Bulletin of the American Physical Society
57th Annual Meeting of the APS Division of Plasma Physics
Volume 60, Number 19
Monday–Friday, November 16–20, 2015; Savannah, Georgia
Session TP12: Poster Session VII (ICF, Mathematical and Simulation Methods, Basic Theory, DIII-D II, Boundary and Plasma-Material Interactions) |
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Room: Exhibit Hall A |
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TP12.00001: ICF |
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TP12.00002: Tailored density profile of heterogeneous underdense medium of a multi-foil assembly for multi-keV x-ray sources optimization Michel Primout, Daniele Babonneau, Laurent Videau, Laurent Jacquet We studied multi-keV x-ray source made of titanium foils assembly. The purpose of this heterogeneous structure is to create a medium with the same hydroradiative properties as an efficient -but yet non existing- pure metallic-like underdense homogeneous medium. We can mimic the multi-keV x-ray emission of an equivalent underdense medium of any density between 5 and 40 $mg/cc$. For both cases, the highest multi-keV x-ray conversion efficiency has been found at density around 20 mg/cc. This optimum is best realized by assembly of a set of 0.1 $\mu m$ titanium foils separated by 20 $\mu m$ of vacuum. Note that the concept can be easily extended to higher Z materials like iron, copper or germanium at higher x-ray emission energy. This approach allows us to build any non uniform homogeneous underdense medium with tailored density profiles : increasing or decreasing ones, both longitudinally and transversally to the laser incident direction. This is a very promising method provided that we can design any foils assembly with thickness as low as 0.1 $\mu m$, what has been proved feasible in recent studies of the reference [Shao-yong Tu et al in PoP, 21, 043107, 2014]. Each configuration has been simulated by the 2D rad-hydro code FCI2 with Arbitrary Eulerian-Lagrangian rezoning option. [Preview Abstract] |
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TP12.00003: Understanding Bright 13 keV Kr K-shell X-ray Sources at the NIF M.J. May, J.D. Colvin, G.E. Kemp, K.B. Fournier, H. Scott, M. Patel, Widmann Barrios, K. Widmann High x-ray conversion efficiency (CE) K-shell Kr sources are being investigated for High Energy Density experiments. These sources are 4.1 mm in diameter 4.4 mm tall hollow epoxy tubes having a 40 $\mu$m thick wall holding either 1.2 or 1.5 atm of Kr gas. The CE of K-shell Kr is dependent upon the peak electron temperature in the radiating plasma. In the NIF experiments, the available energy heats the source to T$_{e}$ = 6-7 keV, well below the temperature of T$_{e}$ $\sim$25 keV needed to optimize the Kr CE. The CE is a steep function of the peak electron temperature. A spatially averaged electron temperature can be estimated from measured He($\alpha$) and Ly($\alpha$) line ratios. Some disagreement has been observed in the simulated and measured line ratios for some of these K-shell sources. Disagreements have been observed between the simulated and measured line ratios for some of these K-shell sources. To help understand this issue, Kr gas pipes have been shot with 3$\omega$ light at ?750 kJ at $\sim$210, $\sim$140 TW and $\sim$120 TW power levels with 3.7, 5.2 and 6.7 ns pulses, respectively. The power and pulse length scaling of the measured CE and K-shell line ratios and their comparison to simulations will be discussed. This work was performed under the auspic [Preview Abstract] |
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TP12.00004: Study of Direct-Drive-targets stability by varying pulse shapes, filling pressure and ablator thickness on the OMEGA laser facility Frederic Girard, Veronique Tassin, Franck Philippe, Charles Reverdin, Olivier Landoas, Tony Caillaud, Stepahen Laffite, Guillaume Legay, Tomline Michel, Wolf Seka, Christian Stoeckl, Vladimir Glebov, Fred Marshall In the context of imploding capsules studies, X-ray heating from laser irradiated hohlraum produces implosions showing that the understanding via simulations of the hydrodynamics (bang times) and performances ( neutron yields) are difficult because the discrepancies with experimental results are important. In the experiments presented here, we studied targets stability in the Direct-Drive heating scheme by varying pulse shapes, filling pressure, ablator thickness and laser energy (non-uniform irradiation). The main objectives have been to get bang-times and neutron yields measurements, neutron images of the imploding core on the OMEGA laser facility. X-ray images have been obtained for the first time on the same axis and lead to direct comparison of the size and location of the X-ray and neutron hot spots. Precise measurements of bang times, X-ray spectra and soft X-ray images from temporally gated imaging systems give large set of data to validate simulations and assess how predictable are the different experimental configurations of implosion. [Preview Abstract] |
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TP12.00005: Development of short pulse laser driven micro-hohlraums as a source of EUV radiation Karl Krushelnick, Thomas Batson, Andrew McKelvey, Anthony Raymond, Alec Thomas, Victor Yanovsky, John Nees, Anatoly Maksimchuk Experiments at large scale laser facilities such as NIF allow the radiativ properties of dens, high-temperature matter to be studied at previously unreachable regime, but are limited by cost and system availability. A scaled system using a short laser pulses and delivering energy to much smaller hohlraum could be capable of reaching comparable energy densities by depositing the energy in a much smaller volume before ablation of the wall material closes the cavit. The laser is tightl focused through the cavity and then expands to illuminate the wall. Experiments were performe using the Hercules Ti:Sapphire laser system at Michiga. Targets include cavities machined in bulk material using low laser power, and then shot in situ with a single full power pulse as well as micron scale pre-fabricate target. Spectral characteristics were measured using a soft X-ray spectromete, K-alpha x-ray imaging system and a filtered photo cathode array. Scalings of the radiation temperature were made for variations in the hohlraum cavit, the pulse duration as well as the focusing conditions. Proof of principle time resolved absorption spectroscopy experiments were also performe. These sources may allow opacity and atomic physics measurements with plasma an radiation temperatures comparable to much larger hohlraums, but with much higher repetition rate and in a university scale laboratory. [Preview Abstract] |
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TP12.00006: Controlling Laser-Plasma Instabilities and CBET Using STUD Pulses in the Strong Coupling Regime for Direct and Indirect Drive ICF Bedros Afeyan, N. Meezan, S. MacLaren, J. Hammer, D. Montgomery, J. Heebner We will show theoretical results on the behavior of SBS in the strong damping regime and CBET in mid-Z plasmas (around 20) where ion Landau damping and collisional damping are both higher order effects and strong coupling is dominant in laser hot spots and near Mach -1 surfaces in appropriately tuned pairs of crossing beams. The spatially dependent frequency shits that ensue and the reductions in growth rate allow the control of LPI even downstream beyond the crossing volumes. Multiple successive crossings between O(100) beams can be used to change the space-time intensity distributions of lasers used entirely differently in direct and indirect drive geometries. In the former case, due to the existence of many angles, a statistical Sqrt(N) gain is expected. with randomly phased beams via STUD pulses. On the other hand, for indirect drive, with 2-4 cone angles to contend with, turning off interactions by staggering crossing beam spikes, achieved with STUD pulses, is a key deterministic element for the success of the plan. Changing the speckle statistics at will and with fine control is a grand challenge of this set of techniques. [Preview Abstract] |
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TP12.00007: Performance of Variable Duration STUD Pulses with Fixed Peal Intensity and their Compliments Stefan H\"uller, Bedros Afeyan The simplest approach to STUD pulse implementation, given the requisite bandwidth of the laser is to keep the peak spike intensities fixed while modulating the lasers on and off on a 1-10 ps time scale. To what extent spatial scrambling is required in this case is compared to cases where the peak spike intensity varies with the duty cycle at fixed pulse width, to preserve the energy of the overall laser pulse. We compare RPP/CPP, SSD and STUD pulses at fixed energy with both variable pulse width and fixed peak intensity configurations and vice versa. This allows us to highlight the effects of speckle statistics, memory accumulation and pump depletion in setting gain saturation levels from the ideal democratized, incoherent sums of small growth spurts equally from all regions of the plasma, vs localized and highly nonlinear growth and re-amplification due to the unchanging or much too slowly changing nature of the illumination strategy, such as RPP/CPP or SSD. [Preview Abstract] |
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TP12.00008: PIC Simulation of Laser Plasma Interactions with Temporal Bandwidths Frank Tsung, J. Weaver, R. Lehmberg We are performing particle-in-cell simulations using the code OSIRIS to study the effects of laser plasma interactions in the presence of temperal bandwidths under conditions relevant to current and future shock ignition experiments on the NIKE laser. Our simulations show that, for sufficiently large bandwidth, the saturation level, and the distribution of hot electrons, can be effected by the addition of temporal bandwidths (which can be accomplished in experiments using smoothing techniques such as SSD or ISI). We will show that temporal bandwidth along play an important role in the control of LPI's in these lasers and discuss future directions. [Preview Abstract] |
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TP12.00009: Progress on CBET Platform at the Nike Laser J.L. Weaver, P. McKenty, J. Oh, D. Kehne, A.J. Schmitt, S. Obenschain, V. Serlin, R. Lehmberg, F. Tsung Cross-beam energy transport (CBET) studies are underway at the Nike krypton-fluoride (KrF) laser at NRL. This facility has unique characteristics that provide an excellent platform for CBET work - including short wavelength (248 nm), large bandwidth (1-3 THz), beam smoothing by induced spatial incoherence (ISI), and full aperture focal spot zooming. Nike's two beam arrays are widely separated (135$^{\circ}$ in azimuth) which facilitates CBET studies in a nearly opposing geometry, relevant to Polar Direct Drive implosions. Various target types are planned: planar slabs, cylindrical and spherical shells, and low-density targets. The solid targets will be used to examine gradient geometries and the latter will access larger volume, more uniform plasmas. The initial campaign is exploring changes observed by scattered light diagnostics for both beam arrays as the probe laser spectrum is modified. [Preview Abstract] |
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TP12.00010: Mitigating the hosing instability in relativistic laser-plasma interactions Luke Ceurvorst, Naren Ratan, Matthew Levy, Muhammad Kasim, James Sadler, Robbie Scott, Raoul Trines, Taiwu Huang, Marija Skramic, Marija Vranic, Luis Silva, Peter Norreys A new physical model of the hosing instability that includes relativistic laser pulses and moderate densities is presented and derives the density dependence of the hosing equation. This is tested against two dimensional particle-in-cell simulations. These simulations further examine the feasibility of using multiple pulses to mitigate the hosing instability in a Nd:glass-type parameter space. An examination of the effects of planar versus cylindrical exponential density gradients on the hosing instability is also presented. The results show that strongly relativistic pulses and more planar geometries are capable of mitigating the hosing instability which is in line with the predictions of the physical model. [Preview Abstract] |
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TP12.00011: The Marble Experiment: Overview and Simulations M.R. Douglas, T.J. Murphy, J.A. Cobble, J.R. Fincke, B.M. Haines, C.E. Hamilton, M.N. Lee, J.A. Oertel, R.E. Olson, R.B. Randolph, D.W. Schmidt, R.C. Shah, J.M. Smidt, I.L. Tregillis The Marble ICF platform has recently been launched on both OMEGA and NIF with the goal to investigate the influence of heterogeneous mix on fusion burn. The unique separated reactant capsule design consists of an ``engineered'' CH capsule filled with deuterated plastic foam that contains pores or voids that are filled with tritium gas. Initially the deuterium and tritium are separated, but as the implosion proceeds, the D and T mix, producing a DT signature. The results of these experiments will be used to inform a probability density function (PDF) burn modelling approach for un-resolved cell morphology. Initial targets for platform development have consisted of either fine-pore foams or gas mixtures, with the goal to field the engineered foams in 2016. An overview of the Marble experimental campaign will be presented and simulations will be discussed. [Preview Abstract] |
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TP12.00012: Results from and Plans for the Development of the MARBLE Platform for Studying Thermonuclear Burn in the Presence of Heterogeneous Mix on OMEGA and the National Ignition Facility T.J. Murphy, M.R. Douglas, J.R. Fincke, J.A. Cobble, B.M. Haines, C.E. Hamilton, M.N. Lee, J.A. Oertel, R.E. Olson, R.B. Randolph, D.W. Schmidt, R.C. Shah, J.M. Smidt, I.L. Tregillis Work is underway to develop the MARBLE ICF platform for use on OMEGA and NIF in experiments to quantify the influence of heterogeneous mix on fusion burn. This platform consists of a plastic (CH) capsule filled with a deuterated plastic foam (CD) with a density of a few tens of milligrams per cubic centimeter, with tritium gas filling the voids in the foam. In order to affect the morphology of the mix, engineered foams with voids of diameter up to 100 microns will be utilized. The degree of mix will be determined from the ratio of DT to DD neutron yield. Experiments have been performed on OMEGA and are planned for NIF to develop techniques and verify that with uniform fine-pore foam, these implosions behave like atomically mixed plastic and gas. Results will be reviewed and future experiments discussed. [Preview Abstract] |
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TP12.00013: Linear stability analysis of the Noh expanding-shock solution M. Murakami, A.L. Velikovich, J.L. Giuliani, B.D. Taylor, S.T. Zalesak, Y. Iwamoto The self-similar one-dimensional (1D) solution of the Noh problem has been used for verification of every code designed to model implosions, explosions and shock waves. The long experience of successful verification of two- and three-dimensional (2D and 3D) hydrocodes against the 1D Noh solution is an implicit confirmation of its hydrodynamic stability. Still, as far as we know, stability analysis of the Noh solution has never been done. Here, such analysis is reported for spherical and cylindrical geometry assuming small-amplitude perturbations. In either case stability of the Noh solution has been demonstrated, all initial perturbations exhibiting a power-law, oscillatory or monotonic, decay with time. The dispersion equation determining the complex eigenvalues of the problem, i. e. the power indices characteristic of this decay, has been derived. Its numerical solution is presented, and the particular and limiting cases when the eigenvalues can be calculated analytically are outlined. Explicit formulas for the eigenfunction profiles corresponding to these eigenvalues are presented. The opportunities of using these new exact solutions for verification of hydrocodes in 2D and 3D are discussed. [Preview Abstract] |
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TP12.00014: Novel, high-pressure instability experiments using imploding cylindrical liners with liquid deuterium fill Patrick Knapp, Matthew Martin, Ryan McBride, Daniel Sinars, Thomas Mattsson We present preliminary results from experiments where a liquid deuterium filled cylindrical liner is imploded onto a perturbed beryllium rod. The liner implosion creates a shock in the deuterium that strikes the interface twice: once as it implodes, and once again after the shock reflects off of the axis. This causes the perturbation to grow due to the Richtmeyer-Meshkov instability and the Rayleigh-Taylor instability while also generating significant vorticity as the shocks cross the interface. In the initial experiments growth of the perturbation is observed after 1$^{\mathrm{st}}$ shock, however, after reshock significant three-dimensional structure is observed at scale lengths much smaller than the initial perturbation. At this time, very little evidence of the seeded mode remains. Pressures exceeding 100 Mbar are predicted at stagnation with an Atwood number at the unstable interface of about 1/3. Analysis of the images will be presented. Additionally, future plans will be discussed. Emphasis in the near future will be on improving image contrast and data collection. [Preview Abstract] |
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TP12.00015: Measurements of Laser Imprint with High-Z Coated targets on Omega EP Max Karasik, J. Oh, C. Stoeckl, Y. Aglitskiy, A. J. Schmitt, J. W. Bates, S. P. Obenschain Previous experiments on Nike KrF laser $(\lambda=248~nm)$ at NRL found that a thin (400--800$\AA$) high-Z (Au or Pd) overcoat on the laser side of the target is effective in suppressing broadband imprint[Obenschain et al., PoP 9, 2234 (2002); Karasik et al.,PRL 114, 085001 (2015)] and reducing ablative Richtmyer-Meshkov growth [ http://meetings.aps.org/link/BAPS.2008.DPP.CO5.9 ]. The overcoat initially absorbs the laser and emits soft x-rays that ablate the target, forming a large stand-off distance between laser absorption and ablation and driving the target at higher mass ablation rate. Implementation of this technique on the frequency-tripled Nd:glass (351~nm) NIF would enable a wider range direct drive experiments there. To this end, we are carrying out experiments using the NIF-like beams of Omega EP. Analogous to experiments on Nike, areal mass perturbations due to RT-amplified laser imprint are measured using curved crystal imaging coupled to a streak camera. High-Z coating dynamics and target trajectory are imaged side-on. First results indicate that imprint suppression is observed, albeit with thicker coatings. Work supported by the Department of Energy/NNSA. [Preview Abstract] |
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TP12.00016: On the numerical simulation of the ablative Rayleigh-Taylor instability in laser-driven ICF targets using the FastRad3D code Jason Bates, Andrew Schmitt, Steve Zalesak The ablative Rayleigh-Taylor (RT) instability is a key factor in the performance of directly-drive inertial-confinement-fusion (ICF) targets. Although this subject has been studied for quite some time, the accurate simulation of the ablative RT instability has proven to be a challenging task for many radiation hydrodynamics codes, particularly when it comes to capturing the ablatively-stabilized region of the linear dispersion spectrum and modeling ab initio perturbations. In this poster, we present results from recent two-dimensional numerical simulations of the ablative RT instability that were performed using the Eulerian code FastRad3D at the U.S. Naval Research Laboratory. We consider both planar and spherical geometries, low and moderate-Z target materials, different laser wavelengths and where possible, compare our findings with experiment data, linearized theory and/or results from other radiation hydrodynamics codes. Overall, we find that FastRad3D is capable of simulating the ablative RT instability quite accurately, although some uncertainties/discrepancies persist. We discuss these issues, as well as some of the numerical challenges associated with modeling this class of problems. [Preview Abstract] |
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TP12.00017: Controlling Laser-Driven Hohlraums-Clues from Experiments with Earlier Lasers William Kruer, Cliff Thomas Better characterized and controlled hohlraums are very important for both implosion and science experiments on NIF. A brief review of some hohlraum and related experiments with earlier lasers is given to search for lessons learned and clues for better understanding NIF hohlraums. For example, surprises associated with heat transport inhibition and improved models for radiation generation have been a recurring theme in indirect drive experiments. In Shiva experiments, the hohlraum filling with plasma with density near quarter-critical was only calculated after inhibited heat transport and improved radiation models were adopted in the design code [1]. Early NIF experiments also led to a change in the heat transport and radiation models. In this case, the heat transport model was changed from one with modest inhibition (which had been used to model Nova experiments) to near classical transport [2]. Most recently, a design model invoking very inhibited transport (at various times and locations) has been proposed by C. Thomas for NIF hohlraums. Other recurring themes will also be discussed. \\[4pt] [1] W. C. Mead (private communication)\\[0pt] [2] M. Rosen, et. al., High Energy Density Physics 7, 180 (2011) [Preview Abstract] |
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TP12.00018: Investigation of beam non-uniformity after cross-beam energy transfer in a gas filled hohlraum Louisa Pickworth, M.B. Schneider, D.E. Hinkel, M.D. Rosen, D.A. Callahan, P.A. Michel, A.S. Moore, J.D. Moody Control of hotspot symmetry in an ignition capsule imploded by the x-ray drive in a high gas-filled cylindrical hohlraum at the NIF currently requires cross-beam energy transfer (CBET) from the outer beams to the inner beams.\footnote{P. Michel et al., PoP 16, 042702 (2009)} CBET occurs in the central region of the laser entrance hole (LEH) where the laser beams overlap. Linear gain models applied to individual rays indicate that CBET is not uniform across the beam profile, producing a non-uniform spatial distribution on the beams that varies in time. This changing spatial distribution could introduce asymmetries in the x-ray drive applied to the ignition capsule and should be quantified. We are investigating the effects of CBET using the Quartraum experimental platform. This platform uses an LEH-only target designed to isolate the effect of CBET on the spatial-intensity distribution of the inner beams by minimizing the effect of absorption and backscatter. A time resolved image of two inner beams is captured on a high Z witness plate. Experimental results showing how the beam's x-ray foot print on the witness plate changes as a function of $\Delta\lambda$ will be shown and compared to models.\footnote{Prepared by LLNL under Contract DE-AC52-07NA27344. LLNL-ABS-67494.} [Preview Abstract] |
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TP12.00019: Hohlraum calculations for the NIF opacity platform E.S. Dodd, T.S. Perry, I.L. Tregillis, J.L. Kline, R.F. Heeter, D.A. Liedahl, Y.P. Opachich A summary of initial hohlraum calculations for planned opacity experiments at the National Ignition Facility (NIF) will be given. The purpose of these experiments is to make LTE opacity measurements of iron at the same conditions as previous experiments on Sandia's Z facility: 156 eV [1] and 190 eV [2]. Ongoing discrepancies between opacity data and theory make corroborating data highly important. The target considered in these calculations is a standard cylindrical hohlraum, with diameter 5.75 mm, but baffles have been placed between the laser hot spot and the sample to maintain the iron in LTE. The hohlraum is driven with a 3 ns flat top laser pulse, but limited to 500 kJ and only the outer beams. The inner beams will be used to drive a capsule implosion, which backlights the iron for the absorption measurements. The iron itself is a thin disk, mixed with magnesium as a spectroscopic tracer, and tamped with beryllium to minimize expansion. A description of the experimental set-up will be given.\\[4pt] [1] J. E. Bailey, et al., \textit{Phys. Rev. Lett}., \textbf{99} 265002 (2007).\\[0pt] [2] J. E. Bailey, et al., \textit{Nature}, \textbf{517 }56 (2015). [Preview Abstract] |
(Author Not Attending)
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TP12.00020: Lasnex Simulations of Axial Power Diagnostic for ZR Heidi Morris The dynamics of energy loss through diagnostic and/or laser entrance holes with or without shine shields is of inertial confinement fusion experiments envisioned for the National Ignition and ZR Facilities. 2-D radiation-hydrodynamic simulations using Lasnex for power diagnostic experiments using a secondary gold hohlraum fielded at the ZR facility are discussed. The axial radiation exiting the aperture of the dynamic hohlraum is modeled as time and spectrum-dependent 1-D and 2-D sources. Hohlraum energy balance and implications for the measured power are discussed. [Preview Abstract] |
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TP12.00021: Kinetic simulation of direct-drive capsule implosions and its comparison with experiments and radiation hydrodynamic simulation Thomas Kwan, Ari Le, Mark Schmitt, Hans Herrmann, Steve Batha We have carried out simulations of direct-drive capsule implosion experiments conducted on Omega laser facility at the Laboratory of Laser energetics of the University of Rochester [1]. The capsules had a glass shell (SiO$_{2})$ with D, T, He-3 fills at various proportions. One-dimensional radiation hydrodynamic calculations and kinetic particle/hybrid simulations with LSP were carried out for the post-shot analysis to compare neutron yield, yield ratio, and shell convergence in assessing the effects of plasma kinetic effects. The LSP simulations were initiated with the output from the rad-hydro simulations at the end of the laser-drive. The electrons are treated as a fluid while all the ion species by the kinetic PIC technique. Our LSP simulations clearly showed species separation between the deuterons, tritons and He-3 during the implosion but significantly less after the compression. The neutron yield, gamma bang-time and --width from the LSP simulations compared favorably with experiments. Detail comparison among the kinetic simulations, rad-hydro simulations, and experimental results will be presented.\\[4pt] [1] H. W. Herrmann, et al., Phys. Plasmas Vol. 16 056312 (2009) [Preview Abstract] |
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TP12.00022: Modeling of NIF Wetted-Foam Capsule Experiments Robert Peterson, Richard Olson, John Kline Wetting of a foam with liquid DT or DD in an ICF capsule provides a mechanism of directly controlling the convergence ratio of the implosion. The density of the DD or DT vapor in the central void in the CH foam is set by the temperature of the liquid fuel, so the convergence ratio is easily adjustable [1]. The capsule is driven by a two step laser pulse on NIF. The ablator is made of high density carbon in these experiments, but it could be beryllium. The experiments will test how well the modeling computer codes agree with experiment as the convergence ratio increases. It is possible that has the convergence ratio increases, a point will be reached were the modeling no longer agree with experiment. We wish to find this limit. In the presentation we will present computer model simulations in 1-D of the performance of NIF wetted-foam capsules, where the vapor density, the ablator type, and the choice of fuel (DD or DT) are varied. \\[4pt] [1] R.E. Olson and R.J. Leeper, Phys. Plasmas 20, 092705 (2013). [Preview Abstract] |
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TP12.00023: High-resolution 3D simulations of NIF ignition targets performed on Sequoia with HYDRA* M.M. Marinak, D.S. Clark, O.S. Jones, G.D. Kerbel, S. Sepke, M.V. Patel, J.M. Koning, C.R. Schroeder Developments in the multiphysics ICF code HYDRA enable it to perform large-scale simulations on the Sequoia machine at LLNL. With an aggregate computing power of 20 Petaflops, Sequoia offers an unprecedented capability to resolve the physical processes in NIF ignition targets for a more complete, consistent treatment of the sources of asymmetry. We describe modifications to HYDRA that enable it to scale to over one million processes on Sequoia. These include new options for replicating parts of the mesh over a subset of the processes, to avoid strong scaling limits. We consider results from a 3D full ignition capsule-only simulation performed using over one billion zones run on 262,000 processors which resolves surface perturbations through modes l $=$ 200. We also report progress towards a high-resolution 3D integrated hohlraum simulation performed using 262,000 processors which resolves surface perturbations on the ignition capsule through modes l $=$ 70. These aim for the most complete calculations yet of the interactions and overall impact of the various sources of asymmetry for NIF ignition targets. *This work was performed under the auspices of the Lawrence Livermore National Security, LLC, (LLNS) under Contract No. DE-AC52-07NA27344. [Preview Abstract] |
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TP12.00024: Monte Carlo Transport for Electron Thermal Transport Jeffrey Chenhall, Duc Cao, Gregory Moses The iSNB (implicit Schurtz Nicolai Busquet\footnote{Schurtz et. al. Phys. Plasmas \textbf{7}, 4238 (2000)} multigroup electron thermal transport method of Cao et. al.\footnote{Cao et. al. BAPS DDP14 UP8.84 (2014)} is adapted into a Monte Carlo transport method in order to better model the effects of non-local behavior. The end goal is a hybrid transport-diffusion method that combines Monte Carlo Transport with a discrete diffusion Monte Carlo (DDMC).\footnote{Chenhall et. al. BAPS DDP14 UP8.81 (2014)} The hybrid method will combine the efficiency of a diffusion method in short mean free path regions with the accuracy of a transport method in long mean free path regions. The Monte Carlo nature of the approach allows the algorithm to be massively parallelized. Work to date on the method will be presented. This work was supported by Sandia National Laboratory -- Albuquerque and the University of Rochester Laboratory for Laser Energetics. [Preview Abstract] |
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TP12.00025: Model Comparison for Electron Thermal Transport Gregory Moses, Jeffrey Chenhall, Duc Cao, Jacques Delettrez Four electron thermal transport models are compared for their ability to accurately and efficiently model non-local behavior in ICF simulations. Goncharov's transport model\footnote{Goncharov et. al. Phys. Plasmas 13, 012702 (2006)} has accurately predicted shock timing in implosion simulations but is computationally slow and limited to 1D. The iSNB (implicit Schurtz Nicolai Busquet\footnote{Schurtz et. al. Phys. Plasmas \textbf{7}, 4238 (2000)}) electron thermal transport method of Cao et. al.\footnote{Cao et. al. BAPS DDP14 UP8.84 (2014)} uses multigroup diffusion to speed up the calculation. Chenhall has expanded upon the iSNB diffusion model to a higher order simplified P3\footnote{Larsen et. al. Nucl. Sci. {\&} Engr. 123, 328-242 (1996)} approximation and a Monte Carlo transport model, to bridge the gap between the iSNB and Goncharov models while maintaining computational efficiency. Comparisons of the above models for several test problems will be presented. This work was supported by Sandia National Laboratory -- Albuquerque and the University of Rochester Laboratory for Laser Energetics. [Preview Abstract] |
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TP12.00026: Very high ratio compression by liquid conical implosion Jianguo Chen, Yian Lei A conical implosion device is designed to achieve very high ratio compression, up to $10^9$ or even higher. The compressing liquid has very low vapor pressure. A novel method is used to prepare the initial pure and thin gas. Some preliminary results proved the concept. The device can produce high energy density (HED) states in a near thermal equilibrium state, heat a gas continuously from very low (a few K) to very high temperature ($\sim$ 100,000 K), or do magnetized target fusion (MTF) with a proper external pulse current or magnetic field. [Preview Abstract] |
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TP12.00027: Implosion Dynamics and Mix in Double-Shell ICF Capsule Designs Mark Gunderson, William Daughton, Andrei Simakov, Douglas Wilson, Robert Watt, Norman Delamater, David Montgomery From an implosion dynamics perspective, double-shell ICF capsule designs have several advantages over the single-shell NIF ICF capsule point design. Double shell designs do not require precise shock sequencing, do not rely on hot spot ignition, have lower peak implosion speed requirements, and have lower convergence ratio requirements. However, there are still hurdles that must be overcome. The timing of the two main shocks in these designs is important in achieving sufficient compression of the DT fuel. Instability of the inner gold shell due to preheat from the hohlraum environment can disrupt the implosion of the inner pill. Mix, in addition to quenching burn in the DT fuel, also decreases the transfer of energy between the beryllium ablator and the inner gold shell during collision thus decreasing the implosion speed of the inner shell along with compression of the DT fuel. Herein, we will discuss practical implications of these effects on double-shell design we carry out in preparation for the NIF double-shell campaign. [Preview Abstract] |
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TP12.00028: iFP: An Optimal, Fully Implicit, Fully Conservative, 1D2V Vlasov-Rosenbluth-Fokker-Planck Code for ICF Simulation William Taitano, Luis Chac{\'o}n, Andrei Simakov We present a new, optimal, fully implicit, and fully conservative 1D2V Vlasov-Rosenbluth-Fokker-Planck (VRFP) code, iFP, which is designed to simulate inertial confinement fusion (ICF) implosions kinetically.\footnote{W.T. Taitano et al, JCP 297, pp.357-380, 2015} Such simulations are difficult to perform because of the disparate time and length scales involved. The challenge in obtaining a credible solution is complicated further by the need to enforce discrete conservation properties (mass, momentum and energy). Our approach uses an optimal, O(N), fully implicit temporal advance to step over stiff collision time-scales. We enforce discrete conservation of mass, momentum, and energy by solving a set of discrete nonlinear constraints, which are derived from continuum symmetries present in the VFP equations. To address the issues of velocity disparity associated with temporal and spatial temperature variations, we have developed a velocity-space meshing scheme, which adapts to the species' local thermal velocity. Length-scale disparity is addressed with a Lagrangian radial mesh, which allows the physical mesh to compress with the capsule. We will demonstrate the performance of the code on several challenging examples, including shock propagation. [Preview Abstract] |
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TP12.00029: Tamping effects and confinement time in NIF experiments Y.M. Wang, B. Cheng, T.J.T. Kwan, F. Merrill, C. Cerjan, S.H. Batha Tamper is expected to play an important role in inertial confinement fusion capsule experiments performed at the National Ignition Facility (NIF). It is expected to increase the confinement time of thermonuclear burning (TN) in the hot spot. In this work, we study the dependence of the capsule performance with respect to the density ratio of the pusher to the hot fuel at the cold-hot interface numerically through LASNEX simulations in one-dimension. Our study shows that the dependence of the capsule performance (neutron yield) with respect to the square root of the density ratio is not linear: the sharper the interface, the higher the tamping effect and neutron yields. Our analysis indicates that the tamping factor in both NIC and NIF experiments has not been appreciable and the tamping factor on yield is less than 1.1. Thus, the tamping factor has not yet played a significant role in the current NIF ignition design. Furthermore, the confinement time in NIF experiments will be discussed. (LA-UR-15-25596). [Preview Abstract] |
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TP12.00030: Proton Radiography of Self-Generated Fields in Strongly-shocked, Low-Density Systems Rui Hua, Sio Hong, Yuan Ping, Christopher Mcguffey, Farhat Beg, Rip Collins Anomalies, in ICF database, have raised challenges to the exclusively hydrodynamic models used in simulation, bringing to light the kinetic effect, which could be a dominated factor in shock phase because of the increasing ion-ion mean free path. Significance of multi-species effects, as well as self-generated electric fields at the shock front has been revealed and considered in recent simulations. [1] First observation of the shock front electric fields was achieved in an ICF implosion experiment [C.K. Li] [2]. To quantify dependence of the field generation on shock parameters, a shock imaging platform by broadband proton radiography was developed on OMEGA-EP. From the first shot day, shock propagation and proton deflection at shock front were clearly seen in gas targets. Varying laser conditions will be carried out on the second shot day in September. The results, as well as the electric field characteristics inferred through analytical and theoretical methods will be presented. This work was performed under DOE contract DE-AC52-07NA27344 with support from OFES Early Career program and LLNL LDRD program \\[4pt] [1] C.Bellei, et al. Phys. Plasmas. 20, 012701, (2013)\\[0pt] [2] C.K.Li, et al., Phys. Rev. Lett .100, 225001 (2008) [Preview Abstract] |
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TP12.00031: Thin Shell evolution of NIF capsule with asymmetric drive and the resulting neutron diagnostics Michael Buchoff, Jim Hammer One of the major impediments to achieving ignition via ICF is the non-spherical implosion arising from small asymmetries in the drive forcing the collapse of the capsule. Likewise, an experimental diagnostic for quantifying the characteristics of the implosion asymmetry is the final state neutrons, whose number and velocity distributions are not experimentally consistent with the expectation of a spherical implosion. In principle, connecting these initial and final state asymmetries could be solved via hydrodynamic simulations, but due to the multiple scales traversed throughout this process, these calculations are difficult and expensive, leaving much of the potential drive asymmetry profiles unexplored. In this work, we solve the resulting analytic equations from the thin-shell model proposed by Ott et. al. to evolve the capsule over a range of different drive asymmetries from its initial state (when the shell aspect ratio is much greater than 1) to a radius of roughly 250 microns, consisting of a layer of dense CH, a cold layer of dense DT, and a warm core of sparsely distributed DT. At this stage, more tractable hydrodynamical simulations are performed in the ARES code suite, determining the distribution of neutron from thermonuclear yield. These and future results allow for a multitude of tests of asymmetric sources to compare with and potentially guide experiment. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
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TP12.00032: Radiative heat transport instability in ICF plasmas W. Rozmus, V. Yu. Bychenkov A laser produced high-Z plasma in which an energy balance is achieved due to radiation losses and radiative heat transfer supports ion acoustic wave instability [1]. A linear dispersion relation is derived and instability is compared to the radiation cooling instability [2]. This instability develops in the wide range of angles and wavenumbers with the typical growth rate on the order of cs/LT (cs is the sound speed, LT is the temperature scale length). In addition to radiation dominated systems, a similar thermal transport driven ion acoustic instability was found before in plasmas where the thermal transport coefficient depends on electron density. However, under conditions of indirect drive ICF experiments the driving term for the instability is the radiative heat flux and in particular, the density dependence of the radiative heat conductivity. A specific example of thermal Bremsstrahlung radiation source has been considered corresponding to a thermal conductivity coefficient that is inversely proportional to the square of local particle density. In the nonlinear regime this instability may lead to plasma jet formation and anisotropic x-ray generation.\\[4pt] [1] V. Yu. Bychenkov and W. Rozmus, Phys. Plasmas 22, (2015).\\[0pt] [2] R. G. Evans, Plasma Phys. Contr. Fusion 27, 751 (1985). [Preview Abstract] |
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TP12.00033: Sudden viscous dissipation in compressing plasma turbulence Seth Davidovits, Nathaniel Fisch Compression of a turbulent plasma or fluid can cause amplification of the turbulent kinetic energy, if the compression is fast compared to the turnover and viscous dissipation times of the turbulent eddies. The consideration of compressing turbulent flows in inviscid fluids has been motivated by the suggestion that amplification of turbulent kinetic energy occurred on experiments at the Weizmann Institute of Science Z-Pinch.\footnote{E. Kroupp et al. PRL \textbf{107}, 105001 (2011)} We demonstrate a sudden viscous dissipation mechanism whereby this amplified turbulent kinetic energy is rapidly converted into thermal energy, which further increases the temperature, feeding back to further enhance the dissipation. Application of this mechanism in compression experiments may be advantageous, if the plasma can be kept comparatively cold during much of the compression, reducing radiation and conduction losses, until the plasma suddenly becomes hot. [Preview Abstract] |
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TP12.00034: Experimental signatures of suprathermal ion distribution in inertial confinement fusion implosions Grigory Kagan, Daniil Svyatskiy, Hans Rinderknecht, Michael Rosenberg, Alex Zylstra, Cheng-Kun Huang, Christopher McDevitt The distribution function of suprathermal ions is found to be self-similar under conditions relevant to inertial confinement fusion hot-spots. By utilizing this feature, interference between the hydro-instabilities and kinetic effects is for the first time assessed quantitatively to find that the instabilities substantially aggravate the fusion reactivity reduction. The ion tail depletion is also shown to lower the experimentally inferred ion temperature, a novel kinetic effect that may explain the discrepancy between the exploding pusher experiments and rad-hydro simulations and contribute to the observation that temperature inferred from DD reaction products is lower than from DT at National Ignition Facility. [Preview Abstract] |
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TP12.00035: MATHEMATICAL SIMULATIONS METHODS AND BASIC THEORY |
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TP12.00036: Quasi-neutral Vlasov theory of magnetized plasmas Cesare Tronci, Enrico Camporeale The low-frequency limit of Maxwell equations is considered in the Maxwell-Vlasov system. This limit produces a quasi-neutral Vlasov system [1] that captures essential features of plasma dynamics, while neglecting radiation effects. Euler-Poincar\'e reduction theory is used to show that the quasi-neutral Vlasov theory possesses a variational formulation in both Lagrangian and Eulerian coordinates. By construction, the new model recovers all collisionless neutral models employed in plasma simulations. Then, comparisons between the quasi-neutral Vlasov system and hybrid kinetic-fluid models are presented in the linear regime. \\[4pt] [1] C. Tronci, E. Camporeale, Neutral Vlasov kinetic theory of magnetized plasmas, Phys. Plasmas, 22 (2015), no. 2, 020704 [Preview Abstract] |
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TP12.00037: Hybrid Vlasov-MHD models: Hamiltonian vs. non-Hamiltonian Enrico Camporeale, Cesare Tronci, Emanuele Tassi, Philip Morrison We investigate hybrid kinetic-MHD models, where a hot plasma (obeying kinetic theory) interacts with a fluid bulk (obeying MHD). Different nonlinear schemes are reviewed, including the pressure-coupling scheme (PCS) used in modern simulations. This latter scheme suffers from being non-Hamiltonian and to not exactly conserve total energy. Upon adopting Vlasov kinetics, the non-Hamiltonian PCS and a Hamiltonian variant are compared. At high frequency, the non-Hamiltonian version exhibits a spurious instability, which is removed in the Hamiltonian version. [Preview Abstract] |
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TP12.00038: The Virtual Casing Principle and Helmholtz's Theorem James Hanson The virtual casing principle is derived for a general vector field with arbitrary divergence and curl. There is no restriction to curl-free or divergence-free fields. The virtual casing principle is shown to be closely related to Helmholtz's theorem. The virtual casing principle [Shafranov V D and Zakharov L E \textit{Nucl. Fusion} \textbf{12} 599-601 (1972), Lazerson S A \textit{Plasma Phys. Control. Fusion} \textbf{54} 122002 (2012)] is used in plasma physics to convert a Biot-Savart integration over a current distribution into a surface integral over a surface that encloses the current. In many circumstances, use of virtual casing to convert a volume integral into a surface integral can significantly speed up the computation of magnetic fields. The virtual casing principle is commonly used for plasma equilibrium computations, magnetic field line tracing, and magnetic diagnostic response calculation. Previous discussion of the virtual casing principle has been specialized to magnetic (divergence-free) fields, and the argumentation has often relied on properties of a virtual superconductor surrounding the volume in question. Extension to vector potentials will also be discussed. [Preview Abstract] |
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TP12.00039: Prediction of plasma rotation and neoclassical toroidal viscosity in KSTAR discharges based on plasma fluid formulation Cheonho Bae, Weston Stacey Braginskii's flow rate of strain tensor formalism [1], as extended first to low collisional plasmas in axisymmetric circular toroidal flux surface geometry [2,3], then to elongated axisymmetric flux surface geometry [4], has recently been extended to 3-D non-axisymmetric toroidal flux surface geometry [5]. In toroidally non-axisymmetric plasmas, the leading order neoclassical parallel viscosity terms in the flow rate of strain tensor do not vanish to cause flux surface averaged toroidal angular momentum damping [5] and eventually slow down the plasma rotation. The formalism of Ref. 5 provides a means to systematically evaluate the ``neoclassical toroidal viscosity (NTV)'' in curvilinear plasma geometry based on the plasma fluid equations. As the first step of its application, a practical formalism for circular plasmas, given in the appendix of Ref. 5, will be applied to KSTAR discharges to predict the rotation and NTV, which can also be compared with actual rotation measurements to numerically validate the NTV damping effects. \\[4pt] [1] S. I. Braginskii, Rev. Plasma Phys., Vol. I, p205 (Consult. Bureau 1965)\\[0pt] [2] W. M. Stacey, D. J. Sigmar, Phys. Fluids 28, 2800 (1985)\\[0pt] [3] W. M. Stacey, A. W. Bailey, D. J. Sigmar, K. C. Shaing, Nucl. Fusion 25, 463 (1985)\\[0pt] [4] C. Bae, W. M. Stacey, W. M. Solomon, Nucl. Fusion 53, 043011 (2013)\\[0pt] [5] W. M. Stacey, C. Bae, Phys. Plasmas 22, 062503 (2015) [Preview Abstract] |
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TP12.00040: Action Principle for Relativistic Magnetohydrodynamics Eric D'Avignon, Philip Morrison, Francesco Pegoraro A covariant action principle for ideal relativistic magnetohydrodynamics in terms of natural Eulerian field variables is given. This is done by generalizing the covariant Poisson bracket theory of Marsden et al., which uses a noncanonical bracket to implement constrained variations of an action functional. Various implications and extensions of this action principle are also discussed. [Preview Abstract] |
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TP12.00041: Common Hamiltonian structure and concomitant topological invariants for extended magnetohydrodynamics models Manasvi Lingam, Hamdi M. Abdelhamid, Yohei Kawazura, Philip J. Morrison, Zensho Yoshida, George Miloshevich, Ko Tanehashi, Noriki Takahashi Extended magnetohydrodynamics (XMHD) includes 2-fluid effects such as electron inertia and the Hall drift absent in ideal MHD. Hamiltonian structure of the XMHD models (Hall MHD, inertial MHD [3] and full XMHD) is presented [1]. Existence of elegant variable transformations that map every XMHD model to a common noncanonical Poisson bracket is highlighted [2]. The bracket is used to derive the existence of two unique helicities (Casimir invariants) for these models, each of which exhibits close similarities with the magnetic and fluid helicities [1,2] - this is highly significant as the latter are important topological invariants. The Lagrangian origins of the helicities and variable transforms, and avenues for future work are outlined.\\[4pt] [1] H.M.~Abdelhamid, Y. Kawazura \& Z. Yoshida, J.~Phys.~A {\bf48}, 235502 (2015) \\[0pt] [2] M.~Lingam, P.J.~Morrison \& G.~Miloshevich, Phys.~Plasmas {\bf22}, 072111 (2015) \\[0pt] [3] M.~Lingam, P.J.~Morrison \& E.~Tassi, Phys.~Lett.~A {\bf379}, 570 (2015) [Preview Abstract] |
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TP12.00042: Low dimensional gyrokinetic PIC simulation by $\delta f$ method C.M. Chen, Yasutaro Nishimura, C.Z. Cheng A step by step development of our low dimensional gyrokinetic Particle-in-Cell (PIC) simulation is reported. One dimensional PIC simulation of Langmuir wave dynamics is benchmarked.\footnote{C.Z.Cheng and G.Knorr, J. Comput. Phys. {\bf 22}, 330 (1976).} We then take temporal plasma echo as a test problem to incorporate the $\delta f$ method. Electrostatic driftwave simulation in one dimensional slab geometry\footnote{S~.E.~Parker and W.~W.~Lee, Phys. Fluids B {\bf 5}, 77 (1993).} is resumed in the presence of finite density gradients. By carefully diagnosing contour plots of the $\delta f$ values in the phase space, we discuss the saturation mechanism of the driftwave instabilities. A $v_\parallel$ formulation is employed in our new electromagnetic gyrokinetic method by solving Helmholtz equation for time derivative of the vector potential.\footnote{H.~Naitou (private communication, 2009); E.~A.~Starstev (private communication, 2014). Electron and ion momentum balance equations are employed in the time derivative of the Ampere's law.} This work is supported by Ministry of Science and Technology of Taiwan, MOST 103-2112-M-006-007 and MOST 104-2112-M-006-019. [Preview Abstract] |
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TP12.00043: Don't worry. Lagrangian drift kinetics is OK Joshua Burby I show that standard Lagrangian (i.e. variational) drift kinetics with $u_{E\times B}\propto v_{\mathrm{th}}$ and $H_{\mathrm{gc}}=H_o+\epsilon H_1+\epsilon^2 H_2$ has an unphysically-large phase space; where a valid initial condition ought to consist of $(F,\mathbf{E},\mathbf{B})$ specified at $t=0$, Lagrangian drift kinetics requires initial time derivatives of the electromagnetic field to be specified as well. This phenomenon occurs because the guiding center coordinate transformation depends on time derivatives of the electromagnetic field, and this leads to the appearance of a time derivative of $\mathbf{E}$ in $H_2$. I also show how to ``renormalize'' the Lagrangian approach to drift kinetics in a way that manifestly preserves the correct structure of the initial value problem. Starting from this modified Lagrangian procedure, I derive the drift kinetic system's Poisson bracket. [Preview Abstract] |
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TP12.00044: Multi-dimensional, fully implicit, exactly conserving electromagnetic particle-in-cell simulations in curvilinear geometry Guangye Chen, Luis Chacon We discuss a new, conservative, fully implicit 2D3V Vlasov-Darwin\footnote{Nielson and Lewis, \textit{Methods Comput. Phys.} 16, p.367 (1976)} particle-in-cell algorithm in curvilinear geometry for non-radiative, electromagnetic kinetic plasma simulations. Unlike standard explicit PIC schemes, fully implicit PIC algorithms are unconditionally stable and allow exact discrete energy and charge conservation.\footnote{Chen, Chac\'on, and Barnes, \textit{J. Comput. Phys.} 230, p.7018 (2011); Chen and Chac\'on, \textit{Comput. Phys. Commun.} 185, p.2391 (2014); Chen and Chac\'on, \textit{Comput. Phys. Commun.}, submitted} Here, we extend these algorithms to curvilinear geometry. The algorithm retains its exact conservation properties in curvilinear grids. The nonlinear iteration is effectively accelerated with a fluid preconditioner for weakly to modestly magnetized plasmas, which allows efficient use of large timesteps, $O(\sqrt{\frac{m_i}{m_e}}\frac{c}{v_{eT}})$ larger than the explicit CFL. In this presentation, we will introduce the main algorithmic components of the approach, and demonstrate the accuracy and efficiency properties of the algorithm with various numerical experiments in 1D (slow shock) and 2D (island coalescense). [Preview Abstract] |
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TP12.00045: Electron dynamics in Hall thruster Samuel Marini, Renato Pakter Hall thrusters are plasma engines those use an electromagnetic fields combination to confine electrons, generate and accelerate ions. Widely used by aerospace industries those thrusters stand out for its simple geometry, high specific impulse and low demand for electric power. Propulsion generated by those systems is due to acceleration of ions produced in an acceleration channel. The ions are generated by collision of electrons with propellant gas atoms. In this context, we can realize how important is characterizing the electronic dynamics. Using Hamiltonian formalism, we derive the electron motion equation in a simplified electromagnetic fields configuration observed in hall thrusters. We found conditions those must be satisfied by electromagnetic fields to have electronic confinement in acceleration channel. We present configurations of electromagnetic fields those maximize propellant gas ionization and thus make propulsion more efficient. [Preview Abstract] |
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TP12.00046: Multisymplectic Integration for Beam and Plasma Simulations Stephen Webb Particle-in-cell methods are a standard tool for simulating charged particle systems such as fusion plasmas, intense beams, and laser- and beam-driven wakefield accelerators. Conventional methods have been successful in studying short-term dynamics, however numerical instabilities and artifacts such as grid heating make long-time simulations unreliable. A similar issue existed in single particle tracking for storage rings in the 1980s, which led to the development of \textit{symplectic algorithms}. The essential insight that if the physical equations of motion derive from a least-action principle, then so too should the numerical equations of motion. The resulting update sequence preserves a symplectic 2-form, which is a strong constraint on the numerical solutions. The resulting algorithms are stable and accurate over very long simulation times. This same structure exists for field theories as well as single-particle dynamics. Such \textit{multisymplectic} integrators have good stability properties and naturally encode conservation laws, making them ideal for simulations over many oscillations of the system. We present here a number of examples where multisymplectic algorithms have been used over very long time scales. [Preview Abstract] |
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TP12.00047: Accelerating particle-in-cell simulations using multilevel Monte Carlo Lee Ricketson Particle-in-cell (PIC) simulations have been an important tool in understanding plasmas since the dawn of the digital computer. Much more recently, the multilevel Monte Carlo (MLMC) method has accelerated particle-based simulations of a variety of systems described by stochastic differential equations (SDEs), from financial portfolios to porous media flow. The fundamental idea of MLMC is to perform correlated particle simulations using a hierarchy of different time steps, and to use these correlations for variance reduction on the fine-step result. This framework is directly applicable to the Langevin formulation of Coulomb collisions, as demonstrated in previous work, but in order to apply to PIC simulations of realistic scenarios, MLMC must be generalized to incorporate self-consistent evolution of the electromagnetic fields. We present such a generalization, with rigorous results concerning its accuracy and efficiency. We present examples of the method in the collisionless, electrostatic context, and discuss applications and extensions for the future. [Preview Abstract] |
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TP12.00048: Variational Formulation of Particle Algorithms for Kinetic E{\&}M Plasma Simulations; A High Fidelity Approach Alexander Stamm, Bradley Shadwick The recent variational technique [1-4] for rigorously deriving discrete, self-consistent equations for electromagnetic particle codes has been further developed in several coordinate systems. The primary advantage of the Lagrangian formulation is the connection between symmetries of the system and conservation laws, which in the present case resolves the grid-heating issue. However, the approach also simplifies coordinate transformations and enables the particle method to be formulated in moving window coordinates and a cylindrical geometry with a truncated Fourier decomposition in angle. For some laser-plasma interaction scenarios, these lead to significant computational savings as compared to the traditional lab frame. New time advance integrators were developed in both the lab frame coordinate system and the moving window. A comparison of symplectic methods to more straightforward explicit and implicit methods allow us to make conclusions about the limits of phase-space fidelity in macro-particle methods. \\[4pt] [1] E. G. Evstatiev and B. A. Shadwick, J. Comput. Phys. 245, 376 (2013).\\[0pt] [2] B. A. Shadwick, A. B. Stamm, and E. G. Evstatiev, Phys. Plasmas 21, 055708 (2014).\\[0pt] [3] A. B. Stamm, B. A. Shadwick, and E. G. Evstatiev, IEEE Trans. Plasma Sci. 42, 1747 (2014).\\[0pt] [4] A. B. Stamm and B. A. Shadwick, AIP Conf. Proc. (Submitted). [Preview Abstract] |
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TP12.00049: Porting plasma physics simulation codes to modern computing architectures using the {\sc libmrc} framework Kai Germaschewski, Stephen Abbott Available computing power has continued to grow exponentially even after single-core performance satured in the last decade. The increase has since been driven by more parallelism, both using more cores and having more parallelism in each core, e.g. in GPUs and Intel Xeon Phi. Adapting existing plasma physics codes is challenging, in particular as there is no single programming model that covers current and future architectures. We will introduce the open-source {\sc libmrc} framework that has been used to modularize and port three plasma physics codes: The extended MHD code MRCv3 with implicit time integration and curvilinear grids; the OpenGGCM global magnetosphere model; and the particle-in-cell code PSC. {\sc libmrc} consolidates basic functionality needed for simulations based on structured grids (I/O, load balancing, time integrators), and also introduces a parallel object model that makes it possible to maintain multiple implementations of computational kernels, on e.g. conventional processors and GPUs. It handles data layout conversions and enables us to port performance-critical parts of a code to a new architecture step-by-step, while the rest of the code can remain unchanged. We will show examples of the performance gains and some physics applications. [Preview Abstract] |
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TP12.00050: High-order continuum Vlasov-Maxwell simulations of collisionless plasmas G.V. Vogman, P. Colella, U. Shumlak Plasma kinetic theory treats each constituent species as a probability distribution function in phase space. Numerically, the velocity dependence of the distribution function can be sampled discretely as in particle-in-cell methods, or represented smoothly as in continuum methods. Continuum methods for solving kinetic theory governing equations are advantageous in that they can be cast in conservation-law form, are not susceptible to noise, and can be implemented using high-order numerical methods, which provide enhanced solution accuracy. A fourth-order accurate finite volume method has been developed to solve the continuum kinetic Vlasov-Maxwell equation system in 2D2V phase space using the Chombo library. The evolving species are collisionless, and are coupled through electromagnetic fields. The algorithm is validated against theoretical predictions using benchmarks based on the Dory-Guest-Harris instability and the Harris current sheet. Extension of the algorithm to cylindrical coordinates and its application to axisymmetric plasma configurations like the Z-pinch are also presented. [Preview Abstract] |
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TP12.00051: Viriato: a Fourier-Hermite spectral code for strongly magnetised fluid-kinetic plasma dynamics Nuno Loureiro, William Dorland, Luis Fazendeiro, Anjor Kanekar, Alfred Mallet, Alessandro Zocco We report on the algorithms and numerical methods used in Viriato, a novel fluid-kinetic code that solves two distinct sets of equations: (i) the Kinetic Reduced Electron Heating Model equations [Zocco \& Schekochihin, 2011] and (ii) the kinetic reduced MHD (KRMHD) equations [Schekochihin et al., 2009]. Two main applications of these equations are magnetised (Alfvn\'enic) plasma turbulence and magnetic reconnection. Viriato uses operator splitting to separate the dynamics parallel and perpendicular to the ambient magnetic field (assumed strong). Along the magnetic field, Viriato allows for either a second-order accurate MacCormack method or, for higher accuracy, a spectral-like scheme. Perpendicular to the field Viriato is pseudo-spectral, and the time integration is performed by means of an iterative predictor-corrector scheme. In addition, a distinctive feature of Viriato is its spectral representation of the parallel velocity-space dependence, achieved by means of a Hermite representation of the perturbed distribution function. A series of linear and nonlinear benchmarks and tests are presented, with focus on 3D decaying kinetic turbulence. [Preview Abstract] |
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TP12.00052: Variational Algorithms for Test Particle Trajectories C. Leland Ellison, John M. Finn, Hong Qin, William M. Tang The theory of variational integration provides a novel framework for constructing conservative numerical methods for magnetized test particle dynamics. The retention of conservation laws in the numerical time advance captures the correct qualitative behavior of the long time dynamics. For modeling the Lorentz force system, new variational integrators have been developed that are both symplectic and electromagnetically gauge invariant. For guiding center test particle dynamics, discretization of the phase-space action principle yields multistep variational algorithms, in general. Obtaining the desired long-term numerical fidelity requires mitigation of the multistep method's parasitic modes or applying a discretization scheme that possesses a discrete degeneracy to yield a one-step method. Dissipative effects may be modeled using Lagrange-D'Alembert variational principles. Numerical results will be presented using a new numerical platform that interfaces with popular equilibrium codes and utilizes parallel hardware to achieve reduced times to solution. [Preview Abstract] |
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TP12.00053: Capturing marginally collisional effects with the 13N-moment plasma model S.T. Miller, U. Shumlak Fluid-based plasma models have typically been applied to parameter regimes where a local thermal equilibrium is assumed. While this parameter regime is valid for low temperature applications, it begins to fail as plasmas enter the collisionless regime and kinetic effects dominate the physics. This research extends the validity of the collisional fluid regime using an anisotropic 13-moment fluid model derived from the Pearson type-IV probability distribution. The model explicitly evolves the heat flux hyperbolically alongside the density, momentum and an energy tensor to capture dynamics usually restricted to costly Boltzmann models. Each particle species is modeled individually and collectively coupled through electromagnetic and collision operators. Electromagnetic fields are evolved using Maxwell's equations. The model is implemented within the University of Washington's WARPXM code for use on accelerated clusters using an unstructured central essentially non-oscillatory finite volume method, and is currently being extended to an unstructured discontinuous Galerkin method. [Preview Abstract] |
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TP12.00054: Multi-fluid plasma modeling with Braginskii collisional transport processes A. Ho, U. Shumlak, S.T. Miller Magnetohydrodynamics (MHD) works well where transport processes are primarily advective. Extensions of the MHD model are capable of capturing some collisional phenomena such as electrical resistivity, which are important in systems with mean free paths less than the characteristic length. However, MHD models have difficulties resolving systems where the Debye length cannot be assumed to approach zero. These systems arise in low density, hot plasmas. By modeling the ions and electrons as distinct fluids, the 5-moment multi-fluid plasma model is able to capture these short-range transport processes that are not accounted for in MHD. To model the transport processes the Braginskii transport terms are added to the 5-moment model, which introduces viscosity, heat conduction, and binary species interactions. These transport properties are affected by strong magnetic fields, resulting in anisotropic collisional effects. The multi-fluid equations are evolved explicitly and are coupled with Maxwell's equations. This research extends the University of Washington's WARPXM code to include the Braginskii terms with the 5-moment multi-fluid plasma model. The implementation is validated against theoretical results from a Hartmann flow benchmark problem. [Preview Abstract] |
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TP12.00055: Non-perturbative closure calculation for fluids and plasmas Xianzhu Tang, Chris McDevitt, Zehua Guo Closure calculation of the Chapman-Enskog type is based on a perturbative expansion in the small parameter of Knudsen number, which is defined as the ratio of the thermal particle mean-free-path and the system gradient length scale. The error in the analysis can be locally measured in phase space using the local Knudsen number, which for the energy squared dependence of the mean-free-path, is much larger for high energy particles. Such breakdown, if occurs at sufficiently high energy, has small impact on closure results, but in cases of strong spatial gradients, can have large effect and invalidate the perturbative calculation. Here we show a non-perturbative closure formulation and its application in calculating standard closure quantitities such as heat flux. This approach applies as long as the thermal bulk is close to a Maxwellian, where a perturbative analysis can be matched onto a non-perturbative treatment of the tail population. [Preview Abstract] |
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TP12.00056: Lorentz boosted frame simulation of Laser wakefield acceleration in quasi-3D geometry Peicheng Yu, Xinlu Xu, Asher Davidson, Adam Tableman, Michael Meyers, Thamine Dalichaouch, Frank Tsung, Viktor Decyk, Frederico Fiuza, Jorge Vieira, Ricardo Fonseca, Wei Lu, Luis Silva, Warren Mori We present results on a systematic study of Particle-In-Cell simulation of Laser Wakefield Acceleration (LWFA) by combining the Lorentz boosted frame technique with the quasi-3D algorithm, in which fields are expanded into azimuthal harmonics and solved on an $r-z$ PIC grid keeping only a few harmonics. The studies emphasize on LWFA in the nonlinear blowout regime, which is more challenging from a computational standpoint. We first discuss strategies for eliminating the numerical Cerenkov instability (NCI) that inevitably arises due to the presence of plasma drifting across the grid with relativistic speeds in quasi-3D geometry. These strategies work for FFT based Maxwell solvers. We have incorporated these mitigation strategies into our PIC code OSIRIS by adding a new hybrid Yee-FFT Maxwell solver. With these strategies, OSIRIS can now be used to combine the quasi-3D algorithm and Lorentz boosted frame technique, and carry out high fidelity LWFA boosted frame simulation with no evidence of the NCI in the quasi-3D geometry, leading to unprecedented speedups. [Preview Abstract] |
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TP12.00057: GPU acceleration of particle-in-cell methods Benjamin Cowan, John Cary, Dominic Meiser Graphics processing units (GPUs) have become key components in many supercomputing systems, as they can provide more computations relative to their cost and power consumption than conventional processors. However, to take full advantage of this capability, they require a strict programming model which involves single-instruction multiple-data execution as well as significant constraints on memory accesses. To bring the full power of GPUs to bear on plasma physics problems, we must adapt the computational methods to this new programming model. We have developed a GPU implementation of the particle-in-cell (PIC) method, one of the mainstays of plasma physics simulation. This framework is highly general and enables advanced PIC features such as high order particles and absorbing boundary conditions. The main elements of the PIC loop, including field interpolation and particle deposition, are designed to optimize memory access. We describe the performance of these algorithms and discuss some of the methods used. [Preview Abstract] |
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TP12.00058: Particle-In-Cell Multi-Algorithm Numerical Test-Bed M.D. Meyers, P. Yu, A. Tableman, V.K. Decyk, W.B. Mori We describe a \textit{numerical test-bed} that allows for the direct comparison of different numerical simulation schemes using only a single code. It is built from the UPIC Framework, which is a set of codes and modules for constructing parallel PIC codes. In this test-bed code, Maxwell's equations are solved in Fourier space in two dimensions. One can readily examine the numerical properties of a real space finite difference scheme by including its operators' Fourier space representations in the Maxwell solver. The fields can be defined at the same location in a simulation cell or can be offset appropriately by half-cells, as in the Yee finite difference time domain scheme. This allows for the accurate comparison of numerical properties (dispersion relations, numerical stability, etc.) across finite difference schemes, or against the original spectral scheme. We have also included different options for the charge and current deposits, including a strict charge conserving current deposit. The test-bed also includes options for studying the analytic time domain scheme, which eliminates numerical dispersion errors in vacuum. We will show examples from the test-bed that illustrate how the properties of some numerical instabilities vary between different PIC algorithms. [Preview Abstract] |
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TP12.00059: Hybrid Envelope Model / Boosted-Frame Simulations of Laser Wakefield Accelerators Adam Higuera, Kathleen Weichman, Dan Abell, Ben Cowan, Michael Downer, John Cary Laser wakefield accelerators use a high-intensity laser pulse to drive a wave in a plasma that traps, transports, and accelerates electrons. The Texas Petawatt Laser experiment measures different electron energies (2 GeV) than predicted (7 GeV) by computer simulations. We present and analyze a method for efficiently performing higher-fidelity 3-D, particle-in-cell simulations of laser wakefield acceleration. This method combines previous work on a Laser Envelope Model, which resolves electron self-injection, and boosted-frame simulation, which efficiently models beam propagation in the regime where the Envelope Model is no longer valid.\\[4pt] This work is supported by the DOE under Grants No. DE-SC0011617 and DE-SC0012444, by DOE/NSF Grant No. DE-SC0012584, and used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. [Preview Abstract] |
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TP12.00060: Comparison of quasi-3D and full-3D laser wakefield PIC simulations using azimuthal mode decomposition Thamine Dalichaouch, Peicheng Yu, Asher Davidson, Warren Mori, Jorge Vieira, Ricardo Fonseca Laser wakefield acceleration (LWFA) has attracted a lot of interest as a possible compact particle accelerator.~~However, 3D simulations of plasma-based accelerators are computationally intensive, sometimes taking millions of core hours on today's computers. A quasi-3D particle-In-cell (PIC) approach has been developed to take advantage of azimuthal symmetry in LWFA (and PWFA) simulations by using a particle-in-cell description in r-z and a Fourier description in $\varphi$. Quasi-3D simulations of LWFA are computationally more efficient and faster than Full-3D simulations because only first few azimuthal harmonics are needed to capture the physics of the problem. We have developed a cylindrical mode decomposition diagnostic for 3D Cartesian geometry simulations to analyze the agreement between full-3D and quasi-3D PIC simulations of laser and beam-plasma interactions. The diagnostic interpolates field data from Full-3D PIC simulations onto an irregular cylindrical grid ($r,\varphi, z$). A Fourier decomposition is then performed on the interpolated 3D simulation data along the azimuthal direction. This diagnostic has the added advantage of separating out the wakefields from the laser field. Preliminary results for this diagnostic of LWFA and PWFA simulations with symmetric and nearly symmetric spot sizes as well as of laser-plasma interactions using lasers with orbital angular momentum (higher order Laguerre-Gaussian modes) will be presented. [Preview Abstract] |
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TP12.00061: A Hamiltonian Five-Field Gyrofluid Model Ioannis Keramidas Charidakos, Francois Waelbroeck, Philip Morrison Reduced fluid models constitute versatile tools for the study of multi-scale phenomena. Examples include magnetic islands, edge localized modes, resonant magnetic perturbations, and fishbone and Alfven modes. Gyrofluid models improve over Braginskii-type models by accounting for the nonlocal response due to particle orbits. A desirable property for all models is that they not only have a conserved energy, but also that they be Hamiltonian in the ideal limit. Here, a Lie-Poisson bracket is presented for a five-field gyrofluid model, thereby showing the model to be Hamiltonian. The model includes the effects of magnetic field curvature and describes the evolution of electron and ion densities, the parallel component of ion and electron velocities and ion temperature. Quasineutrality and Ampere's law determine respectively the electrostatic potential and magnetic flux. The Casimir invariants are presented, and shown to be associated to five Lagrangian invariants advected by distinct velocity fields. A linear, local study of the model is conducted both with and without Landau and diamagnetic resonant damping terms. Stability criteria and dispersion relations for the electrostatic and the electromagnetic cases are derived and compared with their analogs for fluid and kinetic models. [Preview Abstract] |
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TP12.00062: MHD equilibria with incompressible flows: symmetry approach Giampaolo Cicogna, Francesco Pegoraro We identify and discuss a family of azimuthally symmetric, incompressible, magnetohydrodynamic plasma equilibria with poloidal and toroidal flows in terms of solutions of Generalized Grad Shafranov (G-GS) equation. These solutions are derived by exploiting the incompressibility assumption, which allows us to rewrite the G-GS equation in the form of a GS equation in terms of a new dependent variable, and the continuous Lie symmetry properties of the resulting equation and in particular a special type of ``weak'' symmetries. This procedure allows us to construct a family of D shaped MHD equilibrium configurations with both toroidal and poloidal flows. Here we present the simplified case without poloidal flow, in which case the new dependent variable coincides with the standard flux function, and discuss the effect of the gradient of the centrifugal term on the equilibrium configuration.\\[4pt] G. Cicogna, F. Pegoraro, {\it Phys. Plasmas}, {\bf 22}, 022520 (2015). [Preview Abstract] |
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TP12.00063: ABSTRACT WITHDRAWN |
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TP12.00064: Expansion of toroidal MHD equilibria about a given flux surface Harold Weitzner Earlier work, H. Weitzner, Phys. Plasmas 20, 022515 (2014) constructed formal expansions of toroidal ideal MHD equilibria, where the expansion parameter was the amplitude of the ``helical'' magnetic field components. The equilibrium was assumed to have low shear and the resonant components of the boundary flux surfaces were not free but were determined by other components of the magnetic fields and boundary flux surfaces. Here, we consider the topological torus x\textgreater 0, 0\textless y\textless 1, 0\textless z\textless 1, and assume periodicity in y and in z. It is shown that one can give data on the flux surface x$=$0 and expand the equilibrium in powers of x--or an equivalent variable. The rotational transform on x$=$0 is rational, but otherwise free. The magnetic field on x$=$0 must satisfy the well-known condition that the integral of 1/B on every closed magnetic field line must take the same value. Again, convergence is not proven. However, analogies with, and differences from the classic Cauchy-Kowalewski theorem on the existence of analytic solutions of partial differential equations are discussed. [Preview Abstract] |
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TP12.00065: Ertel's vorticity theorem and new flux surfaces in classical fluids Elie Hameiri Following our work on the Ertel surfaces for multi-fluid plasmas [1], we work out the analogous case of an unmagnetized classical fluid. The case of a compressible and nonisentropic fluid is intriguing because it is typically not considered by fluid dynamiscists who mostly deal with incompressible fluids or with equations of state of the form p = p(density), but not when p also depends on the entropy. In the general case, the vorticity is not strictly carried by the fluid, but nevertheless there are still surfaces in which its toroidal and poloidal fluxes are conserved, a notion more familiar in plasma physics than in fluid dynamics. In carrying out this work it is necessary to find all independent constants of the motion, which are used as constraints in a variational principle. It is also necessary to distinguish between toroidal and cylindrical equilibria, since the number of constants of the motion is not the same for both. We are able to show in each case that we have indeed accounted for all the constants. This formulation allows the simple consideration of stability, which will also be discussed.\\[4pt] [1] E. Hameiri, Phys. Plasmas, 20, 092503 (2013). [Preview Abstract] |
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TP12.00066: Current sheet formation in a sheared magnetic field Yao Zhou, Yi-Min Huang, Hong Qin, Amitava Bhattacharjee Recently a variational integrator for ideal magnetohydrodynamics in Lagrangian labeling has been developed using discrete exterior calculus. Its built-in frozen-in equation makes it optimal for studying current sheet formation. We use this scheme to study the Hahm-Kulsrud-Taylor problem, which considers the response of a 2D plasma magnetized by a sheared field under mirrored sinusoidal boundary perturbations. The equilibrium solutions are found to not converge with increasing spatial resolution, which suggests that there exists no smooth equilibrium that preserves the topology of the initial field exactly. Unlike previous studies that examine the current density output, we identify a singular current sheet from the converged part of the fluid mapping. [Preview Abstract] |
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TP12.00067: Temperature gradient formation while axial gas compression V.I. Geyko, N.J. Fisch A spinning gas in equilibrium has a rotation-dependent heat capacity [1]. However, as equilibrium is approached, such as after sudden heating, significant variations in temperature appear. Surprisingly, when fast axial compression or instantaneous gas heating occurs, the temperature does not grow homogeneously in radial direction, but instead has a gradient towards to the maximum of potential energy of external or self potential. The gradient monotonically grows with compression rate and the amplitude of the potential. The gradient builds up due to change of equilibrium density distribution, yet, not due to acoustic waves created by the compression. This result was checked in numerical simulations for particles in an external constant gravitational potential and also for rotating gas in the cylinder with perfect slip boundary conditions on the walls. \\[4pt] [1] V.I Geyko and N.J. Fisch, Phys. Rev. Lett. \textbf{110}, 150604 (2013). [Preview Abstract] |
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TP12.00068: Waves in Relativistic Scalar QED Plasma Yuan Shi, Nathaniel Fisch, Hong Qin When ultra-intense laser is used to compress solid targets, particles are accelerated to relativistic energy and plasma with density higher then solid can be created. In such plasma, the relativistic and quantum nature of particles can no longer be ignored. Using a quantum field theory description of scalar QED plasma, we demonstrate how waves in such plasma are affected. It is found that in unmagnetized plasma, the electrostatic wave can now propagate with nonzero group velocity even when the plasma is cold. In magnetized plasma, Landau levels serve as resonances and polarization due to transitions between these resonances give rise to Bernstein waves even when the plasma is cold. [Preview Abstract] |
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TP12.00069: Nonlinear Model for Thermal Effects in Free-Electron Lasers: Water Bag and Maxwell Velocity Distributions Eduardo Peter, Felipe Rizzato, Antonio Endler This work is an extension of a previously paper produced by our group, in which the nonlinear model to evaluate the role of the space-charge and temperature effects in free-electron lasers has been applied to a water bag distribution. Now, we are interested in other types of velocity distributions and in particular in Maxwell distribution. As in the previous work, we used the concept of compressibility, in a way that its discontinuities, which are equivalent to the discontinuities in the electron density, make possible to estimate the time of the breakdown of particles laminar flow in high-gain regimes with parameters that maximize the laser growth rate. Through wave-particle simulations, we analyze how the mixing processes for water bag and Maxwell distributions occur. Results of particle-wave simulations are compared with the results obtained via nonlinear model. [Preview Abstract] |
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TP12.00070: Rotation and differential confinement effects in magnetized plasmas Renaud Gueroult, Nathaniel J. Fisch For certain plasma configurations and plasma parameters, differential confinement effects can lead to ion separation. An example of such configurations is rotating plasmas. As a matter of fact, plasma rotation leads, through centrifugal forces, to mass differential effects. In the collisionless limit, a maximum rotation velocity exists, the Brillouin limit [1], above which no rigid body equilibrium is possible. In fast magnetic plasma compression experiments, the large electric fields induced locally might be sufficiently large to drive significant plasma rotation. Such conditions are for example anticipated for time resolved plasma wave properties control [2]. In this case, the plasma is essentially collisionless, and charge separation effects result from magnetic field variations on a timescale comparable to or shorter than the ion gyro-period. Interestingly, experimental evidence of ion separation has been reported for similar conditions [3]. Preliminary results aiming at identifying the possible role of rotation on ion separation are presented.\\[4pt] [1] R.C. Davidson, Physics of Nonneutral Plasmas(2001)\\[0pt] [2] P.F. Schmidt and N. J. Fisch, Phys. Rev. Lett. 109, 255003 (2012)\\[0pt] [3] A. Weingarten, R. Arad, Y. Maron and A. Fruchtman, Phys. Rev. Lett. 87, 115004 (2001) [Preview Abstract] |
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TP12.00071: DIII-D II, BOUNDARY AND PLASMA-MATERIAL INTERACTIONS |
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TP12.00072: The DIII-D Boundary/Plasma Materials Interaction Center (BPMIC): Progress and Prospects D. Thomas The boundary of a putative fusion reactor remains a key unresolved issue in the development of useful fusion energy. The BPMIC was established to develop validated boundary/PMI solutions for burning plasma devices by leveraging the existing DIII-D resources in well controlled, variable geometry edge plasmas and extensive boundary diagnostic set. During the first part of the 2015 campaign we have made significant progress in experiments designed to isolate specific known boundary and PMI physics issues and provide data for challenging existing analytical modeling tools such as the SOLPS suite and UEDGE. Topics include characterizing the relation between upstream and divertor parameters, the separate effects of closure and local magnetic geometry on detachment performance, leading edge tungsten erosion studies, and scaling relationships for the divertor heat flux width. This poster summarizes results from these experiments and will describe our high-level goals for the remainder of the 2015 campaign as well as for the 2016 campaign where we plan a campaign to study high-Z material migration and integration. [Preview Abstract] |
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TP12.00073: X-Divertor Geometries for Deeper Detachment Without Degrading the DIII-D H-Mode Brent Covele, M.T. Kotschenreuther, P.M. Valanju, S.M. Mahajan, A.W. Leonard, A.W. Hyatt, A.G. McLean, D.M. Thomas, H.Y. Guo, J.G. Watkins, M.A. Makowski, D.N. Hill Recent DIII-D experiments comparing the standard divertor (SD) and X-Divertor (XD) geometries show heat and particle flux reduction at the divertor target plate. The XD features large poloidal flux expansion, increased connection length, and poloidal field line flaring, quantified by the Divertor Index. Both SD and XD were pushed deep into detachment with increased gas puffing, until core energy confinement and pedestal pressure were substantially reduced. As expected, outboard target heat fluxes are significantly reduced in the XD compared to the SD under similar upstream plasma conditions, even at low Greenwald fraction. The high-triangularity (floor) XD cases show larger reduction in temperature, heat, and particle flux relative to the SD in all cases, while low-triangularity (shelf) XD cases show more modest reductions over the SD. Consequently, heat flux reduction and divertor detachment may be achieved in the XD with less gas puffing and higher pedestal pressures. Further causative analysis, as well as detailed modeling with SOLPS, is underway. These initial experiments suggest the XD as a promising candidate to achieve divertor heat flux control compatible with robust H-mode operation. [Preview Abstract] |
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TP12.00074: Role of cross-field drifts in the onset of divertor detachment Mathias Groth, S.L. Allen, M.E. Fenstermacher, D.H. Hill, M.A. Makowski, A.G. McLean, C.J. Lasnier, G.D. Porter, T.D. Rognlien, A.R. Briesemeister, E.A. Unterberg, A.W. Leonard, J.G. Watkins The impact of cross-field drifts in divertor configurations was investigated in DIII-D L and H-mode discharges. The studies show that the electron temperature at the outer divertor plate is reduced to below 2 eV at about 20$\%$ lower pedestal density in configurations with the ion Bx$\nabla$B direction toward the divertor X-point. When attached, these plasmas have significantly lower electron temperatures and and higher densities in the inner than in the outer divertor as directly measured with divertor Thomson scattering and inferred from line emission imaging using tangentially viewing cameras. Upon reversal of the toroidal field direction, the divertor conditions were observed in-out symmetric. Simulations with the edge fluid code UEDGE show that poloidal flows due to the radial electric field in the private flux region dominate the divertor asymmetries. [Preview Abstract] |
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TP12.00075: Measuring the effect of divertor closure on detachment in DIII-D Auna Moser, A.W. Leonard, T.W. Petrie, C.F. Sang, S.L. Allen, A.G. McLean, M.E. Fenstermacher, I. Joseph, C.J. Lasnier, M.A. Makowski, J.G. Watkins, A.R. Briesemeister Recent experiments compared the open lower divertor and semi-closed upper divertor in DIII-D to measure the effect of divertor closure on detachment onset and heat flux control, extending past work showing reduced core fueling with the more-closed upper DIII-D divertor. Experiments were performed to determine the extent to which closure may facilitate detachment at collisionalities more relevant to future devices. This work builds on previous experiments that quantified effects of divertor magnetic geometry, including connection length, $\nabla$B-drift direction, incidence angle, and flux expansion; efforts were made to match these parameters while comparing single null configurations in the upper and lower divertor in order to isolate the effects of closure. Experimental measurements coupled with simulation results will help weigh the benefits of a more-closed divertor in facilitating detachment and reducing heat flux against the constraints imposed on the magnetic geometry by a more-closed divertor tile structure, aiding in the design of a future advanced divertor for DIII-D. [Preview Abstract] |
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TP12.00076: OEDGE Modeling of Detachment Threshold Experiments on DIII-D J.D. Elder, P.C. Stangeby, A.G. McLean, A.W. Leonard, J.G. Watkins A detachment threshold experiment was performed on DIII-D in which the divertor plasma transitioned from attached to weakly detached at the strike point with minimal changes in upstream parameters. The value of $T_e$ at the outer strike point measured by Thompson scattering decreased from $\sim$ 10eV (attached) to $\sim$ 2 eV (weakly detached). Both the Langmuir probes and the divertor Thomson diagnostics recorded increases in the particle flux on the order of a factor of two between these divertor conditions. OEDGE is used to model both of these plasma regimes for both L-mode and H-mode discharges. The behaviour of molecular hydrogen is assessed using OEDGE and possible roles of hydrogen molecules in the detachment process are examined. [Preview Abstract] |
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TP12.00077: SOLPS Modeling of Slot Divertor Configuration on DIII-D C.F. Sang, P.C. Stangeby, H.Y. Guo, L.L. Lao A major thrust of the DIII-D boundary/PMI initiative is to develop an advanced divertor configuration for next-step devices, such as FNSF and DEMO. We are adopting an integrated approach by optimizing both divertor structure and magnetic shape. Initial SOLPS modeling was carried out to optimize divertor structure shape to enhance divertor power dissipation, focusing on slot configurations. In particular, four different slot divertor structures, i.e., orthogonal-target slot, slanted-target slot, very narrow slot and v-shaped slot have been analyzed and comparisons made with an open divertor structure. It is found that the slot helps to trap recycling neutrals and impurities thus increasing radiative power dissipation in the divertor, reducing the electron temperature $T_e$ and the perpendicular heat flux $q_{\bot}$ at the target plate. As expected, a narrower slot leads to lower $T_e$ and $q_{\bot}$ than a less narrow one. The v-shaped slot appears to be especially effective at redirecting and concentrating recycling neutrals and impurities near the separatrix, thus promoting detachment at a lower upstream density than the other configurations. [Preview Abstract] |
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TP12.00078: Comparing kinetic and fluid simulations of scrape-off layer physics R.M. Churchill, J.M. Canik, C.S. Chang, R. Hager, A.W. Leonard, R. Maingi, R. Nazikian, D.P. Stotler Simulations using the fully kinetic code XGCa were undertaken to explore the impact of kinetic effects on scrape-off layer (SOL) physics in DIII-D H-mode plasmas. XGCa is a total-f, gyrokinetic code which self-consistently calculates the axisymmetric electrostatic potential and plasma dynamics, and includes modules for neutral Monte Carlo transport. Fluid simulations are usually used to simulate the SOL, due to its high collisionality. However, a number of discrepancies have been observed between experiment and leading SOL fluid codes (e.g. SOLPS) [1], including underestimating outer target temperatures, radial electric field in the SOL, parallel ion SOL flows at the low field side, and impurity radiation. Many of these discrepancies may be linked to the fluid treatment, and could be resolved by including kinetic effects in SOL simulations. Status of benchmarking efforts to compare XGCa with the fluid code SOLPS and traditional two-point models will be presented in the sheath-limited and medium-recycling regimes, including future plans to compare results in the high-recycling and detached regimes. \\[4pt] [1] A.V. Chankin, D.P. Coster, the ASDEX-Upgrade Team, JNM Vol. 390--391, pg. 319-324 (2009) [Preview Abstract] |
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TP12.00079: Drift effects on the tokamak power scrape-off width E.T. Meier, R.J. Goldston, E.G. Kaveeva, S. Mordijck, V.A. Rozhansky, I. Yu. Senichenkov, S.P. Voskoboynikov Recent experimental analysis suggests that the scrape-off layer (SOL) heat flux width ($\lambda_{\mathrm{q}})$ for ITER will be near 1 mm, sharply narrowing the planned operating window. In this work, motivated by the heuristic drift (HD) model, which predicts the observed inverse plasma current scaling, SOLPS-ITER is used to explore drift effects on $\lambda _{\mathrm{q}}$. Modeling focuses on an H-mode DIII-D discharge. In initial results, target recycling is set to 90{\%}, resulting in sheath-limited SOL conditions. SOL particle diffusivity (D$_{\mathrm{SOL}})$ is varied from 0.1 to 1 m$^{\mathrm{2}}$/s. When drifts are included, $\lambda_{\mathrm{q}}$ is insensitive to D$_{\mathrm{SOL}}$, consistent with the HD model, with $\lambda_{\mathrm{q}}$ near 3 mm; in no-drift cases, $\lambda _{\mathrm{q}}$ varies from 2 to 5 mm. Drift effects depress near-separatrix potential, generating a channel of strong electron heat convection that is insensitive to D$_{\mathrm{SOL}}$. Sensitivities to thermal diffusivities, plasma current, toroidal magnetic field, and device size are also assessed. These initial results will be discussed in detail, and progress toward modeling experimentally relevant high-recycling conditions will be reported. [Preview Abstract] |
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TP12.00080: $\beta_{p} $ Scaling of the Heat Flux Width in DIII-D M.A. Makowski, C.J. Lasnier, A.W. Leonard, T.H. Osborne The scaling of the heat flux width with poloidal beta at the outer midplane, $\beta_{p} $, is a stringent test of the critical gradient model that posits that the heat flux width is set by an edge stability limit dependent on the separatrix pressure gradient. As $\beta_{p} $ was varied by means of a combined density and power scan, the measured pressure gradient was found to scale linearly with $\beta_{p} $ at both low (0.5 MA) and high (1.5 MA) plasma currents, and lie significantly below the infinite-n ideal ballooning limit critical pressure gradient as computed by the BALOO code. At fixed $I_{p} $, this implies that the separatrix pressure gradient scale length is approximately constant, which is consistent with the kinetic profile measurements. The ballooning limit was found to be constant in the $\beta _{p} $ scan and set by the equilibrium with only a minor dependency on the edge pressure and current profiles. Both the pressure gradient and $\beta _{p} $ varied by more than a factor of 2 in the scans. [Preview Abstract] |
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TP12.00081: Notable Findings From Recent Core-Edge Studies of High Power AT Plasmas on DIII-D T.W. Petrie, A. Leonard, T. Luce, D. Pace, F. Turco, M. Van Zeeland, M.E. Fenstermacher, C. Holcomb, M. Makowski, W. Solomon Recent experiments on DIII-D have focused on specific issues pertaining to high power, high performance hybrid plasmas, as characterized by $\beta_N$ = 3-4, H$_{98}$ = 1.3-1.7, P$_{IN}$ $\leq$ 20 MW, and q$_{95}$ $\cong$ 4.5-6.5. For these near-double null configurations, divertor peak heat flux (q$_P$) $\propto$ I$_P^{0.9}$ P$_{IN}^{0.9}$ at constant B$_T$. Carbon accumulation in the core rose significantly as P$_{IN}$ was increased, particularly when counter-beams were used; prompt beam particle losses by counter-beam injection to the outer midplane walls was 10-20\%. Nearly doubling the poloidal flux expansion at the divertor target resulted in only a fraction of the expected reduction in q$_P$. However, inhibiting particle escape from the divertor by baffling the SOL side of the target reduced q$_P$ by 30-40\%. The puff-and-pump radiating divertor was less effective in reducing q$_P$ while maintaining density control at highest P$_{IN}$ and $\beta_N$ than it was at lower P$_{IN}$ and $\beta_N$. Other features of high power AT operation will also be presented. [Preview Abstract] |
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TP12.00082: High-Z Tile Arrays in the DIII-D Divertor Region for Studying SOL/Edge Transport and Material Migration E.A. Unterberg, P.G. Stangeby, D.A. Buchenaur, E.M. Hollmann, H.Y. Guo, D.M. Thomas, A.W. Leonard Understanding the compatibility of high-Z plasma facing components (PFCs) in the divertor region with high performance (H-mode) tokamak (e.g. AT) operation is still an open issue in fusion research. Specifically with respect to high-Z, it is desirable to determine: (i) impurity transport in the edge plasma and (ii) migration across PFC surfaces as both these mechanisms can in-turn contaminate the confined plasma and limit performance. To address this uncertainty, complete toroidal rows of high-Z metal-coated carbon tiles will be installed at several poloidal locations in the DIII-D divertor. This effort will aid in the identification and characterization of high-Z: (i) source location and (ii) migration pathways. Particularly, experiments will be carried out using matched plasma conditions with/without ELM control to identify the role of ELMs. [Preview Abstract] |
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TP12.00083: Spectroscopic measurements and modeling of tungsten erosion in the DIII-D divertor T.D. Abrams, R. Ding, H.Y. Guo, A.W. Leonard, D.M. Thomas, S.L. Allen, A.G. McLean, A.R. Briesemeister, E.A. Unterberg, C. Chrobak, R.P. Doerner, D.L. Rudakov, J.D. Elder, P.C. Stangeby, W.R. Wampler, J.G. Watkins In situ time-resolved measurements of the gross W erosion rate have been performed in DIII-D by monitoring W/I (400.9 nm) emission in the divertor via a filtered camera and high-resolution spectrometer. The erosion rate of a thin W coating on DiMES, inferred via the S/XB method, was found to be $\sim$ 0.7 nm/s during deuterim L-mode exposure, in fair agreement with post-mortem IBA analysis but lower than REDEP/WBC modeling. During H-mode He bombardment of W disks, average erosion rates of $\sim$ 2.9 nm/s and $\sim$ 9.0 nm/s were estimated during the inter-ELM and intra-ELM phases, using ne and Te from divertor Thomson scattering and Langmuir probes. Results will also be presented from additional W erosion experiments in preparation for the DIII-D mini-campaign to measure high-Z transport in the edge plasma. Comparisons will be made with ERO modeling [Preview Abstract] |
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TP12.00084: Dust remobilization tests in DIII-D divertor I. Bykov, D. Rudakov, R. Moyer, S. Ratynskaia, P. Tolias, M. DeAngeli, A. McLean, K. Bystrov Accumulation of dust on hot surfaces is a safety concern for ITER operation. We studied the life cycle of pre-deposited dust under ITER-relevant conditions by exposing W samples with W, C and Al (surrogate for Be) dust at the outer strike point (OSP) in a few ELMy H-mode discharges using DiMES. The maxima in the dust ejection rate correspond to ELM crashes under both attached and detached OSP conditions, as confirmed by a fast camera monitoring DiMES. SEM mapping of dust before and after exposures shows that $>$95$\%$ of C and $<$5$\%$ of metal dust gets remobilized in a few shots. In discharges with detached OSP, remaining Al particles melt and fuse together, forming larger spherical grains. At elevated heat flux with attached OSP, they melt, destruct and fuse with W substrate, which is not thermally affected. In this mode W grains partly melt and adjacent particles can weld together, forming larger asymmetric agglomerates with increased adhesion to the surface. We show that these results are consistent with recent observations from Pilot-PSI. [Preview Abstract] |
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TP12.00085: SOL and Edge Flows in DIII-D J.A. Boedo, J.S. deGrassie, B.A. Grierson, D.A. Rudakov Recent measurements at DIII-D edge plasmas at the outer midplane show that, in the absence of external torque, the edge and near-SOL plasma flow is largely dominated by the intrinsic source of rotation most likely due to thermal ion loss. We also show that when NBI heating is present, the core momentum competes with the edge intrinsic momentum and can overwhelm it, in short, NBI-heated discharges at high power tend to determine edge and near SOL flows. Experiments performed in the DIII-D tokamak with OH heated, ECH-heated and NBI-heated discharges are diagnosed for core plasma flow with CER and edge/SOL plasma flow with Mach probes. We have changed the amount of NBI, OH and ECH heating while scanning the discharge collisionality. We have compared the experimental measurements to two complementary thermal ion loss theories that explain most of the observed features, including a scaling with Ti. One theory considers passing and trapped particles that are lost via a loss cone purely due to drifts and the other considers turbulence-enhanced loss of passing particles. [Preview Abstract] |
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TP12.00086: Progress in Doppler Velocity Measurements of Ions in the DIII-D Divertor and SOL S.L. Allen, W.H. Meyer, C. Samuel, J. Howard, M. Groth We present recent progress in Doppler velocity measurements of ions using coherence imaging. A new in-situ calibration technique has been developed, focusing on CIII emission (465nm), and an optimized tomographic inversion routine provides time-resolved ($\sim$2 ms) flow images between shots. The CIII flow velocity in the divertor changes direction in response to a change in the sense of the DIII-D toroidal field, indicating the importance of drifts; the details of the flow image also changes near the x-point. Initial comparisons with UEDGE modeling will be presented. A second polarization interferometer system has been installed on the LLNL wide-view periscope, providing a tangential view of the scrape-off region around the plasma core. Initial measurements with a high spatial resolution camera (5.5 Megapixel) with $\sim$10 ms time resolution will be presented. Both systems have a remote filter wheel to select visible impurity lines, e.g. CIII, CII, along with the main ion in Helium plasmas. [Preview Abstract] |
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TP12.00087: Tomographic Reconstruction of Flows in DIII-D William Meyer, Steve Allen, John Howard The DIII-D flow diagnostic produces video of interference images with horizontal fringes that contain spatial emissivity, flow, and temperature information from the lower divertor. Frames are demodulated and compared against a reference interference image to produce phase and contrast images which are the emissivity weighted flow and temperature integrated along the line-of-site, respectively. Inversion of the flow (phase) images require knowledge of the scalar product of the parallel flow vector, from the equilibrium calculations, and each camera pixel line-of-site. Four response matrices are pre-calculated: the emissivity line integral and the line integral of the scalar product of the lines-of-site with the orthogonal unit vectors of parallel flow. Equilibrium data determines the relative weight of the component matrices used in the final flow matrix. Early reconstructions have shown flow reversal during forward and reverse toroidal field plasmas. Ongoing work is to extract temperature information from the contrast images. [Preview Abstract] |
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TP12.00088: System Design of a Coherence Imaging System to Measure Divertor Ion Temperature on DIII-D Cameron M. Samuell, S.L. Allen, W.H. Meyer, J. Howard The interferogram from an imaging polarization interferometer can be used to determine the Doppler shift (phase image) and the Doppler width (contrast image) of a plasma ion species. In the case of a single emission line, the shift is determined at one fixed optical delay, and the width at a second (larger) optical delay. Recent analysis has indicated that it is also possible to recover both the shift and width at the second larger optical delay. However, in the case of a multiplet such as CIII (465 nm), there is an added complication in the width measurement because of the Zeeman splitting of the individual lines; this necessitates taking into account the polarization of the emission. We present progress in the design of a coherence imaging instrument to measure the ion temperature of ion species in the DIII-D divertor and SOL plasma. This uses an interferometer with optical delay optimized for contrast measurements, taking into account polarization effects by either optical switching or polarization mixing. [Preview Abstract] |
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TP12.00089: OEDGE Modeling of Divertor Fueling at DIII-D B.D. Bray, A.W. Leonard, J.D. Elder, P.C. Stangeby Onion-skin-modeling (OSM) is used to assess the affect of divertor closure on pedestal fueling sources. The OSM includes information from a wide range of diagnostic measurements at DIII-D to constrain the model background plasma for better simulation of neutrals and impurity ions and spectroscopy to compare to the results of the simulation. DIII-D has open lower divertor and closed upper divertor configurations which can be run with similar discharges. Progress toward modeling the pedestal fueling in low density plasmas for these cases will be presented as well as initial comparisons of recent lower single null discharges with the outer leg on the divertor shelf (fully open) and divertor floor (partially open). [Preview Abstract] |
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TP12.00090: The EPED Pedestal Model: Validation, Super H-Mode, and Core-Pedestal Coupling P.B. Snyder, E.A. Belli, K.H. Burrell, A. Garofalo, R.J. Groebner, O. Meneghini, T.H. Osborne, W.M. Solomon, J.M. Park, J.W. Hughes, M.N.A. Beurskens, H.R. Wilson The EPED model predicts the H-Mode pedestal height and width by calculating non-local peeling-ballooning and kinetic ballooning mode constraints. Comparisons of EPED predictions to observations in more than 700 cases on 5 tokamaks, show agreement to a standard deviation of $\sim$ 20-25$\%$. The effects of plasma shape, collisionality, and impurities are explored. EPED predicts the pedestal can in some cases have multiple self-consistent solutions, including a higher pressure ``Super H'' solution, which can be reached by controlling density evolution. Comparisons of Super H predictions to DIII-D observations, and Super H predictions for other devices will be presented. Recently, the AToM project has coupled EPED to core transport models, enabling self-consistent prediction of temperature and pressure profiles, and global stored energy, across the confined plasma. Predictions for existing devices and for ITER are discussed. [Preview Abstract] |
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TP12.00091: Recent Advances in Plasma Edge Physics Theory W.M. Stacey This presentation summarizes recent theory developments for interpreting plasma edge physics experiments in DIII-D. i) Radial and poloidal moment balance require that the radial particle flux be of a pinch-diffusive nature with the pinch representing the electromagnetic forces and external momentum input. Ion radial particle fluxes in experiment are found to be a smaller difference between large outward diffusion fluxes and inward pinch fluxes. When the pinch-diffusion relation is used in the continuity equation a new diffusion theory that preserves momentum balance is obtained. ii) The majority of thermalized ions and their energy cross the LCFS on ion loss orbits and are deposited in the SOL near the outboard midplane. The lost ions are predominantly ctr-current, producing a co-current intrinsic rotation of the remaining ions in the edge plasma. iii) While the contribution of the leading order parallel viscosity to toroidal momentum damping vanishes identically in axisymmetric plasmas, non-axisymmetric radial B-fields in the edge plasma enable parallel viscosity to enhance the damping of toroidal rotation. [Preview Abstract] |
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TP12.00092: Determination of the Radial Electric Field in the DIII-D Edge Pedestal Plasma T.M. Wilks, W.M. Stacey, T.E. Evans A self-consistent calculation for the radial electric field in the edge plasma for a representative H-mode DIII-D discharge is presented. The complex interrelationships between edge physics phenomena such as rotation, radial ion fluxes, momentum transport, and the radial electric field are maintained by momentum balance requirements. Modeling efforts include fast and thermal ion orbit loss, return currents, x-transport, and a non-axisymmetric rotation formulation, and their effect on radial particle flux, rotation, and the radial electric field. Recent improvements to the non-axisymmetric rotation model demonstrate a new leading order viscosity term contributing further to toroidal rotation damping via non-axisymmetric magnetic fields, which affect the electric field calculation specifically in the edge pedestal region. The new ion orbit loss and rotation model calculations are compared to experiment to show good agreement with the characteristic ``well'' structure for the radial electric fields in H-mode plasmas. [Preview Abstract] |
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TP12.00093: Comparison of Experimental and Theoretical Thermal Diffusivities in the DIII-D Edge Plasma J.J. Roveto, W.M. Stacey, T.M. Wilks The capability of the Georgia Tech GTEDGE edge transport interpretation code has been upgraded to include improved ion-orbit-loss models for neutral beam and thermalized ions in the edge plasma.\footnote{T. M. Wilks et al., Transport Task Force Workshop 2015.} We are undertaking a new comparison of various theoretical thermal diffusivity models with the improved interpretation of experimental edge transport now possible. The initial effort is examining two DIII-D shots, {\#}123302, a reference ELMing H-mode shot, and {\#}123301, a matched RMP shot. The improved interpretation leads to quite different experimental thermal diffusivity profiles in the edge than previously reported when ion-orbit-loss effects are included. The experimental values are being compared with various theoretical models, including paleoclassical, neoclassical, ITG, drift ballooning mode, TEM, and ETG. [Preview Abstract] |
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TP12.00094: Examination of the Change in Intrinsic Rotation of the DIII-D Edge Pedestal Plasma During the L-H Transition N. Piper, W.M. Stacey, R. Groebner A previous analysis of an L-H transition in DIII-D\footnote{W.M. Stacey, Phys. Plasmas 20, 012509 (2013).} found that the radial particle pinch changed from outward to inward and the co-current edge intrinsic rotation dropped as the plasma went through the L-H transition. Two additional DIII-D discharges are now being examined in the late L-mode and early H-mode stages to determine if these features are characteristic of the L-H transition. A particle-momentum-energy balance analysis of the measured temperature, density, and rotation velocity is being performed to determine if the particle pinch reverses and the co-current intrinsic rotation due to ion orbit loss drops in the edge pedestal region during the L-H transition. [Preview Abstract] |
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TP12.00095: Size Scaling of Intrinsic Rotation in DIII-D J.S. deGrassie, W.M. Solomon Despite the richness in the variety of the profiles of intrinsic rotation in axisymmetric tokamaks, a common feature is a co-Ip directed toroidal velocity on the outboard midplane in the region of $\rho\sim 0.8$ in DIII-D. This feature showed a ``Rice scaling'' (RS) in DIII-D and led to similarity experiments with C-Mod [1]. RS correlates toroidal velocity with W/Ip, where W is the total plasma kinetic energy and Ip the plasma current. Subsequent analysis from DIII-D shows a clear $\rho\ast$ dimensionless scaling of this intrinsic velocity in DIII-D, where $\rho\ast\sim\surd T_i/aB$, multiplying the $\beta$q scaling indicative of RS [1]. The DIII-D scaling is $M_A\sim\beta_N\rho\ast$, where $M_A$ is the Alfv\'en ``Mach'' value and $\beta_N$ is normalized $\beta$. In machine parameters it is very similar to the theoretical ``Parra scaling,'' [2] which emphasizes the correlation of toroidal velocity with ion temperature as seen experimentally, but in this DIII-D scaling having an additional critical dependence on $\surd\beta$. Published data from C-Mod and low power ICRF in JET also fit with this DIII-D scaling. The relation to the RS will be described.\\[4pt] [1] J.S. deGrassie et al., Phys. Plasmas 14, 056115 (2007).\\[0pt] [2] F. Parra et al, Phys. Rev. Lett. 108, 095001 (2012) [Preview Abstract] |
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TP12.00096: Comparison of Flux-Surface Aligned Curvilinear Coordinate Systems and Neoclassical Magnetic Field Predictions T.G Collart, W.M. Stacey Several methods are presented for extending the traditional analytic ``circular'' representation of flux-surface aligned curvilinear coordinate systems to more accurately describe equilibrium plasma geometry and magnetic fields in DIII-D. The formalism originally presented by Miller is extended to include different poloidal variations in the upper and lower hemispheres. A coordinate system based on separate Fourier expansions of major radius and vertical position greatly improves accuracy in edge plasma structure representation. Scale factors and basis vectors for a system formed by expanding the circular model minor radius can be represented using linear combinations of Fourier basis functions. A general method for coordinate system orthogonalization is presented and applied to all curvilinear models. A formalism for the magnetic field structure in these curvilinear models is presented, and the resulting magnetic field predictions are compared against calculations performed in a Cartesian system using an experimentally based EFIT prediction for the Grad-Shafranov equilibrium. [Preview Abstract] |
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TP12.00097: 3D magnetic geometric effects during 3D field application and comparison to measurements in DIII-D R.S. Wilcox, E.A. Unterberg, A. Wingen, M.W. Shafer, M.R. Cianciosa, D.L. Hillis, G.R McKee, T.M. Bird, T.E. Evans Density pumpout during the application of 3D fields in tokamaks may be caused by changes to the plasma equilibrium shaping that destabilize microinstabilities, thereby increasing transport.\footnote{T.M. Bird, PoP 21 (2014) 100702.} Local geometric quantities of the magnetic field that are relevant for microinstabilities (curvature and local shear) are calculated using VMEC equilibria in typical RMP discharges on DIII-D. Measurements of phase-differenced soft X-ray emission in the pedestal region show a clear helical structure that is compared with a model of localized impurity transport based on the 3D geometry. Broadband density fluctuations measured by beam emission spectroscopy also show changes in magnitude with I-coil phase, in support of the theory that microstability changes with the magnetic geometry. A scan of 3D equilibria over a large range of DIII-D geometric parameter space has been preformed in order to map out the operating space of the microstability mechanism. [Preview Abstract] |
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TP12.00098: Burn Control Mechanisms in Tokamaks M.A. Hill, W.M. Stacey Burn control and passive safety in accident scenarios will be an important design consideration in future tokamak reactors, in particular fusion-fission hybrid reactors, e.g. the Subcritical Advanced Burner Reactor. We are developing a burning plasma dynamics code to explore various aspects of burn control, with the intent to identify feedback mechanisms that would prevent power excursions. This code solves the coupled set of global density and temperature equations, using scaling relations from experimental fits. Predictions of densities and temperatures have been benchmarked against DIII-D data. We are examining several potential feedback mechanisms to limit power excursions: i) ion-orbit loss, ii) thermal instability density limits, iii) MHD instability limits, iv) the degradation of alpha-particle confinement, v) modifications to the radial current profile, vi) ``divertor choking'' and vii) Type 1 ELMs. [Preview Abstract] |
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TP12.00099: Local electromagnetic turbulence characterization during type-I ELM cycle in DIII-D H-mode pedestal K.K. Barada, T.L. Rhodes For the first time, local internal perpendicular density and magnetic field fluctuations during the time between type-I ELMs are measured in the pedestal of the DIII-D tokamak. These measurements are made with Doppler backscattering (for \~{n}) and cross polarization scattering (for $\tilde{{B}})$ with $0.26\le k_{\bot } \rho_{i} \le 1$. The broadband magnetic fluctuations exist $\sim$ 9 mm in radial extent from the foot of the pedestal to the steepest gradient. The density fluctuations propagate in the electron diamagnetic direction and appear as a saturated mode when the pedestal gradient starts to develop. The magnetic fluctuations propagate in the ion diamagnetic direction (lab frame) initially and later in electron direction when pedestal gradients are saturated. These fluctuations appear just after the ELM crash in the outermost probed radius and gradually grow towards the inner radius and their times of persistence increase towards the innermost radius of existence. We will present comparisons of experimental results to gyrokinetic predictions of different electrostatic (TEM, ITG) and electromagnetic (KBM and MTM) modes. [Preview Abstract] |
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TP12.00100: Modeling Effects of Toroidal Field Direction on Pedestal Structure in DIII-D Steady-State Discharges using SOLPS A.C. Sontag, J.M. Canik, L.W. Owen, M. Murakami, J.M. Park Core transport models predict that fusion power scales roughly as the square of the pressure at the top of the pedestal, so understanding the effects that determine pedestal structure in steady-state operational scenarios is important in order to project steady-state tokamak operational scenarios developed in DIII-D forward to ITER and other devices. Both experiments and modeling indicate that SOL conditions are important in optimizing the pedestal structure for high-beta steady-state scenarios. The SOLPS code is used to provide interpretive transport analysis of the pedestal and SOL. This work examines the nature of flows, pumping and fueling on the pedestal structure including the effects of drifts in the fluid model. The DIII-D edge diagnostic suite allows for model comparison to density, temperature, flows, impurity transport in the SOL as well as the divertor heat and particle fluxes. This modeling is used to determine why some scenarios in recent DIII-D experiments which require reverse Bt to optimize off-axis neutral beam current drive have reduced pedestal height and width when the grad-B drift is in the unfavorable direction. [Preview Abstract] |
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TP12.00101: Edge MSE measurements on the DIII-D tokamak B.S. Victor, C.T. Holcomb, S.L. Allen, W.H. Meyer, M.A. Makowski The edge motional Stark effect (MSE) diagnostic on DIII-D has recently been upgraded to provide better constraints on the current density in the outer half radius, including high-spatial resolution measurements in the H-mode pedestal. The channels have been upgraded with new bandpass filters with FWHM of 0.3 nm and $>$90$\%$ transmission at the central wavelength, and improved detector positioning. A spectrometer has been used to measure the $\sigma$ and $\pi$ lines for each of the edge channels to optimize the new filter selection. These upgrades are expected to improve our ability to assess current drive and stability in various DIII-D plasmas. Comparisons will be shown between the Sauter and NEO bootstrap current models and these new measurements in fully non-inductive plasmas. We will present calculations of the ideal MHD $\beta_N$-limit in various scenarios that use the improved measurements. Finally, changes in the edge pitch angle during ELMs are examined. [Preview Abstract] |
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TP12.00102: The Effect of Input Torque Ramps on Density Fluctuations Generated in the QH-mode Edge on DIII-D Chris Rost, E.M. Davis, A. Marinoni, M.A. Porkolab, K.H. Burrell Recent studies of Quiescent H-mode with varied input torque have exhibited two regimes of edge density turbulence, as observed with the Phase Contrast Imaging (PCI) density fluctuation diagnostic. The PCI is especially sensitive to turbulence in regions of large velocity shear, as seen in the $E_r$ well in the H-mode edge. QH-modes were first discovered in discharges with large input torque from neutral beams. Such plasmas possess a deep $E_r$ well inside the separatrix and have highly sheared ion-scale turbulence in the outer portion of the well propagating in the electron diamagnetic direction at 50$\%$ of the largest $E\times B$ velocity. As input torque decreases, additional sheared turbulence appears which propagates at 2-3$\times$ the largest $E\times B$ velocity, coincident with discontinuous changes in the velocity shear in the $E_r$ well and the characteristics of the Edge Harmonic Oscillation. Robust performance is observed to continue throughout these qualitative changes in the QH-mode edge parameters and density turbulence. [Preview Abstract] |
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TP12.00103: Disparate-scale coupling of turbulence in QH-mode plasmas on DIII-D C.M. Muscatello, K.H. Burrell, Xi Chen, N.C. Luhmann, Jr., B.A. Grierson, G.J. Kramer, B.J. Tobias Analysis of incoherent fluctuations in quiescent H-mode (QH-mode) plasmas suggests nonlinear coupling between high- and low-frequency turbulence. In QH-mode plasmas with edge harmonic oscillations (EHO), transport levels are enhanced when incoherent fluctuations are present compared to QH-mode plasmas with only EHO. Furthermore, in some cases without EHO, the incoherent fluctuations alone can sustain QH-mode. Bispectral analysis of microwave imaging reflectometer (MIR) data indicates nonlinear 3-wave coupling among disparate spatial scales of the turbulence. The bicoherence is above noise levels for high-frequency (300 $<$ f $<$ 500 kHz), intermediate-scale ($k_\theta \approx$ 0.2 - 0.6 cm $^{-1}$) and low-frequency (f $<$ 50 kHz), large-scale ($k_\theta < $ 0.2 cm $^{-1}$) turbulence. Cross-phase analysis reveals that the high-frequency turbulence rotates in the electron diamagnetic drift direction, while the low-frequency turbulence rotates in the ion diamagnetic drift direction, suggesting coupling between different instabilities. [Preview Abstract] |
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TP12.00104: Accurate Experiment to Computation Coupling for Understanding QH-mode physics using NIMROD J.R. King, K.H. Burrell, A.M. Garofalo, R.J. Groebner, J.D. Hanson, J.D. Hebert, S.R. Hudson, A.Y. Pankin, S.E. Kruger, P.B. Snyder It is desirable to have an ITER H-mode regime that is quiescent to edge-localized modes (ELMs). The quiescent H-mode (QH-mode) with edge harmonic oscillations (EHO) [Garofalo et al, PoP (2015); Burrell et al., PoP (2012); Garofalo et al, NF (2011) and refs. within.] is one such regime. High quality equilibria are essential for accurate EHO simulations with initial-value codes such as NIMROD [Sovinec et al., JCP 195, 355 (2004)]. We include profiles outside the LCFS which generate associated currents when we solve the Grad-Shafranov equation with open-flux regions using the NIMEQ solver [Howell and Sovinec, CPC 185, 1415 (2014)]. The new solution is an equilibrium that closely resembles the original reconstruction (which does not contain open-flux currents). This regenerated equilibrium is consistent with the profiles that are measured by the high quality diagnostics on DIII-D. Results from nonlinear NIMROD simulations of the EHO are presented. The full measured rotation profiles are included in the simulation. The simulation develops into a saturated state. The saturation mechanism of the EHO is explored and simulation is compared to magnetic-coil measurements. [Preview Abstract] |
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TP12.00105: ELM suppression in high-purity DIII-D helium plasmas T.E. Evans, P. Gohil, R.J. Groebner, T.H. Osborne, A. Loarte, E. Unterberg, B. Grierson, M. Fenstermacher ELM suppression in He plasmas with D core concentrations of less than 20$\%$ have been obtained in ECR heated, ITER Similar Shaped plasmas with low pedestal toroidal rotation ($v_\phi<$10 km/s). Here, $n$=3 RMP fields are used to suppress large type-I ELMs at power levels marginally above the PL-H threshold ($P_{ECRH}$=2.9 MW). ELM suppression in He plasmas has also been obtained using balanced co- and counter-Ip injected D neutral beams, with P$_{NBI}$=1.7 MW which is near the P$_{L-H}$ threshold, resulting in $v_\phi\sim$ 0. The electron perpendicular rotation frequency during ELM suppression does not cross zero, assuming no uncertainty in the measurement, but remains slightly negative, with an average frequency of -5 krad/s, between 0.80 and 0.94 in normalized poloidal flux. This suggests that magnetic island screening is weak or nonexistent from the top of the H-mode pedestal inward to surfaces relatively deep in the core plasma. [Preview Abstract] |
(Author Not Attending)
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TP12.00106: Optimization of RMP Coils for ELM Control Someswar Dutta, T.E. Evans, D.M. Orlov Advanced DIII-D RMP coils with improved capabilities are studied using a vacuum island overlap width (VIOW) criterion. Changes in characteristics of the RMP field produced by different geometrical parameters using both ex-vessel (C- and O-) and in-vessel (I- and CP-) coils are discussed. By reducing the poloidal span of each coil, the spacing between them and varying the geometric angle between the coils and the plasma, the resonant field can be adjusted to optimize the edge VIOW criterion while minimizing core resonances. Three separate phase scans using a combination of the as built I-coils and proposed CP-coils are compared for three different equilibria. Two of these equilibria have different edge safety factors and the third one has a different gap between plasma and wall than the standard equilibrium scenario of DIII D. The scan results show that the VIOW correlation criterion is well satisfied in all three cases, resulting in a new way to optimize the RMP coils for the future reactors in order to achieve the ELM suppression criterion over a significantly wider range of fusion plasma operating scenarios. [Preview Abstract] |
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TP12.00107: Spatial decoupling of the n = 1 plasma response during $n$ = 2 RMP ELM suppression on DIII-D J.D. King, E.J. Strait, R.J. Buttery, R.J. La Haye, C. Paz-Soldan, R. Nazikian Experiments in the DIII-D tokamak show a strong $n$ = 1 mode appears when edge localized modes (ELM) are suppressed via an applied $n$ = 2 resonant magnetic perturbation (RMP). The poloidal structure of this $n$ = 1 mode, as it unlocks from the vacuum vessel wall, is discussed. An $n$ = 2 mode is found to be entrained by the rotating RMP and an estimate of the modes m is presented. Previous work suggests these $n$ = 1, 2 modes are magnetic island structures located at the top of the H-mode pedestal.\footnote{R. Nazikian et al., Phys. Rev. Lett., 114, 105002 2015.} The width of these islands is estimated to be 2 $\sim$ 3 cm and the calculated confinement degradation due to their presence is 8 $\sim$ 12\%, which is close to the 13 $\sim$ 14\% measured between the ELMing and RMP suppressed states. This suggests island energy transport may be sufficient to explain the change in peeling-ballooning stability during RMP induced ELM suppression. [Preview Abstract] |
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TP12.00108: Benchmarking extended MHD modeling of resonant magnetic perturbations against measurements of islands and screening on DIII-D M.W. Shafer, J.M. Canik, S.P. Hirshman, T.E. Evans, N.M. Ferraro Experiments where resonant magnetic perturbations (RMPs) are applied to L-mode plasmas are used to test the fundamental plasma response physics on island formation in resistive MHD models. Fine torque scans reveal that large RMP-induced $n$=1 islands open at multiple mode-rational surfaces ($m$=2,3,4) at low rotation, but are screened elsewhere. Time-independent linear resistive MHD simulations with the M3D-C1 code predict a narrow region centered at at $\omega_{e\bot}$ $<$ 0 where resonant fields are found to be weakly screened, though strongly screened elsewhere. Experimentally, the island formation window is wider and centered at $\omega_{e\bot}$ $<$ 0. Nonlinear resistive MHD simulations in the absence of rotation are performed with the SIESTA code. A series of meta-stable equilibria are modeled with an increasing helical 2/1 perturbation. These meta-stable equilibria demonstrate the transition from even-parity screening currents at the rational surface at low perturbation levels to odd-pairty Pfirsch-Schl\"uter currents when a large island is present. [Preview Abstract] |
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TP12.00109: M3D-$C^1$ simulations of plasma response in ELM-mitigated ASDEX Upgrade and DIII-D discharges B.C. Lyons, N.M. Ferraro, S.R. Haskey, N.C. Logan The extended magnetohydrodynamics (MHD) code M3D-$C^{1}$ is used to study the time-independent, linear response of tokamak equilibria to applied, 3D magnetic perturbations. In doing so, we seek to develop a more complete understanding of what MHD phenomena are responsible for the mitigation and suppression of edge-localized modes (ELMs) and to explain why the success of ELM suppression experiments differs both within a single tokamak and across different tokamaks. We consider such experiments on ASDEX Upgrade and DIII-D. We examine how resonant and non-resonant plasma responses are affected by varying the relative magnitude and phase of sets of magnetic coils. The importance of two-fluid effects, rotation profiles, plasma $\beta$, collisionality, bootstrap current profiles, and various numerical parameters are explored. The results are verified against other MHD codes (e.g., IPEC, MARS), correlated to observations of ELM mitigation or suppression, and validated against observed magnetic responses. [Preview Abstract] |
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TP12.00110: Electron temperature fluctuations changes associated with ELM suppression by RMP in DIII-D C. Sung, G. Wang, T. Rhodes, W. Peebles New results in this presentation show an increase in broadband electron temperature fluctuations ($\tilde{T}_e$) during ELM suppression by resonant magnetic perturbations (RMP). This measurement is obtained via correlation ECE (CECE) near the top of the pedestal ($\rho\sim$ 0.9 - 0.96). This $\tilde{T}_e$ increase is significant, ($>$40\%), and occurs after the ELM suppression but not between ELMS. This may imply an increase in thermal transport facilitated by the increased $\tilde{T}_e$ levels. Considering that the changes in gradient scale length during ELMs with RMP are complicated, it is possible that the mechanism responsible for changing $\tilde{T}_e$ is different compared to previously observed changes in $\tilde{n}_e$ [G. R. McKee et al NF 2013]. This possibility, and the nature of the $\tilde{T}_e$, will be studied through profile analysis and linear gyrokinetic analysis using TGLF [J. E. Kinsey et al PoP 2008]. In addition, the relation between the $\tilde{T}_e$ and an observed low frequency coherent mode will be investigated. [Preview Abstract] |
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TP12.00111: Effect of Resonant Magnetic Field Perturbations (RMPs) on Density Profile Evolution in DIII-D L. Zeng, T.L. Rhodes, E.J. Doyle, W.A. Peebles, G. Wang, T.E. Evans The effect of $n$=3 RMPs on density profile evolution is investigated in low collisionality ITER similar discharges. A significant decrease in plasma density is associated with RMP application and subsequent ELM suppression. Profile reflectometer measurements with high temporal (25 $\mu$s) and spatial ($\sim$4 mm) resolution are used to track profile evolution through the ELM suppression. Evidence is presented showing that the enhanced particle transport during RMP operation is not due to more repetitive ELM particle exhaust, but is primarily RMP induced. The magnetic field line loss fraction from the TRIP3D field line tracing code (a proxy for the full plasma response), indicates that the width of the edge stochastic layer exceeds the experimentally observed DIII-D ELM suppression correlation criterion\footnote{Fenstermacher, PoP 15, 56122} when the rate of pedestal density pump-out reaches a maximum. Detailed profile and n evolution during RMPs and their relation to transport changes are also presented and discussed. [Preview Abstract] |
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TP12.00112: High Frequency ELM Pacing by Lithium Pellet Injection on DIII-D A. Bortolon, R. Maingi, D.K. Mansfield, A. Nagy, A.L. Roquemore, R. Lunsford, G.L. Jackson, T.H. Osborne, P.B. Parks Full-shot, high-frequency pacing of edge localized modes (ELM) by lithium pellet injection has been demonstrated in DIII-D. A Lithium Granule Injector (LGI), recently installed on DIII-D to study pacing efficiency dependence on granule size and velocity, was tested in different ELMy scenarios ($\beta _{\mathrm{N}}=$1.2-2.0) injecting granules of nominal diameter 0.3-0.9~mm, with injection speed 50-120~m/s and injection rates up to 500~Hz. Robust ELM pacing was documented on time windows up to 3.5~s, with triggering efficiency close to 100{\%} obtained with 0.9~mm diameter granules, lower with smaller sizes and weakly dependent on granule velocity. Paced ELM frequencies up to 100~Hz were achieved, with a 2-5 fold increase over the natural ELM frequency and a consequent reduction of divertor peak heat flux. Overall, LGI high frequency pacing appeared to be compatible with high plasma performance, in terms of global confinement and pedestal characteristics. [Preview Abstract] |
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TP12.00113: Adapting 3D Equilibrium Reconstruction to Reconstruct Weakly 3D H-mode Tokamaks M.R. Cianciosa, S.P. Hirshman, S.K. Seal, E.A. Unterberg, R.S. Wilcox, A. Wingen, J.D. Hanson The application of resonant magnetic perturbations for edge localized mode (ELM) mitigation breaks the toroidal symmetry of tokamaks. In these scenarios, the axisymmetric assumptions of the Grad-Shafranov equation no longer apply. By extension, equilibrium reconstruction tools, built around these axisymmetric assumptions, are insufficient to fully reconstruct a 3D perturbed equilibrium. 3D reconstruction tools typically work on systems where the 3D components of signals are a significant component of the input signals. In nominally axisymmetric systems, applied field perturbations can be on the order of 1\% of the main field or less. To reconstruct these equilibria, the 3D component of signals must be isolated from the axisymmetric portions to provide the necessary information for reconstruction. This presentation will report on the adaptation to V3FIT for application on DIII-D H-mode discharges with applied resonant magnetic perturbations (RMPs). Newly implemented motional stark effect signals and modeling of electric field effects will also be discussed. [Preview Abstract] |
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TP12.00114: Investigation of torque generated by Test Blanket Module mock-up in DIII-D A. Salmi, T. Tala, M. Lanctot, J.S. deGrassie, C. Paz-Soldan, N. Logan, W.M. Solomon, B.A. Grierson Experiments at DIII-D have investigated the scaling of Test Blanket Module (TBM) torque with plasma pressure and collisionality by performing dimensionless parameter scans. In each configuration, neutral beam torque modulation and TBM torque modulation were sequentially applied to allow experimental characterization of the TBM generated torque and the underlying transport. Calculations of the neoclassical toroidal viscosity (NTV) torque with PENT code of these plasmas find that TBM torque is strongly edge localized while the tentative experimental analysis indicates a more radially broad TBM torque profile. Both the experimental and PENT results will be elaborated and experimental TBM torque scaling with pressure and collisionality presented. Experimental validation of existing plasma response and NTV torque models is an important step toward understanding the impact of magnetic field ripple on plasma rotation, and for predicting the required compensation fields. [Preview Abstract] |
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TP12.00115: Atomic and Molecular Spectroscopic Studies of the DIII-D Neutral Beam Ion Source and Neutralizer B. Crowley, J. Rauch, J.T. Scoville, S.K. Sharma, B. Choksi The neutral beam system is interesting in that it comprises two distinct low temperature plasmas. Firstly, the ion source is typically a filament or RF driven plasma from which ions are extracted by a high voltage accelerator grid system. Secondly the neutralizer is essentially a low temperature plasma system with the beam serving as the primary ionization source and the neutralizer walls serving as conducting boundaries. Atomic spectroscopy of Doppler shifted D-alpha light emanating from the fast atoms is studied to determine the composition of the source and the divergence of the beam. Molecular spectroscopy involves measuring fine structure in electron-vibrational rotational bands. The technique has applications in low temperature plasmas and here it is used to determine gas temperature in the neutralizer. We describe the experimental set-up and the physics model used to relate the spectroscopic data to the plasma parameters and we present results of recent experiments exploring how to increase neutralization efficiency. [Preview Abstract] |
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TP12.00116: Protective Interlocks and Procedures on the DIII-D ECH System J. Lohr, M. Cengher, X. Chen, Y. Gorelov, D. Ponce, R. Prater, A. Torrezan Several new protective interlocks are being installed on the DIII-D ECH system to increase the safety margins for plasma operations at densities approaching cutoff. Inadvertent overdense operation has resulted in reflection of an rf beam back into one of the launchers causing extensive arcing and melt damage on one waveguide line. Therefore, protective steps have been taken to reduce the risk of such damage in the future. These include a density interlock generated by the plasma control system, enhanced video monitoring of the launchers, an ambient light monitor on each of the waveguide systems and versatile rf monitors, measuring forward and reflected power in addition to the mode content of the rf beams, which are installed as the last miter bends in each waveguide line. Calculations of the rf beam trajectories in the plasmas are being performed using the TORAY ray tracing code with input from kinetic profile diagnostics, and strike points for refracted beams on the vacuum vessel are being calculated, which allows evaluating the risk of damage to sensitive diagnostics and hardware. [Preview Abstract] |
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TP12.00117: RF Beam Characterization Measurements on the DIII-D ECH System Y. Gorelov, M. Cengher, J. Lohr, D. Ponce, A. Torrezan Optimum coupling of electron microwave radiation in a tokamak plasma requires polarization control of a high quality Gaussian rf beam. In the DIII\textunderscore D installation, the polarization of the rf beam from each gyrotron is controlled by pairs of grooved mirrors located in transmission line miter bends, The polarizer mirror rotation angles are determined by using a computer model that takes into account the specifics of the transmission line geometry, tokamak magnetic field and equilibrium, the launcher mirror angles and the plasma density. The final polarization of each beam is checked with a polarization measurement at the last miter bend position using an rf power monitor with a rotatable rectangular horn or an orthomode transducer in a newly developed diagnostic miter bend. The Gaussian beam quality is determined by propagating the rf beam into free space and performing a phase retrieval analysis or by measurements using a mode sensitive directional coupler integrated into the diagnostic miter. [Preview Abstract] |
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TP12.00118: Performance Of The DIII-D ECH High Power Transmission Lines And Launching Systems Mirela Cengher, J. Lohr, Y. Gorelov, A. Torrezan, D. Ponce, C. Moeller, R.A. Ellis, E. Kolemen The Electron Cyclotron Heating (ECH) transmission system on the DIII D tokamak consists of corrugated coaxial 31.75 mm waveguide transmission lines and steerable launching mirrors. Total power injected into plasma can reach up to 3.5 MW, with pulse length up to 5 seconds. The ECH power injected to the tokamak from each gyrotron is measured on a shot-to-shot basis and shows individual average injected powers from a gyrotron into the plasma between 540 and 700 kW. The transmission coefficient including the waveguide line and the MOU is between -1.04 dB and -1.43 dB. The maximum ECH energy injected into DIII-D is 16.6 MJ. The HE$_{11}$ mode content is over 85$\%$ for all the lines. The four dual waveguide launchers have increased poloidal scanning speed, and can steer the RF beams 40 degrees poloidally in 200 ms, with real-time poloidal motion control by the plasma control system. A new method of in-situ calibration of the mirror angle was used in conjunction with the upgrading of the encoders and motors for the launchers. [Preview Abstract] |
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TP12.00119: Experiments on Helicon Excitation and Off-Axis Current Drive on DIII-D: Status and Plans R.I. Pinsker, R. Prater, C.P. Moeller, J.S. deGrassie, J.F. Tooker, J.P. Anderson, H. Torreblanca, M. Hansink, A. Nagy, M. Porkolab Fast waves in the LHRF, also called ``whistlers'' or ``helicons,'' will be studied in experiments on the DIII-D tokamak beginning in autumn 2015. In the first stage, a 12-element traveling wave antenna (``comb-line'') is installed in the DIII-D vessel for operation at very low power ($\sim$ 0.1 kW) at 476 MHz, with a well-defined launched $n_{||}$ spectrum peaked at 3.0. The goals of the low-power experiment include: (1) determining the efficiency with which the desired fast waves can be excited under a variety of plasma conditions in discharges relevant to the subsequent high-power current drive experiments and (2) proving that the radial and poloidal location at which the antenna will be mounted does not cause deleterious effects in the DIII-D discharges with high neutral beam power, and that the antenna is not damaged by fast ion losses, etc. Plans for 1 MW-level experiments with a single klystron beginning in FY17 are discussed. In addition to demonstrating off-axis current drive at an efficiency of $\sim$ 60 kA/MW in high-performance plasmas, these experiments will explore non-linear aspects of wave excitation, propagation and absorption such as ponderomotive effects and parametric decay instabilities. [Preview Abstract] |
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TP12.00120: RF Power System and Data Acquisition System for a Helicon Current Drive Antenna on the DIII-D Tokamak H. Torreblanca, J. Anderson A new system to demonstrate efficient off-axis non-inductive current drive from the absorption of a toroidally directed spectrum of very high harmonic fast waves at 476 MHz is being built for installation on the DIII-D tokamak.\footnote{J. Tooker, et al., Development of a Helicon Current Drive System for Installation in the DIII-D Tokamak, to be published in the proceedings of the 26th IEEE Symposium on Fusion Engineering (Austin, TX, 2015).} A high power (1 MW) system is planned but the initial phase of testing will use a low-power antenna powered by a 100 W RF system that has been designed, assembled, and tested. A data acquisition system is also being developed that will acquire data from an array of twelve RF probes in the antenna and digitize them at rates greater than 100 kS/s. Measurements of phase and amplitude of each RF probe signal relative to a fixed reference will be captured to diagnose the performance of the antenna and its interaction with the plasma discharges. Current results on the design and testing of the RF and data acquisition systems will be presented. [Preview Abstract] |
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TP12.00121: Theoretical design of a compact energy recovering divertor D.A. Baver An energy recovering divertor (ERD) is a type of plasma direct converter (PDC) designed to fit in the divertor channel of a tokamak. Such a device reduces the heat load to the divertor plate by converting a portion of it into electrical energy. This recovered energy can then be used for auxiliary heating and current drive, fundamentally altering the relationship between scientific and engineering breakeven and reducing dependence on bootstrap current. Previous work on the ERD concept [1] focused on amplification of Alfven waves in a manner similar to a free-electron laser. While conceptually straightforward, this concept was also bulky, thus limiting its applicability to existing tokamak experiments. A design is presented for an ERD based on sheath-localized waves. This makes possible a device sufficiently compact to fit in the divertor channel of many existing tokamak experiments, and moreover requires no new shaping coils to achieve the desired magnetic geometry or topology. In addition, incidental advantages of this concept will be discussed. \\[4pt] [1] Journal of Fusion Energy 32, 278 (2013). [Preview Abstract] |
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TP12.00122: The Lithium Vapor Box Divertor Robert Goldston, Ammar Hakim, Gregory Hammett, Michael Jaworski, Rachel Myers, Jacob Schwartz Projections of scrape-off layer width to a demonstration power plant\footnote{R.J. Goldston, J. Nucl. Mat. (2015) http://dx.doi.org/10.1016/j.jnucmat.2014.10.080} suggest an immense parallel heat flux, of order 12 GW/m$^{2}$, which will necessitate nearly fully detached operation. Building on earlier work by Nagayama et al.\footnote{Y. Nagayama et al., Fusion Eng. Des. 84 (2009) 1380} and by Ono et al.,\footnote{M. Ono, M.A. Jaworski, R. Kaita et al., Nuc. Fusion 53 (2013) 113030} we propose to use a series of differentially pumped boxes filled with lithium vapor to isolate the buffering vapor from the main plasma chamber, allowing stable detachment. This powerful differential pumping is only available for condensable vapors, not conventional gases. We demonstrate the properties of such a system through conservation laws for vapor mass and enthalpy, and then include plasma entrainment and ultimately an estimate of radiated power. We find that full detachment should be achievable with little leakage of lithium to the main plasma chamber. We also present progress towards solving the Navier-Stokes equation numerically for the chain of vapor boxes, including self-consistent wall boundary conditions and fully-developed shocks, as well as concepts for an initial experimental demonstration-of-concept. [Preview Abstract] |
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TP12.00123: Study of quasi-coherent fluctuations (QCFs) using BOUT$++$ Tianyang Xia, Defeng Kong, Xueqiao Xu, A. Diallo, Xiang Gao, R. Groebner The BOUT$++$ simulations are used to study the characteristics of quasi-coherent fluctuations (QCFs) at different pressure profiles, which are generated by VARYPED tool based on measured plasma profiles from DIII-D. The results show that QCFs can provide the necessary transport to limit and saturate the H-mode pedestal gradient. The simulations predict that (1) QCFs are localized in the pedestal region as observed in DIII-D; (2) the QCFs are near marginal unstable for ideal ballooning modes combined with drift-Alfven wave modes; (3) the frequency of the mode is around 80kHz, close to that of the measured QCF; and (4) particle transport is smaller than the heat transport. (5) Strong nonlinear interactions can be found when the amplitude of QCFs grows to a threshold value. [Preview Abstract] |
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TP12.00124: Studies of Neutral Penetration Depths Variation with Fueling Intensities of SMBI Zhanhui Wang, Yulin Zhou, Xueqiao Xu, Min Xu, Lin Nie, Hao Feng It is very important to find methods of increasing the fueling efficiency and penetration depth of supersonic molecular beam injection (SMBI) for tokamak plasma fueling and other edge plasma physics studies such as ELM control or mitigation. With the new trans-neut module of BOUT$++$ boundary plasma turbulence code, it has further studied neutral penetration depths variation with different fueling intensities of SMBI. The physical model used in the code includes the plasma density, heat and momentum transport equations along with neutral density and momentum transport equations. With the physical model, the molecular transport process during SMBI with various injection speeds and densities, are simulated and compared to study the fueling depth and efficiency. It finds that the radial convection of molecule, rather than the effect of thermal diffusion, dominates the molecular transport process during SMBI. To achieve a better fueling depth and efficiency, it finds increasing the radial injection velocity is more effective than just increasing the molecule injection density. [Preview Abstract] |
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TP12.00125: Comparisons between tokamak fueling of gas puffing and supersonic molecular beam injection in 2D simulations Yulin Zhou, Zhanhui Wang, Xueqiao Xu, Huidong Li, Hao Feng, Weiguo Sun Plasma fueling with high efficiency and deep injection is very important to enable fusion power performance requirements. Two basic fueling methods, gas puffing (GP) and supersonic molecular beam injection (SMBI), are simulated and compared in realistic divertor geometry of the HL-2A tokamak with a newly developed module, named trans-neut, within the framework of BOUT$++$ boundary plasma turbulence code [Z. H. Wang et al., Nucl. Fusion 54, 043019 (2014)]. The physical model includes plasma density, heat and momentum transport equations along with neutral density, and momentum transport equations. Transport dynamics and profile evolutions of both plasma and neutrals are simulated and compared between GP and SMBI in both poloidal and radial directions, which are quite different from one and the other. It finds that the neutrals can penetrate about four centimeters inside the last closed (magnetic) flux surface during SMBI, while they are all deposited outside of the LCFS during GP. It is the radial convection and larger inflowing flux which lead to the deeper penetration depth of SMBI and higher fueling efficiency compared to GP. [Preview Abstract] |
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TP12.00126: Impact of finite beta on kinetic ballooning modes and turbulent transport Chenhao Ma, Xueqiao Xu, Xiaogang Wang We report simulation results of a 3+1 gyro-Landau-Fluid (GLF) model in BOUT++ framework, which contributes to increasing the physics understanding of the edge turbulence. We find that there is no second stable region of kinetic ballooning modes (KBM) in the concentric circular geometry. The unstable threshold of KBM decreases below the ideal ballooning mode threshold with increasing $\eta_i$. In order to study the KBM in the real equilibrium, we find that the approximation of shifted circular geometry ($\beta\ll\varepsilon^2$) is not valid for a high $\beta$ global equilibrium near the second stable region of KBM. Thus we calculate a series of real equilibrium from a global equilibrium solver Corsica, including both Shafranov shift and elongation effects. In these real equilibria, the second stable region of KBM are observed in our global linear simulations. The most unstable mode for different $\beta$ are the same while the mode number spectrum near the second stable region is wider than the case near the first stable region. The impact on the KBM turbulence and transport will be presented based on these equilibria. [Preview Abstract] |
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TP12.00127: Development of test particle module for impurity transport in BOUT++ framework Xiaotao Xiao, Tengfei Tang, Xueqiao Xu Impurity generation and transport are an important topic of research under reactor relevant conditions in order to avoid a significant confinement degradation of the fusion plasmas. It is also a critical issue for RF experiments due to the phenomenon of rf-enhanced impurity generation. Developing the test particle module in BOUT++ framework, which simulates tokamak edge plasmas using fluid models, will enhance the capability to efficiently simulate both turbulence and neoclassical physics in realistic geometry. Firstly, the guiding-center orbit is calculated in cylinder coordinates due to singularity of x-point in flux coordinate. Then evolution of radial profile of impurities in edge plasmas from given sources at the divertor plates and at the protection limiters near RF antennas is obtained by a random walk turbulence model. The characteristics of impurity transport during the ELMs cycles are also simulated by adding a time varying three dimensions perturbed electric field under BOUT++ framework. [Preview Abstract] |
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TP12.00128: Self-consistently simulation of RF sheath boundary condition in BOUT$++$ framework Bin Gui, Xueqiao Xu, Tianyang Xia The effect of the RF sheath boundary condition on the edge-localized modes and the turbulent transport is simulated in this work. The work includes two parts. The first part is to calculate the equilibrium radial electric field with RF sheath boundary condition. It is known the thermal sheath or the rectified RF sheath will modify the potential in the SOL region. The modified potential induces addition shear flow in SOL. In this part, the equilibrium radial electric field across the separatrix is calculated by solving the 2D current continuity equation with sheath boundary condition, drifts and viscosity. The second part is applying the sheath boundary condition on the perturbed variables of the six-field two fluid model in BOUT$++$ framework. The six-field two-fluid model simulates the ELMs and turbulent transport. The sheath boundary condition is applied in this model and it aims to simulate effect of sheath boundary condition on the turbulent transport. It is found the sheath boundary plays as a sink in the plasma and suppresses the local perturbation. Based on this two work, the effect of RF sheath boundary condition on the ELMs and turbulent transport could be self-consistently simulated. [Preview Abstract] |
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TP12.00129: Turbulent Simulations of Divertor Detachment Based On BOUT$++$ Framework Bin Chen, Xueqiao Xu, Tianyang Xia, Minyou Ye China Fusion Engineering Testing Reactor is under conceptual design, acting as a bridge between ITER and DEMO. The detached divertor operation offers great promise for a reduction of heat flux onto divertor target plates for acceptable erosion. Therefore, a density scan is performed via an increase of D$_{\mathrm{2}}$ gas puffing rates in the range of $0.0\sim 5.0\times 10^{23}\mbox{\thinspace s}^{-1}$ by using the B2-Eirene/SOLPS 5.0 code package to study the heat flux control and impurity screening property. As the density increases, it shows a gradually change of the divertor operation status, from low-recycling regime to high-recycling regime and finally to detachment. Significant radiation loss inside the confined plasma in the divertor region during detachment leads to strong parallel density and temperature gradients. Based on the SOLPS simulations, BOUT$++$ simulations will be presented to investigate the stability and turbulent transport under divertor plasma detachment, particularly the strong parallel gradient driven instabilities and enhanced plasma turbulence to spread heat flux over larger surface areas. The correlation between outer mid-plane and divertor turbulence and the related transport will be analyzed. [Preview Abstract] |
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TP12.00130: Benchmark Studies of the Gyro-Landau-Fluid code and Gyro-kinetic Codes on Kinetic Ballooning Modes Tengfei Tang, Xueqiao Xu, Chenhao Ma, Chris Holland, Jeff Candy A Gyro-Landau-Fluid (GLF) 3$+$1 model has been implemented in BOUT$++$ framework recently, which contains full Finite-Larmor-Radius (FLR) effects, Landau damping and toroidal resonance.\footnote{C. H. Ma, X. Q. Xu, et al., \textit{Phys. Plasmas} \textbf{22}, 055903 (2015).} A linear global beta scan has been done using the cbm18 series equilibriums, showing that the unstable modes are kinetic ballooning modes (KBMs). In this work, we use the GYRO code, which is a gyrokinetic continuum code widely used for simulation of the plasma microturbulence, to benchmark with GLF 3$+$1 code on KBMs. As the modes locate in peak pressure gradient region, a linear local beta scan using the same set of equilibriums has been done at this position for comparison. With the drift kinetic electron module in the GYRO code by including small electron-ion collision to damp electron modes, GYRO generated mode structures and parity suggest that they are kinetic ballooning modes, and the growth rate is comparable to the GLF results. However, a radial scan of the pedestal for a particular cbm18 equilibrium shows that the growth rate of the most unstable mode shifts outward to the bottom of pedestal and the real frequency of what was originally the KBMs steadily approaches and crosses over to the electron diamagnetic drift direction. [Preview Abstract] |
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TP12.00131: Drive Mechanisms for Peeling-Ballooning Modes and Implications for Kinetic Ballooning Modes Amelia Lunniss, Howard Wilson, Phil Snyder The EPED model of the H-mode pedestal in tokamak plasmas combines a kinetic ballooning mode (KBM) criterion for the critical pressure gradient with a non-local peeling-ballooning (P-B) mode stability criterion to provide an integrated picture of pedestal structure and ELMs. Employing a set of model tokamak equilibria with pedestal gradients constrained by the KBM criterion, we explore the P-B stability for different pedestal widths. The narrowest widths, corresponding to early in the ELM cycle, are stable. Once a critical width is realised, an intermediate-n P-B mode is destabilised, which we show to be driven by a combination of the kink and curvature contributions to $\delta $W, exceeding field line bending. Although formally of O(n$^{-1})$, we show that the kink term survives to very large n because of steep current density gradients in the pedestal. This kink term is not presently retained in gyro-kinetic codes, but may be important for an accurate prediction of the KBM stability criterion in realistic low collisionality tokamak pedestals. [Preview Abstract] |
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TP12.00132: A theoretical understanding of the scrape-off layer main heat-flux widths multi-tokamak database for limited plasmas Federico Halpern, Jan Horacek, Richard Pitts, Paolo Ricci The Scrape-off Layer and Divertor topical group of the International Tokamak Physics Activity(ITPA) has amassed a database comprising hundreds of measurements of the main scrape-off layer (SOL) heat-flux widthsin inner-wall limited discharges [1]. We have carried out an analysis of the dependence of the heat-flux widthswith respect to the plasma dimensionless parameters, derived fromturbulent transport theory. Restricting our analysis to circular plasmas, we find that a model based on non-linearly saturated turbulence can reproduce the heat-flux width values found in the database with very good agreement. \\[4pt] [1] J. Horacek et al., in 42$^{\mathrm{nd}}$EPS Conference on Plasma Physics, (Lisbon, Portugal, 2015), O2.115. [Preview Abstract] |
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TP12.00133: Analytical and numerical study of the transverse Kelvin-Helmholtz instability in tokamak edge plasmas J.R. Myra, D.A. D'Ippolito, D.A. Russell, M.V. Umansky, D.A. Baver Sheared flows perpendicular to the magnetic field can be driven by Reynolds stresses or ion pressure gradient effects and can potentially influence the stability and turbulent saturation level of edge plasma modes. On the other hand, such flows are subject to the transverse Kelvin-Helmholtz (KH) instability. Here, we first review the linear theory of KH instabilities with an analytic model in the asymptotic limit of long wavelengths compared with the flow scale length. The analytic model treats sheared ExB flows, ion diamagnetism, density gradients and parallel currents in a slab geometry, enabling a unified summary of some well-known results. Second, the important role of realistic toroidal geometry is explored numerically using the 2DX eigenvalue code for KH modes both inside and outside the separatrix. Preliminary results indicate that KH modes are often stable in edge tokamak plasmas, but can also be unstable in some interesting cases. Implications for reduced edge turbulence modeling codes will be discussed. [Preview Abstract] |
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TP12.00134: Study of plasma parameters in expanders of the Gas Dynamic Trap (GDT) E. Soldatkina, M. Korzhavina, A. Dunaevsky, V. Prikhodko, V. Savkin, P. Bagryansky Recent advances in GDT\footnote{A.A.Ivanov and V.V.Prikhodko, Plasma Phys Control. Fusion 55 (2013) 063001} demonstrate the possibility of electron temperatures (Te) of above 1 KeV in mirror machines.\footnote{P.Bagryansky et al, PRL 114 (2015) 205001} Such high Te can be reached if electron heat losses to end walls are limited. Understanding plasma parameters and potential distribution in expanders, common features in GDT and advanced FRCs at Tri Alpha Energy, is crucial for their efficiency. This poster reviews studies of plasma parameters in GDT expanders at on-axis Te of 300-600 eV. Diagnostics includes Langmuir probes, emissive probes, RPAs, and bolometers. Within few centimeters of the end plates, electron energies of $\sim$ 25eV and plasma potentials of several volts are observed. High potential drops are absent in the sheaths, indicative of the role of cold trapped electrons on the potential profiles. New results agree with prior studies at substantially lower on-axis Te. Scaling of plasma parameters with on-axis Te will be reported. This work was supported by the Ministry of Education and Science of Russia (project RFMEFI61914X0003). [Preview Abstract] |
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TP12.00135: Plasma oscillations in impurity seeded divertor R.D. Smirnov, A.S. Kukushkin, S.I. Krasheninnikov, A.Yu. Pigarov, T.D. Rognlien Seeding of divertor plasma with impurities is considered as a method of mitigation of high heat fluxes to divertor targets. In this work we report on self-sustained divertor plasma oscillations induced by seeding with high- and low-Z impurities. The oscillations are demonstrated using modeling with DUSTT/UEDGE and SOLPS4.3 codes of ITER-like divertor plasmas seeded with tungsten or nitrogen impurities. The simulated plasma oscillations for the high- and low-Z impurity seeding cases are characterized by significantly different plasma and impurity dynamics. Correspondingly, two oscillation generation mechanisms of radiation-condensation type are proposed, which are characterized by: i) parallel transport of high-Z impurity ions due to plasma thermal force in SOL, and ii) cross-field transport of low-Z impurity neutrals due to radial pressure gradient in the vicinity of the divertor plates. The both mechanisms are associated with macroscopic plasma-impurity dynamics, which differs the oscillations from intermittent plasma turbulence events. The implications of the plasma oscillations on divertor operation in ITER scale tokamaks, in particular on target plate heat load, are discussed. [Preview Abstract] |
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TP12.00136: Gyrokinetic simulation of edge blobs and divertor heat-load footprint C.S. Chang, S. Ku, R. Hager, M. Churchill, E. D'Azevedo, P. Worley Gyrokinetic study of divertor heat-load width Lq has been performed using the edge gyrokinetic code XGC1. Both neoclassical and electrostatic turbulence physics are self-consistently included in the simulation with fully nonlinear Fokker-Planck collision operation and neutral recycling. Gyrokinetic ions and drift kinetic electrons constitute the plasma in realistic magnetic separatrix geometry. The electron density fluctuations from nonlinear turbulence form blobs, as similarly seen in the experiments. DIII-D and NSTX geometries have been used to represent today's conventional and tight aspect ratio tokamaks. XGC1 shows that the ion neoclassical orbit dynamics dominates over the blob physics in setting Lq in the sample DIII-D and NSTX plasmas, re-discovering the experimentally observed 1/Ip type scaling. Magnitude of Lq is in the right ballpark, too, in comparison with experimental data. However, in an ITER standard plasma, XGC1 shows that the negligible neoclassical orbit excursion effect makes the blob dynamics to dominate Lq. Differently from Lq~1mm (when mapped back to outboard midplane) as was predicted by simple-minded extrapolation from the present-day data, XGC1 shows that Lq in ITER is about 1 cm that is somewhat smaller than the average blob size. [Preview Abstract] |
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TP12.00137: Kinetic dynamics simulation of the detached plasma Theerasarn Pianpanit, Seiji Ishiguro, Hiroki Hasegawa The detached plasma has been proposed to reduce the heat flux to the divertor. Fluid code has been widely used to investigate the detached plasma but the cooling of plasma, trapped particle effects, and other kinetic dynamics in the detached plasma has not been well understood. Particle-in-Cell (PIC) simulation with the Monte Carlo collisions and the cumulative scattering angle coulomb collision are carried out to study dynamical kinetic behavior of the plasma. The constant pressure and temperature of neutral gas box in front of the divertor target model has been used in the simulation. The results show the decrease in electron temperature in front of the divertor plate strongly relate to the Coulomb collision frequency. [Preview Abstract] |
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TP12.00138: 1D Modeling of the Effects of High-Heat Flux ELMs on Partial Detachment in JET and ITER Zhongping Chen, Prashant Valanju, Brent Covele, Mike Kotschenreuther, Swadesh Mahajan, Eva Havlickova, Fulvio Militello Edge localized modes (ELMs) are simulated using a 1D fluid model of the SOL plasma, coupled to a 1 1/2 D neutral model. The simulation is done by using a new code that adapts the SOLF1D code for the plasma and the NUT code for the neutral particles. We simulate ELMs on JET and ITER, starting with a realistic initial condition where the divertor plasma is initially in the partially detached regime. We emphasize heat as the dominant energy input from ELMs, and compare to the case where particles are the dominant energy source, which has been previously studied. We further study the effect of the divertor magnetic field angle with the target in ELMs when the plasma starts in the partially detached regime. [Preview Abstract] |
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TP12.00139: Variable Charge State Impurities in Coupled Kinetic Plasma-Kinetic Neutral Transport Simulations D.P. Stotler, R. Hager, K. Kim, T. Koskela, G. Park A previous version of the XGC0 neoclassical particle transport code with two fully stripped impurity species was used to study kinetic neoclassical transport in the DIII-D H-mode pedestal.\footnote{D. J. Battaglia et al., Phys. Plasmas 21, 072508 (2014).} To properly simulate impurities in the scrape-off layer and divertor and to account for radiative cooling, however, the impurity charge state distributions must evolve as the particles are transported into regions of different electron temperatures and densities. To do this, the charge state of each particle in XGC0 is included as a parameter in the list that represents the particle's location in phase space. Impurity ionizations and recombinations are handled with a dedicated collision routine. The associated radiative cooling is accumulated during the process and applied to the electron population later in the time step. The density profiles of the neutral impurities are simulated with the DEGAS 2 neutral transport code and then used as a background for electron impact ionization in XGC0 via a test particle Monte Carlo method analogous to that used for deuterium.\footnote{D. P. Stotler et al., Comput. Sci. Disc. 6, 015006 (2013).} [Preview Abstract] |
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TP12.00140: Modification of far-SOL flow by substantial gas injection in the inboard poloidal field null configuration on QUEST T. Onchi, H. Zushi, Y. Oyama, K. Mishra, Y. Nagashima, K. Hanada, H. Idei, M. Hasegawa, A. Kuzmin, K. Nakamura, A. Fujisawa, K. Nagaoka Spontaneous plasma flow is generated in the inboard poloidal field null (IPN) configuration on QUEST spherical tokamak. Previous research has found that there is a significant relationship between plasma current and far-SOL flow. Consequently, the SOL flow is influenced by global modification of IPN plasma. For further understanding of the far-SOL flow, a hybrid probe measuring plasma flow, electron density $n_{\mathrm{e}}$, and temperature $T_{\mathrm{e}}$, was installed in the far-SOL. Using the hybrid probe and divertor probe array, two-point observation on an open flux surface was performed to study the SOL-divertor relationship and the particle transport. Substantial gas injection (GI) from private region of the IPN configuration leads to high core density but $I_{\mathrm{p}}$-drop by 50 $\%$. $n_{\mathrm{e}}$ and $T_{\mathrm{e}}$ in the far-SOL and divertor region are modified appreciably due to the GI. Poloidal flow reversal occurs and toroidal velocity drops by about 50 $\%$ in the far-SOL. We investigate modifications of pressure gradient, electric field and particle transport, and study complex structure of the far-SOL flow. [Preview Abstract] |
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TP12.00141: Edge impurity transport in the presence of magnetic islands with EMC3-EIRENE A. Bader, O. Schmitz, M. Kobayashi, F. Effenberg, C.C. Hegna, K. Ida, M. Yoshinuma, Y. Feng Control of edge impurities is a crucial area of research for next generation reactors. These devices require the limitation of impurity accumulation in the core plasma, exhaust of helium ash, and possibly isotropic heat flux distribution via impurity radiation. Simple transport of impurities involves a competition between the plasma friction force and the thermal gradient. The presence of magnetic islands complicates behavior and can lead to impurity accumulation in edge regions while avoiding core contamination in some configurations. Experimental results on LHD have shown differences in helium transport with the introduction of an edge 1/1 island. We present simulations of helium data with the fluid edge code EMC3-EIRENE and compare to spectroscopic measurements from LHD. In addition we show simulation data from alternative magnetic configurations based on the HSX coil system with large edge islands, focusing on the evolution of impurity radiation profiles as a function of plasma density. [Preview Abstract] |
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TP12.00142: 1D2V Kinetic Simulations of Plasmas in the Scrape-Off Layer with Recycling E.L. Shi, T. Stoltzfus-Dueck, A. Hakim, G.W. Hammett The effects of neutral particle recycling into the scrape-off layer in a 1D geometry along the parallel direction are studied with gyrokinetic simulations. This is of interest as a step towards understanding how reduced recycling with lithium can improve confinement. Outflow from the core is represented as a steady source of plasma localized about the midplane, and the ionization of recycled neutrals is represented as a source of cold plasma at the divertor plate. We use the Gkeyll code to calculate the steady-state temperature and density profiles and find the parametric dependencies of upstream and target temperatures on the recycling coefficient. Comparisons are made with a fluid model to identify regimes where a fluid treatment is valid. [Preview Abstract] |
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TP12.00143: Secondary-electron emission effects in a 1D ELM model studied with Gkeyll T. Stoltzfus-Dueck, A. Hakim, E.L. Shi, G.W. Hammett The 1D ELM heat pulse problem of Havl{\'i\v{c}kov\'a} et al (PPCF 54: 045002) is generalized to include secondary electron emission (SEE), an important factor since the secondary electron emission coefficient $\delta$ varies widely for different wall materials, ranging from $\delta <\sim 0.5$ for lithium to $\delta >1$ for standard high-$Z$ metals at large $T_e$. For moderate collisionality regimes, analysis and gyrokinetic simulation with the Gkeyll code show an enhancement of the electron heat flux by $\sim(1-\delta)^{-1}$, a large enhancement for $\delta$ approaching unity. In very collisionless regimes, this enhancement is reduced as secondary electrons escape the plasma before isotropizing in pitch angle. [Preview Abstract] |
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TP12.00144: Properties of Discontinuous Galerkin Algorithms and Implications for Edge Gyrokinetics G.W. Hammett, A. Hakim, E.L. Shi, I.G. Abel, T. Stoltzfus-Dueck The continuum gyrokinetic code Gkeyll uses Discontinuous Galerkin (DG) algorithms, which have a lot of flexibility in the choice of basis functions and inner product norm that can be useful in designing algorithms for particular problems. Rather than use regular polynomial basis functions, we consider here Maxwellian-weighted basis functions (which have similarities to Gaussian radial basis functions). The standard Galerkin approach loses particle and energy conservation, but this can be restored with a particular weight for the inner product (this is equivalent to a Petrov-Galerkin method). This allows a full-$F$ code to have some benefits similar to the Gaussian quadrature used in gyrokinetic $\delta f$ codes to integrate Gaussians times some polynomials exactly. In tests of Gkeyll for electromagnetic fluctuations, we found it is important to use consistent basis functions where the potential is in a higher-order continuity subspace of the space for the vector potential $A_{||}$. A regular projection method to this subspace is a non-local operation, while we show a self-adjoint averaging operator that can preserve locality and energy conservation. This does not introduce damping, but like gyro-averaging involves only the reactive part of the dynamics. [Preview Abstract] |
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TP12.00145: Energy conserving continuum algorithms for kinetic \& gyrokinetic simulations of plasmas A. Hakim, G.W. Hammett, E. Shi, T. Stoltzfus-Dueck We present high-order, energy conserving, continuum algorithms for the solution of gyrokinetic equations for use in edge turbulence simulations. The distribution function is evolved with a discontinuous Galerkin scheme, while the fields are evolved with a continuous finite-element method. These algorithms work for a general, possibly non-canonical, Poisson bracket operator and conserve energy exactly. Benchmark simulations with ETG turbulence in 3X/2V are shown, as well as initial applications of the algorithms to turbulence in a simplified SOL geometry. Sheath boundary conditions with recycling and secondary electron emission are implemented, and a Lenard-Bernstein collision operator is included. Extension of these algorithms to full Vlasov-Maxwell equations are presented. It is shown that with a particular choice of numerical fluxes the total (particle+field) energy is conserved. Algorithms are implemented in a flexible and open-source framework, Gkeyll, which also includes fluid models, allowing potential hybrid simulations of various plasma problems. [Preview Abstract] |
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TP12.00146: Initial development of 5D COGENT R. H. Cohen, W. Lee, M. Dorf, M. Dorr COGENT is a continuum gyrokinetic edge code being developed by the by the Edge Simulation Laboratory (ESL) collaboration. Work to date has been primarily focussed on a 4D (axisymmetric) version that models transport properties of edge plasmas\footnote{M. Dorf, invited talk, this meeting}. We have begun development of an initial 5D version to study edge turbulence, with initial focus on kinetic effects on blob dynamics and drift-wave instability in a shearless magnetic field\footnote{for application, see Wonjae Lee {\it et al.}, this meeting}. We are employing compiler directives and preprocessor macros to create a single source code that can be compiled in 4D or 5D, which helps to ensure consistency of physics representation between the two versions. A key aspect of COGENT is the employment of mapped multi-block grid capability to handle the complexity of diverter geometry. It is planned to eventually exploit this capability to handle magnetic shear, through a series of successively skewed unsheared grid blocks. The initial version has an unsheared grid and will be used to explore the degree to which a radial domain must be block decomposed. We report on the status of code development and initial tests. [Preview Abstract] |
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TP12.00147: Kinetic Effects on Plasma Blob Dynamics with Plasma Sheath Hiroki Hasegawa, Seiji Ishiguro Kinetic effects on plasma blob dynamics with plasma sheath have been studied with a three dimensional electrostatic plasma particle simulation code. In the particle simulation, an external magnetic field {\boldmath $B$} is pointing into the $z$ direction (corresponding to the toroidal direction). The strength of ambient magnetic field increases in the positive $x$ direction (corresponding to the counter radial direction), i.e., $\partial B / \partial x > 0$. A coherent structure is initially set as a column along the external magnetic field\footnote{H. Hasegawa and S. Ishiguro, Plasma Fusion Res. \textbf{7}, 2401060 (2012).}. In our previous study, we investigated kinetic effects on plasma blob dynamics in the system where the periodic boundary condition is applied in the $z$ direction and found that the symmetry breaking in a blob profile occurs by the kinetic effect.\footnote{S. Ishiguro and H. Hasegawa, 55th APS-DPP meeting, PP8.39 (2013).} In this study, we have applied the particle absorbing boundaries to the ends in the $z$ direction and studied such kinetic effects with the plasma sheath. In the simulation, not only the symmetry breaking shown in the previous study but also other properties which were not found in the periodic boundary case have been observed. [Preview Abstract] |
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TP12.00148: Kinetic Modeling of Divertor Plasma Seiji Ishiguro, Hiroki Hasegawa, Theerasarn Pianpanit Particle-in-Cell (PIC) simulation with the Monte Carlo collisions and the cumulative scattering angle coulomb collision can present kinetic dynamics of divertor plasmas. We are developing two types of PIC codes. The first one is the three dimensional bounded PIC code where three dimensional kinetic dynamics of blob is studied and current flow structures related to sheath formation are unveiled.\footnote{S. Ishiguro and H. Hasegawa, J. Plasma Phys. 72, 1233 (2006)}$^,$\footnote{H. Hasegawa and S. Ishiguro, Plasma Fusion Res. 7, 2401060 (2012)} The second one is the one spatial three velocity space dimensional (1D3V) PIC code with the Monte Carlo collisions where formation of detach plasma is studied.\footnote{S. Ishiguro, T. Pianpanit, H. Hasegawa, and R. Kanno, 55th APS-DPP meeting, TP8, 28(2014)} First target of our research is to construct self-consistent full kinetic simulation modeling of the linear divertor simulation experiments. [Preview Abstract] |
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TP12.00149: Graphene's Viability for Fusion Applications Marcos Navarro, Karla Hall, Richard Rojas, John Santarius, Gerald Kulcinski Graphene is a source of interest for multiple applications due to its unusual electronic and physical properties. As a coating material, it has reduced oxidation of the main substrate, though no effort has been reported of testing it under fusion conditions. A number of experimental studies have established that defect-free graphene is an excellent barrier material for gases. We explore its viability to maintain a significant pressure difference under ion irradiation. Deuterium is used as a projectile on graphene coated silicon over a range of 10-50 keV energies and various fluences. The vacancy yield (amount of damage) and natural resonance for graphene are found at around 1350 cm$^{-1}$ and 1550 cm$^{-1}$, respectively. Damage of each sample is quantified via Raman spectroscopy (RS) using the ratio of the intensities at these wavenumbers. Graphene is also tested here as a coating for some fusion components. Though tungsten is a very promising divertor and first wall candidate, after intense irradiation, it is prone to developing fuzz or grass structures, leading to a diminished lifetime. Graphene grown on tungsten is tested under reactor conditions with 30 keV He ions at several fluences, and the sputtering of both materials is studied via RS and Scanning Electron Microscopy. [Preview Abstract] |
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TP12.00150: Examining Helium Line Intensities and Ratios in a Linear Helium Plasma to Identify T$_{\mathrm{e}}$ and n$_{\mathrm{e}}$ H. Ray, T.M. Biewer, E.A. Unterberg, D.T. Fehling, R.C. Isler Oak Ridge National Laboratory's prototype Material Plasma Exposure eXperiment (Proto-MPEX) is a linear plasma device dedicated to the understanding of plasma material interaction physics. A photomultiplier tube (PMT) based diagnostic system called a filterscope examines the visible light emission from Proto-MPEX. The filterscope is a non-invasive, high sensitivity, and high temporal resolution compact system with multiple PMT channels. Three PMTs contain He I narrow bandpass filters of wavelengths 667.9, 723.6, and 706.7 nm for line ratio analysis. Helium line intensities and ratios have been widely applied on astrophysical plasmas and machines such as JET and NSTX to determine profiles of electron temperatures, T$_{\mathrm{e}}$, and densities, n$_{\mathrm{e}}$. Ratios of the He I intensities measured by the filterscope are compared to calculated intensity ratios determined through a collisional radiative model (CRM) as follows: An excited He atom in state P will de-excite to a lower energy level Q by emitting a photon of a specific wavelength. A CRM uses the interactions P has with Q and other energy levels to calculate the population density of P. The calculated population density is used to determine the spectral line intensity of the wavelength analyzed. The aforementioned process is performed for each of the He I bandpass filters, and ratios dependent on T$_{\mathrm{e}}$ and n$_{\mathrm{e}}$ are calculated and compared to the filterscopes measured ratios.\\[4pt] This work was supported by the US. D.O.E. contract DE-AC05-00OR22725. [Preview Abstract] |
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TP12.00151: A 28 GHz ECH/EBW System for the Proto-MPEX plasma source Tim Bigelow, John Caughman, Ian Campbell, Stephanie Diem, Carl Dukes, Richard Goulding, Stephen Killough, Juergen Rapp The Prototype Materials Plasma Exposure Experiment (Proto-MPEX) is a linear high-intensity RF plasma source [1] that requires plasma electron heating in overdense conditions to provide target parameters in the density and temperature range needed for plasma facing material studies. In Proto-MPEX, a dense helicon plasma is produced by 13.56 MHz RF power and is further heated by 28 GHz microwaves via Electron Bernstein Waves (EBW). A 28 GHz 200 kW cw gyrotron system from earlier experiments at ORNL provides the microwave power and has been successful to date at generating \textgreater 150 kW in short pulses into a dummy load and \textgreater 100 kW into the plasma via a 88.9 mm corrugated waveguide system and compact launcher near the plasma edge. For successful coupling via EBW into an overdense plasma, the launcher must be optimized and if possible have adjustable launch angle to maximize the efficiency. Modeling of the EBW coupling has been performed using the GENRAY-C code for the expected plasma profile in order to determine the best beam profile and polarization requirements. A compact HE11 mode waveguide launch with adjustable launch angle has been installed that is tightly coupled to the plasma. \\[4pt] [1] J.B.O. Caughman, et al, this conference [Preview Abstract] |
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TP12.00152: A target station for plasma exposure of neutron irradiated fusion material samples to reactor relevant conditions Juergen Rapp, Dominic Giuliano, Ronald Ellis, Richard Howard, Jeremy Lore, Arnold Lumsdaine, Timothy Lessard, William McGinnis, Steven Meitner, Larry Owen, Venugopal Varma The Material Plasma Exposure eXperiment (MPEX) is a device planned to address scientific and technological gaps for the development of viable plasma facing components for fusion reactor conditions (FNSF, DEMO). It will have to address the relevant plasma conditions in a reactor divertor (electron density, electron temperature, ion fluxes) and it needs to be able to expose a-priori neutron irradiated samples. A pre design of a target station able to handle activated materials will be presented. This includes detailed MCNP as well as SCALE and MAVRIC calculations for all potential plasma-facing materials to estimate dose rates. Details on the remote handling schemes for the material samples will be presented. 2 point modeling of the linear plasma transport has been used to scope out the parameter range of the anticipated power fluxes to the target. This has been used to design the cooling capability of the target. The operational conditions of surface temperatures, plasma conditions, and oblique angle of incidence of magnetic field to target surface will be discussed. [Preview Abstract] |
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TP12.00153: Operation of the Proto-MPEX High Intensity Plasma Source J.B.O. Caughman, R.H. Goulding, T.M. Biewer, T.S. Bigelow, I.H. Campbell, S.J. Diem, E.H. Martin, P.V. Pesavento, J. Rapp, H.B. Ray, G.C. Shaw, M.A. Showers, G.-N Luo The Prototype Materials Plasma Experiment (Proto-MPEX) is a linear high-intensity rf plasma source that combines a high-density helicon plasma generator with electron and ion heating sections. It is being used to study the physics of heating over-dense plasmas in a linear configuration. The helicon plasma is produced by coupling 13.56 MHz rf power at levels up to 100 kW. Microwaves at 28 GHz ($\sim$ 150 kW) are coupled to the electrons in the over-dense helicon plasma via Electron Bernstein Waves (EBW). Ion cyclotron heating ($\sim$ 30 kW) will be via a magnetic beach approach. Plasma diagnostics include Thomson Scattering and a retarding field energy analyzer near the target, while a microwave interferometer and double-Langmuir probes are used to determine plasma parameters elsewhere in the system. Filterscopes are being used to measure D-alpha emission and He line ratios at multiple locations, and IR cameras image the target plates to determine heat deposition. High plasma densities in the helicon region have been produced in He (\textgreater 3x10$^{19}$/m$^{3})$ and D (\textgreater 1.5x10$^{19}$/m$^{3})$, and operation with on-axis magnetic field strength \textgreater 1 T has been demonstrated. Details of the experimental results and future plans for studying plasma surface/RF antenna interactions will be presented. [Preview Abstract] |
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TP12.00154: Overview of diagnostic implementation on Proto-MPEX at ORNL T.M. Biewer, T. Bigelow, J.B.O. Caughman, D. Fehling, R.H. Goulding, T.K. Gray, R.C. Isler, E.H. Martin, S. Meitner, J. Rapp, E.A. Unterberg, R.S. Dhaliwal, D. Donovan, N. Kafle, H. Ray, G.C. Shaw, M. Showers, R. Mosby, C. Skeen The Prototype Material Plasma Exposure eXperiment (Proto-MPEX) recently began operating with an expanded diagnostic set. Approximately 100 sightlines have been established, delivering the plasma light emission to a ``patch panel'' in the diagnostic room for distribution to a variety of instruments: narrow-band filter spectroscopy, Doppler spectroscopy, laser induced breakdown spectroscopy, optical emission spectroscopy, and Thomson scattering. Additional diagnostic systems include: IR camera imaging, in-vessel thermocouples, ex-vessel fluoroptic probes, fast pressure gauges, visible camera imaging, microwave interferometry, a retarding-field energy analyzer, rf-compensated and ``double'' Langmuir probes, and B-dot probes. A data collection and archival system has been initiated using the MDSplus format. This effort capitalizes on a combination of new and legacy diagnostic hardware at ORNL and was accomplished largely through student labor. [Preview Abstract] |
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TP12.00155: Estimates of heat flux to material surfaces in Proto-MPEX with IR imaging M. Showers, T.M. Biewer, T.S. Bigelow, J.B.O. Caughman, D. Donovan, R.H. Goulding, T.K. Gray, J. Rapp, D.L. Youchison, R.E. Nygren The Prototype Material Plasma Exposure eXperiment (Proto-MPEX) at Oak Ridge National Laboratory (ORNL) is a linear plasma device with the primary purpose of developing the plasma source concept for the Material Plasma Exposure eXperiment (MPEX), which will address the plasma material interactions (PMI) science for future fusion reactors. New diagnostics for Proto-MPEX include an infrared (IR) camera, in-vessel thermocouples and ex-vessel fluoroptic probes. The IR camera and other diagnostics provide surface temperature measurements of Proto-MPEX's dump and target plates, located on either end of the machine, which are being exposed to plasma. The change in surface temperature is measured over the duration of the plasma shot to determine the heat flux hitting the plates. The IR camera additionally provides 2-D thermal load distribution images of these plates, highlighting Proto-MPEX plasma behaviors, such as hot spots. The plasma diameter on the dump plate is on the order of 15 cm. The combination of measured heat flux and the thermal load distribution gives information on the efficiency of Proto-MPEX as a plasma generating device. Machine operating parameters that will improve Proto-MPEX's performance may be identified, increasing its PMI research capabilities. [Preview Abstract] |
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TP12.00156: Preliminary Experiments to Develop a He-W Calibration Standard Using Laser Induced Breakdown Spectroscopy Guinevere Shaw, Nicolas Andre, Mark Bannister, Theodore Biewer, Madhavi Martin, Fred Meyer, Brian Wirth To address the needs of future fusion reactors, laser based diagnostic techniques for plasma-material interactions (PMI) are being developed at ORNL. Laser-induced breakdown spectroscopy (LIBS) is a technique for measuring elemental surface composition, and is a possible diagnostic for characterizing elemental concentrations in plasma-facing materials. The purpose of the LIBS system described here is to quantify helium (He) concentration in exposed tungsten (W) targets. To accurately quantify He concentration in situ a calibration stranded must be developed, including extensive calibration of the entire LIBS system. To accomplish this, two LIBS setups were explored: \textit{ex-situ} LIBS and \textit{in-situ} LIBS. \textit{Ex-situ} LIBS experiments used W targets exposed to a He$+$ ion beam to determine laser parameters and calibration settings for in-situ experiments. Results will be discussed. \textit{In-situ} LIBS analysis will be assessed for W targets exposed to He plasma. Preliminary results will be discussed. [Preview Abstract] |
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TP12.00157: Doppler spectroscopy on plasma discharges produced in Proto-MPEX Runpal Dhaliwal, Theodore Biewer, Chris Klepper, Elijah Martin, Juergen Rapp The Prototype Material Plasma Exposure eXperiment (Proto-MPEX) is a linear machine that produces pulsed plasma discharges, and is intended to study plasma-material interactions in conditions similar to those found in future fusion reactors. A high-resolution McPherson Czerny-Turner visible range spectrometer has been installed to study the behavior of ions in the plasma. Together with a Princeton Instruments EMCCD camera and an external trigger box, this system provides excellent spectral and temporal resolution for viewing the emission spectra of the discharges. Around 100 lines of sight have been established for use by this and other diagnostics in the lab. Initial data from recent experiments validate the utility of this setup. Analysis of spectral lines in helium and deuterium plasmas yields valuable information regarding the temperature and density of plasma ions at various locations in the machine as the various RF heating sources are implemented. Differentiating the thermal width of lines from other sources of broadening is an ongoing process. In addition to He I lines, data indicates the presence of the He II line at 468.5 nm, which corresponds to emission from singly ionized atoms at higher temperatures. [Preview Abstract] |
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TP12.00158: Extending Spectroscopic Capabilities for Mo PFC Erosion Rate Diagnostics S.D. Loch, D.A. Ennis, M.S. Pindzola, C.A. Johnson, G.J. Hartwell, D.A. Maurer, D.C. Griffin, C.P. Ballance, M. Reinke, B. Lipschultz, V. Soukhanovskii The use of ionizations per photon coefficients (SXB) provides a useful means of measuring wall erosion rates. Two problems hindering the use of such diagnostics for high-Z materials are a lack of accurate atomic data and determining which lines from the complex spectral features should be used for accurate erosion measurements. We present a new approach for generating and selecting SXB coefficients for high-Z materials. The theoretical spectra show strong agreement with spectra from the Alcator C-Mod and Compact Toroidal Hybrid experiments. Mo II spectral features are identified, including a line ratio suitable for electron temperature measurements which constrains the SXB implementation. Applications of the new SXBs to NSTX-U edge plasmas is described and future plans for Mo and W influx diagnostics are outlined. [Preview Abstract] |
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TP12.00159: Preliminary Design of a Digital Holography PFC Erosion Diagnostic for MPEX C.E. (Tommy) Thomas Jr., T.M. Biewer, G.C. Shaw, L.R. Baylor, S.K. Combs, S.J. Meitner, J. Rapp, D.L. Hillis, E.M. Granstedt, R. Majeski, R. Kaita Preliminary design of a Digital Holography (DH) in-situ Plasma Facing Component (PFC) erosion diagnostic to be used on the proto-MPEX/MPEX experiment is presented. Design trade-offs are discussed including the selection of CO2 laser frequencies and whether/where to use reflective or refractive optical components. The costs and benefits of using a high-speed (expensive) infrared (IR) camera or a lower speed (inexpensive) IR camera, and whether to use simultaneous or sequential acquisition of DH exposures for the dual wavelength system are also described. Expected layout, resolution, and noise figures will be discussed, along with resolution and noise data from previous work at ORNL and PPPL. [Preview Abstract] |
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TP12.00160: Characterization of Secondary Electron Emission Properties of Plasma Facing Materials Marlene I. Patino, Angela M. Capece, Yevgeny Raitses, Bruce E. Koel The behavior of wall-bounded plasmas is significantly affected by the plasma-wall interactions, including the emission of secondary electrons (SEE) from the wall materials due to bombardment by primary electrons. The importance of SEE has prompted previous investigations of SEE properties of materials especially with applications to magnetic fusion, plasma thrusters, and high power microwave devices. In this work, we present results of measurements of SEE properties of graphite and lithium materials relevant for the divertor region of magnetic fusion devices. Measurements of total SEE yield (defined as the~number of emitted secondary electrons per incident primary electron) for lithium are extended up to 5 keV primary electron energy, and the energy distributions of secondary electrons are provided for graphite and lithium. Additionally, the effect of contamination on the total SEE yield of lithium was explored by exposing the material to water vapor. Auger electron spectroscopy (AES) was used to determine surface composition and temperature programmed desorption (TPD) was used to determine lithium film thickness. Results show an order of magnitude increase in total SEE yield for lithium exposed to water vapor. [Preview Abstract] |
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TP12.00161: Secondary Electron Emission Properties of Plasma Facing Ceramic Materials at High Temperatures Yevgeny Raitses, Paul Dourbal, Rostislav Spektor The plasma-wall interaction in the presence of strong secondary electron emission (SEE) has been studied theoretically and experimentally both as a basic phenomenon and in relation to numerous plasma applications such as, for example, fusion devices and Hall thrusters. Herein, we report on SEE measurements for boron nitride (BN) ceramics, which are commonly used as channel wall materials for Hall thrusters. Measurements were conducted for BN ceramics of three different grades with different fractions (0, 1{\%} and 40{\%}) and phases of different binder additions (calcium borate, silicon oxide) and as a function of the sample temperature relevant to the thruster operation (about 600K). For all three grades, the energy at which the yield equals to 1 at room temperature was measured to be near 40 V. This result is in agreement with previous measurements [1,2]. At the elevated temperature, the yield was slightly different for these ceramics, but lower than at the room temperature. This temperature effect was not as strong as measured in [2]. Analysis of these results and their implication on plasma-surface interactions in Hall thrusters and other related devices are presented.\\[4pt] [1] Dunaevsky et al, Phys. Plasmas \textbf{10}, 2574 (2003);\\[0pt] [2] Belhaj et al, J. Phys. D: Appl. Phys. \textbf{42}, 105309 (2009). [Preview Abstract] |
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TP12.00162: Numerical Characterization of Wall Recycling Conditions of the HIDRA Stellarator using EMC3-EIRENE Steven Marcinko, Davide Curreli The wall recycling conditions created by energetic bombardment of plasma-facing components (PFCs) are of critical importance to determining the plasma and impurity profile in the edge region of a magnetically confined plasma. In this work a pre-online numerical characterization of the edge plasma in HIDRA has been carried out. HIDRA is the former WEGA experiment, now relocated to the University of Illinois at Urbana-Champaign. Numerical simulations of the HIDRA edge environment are performed utilizing the 3D edge plasma and neutral transport code EMC3-EIRENE [Y. Feng J. Nucl. Mater 241--243, 930 (1997)]. In our analysis, emphasis is placed on the influence of the neutrals and the impurities on edge plasma profiles and thus on energy and particle fluxes impingent onto PFCs. We examine the effect of different wall types, comparing high recycling conditions to situations of low recycling. The effect of intrinsic impurity screening is also taken into account under the expected HIDRA operating regimes. We report the calculated particle confinement time and fluid moments of both plasma and neutrals at the low recycling regimes expected with lithium-based PFCs, and compare them with the high recycling regimes found with conventional metal-based PFCs. [Preview Abstract] |
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TP12.00163: Flux threshold measurements of nano-fuzz formation by He-ion beam impact on hot tungsten surfaces F.W. Meyer, H. Hijazi, M.E. Bannnister, L.M. Garrison, C.M. Parish, K.A. Unocic We report measurements of the energy dependence of flux thresholds and incubation fluences for He-ion induced nano-fuzz formation on hot tungsten surfaces using real-time sample imaging of tungsten target emissivity change together with accurate ion-beam flux-profile measurements. The measurements were carried out at the Multicharged Ion Research Facility (MIRF) at ion energies from 218 eV to 8.5 keV, using a high-flux deceleration module and beam flux monitor for optimizing the decel optics on the low energy MIRF beamline. The measurements suggest that nano-fuzz formation proceeds only if a critical rate of change of trapped He density in the W target is exceeded. The energy dependence of three factors contributing to the overall energy dependence, ion reflection, ion range and target damage creation, were determined using the SRIM simulation code. The observed energy dependence can be well reproduced by the combined energy dependences of these three factors. The incubation fluences deduced from first visual appearance of surface emissivity change were 2-4x10$^{23}$/m$^{2}$ at 218 eV, and roughly a factor of 10 less at the higher energies, which were all at or above the displacement energy threshold. Additional measurements at 100 and 200 keV, using beams from the MIRF HV-platform-based ECR source will be presented. [Preview Abstract] |
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TP12.00164: Surface morphology changes to tungsten under exposure to He ions from an electron cyclotron resonance plasma source David Donovan, Anurag Maan, Jonah Duran, Dean Buchenauer, Josh Whaley Exposure of tungsten to low energy (\textless 100 eV) helium plasmas at temperatures between 900-1900 K in both laboratory experiments [1] and tokamaks [2] has been shown to cause severe nanoscale modification of the near surface resulting the growth of tungsten tendrils. We used a relatively low flux (2.5x10$^{19}$ ions m$^{-2}$ s$^{-1}$) compact ECR plasma source at Sandia-California to investigate the early stages of helium induced tungsten damage. Exposures of polished tungsten discs were performed and characterized using SEM, AFM, and FIB cross section imaging. Bubbles have been seen on the exposed tungsten surface and in sub-surface cross sections growing to up to 150 nm in diameter. Comparisons were made between exposures of warm rolled Plansee tungsten discs and ALMT ITER grade tungsten samples. A similar He plasma exposure stage has now been developed at the University of Tennessee-Knoxville with an improved compact ECR plasma source. Status of the new UTK exposure stage will be discussed as well as planned experiments and new material characterization techniques (EBSD, GIXRD). \\[4pt] [1] M.J. Baldwin, R.P. Doerner, J. Nucl. Mater. 404 (2010) 165.\\[0pt] [2] G.M. Wright, et al, Nucl. Fusion 52 (2012) 042003. [Preview Abstract] |
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TP12.00165: Diffusion of hydrogen in a hydrogen-saturated tungsten Predrag Krstic, Igor Kaganovich Hydrogen diffusion in monoscrystalline tungsten is studied by molecular dynamics with BOP potential in function of hydrogen concentration and temperature. Tungsten surface is prepared by cumulative irradiation of the 25 eV deuterium atoms at various fluences. The diffusion coefficients for T\textgreater 500K and various D concentrations were calculated from the average slope of the mean square displacements of deuterium as functions of time. The accumulation of deuterium suppresses its diffusion at all temperatures. The results are in a reasonable agreement with the existing experiments. [Preview Abstract] |
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TP12.00166: Displacement energy of the surface layers of tungsten Longtao Han, Predrag Krstic A molecular dynamics study with BOP potential is used to calculate the threshold displacement energy (E$_{\mathrm{D}})$ of primary knock-on atoms in the surface layers of the tungsten bcc crystal lattice at 300 K and at various crystallographic directions. Depending on the direction, E$_{\mathrm{D}}$ is 10{\%} to 75{\%} smaller from the bulk value at the first layer, interfacing vacuum, while it reaches close to the bulk value already at the third tungsten layer. [Preview Abstract] |
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TP12.00167: Evaluating localized surface erosion from rf-sheath interactions in JET with an ITER-like wall C.C. Klepper, A. Lasa, D. Borodin, A. Kirschner, M. Groth, P. Jacquet, V. Bobkov, L. Colas The presence of ion cyclotron resonance frequency (ICRF) heating antennas in JET, presently with an ITER-like Wall (ILW) allows for experimental validation of models for ITER-relevant, ICRH-specific plasma-wall interactions (RF-PWI). Spectroscopic access to neutral and singly ionized beryllium light emission at outboard poloidal limiters in JET-ILW, combined with sequential antenna toggling, led recently to observation of RF-PWI, in the form of enhanced spectral line emission, at limiter spots with $\sim$ 3m magnetic field-line connection to an active antenna [1]. More recently, the measured, locally enhanced Be sources were simulated with an added sheath potential term in the ERO erosion code to account for the RF-PWI [2, 3], leading to good agreement with experiment in terms of relative increase ($\sim$ 2x-3x) in light emission. Furthermore, the added potentials are well in the range of estimated DC RF sheath potentials arising for rectification of near-fields in the SOL. The main uncertainty in absolute Be surface erosion comes from the uncertainty in the local plasma parameters, which are extrapolated to the far-SOL with the aid of edge plasma modelling [3]. Plans for improved measurements in upcoming JET-ILW experimental campaign will be included in this presentation.\\[4pt] [1] C.C. Klepper et al., J. Nucl. Mater. 438 (2013) S594--S598; [2] A. Lasa, same conference; [3] C. C. Klepper et al., PFMC-15, Submitted. [Preview Abstract] |
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TP12.00168: ERO modelling~of RF-Induced Erosion at Antenna-Connected Beryllium Limiters in JET A. Lasa, C.C. Klepper, D. Borodin, A. Kirschner, M. Groth, I. Borodkina, E. Safi, K. Nordlund Experiments at JET showed enhanced, asymmetric Be limiter erosion when magnetically connected, high power ICRH antennas were in use [1,2]. A first modeling exercise of this effect, using the 3D Monte Carlo (MC) code ERO has already been presented [3]. ERO was capable of reproducing the asymmetric Be emission pattern, erosion yields matching best for biasing voltages -- that represent antenna-connection effect -- of 100--200 eV, consistent with experimental findings and code predicted values. However, this model missed finer features, presented here: i) improved angular and energy distributions of impacting particles; ii) a detailed treatment of plasma-shadowed zones, as the connection length varies spatially; iii) additional fluxes due to charge-exchange neutrals -- an important erosion source in shadowed areas and; iv) more accurate Be-D molecular erosion yields, as a function of surface temperature and D concentration, computed in a Molecular Dynamics-Kinetic MC multi-scale approach. With this new database, other important cases, such as erosion of inner wall limiters or during main wall surface temperature scans, can be revisited in the future. \\[4pt] [1] C.C. Klepper et al., J. Nucl. Mater. 438 (2013) S594--S598\\[0pt] [2] C.C. Klepper et al., this conference.\\[0pt] [3] C. C. Klepper et al., PFMC-15. Submitted. [Preview Abstract] |
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TP12.00169: Tritium Plasma Experiment Upgrade for Fusion Tritium and Nuclear Sciences Masashi Shimada, Chase N. Taylor, Robert D. Kolasinski, Dean A. Buchenauer The Tritium Plasma Experiment (TPE) is a unique high-flux linear plasma device that can handle beryllium, tritium, and neutron-irradiated plasma facing materials, and is the only existing device dedicated to directly study tritium retention and permeation in neutron-irradiated materials [M. Shimada et.al., Rev. Sci. Instru. 82 (2011) 083503 and and M. Shimada, et.al., Nucl. Fusion 55 (2015) 013008]. Recently the TPE has undergone major upgrades in its electrical and control systems. New DC power supplies and a new control center enable remote plasma operations from outside of the contamination area for tritium, minimizing the possible exposure risk with tritium and beryllium. We discuss the electrical upgrade, enhanced operational safety, improved plasma performance, and development of tritium plasma-driven permeation and optical spectrometer system. This upgrade not only improves operational safety of the worker, but also enhances plasma performance to better simulate extreme plasma-material conditions expected in ITER, Fusion Nuclear Science Facility (FNSF), and Demonstration reactor (DEMO). [Preview Abstract] |
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TP12.00170: Study of liquid lithium entry into trench structures Kishor Kumar Kalathiparambil, Carlos Sandoval-Rios, Peter Fiflis, Martin Nieto-Perez, David Ruzic Effective handling of high heat and particle flux is one of the key deciding factor in choosing the suitable plasma facing component in future fusion reactors. This is especially critical during disruptive transient plasma events when the PFCs are expected to handle extremely energetic fluxes. The use of low atomic number liquid metals as PFCs have shown to solve many of these problems. In addition, results have indicated that the use of lithium favored better confinement of the plasma in tokamaks. Previous work done with trenches filled with flowing liquid lithium shows they can withstand type-II ELM like transient plasma fluxes. In order to ensure efficient removal of the incident heat energy during such events, it is crucial that the trench material establishes good contact with the liquid lithium; thus, understanding the surface interaction properties is crucial for attaining the ideal trench design. In the present study, the effect of temperature, trench size and trench top surface roughness in the behavior of a liquid lithium droplet deposited on the top of the trench structure is analyzed. The results cover the contact angle measurements, wetting temperatures and identification of critical wetting parameters. [Preview Abstract] |
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