2002) [Preview Abstract] |
|
GP10.00011: Implementation of a 3D version of ponderomotive guiding center solver in particle-in-cell code OSIRIS Anton Helm, Jorge Vieira, Luis Silva, Ricardo Fonseca Laser-driven accelerators gained an increased attention over the past decades. Typical modeling techniques for laser wakefield acceleration~(LWFA) are based on particle-in-cell (PIC) simulations. PIC simulations, however, are very computationally expensive due to the disparity of the relevant scales ranging from the laser wavelength, in the micrometer range, to the acceleration length, currently beyond the ten centimeter range. To minimize the gap between these despair scales the ponderomotive guiding center (PGC) algorithm [1, 2] is a promising approach. By describing the evolution of the laser pulse envelope separately, only the scales larger than the plasma wavelength are required to be resolved in the PGC algorithm, leading to speedups in several orders of magnitude. Previous work was limited to two dimensions [3]. Here we present the implementation of the 3D version of a PGC solver into the massively parallel, fully relativistic PIC code OSIRIS [4]. We extended the solver to include periodic boundary conditions and parallelization in all spatial dimensions. We present benchmarks for distributed and shared memory parallelization. We also discuss the stability of the PGC solver. [1] P. Mora and T.M. Antonsen, Phys. Rev. E 53, R2068 (1996) [2] P. Mora and T.M. Antonsen, Phys. Plasmas 4, 217 (1997) [3] D.F. Gordon et al., IEEE Trans. Plasma Sci. 28, 1224 (2000) [4] R.A. Fonseca et al., Lect. Notes Comput. Sci. 2331, 342 (2002) [Preview Abstract] |
|
GP10.00012: Kinetic Simulations -- Oshun (Vlasov-Fokker-Planck) and PIC (Osiris) -- Physics and Open Source Software In The UCLA PICKSE Initiative. Adam Tableman, Michail Tzoufras, Ricardo Fonseca, W.B. Mori We present physics results and general updates for two plasma kinetic simulation codes developed under the UCLA PICKSE initiative. We also discuss the issues around making these codes open source such that they can be used (and contributed too) by a large audience. The first code discussed is Oshun -- a Vlasov-Fokker-Planck (VFP) code. Recent simulations with the VFP code OSHUN [1] will be presented for all of the aforementioned problems. The algorithmic improvements that have facilitated these studies will be also be discussed. [1] M. Tzoufras, A.R. Bell, P.A. Norreys, F.S. Tsung, JCP 230 (17), 6475-6494 (2011); M. Tzoufras, A. Tableman, F.S. Tsung, W.B. Mori, A.R. Bell, Phys. Plasmas 20, 056303 (2013) The second code discussed is the PIC code Osiris. Osiris is a widely respected code used in hundreds of papers. Osiris was first developed for laser-plasma interactions but has grown into a robust framework covering most areas of plasma research. One defining feature of Osiris is that it is highly optimized for a variety of hardware configurations and scales linearly over 1 million$+$ CPU nodes. We will discuss the recently released version 4.0 written in modern, fully-object oriented FORTRAN. [Preview Abstract] |
|
GP10.00013: Generation of high quality electron beams via ionization injection in a plasma wakefield accelerator Navid Vafaei-Najafabadi, Chan Joshi Ionization injection in a beam driven plasma wakefield accelerator has been used to generate electron beams with over 30 GeV of energy in a 130 cm of lithium plasma. The experiments were performed using the 3 nC, 20.35 GeV electron beam at the FACET facility of the SLAC National Accelerator Laboratory as the driver of the wakefield. The ionization of helium atoms in the up ramp of a lithium plasma were injected into the wake and over the length of acceleration maintained an emittance on the order of 30 mm-mrad, which was an order of magnitude smaller than the drive beam, albeit with an energy spread of 10-20{\%}. The process of ionization injection occurs due to an increase in the electric field of the drive beam as it pinches through its betatron oscillations. Thus, this energy spread is attributed to the injection region encompassing multiple betatron oscillations. In this poster, we will present evidence through OSIRIS simulations of producing an injected beam with percent level energy spread and low emittance by designing the plasma parameters appropriately, such that the ionization injection occurs over a very limited distance of one betatron cycle. [Preview Abstract] |
|
GP10.00014: Linear to non linear analysis for positron acceleration in plasma hollow channel wakefields Ligia Diana Amorim, Weiming An, Warren B. Mori, Jorge Vieira Plasma wakefield accelerators are promising candidates for future generation compact accelerators. The standard regime of operation, non-linear or blowout regime, is reached when a particle bunch space charge or laser pulse ponderomotive force radially expels plasma electrons forming a bucket of ions that defocus positron bunches, thus preventing their acceleration. To avoid defocusing, hollow plasma channels have been considered [L. Yi et al., Scientific Reports 4, 4171 (2014)]. The corresponding wakefields have been examined in the linear [C. B. Schroeder et al., PoP 20, 123115 (2013)] and non-linear [J. Thomas et al.,PoP 23, 053108 (2016)] excitation regimes for electrons. It is therefore important to extend the theory for positron acceleration, particularly in the nonlinear regime where the wakefields strongly differ. In this work we explore the wakefield structure, examine the differences between the electron and positron beam cases, and explore positron acceleration in nonlinear regimes. We support our findings with multi-dimensional particle-in-cell simulations performed with OSIRIS [R.A. Fonseca et al., LNCS 2331, 342 (2002)] and quasi-3D [A. Davidson et al., JoCP 281, 1063 (2015)] and QuickPIC [C. Huang et al., JoCP 217, 658 (2006), W. An et al., JCP 250, 165 (2013)]. [Preview Abstract] |
|
GP10.00015: Modeling multi-GeV class laser-plasma accelerators with INF{\&}RNO Carlo Benedetti, Carl Schroeder, Stepan Bulanov, Cameron Geddes, Eric Esarey, Wim Leemans Laser plasma accelerators (LPAs) can produce accelerating gradients on the order of tens to hundreds of GV/m, making them attractive as compact particle accelerators for radiation production or as drivers for future high-energy colliders. Understanding and optimizing the performance of LPAs requires detailed numerical modeling of the nonlinear laser-plasma interaction. We present simulation results, obtained with the computationally efficient, PIC/fluid code INF{\&}RNO (INtegrated Fluid {\&} paRticle simulatioN cOde), concerning present (multi-GeV stages) and future (10 GeV stages) LPA experiments performed with the BELLA PW laser system at LBNL. In particular, we will illustrate the issues related to the guiding of a high-intensity, short-pulse, laser when a realistic description for both the laser driver and the background plasma is adopted. [Preview Abstract] |
|
GP10.00016: Evolution of beams in a plasma channel due to beam break up Gregory Penn, Remi Lehe, Jean-Luc Vay, Carl Schroeder, Eric Esarey We study the dynamics of beam break-up (BBU) of an accelerated electron beam in a plasma channel. Particle-in-cell simulations using the codes WARP and FBPIC are presented and interpreted in terms of theoretical calculations for the plasma-induced fields and the evolution of the instability. We focus on cylindrical channels for simplicity, and other geometries are considered to better understand the impact of BBU on electron beams undergoing laser-plasma wake field acceleration. We compare our findings with other published results. [Preview Abstract] |
|
GP10.00017: Narrow bandwidth Thomson photon source development using Laser-Plasma Accelerators C.G.R. Geddes, J. van Tilborg, H.-E. Tsai, Cs. Toth, J.-L. Vay, R. Lehe, C.B. Schroeder, E. Esarey, S.G. Rykovanov, D.P. Grote, A. Friedman, W.P. Leemans Compact, high-quality photon sources at MeV energies are being developed based on Laser-Plasma Accelerators (LPAs). An independent scattering laser with controlled pulse shaping in frequency and amplitude can be used together with laser guiding to realize high photon yield and narrow bandwidth. Simulations are presented on production of controllable narrow bandwidth sources using the beam and plasma capabilities of LPAs. Recent experiments and simulations demonstrate controllable LPAs in the energy range appropriate to MeV Thomson sources. Design of experiments and laser capabilities to combine these elements will be presented, towards a compact photon source system. A dedicated facility under construction will be described. [Preview Abstract] |
|
GP10.00018: Analysis of non-Gaussian laser mode guidance and evolution in leaky plasma channels Blagoje Djordjevic, Carlo Benedetti, Carl Schroeder, Eric Esarey, Wim Leemans The evolution and propagation of a non-Gaussian laser pulse under varying circumstances, including a typical matched parabolic channel as well as leaky channels, are investigated. It has previously been shown for a Gaussian pulse that matched guiding can be achieved using parabolic plasma channels. In the low power regime, it can be shown directly that for multi-mode pulses there is significant transverse beating. Given the adverse behavior of non-Gaussian pulses in traditional guiding designs, we examine the use of leaky channels to filter out higher modes as a means of optimizing laser conditions. The interaction between different modes can have an adverse effect on the laser pulse as it propagates through the primary channel. To improve guiding of the pulse we propose using leaky channels. Realistic plasma channel profiles are considered. Higher order mode content is lost through the leaky channel, while the fundamental mode remains well-guided. This is demonstrated using both numerical simulations as well as the source-dependent Laguerre-Gaussian modal expansion. In conclusion, an idealized plasma lens based on leaky channels is found to filter out the higher order modes and leave a near-Gaussian profile before the pulse enters the primary channel. [Preview Abstract] |
|
GP10.00019: Shaped Plasma Lenses for Optical Beam Control at High Laser Intensities R. F. Hubbard, J. P. Palastro, L. A. Johnson, B. Hafizi, D. F. Gordon, J. R. Penano, M. H. Helle, D. Kaganovich A plasma channel is a cylindrical plasma column with an on-axis density minimum. A short plasma channel can focus a laser pulse in much the same manner as a conventional lens or off-axis parabola [R. F. Hubbard, \textit{et al}., Phys. Plasmas \textbf{9}, 1431 (2002)]. If the plasma has an off-axis density maximum (``inverse channel''), it behaves like a negative lens and acts to defocus the pulse. In either case, a shaped plasma lens (SPL) may be placed in the beamline at locations where the laser intensity or fluence is orders of magnitude above the damage threshold for conventional solid optics. When placed after an off-axis parabola, SPLs may provide additional flexibility and spot size control and may also be useful in suppressing laser prepulse. For high power, ultrashort laser pulses, the broad laser bandwidth and extreme intensities produce chromatic and phase aberrations and amplitude distortions that degrade the lens focusing or defocusing performance [J. P. Palastro, \textit{et al}., Phys. Plasmas \textbf{22}, 123101 (2015)]. Although there have been a few experiments that demonstrate laser pulse focusing by a shaped plasma lens, generation and control of the plasma present significant challenges. Potential applications of SPLs to laser-plasma accelerators will be discussed. [Preview Abstract] |
|
GP10.00020: Exploring the orbital angular momentum of betatron radiation Joana Martins, Guenda Hehmann, Ricardo Fonseca, Luis Silva, Jorge Vieira Betatron radiation from laser-wakefield accelerators (LWFA) can be used as a broadband X-ray source. Betatron x-rays have attracted great interest and have applications in biological imaging which have been demonstrated experimentally (see for instance A. Rousse et al., Phys. Rev. Lett. 93, 135005 (2004); S. Kneip et al., Nat. Phys. 6, 980). Endowing betatron radiation with well defined states of orbital angular momentum (OAM), a fundamental property of light by which its wave fronts become twisted, could further enhance the imaging spatial resolution. However, the conditions for the generation of betatron x-rays with OAM, and the fundamental mechanisms underlying the transfer of OAM from electron trajectories to the radiation they emit, remain outstanding open questions. To explore these exciting open challenges, we investigate the OAM spectral content of betatron x-rays in LWFA. We explore the conditions and laser driver characteristics (with/without orbital and spin angular momentum) that can enable the emission of OAM x-rays. We support our studies by 3D numerical modelling, using the particle-in-cell code Osiris [R.A. Fonseca et al, PPCF, 55 124011 (2013)] and using the post processing radiation code jRad [J. L. Martins et al., Proc. SPIE 7359, 73590V (May 07, 2009)]. [Preview Abstract] |
|
GP10.00021: Target optimization for desired X-ray spectra produced by laser plasma accelerated electrons. Maxim Lobok, Andrey Brantov, Valery Bychenkov Different regimes of electron acceleration from low-density targets are investigated using three-dimensional numerical simulations. Multiple spatial target density profiles were examined, including laser pre-pulse modified targets. The size of the plasma corona is shown to be one of the main parameters characterizing the temperature and number of hot electrons, which determine the yield of X-ray radiation and its hardness. The generation of X-ray radiation by laser accelerated electrons, which impact the converter target located behind the laser target, was studied. The X-ray spectra were computed using Monte-Carlo simulations. [Preview Abstract] |
|
GP10.00022: A Stable High-Energy Electron Source from Laser Wakefield Acceleration Ping Zhang, Baozhen Zhao, Cheng Liu, Wenchao Yan, Grigory Golovin, Sudeep Banerjee, Shouyuan Chen, Daniel Haden, Colton Fruhling, Donald Umstadter The stability of the electron source from laser wake-field acceleration (LWFA) is essential for applications, such as novel x-ray sources and fundamental experiments in high field physics. To obtain such a stable source, we used an optimal laser pulse and a novel gas nozzle. The high-power laser pulse on target was focused to a diffraction-limited spot by the use of adaptive wavefront correction and the pulse duration was transform limited by the use of spectral feedback control. An innovative design for the nozzle led to a stable, flat-top profile with diameters of 4 mm and 8 mm with a high Mach-number (\textasciitilde 6). In experiments to generate high-energy electron beams by LWFA, we were able to obtain reproducible results with beam energy of 800 MeV and charge \textgreater 10 pC. Higher charge but broader energy spectrum resulted when the plasma density was increased. These developments have resulted in a laser-driven wakefield accelerator that is stable and robust. With this device, we show that narrowband high-energy x-rays beams can be generated by the inverse-Compton scattering process. This accelerator has also been used in recent experiments to study nonlinear effects in the interaction of high-energy electron beams with ultraintense laser pulses. [Preview Abstract] |
|
GP10.00023: Ionization-injected electron acceleration with sub-terawatt laser pulses Linus Feder, Andy Goers, George Hine, Bo Miao, Fatholah Salehi, Daniel Woodbury, Howard Milchberg The vast majority of laser wakefield acceleration (LWFA) experiments use drive lasers with peak powers \textgreater 10 TW and repetition rates from 10 Hz to less than once an hour. However, it was recently demonstrated that by using a thin, high density gas target, LWFA can be driven by laser pulses well below a TW and with high repetition rates [1,2].We present experiments and particle-in-cell (PIC) simulations of the effect of doping the high density gas jet with higher Z molecules (here nitrogen). Our earlier experiments with low-Z gas relied on self-injection of electrons into the accelerating wake through wave-breaking [1]. In ionization injection, the relativistically self-focused laser pulse ionizes the inner shell of the dopant inside the plasma wake [3]. High energy electrons are then trapped by the wakefield in the earliest potential buckets, which overlap with the laser pulse. PIC simulations show acceleration of these electrons by LWFA and directly by the laser pulse, with the direct contribution significantly increasing the electron energy beyond the LWFA contribution alone. Additionally, ionization injection can be controlled to prevent dephasing of the electron beam, resulting in a narrower energy spectrum and lower spatial divergence. 1. A.J. Goers et al., Phys. Rev. Lett. \textbf{115}, 194802 (2015) 2. F. Salehi et al., submitted for publication 3. A. Pak, et al., , Phys. Rev. Lett. 104, 025003 (2010). [Preview Abstract] |
|
GP10.00024: Laser wakefield signatures: from gas plasma to nanomaterials Deano Farinella, Xiaomei Zhang, Youngmin Shin, Toshiki Tajima The signatures of laser wakefields have become increasingly important in recent years due to the invention of a novel laser compression technique [1] that may enable the creation of single cycle x-ray pulses. This x-ray driver may be able to utilize solid density targets to create acceleration gradients of up to TeV/cm. On the other hand, Laser Wakefield Acceleration (LWFA) has been identified as a potential mechanism for the generation of Extreme High Energy Cosmic Rays (EHECR) in Active Galactic Nuclei (AGN) [2]. Though these disparate density regimes may include different physics, by investigating scalings of the ratio $n_{\mathrm{cr}}$/$n_{\mathrm{e}}$ we are able to survey a wide range of parameters to gain insight into particle acceleration and photon emission properties. The scaling of electron acceleration and photon radiation from wakefields as a function of the parameter $n_{\mathrm{cr}}$/$n_{\mathrm{e}}$ has been studied [3]. Further, acceleration gradient as well as other scalings were investigated in solid density channels and compared to gas plasma [4]. [1] G. Mourou et al., Eur. Phys. J. Spec. Top. 223, 1181 (2014) [2] T. Ebisuzaki and T. Tajima. Astropart. Phys. 56, 9 (2014) [3] D. M. Farinella et al., Phys. Plasmas 23, 073107 (2016) [4] X. M. Zhang et al., ``X-ray wakefield acceleration and betatron radiation in nanotubes,'' (2016), (submitted to Phys. Rev. AB) [Preview Abstract] |
|
GP10.00025: Laser Wakefield Acceleration Experiments in the Self Modulated Regime at Titan Paul King, Felicie Albert, Nuno Lemos, Siddarth Patankar, Joseph Ralph, Jessica Shaw, Manuel Hegelich, John Moody, Chan Joshi Picosecond laser plasma interaction has been studied as a novel source of producing betatron x-rays. In this regime, electrons are accelerated through the interplay of two mechanisms: self-modulated laser wakefield acceleration and direct laser acceleration. The experiment, conducted on the Titan laser system (1 ps and 150 Joules) at Lawrence Livermore National Lab, using electron densities of $0.5-1.5\times10^{19}cm^{-3}$, found electrons accelerated to energies of up to 250 MeV with divergence half angles on order of 10s of milliradians. Corresponding to the electron densities above, frequency shifts of laser light on order $\omega_p\sim 1.5-2\times 10^{14}$ rad/sec were measured using Raman forward scattering diagnostics. [Preview Abstract] |
|
GP10.00026: Backward Raman Amplifier for Laser Wakefield Accelerator Joshua Ludwig, Paul-Edouard Masson-Laborde, Stefan Huller, Wojciech Rozmus, Scott C. Wilks Particle in cell simulations via SCPIC [1] and theoretical work on Raman amplification and laser wake field acceleration will be presented. Laser energy depletion has been shown to be a limiting factor during wake field acceleration [2]. This work focuses on optimizing parameters for Raman amplification to work in conjunction with wake field acceleration in order in order to sustain an accelerating laser pulse as it generates plasma waves. It has been shown that laser pulses undergo red shifting during wake generation [2]. Our work demonstrates that this red shifting results in a detuning between pump and seed in the backward Raman Amplifier. This detuning limits the amount of energy that can be transferred from the pump to the seed, and places new limits on backward Raman amplification. To overcome this limiting factor, this study makes use of a chirped pump allowing for extended coupling to the accelerating pulse. Three wave coupling model of Raman amplifier with a frequency shift term due to wake field will also be discussed and compared with PIC simulations. [1] K. I. Popov et al, PHYSICAL REVIEW LETTERS 105, 195002 (2010) [2] B. A. Shadwick,C. B. Schroeder, and E. Esarey, PHYSICS OF PLASMAS 16, 056704 (2009) [Preview Abstract] |
(Author Not Attending)
|
GP10.00027: Explosion of relativistic electron vortices in laser plasmas Kirill Lezhnin, Fedor Kamenets, Timur Esirkepov, Sergei Bulanov, Yanjun Gu, Stefan Weber, Georg Korn The interaction of high intensity laser radiation with underdense plasma may lead to the formation of electron vortices. Though being quasistationary on an electron timescales, these structures tend to expand on a proton timescale due to Coloumb repulsion of ions. Using a simple analytical model of a stationary vortex as initial condition, 2D PIC simulations are performed. A number of effects are observed such as vortex boundary field intensification, multistream instabilities at the vortex boundary, and bending of the vortex boundary with the subsequent transformation into smaller electron vortices. [Preview Abstract] |
|
GP10.00028: PIC Simulations of direct laser accelerated electron from underdense plasmas using the OMEGA EP Laser Amina Hussein, Thomas Batson, Karl Krushelnick, Louise Willingale, Alex Arefiev, Tao Wang, Phil Nilson, Dustin Froula, Dan Haberberger, Andrew Davies, Wolfgang Theobald, Jackson Williams, Hui Chen The OMEGA EP laser system is used to study channeling phenomena and direct laser acceleration (DLA) through an underdense plasma. The interaction of a ps laser pulse with a subcritical density CH plasma plume results in the expulsion of electron along the laser axis, forming a positively charged channel. Electrons confined within this channel are subject to the action of the laser field as well as the transverse electric field of the channel, resulting the DLA of these electrons and the formation of a high energy electron beam. We have performed 2D simulations of ultra-intense laser radiation with underdense plasma using the PIC code EPOCH to investigate electron densities and self-consistently generated electric fields, as well as electron trajectories. This work was supported by the National Laser Users' Facility (NLUF), DOE. [Preview Abstract] |
|
GP10.00029: Direct laser acceleration of electrons in a strong azimuthal magnetic field Tao Wang, Toma Toncian, David Stark, Alexey Arefiev Recently published particle-in-cell simulations [Phys. Rev. Lett. 116, 185003 (2016)] indicate that a high-intensity laser irradiating an over-critical plasma can induce relativistic transparency and drive a Megatesla magnetic field while propagating into the plasma. At the same time, the quasi-static electric field in this regime is an order of magnitude weaker than the quasi-static magnetic field as a result of ion mobility and the fact that electrons are irradiated by a high intensity laser pulse. We have examined analytically and numerically direct laser acceleration of electrons in such an azimuthal magnetic field. We have considered a general case of a laser beam propagating with a superluminal phase velocity and compared the results to those for a luminal case. Our key finding is that the maximum gamma-factor that can be attained by electrons has a pronounced threshold, with a significant enhancement of the electron energy taking place above the threshold. The threshold is a function of the azimuthal magnetic field and of the initial transverse electron momentum. [Preview Abstract] |
|
GP10.00030: Non-planar electron motion during direct laser acceleration by a linearly/circularly polarized laser pulse. Vladimir Khudik, Alexey Arefiev, Xi Zhang, Gennady Shvets Direct Laser Acceleration (DLA) of electrons in plasma bubbles or ion channels is investigated in the general case of arbitrary polarization of laser pulse. When the laser pulse is linearly polarized, the laser electromagnetic field drives electron oscillations in the polarization plane, intuitively suggesting that the electron trajectory lies in the same plane. We show that strong modulations of the relativistic gamma-factor cause the free oscillations perpendicular to the plane of the driven motion to become unstable. As a consequence, out of plane displacements grow and the electron trajectory becomes strongly three-dimensional, even if it starts out planar during the early stage of the acceleration [1]. For a circularly polarized laser pulse, electron end up moving along a helical trajectory with slowly changing helix radius [2]. By deriving a set of dimensionless equations for paraxial ultra-relativistic electron motion, we have found an estimate for the maximum attainable electron energy for arbitrary laser and plasma parameters. [1] A. V. Arefiev et al, Phys. Plasmas 23, 023111 (2016); [2] V. Khudik et al, arXiv:1603.04757. [Preview Abstract] |
|
GP10.00031: Controlling the Direct Laser Acceleration Inside a Plasma Bubble Using Lasers' Polarization and Wavelength Xi Zhang, Vladimir Khudik, Rafal Zgadzaj, Aaron Bernstein, Mike Downer, Gennady Shvets The combination of the direct laser acceleration and laser wakefield acceleration (DLA and LWFA) mechanisms has been recently proposed [1,2] for increasing the total electron energy gain. Here we will report on the effects of the polarization and wavelength of the DLA pulse on the properties of the accelerated beam. Specifically, we address the moderate-power regime, where the laser powers of the leading LWFA and the trailing DLA pulses are not very much larger than the critical power. Three cases will be discussed: (a) the DLA pulse has the same wavelength and polarization as the LWFA pulse, (b) the wavelengths are the same but the polarizations are orthogonal, and (c) the wavelength of the DLA pulse is twice shorter than that of the LWFA pulse. LWFA via particle-in-cell (PIC) simulations. It is found that both (b) and (c) scenarios result in higher tolerance to pulse-delay jitter. The most promising scenario is (c) because it enables higher final electron bunch energy and charge.This work is supported by the US DOE grant DE-SC0007889 and the AFOSR grant FA9550-14-1-0045. [1] X. Zhang, V. N. Khudik and G. Shvets, Phys. Rev. Lett. , 184801 (2015). [2] X. Zhang, V. N. Khudik, A. Pukhov and G. Shvets, Plasma Phys. Control. Fusion 58 034011 (2016) [Preview Abstract] |
|
GP10.00032: Effects of dimensionality on computer simulations of laser-ion acceleration: When are three-dimensional simulations needed? L. Yin, D. J. Stark, B. J. Albright Laser-ion acceleration via relativistic induced transparency provides an effective means to accelerate ions to tens of MeV/nucleon over distances of 10s of $\mu $m. These ion sources may enable a host of applications, from fast ignition and x-rays sources to medical treatments. Understanding whether two-dimensional (2D) PIC simulations can capture the relevant 3D physics is important to the development of a predictive capability for short-pulse laser-ion acceleration and for economical design studies for applications of these accelerators. In this work, PIC simulations are performed in 3D and in 2D where the direction of the laser polarization is in the simulation plane (2D-P) and out-of-plane (2D-S). Our studies indicate modeling sensitivity to dimensionality and laser polarization. Differences arise in energy partition, electron heating, ion peak energy, and ion spectral shape. 2D-P simulations are found to over-predict electron heating and ion peak energy. The origin of these differences and the extent to which 2D simulations may capture the key acceleration dynamics will be discussed. [Preview Abstract] |
|
GP10.00033: Toward Extrapolating Two-Dimensional High-intensity Laser-Plasma Ion Acceleration Particle-in-Cell Simulations to Three Dimensions D. J. Stark, L. Yin, B. J. Albright, F. Guo A PIC study of laser-ion acceleration via relativistic induced transparency points to how 2D-S (laser polarization in the simulation plane) and -P (out-of-plane) simulations may capture different physics characterizing these systems, visible in their entirety in (often cost-prohibitive) 3D simulations. The electron momentum anisotropy induced in the target by the laser pulse is dramatically different in the two 2D cases, manifesting in differences in polarization shift, electric field strength, density threshold for onset of relativistic induced transparency [1], and target expansion timescales. In particular, a trajectory analysis of individual electrons and ions may allow one to delineate the role of the fields and modes responsible for ion acceleration. With this information, we consider how 2D simulations might be used to develop, in some respects, a fully 3D understanding of the system. [1] D. J. Stark, et al., \textit{Phys. Rev. Lett.} \textbf{115}, 025002 (2015). [Preview Abstract] |
|
GP10.00034: Neutron Generation from Laser-Accelerated Ion Beams: Use of Alternative Deuteron-Rich Targets for Improved Neutron Yield and Control of Neutron Spectra B. J. Albright, L. Yin, A. Favalli Laser-ion-beam generation in the break-out afterburner (BOA) acceleration regime has been modeled for several deuteron-rich solid-density targets using the VPIC particle-in-cell code [1]. Monte Carlo modeling of the transport of these beams in a beryllium converter in a pitcher-catcher neutron source configuration shows significant increases in neutron yields may be achievable through judicious choices of laser target material. Additionally, species-separation dynamics in some target materials during the BOA ion acceleration phase can be exploited to control the shapes of the neutron spectra. [1] K. J. Bowers et al., \textit{Phys. Plasmas} \textbf{15}, 055703 (2008). [Preview Abstract] |
|
GP10.00035: Efficient neutron generation by Coulomb explosions of multi-component cluster targets Masakatsu Murakami, Myles Allen Zosa Irradiating ultra intense ultrashort laser pulses on nano size cluster targets, protons are accelerated due to Coulomb explosion, the energies of which are of the order of a few MeV. In terms of the reactions between Lithium and the protons, neutrons are generated. optimizing the laser and target parameters, we maximize the coupling efficiency of neutron yields. In particular, the cluster targets are made of two or three atomic components in order to produce quasi-monoenergetc protons. The resultant neutrons are expected to have relatively low temperatures lower than a few 100 keV because the endothermic reactions. [Preview Abstract] |
|
GP10.00036: Improved theory for relativistic transmittance of circularly polarized laser pulses in non-ideal, realistic plasmas Teyoun Kang, Young-Kuk Kim, Min Sup Hur Owing to the rapid development of laser technologies, relativistically-induced transmittance (RT) of ultra-intense laser pulses in overdense plasmas is now a practically important matter. RT could give either deleterious or positive effects depending on the kinds of laser-plasma interactions. In radiation-pressure-acceleration (RPA), enhanced transmittance lowers the momentum transfer from the pulse to the ions. Meanwhile, in collisionless-electrostatic-shock, the acceleration efficiency can be increased owing to the effective heating of upstream electrons by transmitted laser fields. Previous theories mostly have handled RT in ideal plasmas, such as an infinitely long uniform plasma or a delta-function-like slab. In the actual applications, however, RT is generally combined with other dynamics, such as plasma density compression, leading to RT under a plasma in other cases. We developed one-dimensional RT theories for circularly polarized laser pulses, which would be used for such realistic plasma profiles. According to our theory, optimal thickness condition should be modified in RPA. Furthermore we developed our theory so that RT in the common two-step density plasma can be modeled. In this poster, we present the derivation and the comparison of the improved theory with PIC simulation results. [Preview Abstract] |
|
GP10.00037: Enhanced proton acceleration in an applied longitudinal magnetic field Toma Toncian, Alexey Arefiev, Gennady Fiksel Using two-dimensional particle-in-cell simulations, we examine how an externally applied strong magnetic impacts proton acceleration in laser-irradiated solid-density targets. We find that a kT-level external magnetic field can sufficiently inhibit transverse transport of hot electrons in a flat laser-irradiated target. While the electron heating by the laser remains mostly unaffected, the reduced electron transport during proton acceleration leads to an enhancement of maximum proton energies and the overall number of energetic protons. The resulting proton beam is much better collimated compared to a beam generated without applying a kT-level magnetic field. A factor of three enhancement of the laser energy conversion efficiency into multi-MeV protons is another effect of the magnetic field. The required kT magnetic fields are becoming feasible due to a significant progress that has been made in generating magnetic fields with laser-driven coils using ns-long laser pulses. The predicted improved characteristics of laser-driven proton beams would be critical for a number of applications. [Preview Abstract] |
|
GP10.00038: Proton and Ion Acceleration on the Contrast Upgraded Texas Petawatt Laser Edward McCary, Rebecca Roycroft, Xuejing Jiao, Rotem Kupfer, Ganesh Tiwari, Craig Wagner, Andrew Yandow, Philip Franke, Gilliss Dyer, Erhard Gaul, Toma Toncian, Todd Ditmire, Bjorn Hegelich Recent upgrades to the Texas Petawatt (TPW) laser system have eliminated pre-pulses and reduced the laser pedestal, resulting in improved laser contrast. Previously unwanted pre-pulses and amplified spontaneous emission (ASE) would ionize targets thinner than 1 micron, leaving an under-dense plasma which was not capable of accelerating ions to high energies. After the upgrade the contrast was drastically improved allowing us to successfully shoot targets as thin as 20 nm without plasma mirrors. We have also observed evidence of relativistic transparency and Break-Out Afterburner (BOA) ion acceleration when shooting ultra-thin, nanometer scale targets. Data taken with a wide angle ion spectrometer (IWASP) showed the characteristic asymmetry of BOA in the plane orthogonal to the laser polarization on thin targets but not on micron scale targets. Thick micron scale targets saw improvement as well; shots on 2 $\mu $m thick gold targets saw ions with energies up to 100 MeV, which broke the former record proton energy on the TPW. Switching the focusing optic from an f/3 parabolic mirror to an f/40 spherical mirror showed improvement in the number of low energy protons created, and provided a source for hundreds of picosecond heating of aluminum foils for warm dense matter measurements. [Preview Abstract] |
|
GP10.00039: A new model for TNSA in the multi-ps laser-foil interactions Kunioki Mima, Natsumi Iwata, Akifumi Yogo, Shota Tosaki, Keisuke Koga, Hideo Nagatomo, Yasuaki Kishimoto, Hiroaki Nishimura, Hiroshi Azechi In laser-matter interactions in the intensity level of $10^{18}$ W/cm$^{2}$, a few tens MeV ions can be generated. Ion acceleration in the interaction of thin foils with sub ps-1 ps laser pulses has been described conventionally by the self-similar plasma expansion theory assuming an isothermal condition [1]. Recently, an ion acceleration experiment using multi-ps laser pulses from kilojoule class laser LFEX was conducted [2] where the large spot size of 70 $\mu$m with the peak intensity $2.3\times10^{18}$ W/cm$^{2}$ results electron heating and ion acceleration exceeding the conventional 1D isothermal model. To understand such an interaction in the multi-ps regime where the electron heating during the laser irradiation is a key ingredient, we here present a new model for plasma expansions that takes the time variation of electron temperature, i.e. sound velocity, into account. Based on the temperature evolution obtained by a PIC simulation corresponding to the LFEX experiment, the theory was validated by comparing the maximum ion energy between theory and simulations. [1] A. Yogo, K. Mima, N. Iwata et al., submitted to Nat. Comm. [2] P. Mora, Phys. Rev. Lett. 90, 185002 (2003). [Preview Abstract] |
|
GP10.00040: Laser-driven Ion Acceleration using Nanodiamonds Luc D'Hauthuille, Tam Nguyen, Franklin Dollar Interactions of high-intensity lasers with mass-limited nanoparticles enable the generation of extremely high electric fields. These fields accelerate ions, which has applications in nuclear medicine, high brightness radiography, as well as fast ignition for inertial confinement fusion. Previous studies have been performed with ensembles of nanoparticles, but this obscures the physics of the interaction due to the wide array of variables in the interaction. The work presented here looks instead at the interactions of a high intensity short pulse laser with an isolated nanodiamond. Specifically, we studied the effect of nanoparticle size and intensity of the laser on the interaction. A novel target scheme was developed to isolate the nanodiamond. Particle-in-cell simulations were performed using the EPOCH framework to show the sheath fields and resulting energetic ion beams. [Preview Abstract] |
(Author Not Attending)
|
GP10.00041: Study of proton generation using thin metal wires with a sharp tip Anatoly Maksimchuk, Gennady Fiksel, Karl Krushelnick, Valery Yu. Bychenkov, Andrey V. Brantov It was suggested to use the highly enhanced electric field close to a low-power-laser-illuminated metal tip for nanometric optical tweezers [1]. Recently, a boost in proton acceleration by high-intensity laser using structured snow-like targets was observed and attributed mostly due to the field enhancement at the whisker tip [2]. Here we report on a more controlled high-intensity laser experiment by using thin metal wires with a diameter of 20 microns with tips of different size ranging from 0.2 to 5.0 microns. 400 fs, 15 TW laser pulses were focused to an intensity of up to 3x10$^{\mathrm{19}}$ W/cm$^{\mathrm{2\thinspace }}$on a tungsten wire at different distances from the tip. We have observed two high-energy proton beams. One beam was produced through the Target Normal Sheet Acceleration (TNSA) mechanism and was perpendicular to the wire and the other one was observed from the wire tip and in the direction along the wire axis. Simultaneous measurements of maximum proton energies using CR-39 nuclear track detectors and high energy electrons using imaging plates in both direction were performed and will be presented. The experimental results were interpreted taking into account the generated electric and magnetic fields near the surface of the wire and at the wire tip as well as a strong collimated surface current along the wire. [1] L. Novotny et al., Phys. Rev. Lett. 79, 645 (1997). [2] A. Zigler et al., Phys. Rev. Lett. \textbf{110}, 215004 (2013). [Preview Abstract] |
(Author Not Attending)
|
GP10.00042: A high repetition rate transverse beam profile diagnostic for laser-plasma proton sources Nicholas Dover, Mamiko Nishiuchi, Hironao Sakaki, Masaki Kando, Keita Nishitani The recently upgraded J-KAREN-P laser can provide $\approx$ PW peak power and intensities approaching 10$^{22}$ Wcm$^{-2}$ at 0.1 Hz. Scaling of sheath acceleration to such high intensities predicts generation of protons to near 100 MeV, but changes in electron heating mechanisms may affect the emitted proton beam properties, such as divergence and pointing. High repetition rate simultaneous measurement of the transverse proton distribution and energy spectrum are therefore key to understanding and optimising the source. Recently plastic scintillators have been used to measure online proton beam transverse profiles, removing the need for time consuming post-processing. We are therefore developing a scintillator based transverse proton beam profile diagnostic for use in ion acceleration experiments using the J-KAREN-P laser. Differential filtering provides a coarse energy spectrum measurement, and time-gating allows differentiation of protons from other radiation. We will discuss the design and implementation of the diagnostic, as well as proof-of-principle results from initial experiments on the J-KAREN-P system demonstrating the measurement of sheath accelerated proton beams up to 20 MeV. [Preview Abstract] |
|
GP10.00043: Neutron Source from Laser Plasma Acceleration Xuejing Jiao, Joseph Shaw, Eddie McCary, Mike Downer, Bjorn Hegelich Laser driven electron beams and ion beams were utilized to produce neutron sources via different mechanism. On the Texas Petawatt laser, deuterized plastic, gold and DLC foil targets of varying thickness were shot with $150J, 150fs$ laser pulses at a peak intensity of ~$2 \times 10^{21}W/cm^{2}$. Ions were accelerated by either target normal sheath acceleration or Breakout Afterburner acceleration. Neutrons were produced via the $^{9}$Be($d$,n) and $^{9}$Be($p$,n) reactions when accelerated ions impinged on a Beryllium converter as well as by deuteron breakup reactions. We observed $2 \times 10^{10}$ neutron per shot in average, corresponding to $5 \times 10^{18} n/s$. The efficiencies for different targets are comparable. In another experiment, $38fs, 0.3J$ UT$^{3}$ laser pulse interacted with mixed gas target. Electrons with energy ~40MeV were produced via laser wakefield acceleration. Neutron flux of $2 \times 10^{6}$ per shot was generated through bremsstrahlung and subsequent photoneutron reactions on a Copper converter. [Preview Abstract] |
|
GP10.00044: What is the surface temperature of a solid irradiated by a Petawatt laser? Andreas Kemp, Laurent Divol When a solid target is irradiated by a Petawatt laser pulse, its surface is heated to tens of millions of degrees within a few femtoseconds, facilitating a diffusive heat wave and the acceleration of electrons to MeV energies into the target. Using numerically converged collisional particle-in-cell simulations, we observe a competition between two surface heating mechanisms - inverse bremsstrahlung in solid density on one hand, and electrons scattering on turbulent electric fields on the other. Collision-less heating effectively dominates above the relativistic intensity threshold. Our numerical results show that a high-contrast 40fs, f/5 laser pulse with 1J energy will heat the skin layer to 5keV, and the inside of the target over several microns deep to a bulk temperature of 100s eV at solid density. [Preview Abstract] |
|
GP10.00045: OSIRIS Modeling of High Energy Electron Transport in Warm Dense Matter J May, T Yabuuchi, C McGuffey, MS Wei, F Beg, WB Mori In experiments on the Omega EP laser, a high intensity laser beam ( $eA/m_ec > 1$ ) is focused onto a gold foil, generating relativistic electrons. Behind the Au foil is a layer of plastic foam through which the electrons are allowed to transport, and on the far side of the CH from the gold is a copper foil; electron fluence is measured by recording the k-$\alpha$ from that foil. The foam layer is either pre-ionized via a shock launched from an ablator irradiated earlier with a beam perpendicular to the high intensity beam; or the foam is in the solid state when the high intensity beam is switched on. In the latter case the foam -- which has an initial density of $200mg / cm^3$ -- heats to a temperature of 40eV and rarifies to a density of $30mg / cm^3$. Results show an order of magnitude decrease in k-$\alpha$ when the CH layer is pre-ionized compared to cold CH. OSIRIS simulations indicate that the primary explanation for the difference in transport seen in the experiment is the partial resistive collimation of the beam in the higher density material, caused by collisional resistivity. The effect seems to be mostly caused by the higher density itself, with temperature having minimal effect. [Preview Abstract] |
|
GP10.00046: Enhancing Understanding of High Energy Density Plasmas Using Fluid Modeling with Kinetic Closures David Hansen, Eric Held, Bhuvana Srinivasan, Robert Masti, Jake King This work seeks to understand possible stabilization mechanisms of the early-time electrothermal instability in the evolution of the Rayleigh-Taylor instability in MagLIF (Magnetized Liner Inertial Fusion) experiments. Such mechanisms may include electron thermal conduction, viscosity, and large magnetic fields. Experiments have shown that the high-energy density plasmas from wire-array implosions require physics modelling that goes well beyond simple models such as ideal MHD. The plan is to develop a multi-fluid extended-MHD model that includes kinetic closures for thermal conductivity, resistivity, and viscosity using codes that are easily available to the wider research community. Such an effort would provide the community with a well-benchmarked tool capable of advanced modeling of high-energy-density plasmas. [Preview Abstract] |
|
GP10.00047: Ultra-intense laser-plasma interaction toward Weibel-mediated collisionless shocks formation Anna Grassi, M Grech, F Amiranoff, A Macchi, C Riconda The rapid developments in laser technology will soon offer the opportunity to study in the laboratory the processes driving Weibel-mediated collisionless shocks, typical of various astrophysical scenarii. The interaction of an ultra-intense laser with an overdense plasma has been identified as the preferential configuration. Yet, the experimental requirements still need to be properly investigated. High performance computing simulations are a necessary tool for this study. In this work, we present a series of kinetic simulations performed with the PIC code \underline {SMILEI}, varying the laser and plasma parameters.~ In particular, we will study the effect of the laser polarisation and plasma density to obtain the best conditions for the creation of a collisionless shock. The role of the electrons heated at the interaction surface and of particles accelerated~ via the Hole Boring (laser-piston) mechanism on the generation of the current filamentation~ instability and the subsequent shock front formation will be highlighted. [Preview Abstract] |
|
GP10.00048: Investigation of the effects of electron plasma frequency on the operation of a helix TWT Lutfi Oksuz, Necati Haytural, Emre Uygun, Ferhat Bozduman, Hakan Yesiltepe, Ali Gulec The oscillations of electrons are an important subject for the design procedure of linear beam tubes such as klystrons and TWTs. These oscillation frequencies may be affected by the finite region of the tube if the plasma wavelength of the electrons are larger than the bounding region of the device, leading to a reduced plasma frequency which further leads to an increase in wavelength [1]. Following the Pierce's theory on traveling wave tubes, it is seen that the reduced plasma frequency takes place in space charge terms which also include the Pierce's gain parameter \textbf{\textit{C }}[2]. In this study the effects of plasma frequency on the operation of a helix TWT are investigated using CST Particle Studio. References: \textbf{1. }A.S. Gilmour Jr., "Klystrons, Traveling Wave Tubes, Magnetrons, Crossed-Field Amplifiers and Gyrotrons", Artech House, 1994 \textbf{2. }J.W. Gewartowski, H. A. Watson, "Principles of Electron Tubes", D. Van Nostrand Company, 1965 [Preview Abstract] |
(Author Not Attending)
|
GP10.00049: Pair production and $\gamma $ ray emission in collisions of e$^{\mathrm{+}}$e$^{\mathrm{-}}$beams. Fabrizio Del Gaudio, Thomas Grismayer, Ricardo Fonseca, Warren Mori, Luis Silva The fast development in Plasma Wakefield Acceleration suggests that high quality relativistic electron/positron beams, having tens of GeV in energy and densities up to 10cm$^{\mathrm{-3}}$, can be delivered by tabletop devices in the near future. With these parameters, the collective electromagnetic field of one beam compares to the Schwinger field in the boosted frame of the oncoming beam, thus the quantum regime is approached where particles are capable to emit hard photons apt to decay in electron positron pairs. Under certain conditions, additionally to quantum effects, the disruption effect takes place altering the beams density profile more or less severely. Estimates regarding beams energy loss and the number of pairs generated in such collisions were addressed by P. Chen with simplified beam geometry. We present a model for the low disruption regime, with realistic beams parameters, that predicts the average radiation intensity, its spectrum and the number of pairs created during the beam-beam interaction. Our model agrees with fully consistent PIC simulations done with the QED module of OSIRIS 3.0. The results obtained qualify this setup as possible $\gamma $ ray source and show that relevant number of pairs, higher than previous estimates, is produced at these beam energies and densities. [Preview Abstract] |
|
GP10.00050: Transforming the Idler for Use in Laser--Plasma Interaction Experiments S. Bucht, D. Haberberger, J. Bromage, D.H. Froula A novel optical system has been designed to transform the idler produced in an optical parametric chirped-pulse--amplification (OPCPA) system into a useable high-power pulse for laser--plasma interaction (LPI) experiments. The idler is an existing byproduct created by all OPCPA systems and has similar bandwidth and energy to the signal; with proper preparation it can be another high-fidelity laser pulse at a shifted or unique wavelength from the signal. This preparation is made difficult because of two issues arising in the optical parametric amplifier: (1) angular dispersion and (2) complex chirp reversal. A two-prism angular dispersion compensator and a grism stretcher show promising analytic solutions to these two problems without significantly sacrificing energy or bandwidth. The unusual wavelength range of this beam makes it ideal for unique LPI experiments such as Raman amplification. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
|
GP10.00051: Pulse Compression in Magnetized Plasmas Yuan Shi, Nathaniel J. Fisch, Hong Qin Laser amplification using Raman or Brillouin backscattering in unmagnetized plasmas is possible only within a window in the intensity-frequency space. However, by applying an external magnetic field transverse to the direction of laser propagation, it is possible to shift this operation window towards high frequency and low intensity side of the parameter space. The compression of laser pulses, whose intensities and frequencies are outside the unmagnetized operation window, might now be made possible by mediating the parametric interaction using waves that are only present in magnetized plasma. This technique introduces new technological challenges in producing the requisite strong magnetic field that may be compared to challenges in producing high density uniform plasmas. [Preview Abstract] |
|
GP10.00052: Scaling solid density high-harmonic generation into the mid-infrared range Tam Nguyen, Franklin Dollar High-order harmonic generation (HOHG) has been observed to be a bright source of coherent x-rays in laser-solid interactions using high intensity, femtosecond laser pulses. The majority of high-intensity experiments investigating HOHG have been with the use of 800 nm wavelength lasers. It is known that the density profile of the solid affects absorption of the initial pulse and the intensity of the re-radiated harmonics. We investigate how conversion efficiency and maximum harmonic order scale into the mid-infrared range. Results and particle-in-cell simulations over the pre-plasma scale length will be shown. [Preview Abstract] |
|
GP10.00053: ABSTRACT WITHDRAWN |
|
GP10.00054: Terahertz Radiation from Laser Created Plasma by Applying a Transverse Static Electric Field Takuya Fukuda, Koji Katahira, Noboru Yugami, Yasuhiko Sentoku, Hitoshi Sakagami, Hideo Nagatomo Terahertz (THz) radiation, which is emitted in narrow cone in the forward direction from laser created plasma has been observed by N.Yugami \textit{et al}. [1]. Additionally, L\"{o}ffler \textit{et al}. have observed that a significantly increased THz emission intensity in the forward direction when the transverse static electric field is applied to the plasma [2]. The purpose of our study is to derive the mechanism of the THz radiation from laser created plasma by applying the transverse static electric field. To study the radiation mechanism, we conducted 2D-PIC simulation. With the static electric field of 10 kV/cm and gas density of 10$^{\mathrm{20}}$ cm$^{\mathrm{-3}}$, we obtain 1.2 THz single cycle pulse radiation, whose intensity is 1.3 \texttimes 10$^{\mathrm{5}}$ W/cm$^{\mathrm{2}}$. The magnetic field called ``picket fence mode'' is generated in the laser created plasma. At the boundary surface between the plasma and vacuum, the magnetic field is canceled because eddy current flows. We conclude that the temporal behavior of the magnetic field at the boundary surface radiates the THz wave. [1] N. Yugami \textit{et al.}, Jpn. J. Appl. Phys. \textbf{45,} L1051 (2006). [2] T. L\"{o}ffler \textit{et al}., Appl. Phys. Lett. \textbf{77}, 453 (2000). [Preview Abstract] |
|
GP10.00055: Harmonic Generation in a Traveling-Wave Tube Patrick Wong, Peng Zhang, Y.Y. Lau, Geoffrey Greening, Ronald Gilgenbach, David Chernin, David Simon, Brad Hoff Crowding of electron orbits in a traveling-wave tube (TWT) may lead to significant harmonic contents in the beam current, even in the linear regime [1]. Here, we consider a wideband TWT that exhibits gain at the second harmonic. We analytically formulate equations governing the evolution of the generation of second harmonic, including axial variations of the Pierce parameters. The second harmonic output is phase-controlled by the input signal which consists only of a fundamental frequency. Several test cases are performed and compared with simulation using the CHRISTINE code. Reasonable agreement between theory and simulation is found. [1] C. F. Dong, et al., \textit{IEEE Trans. ED} 62, 4285 (2015). [Preview Abstract] |
|
GP10.00056: Study on the Before Cavity Interaction in a Second Harmonic Gyrotron Using 3D CFDTD PIC Simulations M. C. Lin, S. Illy, M. Thumm, J. Jelonnek A computational study on before cavity interaction (BCI) in a 28 GHz second harmonic (SH) gryotron for industrial applications has been performed using a 3-D conformal finite-difference time-domain (CFDTD) particle-in-cell (PIC) method. On the contrary to the after cavity interaction (ACI), i.e. beam wave interaction in the non-linear uptaper after the cavity, which has been widely investigated, the BCI, i.e. beam wave interaction in the non-linear downtaper before the cavity connected to the beam tunnel with an entrance, is less noticed and discussed. Usually the BCI might be considered easy to be eliminated. However, this is not always the case. As the SH gyrotron had been designed for SH TE$_{\mathrm{12}}$ mode operation, the first harmonic (FH) plays the main competition. In the 3-D CFDTD PIC simulations, a port boundary has been employed for the gyro-beam entrance of the gyrotron cavity instead of a metallic short one which is not reflecting a realistic situation as an FH backward wave oscillation (BWO) is competing with the desired SH generation. A numerical instability has been found and identified as a failure of the entrance port boundary caused by an evanescent wave or mode conversion. This indicates the entrance and downtaper are not fully cut-off for some oscillations. A further study shows that the undesired oscillation is the FH TE$_{\mathrm{11}}$ BWO mode concentrated around the beam tunnel entrance and downtaper. A mitigation strategy has been found to suppress this undesired BCI and avoid possible damage to the gun region. [Preview Abstract] |
|
GP10.00057: Cherenkov radiation in a surface wave accelerator based on silicon carbide Tianhong Wang, Vladimir Khudik, Gennady Shvets We report on our theoretical investigations of Cherenkov-type emission of surface phonon polaritons (SPPs) by relativistic electron bunches. The polaritons are confined by a planar waveguide comprised of two SiC slabs separated by an air gap [1]. The SPPs are generated in the spectral range known as the reststrahlen band, where the dielectric permittivity of SiC is negative. Two surface modes of the radiation are analyzed: the longitudinal (accelerating) and the transverse (deflecting) ones. Both form Cherenkov cones that are different in the magnitude of the cone angle and the central frequency. However, both exhibits rapid spatial oscillations and beats behind the moving charge. Moreover, the longitudinal mode forms a reversed Cherenkov radiation cone due the negative group velocity for sufficiently small air gaps, but the transverse mode does not. The wakefield acceleration of electron beam inside the structure is also studied. Transverse instabilities and BBU effects can be suppressed by flat driver beam, meanwhile the longitudinal mode can support accelerating fields $>$1 GeV. \ \\ $[1]$ B. Neuner III, D. Korobkin, G. Ferro, and G. Shvets, "Prism-coupled surface wave accelerator based on silicon carbide", Phys. Rev. Sp. Top. - Accelerators and Beams, 15, 031302 (2012). [Preview Abstract] |
|
GP10.00058: Simulations of light-light scattering in quantum vacuum Pedro Carneiro, Thomas Grismayer, Luís Silva, Ricardo Fonseca Facilities such as the Extreme Light Infrastructure (ELI) or the VULCAN 20 PW project, as well as the Petta-Watt SLAC project, coupled with the x-ray LCLSII source will allow to perform the first experiments on the probing of quantum vacuum. In our work, we developed a numerical method to self-consistently solve the nonlinear system of Maxwell's equations including quantum corrections of vacuum polarization. The robustness of our algorithm allied to the ability to integrate this tool within a particle-in-cell (PIC) method, represents an important milestone in modeling future planned experiments to prove the existence of the quantum vacuum. Such experiments aim to measure the induced ellipticity on a x-ray pulse after probing a strong optical pump due to the quantum vacuum fluctuations. We present simulation results of both the ellipticity induced and polarization rotation, using realistic laser parameters of the Petta-Watt SLAC project, and the x-ray LCLSII source, whilst taking into account all finite-size multi-dimensional effects. We show how the ellipticity induced varies as a function of the distance to the axis of the beam, proving the importance of taking into account finite-size effects. This work serves as an important tool to complement existing efforts within the community to probe the effects of the quantum vacuum, in the strong field regime, for the first time. [Preview Abstract] |
|
GP10.00059: MHD, TRANSIENTS AND SIMULATION |
|
GP10.00060: Development of a free-boundary version of the SIESTA MHD equilibrium code H. Peraza-Rodriguez, R. Sanchez, J.M. Reynolds-Barredo, V. Tribaldos, J. Geiger, S.P. Hirshman, M.R. Cianciosa SIESTA is a recently developed MHD stability code that allows for the self-consistent calculation of nonlinear MHD equilibrium solutions for 3D magnetic configurations without the assumption of nested magnetic surfaces. The original version of the code [S.P. Hirshman, R. Sanchez and C.R. Cook, Phys. Plasmas 18, 062504 (2011)] was written as a fixed boundary code that imposed that the normal component of the magnetic field vanished at the prescribed plasma edge. In this contribution, we describe a procedure to extend SIESTA to perform free-boundary equilibrium calculations, thus increasing the range of problems to which the code can be applied. The process requires an automated way to extend the computational domain and mesh all the way to the vacuum vessel and the construction of a reasonable initial guess for the magnetic field, from which SIESTA can iterate towards equilibrium. Examples will be provided for several configurations of the W7-X stellarator. [Preview Abstract] |
|
GP10.00061: Extension of the SIESTA Equilibrium Code to Non-Stellarator Symmetry M.R. Cianciosa, S.P. Hirshman, S.K. Seal Resonant magnetic perturbation (RMP) fields applied for edge localized mode (ELM) mitigation, break the nested flux topology of nominally axisymmetric tokamaks. Understanding the implications of this, requires equilibrium codes that can account for non-nested surface topologies. SIESTA is a 3D equilibrium code that allows for islands and stochastic regions. With the assumption of stellarator symmetry, SIESTA has successfully solved island equilibria in RMP perturbed tokamaks (Hirshman \textit{et. al.} J. Plasma Phys. 2016). However, this assumption of stellarator symmetry limits the application of SIESTA to equilibria with up-down symmetry. Diverted tokamaks and the stellarator symmetry breaking trim coils of W7-X require an extension of SIESTA allowing for up-down asymmetry. We present an initial implementation of SIESTA without stellarator symmetry and study the effects of 3D asymmetry on DIII-D and W7-X plasmas. [Preview Abstract] |
|
GP10.00062: Stabilization of the SIESTA MHD Equilibrium Code Using Rapid Cholesky Factorization S.P. Hirshman, E.A. D'Azevedo, S.K. Seal The SIESTA MHD equilibrium code solves the discretized nonlinear MHD force $F\equiv J$X$B-\nabla p$ for a 3D plasma which may contain islands and stochastic regions. At each nonlinear evolution step, it solves a set of linearized MHD equations which can be written $r\equiv $\textit{Ax}$-b=$0, where $A$ is the linearized MHD Hessian matrix. When the solution norm \textbar $x$\textbar is small enough, the nonlinear force norm will be close to the linearized force norm \textbar $r$\textbar $\approx $ 0 obtained using preconditioned GMRES. In many cases, this procedure works well and leads to a vanishing nonlinear residual (equilibrium) after several iterations in SIESTA. In some cases, however, \textbar $x$\textbar \textgreater 1 results and the SIESTA code has to be restarted to obtain nonlinear convergence. In order to make SIESTA more robust and avoid such restarts, we have implemented a new rapid QR factorization of the Hessian which allows us to rapidly and accurately solve the least-squares problem $ A^{T}r =$ 0, subject to the condition \textbar $x$\textbar \textless 1. This avoids large contributions to the nonlinear force terms and in general makes the convergence sequence of SIESTA much more stable. The innovative rapid QR method is based on a pairwise row factorization of the tri-diagonal Hessian. It provides a complete Cholesky factorization while preserving the memory allocation of $A$. [Preview Abstract] |
|
GP10.00063: Fast Three Dimensional Reconstruction of Magnetohydrodynamic Equilibria in Plasma Confinement Devices S.K. Seal, M.R. Cianciosa, S.P. Hirshman, A. Wingen, R.S. Wilcox, E.A. Unterberg High-fidelity reconstruction of plasma equilibria in confinement devices like stellarators and tokamaks with external three dimensional (3D) fields is computationally very expensive and routinely requires days, even weeks, to complete using serial approaches. Here, we present the performance results of coupling the 3D plasma reconstruction code, V3FIT, with PARVMEC, the recently developed parallel version of VMEC. We present the parallel design of this coupled software along with a scalability analysis to identify its performance bottlenecks. Dependence of its scalability limits on model parameters is derived. These analyses are supported by scaling studies on over 6,000 processor cores of a Cray XC30 supercomputer. PARVMEC, which dominates the total runtime of the reconstruction procedure, is shown to deliver speedup improvements of over one to two orders of magnitude, depending on whether the equilibrium computations are carried out in a free or fixed boundary mode. The overall speedup of the coupled reconstruction code is shown to deliver over 40X improvement enabling fusion scientists to carry out high-fidelity 3D plasma reconstruction analyses in only a few hours instead of in days/weeks for the first time. [Preview Abstract] |
|
GP10.00064: Surface currents on the plasma-vacuum interface in MHD equilibria James D. Hanson The VMEC$^{1}$ non-axisymmetric MHD equilibrium code can compute free-boundary equilibria$^{2}$. Since VMEC assumes that magnetic fields within the plasma form closed and nested flux surfaces, the plasma-vacuum interface is a flux surface, and the total magnetic field there has no normal component. VMEC imposes this condition of zero normal field using the potential formulation of Merkel$^{3}$, and solves a Neumann problem for the magnetic potential in the exterior region. This boundary condition necessarily admits the possibility of a surface current on the plasma-vacuum interface. While this current may be small in MHD equilibrium, this current may be readily computed in terms of a magnetic potential in both the interior and exterior regions. Examples of the surface current for VMEC equilibria will be shown. $^{1}$ Hirshman S P and Whitson J, Phys. Fluids \textbf{26} 3553 (1983) $^{2}$ Hirshman S P, Van Rij W I and Merkel P, Comp. Phys. Comm. \textbf{43} 143--55 (1986) $^{3}$ Merkel P, J. Comp. Phys. \textbf{66} 83--98 (1986) [Preview Abstract] |
|
GP10.00065: Cylindrically-Symmetric Equilibria in Ideal MHD with Fractal Pressure Profiles Brian Kraus, Stuart Hudson In ideal magnetohydrodynamics, unphysical, pressure-driven currents exist where flux surfaces with rational rotational transform coincide with pressure gradients, a situation Grad termed ``pathological'' [1]. As an alternative, we construct a~non-trivial, continuous~pressure profile that is flat on sufficiently wide intervals near each rational surface. Such a profile must be self-similar and thus fractal, because intervals of flat pressure exist around high-order rational surfaces at all scales. This infinite-resolution fractal pressure is analyzed as a homeomorphism of the Cantor set. Additionally, an algorithm has been written to numerically produce an approximation of the pressure profile, where only a finite number of rational surfaces are considered. Using this algorithm, we investigate the magnetic field and current profiles associated with the fractal pressure and a given rotational transform in cylindrical geometry. [1] H. Grad, Phys. Fluids 10 (1), 137 (1967). [Preview Abstract] |
|
GP10.00066: Singular Currents Near Magnetic Islands in MHD Equilibria: Effects of Pressure Variation Within Flux Surfaces and of Symmetry Allan Reiman We present an analytic calculation of the MHD equilibrium current near a magnetic island that includes the effect of the pressure variation on the flux surfaces in that region. The current has logarithmic singularities at the X-lines of magnetic islands in non-stellarator-symmetric equilibria. The singular components vanish in stellarator-symmetric MHD equilibria. (Equilibria invariant under combined reflection in the poloidal and toroidal angles. Tokamaks with balanced double-null divertors are stellarator symmetric, but single-null tokamaks are not.) These equilibrium solutions are to be contrasted with equilibria having ${\rm {\bf B}}\cdot \nabla p=0,$ where the singular components of the pressure-driven currents vanish regardless of the symmetry. They are also to be contrasted with 3D MHD equilibrium solutions that have simply nested flux surfaces, where the pressure-driven current goes like $1/x$ near rational surfaces, where $x$ is the distance from the rational surface. (Except in the case of quasi-symmetric flux surfaces.) We work with a closed subset of the MHD equilibrium equations that involves only perpendicular force balance, and is decoupled from parallel force balance. It is not correct to use the parallel component of the conventional MHD force balance equation, ${\rm {\bf B}}\cdot \nabla p=0,$ near magnetic islands. [Preview Abstract] |
|
GP10.00067: Observed Magnetic Island Rotation and Reconnecting Modes with Phase Velocity in the Ion Diamagnetic Velocity* P. Buratti, B. Coppi, B. Basu The modes that can produce magnetic reconnection in low collisionality regimes and that are driven by the plasma current density gradient are shown to have a characteristic phase velocity in the direction of the ion diamagnetic velocity [1]. Thus the initially formed magnetic islands rotate in the same direction. This result is consistent with the experimental observations of rotating magnetic islands, produced by ``internal modes'' excited in magnetically confined toroidal plasmas, with the caveat that the observed islands have gone through a non-linear evolution that is not covered by the presented theory. In the low collisionality regimes of interest there are to types of singularity to be removed i) that of the perturbed plasma current density removed by a local plasma finite ``inductivity''; ii) that of the perturbed electron temperature (due to a large longitudinal thermal conductivity) removed by a finite transverse electron thermal conductivity. *Sponsored by the U.S. D.O.E. [1] P. Buratti, Nucl. Fusion, 56, 076004, (2016). [2] B. Coppi, Plasma Phys. Report, 42, 5, 383 (2016). [Preview Abstract] |
|
GP10.00068: Variational formulation of Hybrid kinetic-MHD models for energetic particles in tokamak plasmas Alain Brizard The variational formulation of hybrid kinetic-MHD models for energetic particles (e.g., alpha particles or relativistic runaway electrons) in tokamak plasmas is presented [1]. Models involving either current-coupling or pressure-coupling (e.g., through a CGL-type pressure tensor) between the plasma-bulk MHD behavior and the energetic particle distribution are shown to arise naturally under either particle or drift-kinetic dynamical representations for the energetic-particle dynamics, respectively. The role of the wave-action conservation law, which holds even when the background plasma and energetic-particle distribution are time-dependent, will be discussed. \newline The inclusion of three-wave resonant interactions within hybrid kinetic-MHD models through a variational principle that is constructed from a Lagrangian density with cubic nonlinear terms will also be discussed. \newline [1] A.J. Brizard, PoP 1 (1994) 2460. [Preview Abstract] |
|
GP10.00069: Effects of Equilibrium Toroidal Flow on Locked Mode and Plasma Response in a Tokamak Ping Zhu, Wenlong Huang, Xingting Yan It is widely believed that plasma flow plays significant roles in regulating the processes of mode locking and plasma response in a tokamak in presence of external resonant magnetic perturbations (RMPs). Recently a common analytic relation for both locked mode and plasma response has been developed based on the steady-state solution to the coupled dynamic system of magnetic island evolution and torque balance. The analytic relation predicts the size of the magnetic island of a locked mode or a static nonlinear plasma response for a given RMP amplitude, and rigorously proves a screening effect of the equilibrium toroidal flow. To test the theory, we solve for the locked mode and the nonlinear plasma response in presence of RMP for a circular-shaped limiter tokamak equilibrium with constant toroidal flow, using the initial-value, full MHD simulation code NIMROD. The comparison between the simulation results and the theory prediction, in terms of the quantitative screening effects of equilibrium toroidal flow, will be reported and discussed. [Preview Abstract] |
|
GP10.00070: Plasma Density Effects on Toroidal Flow Stabilization of Edge Localized Modes Shikui Cheng, Ping Zhu, Debabrata Banerjee Recent EAST experiments have demonstrated mitigation and suppression of edge localized modes (ELMs) with toroidal rotation flow in higher collisionality regime, suggesting potential roles of plasma density. In this work, the effects of plasma density on the toroidal flow stabilization of the high-$n$ edge localized modes have been extensively studied in linear calculations for a circular-shaped limiter H-mode tokamak, using the initial-value extended MHD code NIMROD. In the single MHD model, toroidal flow has a weak stabilizing effects on the high-$n$ modes. Such a stabilization, however, can be significantly enhanced with the increase in plasma density. Furthermore, our calculations show that the enhanced stabilization of high-$n$ modes from toroidal flow with higher edge plasma density persists in the 2-fluid MHD model. These findings may explain the ELM mitigation and suppression by toroidal rotation in higher collisionality regime due to the enhancement of plasma density obtained in EAST experiment. [Preview Abstract] |
|
GP10.00071: Influence of driven current on resistive tearing mode in Tokamaks Zhiwei Ma, Sheng Wang, Wei Zhang Influence of~driven current on the $m \mathord{\left/ {\vphantom {m n}} \right. \kern-\nulldelimiterspace} n=2 \mathord{\left/ {\vphantom {2 1}} \right. \kern-\nulldelimiterspace} 1$ resistive tearing mode~is studied systematically using~a three-dimensional toroidal~MHD code (CLT). A uniform driven current with Gaussian distribution in the radial direction is imposed around the unperturbed rational surface. It is found that the driven current can locally modify the profiles of the current and safety factor, such that the tearing mode becomes linearly stable. The stabilizing effect increases with increase of the driven current$I_{\mbox{cd}} $or decrease of its width$\delta_{\mbox{cd}} $,unless an excessively large driven current reverses the magnetic shear near the rational surface and drives other instabilities such as double or triple tearing modes. The stabilizing effect can be negligible or becomes reversed if the maximum driven current density is not at the unperturbed rational surface. [Preview Abstract] |
|
GP10.00072: Neoclassical Toroidal Viscosity Torque Induced by Plasma Response in a Low-$\beta$ Tokamak with Edge Pedestal Xingting Yan, Ping Zhu, Youwen Sun The characteristic profile and magnitude are predicted in theory for the neoclassical toroidal viscosity (NTV) torque induced by the plasma response to the resonant magnetic perturbation (RMP) in a tokamak with an edge pedestal, using the newly developed module coupling the NIMROD and the NTVTOK codes. For a low $\beta$ equilibrium, the NTV torque is mainly induced by the dominant toroidal mode of plasma response. The NTV torque profile is radially localized and peaked, which is determined by profiles of both the equilibrium temperature and the plasma response fields. In general, the peak of NTV torque profile is found to trace the pedestal location. The magnitude of NTV torque is extremely sensitive to the $\beta$ of pedestal top; for a given plasma response, the peak value of NTV torque can increase by three orders of magnitude, when the pedestal $\beta$ increases by only one order of magnitude. This suggests a more significant role of NTV torque in higher plasma $\beta$ regimes. [Preview Abstract] |
|
GP10.00073: High-beta equilibria in circular, elliptical and D-shape large aspect ratio axisymmetric configurations with poloidal and toroidal flows Omar Lopez Ortiz, Luca Guazzotto The Grad-Shafranov-Bernoulli system of equations is a single fluid magnetohydrodynamical description of axisymmetric equilibria with mass flows. Using a variational perturbative approach [1], analytic approximations for high-beta equilibria in circular, elliptical and D-shape cross sections in the high aspect ratio limit are found, which include finite toroidal and poloidal flows. Assuming a polynomial dependence of the free functions on the poloidal flux, the equilibrium problem is reduced to a modified Helmholtz PDE subject to homogeneous Dirichlet conditions. An application of the Green's function method leads to a closed form for the circular solution and to a series solution in terms of Mathieu functions for the elliptical case, which is valid for arbitrary elongations. To extend the elliptical solution to a D-shape domain, a boundary perturbation in terms of the triangularity is used. A comparison with the code FLOW [2] is presented for relevant scenarios.\\\\ \noindent [1] Eliezer Hameiri, \textit{Phys. Plasmas}, \textbf{20}, 024504 (2013).\\ \noindent [2] L. Guazzotto \textit{et al.}, \textit{Phys. Plasmas}, \textbf{11}, 604 (2004). [Preview Abstract] |
|
GP10.00074: Tearing Mode Stability with Sheared Toroidal Flows Ryan White, Bruno Coppi Toroidal plasma flow induced by neutral beam heating has been found to increase the stability of tearing modes in tokamak plasmas. The need to extrapolate current (experimentally-based) knowledge of tearing mode onset to future machines, requiresa better understanding of the essential physics. We consider the physics of flow near the rational surfaces. For realistic flow profiles, the velocity shear near the rational surface can be treated as a perturbation, and is found to amplify the dominant stabilizing effect of magnetic curvature. This effect can be seen using a cylindrical model if large-aspect-ratio corrections to the magnetic curvature are incorporated. On the other hand, the physical effects of toroidal rotation are completely absent in a cylinder, and require a fully-toroidal calculation to study. The toroidal rotation near the rational surface is found to couple to a geometrical parameter which vanishes for up-down symmetric profiles. Physically, the dominant effects of rotation arise from a Coriolis force, leading to flow directional dependence. [Preview Abstract] |
|
GP10.00075: Recent Progress on the DCON Code A. H. Glasser, Z. R. Wang, J.-K. Park, D. P. Brennan We report two new developments relating to the MHD stability code DCON. The first concerns ideal stability, for which DCON uses a toroidal generalization of Newcomb crossing criterion for a cylindrical plasma.\footnote{W.A. Newcomb, Ann. Phys.~10, 232 (1960)} While DCON is widely used and has been extensively verified and validated, we have lacked a rigorous mathematical proof of this stability criterion. We present a new proof here.\footnote{A. H. Glasser, Phys.~Plasmas {\bf 23}, 7, 072505 (2016).} Resistive MHD instability is solved by the method of matched asymptotic expansions. Robust convergence of the matching data $\Delta'$ in the ideal outer region has been achieved. Verification against published values of $\Delta'$ has been demonstrated.\footnote{H.P. Furth {\it et al}, Phys.~Fluids {\bf 16}, 7, 1054 (1973)} Systematic numerical benchmarks against the MARS code will be presented, varying equilibrium parameters such as pressure, $q$ profile, aspect ratio, and shaping. This effort helps to reinforce the reliability of DCON for resistive instability and elucidate how these equilibrium parameters affect the asymptotic matching approach in full toroidal geometry. [Preview Abstract] |
|
GP10.00076: Visco-Resistive MHD Modeling Benchmark of Forced Magnetic Reconnection M T Beidler, C C Hegna, C R Sovinec, J D Callen, N M Ferraro The presence of externally-applied 3D magnetic fields can affect important phenomena in tokamaks, including mode locking, disruptions, and edge localized modes. External fields penetrate into the plasma and can lead to forced magnetic reconnection (FMR), and hence magnetic islands, on resonant surfaces if the local plasma rotation relative to the external field is slow. Preliminary visco-resistive MHD simulations of FMR in a slab geometry are consistent with theory [1]. Specifically, linear simulations exhibit proper scaling of the penetrated field with resistivity, viscosity, and flow, and nonlinear simulations exhibit a bifurcation from a flow-screened to a field-penetrated, magnetic island state as the external field is increased, due to the 3D electromagnetic force. These results will be compared to simulations of FMR in a circular cross-section, cylindrical geometry by way of a benchmark between the NIMROD and M3D-C1 extended-MHD codes. Because neither this geometry nor the MHD model has the physics of poloidal flow damping, the theory of [1] will be expanded to include poloidal flow effects. The resulting theory will be tested with linear and nonlinear simulations that vary the resistivity, viscosity, flow, and external field. [1] R. Fitzpatrick, Phys. Plasmas \textbf{5}, 3325 (1998) [Preview Abstract] |
|
GP10.00077: Nonlinear External Kink Computing with NIMROD K. J. Bunkers, C. R. Sovinec Vertical displacement events (VDEs) during disruptions often include non-axisymmetric activity, including external kink modes, which are driven unstable as contact with the wall eats into the $q$-profile. The NIMROD code [Sovinec, et al., JCP 195, 335] is being applied to study external-kink-unstable tokamak profiles in toroidal and cylindrical geometries. Simulations with external kinks show the plasma swallowing a vacuum bubble, similar to [Rosenbluth, et al., Phys. Fluids 19, 1987]. NIMROD reproduces external kinks in both geometries, using an outer vacuum region (modeled as a plasma with a large resistivity), but as the boundary between the vacuum and plasma regions becomes more 3D, the resistivity becomes a 3D function, and it becomes more difficult for algebraic solves to converge. To help allow non-axisymmetric, nonlinear VDE calculations to proceed without restrictively small time-steps, several computational algorithms have been tested. Flexible GMRES, using a Fourier and real space representation for the toroidal angle has shown improvements. Off-diagonal preconditioning and a multigrid approach were tested and showed little improvement. A least squares finite element method (LSQFEM) has also helped improve the algebraic solve. [Preview Abstract] |
|
GP10.00078: Real frequency tearing modes with parallel dynamics and their effect on locking and resistive wall modes Andrew Cole, J.M. Finn, D.P. Brennan Tearing modes with real frequencies in the plasma frame are of potential importance because of their effect on the locking process. In particular, it has recently been shown [1] that the Maxwell torque on the plasma in the presence of an applied error field is modified significantly for tearing modes having real frequencies near marginal stability. In this poster we derive the tearing mode dispersion relation with pressure gradient, field line curvature and parallel dynamics in the resistive-inertial (RI) and visco-resistive regimes, neglecting the divergence of the E × B drift and perpendicular resistivity. The results show that the usual Glasser effect, which involves real frequencies, occurs in this simplified model in both regimes. Moreover, we show that in both regimes the existence of tearing modes with complex frequencies is related to nearby electrostatic resistive interchange modes with complex frequencies. Finally, we find that the lowering of the threshold for destabilization of the resistive wall mode can be much more pronounced than observed for tearing modes in Ref. [2]. References: [1] J. M. Finn, A. J. Cole, and D. P. Brennan, PoP (Letters) 22, 120701 (2015). [2] J. M. Finn and R. A. Gerwin, PoP 3, 2344 (1996). [Preview Abstract] |
|
GP10.00079: A simple rigorous calculation of nonlinear tearing mode Qian Teng, David Gates, Muni Zhou, Roscoe White The nonlinear growth of tearing mode is governed by the Modified Rutherford Equation. This equation can be readily solved numerically. However analytical solution provides more physical intuition and deeper understanding of the instability. R. B. White \textit{et. al.} developed a quasilinear method in 1977 and N. Arcis \textit{et. al.} used a perturbative method to solve the equation in 2006. In this work, we present a simple method to solve the nonlinear problem with Taylor expansion. We calculated $\Delta'_{classic}$, the discontinuity in the first derivative of the perturbed flux across the island, accurate to the second order. $\Delta'_{classic}$ is found to be dominated by three terms: constant $\Delta'_{0}$, $w$, and $wlnw$. This agrees with the results calculated with the perturbative method. [Preview Abstract] |
|
GP10.00080: Study the effect of electrode biasing on m/n$=$2/1 tearing mode on the J-TEXT tokamak. Qiming Hu, Hai Liu, Zhipeng Chen, Qingquan Yu, Lizhi Zhu, Zhifeng Cheng, Ge Zhuang The effects of electrode biasing (EB) on the m/n $=$ 2/1 tearing mode (TM) have been experimentally studied on the J-TEXT tokamak. It is found that, for negative bias with increasing its voltage, the 2/1 TM is accelerated from 4 kHz to 8 kHz and its amplitude is stabilized until complete suppression, and the plasma toroidal rotation is accelerated in counter-Ip direction. While for positive bias with increasing its voltage, the 2/1 TM is decelerated from 4 kHz to less than 1 kHz and its amplitude is destabilized until the occurrence of mode locking, and the toroidal plasma rotation is accelerated in co-Ip direction. Associated with the change in TM dynamic, the plasma particle confinement is found to be improved under negative bias, however, that changes little under positive bias. Statistical results show that the level of TM stabilization (destabilization) and the change in TM frequency caused by EB are found to correlate positively with the value of negative (positive) bias voltage, indicating that the negative (positive) bias stabilizes (destabilizes) the TM through increasing (decreasing) the plasma flow or flow shear. [Preview Abstract] |
|
GP10.00081: Linear instability regimes in L-mode edges using reduced MHD models in BOUT$++$ Eric Bass, Chris Holland, Bruce Cohen, Maxim Umansky We compare linear instabilities in the edge of two DIII-D L-mode discharges using reduced two-fluid MHD models implemented in BOUT$++$ [1]. Discharge 119919, a case used in a previous BOUT$++$ validation study [2], has a cold edge and is dominated by resistive ballooning modes (RBMs). Hotter discharge 128913, an L-mode shortfall benchmark case [3], is drift-wave (DW) dominant. The model captures essential drift wave physics through the electron pressure parallel gradient drive term in the $A_{\vert \vert } $ evolution. At relevant toroidal mode numbers (50-200), the leading DWs in 128913 are flutelike with high $k_{r} $ and require about an order of magnitude greater radial resolution than the leading RBMs in 119919. We quantify when such high $k_{r} $ modes must be resolved in practice. To aid eigenfunction confirmation, and to identify potential subdominant DWs, a companion eigenvalue solver for the BOUT$++$ models is under development. [1] Dudson et al., Comp. Phys. Comm. V.180 (2009) 1467. [2] B. I. Cohen et al., Phys. Plasmas\textbf{ 20}, 055906 (2013) [3] C. Holland et al, Phys. Plasmas \textbf{16},~052301 (2009) [4] J. Candy and R.E. Waltz J. Comput. Phys. 186, 545 (2003) * Prepared by UCSD under contract number DE-FG02-06ER54871. [Preview Abstract] |
|
GP10.00082: Integrated Modeling of Time Evolving 3D Kinetic MHD Equilibria and NTV Torque N.C. Logan, J.-K. Park, B.A. Grierson, S.R. Haskey, R. Nazikian, L. Cui, S.P. Smith, O. Meneghini New analysis tools and integrated modeling of plasma dynamics developed in the OMFIT framework are used to study kinetic MHD equilibria evolution on the transport time scale. The experimentally observed profile dynamics following the application of 3D error fields are described using a new OMFITprofiles workflow that directly addresses the need for rapid and comprehensive analysis of dynamic equilibria for next-step theory validation. The workflow treats all diagnostic data as fundamentally time dependent, provides physics-based manipulations such as ELM phase data selection, and is consistent across multiple machines - including DIII-D and NSTX-U. The seamless integration of tokamak data and simulation is demonstrated by using the self-consistent kinetic EFIT equilibria and profiles as input into 2D particle, momentum and energy transport calculations using TRANSP as well as 3D kinetic MHD equilibrium stability and neoclassical transport modeling using General Perturbed Equilibrium Code (GPEC). The result is a smooth kinetic stability and NTV torque evolution over transport time scales. [Preview Abstract] |
|
GP10.00083: Kinetic Effects on Resistive Tearing Mode Hao Shi, Wenlu Zhang The kinetic effects on stability of resistive tearing mode are investigated by global simulations in cylindrical geometry using Gyrokinetic Toroidal Code(GTC). The fluid simulation of resistive tearing mode agrees well with theory prediction. Kinetic effects are found to reduce the growth rate of the tearing mode and the radial width of mode structure. The drift-tearing mode is obtained when considering density gradient, which has the frequency of the diamagnetic drift frequency. [Preview Abstract] |
|
GP10.00084: Modeling resistive wall modes and disruptive instabilities with M3D-C1 NM Ferraro, SC Jardin, D Pfefferle Disruptive instabilities pose a significant challenge to the tokamak approach to magnetic fusion energy, and must be reliably avoided in a successful reactor. These instabilities generally involve rapid, global changes to the magnetic field, and electromagnetic interaction with surrounding conducting structures. Here we apply the extended-MHD code M3D-C1 to calculate the stability and evolution of disruptive modes, including their interaction with external conducting structures. The M3D-C1 model includes the effects of resistivity, equilibrium rotation, and resistive walls of arbitrary thickness, each of which may play important roles in the stability and evolution of disruptive modes. The strong stabilizing effect of rotation on resistive wall modes is explored and compared with analytic theory. The nonlinear evolution of vertical displacement events is also considered, including the evolution of non-axisymmetric instabilities that may arise during the current-quench phase of the disruption. It is found that the non-axisymmetric stability of the plasma during a VDE depends strongly on the thermal history of the plasma. [Preview Abstract] |
|
GP10.00085: Effect of rotation magnitude and shear on tokamak plasma response to three-dimensional magnetic perturbations B.C. Lyons, N.M. Ferraro, R. Nazikian, C. Paz-Soldan Three-dimensional magnetic perturbations are routinely used in tokamaks to control error fields, particle transport, and edge-localized modes (ELMs). In ELM-suppressed plasmas, a zero-crossing of the electron rotation profile is observed at the top of the pedestal and is believed to play a crucial role in suppression. Using single-fluid, time-independent, linear modeling with the M3D-C1 extended magnetohydrodynamics code, a systematic variation of the zero-crossing and shear of the rotation profile is performed. The resonant tearing drive at a rational surface as the rotation approaches zero is quantified. Furthermore, it is shown that a zero-crossing permits amplification of near-resonant poloidal Fourier harmonics and a reduction of other harmonics at a flux surface. The impact of these phenomena on observables will be assessed. The effect of rotation shear and two-fluid terms will also be explored. [Preview Abstract] |
|
GP10.00086: Fully 3D modeling of tokamak vertical displacement events with realistic parameters David Pfefferle, Nathaniel Ferraro, Stephen Jardin, Amitava Bhattacharjee In this work, we model the complex multi-domain and highly non-linear physics of Vertical Displacement Events (VDEs), one of the most damaging off-normal events in tokamaks, with the implicit 3D extended MHD code M3D-C1. The code has recently acquired the capability to include finite thickness conducting structures within the computational domain [1]. By exploiting the possibility of running a linear 3D calculation on top of a non-linear 2D simulation, we monitor the non-axisymmetric stability and assess the eigen-structure of kink modes as the simulation proceeds. Once a stability boundary is crossed, a fully 3D non-linear calculation is launched for the remainder of the simulation, starting from an earlier time of the 2D run. This procedure, along with adaptive zoning, greatly increases the efficiency of the calculation, and allows to perform VDE simulations with realistic parameters and high resolution. Simulations are being validated with NSTX data where both axisymmetric (toroidally averaged) and non-axisymmetric induced and conductive (halo) currents have been measured. [1] Ferraro, N. et al, PoP 23, 056114 (2016). [Preview Abstract] |
|
GP10.00087: A self-sustaining mechanism that prevents tokamak plasmas from sawtoothing in non-linear 3D MHD simulations I. Krebs, S.C. Jardin, S. G{\"u}nter, K. Lackner, M. Hoelzl, N. Ferraro We use the finite element 3D MHD code M3D-C$^1$ [Jardin et al., Comput. Sci. Discovery 5, 014002 (2012)] to study large-scale instabilities in the center of tokamak plasmas. It has been shown [Jardin et al., Phys. Rev. Lett. 115, 215001 (2015)] that in 3D MHD simulations of plasmas with a flat central $q\approx 1$, an ideal interchange instability can develop that keeps the current density from peaking despite central heating. The instability yields a ($m=1,\,n=1$) perturbation of the core plasma, i.a. a helical flow that flattens the central current density by (1) flattening the temperature profile and (2) combining with the perturbed magnetic field to generate a negative loop voltage through a dynamo effect. This might explain the ``flux-pumping'' effect observed in hybrid discharges [i.a. Petty et al., Phys. Rev. Lett. 102, 045005 (2009)]. We study in which parameter range the two effects are strong enough to prevent sawtoothing. We describe a new regime of quasi-stationary oscillating states and analyze cases in between the stationary and the cycling regime in which the sawtooth behaviour is modified by the current flattening mechanisms. To connect to experimental observations, we have set up simulations starting with a scenario comparable to the current ramp-up phase. [Preview Abstract] |
|
GP10.00088: Steady states of solar coronal loops as nonaxisymmetric toroidal flux ropes Linda Sugiyama, M. Asgari-Targhi Consistent MHD steady states for coronal loops on the surface of the sun, modeled as magnetic flux ropes, are derived for the first time, based on the equilibrium and stability of toroidal magnetically confined fusion plasmas. Coronal loops, like magnetic tori, are unstable to expansion in major radius. The solar gravity and plasma beta, previously ignored, are crucual parameters in the steady state. For loops with a predominantly axisymmetric magnetic axis, three analytical steady states exist in terms of beta and the normalized solar gravity parameter $\hat{G}=ga/v_A^2$, where $g$ is the acceleration due to gravity, ordered in inverse aspect ratio: high beta ($\beta\sim\epsilon$) and small gravity $\hat{G}\sim\epsilon^3$, which resembles a nearly axisymmetric high-beta tokamak, and high beta with larger $\hat{G}\sim\epsilon^2$, and low beta ($\beta\sim\epsilon^2$) with $\hat{G}\sim\epsilon^3$, which are more strongly nonaxisymmetric. Comparison with observations shows that the two high beta states bracket the range of thin coronal loops in solar active regions $\epsilon\sim 0.02$ and $\hat{G}$ orders the loops by height. The low beta solution may describe certain thicker loops $\epsilon\sim 0.1$ that grow to solar flares or Coronal Mass Ejections. [Preview Abstract] |
|
GP10.00089: Proof of a necessary condition for stability in kinetic magnetohydrodynamics J.J. Ramos The linear kinetic magnetohydrodynamic (KMHD) theory of Kruskal-Oberman and Rosenbluth-Rostoker does not have a self-adjointness property. As a result, the standard proofs that an instability follows if some trial perturbation makes the incremental potential energy negative, do not work in KMHD. So, the comparison theorem showing that the Kruskal-Oberman, Rosenbluth-Rostoker KMHD potential energy is less than the potential energy in the double-adiabatic theory of Chew-Goldberger-Low, by itself cannot be construed as a rigorous proof that stability in the double-adiabatic fluid theory is a necessary condition for stability in KMHD. This work derives rigorously a necessary condition for KMHD stability: it proves that, if an isotropic-pressure equilibrium is unstable in the double-adiabatic fluid theory, an explicit initial condition for the KMHD system can be devised which will grow in time without bound. Besides being rigorous, the present necessary condition for KMHD stability is tighter than the one associated with the classic comparison theorem, because it requires the equilibrium to be stable in the double-adiabatic theory including minimization with respect to the parallel component of the fluid displacement instead of just setting the parallel displacement equal to zero. [Preview Abstract] |
|
GP10.00090: Ballooning Stability Of Tokamak Pedestals In The Presence Of Applied 3D Magnetic Perturbations T B Cote, C C Hegna, M Willensdorfer, E Strumberger, W Suttrop, H Zohm Applied 3d magnetic perturbations can destabilize ideal mhd ballooning modes in tokamak pedestals [1]. In this work, we describe techniques for studying infinite-n ballooning stability of 3d equilibria deduced from vmec calculations. Full magnetic profiles from vmec are used to construct local equilibria for flux surfaces in and around the edge pedestal region. These local equilibrium calculations are coupled with ideal ballooning stability analysis to determine stability of the system for given rmp configurations. This theoretical development is motivated by recent asdex-u experiments, where toroidally localized high-n mhd activity is observed in the presence of applied 3d fields. We will attempt to explain these observations. [1] T M Bird and C C Hegna 2013 Nucl. Fusion 013004 [Preview Abstract] |
|
GP10.00091: The effects of safety factor value at pedestal on the MHD stability of ITER H-mode confinement Linjin Zheng, M. T. Kotschenreuther, P. Valanju MHD stability of ITER H-mode confinement is investigated with bootstrap current included for equilibrium for various senarios. We construct ITER equilibria numerically using CORSICA code and study the stability using AEGIS code. The direct consequence of bootstrap current effects on equilibrium is the modification of local safety factor profile at pedestal, so that the magnetic shear can be reduced or reversed locally. This local q value is referred to as $q_s$. This q profile change results in a dramatic change of MHD mode behavior. Both low-n and peeling-ballooning modes are investigated. It is found that the pedestal stability depends not only on the edge current ($J_{ped}$) and pressure gradient ($p'_{ped}$), but also on the $q_s$ value. This shows that the pedestal stability can be affected by the global parameters, not just the local ones at pedestal. Both numerical scheme and results will be presented. The physical interpretation will be explained. [Preview Abstract] |
|
GP10.00092: Toroidal Energy Principle (TEP) and perturbed equilibrium code STB Leonid Zakharov, Di Hu The MHD energy principle TEP is presented in terms of perturbations of the vector potential, rather than plasma displacement. This form makes TEP capable to discribe both the ideal plasmas stability and the perturbed equilibria. The functional is expressed in two terms. The first one represents the energy of magnetic field and is calculated using working equilibrium coordinate system.The second term, containing plasma displacement is expressed in the compact form using Hamada coordinates.This representation uses the same combinations of metric coefficients as in the equilibrium calculations. The STB code implements the TEP for both ideal MHD and perturbed equilibria. In the first case, it uses the matching conditions of the ideal MHD. In the second case, the 2-D equilibrium islands are introduced in order to resolve the singularity and match the solutions across the resonant surfaces [Preview Abstract] |
|
GP10.00093: Sub-microsecond time evolution of edge density inferred from ion cyclotron emission measurements during ELMs in KSTAR plasmas B Chapman, S C Chapman, R O Dendy, K G McClements, G S Yun, M H Kim, S Thatipamula, Y U Nam Spectrally structured ion cyclotron emission (ICE) is detected alongside ELMs in KSTAR deuterium plasmas. For KSTAR ICE where the separation of spectral peak frequencies is close to the proton cyclotron frequency at the outer plasma edge, orbit calculations suggest that the driver may be a subset of centrally-born fusion protons on passing orbits. We report 1D3V PIC code modelling of this scenario for KSTAR ICE. We simulate the self-consistent nonlinear full orbit dynamics of energetic and thermal ions and electrons, in combination with the electric and magnetic fields. Multiple simulation runs enable us to infer the theoretical dependence of ICE spectral structure on bulk plasma parameters, notably density. It is observed on KSTAR that the cyclotron harmonic structure of the ICE spectrum usually chirps down, on sub-microsecond timescales, during an ELM crash; upward chirping is observed in a few cases. By matching these observations to the dependence of ICE on local density that we infer from our PIC simulations, we obtain sub-microsecond resolution of the evolving edge density during the ELM crash. The downward ICE chirps reflect the density collapse during the crash, while the rare upward chirps may be due to locally rising edge density associated with ELM filaments. [Preview Abstract] |
|
GP10.00094: Particle simulation of runaway electrons in rippled tokamaks with pellet suppression effects D. A. Spong, L. Carbajal Gomez, D. del-Castillo-Negrete, L. Baylor, S. Seal Runaway electrons are of significant concern for large tokamak devices both due to gradual acceleration by the Ohmic heating field and the more rapid acceleration and avalanche production that can occur during major disruptions. We have developed a simulation model (KORCGC) that follows large number of runaway guiding center (GC) orbits, taking into account Coulomb collisions, impurities, synchrotron radiation, rippled (3D) fields, and electric field acceleration, including inductive effects. Applications to pellet suppression experiments have been made and show similar effects (current/energy decay rates) as the observations. The model uses a hybrid (MPI/OpenMP) design and shows excellent parallel scaling. The energy parameters of runaway pellet suppression and formation fit within the limits of the GC approximation and the longer time{\-}steps allowed by GC facilitate modeling over relevant timescales. Simulations of impurity injection dissipation experiments on DIII{\-}D and ITER will be discussed. [Preview Abstract] |
|
GP10.00095: Full-orbit effects in the dynamics of runaway electrons in toroidal geometry D. del-Castillo-Negrete, L. Carbajal-Gomez, D.A. Spong, L. Baylor, S. K. Seal The dynamics of RE (runaway electrons) in fusion plasmas spans a wide range of temporal scales from the fast gyro-motion $\sim 10^{-11}$ sec to the observational time scales $\sim 10^{-2}\rightarrow 1$ sec. To cope with this scale separation RE are usually studied within the bounce-average or the guiding center approximations. Although these approximations have yielded valuable insights, a study with predictive capabilities of RE in fusion plasmas calls for the incorporation of full-orbits effects in configuration space in the presence of 3-D integrable and stochastic magnetic fields. Here we present numerical results on this problem using the Kinetic Orbit Runaway electrons Code (KORC) that follows relativistic electrons in general electric and magnetic fields under the full Lorentz force and collisions. At relativistic energies, the main energy loss is due to synchrotron radiation, which we incorporate using the Landau-Lifshitz formulation of the Abraham-Lorentz-Dirac force. Following a study of potential limitations of the bounce-average and the guiding center approximations, we discuss the role of full-orbit effects on the evolution of the pitch-angle, the RE energy limit, the critical electric field, and the emission patterns of synchrotron radiation in toroidal geometry. [Preview Abstract] |
|
GP10.00096: A backward Monte Carlo method for efficient computation of runaway probabilities in runaway electron simulation Guannan Zhang, Diego Del-Castillo-Negrete Kinetic descriptions of RE are usually based on the bounced-averaged Fokker-Planck model that determines the PDFs of RE in the 2 dimensional momentum space. Despite of the simplification involved, the Fokker-Planck equation can rarely be solved analytically and direct numerical approaches (e.g., continuum and particle-based Monte Carlo (MC)) can be time consuming specially in the computation of asymptotic-type observable including the runaway probability, the slowing-down and runaway mean times, and the energy limit probability. Here we present a novel backward MC approach to these problems based on backward stochastic differential equations (BSDEs). The BSDE model can simultaneously describe the PDF of RE and the runaway probabilities by means of the well-known Feynman-Kac theory. The key ingredient of the backward MC algorithm is to place all the particles in a runaway state and simulate them backward from the terminal time to the initial time. As such, our approach can provide much faster convergence than the brute-force MC methods, which can significantly reduce the number of particles required to achieve a prescribed accuracy. Moreover, our algorithm can be parallelized as easy as the direct MC code, which paves the way for conducting large-scale RE simulation. [Preview Abstract] |
|
GP10.00097: KORC: A Kinetic Orbit Runaway Electrons code for tokamak disruptions Leopoldo Carbajal Gomez, Diego del-Castillo-Negrete, Donald Spong, Sudip Seal, Larry Baylor Runaway electrons (RE) resulting from the violent termination of tokamak plasmas pose a serious threat to ITER due to the very high energies they can reach and deposit on the plasma facing components. Most of the current modelling of RE in fusion tokamak plasmas rely on reduced models such as the bounce-average and the test particle equations. In some scenarios, the radiation losses in these models might lead to uncertainties in the RE parameters that determine their confinement and energy limit. In order to study this in detail we have developed a new Kinetic Orbit Runaway electrons Code (KORC). KORC follows the dynamics of ensembles of relativistic electrons in the 6D phase space fully resolving gyro-motion under the influence of the Lorentz force, the Landau-Lifshiftz consistent formulation of the Abraham-Lorentz-Dirac force for radiation damping, and collisions with impurities and the background plasma. KORC is parallelized using open MP/MPI, and benefits from a modified relativistic leap-frog method along with an operator splitting scheme for solving the RE dynamics in different magnetic fields. The code is robust, conservative, and shows nearly linear strong scaling. [Preview Abstract] |
|
GP10.00098: Suppression of runaway electrons with a resonant magnetic perturbation in MST tokamak plasmas. Stefano Munaretto, B.E. Chapman, A.F. Almagri, B.S. Cornille, A.M. DuBois, J.A. Goetz, K.J. McCollam, C.R. Sovinec Runaway electrons generated in MST tokamak plasmas are now being probed with resonant magnetic perturbations (RMP's). An RMP with m$=$3 strongly suppresses the runaway electrons. Initial modeling of these plasmas with NIMROD shows the degradation of flux surfaces with an m$=$3 RMP, which may account for the runaway electron suppression. These MST tokamak plasmas have Bt$=$0.14 T, Ip $=$50kA, and q(a)$=$2.2, with a bulk electron density and temperature of 5x1017 m-3 and 150 eV. Runaway electrons are detected via x-ray emission. The RMP is produced by a poloidal array of 32 saddle coils at the narrow vertical insulated cut in MST's thick conducting shell. Each RMP has a single m but a broad n spectrum. A sufficiently strong m$=$3 RMP completely suppresses the runaway electrons, while a comparable m$=$1 RMP has little effect. The impact of the RMP's on the magnetic topology of these plasmas is being studied with the nonlinear MHD code, NIMROD. With an m$=$3 RMP, stochasticity is introduced in the outer third of the plasma. No such change is observed with the m$=$1 RMP. NIMROD also predicts regularly occurring sawtooth oscillations with a period comparable to MHD activity observed in the experiment. [Preview Abstract] |
|
GP10.00099: Simulation of MST tokamak discharges with resonant magnetic perturbations B.S. Cornille, C.R. Sovinec, B.E. Chapman, A. Dubois, K.J. McCollam, S. Munaretto Nonlinear MHD modeling of MST tokamak plasmas with an applied resonant magnetic perturbation (RMP) reveals degradation of flux surfaces that may account for the experimentally observed suppression of runaway electrons with the RMP. Runaway electrons are routinely generated in MST tokamak discharges with low plasma density. When an $m=3$ RMP is applied these electrons are strongly suppressed, while an $m=1$ RMP of comparable amplitude has little effect. The computations are performed using the NIMROD code and use reconstructed equilibrium states of MST tokamak plasmas with $q(0)<1$ and $q(a)=2.2$. Linear computations show that the $(1,1)$-kink and $(2,2)$-tearing modes are unstable, and nonlinear simulations produce sawtoothing with a period of approximately 0.5 ms, which is comparable to the period of MHD activity observed experimentally. Adding an $m=3$ RMP in the computation degrades flux surfaces in the outer region of the plasma, while no degradation occurs with an $m=1$ RMP. The outer flux surface degradation with the $m=3$ RMP, combined with the sawtooth-induced distortion of flux surfaces in the core, may account for the observed suppression of runaway electrons. [Preview Abstract] |
|
GP10.00100: Enhancement of threshold electric field for relativistic runaway electrons due to magnetic fluctuation and synchrotron radiation Shucai Li, Lu Wang, Zhongyong Chen, Duwei Huang, Ruihai Tong The dynamics of relativistic electrons are analyzed using the relativistic Fokker-Planck equation including deceleration due to synchrotron radiation (SR) [1,2] and radial diffusion loss caused by magnetic fluctuation (MF) [3]. Threshold electric field for avalanche growth is enhanced, and the growth rate is reduced by the combined effect of MF and SR as compared to the case with only SR. The threshold electric field is determined by the time scales balance between momentum evolution and radial diffusion loss induced by MF, and increased with level of MF. More importantly, the hysteresis behavior of runaway pointed out by [2] does not exist anymore. This is because the “seed electrons” cannot be sustained as a result of diffusion loss. [1] J. R. Martín-Solís, R. Sanchez, and B. Esposito, Phys. Plasmas 6, 3925 (1999). [2] P. Aleynikov and B. Breizmann, Phys. Rev. Lett. \textbf{114}, 155001 (2015). [3] P.~Helander,~L.-G.~Eriksson, and~F.~Andersson,~Phys. Plasmas~\textbf{7},~4106~(2000). [Preview Abstract] |
|
GP10.00101: Long slide-away discharges in the COMPASS tokamak Ondrej Ficker, Jan Mlynar, Milos Vlainic, Vladimir Weinzettl, Jakub Urban, Jordan Cavalier, Jaroslav Havlicek, Radomir Panek, Martin Hron, Jaroslav Cerovsky, Petr Vondracek, Richard Paprok, Joan Decker, Yves Peysson, Ondrej Bogar, Adam Stahl In this contribution, long runaway electron (RE) dominated discharges achieved in the COMPASS tokamak are presented. The extensive length is possible due to a low consumption of available volt-seconds of the tokamak transformer in this type of discharge. Energetic electron losses in this regime seems to be modulated mainly by small oscillations of a radial position (controller setting) unlike in the RE discharges at higher electron density, where various MHD phenomena affect the evolution of the losses. The behaviour of the slide-away plasma is studied using magnetic coils, HXR detectors, ECE system and a pair of $^3\mathrm{He}$ proportional counters of neutrons. The plasma scenario is also modelled using Fokker-Planck codes. [Preview Abstract] |
|
GP10.00102: Lifetime of Runaway Electrons at Phase-space Attractor Adrian Fontanilla, Boris Breizman The kinetic theory for relativistic runaway electrons\footnote{P. Aleynikov, B.N. Breizman, PRL {\bf 114}, 155001 (2015).} is extended to find a structure of the distribution function that is peaked around a phase-space attractor. Runaway electron dynamics are examined when the electric field is close to the threshold value required to sustain pre-existing runaways. The near vicinity of predicted stable and unstable points in momentum-space characterize a competition between accumulation and depletion which ultimately determines a finite lifetime for the accumulated runaways, albeit one that can be exponentially long and amenable to avalanche onset. The developed theory is then generalized to the case of stronger driving fields. [Preview Abstract] |
|
GP10.00103: Gyrokinetic ion/fluid electron simulation of nonlinear evolution of multiple Reverse Shear Alfven Eigenmodes Yang Chen, Guo-Yong Fu, Scott Parker We report simulation of simultaneous excitation of multiple Reverse Shear Alfven eigenmodes in DIII-D plasmas (discharge #142111), using the gyrokinetic ion/fluid electron hybrid model of GEM. Thermal ions and beam ions are gyrokinetic, electrons are fluid with finite-mass correction in the Ohm's law. The vorticity equation is solved instead of the quasi-neutrality condition. This improves numerical stability. We extend previous single-n nonlinear simulation \footnote{Chen et. al. Phys. Plasmas {\bf 20}, 012109 (2013)} to simultaneous excitation of toroidal modes with $n=0$ and $2 |
|
GP10.00104: Validation and Continued Development of Methods for Spheromak Simulation Thomas Benedett The HIT-SI experiment has demonstrated stable sustainment of spheromaks. Determining how the underlying physics extrapolate to larger, higher-temperature regimes is of prime importance in determining the viability of the inductively-driven spheromak. It is thus prudent to develop and validate a computational model that can be used to study current results and study the effect of possible design choices on plasma behavior. A zero-beta Hall-MHD model has shown good agreement with experimental data at 14.5 kHz injector operation. Experimental observations at higher frequency, where the best performance is achieved, indicate pressure effects are important and likely required to attain quantitative agreement with simulations. Efforts to extend the existing validation to high frequency (~36-68 kHz) using an extended MHD model implemented in the PSI-TET arbitrary-geometry 3D MHD code will be presented. An implementation of anisotropic viscosity, a feature observed to improve agreement between NIMROD simulations and experiment, will also be presented, along with investigations of flux conserver features and their impact on density control for future SIHI experiments. [Preview Abstract] |
|
GP10.00105: Modeling MHD Equilibrium and Dynamics with Non-Axisymmetric Resistive Walls in LTX and HBT-EP C. Hansen, J. Levesque, J. Bialek, D.P. Boyle, J. Schmitt In experimental magnetized plasmas, currents in the first wall, vacuum vessel, and other conducting structures can have a strong influence on plasma shape and dynamics. These effects are complicated by the 3D nature of these structures, which dictate available current paths. Results from simulations to study the effect of external currents on plasmas in two different experiments will be presented: 1) The arbitrary geometry, 3D extended MHD code PSI-Tet is applied to study linear and non-linear plasma dynamics in the High Beta Tokamak (HBT-EP) focusing on toroidal asymmetries in the adjustable conducting wall. 2) Equilibrium reconstructions of the Lithium Tokamak eXperiment (LTX) in the presence of non-axisymmetric eddy currents. An axisymmetric model is used to reconstruct the plasma equilibrium, using the PSI-Tri code, along with a set of fixed eddy current distributions. Current distributions are generated using 3D time-dependent, thin-wall, eddy current simulations using VALEN or PSI-Tet. Simulations of detailed experimental geometries are enabled by use of the PSI-Tet code, which employs a high order finite element method on unstructured tetrahedral grids that are generated directly from CAD models. Further development of PSI-Tet will also be presented. [Preview Abstract] |
|
GP10.00106: Generalized Weighted Residual Method; Advancements and Current Studies Jan Scheffel, Kristoffer Lindvall The \textit{Generalized Weighted Residual Method} (GWRM) is a time-spectral method for solving initial value partial differential equations [1]. The GWRM treats the temporal, spatial, and parameter domains by projecting the residual to a Chebyshev polynomial space, with the variational principle being that the residual is zero. This treatment provides a global semi-analytical solution. However, straightforward global solution is not economical. One remedy is the inclusion of spatial and temporal sub-domains with coupled internal boundary conditions, which decreases memory requirements and introduces sparse matrices. Only the equations pertaining to the boundary conditions need be solved globally, making the method parallelizable in time. Efficient solution of the linearized ideal MHD stability equations of screw-pinch equilibria are proved possible. The GWRM has also been used to solve strongly nonlinear ODEs such as the Lorenz equations (1984), and is capable of competing with finite time difference schemes in terms of both accuracy and efficiency. GWRM solutions of linear and nonlinear model problems of interest for stability and turbulence modelling will be presented, including detailed comparisons with time stepping methods. \noindent [1] Scheffel J, AJCM 2(2012)173. [Preview Abstract] |
|
GP10.00107: ABSTRACT WITHDRAWN |
|
GP10.00108: A hybrid kinetic hot ion PIC module for the M3D-C$^{\mathrm{1}}$ Code J.A. Breslau, N. Ferraro, S.C. Jardin, K. Kalyanaraman Building on the success of the original M3D code with the addition of efficient high-order, high-continuity finite elements and a fully implicit time advance making use of cutting-edge numerical techniques, M3D-C$^{\mathrm{1}}$ has become a flagship code for realistic time-dependent 3D MHD and two-fluid calculations of the nonlinear evolution of macroinstabilities in tokamak plasmas. It is therefore highly desirable to introduce to M3D-C$^{\mathrm{1}}$ one of the most-used features of its predecessor: the option to use a drift-kinetic delta-$f$ PIC model for a minority population of energetic ions (representing, e.g., beam ions or fusion alpha particles) coupled with the usual finite element advance of the bulk ion and electron fluids through its pressure tensor. We describe the implementation of a module for this purpose using high-order-of-accuracy numerical integration and carefully tuned to take advantage of state-of-the-art multicore processing elements. Verification results for a toroidal Alfv\'{e}n eigenmode test problem will be presented, along with a demonstration of favorable parallel scaling to large numbers of supercomputer nodes. [Preview Abstract] |
|
GP10.00109: Status of the 5D gyrokinetic code COGENT and its initial applications Wonjae Lee, M. Dorf, M. Dorr, R. Cohen, D. Ghosh, T. Rognlien, J. Hittinger, M. Umansky, S. Krasheninnikov We report recent progress with the development of the 5D (3D configuration and 2D velocity space) version of the full-f continuum gyrokinetic code COGENT. The original 2D configuration space has been successfully extended to 3D, with the Cartesian (slab) geometry chosen for verification and initial applications. The code has been successfully verified with drift-wave simulations including drift-kinetic equations for both electrons and ions coupled to the long-wavelength limit of the Gyro-Poisson equation. The initial application of the 5D COGENT is focused on addressing kinetic effects of drift-wave instabilities (e.g., universal instability) on blob dynamics in tokamak edge plasmas. [Preview Abstract] |
(Author Not Attending)
|
GP10.00110: Marker loading effect on ion-temperature-gradient turbulence Hongwei Yang, Yong Xiao, Zhihong Lin Generally, a limited number of particles called ``markers'' are used to simulate a physical system in the particle simulation for plasmas. In this work we find for the first time that different marker particle loading methods would introduce different physical consequences to the global gyrokinetic simulations. The well-studied ion temperature gradient (ITG) turbulence has been chosen to check the results that are carried out by the GTC code from three different particle loading methods in the electrostatic limit: uniform loading, nonuniform temperature and uniform density loading, and nonuniform loading. When applying these three loading methods in GTC code to study the same ITG case, the results from the uniform is very different to the other two methods. Our further study shows that this difference could be contributed by the finite Larmor radius (FLR) effect. The results also suggest that the implementation of global particle equilibrium may play important role on these differences. In addition, this result indicates that such difference among different loading methods may have an important effect on the size scaling of the turbulent transport. \\ \textbf{Keywords:} particle loading, ion temperature gradient mode, finite Larmor radius effec [Preview Abstract] |
|
GP10.00111: RF Wave Simulation Using the MFEM Open Source FEM Package J. Stillerman, S. Shiraiwa, P. T. Bonoli, J. C. Wright, D. L. Green, T. Kolev A new plasma wave simulation environment based on the finite element method is presented. MFEM[1], a scalable open-source FEM library, is used as the basis for this capability. MFEM allows for assembling an FEM matrix of arbitrarily high order in a parallel computing environment. A 3D frequency domain RF physics layer was implemented using a python wrapper for MFEM and a cold collisional plasma model was ported. This physics layer allows for defining the plasma RF wave simulation model without user knowledge of the FEM weak-form formulation. A graphical user interface is built on $\pi $Scope, a python-based scientific workbench [2], such that a user can build a model definition file interactively. Benchmark cases have been ported to this new environment, with results being consistent with those obtained using COMSOL multiphysics, GENRAY, and TORIC/TORLH spectral solvers. This work is a first step in bringing to bear the sophisticated computational tool suite that MFEM provides (e.g., adaptive mesh refinement, solver suite, element types) to the linear plasma-wave interaction problem, and within more complicated integrated workflows, such as coupling with core spectral solver, or incorporating additional physics such as an RF sheath potential model or kinetic effects. [1] http://www.mfem.org [2] http://piscope.psfc.mit.edu [Preview Abstract] |
|
GP10.00112: Development of a fully implicit particle-in-cell scheme for gyrokinetic electromagnetic turbulence simulation in XGC1 Seung-Hoe Ku, R. Hager, C.S. Chang, L. Chacon, G. Chen The cancelation problem [1] has been a long-standing issue for long wavelengths modes in electromagnetic gyrokinetic PIC simulations in toroidal geometry. As an attempt of resolving this issue, we implemented a fully implicit time integration scheme in the full-f, gyrokinetic PIC code XGC1. The new scheme – based on the implicit Vlasov-Darwin PIC algorithm by G. Chen and L. Chacon [2] – can potentially resolve cancelation problem. The time advance for the field and the particle equations is space-time-centered, with particle sub-cycling. The resulting system of equations is solved by a Picard iteration solver with fixed-point accelerator. The algorithm is implemented in the parallel velocity formalism instead of the canonical parallel momentum formalism. XGC1 specializes in simulating the tokamak edge plasma with magnetic separatrix geometry. A fully implicit scheme could be a way to accurate and efficient gyrokinetic simulations. We will test if this numerical scheme overcomes the cancelation problem, and reproduces the dispersion relation of Alfven waves and tearing modes in cylindrical geometry. [1] Y. Chen and S. Parker, Phys. Plasmas 8, 2095 (2001) [2] G. Chen and L. Chacon, Comput. Phys. Comm., 197, 73 (2015) [Preview Abstract] |
|
GP10.00113: A Simulation Model for the Toroidal Ion Temperature Gradient Instability with Fully Kinetic Ions Benjamin Sturdevant, Scott Parker, Yang Chen A simulation model for the toroidal ITG mode in which the ions follow the primitive Lorentz force equations of motion is presented. Such a model can provide an important validation tool or replacement for gyrokinetic ion models in applications where higher order terms may be important. A number of multiple-scale simulation techniques are employed in this work, based on the previous success in slab geometry with an implicit orbit averaged and sub-cycled $\delta f$ model [1]. For the toroidal geometry model, we have derived a particle integration scheme based on variational principles, which is demonstrated to produce stable and accurate ion trajectories on long time scales. Orbit averaging and sub-cycling will be implemented with the variational integration scheme. The inclusion of equilibrium gradients in the fully kinetic $\delta f$ formulation is achieved through the use of a guiding center coordinate transformation of the weight equation. Simulation results for the fully kinetic ion model will be presented for the cyclone base case and comparisons will be made with gyrokinetic ion models. [1] B. Sturdevant, S. Parker, Y. Chen, and B. Hause, J. Comput. Phys., 316 (2016) 519. [Preview Abstract] |
|
GP10.00114: A Nonlinear Gyrokinetic Vlasov-Maxwell System for High-frequency Simulation in Toroidal Geometry Pengfei Liu, Wenlu Zhang, Jingbo Lin, Ding Li, Chao Dong A nonlinear gyrokinetic Vlasov equation is derived through the Lie-perturbation method to the Lagrangian and Hamiltonian systems in extanded phase space. The gyrokinetic Maxwell equations are derived in terms of the moments of gyrocenter phase-space distribution through the push-forward and pull-back representations, where the polarization and magnetization effects of gyrocenter are retained. The goal of this work is to construct a global nonlinear gyrokinetic vlasov-maxwell system for high-frequency simulation in toroidal geometry relevent for ion cyclotron range of frequencies (ICRF) waves heating and lower hybrid wave current driven (LHCD). [Preview Abstract] |
|
GP10.00115: Parallel mesh support for particle-in-cell methods in magnetic fusion simulations Eisung Yoon, Mark S. Shephard, E. Seegyoung Seol, Kaushik Kalyanaraman, Daniel Ibanez As supercomputing power continues to increase Particle-In-Cell (PIC) methods are being widely adopted for transport simulations of magnetic fusion devices. Current implementations place a copy of the entire continuum mesh and its fields used in the PIC calculations on every node. This is in general not a scalable solution as computational power continues to grow faster than node level memory. To address this scalability issue, while still maintaining sufficient mesh per node to control costly inter-node communication, a new unstructured mesh distribution methods and associated mesh based PIC calculation procedure is being developed building on the parallel unstructured mesh infrastructure (PUMI) [1]. Key components to be outlined in the presentation include (i) the mesh distribution strategy, (ii) how the particles are tracked during a push cycle taking advantage of the unstructured mesh adjacency structures and searches based on that structure, and (iii) how the field solve steps and particle migration are controlled. Performance comparisons to the current approach will also be presented. [1] D.A. Ibanez, E.S. Seol, C.W. Smith, M.S. Shephard, PUMI: Parallel Unstructured Mesh Infrastructure, ACM Transactions on Mathematical Software, 42(3), Article No. 17 (2016) [Preview Abstract] |
|
GP10.00116: Simulation of 2D Kinetic Effects in Plasmas using the Grid Based Continuum Code LOKI Jeffrey Banks, Richard Berger, Tom Chapman, Stephan Brunner Kinetic simulation of multi-dimensional plasma waves through direct discretization of the Vlasov equation is a useful tool to study many physical interactions and is particularly attractive for situations where minimal fluctuation levels are desired, for instance, when measuring growth rates of plasma wave instabilities. However, direct discretization of phase space can be computationally expensive, and as a result there are few examples of published results using Vlasov codes in more than a single configuration space dimension. In an effort to fill this gap we have developed the Eulerian-based kinetic code LOKI that evolves the Vlasov-Poisson system in 2+2-dimensional phase space. The code is designed to reduce the cost of phase-space computation by using fully 4th order accurate conservative finite differencing, while retaining excellent parallel scalability that efficiently uses large scale computing resources. In this poster I will discuss the algorithms used in the code as well as some aspects of their parallel implementation using MPI. I will also overview simulation results of basic plasma wave instabilities relevant to laser plasma interaction, which have been obtained using the code. [Preview Abstract] |
|
GP10.00117: Transverse instability of electron plasma waves study via direct 2$+$2D Vlasov simulations Denis Silantyev, Pavel Lushnikov, Harvey Rose Transverse instability can be viewed as initial stage of electron plasma waves (EPWs) filamentation. We performed direct 2$+$2D Vlasov-Poisson simulations of collisionless plasma to systematically study the growth rates of oblique modes of finite-amplitude EPW depending on its amplitude, wavenumber, angle of the oblique mode wavevector relative to the EPW's wavevector and the configuration of the trapped electrons in the EPW. Simulation results are compared to the predictions of theoretical models. [Preview Abstract] |
|
GP10.00118: MINI-CONFERENCE: GYROKINETIC ALGORITHMS |
|
GP10.00119: Verification and validation of electromagnetic instabilities from XGC1 in NSTX and NSTX-U plasmas Peter Porazik, Robert Hager, Seung-Hoe Ku, Walter Guttenfelder, Randy Churchill, Choong-Seock Chang Electromagnetic instabilities will be investigated in the NSTX and NSTX-U plasmas, including edge pedestal area, using the gyrokinetic code XGC1. Cross-verification will be performed with other electromagnetic gyrokinetic codes. Results will be compared with experimental diagnostics data. Relation of the gyrokinetic instabilities to the edge localized instabilities and the maximal pedestal height-gradient will be discussed. [Preview Abstract] |
|
GP10.00120: A Proposed Gyrokinetic Simulation Scheme for the Equilibrium Potential Well due to Finite Larmor Radius Effects at the Tokamak Edge W. W. Lee, Roscoe White A novel mechanism for producing the equilibrium potential well near the edge of a tokamak has been recently proposed [1]. Briefly, because of the difference in gyroradii between electrons and ions, an equilibrium electrostatic potential is generated in the presence of spatial inhomogeneity of the background plasma, which, in turn, produces a well associated with the radial electric field, Er, as observed at the edge of many tokamak experiments. Specifically, this theoretically predicted Er field, which can be regarded as producing a long radial wave length zonal flow, agrees well with recent experimental measurements on JET, NSTX and C-Mod [1]. A possible verification of this new mechanism using a proposed procedure [2] involving global gyrokinetic particle simulation codes and equilibrium MHD codes will be discussed. The approach is iteratively to decouple the transport problem from the equilibrium problem, so that each may be treated accurately, and, then couple them through parameter exchanges. [1] W. W. Lee and R. B. White, “Equilibrium Potential Well due to Finite Larmor Radius Effects at the Tokamak Edge, PPPL-5254 (2016). [2] W. W. Lee, “Magnetohydrodynamics for Collisionless Plasmas from the Gyrokinetic Perspective,” PPPL-5236, to appear in Phys. Plasmas (2016). [Preview Abstract] |
|
GP10.00121: Geometric Algorithms for Relativistic Dynamics Jian Liu, Hong Qin, Yulei Wang Dynamics of relativistic charged particles in external or self-consistent electromagnetic fields serves as the fundamental model underlying many subfields of physics. A series of geometric algorithms for relativistic and nonrelativistic systems have been systematically constructed, such as volume-preserving algorithms and explicit symplectic algorithms $^{\mathrm{1-3}}$. Taking advantage of the long-term conservativeness and accuracy of relativistic geometric algorithms, long-term simulations of runaway electrons in tokamak configurations have been carried out. And a new collisionless pitch-angle scattering process and better confinement of runaway electrons are discovered $^{\mathrm{4,5}}$. The Lorentz covariance of geometric algorithms are strictly defined and investigated for the first time. These geometric algorithms are building blocks for the recently developed structure-preserving geometric algorithms for the Vlasov-Maxwell system, e.g., the canonical and non-canonical PIC methods $^{\mathrm{6-8}}$. [1] R. Zhang et al., PoP 22, 044501 (2015). [2] Y. He et al., JCP 305, 172 (2016). [3] R. Zhang et al., CiCP 19, 1397. (2016) [4] J. Liu et al., NF 56, 064002 (2016). [5] Y. Wang et al., PoP 23, 062505 (2016). [6] J. Xiao et al., PoP 22, 112504 (2015). [7] H. Qin et al. NF 56, 014001 (2016). [8] He et al, PoP 22, 124503 (2015). [Preview Abstract] |
|
GP10.00122: MAGNETO-INERTIAL FUSION |
|
GP10.00123: The Fusion Gain Analysis of the Inductively Driven Liner Compression Based Fusion Akihisa Shimazu, John Slough An analytical analysis of the fusion gain expected in the inductively driven liner compression (IDLC) based fusion is conducted to identify the fusion gain scaling at various operating conditions. The fusion based on the IDLC is a magneto-inertial fusion concept, where a Field-Reversed Configuration (FRC) plasmoid is compressed via the inductively-driven metal liner to drive the FRC to fusion conditions. In the past, an approximate scaling law for the expected fusion gain for the IDLC based fusion was obtained under the key assumptions of (1) D-T fuel at 5-40 keV, (2) adiabatic scaling laws for the FRC dynamics, (3) FRC energy dominated by the pressure balance with the edge magnetic field at the peak compression, and (4) the liner dwell time being liner final diameter divided by the peak liner velocity. In this study, various assumptions made in the previous derivation is relaxed to study the change in the fusion gain scaling from the previous result of $G \propto m_l^{1/2} E_l^{11/8}$, where $m_l$ is the liner mass and $E_l$ is the peak liner kinetic energy. The implication from the modified fusion gain scaling on the performance of the IDLC fusion reactor system is also explored. [Preview Abstract] |
|
GP10.00124: Observations of ETI under dielectric-overcoated aluminum pulsed to hundreds of Tesla Trevor Hutchinson, Bruno Bauer, Stephan Fuelling, Kevin Yates, Thomas Awe, Graham Yelton MagLIF is an inertial confinement concept that takes advantage of relaxed fusion criteria due to premagnetized and preheated fuel. The drive surface is particularly susceptible to highly azimuthally correlated magneto-Rayleigh Taylor (MRT) instabilities, which section the liner wall and compromise confinement. This degree of azimuthal correlation is not due to residual lathe machining or surface roughness and a growing body of evidence suggest electrothermal instabilities (ETI) seed the MRT instability and allow for levels of azimuthal correlation that have been observed experimentally$^{\mathrm{1}}$. Implementation of dielectric coatings on Sandia's Z accelerator has reduced MRT amplitudes by at least a factor of ten$^{\mathrm{2}}$, which simulations suggest is due to mass tamping of the ETI. However, neither ETI nor its theorized suppression via an applied dielectric overcoat has been experimentally observed on a thick wire. We will report on experimental observations of ETI on the surface of 500 um radius aluminum rods with a 70 um parylene-N overcoat pulsed with 1 MA in 100 ns. [1] McBride, et al., PRL 109, 135004 (2012). [2] Peterson, et al., PRL 112, 135002 (2014). [Preview Abstract] |
|
GP10.00125: Study of laser preheat in magnetic liner inertial fusion using the AMR code FLASH Marissa Adams, Christopher Jennings, Stephen Slutz, Kyle Peterson, Pierre-Alexandre Gourdain Magnetic Liner Inertial Fusion (MagLIF) on the Z Pulsed Power Accelerator involves three processes: magnetization, preheat, and compression [S.A. Slutz et al., Phys. Plasmas \textbf{17}, 056303 (2010)]. An issue with this scheme is the development of instabilities during laser preheat, where the Z-Beamlet laser system may not deposit energy into deuterium fuel uniformly. This study explores potential mixing between liner and fuel, and inner imprinting of seeds on a beryllium liner that may generate late instability growth and shear, using the Eulerian AMR code FLASH. We further investigate potential instability implications of an additional layer of deuterium-tritium ice, as has been proposed [S.A. Slutz and R.A. Vesey, PRL \textbf{108} 025003 (2012)] and assess the sensitivity of MagLIF implosions to axial variations in fuel preheat; meanwhile testing the expediency of FLASH for these scenarios. FLASH was developed in part by the DOE NNSA ASC and DOE Office of Science ASCR-supported Flash Center at the University of Chicago. [Preview Abstract] |
|
GP10.00126: Analysis of laser preconditioning experiments on Z Beamlet Laser for MagLIF Michael Glinsky, Matthew Weis, Adam Harvey-Thompson, Matthias Geissel, Christopher Jennings, Taisuke Nagayama, Kyle Peterson Presented is an analysis of a series of laser preconditioning experiments on the Z Beamlet Laser (ZBL). These experiments examine the penetration of the laser through the plastic window (a few microns thick), the energy deposition into a gas behind the window, and the resulting density variations in the gas. The ZBL is a glass laser, frequency doubled to 527 $\mu$m, capable of delivering up to 4 kJ on target with a pulse length of a few ns. This is the same laser that is used to preheat the fuel in the MagLIF scheme before it is magnetically imploded on the Z generator. The design space for the laser pulse is explored in a series of experiments. Diagnostics include transmitted energy, backscattered energy, x-ray self emission images, and density shadowgrams at several times. These results are matched against HYDRA simulations using the uncertainty quantification engine Dakota. The potential for SBS, SRS, and filamentation are evaluated. Estimates of the energy deposition profile and disposition of the window (important because of potential mix with the fuel) are obtained with uncertainty. [Preview Abstract] |
|
GP10.00127: Development And Characterization Of A Liner-On-Target Injector For Staged Z-Pinch Experiments J. C. Valenzuela, F. Conti, I. Krasheninnikov, J. Narkis, F. Beg, F. J. Wessel, H. U. Rahman We present the design and optimization of a liner-on-target injector for Staged Z-pinch experiments. The injector is composed of an annular high atomic number (e.g. Ar, Kr) gas-puff and an on-axis plasma gun that delivers the ionized deuterium target. The liner nozzle injector has been carefully studied using Computational Fluid Dynamics (CFD) simulations to produce a highly collimated ~1 cm radius gas profile that satisfies the theoretical requirement for best performance on the 1 MA Zebra current driver. The CFD simulations produce density profiles as a function of the nozzle shape and gas. These profiles are initialized in the MHD MACH2 code to find the optimal liner density for a stable, uniform implosion. We use a simple Snowplow model to study the plasma sheath acceleration in a coaxial plasma gun to help us properly design the target injector. We have performed line-integrated density measurements using a CW He-Ne laser to characterize the liner gas and the plasma gun density as a function of time. The measurements are compared with models and calculations and benchmarked accordingly. [Preview Abstract] |
|
GP10.00128: Shock formation in Ne, Ar, Kr, and Xe on DD gas puff implosions J. Narkis, H. U. Rahman, F. J. Wessel, P. Ney, F. Beg 1- and 2-D simulations of a 1-cm radius, gas-puff implosion of Ne, Ar, Kr, and Xe liners onto a DD target are conducted using the discharge parameters for the Univ. Nevada, Reno, Zebra (1 MA, 125 ns) voltage driver and the resistive MHD code MACH2. During the run-in phase, initial†shock heating preheats the DD plasma, with subsequent stable, adiabatic compression heating the target to high energy density. The dynamics of the former in both the liner and target are investigated. It is shown that magnetic field transport to the liner/target interface does not occur prior to the run-in phase in Ne and Ar liners, yet does occur in Kr and Xe liners, and that magnetic field transport to the interface is a requirement for shock initiation, thus demonstrating the necessity for using a high-Z material in the Staged Z-pinch. Shock reflection off the axis and subsequent collision with the interface results in partial transmission into the liner, which manifests as current reversal, and consequently an enhanced B$_\theta$ gradient. 2-D simulations show that magneto-Rayleigh-Taylor instability growth decreases with increasing Z, with shock formation providing sufficient isolation to reproduce the current reversal and enhanced B$_\theta$ gradient observed in 1-D simulations. [Preview Abstract] |
|
GP10.00129: Comparison of Staged Z-pinch Experiments at the NTF Zebra Facility with Mach2 simulations E. Ruskov, F. J. Wessel, H. U. Rahman, P. Ney, T. W. Darling, Z. Johnson, E. McGee, A. Covington, E. Dutra, J. C. Valenzuela, F. Conti, J. Narkis, F. Beg Staged Z-pinch experiments at the University of Nevada, Reno, 1MA Z-pinch Zebra facility were conducted. A hollow shell of argon gas liner is injected between 1 cm anode-cathode gap through a supersonic nozzle of 2.0 cm diameter with a throat gap of 240 microns. A deuterium plasma fill is injected inside the argon gas shell through a plasma gun as a fusible target plasma. An axial magnetic field is also applied throughout the pinch region. Experimental measurements such as pinch current, X-ray signal, neutron yield, and streak images are compared with MACH2 radiation hydrodynamic code simulations. The argon liner density profiles, obtained from the CFD (FLUENT), are used as an input to MACH2. The comparison suggests a fairly close agreement between the experimental measurements and the simulation results. This study not only helps to benchmark the code but also suggests the importance of the Z-pinch implosion time, optimizing both liner and target plasma density to obtain the maximum energy coupling between the circuit and the load. [Preview Abstract] |
|
GP10.00130: Neutron and X-ray diagnostics for SZP experiments at Zebra T. Darling, E. McGee, A. Covington, E. Dutra, F. J. Wessel, E. Ruskov, H. U. Rahman, J. C. Valenzuela, F. Conti The Zebra pulsed-power generator at the Nevada Terawatt Facility (NTF) of the University of Nevada produces current pulses of up to a megaamp with a rise time of 70 ns. By passing this current through a structured gas jet target, such as the Staged-Z-pinch (SZP), the project hopes to approach near energy gain conditions from fusion reactions in a pinched plasma. This article describes the setup and instrumentation at Zebra for detecting the neutron and x-ray output of the pinch and the procedures for reducing these signals to a quantitative measurement of the yields. Scintillation detectors with fast PMT detectors and activation decay measurements are the primary neutron diagnostics. These measurements are of prime importance in determining the parameters required to optimize the gas jet conditions for fusion. [Preview Abstract] |
|
GP10.00131: Ion-viscosity effects on plasma-liner formation and implosion via merging supersonic plasma jets Kevin Schillo, Jason Cassibry, Roman Samulyak, Wen Shih, Scott Hsu The PLX-{\$}$\backslash $alpha{\$} project endeavors to study plasma-liner formation and implosion by merging a spherical array of plasma jets as a candidate standoff driver for MIF. Smoothed particle hydrodynamics is used to model the liner formation and implosion processes. SPH is a meshless Lagrangian method to simulate fluid flows by dividing a fluid into a set of particles and using a summation interpolant function to calculate the properties and gradients for each of these particles. Ion viscosity is anticipated to be an important mechanism for momentum transport during liner formation, implosion, and stagnation. To study this, ion viscosity was incorporated into the code.~To provide confidence in the numerical output and to help identify the difference between numerical and physical diffusion, a series of test cases were performed, consisting of Couette flow, Gresho vortex, and a Taylor-Green vortex. An L2-norm analysis was performed to measure the error and convergence. Simulations of conical (6 jets) and 4{\$}$\backslash $pi{\$} (60 jets) liners with and without ion viscosity reveal potential effects of viscosity on ram pressure, Mach-number degradation, and evolution of liner perturbations during jet merging and liner implosion. [Preview Abstract] |
|
GP10.00132: Diagnostics and results from coaxial plasma gun development for the PLX-$\alpha$ project* A. Case, S. Brockington, E. Cruz, F. D. Witherspoon We present results from the diagnostics used during development of the contoured gap coaxial plasma guns [1] for the PLX-$\alpha$ project at LANL. Plasma-jet diagnostics include fast photodiodes for velocimetry, a ballistic pendulum for total plasmoid momentum, and interferometry for line integrated density. Deflectometry will be used for line integrated perpendicular density gradients. Time-resolved high-resolution spectroscopy using a novel detector and time-integrated survey spectroscopy are used for measurements of velocity and temperature, as well as impurities. We will also use a Faraday cup for density, fast imaging for plume geometry, and time-integrated imaging for overall light emission. Experimental results are compared to the desired target parameters for the plasma jets (up to $n \approx 2\times 10^{16}\,\rm{cm}^{-3}$, $v\approx 50\, \rm{km/s}$, mass $\approx 5\, \rm{gm}$, radius$ = 4\, \rm{cm}$, and length $\approx 10\,\rm{cm}$).\newline [1] Witherspoon et al., Rev. Sci. Instr. \textbf{80}, 083506 (2009). [Preview Abstract] |
|
GP10.00133: Engineering design of the PLX-$\alpha$ coaxial gun Edward Cruz, Samuel Brockington, Andrew Case, Marco Luna, Douglas Witherspoon, Samuel Langendorf We describe the engineering and technical aspects of the coaxial gun designed for the 60-gun scaling study of spherically imploding plasma liners as a standoff driver for plasma-jet-driven magneto-inertial fusion [1]. Each coaxial gun incorporates a fast, dense gas injection and triggering system, a compact low-weight pfn with integral sparkgap switching, and a contoured gap designed to suppress the blow-by instability [2]. Alpha1 and Alpha2 guns are compared, with emphasis on the improvements on Alpha2, which include a faster more robust gas valve, an improved electrode contour, a custom 600-$\mu$F, 5-kV pfn, and a set of six inline sparkgap switches operated in parallel. The switch and pfn configurations are mounted directly to the back of the gun, and are designed to reduce inductance, cost, and complexity, maximize efficiency and system reliability, and ensure symmetric current flow. We will provide a detailed overview of the design choices made for the PLX-$\alpha$ coaxial gun. [1] Hsu et al., IEEE Trans. Plasma Sci.~{\bf 40}, 1287 (2012). [2] Witherspoon et al., Rev. Sci. Instr. \textbf{80}, 083506 (2009). [Preview Abstract] |
|
GP10.00134: Semi-analytical model of plasma-jet-driven magneto-inertial fusion Samuel Langendorf, Scott Hsu Plasma-jet-driven magneto-inertial fusion (PJMIF) is an MIF concept in which a spherically imploding plasma liner is formed from the convergence of a large number of discrete supersonic plasma jets, and the assembled liner is employed to compress a magnetized fuel target [Hsu et al., IEEE Trans. Plasma Sci.~\textbf{40}, 1287 (2012)]. We formulate a 1D spherical-geometry MIF model and apply it to PJMIF. The model incorporates compressible hydrodynamics, liner ionization, radiation, D-T fusion burn, heat conduction losses, magnetic pressure, magnetic flux losses via the Nernst effect, and charged-particle energy deposition. We study the effects of different transport outcomes (e.g., optically thin vs.\ optically thick radiation transport, classical vs.\ Bohm-like thermal diffusivity), and scan the liner-target parameter space for configurations with optimal fusion gain at a given total energy. We find that gain-optimal implosion velocity depends significantly on the liner temperature. For liners at approximately room temperature, an implosion speed of roughly 70 km/s is advantageous over faster speeds due to increased dwell time at stagnation. [Preview Abstract] |
|
GP10.00135: Diagnostic Suite for the PLX-$\alpha $ Project Kevin Yates, Mark Gilmore, Samuel Langendorf, John Dunn, Jacquelynne Vaughan, Ricardo Martinez, Scott Hsu The Plasma Liner Experiment--ALPHA (PLX-$\alpha )$ at Los Alamos National Laboratory is demonstrating the viability and scalability of spherically imploding plasma liners as a compression driver for plasma-jet-driven magneto-inertial fusion (PJMIF) [Hsu et al., IEEE Trans. Plasma Sci. 40, 1287 (2012)]. On PLX-$\alpha $, plasma liners will be formed by merging up to 60 supersonic plasma jets. We are currently conducting conical-liner experiments (\textasciitilde $\pi $/2 solid angle with 6 and 7 plasma guns) to diagnose the jet-merging process and determine the values of post-merge Mach-number-degradation and liner uniformity. The diagnostic suite includes 12-chord interferometry, visible-light survey spectroscopy, high-resolution visible spectroscopy, single- and multi-frame intensified visible imaging, and schlieren imaging. The diagnostics suite and a comparison to synthetic data from 3D simulations on the 6- and 7-jet configurations will be presented. [Preview Abstract] |
|
GP10.00136: Numerical Modeling of Plasma-Liner Formation and Implosion for the PLX-{\$}$\backslash $alpha{\$} Project. Jason Cassibry, Roman Samulyak, Kevin Schillo, Wen Shih, Scott Hsu Numerical simulations of the propagation, merging, and implosion of supersonic plasma jets have been performed using the FronTier and smooth particle hydrodynamics (SPH) codes in support of the PLX-{\$}$\backslash $alpha{\$} project. The physics includes radiation, heat conduction using Braginskii thermal conductivities, ion viscosity, and tabular equations of state using LTE and non-LTE models. A parametric analysis provides scaling of peak ram pressure and Mach number ~vs. number of jets, initial density, initial jet velocity, and species including nitrogen, neon, argon, krypton, and xenon. Conical simulations of 6 and 7 jets support near-term experiments, which facilitate diagnostic access for assessing the quality of the liner during merge. Solid angle averaged and standard deviation of ram pressure and Mach number reveal the variation in these properties during formation and implosion. Spherical harmonic mode-number analysis of spherical slices of ram pressure at various radii and times provide a quantitative means to assess the evolution of liner non-uniformity. [Preview Abstract] |
|
GP10.00137: BASIC PLASMA PHYSICS |
|
GP10.00138: Optical Tagging of Ion Beams Accelerated by Double Layers in Laboratory Plasma Timothy Good, Evan Aguirre, Derek Thompson, Earl Scime Experiments in helicon sources that investigate plasma expansion into weakly magnetized, low density regions reveal the production of supersonic ion beams attributed to acceleration by spatially localized double layer structures. Current efforts are aimed at mapping the ion velocity flow field utilizing 2D spatially scanning laser induced fluorescence (LIF) probes that yield metastable ion velocity distribution functions (IVDF) for velocities along and perpendicular to the flow.* Observation of metastable ion beams by LIF renders plausible a Lagrangian approach to studying the field-ion interaction via optical tagging. We propose a tagging scheme in which metastable state ion populations are modulated by optical pumping upstream of the double layer and the synchronous detection of LIF at the ion beam velocity is recorded downstream. Besides the unambiguous identification of the source of beam ions, this method can provide detailed dynamical information through time of flight analysis. Preliminary results will be presented.\newline Reference to Evan Aguirre's poster [Preview Abstract] |
|
GP10.00139: Two Dimensional LIF Measurements and Potential Structure of Ion Beam Formation in an Argon Helicon Plasma Evan Aguirre, Earl Scime, Timothy Good We report 2-dimensional, spatially resolved observations of ion beam formation in an expanding helicon plasma. Previous studies found that a current free double layer (CFDL) spontaneously arises at low pressure, below 1 mT. We use Laser Induced Fluorescence (LIF), a non-perturbative diagnostic to measure the ion velocity distribution functions (IVDFs) of argon ions both parallel and perpendicular to the background magnetic field. We report ion beam formation as a function of the expansion chamber magnetic field (0-108 G). The ion beam appears peaked in the center of the expansion chamber and decays over a few centimeters radially. We also report the potential structure of the plasma obtained with a planar Langmuir probe. To obtain meaningful Langmuir probe measurements, averages of tens of current-voltage are needed to reduce the effects of large electrostatic fluctuations that arise in plasmas that generate ion beams. We report the dependence of density, electron temperature, and floating potential on radial and axial position in the expansion plume. [Preview Abstract] |
|
GP10.00140: ABSTRACT WITHDRAWN |
|
GP10.00141: Agyrotropic pressure tensor induced by the plasma velocity shear Francesco Pegoraro, Danele Del Sarto, Francesco Califano We show that the spatial inhomogeneity of a shear flow in a fluid plasma is transferred to a pressure anisotropy that has both a gyrotropic and a non gyrotropic component. We investigate this process both analytically and numerically by including the full pressure tensor dynamics. We determine the time evolution of the pressure agyrotropy and in general of the the pressure tensor anisotropization which arise from the action of both the magnetic eld and the flow strain tensor. This mechanism can affect the onset and development of shear-induced fluid instabilities in plasmas and is relevant to the understanding of the origin of some of the non-Maxwellian distribution functions evidenced both in Vlasov simulations and in space plasma measurements that exhibit pressure agyrotropy. [Preview Abstract] |
|
GP10.00142: Relativistic Langevin equation for runaway electrons J.A. Mier, J.R. Martin-Solis, R. Sanchez The Langevin approach to the kinetics of a collisional plasma is developed for relativistic electrons such as runaway electrons in tokamak plasmas. In this work, we consider Coulomb collisions between very fast, relativistic electrons and a relatively cool, thermal background plasma. The model is developed using the stochastic equivalence of the Fokker-Planck and Langevin equations [V.I. Tikhonov and M.A. Mironov, Markovian Processes (1977). Soviet Radio, Moscow]. The resulting Langevin model equation for relativistic electrons is an stochastic differential equation, amenable to numerical simulations by means of Monte-Carlo type codes. Results of the simulations will be presented and compared with the non-relativistic Langevin equation for RE electrons used in the past [I. Fernández-Gómez et al., Phys. Plasmas 19 (2012) 102504]. [Preview Abstract] |
|
GP10.00143: Characterizing a multi-MeV e-beam induced plasma through visible spectroscopy and imaging Thierry d'Almeida, Maxime Ribiere, Rémi Maisonny, Sandra Ritter, Damien Plouhinec, Gérard Auriel High energy electrons interaction and propagation mechanisms in solid targets have a broad range of applications in high energy density physics. The latter include fast ignition for inertial fusion research, production of ultra-high mechanical stress levels, plasma interactions with e-beam particles in electron diodes, radiative hydrodynamic models\textellipsis This paper presents the results from recent experiments conducted on the multi-MeV generator ASTERIX operated at CEA-Gramat. This high flux density electron beam was launched from an aluminum cathode onto an aluminum-tantalum target for voltage and current of 2.4 MeV and 55 kA, respectively. A set of optical diagnostics were fielded in all of the experiments, including a UV-visible spectrometers and a fast imaging. The imaging data obtained during the experiment allowed for the ablated species velocity to be determined. based on spectroscopic analysis, the light emission was attributed to aluminum and tantalum excited atoms and ions. The analysis of this time-integrated spectrum based on radiative transfer model clearly unveiled two distinct regions of the plasma over its expansion: a hot core surrounded by a cold vapor. A quantitative analysis of these results is presented. [Preview Abstract] |
|
GP10.00144: A table top experiment to study plasma confined by a dipole magnet Sudeep Bhattacharjee, Anuj Ram Baitha There has been a long quest to understand charged particle generation, confinement and underlying complex processes in a plasma confined by a dipole magnet. Our earth's magnetosphere is an example of such a naturally occurring system. A few laboratory experiments have been designed for such investigations, such as the Levitated Dipole Experiment (LDX) at MIT, the Terella experiment at Columbia university, and the Ring Trap-1 (RT-1) experiment at the University of Tokyo. However, these are large scale experiments, where the dipole magnetic field is created with superconducting coils, thereby, necessitating power supplies and stringent cryogenic requirements. We report a table top experiment to investigate important physical processes in a dipole plasma. A strong cylindrical permanent magnet, is employed to create the dipole field inside a vacuum chamber. The magnet is suspended and cooled by circulating chilled water. The plasma is heated by electromagnetic waves of 2.45 GHz and a second frequency in the range 6 -- 11 GHz. Some of the initial results of measurements and numerical simulation of magnetic field, visual observations of the first plasma, and spatial measurements of plasma parameters will be presented. [Preview Abstract] |
|
GP10.00145: Uniform derivation of Coulomb collisional transport thanks to Debye shielding Dominique Escande, Yves Elskens, Fabrice Doveil The effective potential acting on particles in plasmas being essentially the Debye-shielded Coulomb potential, the particles collisional transport in thermal equilibrium is calculated [1] for all impact parameters $b$, with a convergent expression reducing to Rutherford scattering for small $b$, in agreement with both usual expressions holding for large $b$ [2] and small $b$ [3]. No cutoff at the Debye length scale is needed, and the Coulomb logarithm is only slightly modified. [1] D.F. Escande, Y. Elskens, F. Doveil, J. Plas. Phys. 81 (2015) 305810101 (9 pp.). [2] S. Gasiorowicz, M. Neuman, R.J. Riddell Jr, Phys. Rev. 101 (1956) 922-934. [3] M.N. Rosenbluth, W.M. MacDonald {\&} D.L. Judd, Phys. Rev. 107 (1957) 1-6. [Preview Abstract] |
|
GP10.00146: Study of a dual frequency capacitively coupled rf discharge in the background of multi-component plasma and its validation by a simple analytical sheath model Heman Bhuyan, Partha Saikia, Mario Favre, Edmundo Wyndham, Felipe Veloso The behavior of a phase-locked dual frequency capacitively coupled rf discharges (2f-CCRF) in the background of multi-component plasma is experimentally studied by rf current-voltage measurements and optical emission spectroscopy (OES). The multi-component plasma is produced by adding hydrogen to the argon CCRF discharge. Variation of experimental parameters, like working pressure, low frequency (LF) and high frequency (HF) rf power indicate significant changes in the electron density and temperature as well as the DC self-bias developed on the power electrode. It is observed that the electron density decreases as the percentage of hydrogen increases in the argon plasma while the electron temperature follows opposite trend. An analytical sheath model for the 2f-CCRF discharge in the background of multi-component plasma is developed and its prediction on the observed variation of DC self-bias is well agreed with the experimental observations. [Preview Abstract] |
|
GP10.00147: The Fokker-Planck approach to derive the magnetized Balescu-Lenard-Guernsey equation. Chao Dong, Wenlu Zhang, Ding Li The Fokker-Planck coefficients are expressed in terms of the power spectral function of the electric field fluctuations and the dielectric response function based on their new definitions for a plasma in the magnetic field. For a quiescent plasma, the power spectral function is calculated analytically by generalizing Hubbard's approach. The magnetized Balescu-Lenard-Guernsey equation is thus obtained which is shown to be the same as the kinetic equation derived from the BBGKY hierarchy of equations and reduce to our previous results within the binary collision model when the collective effects are neglected. Compared to the BBGKY approach, the Fokker-Planck approach is apparently simple in mathematical calculations in deriving the kinetic equation for magnetized plasmas. [Preview Abstract] |
|
GP10.00148: Two-fluid effects and shear in large-scale dynamos Manasvi Lingam, Amitava Bhattacharjee In recent times, two-fluid effects (especially the Hall term) have been increasingly explored in space and astrophysical plasmas. The large-scale and small-scale dynamos with the Hall term were explored in [1] and [2]. Here, we consider the role of shear (and rotation) in conjunction with the Hall term. It was recently shown, by means of a resistive MHD analysis, that the turbulent resistivity becomes tensorial in nature with negative off-diagonal components [3]. However, the Hall term leads to additional couplings, and introduces on-diagonal contributions [1] which can make the diagonal terms negative and drive dynamo growth. Lastly, electron inertia (a hitherto unconsidered two-fluid effect) is shown to further enhance the possibility of a turbulent anti-diffusivity, and thereby drive the large-scale dynamo.\\ \noindent [1] M. Lingam, A. Bhattacharjee, ApJ, in press (2016) \\ \noindent [2] M. Lingam, A. Bhattacharjee, MNRAS, 460, 478 (2016) \\ \noindent [3] J. Squire, A. Bhattacharjee, J. Plasma Phys., 82, 535820201 (2016) [Preview Abstract] |
|
GP10.00149: The role of the Hall current in mean-field dynamo theory Amitava Bhattacharjee, Manasvi Lingam It is now well established that the Hall current plays a significant role in astrophysical environments. Hence, the role of the Hall term in classical mean-field dynamo theory is investigated [1]. The standard alpha coefficient is modified, and shown to vanish only when a specific double Beltrami state (an outcome of certain Hall MHD relaxation theories) is attained. The dynamics of alpha quenching is also elaborated, and shown to exhibit both similarities and dissimilarities with its resistive MHD counterpart. A noteworthy and unusual feature of this analysis is the emergence of a turbulent resistivity that is not necessarily positive-definite. It implies that, even in the absence of shear and rotation, Hall effects may enable the growth of large-scale magnetic fields. Connections with the Hall MRI dynamo are also briefly discussed via a heuristic model [2].\\ \noindent [1] M. Lingam, A. Bhattacharjee, ApJ, in press (2016) \\ \noindent [2] M. Lingam, A. Bhattacharjee, MNRAS, 460, 478 (2016) [Preview Abstract] |
|
GP10.00150: Study of axisymmetric Z-pinch compression using continuum kinetic simulations G.V. Vogman, P. Colella, U. Shumlak Plasma kinetic theory treats each constituent species as a probability distribution function in phase space. Numerically, the velocity dependence of the distribution function can be sampled discretely as in particle-in-cell methods, or represented smoothly as in continuum methods. Continuum methods for solving kinetic theory governing equations are advantageous in that they can be cast in conservation-law form, are not susceptible to noise, and can be implemented using high-order numerical methods, which provide enhanced solution accuracy. A conservative fourth-order finite-volume algorithm has been developed to solve the Vlasov-Maxwell equation system in cylindrical phase space coordinates. This new platform is used to investigate the kinetic physics associated with compression in a collisionless axisymmetric Z-pinch. These kinetic simulations provide a means to assess the accuracy of the polytropic assumptions often made when analyzing Z-pinch stability and scaling properties. [Preview Abstract] |
|
GP10.00151: Arc Voltage Between Deion Grid Affected by Division of Arc in Magnetic Driven Arc. Yutaro Inuzuka, Takashi Yamato, Shinji Yamamoto, Toru Iwao Magnetic driven arc has been applied to DC breaker and fault current limiters. However, it has not been researched, especially stagnation and re-strike of the arc. In this paper, the arc voltage between deion grid affected by division of arc in magnetic driven arc and arc behavior are measured by using the oscilloscope and HSVC (High Speed Video Camera). As a result, arc voltage increased because of division of the arc. The arc mean moving speed increases with increasing the external magnetic field. However, when the arc was not stalemate, the arc moving speed does not change so much. The arc re-strike time increases and stalemate time decreases with increasing the external magnetic field. Therefore, the anode spot moving speed increases 8 times because arc re-strike occurs easily with the external magnetic field. Thus, the erosion of electrodes decreases and the arc movement becomes the smooth. When the arc is divided, the arc voltage increased because of the electrode fall voltage. Therefore, the arc voltage increases with increasing the number of deion grid. [Preview Abstract] |
|
GP10.00152: Heat Transfer Affected by Transverse Magnetic Field using 3D Modeling of Arc Plasma. Yoshifumi Maeda, Tatsuro Tanaka, Shinji Yamamoto, Toru Iwao Gas shielded metal arc welding is used to join the various metal because this is the high quality joining technology. Thus, this welding is used for a welding of large buildings such as bridges and LNG tanks. However, the welding defect caused by the heat transfer decrement may occur with increasing the wind velocity. This is because that the convection loss increases because the arc deflects to leeward side with increasing the wind velocity. In order to prevent from the arc deflection, it is used that the transverse magnetic field is applied to the arc. However, the arc deflection occurs with increasing the transverse magnetic field excessively. The energy balance of the arc is changed with increasing the convection loss caused by the arc deflection, and the heat transfer to the anode decreases. Therefore, the analysis including the arc and anode is necessary to elucidate the heat transfer to the anode. In this paper, the heat transfer affected by the transverse magnetic field using 3D modeling of the arc plasma is elucidated. The heat transfer to the anode is calculated by using the EMTF(electromagnetic thermal fluid) simulation with increasing the transverse magnetic field. As a result, the heat transfer decreased with increasing the transverse magnetic field. [Preview Abstract] |
|
GP10.00153: Transient Response of Arc Temperature and Iron Vapor Concentration Affected by Current Frequency with Iron Vapor in Pulsed Arc Tatsuro Tanaka, Yoshifumi Maeda, Shinji Yamamoto, Toru Iwao TIG arc welding is chemically a joining technology with melting the metallic material and it can be high quality. However, this welding should not be used in high current to prevent cathode melting. Thus, the heat transfer is poor. Therefore, the deep penetration cannot be obtained and the weld defect sometimes occurs. The pulsed arc welding has been used for the improvement of this defect. The pulsed arc welding can control the heat flux to anode. The convention and driving force in the weld pool are caused by the arc. Therefore, it is important to grasp the distribution of arc temperature. The metal vapor generate from the anode in welding. In addition, the pulsed current increased or decreased periodically. Therefore, the arc is affected by such as a current value and current frequency, the current rate of increment and the metal vapor. In this paper, the transient response of arc temperature and the iron vapor concentration affected by the current frequency with iron vapor in pulsed arc was elucidated by the EMTF (ElectroMagnetic Thermal Fluid) simulation. As a result, the arc temperature and the iron vapor were transient response as the current frequency increase. Thus, the temperature and the electrical conductivity decreased. Therefore, the electrical field increased in order to maintain the current continuity. The current density and electromagnetic force increased at the axial center. In addition, the electronic flow component of the heat flux increased at the axial center because the current density increased. However, the heat conduction component of the heat flux decreased. [Preview Abstract] |
|
GP10.00154: Time and space correlated investigations of confinement effects due to static axial magnetic fields acting on laser produced carbon plasmas Mario Favre, Edmund Wyndham, Felipe Veloso, Heman Bhuyan, Sebastian Reyes, Hugo Marcelo Ruiz, Luis Sebastian Caballero-Bendixsen We present further detailed studies of the dynamics and plasma properties of a laser produced Carbon plasma expanding in a static axial magnetic field. The laser plasmas are produced in vacuum, $\sim1\cdot10^{-6}$ Torr, using a graphite target, with a Nd:YAG laser, 3.5 ns, 340 mJ at 1.06 $\mu$m, focused at $\sim2\cdot10^9$ W/cm$^2$, and propagate in static magnetic fields of maximum value $\sim$0.2 T. 15 ns time and spaced resolved OES is used to investigate plasma composition. 50 ns time resolved plasma imaging is used to visualize the plasma dynamics. A mm size B-dot probe is used, in combination with a Faraday cup, to characterize the interaction between the expanding plasma and the magnetic field. As a result of time and space correlated measurements, unique features of the laser plasma dynamics in the presence of the magnetic field are identified, which highlight the confinement effects of the static magnetic field [Preview Abstract] |
|
GP10.00155: Galerkin GX Vlasov-Maxwell system Joshua Burby A variety of "gyroaveraged" kinetic plasma models are structurally very similar, even though they describe different physical processes. I will show that drift kinetics, gyro kinetics, and Vlasov-Maxwell theory are all particular examples of a much more general theory that I call GX Vlasov-Maxwell theory. The GX Vlasov-Maxwell system is an infinite-dimensional Hamiltonian system. Starting from the GX Vlasov-Maxwell system, I will derive a finite-dimensional version of the theory called Galerikin GX Vlasov-Maxwell theory. By representing the electromagnetic field using finite element exterior calculus, and replacing the one-particle distribution function with the Klimontovich distribution, the partial differential-integral equation that comprises the GX Vlasov-Maxwell system will be replaced with a finite dimensional ordinary differential equation. The conserved Hamiltonian and Poisson bracket for this system will be presented. While infinite-dimensional Hamiltonian systems do not possess (functional) Liouville measures, finite dimensional approximations of these systems do. The expression for Galerkin GX Vlasov-Maxwell theory's Liouville volume will be presented. [Preview Abstract] |
|
GP10.00156: Current sheet formation in a 3D line-tied plasma Yao Zhou, Yi-Min Huang, Hong Qin, Amitava Bhattacharjee Recently a variational integrator for ideal MHD in Lagrangian labeling has been developed by discretizing Newcomb's Lagrangian on a moving mesh using discretized exterior calculus. With the frozen-in equation built-in, the method is free of artificial reconnection, and therefore optimal for studying current sheet formation. Using this method, it is confirmed that the nonlinear solution to the ideal Hahm-Kulsrud-Taylor problem in 2D yields a singular current sheet. We identify it by showing that the equilibrium solution converges with increasing resolution, except where there is singularity. This approach is in contrast to previous studies which use diverging peak current density as sole evidence of current singularity. We then extend the problem to 3D line-tied geometry. The linear solution, which is singular in 2D, is found to be smooth, but pathological when the system is sufficiently long. Accordingly, the nonlinear solution turns out to be smooth for short systems, but tends to become more singular when the system length increases. A resolution to this problem can potentially settle the long-standing controversy over Parker's conjecture on the formation of current singularity in 3D line-tied geometry. [Preview Abstract] |
|
GP10.00157: Plasma Rotation Control Experiment in a Strongly Diverging Magnetic Field Kenichiro Terasaka, Kanshi Furuta, Shinji Yoshimura, Mitsutoshi Aramaki, Masayoshi Y. Tanaka It has been recognized that the plasma rotation affects the plasma flow structure along the magnetic field line. However, the effect of plasma rotation on structure formation in a strongly diverging magnetic field with magnetized electrons and unmagnetized ions has not been fully understood, so far. Understanding the flow structure formation in an ion-unmagnetized plasma is essential to control ion streamline detachment from the magnetic field line and also necessary to study the astrophysical phenomena in laboratory. In order to clarify the effect of plasma rotation in a diverging magnetic field, we have performed the plasma rotation control experiment in the HYPER-II device at Kyushu Univ., Japan. A set of cylindrical electrode was utilized to control the radial electric field, and the profile of azimuthal $\bm E \times \bm B$ rotation has been changed. We present the experimental results on the electron density pileup and the flow reversal appeared in the rotating plasma. [Preview Abstract] |
|
GP10.00158: Heat Transport Effects in Rotating Gases and Plasmas Elijah Kolmes, Vasily Geyko, Nathaniel Fisch In some contexts, rotating gases and plasmas exhibit heat transport effects that are substantially different from what would be found in the absence of rotation. For instance, a Ranque-Hilsch vortex tube is a device which separates an input stream of (neutral) gas into hot and cold streams by setting up a rotating flow in a specially designed cylindrical chamber. One class of vortex tube models involves radial motion that carries gas up and down the pressure gradients set up by the centrifugal potential inside the tube and which results in adiabatic heating and cooling of the radially moving material. The approach of producing heat transport in a rotating flow using pressure gradients and motion along those gradients may have applications in plasma systems. We discuss possible applications for rotational heat transport effects in plasma systems, including Z-pinch configurations. [Preview Abstract] |
|
GP10.00159: Observations on the dynamics of the plasma sheath axial acceleration phase on a Plasma Focus Discharge of hundreds of Joules Gonzalo Avaria, Alejandro Clausse, Osvaldo Cuadrado, Nelson Villalba, Jose Moreno, Cristian Pavez, Leopoldo Soto The plasma sheath evolution in the axial acceleration phase of plasma focus discharges is of interest for fundamental studies of the ionization and electron density evolution at the early stages of plasma formation, in order to improve the understanding of its influence in pinch development characteristics. We present spatial and temporal resolved measurements performed with a 0.5 m imaging spectrometer that captures the emission of the interelectrode region in the PF-400J (176-539 J, 880 nF, 20-35 kV, quarter period \textasciitilde 300 ns) Plasma Focus Discharge. Spectral images of the plasma sheath at different times of the current pulse evolution were acquired with an ICCD integrating over a 3 ns window. The sheath speed was determined to be approximately 43.6 km/s for discharges in Hydrogen at 9 mbar. Comparison of these measurements with numerical calculations, based on a lumped parameter model, show excellent correspondence. Electron density calculations at different stages of the plasma evolution are also presented. [Preview Abstract] |
|
GP10.00160: Experimental Observation of Sheath-presheath Instabilities Vara Prasad Kella, Joydeep Ghosh, Devendra Sharma, Prabal k Chattopadhyay Instabilities in the Sheath-presheath regime are most important phenomena that can affect the plasma-wall interaction. These instabilities can modify the particle flow velocities and distribution functions in that regime. In this present work, instabilities exists in the sheath-presheath in a low temperature plasma are observed. Experiments are carried in single ion species argon plasma and multi ion species Ar-He plasma. Experiments are carried in a stainless steel chamber with filament discharge plasma. Sheath is produced around a stainless steel grid at center of the chamber. Fluctuations from the grid and cylindrical Langmuir probe are recorded. Langmuir probe is used to get the floating potential fluctuations from presheath and bulk plasma as well. In single ion species argon plasma, there are two instabilities observed namely ion-ion counter streaming instability through mesh grid and ion acoustic instability respectively arises in the presheath. In case of multi-ion Ar-He plasma, two stream instability also explored. The neutral pressure threshold for the sustain of these instabilities also observed. [Preview Abstract] |
|
GP10.00161: Characterization and Stability of High Beta Spherical Flows Robert Siller, Vladimir Mirnov, Cary Forest The fluid response of a fully compressible, isothermal plasma in investigated in a spherical system with application to the Madison Plasma Dynamo Experiment (MPDX). Numerical results are found in a fully spectral code, solving separately for the equilibrium profile of a given drive, and the linear eigenmodes of the system. The example flows are driven by a large radial current drawn across a small axial field generating torque across the system. Numerical calculations show sample conditions near achievable conditions for exciting various instabilities, with the MRI and the dynamo instabilities of primary focus. [Preview Abstract] |
|
GP10.00162: Electron Cyclotron Heating Plasmas at the Wisconsin Plasma Astrophysics Laboratory Jason Milhone, Michael Clark, John Wallace, David Weisberg, Cary Forest A 2.45 GHz CW electron cyclotron heating (ECH) system has been added to the Wisconsin Plasma Astrophysics Laboratory (WiPAL) to increase plasma performance, allowing access to regimes where flow driven plasma instabilities are relevant. Plasma breakdown has been achieved with 10-15 kW of injected RF power at neutral pressures as low as $5 \times 10^{-6}$ Torr demonstrating good confinement from WiPAL’s multi-cusp scheme. Fast electrons have been observed visually in magnet rings where nothing has been inserted. Both over- and under-dense plasmas have been observed as the neutral fill pressure is varied. With ECH, the plasma density and cathode stirring are decoupled; This allows for high performance plasmas at lower neutral fill pressures with high ionization fraction, thus reducing the momentum loss to charge-exchange collisions. Initial results from the full suite of WiPAL diagnostics, including langmuir/mach probes, optical emission spectroscopy (OES), and millimeter-wave interferometry, will be shown. [Preview Abstract] |
|
GP10.00163: Water cooling system leak proofing strategy for the Plasma Couette Experiment Upgrade (PCX-U) Mike Clark, Ken Flanagan, Wilson Hernandez, Austin Jaeger, Lauren Laufman-Wollitzer, Ethan Nikolau, Megan Tabbutt, Roger Waleffe, John Wallace, Yufan Xu, Cary Forest An improved system for water cooling several experimental components has been installed for the Plasma Couette Experiment Upgrade (PCX-U). The most important aspect of the upgrade was to cool the new SmCo permanent magnet cage array. Many methods of connecting water cooling pipes, tubes, and fittings were employed balancing several factors. These factors included ease of assembly/disassembly, reliability, operating pressure, operating temperature, chemical reactivity, and cost. The actions taken to develop the water cooling system will be discussed and illustrated. A focus will be made on sealing cooling water leaks from the inside out on small diameter metal passages (including extrusions, tubing, and fittings). These passages were located inside a vacuum environment, and only the ends of each passage were accessible to do the work. The vacuum vessel of PCX-U is a 1 meter diameter, 1 meter tall cylinder comprised of 0.25'' thick stainless steel. PCX-U has one removable end. Rings of SmCo magnets attached to a removable frame create a cusp field to contain the plasma and provide a resonance surface for the RF. [Preview Abstract] |
|
GP10.00164: Volumetric-driven flows on the Plasma Couette Experiment Ken Flanagan, M.M. Clark, J. Lynn, R. Siller, M. Tabbutt, J. Wallace, Y. Xu, C.B. Forest Experiments for driving Keplerian-like flow profiles with the goal of exciting the magnetorotational instability (MRI) on the Plasma Couette Experiment Upgrade (PCX-U) are described. Instead of driving flow at the boundaries as is typical in many liquid metal Couette experiments, a global drive is implemented. A large (20+ A) radial current is drawn across a small (1-3 G) axial field generating torque across the whole profile. This volumetric-driven flow (VDF) is capable of producing profiles similar to Keplerian flow with Alfvén Mach numbers of order unity--ideal for MRI studies. Experimental measurements will be compared to numerical calculations that show that at sufficiently high magnetic and fluid Reynolds numbers, VDF can drive the MRI. [Preview Abstract] |
|
GP10.00165: Improved Cooling Methods for Magnetized Electron Plasmas Eric Hunter, Nathan Evetts, Joel Fajans Cavity and lumped-element resonators have been designed for coupling electron plasmas to a 4 K thermal bath via their cyclotron modes and Trivelpiece-Gould modes. Plasmas cooled this way can reach lower temperatures and can be manipulated at lower magnetic fields than those cooled via free-space cyclotron radiation. We are exploring gradient enhanced cyclotron-cavity resonance, resistive cooling, and phase-space tailoring schemes with the goal of optimizing cooling of $N > 10^5$ electrons from $\sim 1 \, \mathrm{eV}$ to $\sim 10 \, \mathrm{K}$ temperatures in a few seconds. [Preview Abstract] |
|
GP10.00166: Electron Plasmas Cooled by Cyclotron-Cavity Resonance Lenny Evans, N. DeTal, Nathan Evetts, Joel Fajans, Walter Hardy, Eric Hunter, Isaac Martens, Francis Robicheaux, Sabrina Shanman, Chukman So, X. Wang, Jonathan Wurtele We observe that high-Q electromagnetic cavity resonances increase the cyclotron cooling rate of pure electron plasmas held in a Penning-Malmberg trap when the electron cyclotron frequency, controlled by a tunable magnetic field, matches the frequency of a standing wave mode in the cavity. The cooling rate and equilibrium plasma temperatures depend on the spatial distribution of electrons in the cavity and the magnetic field. These dependencies have been modeled analytically, and good agreement is found between theoretical and experimental spatial-magnetic profiles. [Preview Abstract] |
|
GP10.00167: Integral parallel closures for various ions Jeong-Young Ji, Hankyu Lee, Eric Held Ion parallel closures for heat flow and viscosity are expressed as kernel-weighted integrals of temperature and flow-velocity gradients. Due to the ion-electron collisions, the closures depend on the ion charge number and ion-electron temperature and mass ratios. Simple, fitted kernel functions are obtained for arbitrary collisionality from the 1600 moment solution and the asymptotic behavior in the collisionless limit. The fitted parameters are presented for hydrogen and helium isotopes with various temperature ratios. A generalization to arbitrary ions is discussed. [Preview Abstract] |
|
GP10.00168: Phase Space Velocy Correlation and Degrees of Freedom Sean Mattingly, Jorge Berumen, Feng Chu, Ryan Hood, Fred Skiff We measure the phase space distribution function's velocity correlation function $C(v, v', \tau) = \langle f(x, v,t)f(x'=x, v', t - \tau \rangle_t$ in a cylindrical axially magnetized laboratory plasma ($n\sim10^9, T_e\sim5eV, T_i\sim0.08eV$) generated with an inductively coupled RF source. We use Laser Induced Fluorescence (LIF) with two lasers that each have their own atomic transition scheme and collection optics to simultaneously measure distinct ion subpopulations at differing velocities $v$ and $v'$. A separately mounted antenna facilitates the velocity correlation measurement through either single mode excitation with a sinusoidal signal or broadband excitation with white noise. LIF photon acquisition is synchronized with digitizer sampling of the signal driving the fluctuation excitation antenna. With this we explore phase space degrees of freedom in $v$ and $v'$ with either monochromatic or broadband excitation. Finally, driving a sinusoidal wave near the ion cyclotron frequency causes linear wave - particle resonance $\omega - n \Omega_{ci} = k_{||}(\omega)v_{||}$ that results in a tunable ion resonance velocity located within the Doppler broadened IVDF - making it measureable by LIF. [Preview Abstract] |
|
GP10.00169: Thermal and electrical conductivity and temperature relaxation for dense plasma using a multi-species BGK model Cory Hauck, Jeffrey Haack, Michael Murillo We derive thermal and electrical conductivity transport coefficients using a new multi-species multi-temperature BGK model. This model conserves mass, momentum, and kinetic energy and allows for a more clear connection to the underlying cross sections and inter-species collision rates. We use the molecular dynamics validated dense plasma effective potential model of Stanton and Murillo and numerically generated QM cross sections as inputs for these collision rates and compare with the results of Lee and More as well as of election-hydrogen temperature relaxation and electron-ion relaxation in an ionized $SF_6$ mixture. [Preview Abstract] |
|
GP10.00170: Intense laser-driven proton beam energy deposition in compressed and uncompressed Cu foam Christopher McGuffey, C M Krauland, J Kim, F N Beg, M S Wei, H Habara, S Noma, T Ohtsuki, A Tsujii, K Yahata, Y Yoshida, Y Uematsu, S Nakaguchi, A Morace, A Yogo, H Nagatomo, K Tanaka, Y Arikawa, S Fujioka, H Shiraga We investigated transport of intense proton beams from a petawatt laser in uncompressed or compressed Cu foam. The LFEX laser (1 kJ on target, 1.5 ps, 1053 nm, I \textgreater 2\texttimes 10$^{\mathrm{19}}$ W/cm$^{\mathrm{2}})$ irradiated a curved C foil to generate the protons. The foil was in an open cone 500 $\mu $m from the tip where the focused proton beam source was delivered to either of two Cu foam sample types: an uncompressed cylinder (1 mm L, 250 \textmu m $\phi )$, and a plastic-coated sphere (250 \textmu m $\phi )$ that was first driven by GXII (9 beams, 330 J/beam, 1.3 ns, 527 nm) to achieve similar $\rho \phi $ to the cylinder sample's $\rho $L as predicted by 2D radiation hydrodynamic simulations. Using magnetic spectrometers and a Thomson parabola, the proton spectra were measured with and without the Cu samples. When included, they were observed using Cu K-shell x-ray imaging and spectroscopy. This paper will present comparison of the experimentally measured Cu emission shape and proton spectrum changes due to deposition in the Cu with particle-in-cell simulations incorporating new stopping models. [Preview Abstract] |
|
GP10.00171: Heat Transfer to Anode of Arc as Function of Transverse Magnetic Field and Lateral Gas Flow Velocity Yoshiyuki Zama, Toru Shiino, Yoko Ishii, Yoshifumi Maeda, Shinji Yamamoto, Toru Iwao Gas tungsten arc welding has useful joining technology because of high-energy and high-current characteristics. It can be flexible from the transverse magnetic field and lateral gas flow velocity. In this case, the weld defect occurs. In this research, the heat transfer to the anode of the arc as a function of the transverse magnetic field and lateral gas flow velocity is elucidated. That magnetic flux density and lateral gas velocity were varied from 0 to 3 mT and 0 to 50?m?s$-$1, respectively. The axial plasma gas argon flow rates were 3?slm. A transverse magnetic field is applied to the arc using Helmholtz coil. The anode is used by a water-cooled copper plate, and the heat transfer is measured by temperature of cooled water. As a result, the arc is deflected by the Lorentz force and lateral gas convection. Thus, the heat transfer to the anode of the arc decreases with increasing the transverse magnetic field and lateral gas flow velocity. In addition, the heat transfer to the anode changes with different attachments modes. The lateral gas flow causes a convective heat loss from the arc to the chamber walls. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700