Bulletin of the American Physical Society
60th Annual Meeting of the APS Division of Plasma Physics
Volume 63, Number 11
Monday–Friday, November 5–9, 2018; Portland, Oregon
Session BP11: Poster Session I: HEDP; General Stellarator; Wendelstein 7-X; Heating, Current Drive, and Energetic Ions (9:30am-12:30pm) |
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Room: OCC Exhibit Hall A1&A |
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BP11.00001: Atomic and Electronic Structure of Warm Dense Silicon Rahul Saha, Jacob Topp-Mugglestone, Thomas G White, Gilbert W Collins, Sean P Regan, Gianluca Gregori, Thomas Boehly, James Ryan Rygg We present experimental data to determine the atomic and electronic structure of warm dense silicon using simultaneous spectrally and angularly resolved measurements of the x‑ray scattering. A variety of uniform warm dense states spanning the solid–liquid boundary were generated through laser shock compression of silicon samples. A unified analysis of the x-ray scattering, combining spectral (x-ray Thomson scattering) and angular (x-ray diffraction) scattering data, reduced the necessary model assumptions used to determine the ion and electron structure factors, thereby reducing systematic uncertainties. |
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BP11.00002: Interpreting EXAFS Spectra: Toward Ramp-Compression Studies of Iron Oxide David Alexander Chin, Philip Michael Nilson, James R Rygg, Thomas Boehly, Gilbert W Collins, Federica Coppari, Yuan Ping Extended x-ray absorption fine structure (EXAFS) spectroscopy will be used to characterize iron oxide (FeO) ramp compressed to Mbar pressures. EXAFS spectroscopy provides information about the local structure, density, and temperature in compressed materials. The interpretation of the EXAFS spectral modulations was studied using ambient temperature FeO absorption data obtained with the Advanced Photon Source (APS) synchrotron[1] and theoretical predictions. The FeO data were evaluated by degrading the signal-counting statistics and background levels to understand how future spectrometer development will improve the data obtained in planned experiments at the Omega Laser Facility.[2] This analysis and spectrometer development will be applied to experiments using FeO compressed to conditions analogous to the Earth’s lower mantle and core in order to increase our understanding of the formation and evolution of the Earth and iron-rich exoplanets. [1] XAFS Spectral Library, Spectrum: FeO data, 13 June 2018, http://cars.uchicago.edu/xaslib/spectrum/611. [2] T. R. Boehly et al., Opt. Commun. 133, 495 (1997). |
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BP11.00003: Effective Potential Theory Applied to Electron-Ion Temperature Equilibration in Dense Plasmas Shane Rightley, Charles Starrett, Scott D Baalrud We present calculations of electron-ion temperature equilibration rates in dense plasmas with degenerate electrons. This and other transport processes have consequences for the modeling of inertial confinement fusion. Modeling transport in dense plasmas is complicated by the combined effects of electron degeneracy and strong coupling of ions. Recently, an approach known as Effective Potential Theory (EPT) has been developed to extend Boltzmann transport theory to stronger coupling. EPT utilizes an effective potential that incorporates many-body physics into a binary collision integral and has had success in predicting ion-ion transport for Γ up to ~20. Extending EPT to address electron-ion transport will greatly increase its range of applications, but in many cases this must account for quantum electrons. We explore the application of the EPT concept to the quantum Boltzmann equation in order to model electron-ion temperature relaxation. Our model incorporates quantum statistics and dynamics into the scattering, in addition to utilizing a realistic quantum effective potential. This is relevant to recent experiments and prepares for further investigations of inter-species transport in strongly coupled and degenerate plasmas. |
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BP11.00004: Improvements to the VISAR and Streaked Optical Pyrometer at the Omega Laser Facility Mary Kate Ginnane, Andrew Sorce, Joshua Kendrick, Robert Boni, Benjamin Saltzman, David Weiner, Mohamed Zaghoo, Danae N Polsin, Brian Henderson, Jun Zou, Michael Couch, Michelle C Gregor, Craig Rogoff, Thomas Boehly, James R Rygg, Gilbert W Collins VISAR and optical pyrometry measurements are mainstay of high-energy-density physics experiments. These systems on OMEGA share an optical telescope to relay probe beam and optical signals to their respective detectors. The telescope, originally optimized for 532 nm, was redesigned to provide better performance for 600 to 900 nm and include a baffle system that reduces signal from outside the field of view. This poster provide an overview of these refinements and an update on recent calibrations. |
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BP11.00005: Development and utilization of the DT3He multi-particle backlighter for stopping-power experiments and for radiography of strong fields at the NIF Graeme Sutcliffe, Neel Kabadi, Cody E Parker, Mason Black, Brandon J Lahmann, Raspberry Simpson, Patrick J Adrian, Johan Frenje, Maria Gatu Johnson, Hong Sio, Chikang Li, Fredrick Seguin, Richard David Petrasso, Michael J Rosenberg, Arijit Bose, Hye-Sook Park, Daniel T Casey, Bradley B Pollock, Scott Wilks, John D Moody, Otto L Landen, Frank R Graziani, Daniel B Sinars, Ramon J Leeper, Alex Zylstra The D3He backlighter platform, based on implosions of D3He-filled capsules that generate mono-energetic 14.7-MeV and 3.0-MeV protons, has been used extensively with success at OMEGA and the NIF both for radiography of plasmas and their electromagnetic fields and for stopping-power studies. To significantly advance this work, a new tri-particle mono-energetic backlighter based on a DT3He gas-filled capsule implosion that provides 14.7-MeV and 3.0-MeV protons plus 9.5-MeV deuterons from the T+3He reaction is being developed at OMEGA. Initial tests using 860-µm-diameter, thin-glass capsules filled with 20:40:40 DT3He (atomic ratios) fuel produced 1e10,1e9, and 8e8 D3He-p, DD-p, and T3He-d yields, respectively. Other performance characteristics of the backlighter, including source size, burn duration and line widths are discussed. Radiographs of laser-driven foils and measurements of stopping power in cold beryllium were made with the backlighted particles and the results are shown. This work was supported by DOE, NLUF, LLNL, and LLE. |
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BP11.00006: Magnetically driven flyer launch optimisation Savva P Theocharous, Simon N Bland, David Yanuka We present multi-point velocity measurements for 1cm scale flyer plates launched using the 1MA MACH facility at Imperial College London. Magnetically driven flyer plate impact is of interest for material equation of state research, and more recently for a novel fusion ignition scheme. Generation of the desired planar shock is dependent on maintaining a flat flyer face, with the front surface of the flyer in a solid state at impact. However, flatness can be compromised by material effects and non-uniform magnetic field, while melting can be caused by formation of shocks within the flyer or joule heating related to the magnetic field. We show results of planar 1cm2 Al flyers diagnosed using 6-point Photon Doppler Velocimetry (PDV) to probe velocity across a width of 3mm on the flyer to return front surface uniformity and qualitative reflectivity data. Maximum velocity of 3000ms-1 was achieved with a flyer of thickness 0.3mm, at maximum current of 991kA. Velocity variation along the direction of current through the flyer was found to be ~6% over 3mm. 2D simulations, performed in COMSOL, of both equivalent and novel non-planar geometries, are also presented. |
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BP11.00007: Development of predictive modelling for ICF plasmas produced by high-velocity projectiles Dave A Chapman, James D Pecover, Nikita Chaturvedi, Adam R Fraser, Nicolas P.-L Niasse First Light Fusion is undertaking research into novel approaches to ICF using intense shock waves driven by high-velocity projectile impactors, requiring a multi-physics approach including microphysics and equation of state (EoS) models for our main simulation tool Hytrac. The detailed processes governing the microscopic evolution of such states, i.e. heating due to conduction, equilibration and radiation, all require the simple models featured in most hydrocodes to be substantially modified. Here, we focus on thermal conductivity and temperature equilibration. Hytrac uses EoS tables produced by FEOS, a code based on the well-known QEOS approach. We present improvements to FEOS resulting in robust transitions between model components. Ionisation models beyond the Thomas-Fermi model are also investigated, focusing particularly on detailed atomic shell structure. A broad sensitivity study has been undertaken, informing prioritisation for physics model development. Comparisons between Hytrac and Helios in idealised geometries show that radiation transport is the greatest source of sensitivity, with EoS also being significant. We find that microphysics is generally unimportant for these simulations, even when processes such as conduction are important to the fuel energetics. |
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BP11.00008: Laser-Plasma Particle-in-Cell Simulation Reproducibility and QED Effects Scott V. Luedtke, Lin Yin, Lance Labun, Ou Z. Labun, Brian James Albright, David Stark, Robert F. Bird, William D Nystrom, B. Manuel Hegelich The goals of discovering quantum radiation dynamics in high-intensity laser-plasma interactions and engineering new laser-driven high-energy particle sources both require accurate and robust predictions. We demonstrate a characteristic dipole pattern of high-energy photon emission that results when the laser pulse bores through the target, forming a channel that enhances the laser field. We establish that the phenomenon is robust by showing that the dipole pattern gradually switches on as a function of target density, and with no need for wavelength-scale structure in the target, as used in previous work. This phenomenon is robust to experimentally motivated perturbations including a preplasma and non-normal laser-target angle. We analyze statistical uncertainties that result from running the same simulation multiple times with different random seeds and compare results from two particle-in-cell codes. |
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BP11.00009: X-ray diagnostic development for electron beam driven WDM studies Nicholas Ramey, Joshua Coleman, James P Colgan, John Perry, Heidi E Morris, Ronald Matthew Gilgenbach, Ryan D McBride A platform for characterizing the equation-of-state of the warm dense matter (WDM) regime is being developed on an intense, relativistic electron accelerator1,2. An 80-ns (FWHM) electron pulse with a beam current of 1.7 kA and energy of 19.8 MeV deposits energy into a thin, low-Z metal foil heating it to a warm dense plasma. LASNEX hydrodynamic simulations suggest the relatively large scale (~100 μm) WDM lasts 100 ns after the beam pulse providing a background-free measurement window for deploying X-ray diagnostics. Time-resolved X-ray shadowgraphy will be used to measure the spatial extent of the WDM. X-ray absorption spectroscopy can measure the temperature and density of the WDM by tailoring backlighter/crystal pairs to the plasma under interrogation. Calculations and simulations will be presented to evaluate the feasibility of different X-ray sources and detection techniques for each diagnostic. Full scale Geant4 simulations to estimate photon yields and guide experimental deployment are underway. |
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BP11.00010: Exploring the universe through Discovery Science on NIF Bruce Remington An overview of recent research done on the 2 MJ, 192 beam NIF laser facility at LLNL through the NIF Discovery Science program will be presented. A selection of examples will be drawn from experiments on nuclear reactions in capsule implosions relevant to stellar nucleosynthesis [1]; studies of equations of state and phase studies of H, C, CH, and Fe at high pressures (1-60 Mbar) and densities relevant to planetary interiors [2, 3, 4]; heat conduction stabilized Rayleigh-Taylor instability growth relevant to supernova remnants [5]; and high velocity, low density interpenetrating plasma flows that can lead to collisionless astrophysical shocks relevant to galactic collisions and particle acceleration relevant to cosmic ray generation mechanisms. [6] |
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BP11.00011: Experimental investigation of solid-vapour phase transitions in metals during shock release Matthew R Betney, Tim J Ringrose, Lucy Donora, Nikita Chaturvedi, Jonathan Skidmore This work presents the results of an experimental investigation into the solid-gas phase transitions that occur when strong shocks in metals release to a low pressure state, on reflection from an interface with a low pressure gas. Such processes occur frequently in ICF targets, and this work was motivated primarily by a need to validate our equations of state in and around the phase change boundary. In this study, shock waves are created in targets by the impact of a Cu tipped projectile from a two-stage light gas gun. Conditions in the released material are then varied from shot to shot by the modification of both the projectile velocity (between 5 and 7 km/s), and the pressure of the gas surrounding the material. Point projection X-ray imaging is performed approximately 100 ns after the incident shock has reflected from the rear face of the target; the subsequent image can then be Abel inverted to form a map of the density of the released material. X-rays are generated for the backlighting source using a 100 kA X-pinch. This provides a broadband X-ray source, with energies up to 30 keV, a 200-300 um source size and a 20 ns pulse length. This enabled us to achieve a spatial resolution of 50 to 100 um, in a 30 mm field of view. |
(Author Not Attending)
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BP11.00012: Shock Dynamics and Magneto-Rayleigh-Taylor Instability in Gas-Puff Z-Pinch Experiments Eric Sander Lavine Gas-puff z-pinch experiments on Cornell University’s 1 MA COBRA generator are conducted using a custom triple-nozzle gas-puff valve. Laser shearing interferometry and time-gated XUV cameras are used to observe shock formation and evolution early during the implosion while Thomson scattering, x-ray pinhole cameras, and x-ray spectroscopy provide additional measurements at stagnation. Radial implosion velocities suggest the shocks are driven by a current layer at the outer radius of the imploding plasma which acts as piston moving inward at several hundred km/s (Potter 1978). The outer surface of this current layer is Magneto-Rayleigh-Taylor (MRT) unstable. Previous observations have demonstrated a working fluid dependence on shock structure, MRT growth rate, and x-ray yield (de Grouchy et al. 2018). Here, we build upon these observations by investigating shock evolution for various working gasses, initial density profiles, and applied axial magnetic field strengths. The effect of these parameters on the MRT growth rate and x-ray yield is discussed. |
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BP11.00013: Ion Beam Driven Isochoric Heating on Texas Petawatt and Trident Laser Facilities Rebecca Roycroft, Frances Aymond, Brant Bowers, Herbie Smith, Edward McCary, Hernan J Quevedo, Gilliss Dyer, Brian James Albright, Juan Carlos Fernandez, Paul A. Bradley, Erik L. Vold, Lin Yin, B. Manuel Hegelich The results of several experimental campaigns at the Texas Petawatt and Trident laser facilities on isochoric heating of solids and foams to warm dense matter conditions are presented. At the Texas Petawatt Laser Facility, we used the f/40 beamline of the petawatt laser to illuminate 5mm Au foils to drive large numbers of 1-20MeV protons via TNSA. The proton beam then heats a secondary target (Al foil or carbon foam). The time-dependent brightness temperature of the secondary target is measured by a streaked optical pyrometer, which images the rear surface of the secondary target. We have observed peak brightness temperatures from 1-20eV, with a heating time of ~20ps. We have simulated the heating of these targets in the radiation-hydrodynamics codes HYADES and RAGE. In experiments at the Trident laser facility, the secondary target contains a high Z and a low Z material arranged to make a sharp interface. It is heated isochorically to ~2eV by laser accelerated Al ions. This target is similarly diagnosed with time-resolved pyrometry while the evolution of the high-Z/low-Z interface is measured with an X-ray framing camera. We have simulated the heating and expansion of these materials in RAGE coupled to the SESAME equation of state tables. |
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BP11.00014: Molecular Dynamics Simulations of Diffusion in the Strong Magnetized One-Component Plasma. Keith R. Vidal, Scott D Baalrud, Jerome Daligault The degree to which plasma is magnetized affects its transport properties. Future magnetized ultracold plasma experiments may enable experimental test of the degree to which magnetization affects transport across a wide range of coupling strengths. Recent simulations appear to observe Bohm scaling (1/B) of the diffusion coefficient perpendicular to the magnetic field when the plasma is strongly magnetized [1]. Here we define strongly magnetized plasmas to be when the gyroradius is smaller than the Debye length but larger than the distance of closest approach. However, these simulations were done near the strongly coupled regime. Here we extend the previous work to weaker coupling. We present perpendicular and parallel diffusion results measured from molecular dynamic (MD) simulations of the one-component plasma in the strongly magnetized, weakly coupled regime (Γ≤0.25). These results are applicable to the electrons in an ultracold plasma experiment with a strong magnetic field. They indicate that the traditional Braginskii transport theory does not accurately describe transport of strongly magnetized plasmas, and a new transport theory is needed to address these conditions. [1] Phys. Rev. E 96, 043202 (2017). |
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BP11.00015: UPIC-EMMA 2.0: A parallel spectral Particle-In-Cell code designed for heterogeneous architectures and PIC algorithms benchmarking Michael Touati, Frank Shih-Yu Tsung, Han Wen, Xinlu Xu, Viktor K Decyk UPIC-EMMA 2.0 is an open source (available on GitHub) fully parallelized spectral Particle-In-Cell (PIC) code. Inherited from the time-tested UPIC framework, it utilizes the message-passing interface (MPI) for parallel processing and a multi-tasking library was implemented to enable mixed multi-tasking and MPI. The full set of Maxwell equations is computed using the Fast Fourier Transform (FFT) method and a Perfectly Matched Layers (PML) technique has been developed to damp the fields at the simulation box boundaries. Numerical schemes usually used in non-spectral PIC codes can be emulated such as the Yee scheme or other higher order finite-difference time domain (FDTD) schemes. This capability makes UPIC-EMMA a unique testbed for testing new algorithms. Initially developed for studying the interaction between a highly intense laser pulse and underdense plasmas (in boosted frame, moving antenna and numerical Cherenkov instability mitigation), it has been extended to simulate the interaction between a highly intense laser pulse and an overdense plasma (relativistic binary collisions, particle antenna) and other HEDP applications. We will present the latest physics results at the poster. |
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BP11.00016: The PSC Code: Particle-in-cell Simulations on GPU-based Supercomputers Kai Germaschewski, William Randolph Fox, Jackson VH Matteucci We present the latest version of the Plasma Simulation Code (PSC). PSC has been adapted to use generic programming techniques provided by the C++11 standard. PSC provides flexibility in selecting algorithms for particle-in-cell simulations, e.g., switching between 1st and 2nd order particle shape functions, and options for charge-conservative current deposition. At the same time, PSC supports modern hardware architectures, e.g., Nvidia GPUs and SIMD-based CPUs. Our applications, in particular in the area of HEDLP, also require custom functionality like localized heating and particle injection operations. Generic programming helps in that algorithms can be defined and implemented once, in generic form, and then can be compiled into, e.g., CPU and GPU code as required. |
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BP11.00017: Hydrodynamic motion of electron beam heated warm dense foils Payson Dieffenbach, David Montgomery, Heidi Morris, Nicholas Ramey, Joshua Coleman Foils of Al, Ti, Ni, and Cu are heated isochorically by an electron bunch with ~100 ns pulse length, energy of 19.8 MeV and current of 1.7 kA. The electron bunch is focused to a spot size of ~2 mm onto each foil. The corresponding hydrodynamic motion of the foils is measured instantaneously once electron energy deposition begins with Photonic Doppler Velocimetry (PDV). PDV provides a measurement of the foil velocity, inferred pressure within the foil, and hydrodynamic disassembly time. Electron energy deposition into these foils was simulated utilizing a new capability in the HELIOS-CR code. Simulated results of velocity and plasma pressure profiles of the various foils are compared with LASNEX simulations and experimental measurements. |
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BP11.00018: Thermoelectric Transport with an 8-Moment Plasma Model in PERSEUS Jason Hamilton, Charles E Seyler The PERSEUS code originally solved a 5-moment 2-fluid Extended MHD model including electron inertia and Hall terms in the Generalized Ohm's Law. However, this model lacks non-equilibrium transport such as heat flow and thermoelectric effects. In the high temperature and density regimes found in High Energy Density plasmas, these higher-moment transport phenomena may have substantial influences on the evolution of the system.
An 8-moment model has been developed that inherently includes non-equilibrium transport and retains conservation form of the equations. The closure is handled by ansatz that the distribution function is completely described by only 8 degrees of freedom, with a functional form that gives consistent definitions for the moment variables. This results in the non-factorable parts of the third-rank (heat flow) and fourth-rank tensor moments being identically zero. Transport effects such as thermal conductivity and Nernst are shown to be present in this formulation, while maintaining the time derivative terms that give physical propagation speeds. Results from 1D validation tests will be presented. |
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BP11.00019: Crossover from Classical to Fermi Liquid Behavior in Dense Plasmas Jerome Daligault We explore the crossover from classical plasma to quantum Fermi liq- uid behavior of electrons in dense plasmas. To this end, we analyze the evolution with density and temperature of the momentum lifetime of a test electron introduced in a dense electron gas. This allows us 1) to determine the boundaries of the crossover region in the temperature-density plane and to shed light on the evolution of scattering properties across it, 2) to quantify the role of the fermionic nature of electrons on electronic collisions across the crossover region, and 3) to explain how the concept of Coulomb logarithm emerges at high enough temperature but disappears at low enough temperature. |
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BP11.00020: Recent OSIRIS development for improved high-energy- density plasma simulations Roman Lee, Kyle Glen Miller, Ricardo Fonseca, Benjamin J Winjum, Anton Helm, Frank Shih-Yu Tsung, Warren B Mori OSIRIS is a parallelized, fully relativistic, explicit particle-in-cell code. Some studies of nonlinear optics of plasmas are very sensitive to noise and numerical instabilities. OSIRIS uses charge conserving current deposit to maintain Gauss’s law but recent implementation of a Boris correction (including a multigrid Poisson solver) combined with a direct current deposit provides a low-noise alternative. In addition, the Boris correction may allow for the use of higher-order stencils in the field solver to limit the numerical Cerenkov instability. Preliminary results comparing these options for SRS and the Weibel instability will be presented. Finally, performance on both a future single many core node and on large parallel simulations can be improved by partitioning data into small units known as tiles. This provides the ability to stream particle operations and provides flexibility needed to implement load-balancing schemes which may improve the parallel scalability of problems such as laser wakefield and laser-solid interactions. Details for this implementation into the non-CUDA version of OSIRIS 4.0 as well as preliminary timings of OSIRIS on the Intel Xeon Phi with and without tiling will be presented. |
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BP11.00021: Observation of ultra-high energy density state in nanowire allay target with x-ray free electron laser SACLA Keisuke Shigemori, Kohei Miyanishi, Yuji Fukuyama, Neo Fujiwara, Yuto Maeda, Youichi Sakawa, Yoichiro Hironaka, Yasuhiko Sentoku, Takeshi Matusoka, Toshihiro Somekawa, Norimasa Ozaki, Ryosuke Kodama, Toshinori Yabuuchi, Akira Kon, Yuichi Inubuchi We have been developing to measure the heating process of nano-wire foils irradiated with ultra-high intensity laser. Experiments were performed at SACLA x-ray free electron laser facility. Ni Nano-wire alley foils (diameter ~ 0.2 μm, length ~ 10-20 μm, with Ni 20-μm-thickness substrate) were irradiated with Ti:Sappire laser at an intensity of around 1019 W/cm2. The focal spot size and the pulse duration were around 20 μm and 25 fs, respectively. We measured shadowgraph images from side view with point source x-ray backlighting by SACLA at a photon energy around the K absorption edge of the Ni (~8.3 keV). The duration of the x-ray pulse was ~ 30 ps. We observed a temporal evolution of the heating region in the nano-wire region by changing the delay timing of the laser and the x-ray pulses. |
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BP11.00022: Using Variable Size Particles in Phase Space: Towards Machine-Assisted Learning of Optimal, Dynamical, Phase-Space Tiling Sean Young, Bedros Afeyan, Archis Joglekar, David Jeffrey Larson We Use the Code SFK (Shape Function Kinetics) as our baseline and test its efficacy and accuracy on nonlinear driven plasma wave simulations. Our aim is to test a set of ideas on variable size particle tiling which match form to function. Mutually incoherent families of particle shapes and sizes are constructed which try and adapt to multiscale behavior by homing in on different features adaptively. We compare various choices of phase space tiling and various selection criteria wherein optimality is based on sparsity promotion. Test particle orbit reconstructions of phase space density are used to evaluate the relative merits of different heterogeneous tiling choices. The real goal is to learn proper tiling which can then be used in nearby problems with little change, facilitating families of simulations to be far more robust and efficient than following the old paradigm of von Neumann computing starting from scratch each and every time. Examples from electron plasma waves, Kinetic Electrostatic Electron Nonlinear (KEEN) waves and their pair plasma analogs KEEPN (Positron) waves will be given. |
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BP11.00023: Stellarator Research Opportunities David Gates, and the National Stellarator Coordinating Committee This poster is the product of a stellarator community workshop, organized by the National Stellarator Coordinating Committee and referred to as Stellcon, that was held in Cambridge, Massachusetts in February 2016. The workshop was attended by 40 scientists from 12 different institutions including national labs, universities and private industry, as well as a representative from the Department of Energy. This poster describes areas of community-wide consensus that were developed as a result of the discussions held at that workshop. Areas where further study would be helpful to generate a consensus path forward for the US stellarator program are also discussed. The program outlined in this document is directly responsive to many of the strategic priorities of FES as articulated in “Fusion Energy Sciences: A Ten-Year Perspective (2015-2025)”. The natural disruption immunity of the stellarator directly addresses “Elimination of transient events that can be deleterious to toroidal fusion plasma confinement devices” and “Strengthening our partnerships with international research facilities,” is being significantly advanced on the W7-X stellarator in Germany. |
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BP11.00024: Plans for Neutral Beam Injection on HSX David Anderson, F.S.B. Anderson, K.M. Likin, S. Murakami, Evan M Schilling, A.L.F. Thornton A neutral beam injector on loan from the MST group is being refurbished for use on HSX (1.2 ms, 20 keV, 40A). Initially the beam will be used to produce a population of fast ions so that the confinement can be studied as a function of the magnetic field geometry (quasisymmetric or with introduced changes to the spectrum). Confinement will be measured through neutron production rates from deuterium beams injected into a deuterium plasma. This has been successfully done under similar parameters in the CHS device [1]. The HFREYA and GNET [2] codes have been used to look at fast-ion birth profiles, confinement and thermalization. Under attainable conditions a geometrical restrictions 15-25 % of the beam energy can be coupled to the plasma in the QHS mode, including charge exchange losses. To improve efficiency of NBI, an upgrade to HSX is under consideration to increase plasma density, reduce neutral population, and increase ion temperature. The magnetic field will be increased to 1.25 T to be able to utilize a 1 second 500 kW 70 GHz gyrotron for plasma heating. [1] M. Isobe et al. Rev. Sci. Instrum. 68 (1) 1997 [2] H. Yamaguchi and S. Murakami, Plasma and Fusion Research, 9 (2014) 3403127 |
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BP11.00025: Evolution of plasma flow and radial electric field in HSX Joseph Nathan Talmadge, Santhosh T. A. Kumar, Jeremy Lore The fluid equation approach is used to model the evolution of the plasma flow and ambipolar radial electric field in HSX subject to a steady-state radial current, such as that induced by a biased electrode. Previous work [1] on this problem assumed that the drag due to the parallel viscosity increases linearly with the velocity. Here we extend the approach to include regimes where the viscosity is nonlinear using the approach of Sugama and Nishimura [2]. Included in the model is momentum damping due to neutrals, which is significant for the low density, ECRH heated plasmas of HSX. We also examine the role of electron viscosity which was previously neglected in the momentum balance equation when summed over species. Results are presented for the evolution and steady-state plasma flow and radial electric field and compared to the previous formulation which assumed that the viscosity was linear. [1] M. Coronado and J.N. Talmadge, Phys. Fluids B 5 1200 (1993). [2] H. Sugama and S. Nishimura, Phys. Plasmas 9 4637 (2002). |
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BP11.00026: The effect of neutrals on intrinsic flows and radial electric fields in the HSX stellarator Tom J Dobbins, Santhosh T. A. Kumar, Joseph Nathan Talmadge, David Thomas Anderson The ion parallel flow and radial electric field measured in the HSX stellarator using Charge Exchange Recombination Spectroscopy have been found to be inconsistent with those calculated by the neoclassical code, PENTA. In this work, we show that this discrepancy can in part be explained by including the effect of collisions with background neutrals in the neoclassical calculations. An analytical model, benchmarked with the PENTA code in the linear neoclassical viscosity regime, has been used for this purpose. It has been found that, in the quasi-helically symmetric configuration, the experimental values of ion parallel flow and the radial electric field are in better agreement with the neoclassical calculation if the neutral effects are included. The change in the experimentally measured parallel flow with two plasma density levels in the plasma agrees with the neoclassical calculations when neutral effects are included. In a symmetry degraded configuration, higher neoclassical viscosity makes the viscosity arising from neutrals insignificant. |
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BP11.00027: Dependence of Impurity Transport on Absorbed Power in the HSX Stellarator J.F. Castillo, A. Bader, S.T.A. Kumar, K.M. Likin, D.T. Anderson, F.S.B. Anderson, J.N. Talmadge Results from the HSX laser blow-off experiment show impurity confinement times from a 50 kW plasma to be 1.5 ms with on-axis heating (core electron temperature Te~1000 eV) and 2.1 ms with off-axis heating (core Te~500 eV). A laser blow-off impurity injection system is used to rapidly deposit a small, controlled quantity of aluminum into the confinement volume. Global confinement time (τ) has been calculated from the exponential decay of the line-integrated impurity radiation measured using AXUV photodiodes. Experiments are done in hydrogen plasmas of line-average density 3e+12 cm-3 with core electron temperatures in the range of 500-1500 eV. The heating location has been varied from on-axis to off-axis to change the absorbed ECH power (P). Analysis of the confinement time dependence on the absorbed power, τ~Px, suggests that x is on the order of -1. The spatially one-dimensional impurity transport code STRAHL is used to obtain experimental impurity transport coefficients, diffusivity and convective velocity. |
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BP11.00028: Neoclassical impurity transport coefficients in HSX configurations Santhosh Kumar, Joseph Nathan Talmadge, Juan F Castillo, David Thomas Anderson, Matt Landreman Neoclassical impurity transport coefficients are calculated for quasi-helically symmetric and symmetry degraded configurations of HSX stellarator using SFINCS and PENTA codes. Trace amounts of carbon+6 and aluminum+12 impurities in a background hydrogen plasma are used in the calculations. Transport coefficients are found to be similar for both configurations for experimentally relevant plasma parameters. Calculations for aluminum impurity are compared with experimental transport coefficients obtained using a laser blow-off system. Initial experimental results indicate a much higher diffusivity and convection velocity than neoclassical values. |
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BP11.00029: Perturbative Particle Transport Study in the HSX Stellarator Chuanbao Deng, D.L. Brower, D.T. Anderson, F.S.B. Anderson, A. Bader, K.M. Likin, J.N. Talmadge, K. Tanaka Particle transport studies using modulated gas puffing have been conducted in the HSX stellarator. Transport coefficients D and V calculated from the amplitude and phase of the density modulations will be reported. The equilibrium plasma density and its modulations are measured by a multichannel interferometer system. For data analysis, the particle source rates are modelled using both the DEGAS and EMC3-EIRENE codes. Preliminary analysis of experimental data shows that the density modulation data are fit very well with the model when using EMC3-EIRENE calculated particle source rates. The results of particle transport experiments for different plasma density, and magnetic configurations (with and without quasi-helical symmetry) are compared. |
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BP11.00030: Electron thermal diffusivity differences due to magnetic geometry on the HSX stellarator E.M. Schilling, J.N. Talmadge, J. Smoniewski, K.M. Likin, F.S.B. Anderson, D.T. Anderson Electron thermal diffusivity profiles χe have been experimentally determined on the Helically Symmetric eXperiment (HSX) over a scan of magnetic configurations defined by adding or subtracting toroidal field. HSX is a Quasi-Helically Symmetric (QHS) stellarator with a flexible magnetic geometry owing to a set of auxiliary coils surrounding its main field coils. The performed configuration scan suggests that additional toroidal field (decreased well depth and rotational transform) enhances electron heat flux relative to the standard QHS configuration. These diffusivity profiles are obtained by fitting electron temperature Te and density ne data from a 10-channel Thomson Scattering system and assuming a steady-state energy balance and diffusive transport model. Due to the sensitivity of χe to the fit of ne and gradient of Te, Gaussian Process Regression (GPR) has been used to better estimate the uncertainty in the fit. GPR is the first step in upgrading the Thomson system to a Bayesian probability theory based analysis technique, allowing for further confidence in determining sensitive transport coefficients. |
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BP11.00031: Collisionless zonal flow damping in quasisymmetric stellarators J Smoniewski, E Sanchez, I Calvo, M J Pueschel, J N Talmadge Quasisymmetric stellarators are similar to tokamaks in radial particle drift and flow damping, but exhibit zonal flow oscillations and the residual zonal flow RZF -> 0 as radial wavenumber kx -> 0 due to deviations from perfect quasisymmetry. In collisionless linear calculations, damped GAM oscillations settle to a long-time zonal flow residual. This often describes the strength of zonal flows in tokamaks, and thus their effect on turbulence saturation. In stellarators, long-time damping and zonal flow oscillations complicate this simple picture. We present calculations of zonal flow damping in realistic geometry from the National Compact Stellarator eXperiment and symmetric and broken-symmetry configurations of the Helically Symmetric eXperiment. Using GENE and EUTERPE, a comparison of flux tube, flux surface, and full volume geometry representations demonstrates that a local flux tube captures the zonal flow residual, but that full volume simulations are required for damping at small kx. The zonal flow residual does not correlate to reduced heat flux in nonlinear simulations, but the oscillation and short-time damping may provide a proxy. |
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BP11.00032: Comparison of Turbulent Transport in Quasi-Helically Symmetric and Quasi-Axisymmetric Stellarators I.J. McKinney, M.J. Pueschel, C.C. Hegna, J.N. Talmadge, D.T. Anderson, B.J. Faber Employing the gyrokinetic turbulence code GENE, a comparison of ion temperature gradient mode growth rates and saturated heat fluxes between two quasi-symmetric (QS) stellarator configurations is conducted. We focus on the quasi-axisymmetric stellarator NCSX and the quasi-helically symmetric stellarator HSX in both the adiabatic and kinetic electron limits. Here, our results show that despite higher growth rates in HSX, saturated heat fluxes are lower in HSX compared to NCSX. Study of nonlinear frequencies reveal more broadband frequency structures in HSX versus NCSX where frequencies more closely match linear results. HSX exhibits a large number of subdominant modes characterized by eigenmodes extending along a field line while NCSX exhibits only a few subdominant modes that are more localized in the bad curvature region. This highlights the necessity for including nonlinear physics in the modeling of non-axisymmetric magnetic configurations, as linear growth rates alone prove to be a poor proxy for predicting turbulent transport across QS stellarator designs. |
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BP11.00033: Physics Optimization for a New UW-Madison Mid-Scale Quasi-Helically Symmetric Stellarator Aaron Bader, Benjamin Faber, John C Schmitt, David Thomas Anderson, Cary B Forest, Raymond John Fonck, Heinke G Frerichs, Chris C Hegna, Thomas Kruger, Matt Landreman, Samuel A Lazerson, Ian J McKinney, M.J. Pueschel, John Stephen Sarff, Evan M Schilling, Oliver Schmitz, Joseph Nathan Talmadge, Paul Willis Terry, Andrew Simon Ware A new mid-scale quasi-helically symmetric (QHS) stellarator should be designed and built as soon as possible to realize the potential of QHS configurations for fusion energy. New advancements in stellarator design greatly improve optimization pathways. In this poster we describe novel metrics for stellarator optimization and present their effects on stellarator design. Design considerations include: turbulent optimization, specifically by maximizing coupling from unstable to stable modes; simplified coil design and plasma boundaries conducive to simpler coils; self-consistent bootstrap current calculations; divertor solutions capable of operating over a range of plasma current; and improvements to energetic particle confinement. Incorporating these elements into a design will lead to a US experimental facility capable of significantly advancing the stellarator concept. |
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BP11.00034: Using geometry to optimize turbulence in quasi-helically symmetric stellarators Benjamin J Faber, Chris C Hegna, Paul Willis Terry, Aaron Craig Bader, John C Schmitt Reducing turbulent energy and particle losses represents one of the major challenges facing magnetically confined fusion and is a major design goal of a future stellarator. Observations from fluid modeling and gyrokinetic simulations indicates saturation of microturbulence in the quasi-helically symmetric stellarator HSX is strongly influenced by three-wave, nonlinear energy transfer between stable and unstable eigenmodes mediated by a third, non-zonal mode[1,2]. The nonlinear coupling strength is sensitive to the local magnetic geometry on the flux surface and suggests that geometry may be manipulated to optimize energy transfer between unstable and stable modes, reducing microturbulence levels and fluxes. To help design a turbulence-optimized stellarator, an analytic, three-field fluid model for ITG turbulence saturation in general 3D geometry has been implemented within an optimization framework. The first turbulence optimization results will be presented and compared against nonlinear gyrokinetic simulations, focusing on identifying geometric mechanisms responsible for increased confinement. |
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BP11.00035: Optimized stellarators without optimization Matt Landreman, Wrick Sengupta, Gabriel Plunk The work of Garren and Boozer [Phys Fluids B, 3, 2805 (1991)] is usually cited as a proof that perfectly quasisymmetric magnetic fields do not exist, but in fact their work also contains a useful procedure to construct and parameterize magnetic fields that are approximately quasisymmetric. For quasisymmetry to first order in distance from the magnetic axis, the Garren-Boozer construction involves solution of a single nonlinear ordinary differential equation. We demonstrate the usefulness and accuracy of this approach by providing the results of this construction as input to the codes VMEC and BOOZ_XFORM, confirming the purity and scaling of the magnetic spectrum. The space of magnetic fields that are quasisymmetric to this order is parameterized by the magnetic axis shape along with three other real numbers, one of which reflects the toroidal current, and another one of which is zero for stellarator symmetry. Compared to the previous approach to finding quasisymmetric stellarator fields - iteration of MHD equilibria calculations inside a black-box optimization algorithm - the Garren-Boozer construction is many orders of magnitude faster, enabling wide high-resolution scans over parameter space. |
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BP11.00036: An adjoint method for neoclassical stellarator optimization Elizabeth Paul, Ian Abel, Matt Landreman, William D Dorland The design of modern stellarators often employs gradient-based optimization to navigate the high-dimensional space describing the magnetic field geometry. However, computing the gradient of a target function is typically quite expensive, necessitating the use of simplified physics models. The adjoint method provides a means to efficiently compute analytic gradients of a target function with respect to many input parameters. We implement the adjoint method in the SFINCS drift kinetic solver to compute gradients of moments of the distribution function, such as the bootstrap current and radial particle fluxes, with respect to input parameters, such as the Boozer spectrum. We perform adjoint-based optimization with the STELLOPT framework using a quasi-Newton method, targeting the neoclassical quantities computed with SFINCS. To demonstrate, we present a W7X-like configuration optimized for minimal bootstrap current. In addition, we use the gradients to compute the sensitivity of moments of the distribution function to local perturbations of the magnetic field strength on a surface. This local sensitivity information provides greater insight into the optimization and engineering tolerances than parameter derivatives. |
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BP11.00037: Alpha particle confinement in quasi-helically symmetric stellarators Travis Griffin, Andrew Simon Ware, William Sutherland, Aaron Craig Bader, Chris C Hegna Computational modeling is used to determine the confinement of high energy alpha particles in quasi-helically symmetric stellarator configurations. Work is being undertaken to optimize a possible new mid-scale quasi-helically symmetric stellarator experiment, targeting lower neoclassical transport, ballooning stability, and resilience to bootstrap current. Configurations with different rotational transform profiles have been obtained that are able to withstand the addition of bootstrap current at higher beta. That is, the unwinding of the rotational transform due to bootstrap current in quasi-helical configurations does not cause low order rational surfaces to enter the plasma. In this work, alpha particle confinement in these configurations is explored. The code used follows guiding-center orbits of high-energy alpha particles [V.V. Nemov, et al., Phys. Plasmas 21, 062501 (2014)]. This work will test the impact increasing plasma beta, bootstrap current, and different rotational transform profiles have on alpha confinement. Results will be presented for both the confinement of trapped alpha particles as a function of time and the regions of alpha particle losses. |
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BP11.00038: Error field analysis for modular coils of China First Quasi-axisymmetric Stellarator (CFQS) Caoxiang Zhu, David Gates, Stuart Hudson, Haifeng Liu, Yuhong Xu, Shoichi Okamura The presence of error fields has been shown to degrade plasma confinement and drive instabilities. Error fields are predominantly attributed to deviations in the coil geometry. To determine how coil misplacements drive error fields, previous investigations scanned through all the possible rigid displacements of coils to provide insights prior to construction. Here we employ a Hessian matrix approach [C. Zhu et al., PPCF 60, 054016 (2018)] for determining error field sensitivity to coil deviations. A primary cost function used for designing stellarator coils was adopted to evaluate the deviation of the generated magnetic field from the desired magnetic field. The sensitivities of error fields to coil displacements are then determined by the eigenvalues of the Hessian matrix, calculated by the FOCUS code. The new method is applied to analyze the possible errors fields in the CFQS experiment, which will be constructed in SWJTU in China, in collaboration with NIFS, Japan. The results provide information required to avoid dangerous coil misalignments, and that each modular coil has different tolerance. Relaxing unnecessary tolerances could lead to significant cost and schedule savings. |
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BP11.00039: High-beta Extended MHD Simulations of Stellarators with Finite Parallel Transport Torrin A Bechtel, Chris C Hegna, Carl R Sovinec The nonlinear, extended MHD code NIMROD is employed to simulate high beta stellarator physics. This work concentrates on the dependency of the resulting MHD equilibrium configuration on finite parallel transport processes. In particular, the numerical simulations show that anisotropic heat conduction has a significant effect on the pressure profile in regions with stochastic magnetic fields. As a consequence, the attained stored energy is sensitive to the size of the anisotropy in this region, underscoring that equilibrium beta limits are sensitive to anisotropic transport properties. The configuration under investigation is an l=2, M=10 torsatron with vacuum rotational transform near unity. Finite-beta plasmas are created using a volumetric heating source and temperature dependent resistivity. Extended MHD simulations are performed to generate steady state solutions that represent 3D equilibria. Due to the presence of finite parallel heat conduction, pressure profiles can exist in regions of magnetic stochasticity. Here, we present results of varying the parallel and perpendicular thermal anisotropy. Preliminary studies with temperature dependent Braginskii conduction closures are also presented. |
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BP11.00040: DCON for Stellarators Alan Herbert Glasser DCON is a widely-used code for determining the ideal MHD stability of a tokamak.1 We present a new version of DCON for application to stellarators. DCON originally worked by integrating the Euler-Lagrange equation for minimizing the ideal potential energy functional delta-W, using the solution matrix U to construct a plasma response matrix P, then using P to determine stability to ideal modes. Fixed-boundary instability is indicated by the vanishing of 1/(det P), a generalization of the Newcomb crossing criterion. Free-boundary instability is determined by adding P at the plasma-vacuum interface to a vacuum response matrix computed by another code and determining whether any of the eigenvalues is negative. Integration of U is numerically unstable, requiring occasional re-organization to maintain linear independence of the column vectors of U. A new method has been developed to integrate P rather than U, a numerically stable matrix Riccati equation. An interface has been developed to the VMEC stellarator equilibrium code. DCON also computes the Mercier criterion for local ideal interchange stability. Results will be presented for the Wendelstein 7-X stellarator. 1 A. H. Glasser, Phys. Plasmas 23, 072505 (2016). |
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BP11.00041: Latest advances in the V3FIT 3D equilibrium reconstruction code Mark Cianciosa, Eric C Howell, Sudip Seal, Steven P Hirshman, James David Hanson V3FIT is a code that determines the optimal inputs of a 3D equilibrium model by searching parameter space to minimize differences between observable and synthetic signals. Initially built around VMEC, V3FIT is routinely used to reconstruct many 3D equilibria in stellarators, tokamaks, and RFPs. A recent inclusion of the SIESTA equilibrium code has expanded 3D reconstruction to cases with magnetic islands and stochastic fields. In an effort to make V3FIT suitable for island divertor reconstruction of W7-X, this presentation will focus on the implementation of new capabilities. Among these are Gaussian process profiles, a machine learning method for automatic generation of radial profiles; and implementation of the TRAVIS ECE diagnostics for non-optically thick conditions and its extension to non-nested equilibria. We will also report progress on the implementation of a SIESTA free boundary mode for extension of reconstruction into the scrape-off layer. |
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BP11.00042: Non-planar elasticae as optimal curves for the magnetic axis of stellarators David Pfefferlé, Lee Gunderson, Stuart R. Hudson, Lyle Noakes The Euler-Lagrange equations are derived for finite length three-dimensional curves that optimize their bending energy while yielding fixed integrated torsion. The obvious translational and rotational symmetry is exploited to express solutions in a preferred cylindrical coordinate system in terms of elliptic Jacobi functions. These solution curves, which, up to similarity transformations, depend on three dimensionless parameters, do not necessarily close. Two closure conditions are obtained for the vertical and toroidal displacement (the radial coordinate being trivially periodic) to yield a countably infinite set of one-parameter families of closed non-planar curves. The behaviour of the integrated torsion (Twist of the Frenet frame), the Linking of the Frenet frame and the Writhe of the solution curves is studied in light of the Caluguareanu theorem. A refreshed interpretation of Mercier's formula for the on-axis rotational transform of stellarator magnetic field-lines is proposed. |
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BP11.00043: Nonlinear ideal interchanges in shaped 3D magnetic geometry A. B. Hassam, Sergio DeSouza Machado, Yi-Min Huang A 3D nonlinear ideal MHD code is developed to study interchange modes in 3D shaped magnetic geometry. Heat sources are introduced to allow possible non-stationary convection depending on the MHD stability properties. The initial code development is done using UMHD (Guzdar et al, PF, 1993). As a first example, unstable interchanges are shown in 2D Mirror geometry, from equilibrium to linear growth to nonlinear collapse. An axisymmetric cylindrical mirror is considered next. The code is Cartesian and thus has Cartesian hard-wall conducting boundaries. An initial equilibrium, under heating, is shown to transit to a convecting quasi-equilibrium, with collapse of the density annulus. At the next level, a 3D cylindrical helical stellarator geometry with single period will be considered. Here, the rotational transform being weak, static equilibrium may not exist and a convecting state is expected. Finally, stellarator geometry with multiple periods and more robust rotational transform will be considered. As in an earlier NIMROD simulation (Schlutt et al., PoP, 2013), B.n is held constant on the boundary. Perfectly conducting hard-walls are assumed. |
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BP11.00044: Non-axisymmetric divertor topology Allen Boozer, Alkesh Punjabi The fundamental properties of divertors on toroidal plasmas are determined by the magnetic flux tubes that carry the plasma from the plasma edge to the divertor chamber. In non-axisymmetric magnetic fields, they are defined by the flux that passes through holes that arise in what would otherwise be irrational magnetic surfaces outside of the confining surfaces. The name for these holes is turnstiles and for the surfaces on which they lie is cantori. The holes can be characterized by the probability per toroidal transit that a magnetic field line will escape confinement and strike the surrounding wall. This probability increases as the separation from outermost confining magnetic surface becomes greater. For a sufficiently large perturbation to an axisymmetric divertor, the escaping flux tubes go from being determined by the singularity in q to being determined by cantori/turnstiles as are stellarator divertors. Non-axisymmetric divertors are subject to more control than tokamak. The method of studying divertor topology and divertor control will be discussed. |
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BP11.00045: Non-resonant island divertor for stellarators Alkesh Punjabi, Allen Boozer The shape parameters in the optimized Hamiltonian for field lines in nonresonant stellarator divertor control the magnetic topology. It is found that a nonresonant island divertor for stellarators can be designed with appropriate choice of these parameters. The field lines are lost to the walls in the magnetic turnstiles escaping through the holes in cantori. The probability function for the escape determines the scaling of full confinement time, the loss time, and the decay of confinement for field lines. These can be estimated by giving the field lines an artificial radial velocity. Results of our study of the nonresonant nonaxisymmetric island divertor for stellarator will be presented. Non-axisymmetric divertors have greater scope for control than axisymmetric divertors. |
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BP11.00046: Overview, Progress, and Plans for the Compact Toroidal Hybrid Experiment D. A. Maurer, N. Allen, D. A. Ennis, J. D. Hanson, C. A. Johnson, S. F. Knowlton, J. D. Kring, X. Ma, K. G. Ross, J. C. Schmitt, P. J. Traverso, E. Williamson The Compact Toroidal Hybrid (CTH) is an ℓ=2,m=5 torsatron/tokamak hybrid (R0=0.75m, a∼0.2m, and |B|≤0.5T) with the ability to vary the confining magnetic field configuration and generate rotational transform profiles that are tokamak-like with ohmically driven plasma current for disruption and MHD studies. The main goals of the CTH experiment are to study disruptive behavior as a function of applied 3D magnetic shaping, and to test and advance the V3FIT reconstruction code and NIMROD modeling of CTH. Past and recent disruption studies will be overviewed and their relevance to tokamaks and quasi-axisymmetric stellarators discussed. Current new diagnostic development for the experiment includes an upgrade to the interferometer, new spectroscopic studies, and coherence imaging of plasma flows. CTH also serves as a test bed for diagnostic development for our collaborations on the larger facilities like DIII-D and W7-X. These facility collaborations will be briefly summarized along with a new research direction to explore low temperature plasmas on magnetic surfaces. |
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BP11.00047: Enhanced density profile resolution and equilibrium reconstruction with Compact Toroidal Hybrid interferometer upgrade Kevin Ross, David A Ennis, Gregory J Hartwell, Stephen F Knowlton, David A Maurer The Compact Toroidal Hybrid (CTH) experiment investigates the physics of disruptions in ohmically-driven torsatron/tokamak hybrid plasmas. To aid in characterizing these plasmas, a three channel 1-mm, 244GHz interferometer system has been upgraded by adding a fourth channel with an independent 280 GHz source. The system provides line-integrated plasma density measurements which are used by the V3FIT code for reconstructing the equilibrium 3-D flux surfaces and density profiles of CTH hybrid plasmas. The original interferometer configuration provided only 2 independent density measurements given the symmetric placement of two chords. The new design has 4 independent measurement beams spaced asymmetrically across the plasma to maximize resolution of the density profile. New beam locations were chosen based upon a refraction study using the TRAVIS code. An overview of the new design, implementation, and initial results of experiments performed to evaluate performance will be presented. |
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BP11.00048: Design and Implementation of a 200kW, 28GHz gyrotron system for the Compact Toroidal Hybrid Experiment Gregory J. Hartwell, Stephen F. Knowlton, David A. Maurer, David A. Ennis The Compact Toroidal Hybrid (CTH) is an l = 2,m = 5 torsatron/tokamak hybrid (R0 = 0.75 m, ap ∼0.2 m, and |B| ≤ 0.7 T). It can generate its highly configurable confining magnetic fields solely with external coils, but typically operates with up to 80 kA of ohmically-generated plasma current for heating. A 28 GHz, 200kW pulsed gyrotron system operating at 2nd harmonic for ECRH is being installed to supplement the existing 10kW klystron system operating at the fundamental frequency; the latter will be used to generate the target plasma. Ray-tracing calculations that guide the selection of launching position, antenna focal length, and beam-steering characteristics of the ECRH have been performed with the TRAVIS code[1]. The calculated absorption is up to 95.7% for vertically propagating rays; however, the absorption is more sensitive to magnetic field variations than for a side launch where the field gradient is tokamak-like. The design of the waveguide path and components for the top-launch scenario will be presented, as well as initial ECRH results. [1]N.B. Marushchenko, Y. Turkin, H. Maassberg, Comp. Phys. Comm. 185 165 (2014) |
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BP11.00049: Characterization of Coherence Imaging Instruments for the CTH and W7-X Experiments* N.R. Allen, D.A. Ennis, G.J. Hartwell, C.A. Johnson, D.A. Maurer, C.M. Samuell, S.L. Allen, D. Gradic, R. König, V. Perseo, W7-X Team Two dimensional profiles of line-integrated impurity emissivity and velocity in the Compact Toroidal Hybrid (CTH) experiment are obtained with Coherence Imaging Spectroscopy (CIS), a polarization interferometry technique with fixed delays. The 2D measurement output provided by a recently developed analysis routine, is necessary for the non-axisymmetric geometry of CTH plasmas. Bench tests of the CIS instrument demonstrate externally applied magnetic fields induce spurious flows of order 1 km/s for field strengths of up to 200 G irrespective of field direction. Additionally, two new CIS instruments designed to investigate the 3D physics of the W7-X island divertor by providing ion impurity flow measurements in orthogonal directions are fully operational. Further, a continuously tunable laser over most of the visible region now provides immediate and accurate calibrations of both CIS systems during W7-X plasma operations. |
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BP11.00050: In-situ wavelength calibration system for the X-ray Imaging Crystal Spectrometer (XICS) on W7-X James D Kring, Novimir A Pablant, Andreas Langenberg, John Edward Rice, Luis F. Delgado-Aparicio, David Alan Maurer, Peter J Traverso, Manfred Ludwig Bitter, Kenneth Wayne Hill, Matthew L Reinke An in-situ wavelength calibration system for the X-ray Imaging Crystal Spectrometer (XICS) on W7-X has been developed to provide routine calibrations between plasma shots. XICS determines plasma flow profiles by measuring the Doppler shift of x-ray line emission from high charged impurity species. A novel design is described that uses an x-ray tube with a cadmium anode placed in front of the diffracting spherically bent crystal to provide calibration lines over the full detector extent for the spectrometer channels. This calibration system can provide a relative wavelength accuracy of 3x10^-7 angstroms, corresponding to 50 m/s in the W7-X system over the wavelength range of 3.95 to 4.00 angstroms. An absolute wavelength calibration of 1x10^-5 angstroms (1 km/s) is expected based on the known cadmium wavelengths and the absolute positioning of the hardware. This independent calibration system can be used on both stellarators and tokamaks. Experimental and simulated results are shown along with expected results and a complete design of the calibration hardware to be installed in the W7-X XICS system. |
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BP11.00051: STRAHL modeling of impurity transport on Wendelstein 7-X during the first divertor campaign P. J. Traverso, N. A. Pablant, A. Langenberg, R. Burhenn, T. Wegner, B. Geiger, D. Zhang, B. Buttenschön, J. D. Kring, J. C. Schmitt, D. A. Maurer, W7-X Team In the first divertor operational phase (OP 1.2a) of Wendelstein 7-X, impurity transport experiments were performed with iron via laser blow-off injection. The x-ray imaging spectrometer systems, HR-XIS and XICS, were used to measure the He-like spectra at various input ECRH heating powers and plasma densities. These measured He-like iron spatial and temporal emissivities can then be used to estimate the He-like iron’s diffusion and convective velocity parameters. Utilizing the 1D transport code STRAHL, the spatial and temporal evolution of the He-like iron charge state is modeled for assumed stationary anomalous diffusion and convective velocity profiles. To match both the observed total He-like iron emissivity and a corresponding spatially inverted emissivity profile, a chi-squared minimization is done on the experimental data by varying the input spatial values of the anomalous diffusion and convective velocity parameters in STRAHL. |
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BP11.00052: Status of Equilibrium Reconstructions for Wendelstein 7-X J. C. Schmitt, S. A. Lazerson, J. Geiger, J. Schilling The reconstruction of the plasma equilibrium is a vital tool for toroidal fusion experiments to understand plasma performance and interpret diagnostic signals. The procedure involves solving the MHD equilibrium, computing synthetic diagnostic signals, and comparing these signals to measured signals. The parameters that describe the equilibrium are adjusted to match the synthetic signals to the measured ones. Information gained from the reconstruction includes the shape and location of the plasma and profile information regarding the plasma pressure, current, and individual plasma species which are subsequently used to interpret diagnostic information and for further analyses. Constraints for plasma reconstructions at Wendelstein 7-X (W7-X) include magnetic diagnostics, Thomson Scattering, interferometry, electron cyclotron emission, soft x-ray arrays and x-ray imaging crystal spectroscopy. Treatments of edge constraints related to the edge rotational transform and divertor location are also presented. The MHD equilibrium solution is provided by VMEC, which assumes solutions with nested, closed flux surface. The current status and future plans for equilibrium reconstructions for W7-X are shown and discussed. |
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BP11.00053: Characterization of a microwave generated plasma with varying fractional ionization on magnetic surfaces Eleanor N Williamson, David A Maurer, David A Ennis, Gregory J Hartwell Understanding the transition region between fully ionized and neutral dominated plasma is important to the study of the magnetosphere/ionosphere as well as the solar corona/chromosphere transition regions of the earth and sun. We are exploring the use of the magnetic surfaces of a stellarator confinement device, the Compact Toroidal Hybrid, to contain plasmas with widely varying fractional ionization with the goal of studying those naturally occurring transitions. These plasmas are heated using ECRH at two fundamental cyclotron frequencies of 17.65GHz and 18GHz with up to 10kW of power. A radially scannable triple probe and optical diagnostics are used to characterize plasma parameters. Results from a systematic study of varying ECRH input power and neutral gas fill pressure to access a range of plasma fractional ionization levels and electron temperatures will be presented. |
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BP11.00054: Radiative power exhaust with impurity seeding at Wendelstein 7-X Florian Effenberg, Tullio Barbui, Sebastijan Brezinsek, Yühe Feng, Heinke G Frerichs, Marcin Jakubowski, Ralf König, Maciej Krychowiak, Holger Niemann, Oliver Schmitz, Thomas Sunn Pedersen, Victoria Winters, Daihong Zhang First radiative power exhaust experiments with impurity seeding have been conducted in the 3D island divertor at W7-X. Ne seeding showed substantial enhancement of 𝑃𝑟𝑎𝑑 by up to 40% in plasma scenarios with 𝑛𝑒≈1−2×1019m−3 and ECRH heating of 2.5-5MW accompanied by a drop of divertor peak heat fluxes from 𝑞𝑑𝑖𝑣, 𝑚𝑎𝑥≈2MWm−2 to 1MWm−2. 𝑃𝑟𝑎𝑑 decays over seconds after termination of Ne injection. N2 seeding results in weaker 𝑃𝑟𝑎𝑑 enhancement and fast recovery of divertor heat fluxes and SOL electron temperatures measured with the He beam in the seeded island. The effects of seeding on heat and particle fluxes will be analyzed. The impact of changes of the island geometry on 𝑃𝑟𝑎𝑑 will be discussed based on 3D transport calculations with EMC3-EIRENE and particle balance analysis.
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BP11.00055: Comparison of measured and simulated heat and particle fluxes during high-beta mimic experiments on the W7-X stellarator Jeremy Lore, Yu Gao, Holger Niemann, Tullio Barbui, Glen A Wurden, Alexandra LeViness, Florian Effenberg, Heinke G Frerichs, Joachim Geiger, Marcin Jakubowski, W7-X Team Heat and particle fluxes to the Wendelstein 7-X (W7-X) divertor are compared to 3D simulations for a set of magnetic configurations designed to mimic the evolution of plasma currents in future high-beta plasmas which have been predicted to result in overloaded components. The experiments were performed in the first diverted operational phase of W7-X (OP1.2a) before the installation of divertor “scraper elements” which are designed to intercept the flux to the otherwise overloaded components. Comparison of heat fluxes from the OP1.2a experiments indicates that the patterns and relative magnitudes can be well reproduced with fluid transport simulations using EMC3-EIRENE, as well as simple field line diffusion models. These results indicate that the approach of mimicking otherwise inaccessible high-beta conditions using vacuum magnetic configurations is valid and increase confidence that scraper elements will protect the overloaded components as designed. The scraper elements have the disadvantage of intercepting flux in steady state OP2 configurations, reducing pumping efficiency and affecting particle transport. This effect is investigated using filtered cameras and spectroscopy. Initial results from experiments with two scraper elements installed (OP1.2b) will also be presented. |
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BP11.00056: Initial measurements of turbulence with a PCI diagnostic on Wendelstein 7-X Miklos Porkolab, Eric Edlud, Zhouji Huang, Adrian von Stechow, Olaf Grulke, Lukas Boettger, Evan M Davis, Alessandro Marinoni A phase contrast imaging (PCI) diagnostic has been implemented on the W7-X stellarator at IPP Greifswald and operated during the 2017 OP1.2a campaign [1]. The diagnostic has been upgraded recently as described in a companion presentation [2], and its operation will be continued in the 2018 OP1.2b campaign. New results from OP1.2b will be presented, expanding on previous observations of broadband turbulence as well as Alfvén wave phenomena in ECR heated plasmas collected during OP1.2a. The low frequency broad-band turbulence is believed to be evidence of nonlinearly saturated spectrum of ion scale turbulence driven by temperature gradients, while the band of nearly coherent modes observed in the Alfvén frequency range under different ECRH conditions may be manifestation of the presence of energetic electrons. Possible driving mechanisms include gradients of the thermal electron pressure or more likely, trapped electrons [3]. [1] E. Edlund, M. Porkolab, O. Grulke, et al, Proc. HTPD2018 Conference, San Diego, 2018. [2] Z. Huang, et al, presented at this conference. [3] M. Valovic, et al, Nuclear Fusion 40, 1569 (2000) |
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BP11.00057: Optimization and operation of the upgraded PCI system on Wendelstein 7-X Zhouji Huang, Eric M Edlund, Adrian von Stechow, Miklos Porkolab, Olaf Grulke, Lukas Georg Boettger A phase contrast imaging (PCI) diagnostic using a CO2 laser at 10.6 µm wavelength has been implemented on the W7-X stellarator during the 2017 OP1.2a campaign [1]. PCI is an internal reference beam interferometric technique which provides a direct image of line integrated electron density fluctuations in a plasma. In OP1.2a, typical wave numbers and frequencies that could be measured with PCI were in the range of 1.5 < k(cm-1) < 14 at 2 <f(kHz) < 2000. The diagnostic has been upgraded during the past year and new physics results will be presented in a companion paper [2]. Key upgrades during the past year include a new telescope design to expand the upper limit of measurable wave-numbers to 30 cm-1, an additional detector employing a spatial filter to focus on different locations in the plasma, a feedback-controlled vibration compensation system, a new sound wave calibration system, and a memory upgrade to allow data acquisition for 75 s compared to 4 s in OP1.2a. References [1] E. Edlund, M. Porkolab, O. Grulke et al, Proc. HTPD2018 Conference, San Diego, 2018. [2] M. Porkolab, E. Edlund, et al, at this conference. |
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BP11.00058: The 1-2 kHz Divertor-Area Fluctuation in the W7-X Stellarator Sean B Ballinger, James Layton Terry, Seung Gyou Baek, Adrian von Stechow, Carsten Killer, Olaf Grulke, the W7-X team Passive fast-camera imaging has been used to study the characteristics and motion of filamentary fluctuations in tokamak edge plasmas, shedding light on their role in boundary layer transport. Fast cameras may also help to understand transport in the unique magnetic topology of stellarators. A camera viewing a full plasma cross section of Wendelstein 7-X was used to record integrated visible light at up to 30,000 frames per second during the OP 1.2a campaign, featuring a passively cooled divertor. Videos from the camera reveal a quasi-coherent fluctuation in divertor-area emissions, with a steady peak frequency varying between 1-2 kHz from shot to shot. The fluctuation occurs during steady-state operation over hundreds of helium and hydrogen shots with the standard magnetic configuration but is absent from other magnetic configurations such as the high iota and high mirror configurations. Langmuir probes in the far scrape-off layer also observe the fluctuation with a close match in peak frequency. Videos of OP 1.2b campaign shots filtered for Dα, He I and C III line emission are also analyzed. We attempt to understand the physics underlying this fluctuation and its impact on the plasma and machine. |
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BP11.00059: Design of a Gas-Puff Imaging Diagnostic for W7-X James Terry, Seung Gyou Baek, Sean B Ballinger, Adrian von Stechow, Olaf Grulke, Paolo Scarin, the W7-X team A Gas-Puff-Imaging (GPI) diagnostic is being designed for use on the W7-X Stellarator. It will allow for detailed study of boundary and scrape-off-layer physics during the long-pulse W7-X operation period OP 2. GPI requires an in-vessel nozzle close to the plasma, through which non-perturbing amounts of H2 or He gas are puffed; a re-entrant view of the puff, ideally along local field lines; and an optical system that relays the imaged emission to a fast 2D detector. This must be done in the harsh long-pulse environment, where re-entrant hardware must be actively cooled and collecting optics shuttered. We have prepared a design for viewing a roughly 50 x 70 mm region at the plasma boundary with spatial resolution of ~5 mm and time resolution of ~1 μsec. We propose to puff the gas through 4 converging-diverging nozzle apertures in order to collimate the gas cloud. We have performed finite-element analysis of fluid flow down the feed tube and through the nozzles. We have modeled the puff emission using DEGAS 2. We have designed a water-cooled re-entrant tube to hold the light collection optics and modeled the optics to attain sufficient resolution and light throughput. The details of the design and modeling will be shown. |
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BP11.00060: Neutral particle fluxes on the divertor during overload mimic scenarios in Wendelstein 7-X Alexandra LeViness, Peter Drewelow, Jeremy Lore, Kenneth Hammond, Glen A Wurden, Marcin Jakubowski, Georg Schlisio, W7-X Team Neutral particle fluxes on the Wendelstein 7-X (W7-X) divertor are compared for the first two diverted operational phases (OP1.2a and OP1.2b). In OP1.2b, two test divertor unit scraper elements (TDU-SE) have been implemented in an attempt to prevent heat flux overload on divertor edges during long, high-beta discharges. While these conditions cannot be reached with current operating parameters, magnetic configurations have been designed to mimic the effect of high bootstrap current. Experiments are performed in each phase in order to compare divertor fluxes with and without the TDU-SE in these configurations, as well as in standard operational configurations. The SE have been predicted to have the side effect of reducing neutral pumping, which could have an adverse effect on density control. Results are presented from the H-α spectroscopic cameras, which measure flux of neutral hydrogen at the divertor in order to quantify the effect on pumping efficiency. This work also covers calibration of the H-α cameras to absolute neutral flux. |
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BP11.00061: Targeting measurements of electric fields and turbulence in the Wendelstein 7-X stellarator through optimization of ion beam probe design Peter J Fimognari, Thomas P Crowley, Diane R Demers, Olaf Grulke, Ralph Laube Experiments with a Heavy Ion Beam Probe (HIBP) on Wendelstein 7-X (W7‑X) would advance understanding of core neoclassical and turbulent particle and energy transport. We have carried out a feasibility study for the diagnostic which assumes use of former TEXT-U 2 MeV HIBP equipment. It assumed predictions of electron density (ne) and temperature profiles with central ne ≤ 1020 m-3. We have performed, for the W7-X standard magnetic configuration, detailed simulations of sample volume shapes, sizes, orientations, and locations. They indicate that the radial extent of sample volumes (Δreff/a) is typically 0.02, which is advantageous when measuring profiles. They also predict the diagnostic will be sensitive in the plasma region reff/a ≥ 0.25 to fluctuations of ne and electric potential (φ) at levels > 0.5%, widely covering ion-scale turbulence with k⊥ρs < 1. More broadly, simulations for all eight W7-X reference magnetic configurations predict signal levels will be sufficient for study of equilibrium electric fields at all radii and ion-scale fluctuations of ne and φ for the outer region of the plasma. Relationships between HIBP design parameters, sample volume properties, and the scope of possible physics studies will be discussed. |
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BP11.00062: Scraper Element Observations in the W7-X Stellarator Glen A. Wurden, Jeremy Lore, Victoria Winters, Peter Drewelow, Marcin Jakubowski, Kenneth Hammond, Holger Niemann, Joris Fellinger, Adnan Ali, Christoph Biedermann, Simppa Akaslompolo, Alexandra LeViness, and the W7X Team We report on plasma interactions with the new graphite scraper element in Module 5 of the Wendelstein 7-X stellarator during the recent OP1.2b campaign in 2018. Infrared and filtered visible imaging [1], spectroscopy, Langmuir probes and thermocouples provide a suite of measurements to compare a variety of plasma configurations, including a specially designed “high-beta mimic” configuration to put up to 10 MW/m^2 on the scraper element. Additionally, we looked for localized low-level hot spots with the new high-resolution IR capability, due to expected localized lost fast hydrogen ions injected from the new neutral beam injection (NBI) system. [1] G. A. Wurden, et al, Rev. Sci. Instruments 2018 |
(Author Not Attending)
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BP11.00063: Coordinate-free Grad-Shafranov equation on a Riemannian manifold with Killing field Vishnu Mangalath, David Pfefferlé, Lyle Noakes The MHD force-balance equation is formulated on a Riemannian manifold with the vector potential, the magnetic field, the current density and the pressure gradient as differential forms. In the presence of a non-vanishing Killing field (isometry) and the assumption that the physics is invariant under its flow, a coordinate-free Grad-Shafranov equation (GSE) is derived for the interior product of the vector potential with the Killing field. The symmetry reduces the MHD equilibrium problem to a two-dimensional elliptic boundary value one, where flux-surfaces are effectively iso-contours extruding along the Killing flow. The usual axisymmetric tokamak GSE and its helical extension are obtained as special cases from Euclidean three-space. Examples of GSE on non-flat Riemannian manifolds are explored. |
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BP11.00064: First results of the Coherence Imaging Spectroscopy Systems on Wendelstein 7-X Valeria Perseo, Dorothea Gradic, Ralf Koenig, Oliver Ford, Florian Effenberg, David Ennis, Thomas Sunn Pedersen, W7-X Team Coherence Imaging Spectroscopy (CIS) is a camera-based interferometry diagnostic capable of measuring 2D impurity flow velocities for a selected visible emission line from the plasma. A spatial interference pattern encoding the spectral line properties is overlaid on the plasma image by the use of α-BBO birefringent crystals. The CIS 2D measurement capability and high optical throughput are attractive for the complex magnetic island topology of Wendelstein 7-X (W7-X). Two CIS systems have been designed and set up to face the challenging experimental conditions of W7-X. The field of view of each system images the same island divertor portion from nearly perpendicular directions for improved emission and flow interpretation in the W7-X geometry. The diagnostic has been used during the OP1.2 experimental campaign, observing the plasma at different magnetic configurations, densities, and heating powers. Measured velocities on the order of km/s were observed and the behavior of multiple impurities, mainly carbon and helium, will be presented. The diagnostic output is also compared with dedicated EMC3-EIRENE simulations. |
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BP11.00065: The He/Ne beam diagnostic for line ratio spectroscopy in the island divertor of Wendelstein 7-X Tullio Barbui, Erik Flom, Florian Effenberg, Oliver Schmitz, Marcin Jakubowski, Maciej Krychowiak, Ralf Koenig, Jorge M. Muñoz Burgos, Stuart Loch The line-ratio spectroscopy system on thermal He and Ne enables measurement of ne and Te in front of the horizontal divertor target, across the 5/5 island in the standard magnetic configuration. For the second divertor campaign of Wendelstein 7-X the observation system has been upgraded adding 27 new vertical lines-of-sight to the existing 27 horizontal ones. These lines are channeled to a 20cm and a 32cm focal length Czerny-Turner spectrometer allowing observation of the He and Ne lines as well as impurities and H lines with high spectral resolution. Gas injection is enabled by two boxes with 5 fast piezo valves each, mounted directly behind the divertor plates in one upper and one lower divertor module, which are magnetically connected in the 5/5 island configuration. He was used as routine gas while Ne was tested in order to extend the applicability of the diagnostic to the detached divertor regime at very low Te (≪10eV). In this work Te/ne profiles across the divertor island are shown for different island widths. Results from Ne injection and the status of the development of a dedicated collisional-radiative model for Ne will be discussed. |
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BP11.00066: Impurity studies with Charge Exchange Spectroscopy on W7-X Lilla Vanó, Jürgen Baldzuhn, Sergey Bozhenkov, Oliver Ford, Robert C Wolf Charge Exchange Recombination Spectroscopy (CXRS) is a diagnostic suitable for impurity studies as it measures the emission of low-Z ions. This presentation will give an overview of the passive and active CXRS systems on the Wendelstein 7-X (W7-X) stellarator. The understanding of the behavior of impurities and the control of their concentrations is necessary to adjust the desired radiation level. During the 2017 experiment campaign of W7-X, a passive CXRS system was employed to observe impurity line emission from the plasma edge. The analysis focuses on inverted spectral radiance, temperature and rotational velocity profiles of carbon-VI. In the analyzed plasmas, the carbon-VI emission seems to be localized inside the last closed flux surface. The experimental observations will be supported by the first comparison with transport modeling using the STRAHL-code. The active CXRS system requires an external neutral source to give an electron to the fully stripped low-Z ions in the plasma. Neutral Beam Injection (NBI) will be in operation for the first time during the 2018 W7-X campaign. A dedicated spectroscopic observation system has been set-up and the first active measurements will be carried out. |
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BP11.00067: Collective Thomson Scattering at Wendelstein 7-X: design, commissioning, and first results Dmitry Moseev A Collective Thomson Scattering (CTS) diagnostic was installed on the Wendelstein 7-X (W7-X) stellarator for ion temperature measurements. The diagnostic setup features measurement capabilities at two plasma cross-sections with different topology of the magnetic field. CTS uses 140 GHz heating gyrotrons as sources of probing radiation. It also uses a remote steering quasi-optical receiving antenna where the movable parts are located outside the vacuum vessel. The CTS signal at sub-nW power levels shares a quasi-optical transmission line with the heating beams of ECRH at MW-power levels. In the commissioning phase, electron cyclotron emission (ECE) measured by CTS showed good agreement with the measurements of the ECE radiometer. CTS spectra were also measured and the spatial localization of the measurements was demonstrated by sweeping the receiver beam across the probing beam using the moveable quasi-optical components. We fitted the scattering spectra using the electrostatic CTS code eCTS, which is integrated into the MINERVA system for Bayesian analysis at W7-X. The obtained ion temperatures showed good agreement with X-ray spectroscopy measurements. |
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BP11.00068: The role of particle drifts in W7-X divertor heat load asymmetries K.C. Hammond, P. Drewelow, P. Drews, M. Endler, Y. Gao, M. Jakubowski, C. Killer, H. Niemann, L. Rudischhauser, the W7-X team In both tokamaks and stellarators, particle drifts in the scrape-off layer are known to affect the heat load distribution on the divertor targets. In W7-AS, for example, divertor heat loads were sometimes observed to be greater by up to a factor of two in regions downstream relative to the ExB drift [1]. In the first divertor experiments in W7-X we have observed indications of similar asymmetries, which appear to shift in response to reversal of the magnetic field. We present probe measurements from the upper and lower W7-X divertors, as well as measurements of upstream scrape-off layer parameters including plasma flow and radial electric field. |
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BP11.00069: Analysis of SOL carbon radiation distributions with changing plasma parameters at W7-X Victoria R Winters, Tullio Barbui, Sebastijan Brezinsek, Florian Effenberg, Jeffrey H Harris, Marcin Jakubowski, Ralf Koenig, Petra Kornejew, Maciej Krychowiak, Oliver Schmitz, Thomas Sunn Pedersen, Erhui Wang, Daihong Zhang The plasma-surface interaction (PSI) on both the divertors of the island divertor at W7-X and on the first wall leads to carbon erosion and carbon as the main plasma impurity. It is of importance to quantify the amount and distribution of carbon in the scrape-off layer (SOL), as an excess of radiating impurities can move the radiation boundary layer inward, and in extreme cases can ultimately lead to radiative collapse of the plasma. This work provides insight into the distribution of carbon in the SOL using a combination of a sophisticated divertor spectroscopy set-up and ORNL Filterscopes viewing the first wall. The C-II (514.5nm) and C-III (465nm) radiation distributions in the SOL will be analyzed as a first attempt to infer a partial carbon particle flux distribution using electron density and temperature measurements provided by the Helium-beam diagnostic. The effects of plasma parameters on the carbon radiation and particle flux distribution and how these distributions are correlated to plasma performance is assessed. |
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BP11.00070: Light impurity exhaust with the Wendelstein 7-X island divertor Thierry Kremeyer, Florian Effenberg, Jeffrey H Harris, Marcin Jakuboski, Ralf König, Petra Kornejew, Maciej Krychowiak, Oliver Schmitz, Victoria R Winters, the W7-X team Preventing impurity accumulation in improved energy confinement regimes to maintain stationary plasma conditions is of great importance in fusion research. This is important for helium, the ash of the fusion process as well for impurities used for radiative edge cooling. A set of ORNL Filterscopes [1] was used to observe line integrated intensities of the impurity lines while a set of WISP gauges [2] was used to monitor the partial neutral pressure in the divertor pump gap of the island divertor at W7-X. The results from experiments utilizing short injections of moderate flux densities of He, N and Ne will be discussed. The response of these injections in the line-averaged density and in the partial neutral pressure in the pump gaps will be discussed. In particular the effective confinement time τp* will be analysed for hydrogen as well as for He. References [1] Colchin, R J et al, Rev. Sci. Instrum. 74 (2003) [2] Flesch, K et al, Rev. Sci. Instrum. 87 (2016)
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BP11.00071: Overview of Research in the SciDAC Center for Simulation of Fusion Relevant RF Actuators P. T. Bonoli, D. L. Green, E. D'Azevedo, N. Bertelli, A. Dimits, T. Kolev, D. N. Smithe, R. W. Harvey, J. R. Myra, M. S. Shephard, D. Curreli, RF SciDAC Team An overview is given of research which has the goal of developing a predictive simulation capability of the self-consistent interaction of RF power with the scrape-off layer (SOL), including the effects of plasma sheaths, ponderomotive forces near an antenna, and turbulence and transport. This will make it possible to answer critical questions related to how RF power modifies properties of the SOL and how, in turn, the SOL affects the propagation and absorption of RF waves. Targeted problems include the impact of high power RF systems on plasma facing materials, including high-Z impurity sputtering and transport induced by large RF-induced sheath potentials, localized thermal loads, and antenna damage. In order to address the high geometric fidelity required to describe the 3D magnetic field and 3D solid geometry of the RF launching structures and the surrounding vacuum vessel the open-source scalable Modular Finite Element Framework (MFEM) is being utilized. The Parallel Unstructured Mesh Infrastructure (PUMI) is also being used in order to move to global solution domains, which are presently precluded by existing meshing approaches. |
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BP11.00072: RF Modeling and Simulation using the IPS system Donald Batchelor, David L Green, Wael Elwasif, Robert Walter Harvey, Yuri Petrov, Paul Thaddeus Bonoli, John Wright, Samuel Frank The Integrated Plasma Simulator (IPS) is a computational framework that permits serial and or parallel simulation codes to function inter-operably on the most advanced massively-parallel computers so as to provide a flexible capability for integrated, multi-physics simulation. In a joint effort between the Advanced Tokamak Modeling (AToM) and RF SciDAC projects some of the most capable RF code are being made openly available using the IPS. Examples will be shown using the GENRAY, CQL3D, and TORLH codes and how these can be coupled together and also with other non-RF codes. A workflow with which TORLH iteratively coupled with CQL3D was used to study lower hybrid current drive in EAST tokamak will be described. |
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BP11.00073: Coupling Simulations of RF, Antenna Effects and Scrapeoff-Layer Turbulence A.M. Dimits, M.V. Umansky, Ilon Joseph, T.D. Rognlien, J.R. Myra, S. Shiraiwa, C. Lau, E.H. Martin, T.G. Jenkins, D.N. Smithe, R.W. Harvey, Y. Petrov We discuss progress on coupling tokamak edge/scrapeoff-layer (SOL)-relevant turbulence and ICRF codes and models. A 3D extension of the SOLT model is being developed under the BOUT++ framework [1]. The effects of boundary conditions representing RF launching structures on transport equilibria and turbulence in this model are studied. The simplest such boundary conditions involve regions on the outer flux surface biased at an effective RF sheath potential relative to the parts of the surface not in these regions. We compare solutions between the BOUT++ models and the UEDGE code for cases where these regions are axisymmetric. For cases where they are not, e.g., a rectangle in the toroidal-poloidal plane, the solutions are studied, beginning with steady solutions and proceeding to fluctuating ones. Simulations in which biased structures impinge in an outer layer of the BOUT++ model are also studied and compared with those obtained with biased regions (only) on the outer surface. We also report work on the transfer of data from BOUT++ simulations to the edge-relevant ICRF codes, including development of a generally useful data format and API. [1] M.V. Umansky et al., this meeting.
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BP11.00074: Calculation of RF sheath properties from surface wave-fields: a post-processing method J. R. Myra, J. C. Wright, S. Shiraiwa, H. Kohno It is important to understand and quantitatively model the RF sheaths that form on material surfaces when intense ICRF waves are present. A method, outlined in [1], enables calculation of the RF sheath voltage by post-processing surface RF quantities, such as the normal plasma current into the surface, from codes which employ conducting wall, rather than sheath boundary conditions (BCs). Essentially, the method recalculates the local fields near the surface in the presence of the sheath under some assumptions. The previous implementation [1], valid for slow waves when the background magnetic field was normal to the surface has now been generalized to arbitrary electromagnetic fast and slow wave polarizations and oblique magnetic fields. Verification tests and an application to the modeling of far-field sheaths in the LAPD experiment will be presented. In addition, progress on incorporating the nonlinear sheath BC directly into the Petra-M global code will be discussed. [1] J.R. Myra and H. Kohno, EPJ Web of Conferences 157, 03037 (2017). |
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BP11.00075: The impact of the edge density fluctuations on the electron cyclotron wave propagation in tokamaks Nicola Bertelli, Gerrit J Kramer, Ernest J Valeo In this work we show a numerical analysis of the electron cyclotron (EC) wave beam propagation in the presence of the edge density fluctuations for DIII-D-like and ITER-like plasmas. Particular attention will be paid on the broadening of the EC beam at the EC resonance location with and without edge density fluctuations. In fact, such broadening might affect the ECCD efficiency to suppress the neoclassical tearing mode (NTM) instability particularly for ITER plasmas where the distance between the SOL density fluctuations and the EC resonance is larger than one meter. In this paper we make use of a 2D/3D code originally developed for reflectometer simulations. This code was extensively used for reflectometer antenna-plasma coupling calculations that include density fluctuations. One important capability of the code is that the numerical domain can be divided in three regions: a vacuum region, a paraxial region, and a full-wave region, for high computation efficiency. A scan in the amplitude of edge density fluctuations will be performed together with the evaluation of the 3D electric field pattern at the location of the EC resonance. The importance of these results on NTM suppression by ECCD on ITER will be discussed. |
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BP11.00076: Cold plasma finite element methods for RF waves with synthetic turbulence Cornwall Lau, Ed D'Azevedo, David L Green, Elijah Henry Martin, Syun'ichi Shiraiwa, Gregory Marriner Wallace New 2-D and 3-D cold plasma finite element simulations of lower hybrid (LH) waves with synthetic turbulence will be shown that can modify the polarization, penetration and scrape-off-layer (SOL) absorption of the LH wave. The key parameters required to observe these modifications are that the turbulence amplitude needs to be large and that the turbulence scale length is close to the LH wavelength. This may have implications for the coupling of LH waves through the SOL into the core plasma given that these parameters are necessary to reproduce LH electric field polarization measurements on Alcator C-Mod. Because these finite element simulations are computationally expensive, different solvers are being developed and tested to increase the maximum solvable problem size. This problem is typically solved using a direct solver, but it will be shown that an iterative domain decomposition with a multiplicative Schwarz method can possibly reduce the memory footprint for cold plasma finite element problems. Scaling studies for direct and iterative solvers on both shared and distributed memory system will be shown. |
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BP11.00077: Simulation of anisotropic heat transport in fusion plasma with MFEM Mark Stowell, Jeremy Lore, Kazem Kamran, Mark S Shephard, David L Green The simulation of magnetized plasma transport using the fluid approximation is ubiquitous in the study of fusion devices. However, the extreme anisotropy in the ratio of heat diffusivity in the direction parallel to the confining magnetic field to that in the perpendicular direction can range from 10^6 at the periphery of the plasma, to more than 10^12 in the core. This presents a challenge to discretization schemes which must avoid contamination of the perpendicular transport by numerical errors associated with the parallel transport. Typical solutions to this problem involve aligning the coordinate system with the magnetic field. However, for problems requiring complex domain boundaries (non-coordinate aligned), alternatives must be investigated. Here we present progress on the utilization of the high-order capabilities of the MFEM [1] library, combined with advanced meshing techniques to address this problem targeting simulations of the far scrape-off-layer (SOL) for investigating the nonlinear modification of the SOL properties by application of RF power. [1] R. Anderson, et al., MFEM: A Modular Finite Element Library, in preparation, 2018 (http://mfem.org/publications/). |
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BP11.00078: A Monte-Carlo particle code for modeling of RF and NB heating in tokamaks Yuri Petrov, R. W. Harvey The MCGO [1] MC code has been further developed, aimed at calculations of heating and losses of fast ions that are produced by RF and/or NBI. The code will be available as an Open Source. Ion orbits from a thermal, and an included NB source, are integrated in time under effects of collisions, charge exchange events and RF kicks. A procedure for accumulation of particle loss distributions to the wall of test ions and neutrals is included as a function of poloidal and toroidal distance along the wall, particle energy and the incident angle. As a verification study, a comparison is made between the results from the MC code and the finite-orbit-width bounce-average continuum Fokker-Planck code CQL3D-FOW [2], in case of minority ICRH in C-Mod. The developed MC methods are being adapted to the multi-processor and GPU compute platforms supported by the US DOE. [1] http://www.compxco.com/mcgo.html; [2] Yu.V. Petrov and R.W. Harvey, PPCF 58, 115001 (2016). |
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BP11.00079: The impact of fluctuations on the propagation of radio frequency waves A. K. Ram, K. Hizanidis, F. Bairaktaris, S. I. Valvis, A. Zisis, A. Papadopoulos, I. G. Tigelis In fusion devices, radio frequency (RF) electromagnetic waves encounter turbulent plasmas in the edge region as they propagate from the excitation structures to the core of the device. In order to optimize heating and current drive by RF waves, it is necessary to understand the effect of turbulence on the propagation characteristics of the waves. A common approach towards quantifying the effects of turbulence on RF waves is the Kirchhoff technique. The wave fields and their normal derivatives are evaluated at a surface, separating two different densities, using physical optics. The fields at any point on this surface are approximated to be the same as the fields on a tangent plane at that point. The results from this theoretical analysis are compared with full-wave numerical simulations. A complementary set of studies is to construct the effective permittivity of turbulent plasmas that are a mix of coherent (blobs and filaments) and incoherent fluctuations. Towards this end, we use the effective medium approximation and the homogenization theory for magnetized plasmas to formulate the plasma permittivity. This effective permittivity will be implemented in the full-wave and in the physical optics studies of RF scattering. |
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BP11.00080: A 1D coupled RF / transport model for ponderomotive density modification of the SOL near high power RF actuators. Rhea L Barnett, David L Green, Jeremy Lore, David N Smithe, James Richard Myra, Colin L Waters The efficiency of coupling RF power to magnetically confined fusion plasmas depends on the properties of the scrape off layer, which are in turn altered by the application of RF power. The dynamics of this nonlinear interaction are not well understood such that designing a robust coupling scheme has proven elusive. Here we present a simplified 1D (parallel to the confining magnetic field), coupled RF and fluid plasma transport simulation to examine density modification near RF actuators via ponderomotive effects. Our initial approach includes only the parallel component of the force (associate with electron density modification). Progress towards including a ponderomotive tensor term capturing both parallel and perpendicular (associated with ion drifts) components, and determining the requirements on the numerical scheme required for such a coupling will also to be discussed. |
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BP11.00081: Computing tensor ponderomotive force terms in Vorpal for use in BOUT++ David Smithe, Thomas G Jenkins, James Richard Myra, Rhea L Barnett, Andris Martin Dimits, Maxim V Umansky A literature survey across numerous references discussing ponderomotive force leads us to a single consolidated formulation, utilizing the fluid force equation and separation of time-scales. We present analytical case studies from various scenarios of the formulation where ponderomotive forces can occur. Of significance is that this work includes the presence of collisions with neutral gas (RF drag and absorption) in the far-SOL and partial ionization regions. The formulation includes a classical term with ∇|VRF|2, thus permitting force in non-drift directions. Other terms include cyclotron motion and neutral gas collisions, and have gradients of plasma and neutral gas density, and thus can be significant in the steep density gradients. We implement diagnostic calculation of these ponderomotive forces in the Vorpal time-domain plasma model, and starting from the description of the BOUT equations in Reference [1], we formulate the computed ponderomotive forces terms needed for use in the BOUT++ code. We also set up simple launcher geometries, and visualize the analytical case studies in higher dimensions. [1] Umansky et al., Computer Physics Communications 180 (2009) 887. |
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BP11.00082: Development of Petra-M Framework: toward OS integrated FEM analysis Syun'ichi Shiraiwa, Nicola Bertelli, Paul Thaddeus Bonoli, Eun-Hwa Kim, T. Kolev, James Richard Myra, M. Stowell, John Christopher Wright, Satoru Yajima Progress in the Petra-M framework development is presented. Petra-M was originally built to solve the RF wave propagation problem. It uses the MFEM finite element library, and has been used for the variety of cold plasma wave simulations. Recent modeling needs motivated to extend Petra-M in order to allow for performing more complicated multi-physics type simulations. Such needs include integrating the RF rectified potential model, estimating the density modulation due to the Ponderomotive forces in front of antenna, investigating the scattering of RF wave due to the turbulence, and solving an electro-magnetic-thermal coupled problem in time domain. Petra-M now supports an arbitrary number of finite element space discrete variables defined over a part of input geometry, and a user-friendly interface allows for defining a coupling PDE system using MFEM Linear/BilinearForm integrators. A custom operator and preconditioner can be defined using Python decorator. In RF SciDAC4, these new features are used to integrate the RF sheath and the turbulence predicted by the BOUT++ code to SOL wave simulations. We discuss how to use Petra-M to prepare MFEM simulations using the variety of existing models. |
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BP11.00083: An investigation of projective integration methods for scrape off layer density modification by nonlinear RF ponderomotive effects. David Green, Sebastian De Pascuale, Rhea L Barnett Fusion relevant plasmas exhibit multiscale processes in both space and time. Here we present progress on the application of projective integration methods to address the temporal multiscale issue. As a test case, we use the scenario of electron density modification via parallel ponderomotive effects during the application of high power RF waves in the scrape off layer of magnetic confinement fusion devices. In this scenario there is a clear separation of timescales between a fast RF oscillatory component to the density, and a subsequent slow non-oscillatory modification. These physics are traditionally addressed by a formal separation of timescales, allowing the comparison of our projective integration treatment to integrating the slow non-oscillatory density modification with that approach. |
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BP11.00084: Solid-State Klystron Driver for Lower Hybrid Current Drive (Phase I) James R Prager, Timothy Ziemba, Kenneth E Miller, Alex Henson Launching radio frequency (RF) waves from the high-field side (HFS) of a tokamak has the potential to be an efficient off-axis current drive method while reducing the plasma-material interaction issues of the antenna. Researchers at the Plasma Science and Fusion Center (PSFC) at the Massachusetts Institute of Technology (MIT) have proposed to reuse existing equipment at MIT, including CPI klystrons, to demonstrate HFS RF launching at DIII-D. Eagle Harbor Technologies (EHT), Inc. is developing the next-generation klystron driver for use by MIT for HFS RF launching experiments at DIII-D. The next-generation klystron driver will take advantage of the high frequency solid-state switching capabilities developed by EHT with support of the DOE SBIR program. The high frequency nature will allow for the development of a more compact system, which can be placed closer to the klystrons. This system will be designed so that there is one driver per klystron, which will allow the system to scale as more klystrons are added and experiments to continue in the event of a klystron fault. We will present the Phase I project plan and overview of progress to date. |
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BP11.00085: A Real-Time Imaging Diagnostic for Electron Cyclotron Heating Systems Elijah H Martin, Cornwall Lau, Michael W Brookman In this presentation we describe a spectroscopic diagnostic capable of imaging high frequency waves (>50 GHz) in real time with sub-mm spatial resolution. The diagnostic relies on measuring passive Dβ optical emission and requires a minimum wave electric field magnitude of ~500 V/cm. These requirements limit the technique to the edge plasma and yield an optimal response when implemented to diagnose the microwave beam of electron cyclotron heating systems. The diagnostic was specifically designed to investigate electron cyclotron beam scattering from turbulence. This effect is important because it can lead to a substantial increase in the power required to stabilize neoclassical tearing modes. The dynamic Stark effect is used to extract the electric field vector from the Dboptical emission. This effect produces a satellite structure in the spectrum. For high frequency waves, individual satellite intensities can be imaged using ultra-narrow bandpass filtered cameras. The electric field vector is determined from the intensity ratio of two satellites in two orthogonal polarization states. The details associated with the diagnostic and its application for physics studies will be discussed. |
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BP11.00086: Heating and current drive by high intensity, pulsed, electron cyclotron Gaussian beams Kyriakos Hizanidis, A. K. Ram, A. Kirsch, R. J. Temkin, G. Anastassiou Inspired by the experiments in the Microwave Tokamak Experiment (MTX) in the early 1990s, and the theoretical studies associated with these experiments, we have been studying the nonlinear relativistic interaction of electrons with high intensity, pulsed, Gaussian beams in the electron cyclotron range of frequencies. The Gaussian beam is analytically constructed so as to satisfy the full complement of Maxwell's equations for a cold, magnetized plasma. For electrons interacting with the beam, the components of the electron momentum, along and across the magnetic field, vary with the power and the direction of propagation of the beam. There are two effects that lead to changes in the momenta of electrons -- the ponderomotive force due to the spatial variation of the beam, and transit time interaction, including trapping, as electrons traverse the beam. We consider both the ordinary and the extraordinary waves and find that the wave polarizations lead to different dynamics of the electrons. There is no saturation of the momenta as the beam power is increased. The interaction of electrons with a beam is distinctly different from their interaction with a plane wave. Results for the gain in energy and momentum of the electrons as a function of beam parameters will be presented. |
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BP11.00087: Second harmonic EC plasma ramp-up through production of energetic electrons on the QUEST spherical tokamak Takumi Onchi, Hiroshi Idei, Makoto Hasegawa, Kengoh Kuroda, Ryuya Ikezoe, Kazuaki Hanada, Akira Ejiri, Tsuyoshi Kariya, Shin Kubo, Toru Tsujimura, Atsushi Fukuyama Plasma current is started up solely by electron cyclotron wave (ECW) in QUEST spherical tokamak. The beam of 28 GHz second harmonic ECW is injected obliquely from low field side, and hence parallel refractive index at the resonant layer is N|| = 0.78. A steering antenna, consisting of two quasi-optical mirrors, can turn direction of the focused Gaussian beam. Polarization of the ECW is controlled as X-mode is superior to O-mode. In the experiment, achieved peak plasma current has been high record, as non-inductive RF start-up, reaching Ip > 75 kA. The count number and the averaged energy of Hard X-ray attributed to energetic electrons increase with Ip ramp-up. Analysis through TASK-WR simulation demonstrates that power of 28 GHz ECW is absorbed dominantly to energetic electrons via single-pass cyclotron damping, and that X-mode absorption into particle is adequately higher than that of O-mode. |
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BP11.00088: High Voltage Solid-State Switch for Magnetron Driving Ilia Slobodov, Timothy Ziemba, Kenneth E Miller, James R Prager, Connor Liston, Alex Henson Eagle Harbor Technologies, Inc. (EHT) is developing a 35-kV solid-state switch that can be utilized for driving magnetrons. This 35-kV solid-state switch will be packaged into a driver for a pulsed magnetron at the Lithium Tokamak Experiment (LTX) at Princeton Plasma Physics Laboratory. This system will be delivered to LTX for testing the Year 2 of the Phase II program, where it will remain at the completion of the Phase II program. EHT will present high voltage switch results, including voltage sharing, fault mitigation, and resistive load driving. On the road to 35 kV, EHT has developed a 10-kV switch that has been packaged and commercialized. Results from the lower voltage switch will also be presented. |
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BP11.00089: Advanced Auxiliary Fast Ion Heating Schemes in the W7-X stellarator Hamish Patten, Jonathan P Graves, Jonathan Faustin, Alfred Cooper, David Pfefferlé Improving the fusion performance of fusion devices depends on the fast ion pressure in order to generate the fusion reactions, and thus also on the effectiveness of the auxiliary ion heating methods applied. It is foreseen that the ITER, LHD and Wendelstein 7-X reactors will utilise two main ion heating methods: Neutral Beam Injection (NBI) and Ion Cyclotron Range of Frequency (ICRF) heating. The options for NBI are limited by the lack of flexibility due to experimental limitations. ICRF heating, in principle, permits a broad range of scenarios [1-6]. This work utilises the Self ConsistENt Ion Cyclotron (SCENIC) code package to simulate various heating schemes involving both NBI and ICRF (including synergetic 3-ion species RF-NBI heating) in 3D magnetic geometry. Work presented will illustrate the numerical development and application of different advanced heating schemes in the JET and W7-X reactors. |
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BP11.00090: The Travelling Wave Antenna: an important option for ICRH in DEMO Jozef Ongena, Andre Messiaen, Riccardo Ragona, Yevgen Kazakov For the central heating of a commercial fusion reactor Ion Cyclotron Radio Frequency Heating (ICRH) is an important option as it is the only heating method capable of coupling Radiofrequency (RF) power directly to the D and T plasma ions without a cut-off density limit. However the excitation of the magnetosonic waves through the boundary layer can lead to voltage standoff problems with traditional ICRH antenna systems. In this paper we present a new antenna concept that optimizes the antenna excitation. It consists of a number of Travelling Wave Antenna (TWA) sections distributed all around the first wall, embedded in the blanket modules. The sections are fed using a resonant ring system. We will present the underlying physics of a TWA section, details of the resonant ring feeding, will show the wave patterns from antenna, through the evanescent layer, up to the bulk plasma. The proposed system is totally load resilient and is characterized by a very selective spectrum for the toroidal wavenumbers k//. Coupling of the power scales as the ratio of the total number of straps divided by the distance between the straps, for a given geometry and applied voltage. |
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BP11.00091: Interaction between high-power ICRF waves and drift-wave turbulence in LAPD Troy Carter, Bart G.P. Van Compernolle, Rory James Perkins, Patrick Pribyl An experimental campaign on the physics of ICRF waves has recently begun using the Large [1] Martin, et al., PRL 119, 205002 (2017)
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BP11.00092: Vorpal Modeling of RF Plasma Sheaths in Fusion Devices Thomas G Jenkins, David N Smithe The development of robust radiofrequency (RF) actuators for plasma heating and current drive will be critical in sustaining steady-state operation of future magnetic fusion devices. As part of this development, we explore via numerical simulation how applied RF power interacts with plasma sheaths which form near antenna hardware and other plasma-facing material surfaces. These interactions are modeled using Vorpal, a high-performance PIC/finite-difference time-domain code. Efforts include (a) PIC modeling of sheath rectification in time-varying RF sheaths near antenna surfaces, and the benchmarking of these results with other RF-SciDAC codes; (b) comparison of experimentally measured RF sheath electric fields with Vorpal models; and (c) implementation of a capacitive/resistive sheath model [Kohno & Myra, CPC 220, 129 (2017)] as a sub-grid boundary condition for macroscopic-scale time-domain modeling of RF wave propagation in a tokamak. Ongoing progress in these areas will be presented. Eventual couplings of Vorpal with materials, turbulence, and transport codes are anticipated as part of ongoing RF-SciDAC work; future efforts in these areas will also be discussed. |
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BP11.00093: Hybrid iterative approach for simulation of radio-frequency fields in plasma Vladimir Svidzinski, Jin-Soo Kim, Liangji Zhao, Sergei A Galkin, Joseph A. Spencer A novel iterative approach for solving discretized linear wave equations in frequency domain, which combines time evolution with iterative relaxation schemes, is presented. In this hybrid approach each iteration cycle consists of evolution of the electromagnetic (EM) fields in time over specified number of field periods followed by several iterative relaxations. Provided that there is sufficient dissipation, both the time evolution and the iterative relaxations contribute to the convergence of the EM fields to the solution of the formulated full wave boundary value problem. Time evolution rapidly distributes EM fields, propagating with group velocity, over simulation domain, while the iterative relaxations smooth the fields reducing the numerical errors such that iteration cycles converge to a steady state solution approximating the solution of the formulated problem. This approach is intended for large scale simulations which are beyond the capabilities of direct solvers presently used for solving wave equations in frequency domain. Formulation of the hybrid iterative approach and its application to cold plasma modeling of RF beam in ECR frequency range for realistic tokamak parameters, will be presented. |
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BP11.00094: Application of hybrid iterative approach for full wave modeling of hot tokamak plasma Liangji Zhao, Vladimir Svidzinski, Jin-Soo Kim, Joseph A. Spencer Hybrid iterative approach for solving linear wave equations with nonlocal hot plasma dielectric response in frequency domain in configuration space is being developed. It combines time evolution of the electromagnetic fields and iterative relaxation schemes into iteration cycles. Provided sufficient dissipation of RF power in simulation domain, the iteration cycles converge to solution of full wave boundary value problem. The approach is tested for modeling RF beam propagation and absorption in tokamaks in ECR frequency range. The nonlocal hot plasma dielectric response is formulated by calculating the plasma conductivity kernel based on the solution of the linearized Vlasov equation in inhomogeneous magnetic field. In this scenario the hot plasma dielectric response is mostly limited to the distance of a few particles' Larmor radii, near the magnetic field line passing through the test point, which significantly reduces the size of the problem. The details of the hybrid iterative algorithm will be presented along with the initial results of simulations of RF waves in a realistic hot tokamak plasma. |
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BP11.00095: Reduced modelling of the X-B mode-conversion for RF heating and current drive applications Nicolas A Lopez, Abhay K Ram, Ilya Y Dodin Mode-conversion of vacuum-launched electromagnetic waves to the electrostatic electron Bernstein wave (EBW) has gained renewed interest due to an increasing need to non-inductively drive current & heat overdense plasmas (fpe > fce). One such method is the direct X-B conversion, where a vacuum-launched fast X-mode converts to the EBW via evanescent mode-coupling to the slow X-mode as an intermediary step. The efficiency of this process is typically computed with full-wave methods. However, these methods can be troublesome, as any spurious reflections due to a finite computational domain will cause non-physical interference, which affects the conversion efficiency. To avoid this issue, we re-formulate the X-B conversion problem as a boundary-value problem on a finite domain. Reflections off the domain edge are then exactly accounted for. This model is studied analytically in one dimension (1-D) and is shown to agree with the main features of previous 1-D models of the X-B conversion. The model is then used computationally to study the effect of density fluctuations on the 1-D X-B conversion efficiency. Finally, recent theoretical results suggest the possibility of incorporating mode-conversion into a ray-based code, which may help model the X-B conversion more efficiently. |
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BP11.00096: Review and perspectives of helicon waves and negative ion studies in the basic plasma physics device RAID Ivo Furno, Riccardo Agnello, Alan Arthur Howling, Remy Jacquier, Gennady Plyushchev, Philippe Guittienne, Marco Barbisan, Roberto Pasqualotto, Stephane Béchu, Iaroslav Morgal, Alain Simonin The Resonant Antenna Ion Device (RAID) is a basic plasma physics device located at the SPC-EPFL in Lausanne. In RAID, a novel radio frequency helicon plasma source, based on a resonant network antenna delivering up to 10 kW power at 13.56 MHz, is presently under study as a negative ion source for neutral beam applications for fusion. RAID is equipped with an extensive set of diagnostics, including laser photo detachment, cavity ring down spectroscopy and optical emission spectroscopy for negative ion characterization, movable Langmuir probes and interferometry for plasma profiles, as well as magnetic probes for helicon wave studies. We review recent advances in the understanding of helicon wave physics and negative hydrogen/deuterium production by volumetric processes and outline future research towards the use of resonant antennas for neutral beams. |
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BP11.00097: IShTAR plasma operation and diagnostics Kristel Crombe, Ana Kostic, Anton Nikiforov, Roman Ochoukov, Ilya Shesterikov, Mari Usoltceva, Helmut Faugel, Helmut Fuenfgelder, Jean-Marie Noterdaeme, Stephane Heuraux IShTAR (Ion cyclotron Sheath Test ARrangement) is a linear magnetized plasma test facility for RF sheaths studies. The device consists of a cylindrical vacuum vessel with a diameter of 1 m and length of 1.1 m. The plasma is created by an external cylindrical plasma source equipped with a helical antenna. Progress is reported on the plasma optimization and electric field measurements. An interferometer and probes have recently been added to the IShTAR suite of diagnostics. A sensitivity of the electron density to the magnetic topology has been observed, possibly related to the helicon wave propagation. E-field measurements by Passive Optical Emission Spectroscopy were achieved at two viewing locations: a DC biased electrode and the ICRF antenna. The installation of an active Doppler-Free Saturation Spectroscopy diagnostic has started. |
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BP11.00098: Kinetic full wave analysis of electromagnetic waves in plasmas using integral operator form of dielectric tensor Atsushi Fukuyama In order to describe kinetic response of plasmas to electromagnetic waves, dielectric tensor is formulated as an integral operator without introducing wave number. In magnetized plasmas, the response due to the particle motion along the magnetic field line and the cyclotron motion are expressed separately. This formulation is implemented in two-dimensional full wave analyses using the finite element method, and applied to the analyses of plasma production and heating in various configurations, e.g. Landau damping of helicon waves, cyclotron damping in a magnetic mirror, and mode conversion to Bernstein waves. |
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BP11.00099: Nonlinear wave coupling within the ion cyclotron emission spectrum from fusion-born protons in KSTAR tokamak plasmas Benjamin Chapman, Richard Dendy, Sandra Chapman, Gunsu Yun, Minho Kim, Ken McClements, Shekar Thatipamula During ELM crashes in D plasmas in the KSTAR tokamak, the RF emission includes sharp spectral peaks in the frequency range up to ~900MHz. Peaks below ~500MHz that correspond to proton cyclotron harmonics at the outer midplane edge are explained as ion cyclotron emission (ICE) driven by the magnetoacoustic cyclotron instability (MCI) of 3MeV protons born in DD fusion reactions, lying on deeply passing drift orbits. The MCI drives waves on the fast Alfvén-cyclotron harmonic branch, observed as ICE at the antenna. The proton ICE features last a few μs, and exhibit frequency chirping due to rapid changes in the plasma density. Some ICE features below ~500MHz are accompanied, after < 1μs, by a detached “ghost” chirping feature in the range 500-900MHz. This exceeds the local lower hybrid frequency; hence cold plasma waves should be evanescent here. We show that the “ghost” chirping ICE feature is driven by strong nonlinear wave coupling between different spectral peaks within the ICE feature below ~500MHz. This follows from bicoherence analysis of: KSTAR data files for ICE field magnitudes; and the fields generated from a particle-in-cell code, which solves the Maxwell-Lorentz system of equations for the full gyro-orbit kinetics of the plasma particles and their self-consistent fields. |
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BP11.00100: Plasma Fueling, Heating, and Neutrality Control Using Magnetic Plasma Expulsion Angelica Martinez, Ryan E Phillips, Carlos A Ordonez Plasma fueling, heating, and neutrality control using particle sources located inside of a magnetically confined plasma is simulated by computer. Magnetic plasma expulsion [R. E. Phillips, C. A. Ordonez, Phys. Plasmas 25 (2018) 012508] is used to keep the magnetically confined plasma away from the particle sources. The research aims to show how particles can be injected into a plasma using particle sources located directly between two current-carrying wires that create the magnetic expulsion field. Three different cases are considered. (1) Plasma fueling. For this case, the average energy of injected particles is the same as the average energy of plasma particles, so that no heating occurs. Also, positive and negative particles are injected simultaneously at equal rates, so that no change in neutrality occurs. (2) Plasma heating. This case is different from the first in that the average energy of injected particles is larger than the average energy of plasma particles. (3) Plasma neutrality control. This case is different from the first in that positive and negative particles are injected at different rates. |
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BP11.00101: Neural-Network Version of NUBEAM for Real-time Control and Scenario Optimization in DIII-D. Shira Morosohk, Dan Boyer, Eugenio Schuster NUBEAM is a code developed to simulate with high accuracy the effects of neutral beams on the plasma. However, the code is computationally demanding due to its Monte Carlo nature. This makes it impractical for use in real-time control or in applications where a large number of simulation runs are required such as in model-based scenario optimization. Recently, a neural network model (NubeamNet) has been developed to closely approximate NUBEAM results for NSTX-U at drastically reduced run time [1]. In this work, a database of NUBEAM runs has been developed for DIII-D and used to train a neural network for model-based control and optimization applications. The trained neural network can match the NUBEAM data with high accuracy, and requires only microseconds to compute the outputs for a given set of inputs, making it a powerful tool for use in real-time control or in model-based scenario planning where a large number of simulation runs are required by the optimization algorithm. Simulation results illustrate the potential of the trained neural-network version of NUBEAM. [1] M.D. Boyer et al., “Real-time capable neural network approximation of NUBEAM for use in the NSTX-U control system,” 45th EPS Conference on Plasma Physics, 2018. |
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BP11.00102: Assessment of Performance of Burn Control Schemes under Confinement Mode Transitions in ITER Vincent Graber, Eugenio Schuster A nonlinear simulation study has been carried out to identify challenges associated with burn control during the entry and exit of the burning phase in ITER. The objective of a burn control scheme is to regulate the amount of fusion power produced by the burning plasma using fueling and heating actuators while avoiding thermal instabilities. The model nonlinearities and uncertainties of the multi-variable plasma system makes burn control design extremely challenging. To add to the challenge, burn control strategies for ITER need to effectively maintain desired operating points in spite of sudden changes in energy confinement after entering or exiting the burning phase. In ITER, the transition to the H-mode is synonymous with the entry to the burning phase for a 50/50 DT fuel mixture, since the higher energy confinement and density of the H-mode significantly increases the fusion reactivity. A simulation study is presented to illustrate the performance of present nonlinear feedback controllers during the entry and exit of the burning phase in ITER as well as to identify critical control challenges that need to be addressed during these transitions. |
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BP11.00103: Neutral Beam Power System for Fast Grid Modulation Timothy Ziemba, Alex Henson, Kenneth E Miller, James R Prager Neutral beam injection (NBI) is an important tool for plasma heating, current drive and a diagnostic at fusion science experiments around the United States. Producing power systems that can respond rapidly (10 µs) at high voltage (10 to 100 kV) for beam acceleration or high currents (~ 1 to 5 kA) for control of the ion source is non-trivial. Currently, there are no vendors in the United States for NBI power systems. Eagle Harbor Technologies (EHT), Inc. is developing new solid-state switching power systems for NBI that takes advantage of the latest developments in solid-state switching. EHT has developed a series resonant converter that can be scaled to the power required for NBI at small-scale validation platform experiments. This power system can modulate the injection beam current during a plasma shot, which can lead to improved control over the plasma. Additionally, these modern solid-state supplies can be made smaller and lower cost that previous generations of NBI power systems. We will present the Phase I testing results, including demonstration of current modulation on a neutral beam system. |
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BP11.00104: Recent updates of the energetic particle gyro-Landau fluid model Donald Spong, Jacobo Varela, Luis Garcia Reduced dimensionality physics models provide useful tools for parameter surveys, profile optimization studies, and long-term nonlinear simulations. The FAR3D model uses Landau fluid closures coupled with finite Larmor radius and drift orbit effects to excite and analyze fast ion resonant Alfvén instabilities in both tokamaks and stellarators. Recently, a number of improvements have been made to this model: extension to up-down asymmetric equilibria, higher order closures (allowing simulation of non-Maxwellian components), improved accuracy of FLR effects, multiple fast ion components, and global eigenmode solver options. These improvements significantly extend the applicability of such a model. The techniques used will be described and examples given of the application of these new capabilities for both tokamaks and stellarators. |
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BP11.00105: Status and plans of neutral particle analyzers as fast ion diagnostics for HL-2A/M Linge Zang, Xiaobing Luo, Yi Liu, Qingwei Yang, Zhongbing Shi A multi-spatial channel compact pinhole NPA (CP-NPA) has been developed for HL-2A tokamak. CP-NPA has 11 spatial channels, with 2.2° view angle for each channel. CP-NPA will be maintained and calibrated using an energetic neutral source. This neutral source is based on a small ion accelerator system. The source has the following features: Source Particle: H or/and D; Energy range: 0.5keV-70keV; Beam current: ~ 20μA; Small energy dispersion; Beam spot: < 5mm2. To calibrate different channels of CP-NPA, we will side lay the CP-NPA and rotate it on a platform. Axis of rotation is at the center line of the slit. In this way, with a single neutral beam, all the detectors could be calibrated. Diamond detectors have been used for NPA on some devices such as LHD, and will be built for HL-2A/M. Diamond detector has many important advantages to be used as a CX neutral particle spectrometer, for example, very compact size, high energy resolution, and high radiation hardness etc. A new NPA capable of mass resolution and broader energy resolution is planned to be built. The analyzer will be the combination of a permanent magnet and a condenser, to resolve both momentum and energy. |
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BP11.00106: Nonlinear Particle-in-Cell simulation of Toroidal Alfven Eigenmodes with gyrokinetic ions and drift-kinetic electrons Yang Chen, Lei Ye, Guo-Yong Fu, Scott Edward Parker The GEM code is used to study the nonlinear evolution of reverse shear Alfven eigenmodes (RSAE) driven by energetic particles. Recent simulations with fluid electrons [Y.~Chen et. al. Physics of Plasmas 25, 032304 (2018)] show that zonal structures are force generated, and reduce the saturation amplitude. The reduction is not caused by the zonal flow shearing of the RSAE, but by the force-generated n=0 component in the thermal ion distribution function and the electron density. These n=0 perturbations lead to nonlinear evolution of the RSAE mode structure and enhance damping. Here we continue to investigate this problem with kinetic electrons. Although the problem, in essence, involves a single-n RSAE excited by resonant beam particles, we find it necessary to include harmonics of the unstable RSAE in PIC simulations. The reason is, parallel electron current of high-n harmonics is nonlinearly generated and causes Joule heating and damping of RSAE, if the high-n EM fields are removed. Low-n modes are difficult with kinetic electrons. To verify simulation results, we have implemented Mishchenko's mixed variable method with pull-back transformations, and will compare it with the split-weight scheme. |
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BP11.00107: Machine learning driven correlation studies: Alfvénic and sub-Alfvénic frequency chirping at NSTX Benjamin Woods, Vinicius Duarte, Eric Donald Fredrickson, Nikolai Gorelenkov, Mario L. Podesta Magnetic perturbations in a very broadband range (<30 kHz to >1 GHz) are commonly measured on tokamaks such as NSTX by using Mirnov coils. The spectral behaviour can be categorised as quiescent, fixed-frequency, chirping, or avalanching. Here, ‘chirping’ modes experience a time-dependent frequency shift due to non-linear effects – in some cases, multiple plasma modes chirp in a near-concurrent fashion (mode 'avalanching'). |
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BP11.00108: Collisional scattering effects on the quasilinear dynamics of Alfvénic eigenmodes V. Duarte, Nikolai Gorelenkov The interaction between fast ions and Alfvénic eigenmodes has proved to be numerically expensive to be modeled in realistic tokamak configurations. Therefore it can be convenient to exploit reduced modeling for simulations where the simultaneous excitation of multiple unstable Alfvénic instabilities is expected. The Resonance Broadening Quasilinear (RBQ) code, which is capable of modeling the fast ion distribution function while self-consistently evolving the amplitude of modes, is presently under development. RBQ is interfaced with linear ideal/kinetic codes, NOVA/NOVA-K, which provide eigenstructures, damping rates and wave-particle interaction matrices for resonances in the constant of motion space. We report recent progress on the development and verification of the diffusion solver used in the RBQ code by comparing numerical outputs with known analytical solutions. Both the wave-particle resonant diffusion and Coulomb collisional scattering diffusion operator are thoroughly verified. In addition, we report detailed analyses of the collisional scattering frequency dependence of the modes saturation level. |
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BP11.00109: Experimental measurements of Toroidal Alfvén Eigenmodes (TAEs) on JET with the AEAD system and simulations with the Gyrokinetic Toroidal Code (GTC). Nicolas Fil, Miklos Porkolab, Valentin Aslanyan, Paulo Puglia, Sergei Sharapov, Stuart Dowson, Haroon Sheikh, Sam Taimourzadeh, Lei Shi, Zhihong Lin, Patrick Blanchard, Ambrogio Fasoli, Duccio Testa, Joelle Mailloux, Maximos Tsalas, Mikhail Maslov, Adrian Whitehead, Rory Scannell, Sergei Gerasimov, Simon Dorling, Trevor Blackman, Krassimir Kirov, Remi Jean Dumont, Ge Dong, JET Contributors The resonant detection and measurement of the damping rates of Alfvén Eigenmodes (AEs) is of critical importance to the design of experiments and development of models of AE stability [1]. We performed experimental measurements on JET with the AE Active Diagnostic (AEAD), and theoretical modelling using state of the art MHD and gyrokinetic codes. The AEAD has undergone a major upgrade [2]. It can provide a state of the art excitation and real-time detection system thanks to its new amplifiers, filters, digital control system and to the newly installed magnetic probes. Weakly-damped AEs have been resonantly probed with external antennas. With GTC [3] we have simulated both stable and unstable AEs by using equilibria and diagnostic data from JET pulses dedicated to TAEs studies. Good agreement was obtained between simulations and experiments which adds confidence to further predictions for next-step burning plasma experiments, including JET and ITER.
[1] W.W.Heidbrink, Phys. Plasmas 15, 055501 (2008) [2] P.Puglia et al., Nucl. Fusion 56, 112020 (2016) [3] Z.Wang et al., Phys. Rev. Lett. 111, 145003 (2013) |
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