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 JP12: Poster Session IV (Education and Outreach; Undergraduate/High School Research; DIII-D I, Diagnostics and Simulation Methods; Low Temperature Plasmas, Breakdown, Thrusters, and Sheaths)Poster
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Room: Exhibit Hall A |
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JP12.00001: EDUCATION AND OUTREACH |
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JP12.00002: New Outreach Initiatives at the Princeton Plasma Physics Laboratory Andrew Zwicker, Arturo Dominguez, Shannon Greco, Deedee Ortiz, John DeLooper In FY15, PPPL concentrated its efforts on a portfolio of outreach activities centered around plasma science and fusion energy that have the potential to reach a large audience and have a significant and measurable impact. The overall goal of these outreach activities is to expose the public (within New Jersey, the US and the world) to the Department of Energy's scientific endeavors and specifically to PPPL's research regarding fusion and plasma science. The projects include several new activities along with upgrades to existing ones. The new activities include the development of outreach demos for the plasma physics community and the upgrade of the Internet Plasma Physics Experience (IPPEX). Our first plasma demo is a low cost DC glow discharge, suitable for tours as well as for student laboratories (plasma breakdown, spectroscopy, probes). This has been field tested in a variety of classes and events. The upgrade to the IPPEX web site includes a new template and a new interactive virtual tokamak. Future work on IPPEX will provide users limited access to data from NSTX-U. Finally, our Young Women's Conference was expanded and improved. These and other new outreach activities will be presented. [Preview Abstract] |
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JP12.00003: Do They Enter the Workforce? Career Choices after an Undergrad Research Experience S. Greco, S. Wissel, A. Zwicker, D. Ortiz, A. Dominguez Students in undergrad research internships go on to grad school at rates of 50-75{\%} (Lopatto, 2007;Russell, 2005). NSF studied its undergrad program and found that 74{\%} of physics interns (67{\%} for engineering) go to grad school. PPPL undergrad interns were tracked for 10 years. Only 3{\%} of physics PhD candidates are studying plasma physics, but 23{\%} of our alumni that entered grad school did so in plasma. AIP reports that 60{\%} of physics majors go to grad school (AIP, 2012), but 95{\%} of PPPL interns have gone on to grad schools. Several programs track enrollment in grad school. AIP compiles statistics of undergrads who enter grad school and PhD students who work in the field. There has been no study of interns that follows the path from undergrad to grad school and then on to employment. Our tracking shows that most not only complete their advanced degrees but also stay in STEM fields following their academic careers. 88{\%} of them become part of the STEM workforce, higher than the 82{\%} of all physics PhDs employed in physics after obtaining their degree (AIP, 2014). PPPL puts more students in grad school in physics, and specifically plasma physics, and a higher percentage of those grad students stay in the STEM workforce. [Preview Abstract] |
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JP12.00004: Young Women's Conference in STEM: Our starting point for getting women into STEM fields Deedee Ortiz, Andrew Zwicker, Shannon Greco, Arturo Dominguez The number of women in STEM careers is staggeringly low; just one in seven engineers are female and only 27{\%} of all computer science jobs are held by women. A recent US Dept. of Commerce report found that between 2000-2011 women experienced no employment growth in STEM jobs. According to the AIP, the percentage of women in physics overall is 26{\%} and women make up less than 7{\%} of the fusion energy workforce. To address this problem we have, since 2001, run a conference to introduce young women to the wide range of careers in STEM fields in a way that is not part of their typical education. By introducing students in a meaningful way to successful women in STEM, from graduate students to senior researchers, the intent of the conference is to foster interest, develop mentoring relationships, and to provide role models that will have a positive influence on future educational and career choices. Data from surveys indicate that this is indeed the impact. For example, 86{\%} of~2015 attendees indicated they are more likely to major in a scientific field after attending the~conference then they were before. We are now in the process of expanding and improving the conference in order to reach more students and increase the overall impact. [Preview Abstract] |
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JP12.00005: Relaunch of the Interactive Plasma Physics Educational Experience (IPPEX) A. Dominguez, L. Rusaitis, A. Zwicker, D.P. Stotler In the late 1990's PPPL's Science Education Department developed an innovative online site called the Interactive Plasma Physics Educational Experience (IPPEX). It featured (among other modules) two Java based applications which simulated tokamak physics: A steady state tokamak (SST) and a time dependent tokamak (TDT). The physics underlying the SST and the TDT are based on the ASPECT code [1] which is a global power balance code developed to evaluate the performance of fusion reactor designs. We have relaunched the IPPEX site with updated modules and functionalities: The site itself is now dynamic on all platforms. The graphic design of the site has been modified to current standards. The virtual tokamak programming has been redone in Javascript, taking advantage of the speed and compactness of the code. The GUI of the tokamak has been completely redesigned, including more intuitive representations of changes in the plasma, e.g., particles moving along magnetic field lines. The use of GPU accelerated computation provides accurate and smooth visual representations of the plasma. We will present the current version of IPPEX as well near term plans of incorporating real time NSTX-U data into the simulation.\\[4pt] [1] D.P. Stotler et al., Comp. Phys. Comm. 81, 261 (1994) [Preview Abstract] |
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JP12.00006: CASPER's New Educational Research Group: The Physical Environment and Educational Interventions Jorge Carmona-Reyes, Li Wang, Lorin Matthews, Truell Hyde CASPER research has long included the area of STEM science education, which grew out of its history in educational intervention and curriculum development and its NSF funded REU summer program, which has been active since 1994. Recently CASPER's Educational Research group entered into a partnership with the Region 12 Educational Service Center and Huckabee, Inc. to examine the role the physical environment plays in educational intervention and the impact this combination has on student engagement and learning. This talk introduces the partnership, explains the framework guiding the research and presents the roles each partner plays in the research. [Preview Abstract] |
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JP12.00007: UNDERGRADUATE OR HIGH SCHOOL RESEARCH |
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JP12.00008: The diagnosing of plasmas using spectroscopy and imaging on Proto-MPEX K.A. Baldwin, T.M. Biewer, J. Crouse Powers, R. Hardin, S. Johnson, A. McCleese, G.C. Shaw, M. Showers, C. Skeen The Prototype Material Plasma Exposure eXperiment (Proto-MPEX) is a linear plasma device being developed at Oak Ridge National Laboratory (ORNL). This machine plans to study plasma-material interaction (PMI) physics relevant to future fusion reactors. We tested and learned to use tools of spectroscopy and imaging. These tools consist of a spectrometer, a high speed camera, an infrared camera, and a thermocouple. The spectrometer measures the color of the light from the plasma and its intensity. We also used a high speed camera to see how the magnetic field acts on the plasma, and how it is heated to the fourth state of matter. The thermocouples measure the temperature of the objects they are placed against, which in this case are the end plates of the machine. We also used the infrared camera to see the heat pattern of the plasma on the end plates. Data from these instruments will be shown. [Preview Abstract] |
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JP12.00009: Visible camera imaging of plasmas in Proto-MPEX R. Mosby, C. Skeen, T.M. Biewer, R. Renfro, H. Ray, G.C. Shaw The prototype Material Plasma Exposure eXperiment (Proto-MPEX) is a linear plasma device being developed at Oak Ridge National Laboratory (ORNL). This machine plans to study plasma-material interaction (PMI) physics relevant to future fusion reactors. Measurements of plasma light emission will be made on Proto-MPEX using fast, visible framing cameras. The cameras utilize a global shutter, which allows a full frame image of the plasma to be captured and compared at multiple times during the plasma discharge. Typical exposure times are $\sim$10-100 microseconds. The cameras are capable of capturing images at up to 18,000 frames per second (fps). However, the frame rate is strongly dependent on the size of the ``region of interest'' that is sampled. The maximum ROI corresponds to the full detector area, of $\sim$1000x1000 pixels. The cameras have an internal gain, which controls the sensitivity of the 10-bit detector. The detector includes a Bayer filter, for ``true-color'' imaging of the plasma emission. This presentation will exmine the optimized camera settings for use on Proto-MPEX. [Preview Abstract] |
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JP12.00010: Drift Wave Chaos and Turbulence in a LAPTAG Plasma Physics experiment Cami Katz, Walter Gekelman, Patrick Pribyl, Joe Wise, Henry Birge-Lee, Bob Baker, Ken Marmie, Sam Thomas, Samuel Buckley-Bonnano Whenever there is a pressure gradient in a magnetized plasma drift waves occur spontaneously. Drift waves have density and electrical potential fluctuations but no self magnetic field. In our experiment the drift waves form spontaneously in a narrow plasma column. ($n_{e} =5X10^{11}cm^{3},\mbox{\, }T_{e} =3eV,B=200Gauss,\mbox{\, d}ia=\mbox{\, 5\, cm,\, L=\, 1.5\, m})$. As the drift waves grow from noise simple averaging techniques cannot be used to map them out in space and time. The ion saturation current $I_{sat} \propto n\sqrt{T_{e}}$ is recorded for an ensemble of 50 shots on a fixed probe located on the density gradient and for a movable probe. The probe signals are not sinusoidal and are filtered to calculate the cross-spectral function CSF $ = \int {\sum\limits_{nshot} Ifix_{,\omega } (\vec{{r}}_{1} } ,t)I_{mov,\omega} \left( {\vec{{r}}_{1} + \Delta \vec{{r}},t+\tau } \right)dt$, which can be used to extract the temporal and spatially varying wave patterns. The dominant wave at 18 kHz is a rotating spiral with m$=$2. LAPTAG is a university-high school alliance outreach program, which has been in existence for over 20 years. Work done at the BaPSF and supported by NSF/DOE. [Preview Abstract] |
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JP12.00011: The Entropy and Complexity of Drift waves in a LAPTAG Plasma Physics Experiment Henry Birge-Lee, Walter Gekelman, Patrick Pribyl, Joe Wise, Cami Katz, Bob Baker, Ken Marmie, Sam Thomas, Samuel Buckley-Bonnano Drift waves grow from noise on a density gradient in a narrow (dia $=$ 3 cm, L $=$ 1.5 m) magnetized ( B$_{oz} = $ 160G) plasma column. A two-dimensional probe drive measured fluctuations in the plasma column in a plane transverse to the background magnetic field. Correlation techniques determined that the fluctuations were that of electrostatic drift waves. The time series data was used to generate the Bandt-Pompe/Shannon entropy, H, and Jensen-Shannon complexity, C$_{JS}$. C-H diagrams can be used to tell the difference between deterministic chaos, random noise and stochastic processes and simple waves, which makes it a powerful tool in nonlinear dynamics. The C-H diagram in this experiment, reveal that the combination of drift waves and other background fluctuations is a deterministically chaotic system. The PDF of the time series, the wave spectra the spatial dependence of the entropy wave complexity will be presented. LAPTAG is a university-high school alliance outreach program, which has been in existence for over 20 years. [Preview Abstract] |
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JP12.00012: Whistler Wave generation by an electron beam in a LAPTAG Plasma Physics experiment Gabriel Bridges, Patrick Pribyl, Walter Gekelman, Sam Thomas, Henry Birge-Lee, Joe Wise, Cami Katz, Bob Baker, Ken Marmie, Ben Wolman, Samuel Buckley-Bonnano A multi-grid pulsed electron beam (E$_{beam}=$ 1-4.8 KV, area $=$1.32 cm$^{2}$, $\tau $ \textgreater 5 $\mu $s ) is inserted into a background plasma (He, n $=$ 5X10$^{10}$ cm$^{3}$, B$_{0z} \quad =$ 80 G, L $=$ 1.5 m, dia $=$ 40 cm). The pulsed electron beam power supply, can generate up to 4800 Volts at 10 Amps and was constructed by the LAPTAG high school students. The beam can be oriented at any angle with respect to the background magnetic field. The pulsed beam generates whistler waves by Cherenkov radiation. The waves are detected with 3 axis magnetic pickup probes which can be moved in planes transverse or parallel to the background magnetic field under computer control. The whistler wave pattern is used to determine the wavenumber ${\rm {\bf \vec{{k}}}}$ and Fourier analysis of the signal determines $\omega $. The wave dispersion relation is compared to theory. [Preview Abstract] |
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JP12.00013: Langmuir Probe Analysis of Maser-Driven Alfven Waves Using New LaB6 Cathode in LaPD Mary Clark, Seth Dorfman, Ziyan Zhu, Giovanni Rossi, Troy Carter Previous research in the Large Plasma Device shows that specific conditions on the magnetic field and cathode discharge voltage allow an Alfven wave to develop in the cathode-anode region. When the speed of bulk electrons (dependent on discharge voltage) entering the region exceeds the Alfven speed, the electrons can excite a wave. This phenomenon mimics one proposed to exist in the Earth's ionosphere. Previous experiments used a cathode coated with Barium Oxide, and this project uses a new cathode coated with Lanthanum Hexaboride (LaB6). The experiment seeks to characterize the behavior of plasmas generated with the LaB6 source, as well as understand properties of the driven wave when using the new cathode. Langmuir probes are used to find electron temperature, ion saturation current, and plasma density. These parameters determine characteristics of the wave. Preliminary analysis implies that density increases with LaB6 discharge voltage until 170 V, where it levels off. A linear increase in density is expected; the plateau implies cathode power does not ionize the plasma after 170 V. It is possible the power is carried out by the generated Alfven wave, or heats the plasma or cathode. This ``missing'' power is currently under investigation. [Preview Abstract] |
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JP12.00014: Properties of Maser Generated Alfv\'{e}n Wave in a Large Laboratory Device Ziyan Zhu, Troy Carter, Seth Dorfman, Giovanni Rossi, Mary Clark This research is motivated by the investigations of the natural Alfv\'{e}n wave maser, which refers to the resonant amplification of Alfv\'{e}n wave in the earth-surrounding plasmas. A resonant cavity that results from applying a locally non-uniform magnetic field to a plasma source region between the anode and cathode of the Large Plasma Device creates the maser. In this research, a lanthanum hexaboride (LaB6) cathode is used as the plasma source. When a threshold in the plasma discharge current is exceeded, selective amplification produces a highly coherent, large amplitude shear Alfv\'{e}n wave that propagates out of the resonator through a semitransparent mesh anode into the plasma column where the magnetic field is uniform [1]. The discharge current threshold for maser action increases as background magnetic field strength B0 increases; this threshold influences the maser behaviors, including amplitude modulations. This maser with LaB6 source has only m $=$ 1 mode, while the maser with BaO source has a mode transition from m $=$ 0 to m $=$ 1 mode. The LaB6 maser wavelength is insensitive to parameters except for discharge voltage, which is under investigation. The experimental results will motivate future Alfv\'{e}n wave study in laboratory device and thus help better understand space plasma physics such as testing the theory of Alfv\'{e}n-wave-induced heating of stellar atmosphere. \\[4pt] [1] J. E. Maggs and G. J. Morales and T.A. Carter, Phys. Plasmas. 12, 013103 (2005) [Preview Abstract] |
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JP12.00015: Progress on Development of Low Pressure High Density Plasmas on the Helicon Plasma Experiment (HPX) Phillip Azzari, Jordan Hopson, Paul Crilly, Omar Duke-Tinson, Jackson Karama, Richard Paolino, Eva Sandri, Justin Sherman, Erin Wright, John Frank, Jeremy Turk HPX Plasmas are created by imparting directed energy into a Pyrex tube preloaded with Ar gas at fill pressures on the order of 10$^{4}$ mTorr utilizing an RF power supply and matching box that can deliver about 250 W of power in the 20 MHz to 100 MHz frequency range. It has been demonstrated [1] that a uniform magnetic field in lower energy level plasmas can facilitate a decrease in inertial effects, which promotes energy conservation within the plasma to provide the necessary external energy in the plasma's magnetic field required to reach the Helicon Mode. This uniform magnetic field will be created by a set of electromagnets capable of producing 1000 gauss. These electromagnets, provided by Princeton Plasma Physics Laboratory will facilitate W-mode production. After reaching the Helicon Mode, the plasma must be forced along the Pyrex tube by an acceleration coil in order to come in contact with several diagnostic probes and to be propelled into a viewing port so Thompson Scattering can be conducted. The progress on the development of the acceleration coil and electromagnets will be presented.\\[4pt] [1] K. Toki, et al., Thin Solid Films 506-507 (2005). [Preview Abstract] |
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JP12.00016: Updates on Optical Emission Spectroscopy {\&} Langmuir Probe Investigations on the Helicon Plasma Experiment (HPX) Jackson Karama, John Frank, Phillip Azzari, Jordan Hopson, Royce James, Omar Duke-Tinson, Richard Paolino, Eva Sandri, Justin Sherman, Eva Wright, Jeremy Turk HPX is developing a to shorter lifetime (20 - 30 ns) more reproducible plasma at the Coast Guard Academy Plasma Laboratory (CGAPL). Once achieved, spectral and particle probes will help to verify plasma mode transitions to the W-mode. These optical probes utilize movable filters, and ccd cameras to gather data at selected spectral frequency bands. Once corrections for the RF field are in place for the Langmuir probe, raw data will be collected and used to measure the plasma's density, temperature, and potentially the structure and behavior during experiments. Direct measurements of plasma properties can be determined with modeling and by comparison with the state transition tables, both using Optical Emission Spectroscopy (OES). The spectral will add to HPX's data collection capabilities and be used in conjunction with the particle probes, and Thomson Scattering device to create a robust picture of the internal and external plasma parameters on HPX. Progress on the implementation of the OES and Langmuir probes will be reported. [Preview Abstract] |
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JP12.00017: Enhancement of Particle Track Etch Rate in CR-39 by UV Exposure Micah Wiesner, Rubab Ume, James McLean, Craig Sangster, Sean Regan The use of CR-39 plastic as a Solid State Nuclear Track Detector is effective for obtaining data in high-energy particle experiments including inertial confinement fusion. To reveal particle tracks after irradiation, CR-39 is chemically etched at elevated temperatures with NaOH, producing signal pits at the nuclear track sites that are measurable by an optical microscope. CR-39 pieces sometimes also exhibit etch-induced noise, either surface features not caused by nuclear particles. When CR-39 is exposed to high intensity UV light after nuclear irradiation with 5.4~MeV alpha particles and before etching, an increase in etch rates and pit diameters is observed, though UV exposure can also increase noise. We have determined that light from a low pressure mercury vapor lamp (predominantly wavelength 253.7~nm) increases etch rates and pit diameters while causing minimal background noise. Heating CR-39 to elevated temperatures ($\sim$80 $^{\circ}$C) during UV exposure also improves the signal-to-noise ratio for this process. Surprisingly, initial data from CR-39 irradiated with 3.4~MeV protons and exposed to UV show reduced pit diameters. This material is based in part upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
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JP12.00018: Time-Resolved Tandem Faraday Cup Development for High Energy TNSA Particles S. Padalino, A. Simone, E. Turner, M.K. Ginnane, M. Glisic, B. Kousar, A. Smith, C. Sangster, S. Regan MTW and OMEGA EP Lasers at LLE utilize ultra-intense laser light to produce high-energy ion pulses through Target Normal Sheath Acceleration (TNSA). A Time Resolved Tandem Faraday Cup (TRTF) was designed and built to collect and differentiate protons from heavy ions (HI) produced during TNSA. The TRTF includes a replaceable thickness absorber capable of stopping a range of user-selectable HI emitted from TNSA plasma. HI stop within the primary cup, while less massive particles continue through and deposit their remaining charge in the secondary cup, releasing secondary electrons in the process. The time-resolved beam current generated in each cup will be measured on a fast storage scope in multiple channels. A charge-exchange foil at the TRTF entrance modifies the charge state distribution of HI to a known distribution. Using this distribution and the time of flight of the HI, the total HI current can be determined. Initial tests of the TRTF have been made using a proton beam produced by SUNY Geneseo's 1.7 MV Pelletron accelerator. A substantial reduction in secondary electron production, from 70{\%} of the proton beam current at 2MeV down to 0.7{\%}, was achieved by installing a pair of dipole magnet deflectors which successfully returned the electrons to the cups in the TRTF. Ultimately the TRTF will be used to normalize a variety of nuclear physics cross sections and stopping power measurements. Based in part upon work supported by a DOE NNSA Award{\#}DE-NA0001944. [Preview Abstract] |
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JP12.00019: Characterizing ICF Neutron Scintillation Diagnostics on the nTOF line at SUNY Geneseo Pat Lawson-Keister, Jonah Padawar-Curry, Hannah Visca, Kurt Fletcher, Stephen Padalino, T. Craig Sangster, Sean Regan Neutron scintillator diagnostics for ICF and HEDP can be characterized using the neutron time-of-flight (nTOF) line on Geneseo's 1.7 MV tandem Pelletron accelerator. Neutron signals can be differentiated from gamma signals by employing coincidence methods. A 1.8-MeV beam of deuterons incident on a deuterated polyethylene target produces neutrons via the $^{2}$H(d,n)$^{3}$He reaction. Neutrons emerging at a lab angle of 88$^{\circ}$ have an energy of 2.96 MeV; the $^{3}$He ions associated with these neutrons are detected at a scattering angle of 43$^{\circ}$ using a surface barrier detector. The time of flight of the neutron can be measured by using the $^{3}$He detection as a ``start'' signal and the scintillation detection as a ``stop'' signal. This time of flight requirement is used to identify the 2.96-MeV neutron signals in the scintillator. To measure the light curve produced by these monoenergetic neutrons, two photomultiplier (PMT) tubes are attached to the scintillator. The full aperture PMT establishes the nTOF coincidence. The other PMT is fitted with a pinhole to collect single events. The time between the full aperture PMT signal and the arrival of the signal in the pinhole PMT is used to determine the light curve for the scintillator. This system will enable the neutron response of various scintillators to be compared. [Preview Abstract] |
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JP12.00020: Radiochromic film sensitivity calibrations using ion beams from a Pelletron accelerator T.M. Filkins, Jessica Steidle, R.J. Ward, C.G. Freeman, S.J. Padalino, S.P. Regan, T.C. Sangster Radiochromic film (RCF) is a transparent detector film that permanently changes color following exposure to ionizing radiation. The optical density of the film increases with increasing absorbed dose. RCF is convenient to use because it requires no chemical processing and can be scanned using commercially available document scanners. RCF is used frequently in medical applications, but is also used in a variety of diagnostics in high energy density physics. The film consists of a single or double layer of radiation-sensitive organic microcrystal monomers placed onto a polyester backing. Gafchromic\textsuperscript{TM} manufactures a large number of different types of RCF, and new types of film frequently replace older products. In this study, the sensitivity of several types of RCF to ion beams of different energies was measured. Ion beams produced by the SUNY Geneseo 1.7 MV Pelletron accelerator were directed into a target chamber where they scattered off of a gold foil. A sample of RCF was exposed to the scattered ions. The fluence of incident particles on the film was measured using a surface barrier detector. Results of these calibrations will be presented. This work was funded in part by a grant from the DOE through the Laboratory for Laser Energetics. [Preview Abstract] |
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JP12.00021: Efficiency Calibration for Measuring the $^{12}$C(n, 2n)$^{11}$C Cross Section Thomas Eckert, August Gula, Laurel Vincett, Mark Yuly, Stephen Padalino, Megan Russ, Mollie Bienstock, Angela Simone, Drew Ellison, Holly Desmitt, Craig Sangster, Sean Regan, Ryan Fitzgerald One possible inertial confinement fusion diagnostic involves tertiary neutron activation via the~$^{12}$C(n, 2n)$^{11}$C reaction. A recent experiment to measure this reaction cross-section involved coincidence counting the annihilation gamma rays produced by the positron decay of~$^{11}$C. This requires an accurate value for the full-peak coincidence efficiency of the NaI detector system.~The GEANT 4 toolkit was used to develop a Monte Carlo simulation of the detector system which can be used to calculate the required efficiencies. For validation, simulation predictions have been compared with the results of two experiments. In the first, full-peak coincidence positron annihilation efficiencies were measured for~$^{22}$Na decay positrons that annihilate in a small plastic scintillator. In the second, a NIST-calibrated~$^{68}$Ge source was used. A comparison of calculated with measured efficiencies, as well as $^{12}$C(n, 2n)$^{11}$C cross sections are presented. [Preview Abstract] |
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JP12.00022: Cost-effective containment of unmagnetized argon plasma using a magnetic bucket and a helicon source Miguel Henriquez, M. Umair Siddiqui, Earl Scime We demonstrate highly-ionized and unmagnetized plasma production in the low-power Compact HElicon for Waves and Instabilities Experiment (CHEWIE) at West Virginia University. To achieve this, the argon helicon is injected plasma into a multidipole-confined expansion chamber. Using Langmuir probes and optical emission spectroscopy, we calculate ionization fractions in the unmagnetized volume as a function of input power and fill pressures. Finally, we investigate the ionization efficiency power scaling to determine if helicons are cost-efficient plasma sources for larger highly-ionized, unmagnetized plasma experiments. [Preview Abstract] |
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JP12.00023: Three-dimensional laser-induced fluorescence measurements in a helicon plasma source Julianne McIlvain, M. Umair Siddiqui, Zachary Short, Miguel Henriquez, Earl Scime We describe an upgrade to our two-dimensional laser-induced fluorescence (LIF) diagnostic that enable measurements over a three dimensional volume of plasma. With this new capability, we have measured the flow of ions and neutrals in an argon plasma toward a stainless steel, grounded plate aligned perpendicular to the magnetic field in a helicon plasma source. We present measurements of the three-dimensional flow field in this three-dimensional volume as a function of the magnetic field strength and rf power in the helicon source. [Preview Abstract] |
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JP12.00024: Comparison of established and novel laser-induced fluorescence schemes for Ar I Zachary Short, M. Umair Siddiqui, Miguel Henriquez, John McKee, Julianne McIlvain, Earl Scime, Amy Keesee, Drew Elliott To explore ion-neutral coupling in plasmas, it is advantageous to be able to measure the velocity distribution function (VDF) of ions and neutrals simultaneously at a single spatial location. While in previous experiments we have successfully performed neutral and ion VDF measurements with a single laser, the Ar I neutral laser induced fluorescence (LIF) scheme used was limited to operational regimes that were unsuitable for LIF measurements of Ar II. Here we describe a novel infrared LIF scheme for Ar I using a Sacher tunable diode laser and compare it to the previous Ar I LIF scheme [\textit{Keesee et al.} Rev. Sci. Instrum. \textbf{75}, 4091 (2004)]. In contrast to the previous method, our LIF scheme directly pumps a metastable Ar I state which is expected to be well populated for a wide range of plasma source operating parameters. In addition, we present iodine cell spectra for the infrared Ar I LIF scheme and corrected iodine cell spectra for the previous Ar I LIF scheme. [Preview Abstract] |
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JP12.00025: Miniaturized Energy Spectrometer for Space Plasma Measurements Raphaela Goes de Lima, Earl Scime, Amy Keesee, Greg Lusk Taking advantage of technological developments in lithographic fabrication techniques over the past two decades, we have designed an ultra-compact plasma spectrometer that requires only low voltage power supplies, no microchannel plates, and has a high aperture area to instrument area ratio. The designed target is for ions in the 3- 20 keV range with a highly directional field of view. In addition to reducing mass, size, and voltage requirements, the new design will revolutionize the manufacturing process of plasma spectrometers, enabling large quantities of identical instruments to be manufactured at low individual unit cost. Such a plasma spectrometer is ideal for Heliophysics plasma investigations, particularly for small satellite and multi-spacecraft missions. Here we present initial measurements of the performance of the instrument components and designs of the electronics for the low energy threshold solid state detector. [Preview Abstract] |
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JP12.00026: Optical Characterization of Plasma Generated in a Commercial Grade Plasma Etching System Ashley Hardy, Dereth Drake The use of plasma for etching and cleaning of many types of metal surfaces is becoming more prominent in industry. This is primarily due to the fact that plasma etching can reduce the amount of time necessary to clean/etch the surface and does not require large amounts of environmentally hazardous chemicals. Most plasma etching systems are designed and built in academic institutions. These systems provide reasonable etching rates and easy accessibility for monitoring plasma parameters. The downside is that the cost is typically high. Recently a number of commercial grade plasma etchers have been introduced on the market. These etching systems cost near a fraction of the price, making them a more economical choice for researchers in the field. However, very few academics use these devices because their effectiveness has not yet been adequately verified in the current literature. We will present the results from experiments performed in a commercial grade plasma etching system, including analysis of the pulse characteristics observed by a photo diode and the plasma parameters obtained with optical emission spectroscopy. [Preview Abstract] |
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JP12.00027: The Effects of Using a Commercial Grade Plasma Etching Chamber to Etch Anodized Niobium Surfaces Christiana Epperson, Dereth Drake, Kalina Winska Anodized niobium surfaces are used in particle accelerators for construction of the superconducting cavities. These surfaces must be cleaned regularly to remove containments and maintain the surface smoothness. The most common method used is that of chemically etching the surface using acid baths; however, this process can affect the smoothness of the layer and is extremely time consuming and hazardous. Plasma etching is one alternative that has shown great promise. We are using a commercial grade plasma etching chamber to clean anodized niobium samples that have varying oxide layer thicknesses. Spectral profiles of the surfaces of the samples are taken before and after etching. All measured results are compared to a simple theoretical model in order to determine the effects of the etching process on each surface. [Preview Abstract] |
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JP12.00028: Kinetic Modeling of Martian Atmosphere Aerobraking Plasma Nathaniel Slayton, Dereth Drake During Martian atmospheric aerobraking the plasma that forms around a spacecraft can be considered a high-volume plasma reactor that is sustained by the dissipation of the spacecraft's kinetic energy. At altitudes below 100 km, it has been shown that the plasma parameters vary considerably depending on the spacecraft's trajectory. However, in the range which is applicable to aerobraking, 100 km \textless $h$ \textless 200 km, little of this work has been completed. We have evaluated a simple kinetic model to determine a probable range of plasma parameters for altitudes between 100 and 200 km using existing Martian atmospheric data and recorded probe trajectories. [Preview Abstract] |
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JP12.00029: Symmetric Quartic Map in natural canonical coordinates Danielle Baldwin, Bilal Jones, Talise Settle, Halima Ali, Alkesh Punjabi The generating function for the simple map [A. Punjabi, A. Verma, and A. Boozer, Phys. Rev. Lett. 69, 3322 (1992)] is modified by replacing the cubic term in canonical momentum by a quartic term. New parameters are introduced in the modified generating function to control the height and the width of ideal separatrix surface and the poloidal magnetic flux inside ideal separatrix. The new generating function is the generating function for the Symmetric Quartic Map (SQM) [M. Jones et al, Phys. Plasmas 16, 042511 (2009)]. The new parameters in the generating function are chosen such that the height, width, elongation, and the poloidal flux inside the separatrix for the SQM are same as the simple map. The resulting generating function for the SQM is then transformed from the physical coordinates to the natural canonical coordinates [A. Punjabi, Nucl. Fus. 49, 115020 (2009)]. The equilibrium separatrix of the SQM is calculated in the natural canonical coordinates. The purpose of this research is to calculate the homoclinic tangle of the SQM and compare with the simple map. The separatrix of the simple map is open and unbounded; while the separatrix of the SQM is closed and compact. Motivation is to see what role the topology of the separatrix plays in its homoclinic tangle in single-null divertor tokamaks. This work is supported by grants DE-FG02-01ER54624, DE-FG02-04ER54793, and DE-FG02-07ER54937. [Preview Abstract] |
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JP12.00030: Homoclinic tangle of the separatrix of the Symmetric Quartic Map Talise Settle, Bilal Jones, Danielle Baldwin, Halima Ali, Alkesh Punjabi The equilibrium separatrix of the Symmetric Quartic Map (SQM) [M. Jones et al, Phys. Plasmas 16, 042511 (2009)] is calculated in natural canonical coordinates (NCC) [A. Punjabi, Nucl. Fus. 49, 115020 (2009)] and the SQM is constructed in NCC. The map parameter of the SQM is used to represent the magnetic perturbation as in the Standard Map [B. V. Chirikov, Phys. Rep. 52, 263 (1979)]. The homoclinic tangle of the ideal separatrix of the SQM is calculated for different values of the map parameter. The parameters in the generating function of the SQM are so chosen that the height, the width, the elongation and the poloidal flux inside the separatrix is same as in the simple map [A. Punjabi, A. Verma, and A. Boozer, Phys. Rev. Lett. 69, 3322 (1992)]. The purpose of this research is to compare the homoclinic tangle of the SQM with that of the simple map. The separatrix of the simple map is open and unbounded; while the separatrix of the SQM is closed and compact. Motivation is to see what role the topology of the separatrix plays in its homoclinic tangle in single-null divertor tokamaks. This work is supported by grants DE-FG02-01ER54624, DE-FG02-04ER54793, and DE-FG02-07ER54937. [Preview Abstract] |
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JP12.00031: Correlation analysis of magnetic field and density fluctuations in SSX Holden Parks, Ariel Rock, David Schaffner, Michael Brown The cross correlation and cross spectrum of magnetic field and density fluctuations of plasmas created by the Swarthmore Spheromak Experiment (SSX) MHD wind tunnel are examined. The SSX MHD wind tunnel produces dynamic magnetized plasma plumes with typical values $B\approx 0.2$ T, $n\geq 10^{21}$ m$^{-3}$, and $T_i\geq 20$ eV. Magnetic field fluctuations of these plasmas are measured with a $\dot{B}$ probe and local density fluctuations are measured with a double Langmuir probe inserted radially within $1$ mm of the $\dot{B}$ probe. The axial distance of both probes from the plasma source is varied to examine plasmas of different ``turbulent ages." Linearized MHD theory admits three types of waves - slow, fast, and Alfv\'{e}n - each with different correlation values between magnetic field and density. By taking the Fourier transforms of $B(t)$ and $n(t)$ time series data, the cross spectrum $\widetilde{B}^*(f)\widetilde{n}(f)$ is calculated, and the correlation between magnetic field and density can be determined as a function of frequency. Preliminary results of SSX data analysis indicate a pressure balanced structure present at $100$ kHz, likely in the form of a flux tube, as well as predominately positive correlations in the frequency range $100$ kHz to $10$ MHz. [Preview Abstract] |
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JP12.00032: Physical interpretations of permutation entropy scaling analyses of turbulent space and laboratory fluids Ariel Rock, Holden Parks, David Schaffner, Michael Brown Statistical properties of various turbulent laboratory and natural (magneto)fluids are investigated using both complexity measures of ordinal pattern distribution\footnote{Rosso \textit{et al.}, PRL \textbf{99}, 154102 (2007).} and temporal increments. The systems analyzed are solar wind $|B|$ time series from the Cluster satellites, streamwise velocity time series from the Johns Hopkins University Corsin Wind Tunnel, and $|\dot{B}|$ time series from the Swarthmore Spheromak Experiment (SSX) MHD wind tunnel. Plasma in the SSX wind tunnel has parameters $B \approx .2$T, $n\geq 10^{21}$ m$^{-3}$, and $T_{i}\geq 20$ eV. By comparing the permutation entropy and Jensen-Shannon complexity with the behavior of the structure functions derived from the intermittency analysis, the connections between the complexity measures and dissipation mechanisms can be determined. The Corsin Wind Tunnel velocity data is used to compare the statistical signatures of dissipation in conventional hydrofluids with that seen in magnetofluids. [Preview Abstract] |
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JP12.00033: Studying Simple Molecular Ionization using Radiation Emission Spectroscopy Christopher Proctor, Kristina Lemmer This study focuses on radiation emission from the formation of simple molecular plasma using a DC glow discharge. The purpose is to measure the emission from argon and molecular nitrogen gas as a function of time with an optical emission spectroscopy system operating in kinetic mode as the gases go from their neutral state to ionized state. The end goal of the research is to develop a diagnostic tool that will be used to study the formation of plasma discharges from complex molecules. The kinetic mode of the CCD camera allows for fast data acquisition so that the species present and their relative concentrations as a function of time can be measured as the plasma is forming. The primary difficulty in the development of this diagnostic tool is designing a device and data analysis technique to allow for kinetic mode operation of the CCD camera. Experimental devices have been designed and built to enable the CCD to operate in kinetic mode, including a fiber optic adapter, camera mount, and twin razor blade system. The twin blades allow for the reduction of exposed pixels on the CCD camera and thereby allow the camera to store data on rows of pixels, rather than imaging the entire camera, allowing for faster data transfer. [Preview Abstract] |
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JP12.00034: Analysis of particle trajectories in a simulated, magnetized dusty plasma in a radially-increasing electric field Brian Bender, Edward Thomas Using the DEMON (Dynamic Exploration of Microparticle clouds Optimized Numerically) code, a molecular dynamics simulation of dusty plasma was performed. In this simulation, an initial grid of particles is subjected to a uniform magnetic field and a perpendicular electric field whose magnitude increases radially. To analyze the output of the simulation, a single particle was chosen and a Fourier analysis of its trajectory is performed, revealing two primary frequencies that contain information about the E x B drift motion and the gyromotion of the particle. If the electric field only increases linearly, the difference between the two frequencies is the cyclotron frequency, which agrees with analytical results. If the electric field model is modified by an exponential decay term, then the frequencies depend on the particle's initial conditions. These results will help us to understand the electric field configuration of the MDPX device as well as highlight interesting parameter regimes for further study. [Preview Abstract] |
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JP12.00035: First Absolutely Calibrated Localized Measurements of Ion Velocity in the MST in Locked and Rotating Plasmas M. Baltzer, D. Craig, D.J. Den Hartog, M.D. Nornberg, S. Munaretto An Ion Doppler Spectrometer (IDS) is used on MST for high time-resolution passive and active measurements of impurity ion emission. Absolutely calibrated measurements of flow are difficult because the spectrometer records data within 0.3 nm of the C$^{\mathrm{+5}}$ line of interest, and commercial calibration lamps do not produce lines in this narrow range$.$ A novel optical system was designed to absolutely calibrate the IDS. The device uses an UV LED to produce a broad emission curve in the desired region. A Fabry-Perot etalon filters this light, cutting transmittance peaks into the pattern of the LED emission. An optical train of fused silica lenses focuses the light into the IDS with f/4. A holographic diffuser blurs the light cone to increase homogeneity. Using this light source, the absolute Doppler shift of ion emissions can be measured in MST plasmas. In combination with charge exchange recombination spectroscopy, localized ion velocities can now be measured. Previously, a time-averaged measurement along the chord bisecting the poloidal plane was used to calibrate the IDS; the quality of these central chord calibrations can be characterized with our absolute calibration. Calibration errors may also be quantified and minimized by optimizing the curve-fitting process. Preliminary measurements of toroidal velocity in locked and rotating plasmas will be shown. This work has been supported by the US DOE. [Preview Abstract] |
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JP12.00036: Fluctuation-induced shear flow and energy transfer in plasma interchange turbulence Ao Zhou, Huaxiang Zhang, Chuankui Sun, Xueyun Wang, Bo Li, Xiaogang Wang A flux-driven system of plasma interchange turbulence is developed to study the energy transfer and shear flow generation. Large-scale eddies are found during the nonlinear evolution of the interchange instability. With the increased heat flux, the system responds with higher pressure fluctuations. This gives rise to the stronger energy transfer from the thermal energy to the ${\mathbf E} \times {\mathbf B}$ kinetic energy, and then from the fluctuating to the mean ${\mathbf E} \times {\mathbf B}$ flow. As a result, stronger mean ${\mathbf E} \times {\mathbf B}$ shear flow is generated in the edge. [Preview Abstract] |
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JP12.00037: Characterization of Mode Activity during Disruptions for Shaped and Unshaped Plasmas T.G. Cowan, J.P. Levesque, M.E. Mauel, P.E. Hughes Understanding the dynamics of disruptions in the High Beta Tokamak -- Extended Pulse (HBT-EP) is the first step in modeling the halo currents that occur due to plasma-wall contact. Analysis of poloidal and toroidal magnetic diagnostic arrays shows that disruptions in circular, limited plasmas typically occur in two distinct phases. These phases are characterized by progressive increases in the current quench rate and a mode transition from m $=$ 3 to m $=$ 2. During the second phase, the plasma moves radially inward, contacts the inner edge of the vessel, and then decreases in minor radius. In the case of single-null diverted (shaped) plasmas, only the latter phase is observed. In both cases, an n $=$ 1 kink mode is found to persist through the entirety of the disruption, with a frequency that increases over time. These findings warrant a measurement of the halo currents during the disruption, as well as an x-ray analysis of the plasma's interior. [Preview Abstract] |
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JP12.00038: Plasma Emission Profile Recreation using Soft X-Ray Tomography J.W. Page, M.E. Mauel, J.P. Levesque With sufficient views from multiple diode arrays, soft X-ray tomography is an invaluable plasma diagnostic because it is a non-perturbing method to reconstruct the emission within the interior of the plasma. In preparation for the installation of new SXR arrays in HBT-EP, we compute high-resolution tomographic reconstructions of discharges having kink-like structures that rotate nearly rigidly. By assuming a uniform angular mapping from the kink mode rotation, $\Delta \phi \approx \omega \Delta $t, a temporal sequence from a single 16-diode fan array represents as many as 16 x 100 independent views. We follow the procedure described by Wang and Granetz [1] and use Bessel basis functions to take the inverse Radon transform. This transform is fit to our data using a least-squares method to estimate the internal SXR emissivity as a sum of polar functions. By varying different parameters of the transformation, we optimize the quality of our recreation of the emission profile and quantify how the reconstruction changes with the azimuthal order of the transform. \\[4pt] [1] L. Wang and R.S. Granetz, Rev. Sci. Instrum. 62, 4 (1991) [Preview Abstract] |
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JP12.00039: Multiple-Probe Excitation and Control of Low-Frequency Fluctuations in a Laboratory Magnetosphere Alexander Battey, Melissa C. Abler, Michael Mauel Plasma confined by a magnetic dipole have both astrophysical and laboratory applications and exhibit complex flute-like low-frequency turbulence. In this poster, we present new experiments, conducted with the Collisionless Terella Experiment (CTX), where both single and dual electrodes are used to inject currents and drive electrostatic potentials. These probes are driven either open-loop, to excite waves, or closed-loop, to implement multiple-point feedback control of the plasma's interchange turbulence. Our measurements of interchange turbulence show that two probes with a 90 degree spatial separation are able to manipulate the interchange modes differently than a system using only one probe. [Preview Abstract] |
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JP12.00040: Chaotic Magnetic Coordinates in W7-X Ben Israeli, Samuel Lazerson, Stuart Hudson The effect of symmetry breaking magnetic fields on the island divertor of W7-X is characterized using chaotic magnetic coordinates (Hudson et al., Phys. Plasmas 21, 102505). The ability of the W7-X device to reach high beta long-pulse operation relies on the proper operation of the island divertor. Analysis has suggested that intrinsic error fields may significantly modify the island divertor magnetic field structure in W7-X (Andreeva et al., Nuclear Fusion 55(6), 063025). The impact of such fields on the vacuum field structure is assessed via construction of chaotic coordinates for various W7-X vacuum configurations using code developed by Hudson et al. Chaotic coordinates are constructed via the calculation of a radial framework of quadratic flux minimizing surfaces, which allow the approximation of field geometry in non-integrable fields. This approach enables the analysis of chaotic fields near the island divertor under varied error and trim fields. The effect of the trim and sweep coils on the island divertor is explored in this work. [Preview Abstract] |
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JP12.00041: Real-time error field control via rotation optimization in DIII-D M. Capella, M.J. Lanctot DIII-D experiments have demonstrated a new approach to tokamak error field control based on maximizing the toroidal angular momentum. The technique uses extremum seeking control theory to optimize the error field in real time without inducing instabilities. Slowly-rotating n$=$1 fields (the dither), generated by non-axisymmetric coils, are used to perturb the angular momentum, monitored in real-time using a charge-exchange spectroscopy diagnostic. Signal processing of the rotation measurements is used to extract information about the rotation gradient with respect to the control coil currents in order to identify the control currents that minimize the relevant error fields. Previous results show the algorithm identifies the optimal control currents during a single plasma discharge; however, the timescale for convergence projects poorly to devices with seconds long angular momentum confinement times. Here, we report on the development of parameter estimation methods which, when used in conjunction with extremum seeking, allow improved algorithm performance. [Preview Abstract] |
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JP12.00042: Langmuir Probe and Spectroscopic Studies of the DIII-D Neutral Beam Ion Source S. Guerra, B. Crowley The Neutral Beam system on DIII-D consists of eight ion sources on four beamlines. The ion source is a filament driven magnetic bucket design. The source utilizes a Langmuir probe in a feedback control loop to set the plasma density for the desired beam extraction current. Until now, the probe operated in single-point ion-saturation mode which provided information about the plasma ion density $N_i$. Here we report on a project in which the probe circuitry is modified to enable I-V probe traces to be obtained. Hence, plasma parameters that were not measured heretofore are now available. Namely, the electron temperature $T_e$ and plasma potential $V_p$. The Energy Electron Distribution Function (EEDF) is also obtained and provides further information about the source. Additionally atomic spectroscopy of Doppler shifted D-alpha light emanating from the fast atoms is studied to determine the ion species composition in the source and the divergence of the beam. The new measurements will be used to optimize ion source performance. [Preview Abstract] |
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JP12.00043: Scaling Relationships for ELM Diverter Heat Flux on DIII D E.A. Peters, M.A. Makowski, A.W. Leonard Edge Localized Modes (ELMs) are periodic plasma instabilities that occur during H-mode operation in tokamaks. Left unmitigated, these instabilities result in concentrated particle and heat fluxes at the divertor and stand to cause serious damage to the plasma facing components of tokamaks.\footnote{A.W. Leonard Phys. Plasmas 21, 090501 (2014)} The purpose of this research is to find scaling relationships that predict divertor heat flux due to ELMs based on plasma parameters at the time of instability. This will be accomplished by correlating characteristic ELM parameters with corresponding plasma measurements and analyzing the data for trends. One early assessment is the effect of the heat transmission coefficient ? on the in/out asymmetry of the calculated ELM heat fluxes. Using IR camera data, further assessments in this study will continue to emphasize in/out asymmetry in ELMs, as this has important implications for ITER operation. [Preview Abstract] |
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JP12.00044: CMApp: a Dynamic Presentation of the Clemmow-Mullaly-Allis Diagram for Cold Plasma Waves P. Adrian, R.I. Pinsker, R. Prater, M. Porkolab The CMA diagram contains a great deal of qualitative information about the character of cold-plasma waves and their evolution in spatially varying magnetic fields and electron density [1]. We create a dynamic version of the CMA diagram which we call CMApp, in which the user can navigate the CMA diagram and arbitrarily zoom into chosen regions. The user chooses the form of the variables and many other aspects of the generalized diagram, including whether wave-normal surfaces or inverse-wave-normal surfaces are plotted. Since the zoom level is dynamic, there is no need to use logarithmic axes, an unrealistic ion-to-electron mass ratio, or to normalize magnetic field and density to wave frequency, thus enabling the diagram to be more of a quantitative tool. Examples of the applications of CMApp to wave propagation in different parameter regimes are illustrated.\\[4pt] [1] T.H. Stix, ``The Theory of Plasma Waves'' (McGraw-Hill, New York, 1962). [Preview Abstract] |
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JP12.00045: Characterization of the Radiation Environment During and Following Operation of the DIII-D Tokamak Victoria Riso, D.C. Pace, C.M. Cooper A survey of the gamma ray spectrum throughout the machine hall of the DIII-D tokamak provides a detailed mapping of its energy and temporal evolution. Engineering issues related to the structural effects of radiation produced by a fusion power plant will significantly affect the cost-effectiveness of the resulting energy. While existing magnetic confinement facilities produce considerably less neutron and gamma radiation than that expected from a power plant-scale facility, it remains useful to examine the latent gamma spectrum of the surrounding structures. The DIII-D tokamak produces $\sim$10$^{16}$ neutrons per run day (resulting primarily from beam-target DD fusion), with $\sim$75 run days per year, leading to the activation of support structures with a short half-life. Measurements are made using bismuth germinate scintillator detectors operated in pulse height analysis mode. These detectors are placed throughout the machine hall and acquire gamma data both during experiments and for some time afterward. Results of these surveys from the 2015 experiments will be presented. [Preview Abstract] |
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JP12.00046: Energetic Ion Losses During Edge Localized Modes in DIII-D B. Lee, D.C. Pace, X. Chen Energetic ions lost from tokamak plasmas due to perturbations associated with edge localized modes (ELMs, periodic plasma ejections occurring when tokamaks operate in the high confinement regime known as H-mode) may contribute a damaging heat load to the vessel walls. Two scintillator-based fast ion loss detectors (FILD) located on the outer wall of the DIII-D tokamak measure the energy and pitch angle of energetic ions ejected from the bulk plasma. A large data set of ELMs and energetic ion losses has been compiled from many experiments featuring neutral beam heated H-mode plasmas (beam injection is the source of the energetic ions). This data set will be analyzed to identify the energetic ion loss activity and other plasma parameters before, during, and after ELMs. Initial observations identify cases in which energetic ion losses appear as few millisecond precursors to ELMs. An assessment of loss properties and the conditions under which ions are expelled in advance of the ELMs is intended to provide guidance for future studies in which the wall heat load resulting from ELM-induced energetic ion losses will be investigated. [Preview Abstract] |
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JP12.00047: Field Line modeling of divertor footprints due to RMPs Benjamin P. Riviere, D.M. Orlov, R.A. Moyer, S.P. Dutta, T.E. Evans In ITER H-mode plasmas, high pedestal pressures form edge-localized modes (ELMs) which must be controlled. Resonant Magnetic Perturbations (RMPs) produce magnetic fields that have proven to be effective for ELM suppression in several existing tokamaks, but the effects of RMPs on divertor conditions are not yet well understood. Plasma response modeling has shown that RMPs can cause pedestal field lines to strike the divertor targets, potentially resulting in high heat fluxes due to the large parallel heat transport along these open field lines. Here, we use the TRIP3D code to study the properties of field lines connecting the inner and outer divertor plates through the pedestal plasma. TRIP3D results are compared to M3D-C1 simulation showing differences in the density of field lines hitting the divertor targets. These studies are needed to develop tools for quantifying the target heat flux from plasma response codes such as M3D-C1. These results will also serve as standard test cases for verifying future model iterations being implemented in both the vacuum field and plasma response codes. [Preview Abstract] |
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JP12.00048: 2D Imaging of ELM Structures Using Microwave Imaging Reflectometry Hannah Hoffmann, B.J. Tobias 2D images of density perturbations due to plasma edge instabilities have been obtained using microwave imaging reflectometry (MIR). Edge-localized modes (ELMs) are fast-growing instabilities that can locally deposit heat and particles, which can be very detrimental to plasma-facing components. The mitigation and suppression of these instabilities on ITER is therefore an area of active research. Prior attempts to image temperature perturbations of ELMs with the electron-cyclotron emission imaging (ECEI) diagnostic were confounded by unexplained ``bursts'' of intense millimeter wave emission. However, MIR is not sensitive to these bursts, and provides long-sought 2D density fluctuation data. Using this data, changes in mode structure have been correlated with changes in ELM behavior, providing important insights into possible methods of ELM control. Additionally, by simultaneously using ECEI we can explore the relationship between mode structure and the bursts. [Preview Abstract] |
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JP12.00049: Comparison of a 3-D GPU-Assisted Maxwell Code and Ray Tracing for Reflectometry on ITER Sarah Gady, Shigeyuki Kubota, Irena Johnson Electromagnetic wave propagation and scattering in magnetized plasmas are important diagnostics for high temperature plasmas. 1-D and 2-D full-wave codes are standard tools for measurements of the electron density profile and fluctuations; however, ray tracing results have shown that beam propagation in tokamak plasmas is inherently a 3-D problem. The GPU-Assisted Maxwell Code utilizes the FDTD (Finite-Difference Time-Domain) method for solving the Maxwell equations with the cold plasma approximation in a 3-D geometry. Parallel processing with GPGPU (General-Purpose computing on Graphics Processing Units) is used to accelerate the computation. Previously, we reported on initial comparisons of the code results to 1-D numerical and analytical solutions, where the size of the computational grid was limited~by the on-board memory of the GPU. In the current study, this limitation is overcome by using domain decomposition and an additional GPU. As a practical application, this code is used to study the current design of the ITER Low Field Side Reflectometer (LSFR) for the Equatorial Port Plug 11 (EPP11). A detailed examination of Gaussian beam propagation in the ITER edge plasma will be presented, as well as comparisons with ray tracing. [Preview Abstract] |
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JP12.00050: Spectroscopic Analysis of Wall Conditioning Methods in NSTX Eleanor Forbes, Vlad Soukhanovskii Plasma confinement and performance in NSTX are reliant upon well-conditioned plasma facing components (PFCs). Past conditioning techniques used in NSTX include hot and cold boronization, lithium pellet injection (LPI), and lithium evaporation. The influx of hydrogen-containing molecules and radicals can be studied through spectroscopic observation of the hydrogen to deuterium (H/D) intensity ratio in the edge plasma. A code to determine H/D ratios has been developed and tested on known light sources before being applied to data from prior NSTX experiments. In general, boronization was found to reduce the H/D ratio, with further H reduction seen from cold boronization when compared to hot boronization. No correlation between LPI and H/D ratio was observed. Lithium evaporation produced a significant H decrease. In the future this analysis will be applied immediately following NSTX-U pulses to provide data on plasma-surface interactions. [Preview Abstract] |
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JP12.00051: Observations of 0.3 kHz to 500 MHz Periodic Oscillations in the PFRC-2 Device Emily Ho, Charles Swanson, Sam Cohen With a capacitively coupled 27-MHz RF system supplying up to 400 W of power in one end cell, hydrogen plasma was generated in the PFRC-2 device, operating in a tandem mirror mode. Langmuir probes were used to measure oscillations in the floating potential in the other PFRC-2 end cell, with a conducting floating boundary slightly downstream of the probe. A fast camera was used to examine visible plasma oscillations in PFRC-2 central cell. At speeds between 5 and 13 kfps, the fast camera showed oscillations below 700 Hz; corresponding peaks were found in the floating potential spectrum. Radial probe measurements suggest it to be a bursting m=0 mode. The probe data also showed large periodic MHz-range oscillations, at 27 MHz and its first 10 harmonics. A broad peak is near 300 MHz, close to the central-cell electron cyclotron frequency. The frequency of FFT peaks near 1 MHz, in the ICRF of the mirror-coil field, varied linearly with the magnetic field strength. These intermodulate with the RF signal, creating sidebands to the 27 MHz peak and its harmonics. We present findings from the fast camera of the rotating magnetic field mode. [Preview Abstract] |
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JP12.00052: Characteristics of X-Ray Emission from the PFRC-2 Capacitively Coupled Plasma Richard Oliver, Jacob Pearcy, Peter Jandovitz, Charles Swanson, Jackson Matteucci, Samuel Cohen It is uncertain what causes keV X-rays emitted from the central-cell region of a cool (bulk $T_e \approx 4$ eV), tenuous ($n_e \approx 10^{10}$ cm$^{-3}$), 5 cm diameter, weakly ionized hydrogen plasma column generated in a tandem high-mirror-ratio mirror machine (PFRC-2 device) by a low-power, external, capacitively-coupled RF (27 MHz) antenna. We explored whether the energetic electrons responsible for the X-rays exist only in the central cell (ER) or also in the asymmetric mirror regions at opposite ends of the machine, as well as how the spectra compare if they do exist in both regions. To address this, we have designed, built, calibrated, installed and operated an X-ray detector system to view the PFRC-2 region near the RF antenna in one end cell (MC). We observe somewhat different X-ray spectra emanating from the two regions. The system comprises two Amptek XR-100CR detectors with moveable slits that scan across the plasma column. Further control of radial resolution (to 0.4 cm) is afforded by changing the detector-to-slit distance. Calibrations were performed with an $^{55}$Fe source. These data are being used to understand the source of the fast electrons that create the X-rays in the MC and in the ER. [Preview Abstract] |
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JP12.00053: Using Single-Particle Motion Simulation to Optimize Coil Parameters for Inducing Autoresonant Heating in the PFRC Jackey Liu, Samuel Cohen, Alan H. Glasser, Ido Barth The heating of ions confined in a field-reversed configuration (FRC) equilibrium magnetic geometry subject to a small-amplitude, odd-parity rotating magnetic field (RMF) has previously been observed in single-particle Hamiltonian simulations. We consider a form of the autoresonance method to provide added heating capabilities. Two coils encircling the FRC were added near the X-points of the FRC, co-axial with the major axis; these may be used to add oscillating components, primarily to the axial field, stiffening or relaxing the field, shortening or lengthening the x-point distance. Various parameters of the simulations were modified, including the positions of the coils along the axis, the amplitude and frequency of the oscillations, as well as other FRC parameters to determine whether autoresonant heating is a feasible method for increasing ion heating. This work was support, in part, by DOE contract DE-AC02-09CH11466 and the Princeton Environmental Institute. [Preview Abstract] |
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JP12.00054: The effect of impurities and incident angle on the secondary electron emission of Ni(110) Hadar Lazar, Marlene Patino, Yevgeny Raitses, Bruce E. Koel, Charles Gentile, Eliot Feibush The investigation of secondary electron emission (SEE) of conducting materials used for magnetic fusion devices and plasma thrusters is important for determining device lifetime and performance. Methods to quantify the secondary electron emission from conducting materials and to characterize the effects that impurities and incident angles have on secondary electron emission were developed using 4-grid low energy electron diffraction (LEED) optics. The total secondary electron yield from a Ni(110) surface was continuously measured from the sample current as surface contamination increased from reactions with background gases in the ultrahigh vacuum chamber. Auger electron spectroscopy (AES) and temperature programmed desorption (TPD) were used to examine the composition and impurity levels on the Ni(110) surface. The total secondary electron yield was also measured at different incident angles. [Preview Abstract] |
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JP12.00055: Excitation of Ion Acoustic Waves in Confined Plasmas with Untrapped Electrons Hanna Schamis, Ansel Dow, Johan Carlsson, Igor Kaganovich, Alexander Khrabrov Various plasma propulsion devices exhibit strong electron emission from the walls either as a result of secondary processes or due to thermionic emission. To understand the electron kinetics in plasmas with strong emission, we have performed simulations using a reduced model with the LSP particle-in-cell code. This model aims to show the instability generated by the electron emission, in the form of ion acoustic waves near the sheath. It also aims to show the instability produced by untrapped electrons that propagate across the plasma, similarly to a beam, and can drive ion acoustic waves in the plasma bulk. [Preview Abstract] |
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JP12.00056: Quantifying non-axisymmetric perturbations of the plasma edge Joe Milliano, N. Ferraro, C. Paz-Soldan, B. Lyons Tokamak plasmas, while largely axisymmetric, exhibit non-axisymmetric displacements due to both internal instabilities and intentionally applied 3D magnetic fields. Measuring these displacements can identify non-axisymmetric sensitivities to various parameters and benchmark existing plasma response models. We measure the spatial shift of temperature, density, and rotation profiles as n=2 3D fields are applied by the DIII-D upper and lower in-vessel coils. The currents are quickly inverted, shifting the toroidal phase of the applied fields by 180 degrees. These quick polarity inversions allow us to extract information about the non-axisymmetric properties of the plasma, while keeping the overall axisymmetric properties unchanged. Using a variety of toroidally separated high-resolution kinetic profile diagnostics, we infer the magnitude of these displacements at various toroidal angles. The absolute magnitude of the displacements are compared against different applied n=2 field pitches and different underlying axisymmetric equilibrium conditions, such as plasma pressure and collisionality. [Preview Abstract] |
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JP12.00057: Kinetic Simulations of Ladder Climbing and Autoresonance of Plasma Waves Erez Kaminski, Ido Barth, Nat Fisch, Ilya Dodin Quantum like Ladder Climbing and Autoresonance of classical Langmuir waves in bounded plasmas are numerically studied within a kinetic model and compared with earlier fluid model simulations. Both dynamical solutions are excited and controlled via chirped modulations of the background density that preserve the plasma wave quanta. Landau damping determines the system's maximal stable level, imposing a kinetic limit on the maximal level of the Ladder Climbing or Autoresonance dynamics. Vlasov simulations are employed to test the kinetic stability of both dynamics and to find the kinetic limit for different system's parameters. [Preview Abstract] |
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JP12.00058: Development and application of a multi-fluid simulation code for modeling interpenetrating plasmas M. Khodak, R.L. Berger, T. Chapman, J.A.F. Hittinger A multi-fluid model, with independent velocities for all species, is developed and implemented for the numerical simulation of the interpenetration of colliding plasmas. The Euler equations for fluid flow, coupled through electron-ion and ion-ion collisional drag terms, thermal equilibration terms, and the electric field, are solved for each ion species with the electrons treated under a quasineutrality assumption. Fourth-order spatial convergence in smooth regions is achieved using flux-conservative iterative time integration and a Weighted Essentially Non-Oscillatory (WENO) finite volume scheme employing an approximate Riemann solver. Analytic solutions of well-known shock tube tests and spectral solutions of the linearized coupled system are used to test the implementation, and the model is further numerically compared to interpenetration experiments such as those of J.S. Ross et al. [Phys. Rev. Lett. 110 145005 (2013)]. This work has applications to laser-plasma interactions, specifically to hohlraum physics, as well as to modeling laboratory experiments of collisionless shocks important in astrophysical plasmas. [Preview Abstract] |
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JP12.00059: Using Betatron Emissions from Laser Wakefield Accelerated Electrons to Probe Ultra-fast Warm Dense Matter Jordan Kotick, Will Schumaker, Florian Condamine, Felicie Albert, Benjamin Barbrel, Eric Galtier, Eduardo Granados, Alessandra Ravasio, Siegfried Glenzer Laser wakefield acceleration (LWFA) has been shown to produce short X-ray pulses from betatron oscillations of electrons within the plasma wake. These betatron X-rays pulses have a broad, synchrotron-like energy spectrum and a duration on the order of the driving laser pulse, thereby enabling probing of ultrafast interactions. Using the 1 J, 40fs short-pulse laser at the Matter in Extreme Conditions experimental station at LCLS, we have implemented LWFA to generate and subsequently characterized betatron X-rays. Notch filtering and single photon counting techniques were used to measure the betatron X-ray spectrum while the spatial profile was measured using X-ray CCDs and image plates. We used an ellipsoidal mirror to focus the soft betatron X-rays for pump-probe studies on various targets in conjunction with LCLS X-ray and optical laser pulses. This experimental platform provides the conditions necessary to do a detailed study of warm-dense matter dynamics on the ultrafast time-scale. [Preview Abstract] |
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JP12.00060: Molecular Dynamics Simulations of Ion Equilibration in Ultracold Neutral Plasmas Nikola Maksimovic, Thomas Langin, Trevor Strickler, Thomas Killian Understanding transport and equilibration in strongly coupled plasmas is important for modeling plasmas found in extreme environments like inertial confinement fusion plasmas and interiors of gas-giant planets. We use molecular dynamics simulations of Yukawa one component plasmas under periodic boundary conditions to study the evolution of strongly coupled ultracold neutral plasmas (UNPs) at early times. Simulations provide access to observable quantities in strongly coupled plasmas, namely correlation functions. Experimentally, the average velocity of an ion subset with a skewed velocity profile has been used to measure velocity autocorrelation functions and provide access to diffusion coefficients and other transport processes in UNPs. Using the simulation, we verify the experimental measurements of average velocities of ion subsets in UNPs and confirm their agreement with the velocity autocorrelation function. Finally, we examine the collective mode behavior of the ions during their equilibration phase by calculating the longitudinal current correlation function at various times during equilibration. This allows us to study the collective mode coupling behavior of the equilibration of ions in UNPs and its dependence on screening parameter. [Preview Abstract] |
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JP12.00061: Finite Element Analysis of Transverse Compressive Loads on Superconducting Nb3Sn Wires Containing Voids Luc D'Hauthuille, Yuhu Zhai High field superconductors play an important role in many large-scale physics experiments, particularly particle colliders and fusion devices such as the LHC and ITER. The two most common superconductors used are NbTi and Nb3Sn. Nb3Sn wires are favored because of their significantly higher Jc, allowing them to produce much higher magnetic fields. The main disadvantage is that the superconducting performance of Nb3Sn is highly strain-sensitive and it is very brittle. The strain-sensitivity is strongly influenced by two factors: plasticity and cracked filaments. Cracks are induced by large stress concentrators due to the presence of voids. We will attempt to understand the correlation between Nb3Sn's irreversible strain limit and the void-induced stress concentrations around the voids. We will develop accurate 2D and 3D finite element models containing detailed filaments and possible distributions of voids in a bronze-route Nb3Sn wire. We will apply a compressive transverse load for the various cases to simulate the stress response of a Nb3Sn wire from the Lorentz force. Doing this will further improve our understanding of the effect voids have on the wire's mechanical properties, and thus, the connection between the shape {\&} distribution of voids and performance degradation. [Preview Abstract] |
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JP12.00062: DIII-D I DIAGNOSTICS AND SIMULATION METHODS |
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JP12.00063: Impact of Central ECCD on Steady-State Hybrid Scenario in DIII-D C.C. Petty, M.A. Van Zeeland, D.C. Pace, Xi Chen, R. Prater, R. Nazikian, B.A. Grierson, E. Kolemen, G.R. McKee, F. Turco In steady-state hybrid plasmas with zero surface loop voltage, 3.4 MW of central ECCD drives $\approx$ 0.2 MA out of $\approx$ 1.0 MA plasma current with concurrent changes in sawteeth, Alfv\'en eigenmodes (AE) and thermal transport. While the hybrid scenario normally does not sawtooth because $q_{min}>$1, localized ECCD (with calculated peak magnitudes of $\sim$ 6 MA/$m^2$) causes sawteeth to appear, indicating that the intense ECCD overwhelms the flux pumping mechanism. In hybrid plasmas with NBI heating only, strong AE activity leads to high beam ion transport coefficients of $\sim$ 2 /$m^2$/s. During central ECCD, this AE activity is suppressed, replaced by a bursty n=1 energetic particle mode with low beam ion transport coefficients of $\sim$ 0.3 /$m^2$/s. While central electron heating raises electron thermal transport, increasing $\chi_e$ by $\approx$100\% for 3.4 MW of ECCD, the confinement factor is little changed as the higher thermal transport is offset by the decreased fast ion transport resulting from AE suppression. [Preview Abstract] |
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JP12.00064: Elevated-$q_{min}$ Steady-State Scenarios on DIII-D: New Controls and Stability Characterization C.T. Holcomb, B.S. Victor, J.R. Ferron, T.C. Luce, R.J. LaHaye, E. Schuster, W.P. Wehner, F. Turco, W.M. Solomon Fully non-inductive, high performance plasmas with $q_{min} > 1.5$ have been sustained on DIII-D at $\beta_N > 3.5$. Since the formation and sustainment of such plasmas entails operation near multiple stability limits there is a sensitivity to variations in conditions that can lead to different results, i.e. tearing modes. The parameter range for stability at high-$\beta_N$ is not well known, and it is unclear if tearing modes are destabilized by neoclassical (i.e. missing bootstrap current) or classical (i.e. tearing too close to ideal-wall kink limit) $\beta_N$ limits. We present work to improve reproducibility and stability understanding using new controls. Electron cyclotron heating applied at breakdown improves repeatability. Current profile and $\beta_N$ feedback control are used to obtain equilibria that are assessed for stability and steady-state potential. We discuss the range of stable operation found using these tools and tests designed to identify tearing destabilization mechanisms. [Preview Abstract] |
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JP12.00065: Modeling the effects of plasma rotation on RWM stability in high current and high $\beta_N$ DIII-D plasmas F. Turco, J.M. Hanson, G.A. Navratil, A.D. Turnbull, C. Paz-Soldan Operation at ITER relevant $q_{95}\sim$3 and torque ($<$1 Nm) is challenging, despite the moderate $\beta_N$ values (30-40\% below the no-wall limit). Measurements obtained with active MHD spectroscopy in ITER baseline discharges in DIII-D show a large increase in the n=1 plasma response to a low frequency applied field, when the rotation decreases to $<$10 krad/s. This increased plasma response, along with an abrupt change in the response phase, usually indicates the approach to an ideal stability limit, despite the low $\beta_N < \beta_{N,nowall}$. Modeling with the MARS-K code allows us to test the hypothesis that non-ideal effects become important for RWM stability at very low rotation. Initial modeling results reproduce the high to moderate rotation measurements correctly when collisionality and drift kinetic effects are included, but the code does not predict the rise in amplitude and change in phase at low rotation. These results will be compared to rotation scans at higher $\beta_N > \beta_{N,nowall}$, to complement the model validation for ITER stability predictions. [Preview Abstract] |
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JP12.00066: Particle transport in DIII-D and JET plasmas S. Mordijck, X. Wang, L. Zeng, E.J. Doyle, T. Tala, A. Salmi Particle transport is currently still poorly understood in magnetic confinement devices. Using a perturbative gas puff technique, both DIII-D and JET are able to extract perturbative transport coefficients D and v for various plasma conditions, ranging from L- to H-mode (with and without RMPs), scanning collisionality, torque and electron dominant versus ion dominant heating. We find that heating as well as torque changes affect the individual v/D contributions in the core, while keeping the ratio fairly constant. However, we find that when comparing the v/D ratio with the peaking of the density and fueling profile that the absolute v/D values are larger than the steady state conditions would merit. Using 1.5 D transport simulations along with SOLPS simulations we will address whether the ratio of v/D along with the changes in fueling are a good representation of the changes in steady-state transport. [Preview Abstract] |
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JP12.00067: Electron Density Change and Turbulent Particle Transport During the ITG-TEM Transition Process in DIII-D Plasmas X. Wang, S. Mordijck, E.J. Doyle, M.E. Austin, O. Meneghini, S.P. Smith, G.M. Staebler Periodic ECH power is added into neutral beam heated H-mode plasmas in DIII-D, leading to periodic changes in the electron density, electron temperature, and ion temperature. The changes strengthen the drive for trapped electron modes (TEM) while reducing the ion temperature gradient (ITG) instability drive. Linear gyrokinetic calculations show the instability transitions from ITG to TEM only after all these changes. We find the density gradient in the core shows a dependence on the most unstable mode frequency, where R/Ln maximizes near the ITG-TEM transition and decreases towards both the ITG and TEM regions. However, such dependence does not exist near the plasma edge. Both the density rate of change over time and the most unstable mode growth rate increase substantially around $\rho$=0.75 when ECH is turned on. These results indicate that the particle transport in the plasma edge is different from what was found previously in the plasma core. [Preview Abstract] |
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JP12.00068: Sensitivity of core transport to parallel velocity gradient in DIII-D H-modes W. Guttenfelder, B.A. Grierson, D.R. Ernst, A. Marinoni, A.M. Dimits, R. Bravenec, J.M. Candy, G.M. Staebler In DIII-D QH modes with NBI+ECH, nonlinear GYRO simulations of density-gradient-driven trapped electron mode (TEM) turbulence at $\rho$=0.3 predict thermal fluxes and synthetic Doppler backscattering spectra that agree with experimental measurements within uncertainties [1]. With only NBI heating, TEM simulations are also found to agree with experimental measurements, but only if the destabilizing influence of the parallel velocity gradient (PVG) is included. The influence of the PVG increases the predicted transport in this case by lowering the effective density gradient threshold, as the rotation shear (u$^{\prime}$ = -R$^2$$\nabla\Omega/c_s$=3.4) is much larger than the case with ECH added (u$^{\prime}$=2.0). A similar destabilizing influence of PVG has been predicted in high-$\beta_{pol}$ discharges [2] with similar values of rotation shear (u$^{\prime}>3$), which will also be presented.\par \vskip6pt \noindent [1] D.R.\ Ernst, IAEA 2014; APS 2015\par \noindent [2] A.M.\ Garofalo, IAEA 2014 [Preview Abstract] |
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JP12.00069: Connecting Microscopic L-H Transition Physics to the Power Threshold L. Schmitz, T.L. Rhodes, L. Zeng, B.A. Grierson, D. Eldon, Z. Yan, G.R. McKee, J. Boedo, C. Chrystal, P. Gohil, J.R. Groebner, K.H. Burrell A physics-based model of the L-H transition power threshold $P_{th}$ is needed to confidently extrapolate auxiliary heating requirements for burning plasmas. The goal of this work is to link differences in the microscopic turbulence-flow interaction to the macroscopic power threshold density and isotope scaling. The turbulence-driven poloidal ion flow is found decisive for initial turbulence suppression, with a Reynolds stress gradient (evaluated from BES data) sufficiently large to account for the measured poloidal flow acceleration. The turbulence-flow energy transfer rate $P_E=< \tilde{v}_r\tilde{v}_q>\partial (v_q + v_{dia})$ / $\partial r$ depends on the L-mode seed flow shear, which shows a similar density dependence than $P_{th}$ (increasing below the density $n_{min}$ where $P_{th}$ has a minimum, and increasing above $n_{min}$). Differences in turbulence properties, and lower L-mode diamagnetic seed flow shear are found in hydrogen plasmas compared to deuterium plasmas, supporting the experimentally observed $P_{th}$ isotope scaling. [Preview Abstract] |
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JP12.00070: Testing of the ITER-ECE prototype receiver and related components on DIII-D M.E. Austin, M.W. Brookman, P.E. Phillips, W.L. Rowan, S. Danani Real-world testing of advanced plasma diagnostic instruments and techniques intended for use on ITER is crucial to ensure their success. A prototype millimeter-wave receiver developed by Virginia Diodes, Inc. was brought to DIII-D to check its performance by measuring third harmonic ECE in high temperature plasmas. The receiver is state-of-the art, employing a waveguide based triplexer and a DRO-based local oscillator with an integrated tripler, subharmonic mixer and amplifier to detect emission in the 200-300 GHz range. Comparisons of ECE measurements with those from the DIII-D Michelson interferometer will evaluate linearity, sensitivity, and noise temperature. Also, transmission measurements of a double wedged quartz window, very similar to that proposed for the ITER vacuum interface, are given, showing no interference effects and good broadband performance. Additionally, results of the testing of a new high intensity LED light source for alignment of transmission line components are shown. [Preview Abstract] |
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JP12.00071: Calculation of Modulated Transport Coefficients for Recovery of ECH Deposition Profiles M.W. Brookman, M.E. Austin, C.W. Horton, C.C. Petty Ray tracing of ECRF power through fixed plasma profiles may significantly underestimate the ECH and ECCD deposition profile width. Density fluctuations present in tokamak plasmas modify the path of radiation on a fluctuation timescale, spreading the heating power over a wide area. Deposition is hard to measure as transport quickly spreads power, and transport effects are difficult to separate from a truly broadened profile. While the total power deposited should be unchanged in an ITER-like scenario, tearing mode suppression is sensitive to the alignment and width of the ECCD profile. A novel integral method for calculating thermal transport coefficients based on ECE measurements of $T_e$ is presented and applied to DIII-D data. These are compared with computational predictions of broadening from the ray tracing code C3PO and distribution code LUKE [1]. This work will provide the analytical framework for measuring fluctuation broadening in a future DIII-D experiment.\\[4pt] [1] Y. Peysson, J. Decker, L. Morini, and S. Coda, Plasma Phys. Control. Fusion 53 124028 (2011) [Preview Abstract] |
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JP12.00072: Collisionality scaling of turbulence and transport in advanced inductive plasmas in DIII-D Z. Yan, G.R. McKee, C. Petty, T. Luce, X. Chen, C. Holland, T. Rhodes, L. Schmitz, G. Wang, L. Zeng, A. Marinoni, W. Solomon The collisionality scaling of multiscale turbulence properties and thermal transport characteristics in high-beta, high confinement Advanced Inductive (AI) plasmas was determined via systematic dimensionless scaling experiments on DIII-D. Preliminary estimate indicates a weak collisionality dependence of energy confinement as $v^*$ varied by a factor of $\sim$2. Electron density and scaled ($\sim{B_t} ^2$) temperature profiles are well matched in the scan. Interestingly, low-k density fluctuation amplitudes are observed to decrease at lower $v^*$ near $\rho\sim 0.75$. Ion and electron thermal transport values, computed with ONETWO using experimentally measured profiles and sources, will be presented, along with multi-scale turbulence measurements obtained with various fluctuation diagnostics. Altering collisionality should change the relative contribution of different modes to transport. [Preview Abstract] |
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JP12.00073: Bispectral Analysis of the Edge Harmonic Oscillation and Turbulence Measured with BES on DIII-D M. Ono, K. Ida, M. Yoshinuma, T. Kobayashi, C. Moon, G.R. McKee, K.H. Burrell An edge harmonic oscillation (EHO) provides continuous particle transport to allow ELM-free operation at good energy confinement, constant density, and radiated power in Quiescent H-mode plasmas (QH). The nonlinear interaction between harmonics of the EHO and turbulence is important to understanding the mechanisms and dynamics of enhanced particle transport in QH-mode. Bispectral analysis has been applied to the localized density fluctuation data in QH-mode plasmas measured using BES in DIII-D. The fundamental frequency of the EHO was typically $\sim$ 10 kHz with long poloidal wavelength ($k_\theta \sim$ 0.02 cm $^{-1}$) and broadband turbulence in the range of 50-300 kHz with correlation lengths of a few cm. The cross-bicoherence among the BES channels showed radially varying magnitude of phase coherence well above the noise floor between the EHO and broadband turbulence in the region of 0.8$ < \rho < $1.0. This indicates that the EHO and turbulence interact with each other, potentially impacting the observed particle transport. [Preview Abstract] |
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JP12.00074: A combined phase contrast imaging-interferometer system for the detection of multiscale density fluctuations on DIII-D E.M. Davis, J.C. Rost, M. Porkolab, A. Marinoni, M.A. Van Zeeland A combined phase contrast imaging (PCI) and heterodyne interferometer system has been implemented on DIII-D, extending the physics capabilities of the pre-existing PCI and acting as a prototypical fluctuation diagnostic for next-step devices. The combined PCI-interferometer uses a single 10.6 $\mu$m laser beam, two interference schemes, and two detectors to measure $\int \tilde{n}_edl$ over a large spatiotemporal bandwidth $\mbox{(10 kHz $< f < $2 MHz and 0 $\le k \le$ 20 cm$^{-1}$)}$, allowing simultaneous measurement of ion- and electron-scale instabilities. Further, time-correlating our interferometer's measurements with those of DIII-D's pre-existing, toroidally separated ($\Delta\zeta$ = 45$^{\circ}$) interferometer will allow novel studies of low-$n$ Alfv\'en eigenmodes. The combined diagnostic's small port requirements and minimal access restrictions make it well-suited to the harsh neutron environments and limited port space expected in next-step devices. Measurements from sound wave calibrations and DIII-D operations will be presented. [Preview Abstract] |
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JP12.00075: Faraday-Effect Polarimeter Diagnostic for Internal Magnetic Field Fluctuation Measurements in DIII-D Jie Chen, W.X. Ding, D.L. Brower A high-resolution Faraday-effect polarimeter-interferometer diagnostic currently under construction at the DIII-D tokamak has three overall measurement goals: (1) determine the current density dynamics at the magnetic axis, J(0,t), for torque-free plasmas (no NBI) and bootstrap current in the pedestal region; (2) resolve both coherent and broadband magnetic fluctuations [at the level $\delta$b$\leq$1Gauss with up to 2 MHz bandwidth] associated with MHD perturbations, energetic particle driven modes and broadband turbulence (e.g. microtearing modes), and (3) identify non-axisymmetric structures and plasma response to externally applied RMP (resonant magnetic perturbation) fields being developed for ELM control as well as MHD events. These goals will be achieved using a 650-700 GHz source and heterodyne receiver system to measure the line-integrated Faraday-effect and density along three horizontal chords positioned at the magnetic axis and $\pm$15 cm off-axis. The system will be double-pass and cornercube retroreflectors have already been installed. Simultaneous measurement of density and Faraday effect allows isolation of the fluctuating magnetic field component in the radial direction. [Preview Abstract] |
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JP12.00076: Ions lost on their first orbit can impact Alfv\'en eigenmode stability and experience nonlinear wave-particle interactions W.W. Heidbrink, E.A.D. Persico, Xi Chen, D.C. Pace, M.A. Van Zeeland, G.Y. Fu Some neutral-beam ions are deflected onto loss orbits by Alfv\'en eigenmodes on their first bounce orbit and detected by a fast-ion loss detector (FILD). The resonance condition for these ions differs from the usual resonance condition for a confined fast ion. Estimates indicate that particles on single-pass loss orbits transfer enough energy to the wave to alter mode stability. When these ions interact with more than one mode, oscillations in the FILD signal often appear at the sum and difference frequencies of the independent modes. A wide variety of FILD spectra are observed. [Preview Abstract] |
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JP12.00077: Characterizing Critical Gradient Threshold for Alfv\'en Eigenmode Induced Fast-Ion Transport C.S. Collins, W.W. Heidbrink, L. Stagner, M.A. Van Zeeland, D.C. Pace, C.C. Petty Recent experiments on DIII-D indicate a sudden increase in fast-ion transport in the presence of many simultaneous Alfv\'en eigenmodes (AEs) at a threshold in neutral beam power. The threshold is beyond the AE linear stability limit and appears to differ between various fast-ion diagnostics, indicating phase-space dependent transport. Above threshold, transport becomes stiff, resulting in virtually unchanged fast-ion density profiles despite increased beam drive. In the experiment, a beam power scan (2-9 MW) varies AE activity, while the fast-ion pressure profile is modulated using an off-axis neutral beam. Measurements of the fast-ion density evolution are used to infer flux. Fast-ion D$\alpha$ (FIDA) spectroscopy indicates the peak of the modulated fast-ion flux is localized to mid-core radii, corresponding to the radial location of AEs. These measurements facilitate numerical model validation studies, giving greater confidence in predicting the fusion alpha density profiles and losses in future burning plasma devices. [Preview Abstract] |
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JP12.00078: Sensitivity of FIDA Diagnostics to Individual Fast-ion Orbits for use in Forward Modeling and Tomography L. Stagner, W.W. Heidbrink, C.S. Collins, R.B. White, G. Kramer, Y. Todo, M.A. Van Zeeland In recent years, there has been an effort to determine the phase-space sensitivities of different fast-ion diagnostics--these ``weight functions'' are used for interpreting diagnostic signals, forward modeling, and in calculating tomographic reconstructions of the fast-ion distribution from Fast-ion D$\alpha$ (FIDA) measurements. Currently, weight functions are only calculated in velocity-space, neglecting spatial dimensions. This lack of spatial dependence makes it difficult to use weight functions in transport studies which require understanding of the interplay between different regions of configuration space. Extending velocity-space weight functions to constants of motion space allows different spatial locations to be coupled together via their intersecting orbits. These ``orbit weights functions'' offer a more fundamental approach toward interpreting and calculating fast-ion diagnostic signals. Calculation of orbit weight functions for the FIDA diagnostic are validated in a low-power, MHD-quiescent discharge. In a forward modeling application, theoretical predictions of distributions during Alfv\'en eigenmode activity are compared with data from the ``critical gradient'' experiment. The extension of velocity-space tomography to utilize orbit weight functions will also be outlined. [Preview Abstract] |
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JP12.00079: A New Neutron Calibration Technique with Fast Scintillators on DIII-D Tokamak Y.B. Zhu, W.W. Heidbrink, P.L. Taylor, W. Carrig Absolute calibrations are necessary for conventional neutron measurements based on proportional counters and fission chambers, at regular intervals. For the DIII-D tokamak, the wide span of fusion rates, approximately between 1.e9 - 1.e17 neutrons per second, from pure Ohmic to high power auxiliary heating plasmas requires careful cross-calibrations of a variety of neutron detectors with stepwise and overlapped sensitivities,\footnote{W.W. Heidbrink, P.L. Taylor, J.A. Phillips, Rev. Sci. Instrum. 68 (1997) 536.} with an intense isotope neutron source, e.g. californium-252 and real plasmas. Scintillators have been successfully utilized for fast time resolved neutron detection for decades.\footnote{W.W. Heidbrink, Rev. Sci. Instrum. 57 (1986) 1769.} A new calibration approach with the help of scintillators is shown to be straightforward, simpler and trustworthy while the conventional approach is complicated, time consuming and costly. Details on the calibration setup and results will be presented. [Preview Abstract] |
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JP12.00080: Integrated modeling of high poloidal beta scenario for a next-step reactor J. McClenaghan, A.M. Garofalo, O. Meneghini, S.P. Smith In order to fill the scientific and technological gaps between ITER and a nuclear fusion power plant DEMO, a next-step integrated nuclear test facility is critical. A high poloidal beta tokamak regime investigated in recent DIII-D experiments is a promising candidate for steady state operation in such a next-step device because the large bootstrap current fraction ($\sim$~80$\%$) reduces the demands on the external current drive. Despite the large values of q$_{95}$$\sim$10, the normalized fusion performance observed in the experiments meet the target for an economically attractive fusion power plant such as ARIES-ACT2. In this work, we will project the performance for a conducting and superconducting coil next-step steady state reactor using theory-based 0-D modeling and full 1.5D transport modeling. [Preview Abstract] |
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JP12.00081: Advances towards high performance low-torque $q_{min} > 2$ operations with large-radius ITB on DIII-D G.S. Xu, W.M. Solomon, A.M. Garofalo, J.R. Ferron, A.W. Hyatt, Q. Wang, Z. Yan, G.R. McKee, C.T. Holcomb A joint DIII-D/EAST experiment was performed aimed at extending a fully noninductive scenario with high $\beta_P$ and $q_{min} > 2$ to inductive operation at lower torque and higher I$_p$ (0.6 $\rightarrow$ 0.8 MA) for better performance. Extremely high confinement was obtained, i.e., H$_{98y2}$ $\sim$ 2.1 at $\beta_N \sim$ 3, which was associated with a strong ITB at large minor radius ($\rho\sim$ 0.7). Alfv\'en Eigenmodes and broadband turbulence were significantly suppressed in the core, and fast-ion confinement was improved. ITB collapses at 0.8 MA were induced by ELM-triggered n = 1 MHD modes at the ITB location, which is different from the ``relaxation oscillations'' associated with the steady-state plasmas at lower current (0.6 MA). This successful joint experiment may open up a new avenue towards high performance low-torque $q_{min} > 2$ plasmas with large-radius ITBs, which will be demonstrated on EAST in the near future. [Preview Abstract] |
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JP12.00082: Energy Transport Analyses of DIII-D High-$\beta_P$ EAST-Demonstration Discharge C.K. Pan, G.M. Staebler, L.L. Lao, A.M. Garofalo, X.Z. Gong, Q. Ren, G.Q. Li, S.Y. Ding, J.P. Qian, B.N. Wan, G.S. Xu, W.M. Solomon, O. Meneghini, S.P. Smith Energy transport analyses of DIII-D high-$\beta_P$ EAST-demonstration discharge have been performed using the TGYRO transport package with TGLF turbulent and NEO neoclassical transport models under the OMFIT integrated modeling framework. Ion energy transport is dominated by neoclassical transport and TGYRO predicted ion temperature profiles agree closely with the experimental measured profiles for these high-$\beta_P$ discharges. A significant shortfall in the electron energy transport is found for these high-$\beta_P$ discharges with TGYRO prediction. Increasing the saturated turbulence level for high-wavenumber electron temperature gradient (ETG) driven modes used in TGLF can successfully reproduce the experimental electron temperature profiles. Both the ion and electron energy transport are largely insensitive to reductions in the $E\times B$ flow shear or Shafranov shift stabilization. [Preview Abstract] |
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JP12.00083: ELM Behavior in High-$\beta$p EAST-Demonstration Plasmas on DIII-D G.Q. Li, X.Z. Gong, A.M. Garofalo, L.L. Lao, O. Meneghini, P.B. Snyder, Q.L. Ren, S.Y. Ding, W.F. Guo, J.P. Qian, B.N. Wan, G.S. Xu, C.T. Holcomb, W.M. Solomon In the DIII-D high-$\beta$p EAST-demonstration experiment, for several similar discharges when the experimental parameters such as the toroidal magnetic field or ECH power are varied slightly, the changes in ELM frequency response are observed to be much larger. Kinetic EFIT equilibrium reconstructions for these discharges have been performed, which suggest that the ELM frequency changes are likely due to the variations of pedestal width, height, and edge current density. Kinetic profile analyses further indicate that the strong ITB that are located at large minor radii (rho=0.6$\sim$0.7) in these discharges are affecting the pedestal structure. The ITB could broaden the pedestal width and decrease the pedestal height, thus changing the ELM frequency and size. With the GATO and ELITE MHD codes, the linear growth rates and mode structures of these ELMs are analyzed. The impact of ITB on the ELMs behavior will be discussed. [Preview Abstract] |
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JP12.00084: Computational Study of Anomalous Transport in High Beta DIII-D Discharges with ITBs Alexei Pankin, Andrea Garofalo, Brian Grierson, Arnold Kritz, Tariq Rafiq The advanced tokamak scenarios require a large bootstrap current fraction and high $\beta$. These large values are often outside the range that occurs in ``conventional'' tokamak discharges. The GLF23, TGLF, and MMM transport models have been previously validated for discharges with parameters associated with ``conventional'' tokamak discharges. It has been demonstrated that the TGLF model under-predicts anomalous transport in high $\beta$ DIII-D discharges~[A.M.~Garofalo {\em et al.} 2015 TTF Workshop]. In this research, the validity of MMM7.1 model~[T.~Rafiq {\em et al.} {Phys.~Plasmas} {\bf 20} 032506 (2013)] is tested for high $\beta$ DIII-D discharges with low and high torque. In addition, the sensitivity of the anomalous transport to $\beta$ is examined. It is shown that the MMM7.1 model over-predicts the anomalous transport in the DIII-D discharge 154406. In particular, a significant level of anomalous transport is found just outside the internal transport barrier. Differences in the anomalous transport predicted using TGLF and MMM7.1 are reviewed. Mechanisms for quenching of anomalous transport in the ITB regions of high-beta discharges are investigated. [Preview Abstract] |
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JP12.00085: Rotation effects in MHD simulations of disruption mitigation V.A. Izzo, E. Hollmann, D. Shiraki, N. Commaux, N. Eidietis, P.B. Parks DIII-D experiments with massive gas injection (MGI) for disruption mitigation have confirmed results from MHD simulations predicting the n$=$1 mode as the primary source of radiation asymmetry during the thermal quench (TQ), and have shown that the n$=$1 mode phase is controllable with external coils when the target plasma has very low initial rotation. New MHD simulations including rotation help to address several open questions concerning the role of plasma rotation during an MGI shutdown. First, the edge plasma rotation is found to strongly influence the toroidal spreading of impurities. Second, the slowing of the core plasma rotation is found to depend less on direct penetration of the massive impurities deep into the core than on interaction between large islands in the edge and core. Finally, the simulations support the observation that the n$=$1 phase can rotate by a significant fraction of 2$\pi $ between its appearance and the end of the TQ, thus decoupling the radiation peak location from the injection location. [Preview Abstract] |
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JP12.00086: Prospects for MHD Spectroscopy in Disruption Avoidance A.D. Turnbull, F. Turco, J.D. Hanson, N.M. Ferraro, L.L. Lao MHD spectroscopy has been shown experimentally to reveal the presence of stable Alfv\'en eigenmodes and simulations using the MARS-F code have shown that the spectroscopy scan can be modeled. The peaks in the response impedance reveal the frequencies corresponding to normal modes of the plasma and the width of the peak is a measure of the damping. The same techniques can be used to probe lower frequencies to investigate ideal MHD kink instabilities near marginal stability. MARS-F is used to probe the stable spectrum. It is well known that the ideal MHD spectrum consists of a pair of stable continua corresponding to the shear Alfv\'en and acoustic waves, and a discrete unstable component. When the equilibrium is Mercier unstable, a Sturmian sequence of unstable modes is also present with an accumulation point at the edge of the continuum. Discrete marginally stable kink modes also exist within gaps in the continua. The ultimate aim is to evaluate proximity to stability limits and develop techniques to steer the equilibrium away from those limits. [Preview Abstract] |
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JP12.00087: A new gamma ray imaging diagnostic for runaway electron studies at DIII-D C.M. Cooper, D.C. Pace, N.W. Eidietis, C. Paz-Soldan, N. Commaux, D. Shiraki, E.M. Hollmann, R.A. Moyer, V. Risov A new Gamma Ray Imager (GRI) is developed to probe the electron distribution function with 2D spatial resolution during runaway electron (RE) experiments at DIII-D. The diagnostic is sensitive to 0.5 - 50 MeV gamma rays, allowing characterization of the RE distribution function evolution during RE dissipation from pellet injection. The GRI consists of a lead ``pinhole camera'' mounted on the midplane with 11x11 counter-current tangential chords 20 cm wide that span the vessel. Up to 30 bismuth germanate (BGO) scintillation detectors capture RE Bremsstrahlung radiation. Detectors operate in current saturation mode at 10 MHz, or the flux is attenuated for Pulse Height Analysis (PHA) capable of discriminating up to $\sim$10k pulses per second. Digital signal processing routines combining shaping filters are performed during PHA to reject noise and record gamma ray energy. The GRI setup and PHA algorithms will be described and initial data from experiments will be presented. [Preview Abstract] |
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JP12.00088: Role of non-thermal electrons in the DIII-D low density stability limit Carlos Paz-Soldan, N. Eidietis, R.J. La Haye, E.J. Strait, D.L. Shiraki, R.A. Moyer, E.M. Hollmann, C.M. Cooper The lowest achievable density in tokamaks is thought to be linearly related to uncorrected error fields, with the limiting instability being n=1 error field penetration (the low-density locked mode). Experiments at DIII-D over the past two decades find that despite various types of optimized error correction, operation below densities of $\sim$ (4-5)x10$^{18}$ m$^{-3}$ yields penetration. Recent experiments show that at similar densities, non-thermal electrons are robustly excited by the Dreicer mechanism. Unexpectedly, locked mode onset in these discharges occurs at similar levels of non-thermal emission intensity, despite application of gas puffing to raise the density. Penetration is preceded by 1) non-thermalization of the electron cyclotron emission, 2) decrease in the bulk electron temperature, 3) anisotropization of the total pressure, and 4) appearance of new structures in synchrotron emission images. These observations will be detailed to assess the degree to which the non-thermal electron population sets the low-density limit. [Preview Abstract] |
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JP12.00089: Probing plasma response models with $n$=2 measurements J.M. Hanson, J. Bialek, G.A. Navratil, F. Turco, J. King, M. Lanctot, C. Paz-Soldan, E. Strait Frequency-dependent measurements of the magnetic plasma response to n=2 perturbations in DIII-D exhibit multiple resonances. In contrast with n=1 response measurements, which have been shown to exhibit a single frequency resonance in several tokamak devices, high-field side n=2 response measurements show a double resonance in the investigated frequency range of -100 to 100 Hz, in discharges near the n=2 no-wall $\beta_N$ limit. The poloidal structure of the response varies with the perturbation frequency, becoming more peaked on the midplane at low frequency. The validation of response models remains an open issue for predicting the sensitivity of future devices to non-axisymmetric perturbations. In this case, the response amplitude is over-predicted by ideal MHD theory, evaluated using the MARS code. Incorporating kinetic modifications brings the predictions into closer agreement with measurements. However, aspects of the measurements, such as the multi-resonant behavior and vacuum field pattern, are not presently captured by the simulations. [Preview Abstract] |
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JP12.00090: Resistive Wall Mode feedback on DIII-D using Linear Quadratic Gaussian control and a GPU powered control system M.D. Clement, G.A. Navratil, J.M. Hanson, J. Bialek, D.A. Piglowski, B.G. Penaflor A Graphics Processing Unit (GPU) based control system has been installed on the DIII-D tokamak for Resistive Wall Mode (RWM) control similar to one implemented at the HBT-EP tokamak [1]. DIII-D can excite RWMs, which are strong, locked or nearly locked kink modes whose rotation frequencies do not evolve quickly and are slow compared to their growth rates. Simulations have predicted that modern control techniques like Linear Quadratic Gaussian (LQG) control will perform better than classical control techniques when using control coils external to the vacuum vessel. An LQG control algorithm based on the VALEN model for the RWM [2] has been developed and tested on this system. Early tests have shown the algorithm is able to track and suppress with external control coils the plasma response of an n=1 perturbation driven by internal control coils. An overview of the control hardware, VALEN model, control algorithm and initial results will be presented. \par \vskip6pt \noindent [1] Rath, Nikolaus 2013, Plasma Phys. Control. Fusion, {\bf55}, 084003 \par \noindent [2] Bialek, James 2001, Physics of Plasmas, {\bf8}, 2170 [Preview Abstract] |
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JP12.00091: DIII-D Equilibrium Reconstructions with New 3D Magnetic Probes Lang Lao, E.J. Strait, N.M. Ferraro, J.R. Ferron, J.D. King, X. Lee, O. Meneghini, A.D. Turnbull, Y. Huang, J.G. Qian, A. Wingen DIII-D equilibrium reconstructions with the recently installed new 3D magnetic diagnostic are presented. In addition to providing information to allow more accurate 2D reconstructions, the new 3D probes also provide useful information to guide computation of 3D perturbed equilibria. A new more comprehensive magnetic compensation has been implemented. Algorithms are being developed to allow EFIT to reconstruct 3D perturbed equilibria making use of the new 3D probes and plasma responses from 3D MHD codes such as GATO and M3D-C1. To improve the computation efficiency, all inactive probes in one of the toroidal planes in EFIT have been replaced with new probes from other planes. Other 3D efforts include testing of 3D reconstructions using V3FIT and a new 3D variational moment equilibrium code VMOM3D. Other EFIT developments include a GPU EFIT version and new safety factor and MSE-LS constraints. The accuracy and limitation of the new probes for 3D reconstructions will be discussed. [Preview Abstract] |
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JP12.00092: Development of GPU-Optimized EFIT for DIII-D Equilibrium Reconstructions Y. Huang, L.L. Lao, B.J. Xiao, Z.P. Luo, X.N. Yue The development of a parallel, Graphical Processing Unit (GPU)-optimized version of EFIT for DIII-D equilibrium reconstructions is presented. This GPU-optimized version (P-EFIT) is built with the CUDA (Compute Unified Device Architecture) platform to take advantage of massively parallel GPU cores to significantly accelerate the computation under the EFIT framework. The parallel processing is implemented with the Single-Instruction Multiple-Thread (SIMT) architecture. New parallel modules to trace plasma surfaces and compute plasma parameters have been constructed. DIII-D magnetic benchmark tests show that P-EFIT could accurately reproduce the EFIT reconstruction algorithms at a fraction of the computational cost. The acceleration factor continues to increase as the (R, Z) spatial grids are increased from $65\times 65$ to $257\times 257$, suggesting there may be rooms for further optimization by further reducing the communication cost. Details of the P-EFIT optimization algorithms will be discussed. [Preview Abstract] |
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JP12.00093: Effects of plasma rotation in reconstructed 3-D equilibria for DIII-D A. Wingen, M.W. Shafer, E.A. Unterberg, R.S. Wilcox, M.R. Cianciosa, S.P. Hirshman, D.L. Hillis, L. Lao, C. Paz-Soldan A technique for tokamak equilibrium reconstructions when weakly 3-D fields ($\delta$B/B $\sim$ 10$^{-3}$) are applied is used for inner-wall-limited DIII-D discharges. The technique couples diagnostics to the non-linear, ideal MHD equilibrium solver VMEC, using the V3FIT code, to find the most likely 3-D equilibrium based on a suite of measurements. Observations at DIII-D show that plasma rotation larger than 20 krad/s changes the relative phase between the applied 3-D fields and the measured plasma response. Numerical simulations of linear, resistive, 2-fluid MHD show, that large plasma rotation increases flux surface corrugations1. Discharges with low averaged ($\sim$10 krad/s) and peaked rotation profiles ($\sim$40 krad/s) are reconstructed. Similarities and differences to forward modeled VMEC equilibria, which do not include rotational effects, are shown. The resulting significance of including rotational effects in VMEC is discussed. [Preview Abstract] |
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JP12.00094: Tracking Inductive Flux Usage to Evaluate Flux Pumping in the Hybrid Scenario in DIII-D N.Z. Taylor, T.C. Luce, R.J. La Haye, C.C. Petty, R. Nazikian In hybrid scenarios the presence of amplitude modulation of tearing modes by nonaxisymmetric instabilities (usually m/n=3/2 by ELMs) is necessary for the redistribution of magnetic poloidal flux (flux pumping) to be observed. The physical mechanism through which the poloidal flux is redistributed (current profile anomalously broadened) is not well understood. The evolution of normalized flux states is used to track the rate at which poloidal flux is provided by the coils, and the rate it is converted to kinetic energy in the plasma. In DIII-D hybrid discharges there is a measured deficit in the rate of change in the flux states indicating that poloidal flux is being consumed at a higher rate than it is being provided by the coils. This unexpected deficit disappears when the tearing mode is suppressed with electron cyclotron current drive (ECCD) and increases with the level of flux pumping present. One explanation for the deficit could be that the beneficial tearing mode facilitates the conversion of toroidal to poloidal magnetic flux. [Preview Abstract] |
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JP12.00095: Multi-layered mode structure of locked-tearing-modes after unlocking Michio Okabayashi, N. Logan, B. Tobias, Z. Wang, B. Budny, R. Nazikian, E. Strait, R. La Haye, C.J. Paz-Soldan, N. Ferraro, D. Shiraki, J. Hanson, P. Zanca, R. Paccagnella Prevention of m/n=2/1 tearing modes (TM) by electro-magnetic torque injection has been successful in DIII-D and RFX-mod where plasma conditions and plasma shape are completely different [1]. Understanding the internal structure in the post-unlocked phase is a pre-requisite to its application to reactor relevant plasmas such as in ITER. T$_i$ and toroidal rotation perturbations show there exist several radially different TM layers. However, the phase shift between the applied field and the plasma response is rather small from plasma edge to the q$\sim$3 domain, indicating that a kink-like response prevails. The biggest threat for sustaining an unlocked 2/1 mode is sudden distortion of the rotational profile due to the internal mode reconnection. Possible TM layer structure will be discussed with numerical MHD codes and TRANSP. This work is supported in part by the US Department of Energy under DE-AC02-09CH11466, DE-FG02-99ER54531, DE-SC0003913, and DE-FC02-04ER54698.\par \vskip6pt \noindent [1] 2014 IAEA. M. Okabayashi et al. [Preview Abstract] |
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JP12.00096: Simulated dynamics and feed-back control of locked and nearly-locked islands W. Choi, K.E.J. Olofsson, R. Sweeney, F.A. Volpe, M. Okabayashi A model has been developed at DIII-D to model and predict the dynamics of saturated m/n = 2/1 tearing modes subject to various torques. The modes are modeled as surface currents with finite moment of inertia, interacting with the error fields, magnetic perturbations applied by internal and external active coils, the conducting wall, and the graphite tiles. This model also accepts input auxiliary torques (viscous drag, neutral beam torque, etc). Using this island dynamics model, a feed-back controller has been designed to control the phase of locked modes in the presence of drag from the wall and other disturbances. Preliminary results show a simple fixed-gain controller connected to realistic external coils can follow the desired phase for a range of island sizes. For a given current in the control coils, a maximum entrainment frequency exists and is dependent on island width. The controller is expected to be useful in assisting island suppression with electron cyclotron current drive, as well as to prevent mode locking and possible disruption. [Preview Abstract] |
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JP12.00097: Analysis of m/n=2/1 locked mode disruption database on the DIII-D Tokamak R. Sweeney, W. Choi, K.E.J. Olofsson, F.A. Volpe, R.J. La Haye, S. Mao A study of $\sim$2,800 m/n=2/1 locked modes (LMs) with rotating precursors at DIII-D reveals that LMs near the plasma edge are the most disruptive, and a period of exponential growth with $\tau \approx$10 ms precedes the disruption. LM durations are also correlated with edge proximity, with modes near the core living longer and/or not disrupting. Non-disruptive LMs are on average larger than disruptive LMs, but the latter degrade the normalized plasma beta more throughout their evolution. The edge proximity, smaller island size, and larger reduction in plasma beta characteristic of disruptive LMs, are all consistent with the observed linear dependence of island width on ($r / a$)$\beta_{\theta}$/(${dq} / {dr}$) $ \propto$ $\beta_{\theta}$ ($a^2 / r$) ($\beta_{\theta}$ is poloidal beta, $q$ is safety factor, $a$ is minor radius). The disruptive exponential growth is consistent with the radiation-driven tearing mode model, or alternatively, might be explained by the evolution of the classical stability index. This work suggests a basis for scenario design and profile control in ITER and future devices, as a simple means to avoid locked mode disruptions. [Preview Abstract] |
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JP12.00098: Interaction of 2/1 Neoclassical Tearing Modes, Turbulence and Thermal Transport in the DIII-D Tokamak L. Bardoczi, T.L. Rhodes, T.A. Carter, A. Banon-Navarro, N.A. Crocker, W.A. Peebles, F. Jenko, G. McKee The interaction of neoclassical tearing modes (NTM), turbulence and transport has received increased attention, e.g. magnetic islands are predicted to modify turbulence that in turn affects cross-field transport ($\chi_{\bot}$) and NTM stability [1]. We present two non-perturbative experimental approaches to examine the problem: (i) Comparison of measured electron temperature to anisotropic heat transport models employing spatially non-uniform $\chi_{\bot}$ shows reduction of anomalous $\chi_{\bot}$ at the O-point. (ii) Far Infrared Scattering and Beam Emission Spectroscopy measurements are consistent with density fluctuation amplitude being modified by the NTM while GENE linear gyrokinetic simulations show that these density fluctuations are driven by ion temperature gradient turbulence. This study suggests an interesting correlation between the reduction of turbulence and anomalous transport across the island.\par \vskip6pt \noindent [1] Wilson and Connor, PPCF 51 115007 (2009). [Preview Abstract] |
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JP12.00099: Current/Pressure Profile Effects on Tearing Mode Stability in DIII-D Hybrid Discharges K. Kim, J.M. Park, M. Murakami, R.J. La Haye, Yong-Su Na It is important to understand the onset threshold and the evolution of tearing modes (TMs) for developing a high-performance steady state fusion reactor. As initial and basic comparisons to determine TM onset, the measured plasma profiles (such as temperature, density, rotation) were compared with the calculated current profiles between a pair of discharges with/without n=1 mode based on the database for DIII-D hybrid plasmas. The profiles were not much different, but the details were analyzed to determine their characteristics, especially near the rational surface. The tearing stability index calculated from PEST3, $\Delta$' tends to increase rapidly just before the n=1 mode onset for these cases. The modeled equilibrium with varying pressure or current profiles parametrically based on the reference discharge is reconstructed for checking the onset dependency on $\Delta$' or neoclassical effects such as bootstrap current. Simulations of TMs with the modeled equilibrium using resistive MHD codes will also be presented and compared with experiments to determine the sensibility for predicting TM onset. [Preview Abstract] |
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JP12.00100: Development of Negative Triangularity Plasmas in DIII-D M.L. Walker We report on development of DIII-D electron cyclotron heated discharges with controlled negative triangularity plasma shapes, to evaluate the effect on electron heat transport in L-mode plasmas, as reported on TCV [1]. Analysis of TCV data found that negative triangularity exerts a stabilizing influence on the trapped electron mode, the dominant instability in the conditions of those experiments [2]. Major objectives of the DIII-D development are producing complementary plasmas, one with negative and one with positive triangularity, approximately symmetric in major radial coordinate and having similar density and current profiles. Major constraints include selection of plasma parameters and toroidal field to optimize fluctuation diagnostic measurements while preventing transition from L- to H-mode and deposition of EC heating power near the q=1 surface to limit sawteeth. Issues discussed are definition of control scenarios by which the pair of shapes are accessed and their resulting controllability under the constraints imposed by DIII-D shaping control.\par \vskip6pt \noindent [1] Y. Camenen, et al, 2007 Nuc. Fus. 47 510\par \noindent [2] A. Marinoni, et al, 2009 PPCF 51 055016 [Preview Abstract] |
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JP12.00101: Implementing a Finite-State Off-Normal and Fault Response System for Robust Tokamak Operation N.W. Eidietis, D.A. Humphreys, B. Sammuli, M.L. Walker The initial implementation and testing of a finite state off-normal \& fault response (ONFR) system on the DIII-D and KSTAR tokamaks is presented. Robust ONFR will be critical to the operation of ITER as the physical consequences of unexpected events will be far more extreme than in present devices. ``Off-normal'' refers to unexpected plasma events (e.g. disruptions) and plasma events that are expected but still require asynchronous response (e.g. neoclassical tearing modes). ``Fault'' refers to hardware failure. ONFR priorities are to (1) protect the device from damage, (2) minimize recovery time between shots by avoiding unnecessary initiation of mitigation procedures, and (3) maximize the useful pulse length of a given shot by providing for discharge recovery after deleterious events. The detailed implementation of finite-state ONFR using Matlab/Simulink and Stateflow exported to the DIII-D and KSTAR plasma control systems is described, as are initial tests of multi-stage locked mode handling on both devices. [Preview Abstract] |
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JP12.00102: ITER Baseline Scenario with ECCD Applied to Neoclassical Tearing Modes in DIII-D A.S. Welander, R.J. La Haye, J.M. Lohr, D.A. Humphreys, R. Prater, C. Paz-Soldan, E. Kolemen, F. Turco, E. Olofsson The neoclassical tearing mode (NTM) is a magnetic island that can occur on flux surfaces where the safety factor $q$ is a rational number. Both m/n=3/2 and 2/1 NTM's degrade confinement, and the 2/1 mode often locks to the wall and disrupts the plasma. An NTM can be suppressed by depositing electron cyclotron current drive (ECCD) on the $q$-surface by injecting microwave beams into the plasma from gyrotrons. Recent DIII-D experiments have studied the application of ECCD/ECRH in the ITER Baseline Scenario. The power required from the gyrotrons can be significant enough to impact the fusion gain, $Q$ in ITER. However, if gyrotron power could be minimized or turned off in ITER when not needed, this impact would be small. In fact, tearing-stable operation at low torque has been achieved previously in DIII-D without EC power. A vision for NTM control in ITER will be described together with results obtained from simulations and experiments in DIII-D under ITER like conditions. [Preview Abstract] |
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JP12.00103: Combined Current Profile and $\beta_N$ Control to Facilitate Accessibility and Reproducibility Testing of High-qmin Steady-State Scenarios W. Wehner, J.M. Barton, M.E. Lauret, E. Schuster, C.T. Holcomb, B. Victor, J.R. Ferron, T.C. Luce, R. La Haye, F. Turco, W. Solomon The capability of combined current profile and $\beta_N$ control to enable access and repeatability of steady-state scenarios for high $q_{min}>1.5$ discharges is studied in both nonlinear simulations and experiments. The presentation focuses on model-predicted $q$-profile+$\beta_N$ control, which numerically solves successive optimal control problems over a receding time horizon by exploiting efficiently solvable quadratic programming techniques. One of the key advantages of this control approach is that it allows for explicit incorporation of state/input constraints to prevent the controller from driving the plasma outside of stability/performance limits and obtain, as closely as possible, steady state conditions in the $q$ profile. To characterize the $q$ profile+$\beta_N$ response, empirical correlations are combined with first-principles laws to arrive at a control-oriented model, which captures the dominant physics that is necessary for model-based optimal control design. [Preview Abstract] |
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JP12.00104: Current Density Profile Estimation via Model-based Extended Kalman Filtering in the DIII-D Tokamak H. Wang, J.E. Barton, E. Schuster A closed-loop observer for the estimation of the poloidal magnetic flux profile has been proposed for tokamaks in which the profile cannot be accurately estimated in real time via equilibrium reconstruction methods due to the lack of internal diagnostics. The observer has been synthesized by applying extended Kalman filtering theory and using a discrete lumped-parameter nonlinear model based on the magnetic diffusion equation. The observer makes use of measurements of the total plasma current and the poloidal magnetic flux at both the magnetic axis and the plasma boundary to estimate the full magnetic flux profile. DIII-D has the capability of accurately reconstructing the poloidal magnetic flux profile by incorporating measurements by the Motional Stark Effect (MSE) diagnostics in the real-time equilibrium code, which makes it a perfect testbed for the proposed observer. Comparisons between estimated (by observer) and reconstructed (by equilibrium code) magnetic flux profiles are carried out for several discharges in the DIII-D tokamak, demonstrating the potential of the proposed observer. [Preview Abstract] |
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JP12.00105: Gyrokinetic Simulations of Electrostatic Turbulence Reduction due to Radial Electric Field Shear in DIII-D Sam Taimourzadeh, Ihor Holod, Zhihong Lin, Raffi Nazikian It has been demonstrated that edge localized modes (ELMs) can be fully suppressed in DIII-D H-mode plasmas with the application of resonant magnetic perturbations (RMPs), and that there is a corresponding reduction of pedestal gradients, changes in rotation, and changes in the radial electric field (Er) profile [Nucl. Fusion \textbf{55}, 023002 (2015)]. However, with the application of RMPs there is also an increase in short wavelength, electrostatic turbulence on top of the pedestal, as observed with BES, DBS, and other fluctuation diagnostics. The effects of Er shear on this turbulence, is investigated using gyrokinetic simulations via the gyrokinetic toroidal code (GTC) for in DIII-D shot 158103, at times 3750 ms (RMP on, ELM active) and 3050 ms (RMP on, ELM suppressed). [Preview Abstract] |
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JP12.00106: Model Predictive Control of the Current Profile and the Internal Energy of DIII-D Plasmas M. Lauret, W. Wehner, E. Schuster For efficient and stable operation of tokamak plasmas it is important that the current density profile and the internal energy are jointly controlled by using the available heating and current-drive (H\&CD) sources. The proposed approach is a version of nonlinear model predictive control in which the input set is restricted in size by the possible combinations of the H\&CD on/off states. The controller uses real-time predictions over a receding-time horizon of both the current density profile (nonlinear partial differential equation) and the internal energy (nonlinear ordinary differential equation) evolutions. At every time instant the effect of every possible combination of H\&CD sources on the current profile and internal energy is evaluated over the chosen time horizon. The combination that leads to the best result, which is assessed by a user-defined cost function, is then applied up until the next time instant. Simulations results based on a control-oriented transport code illustrate the effectiveness of the proposed control method. [Preview Abstract] |
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JP12.00107: Recent Progress and Future Plans for Fusion Plasma Synthetic Diagnostics Platform Lei Shi, Gerrit Kramer, William Tang, Benjamin Tobias, Ernest Valeo, Randy Churchill, Loic Hausammann The Fusion Plasma Synthetic Diagnostics Platform (FPSDP) is a Python package developed at the Princeton Plasma Physics Laboratory. It is dedicated to providing an integrated programmable environment for applying a modern ensemble of synthetic diagnostics to the experimental validation of fusion plasma simulation codes. The FPSDP will allow physicists to directly compare key laboratory measurements to simulation results. This enables deeper understanding of experimental data, more realistic validation of simulation codes, quantitative assessment of existing diagnostics, and new capabilities for the design and optimization of future diagnostics. The Fusion Plasma Synthetic Diagnostics Platform now has data interfaces for the GTS and XGC-1 global particle-in-cell simulation codes with synthetic diagnostic modules including: (i) 2D and 3D Reflectometry; (ii) Beam Emission Spectroscopy; and (iii) 1D Electron Cyclotron Emission. Results will be reported on the delivery of interfaces for the global electromagnetic PIC code GTC, the extended MHD M3D-C1 code, and the electromagnetic hybrid NOVAK eigenmode code. Progress toward development of a more comprehensive 2D Electron Cyclotron Emission module will also be discussed. [Preview Abstract] |
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JP12.00108: EHT Integrator Demonstration at DIII-D Ilia Slobodov, Ken Miller, Timothy Ziemba, James Prager Eagle Harbor Technologies, Inc. (EHT) has developed a series of analog integrators for magnetic diagnostics for the fusion science and plasma physics communities. Three varieties of EHT integrators: short pulse, high dynamic range, and long pulse, have all been tested at DIII-D. The EHT short pulse integrators were used to measure the poloidal magnetic field coil, saddle coil, and Rogowski coil. The results were compared to existing diagnostics at DIII-D. The EHT high dynamic range integrator was used to measure the toroidal magnetic field. Due to the high dynamic range, this integrator resolved features in the signal that could not otherwise be observed. Additionally, preliminary testing of the EHT long pulse integrator was conducted. The long pulse integrator meets all the specifications for ITER. [Preview Abstract] |
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JP12.00109: Electron Cyclotron Emission Imaging on ITER with Rowland Circle Optics Jason Liu, Woochang Lee, June-eok Leem, Manfred Bitter, Hyeon Park, Gunsu Yun The implementation of advanced electron cyclotron emission imaging (ECEI) systems on the major tokamaks TEXTOR$^{\mathrm{1}}$, DIII-D$^{\mathrm{2,3}}$, KSTAR$^{\mathrm{4}}$, EAST$^{\mathrm{5}}$, and ASDEX Upgrade$^{\mathrm{6}}$ has revolutionized the diagnosis of MHD activities$^{\mathrm{\thinspace }}$and improved our understanding of various instabilities. However, the conventional ECEI systems cannot be applied to ITER because of the space constraints and excessive radiation that would be encountered in the diagnostic port plugs. This paper describes an alternative optical concept that employs the Rowland circle imaging geometry to implement an advanced ECEI system on ITER that is suitable for the tight space and harsh environments of the diagnostic port plugs. Such a system would match the capabilities of conventional ECEI diagnostics and would be capable of simultaneous core and edge measurements. [Preview Abstract] |
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JP12.00110: Fast, Deep-Record-Length, Fiber-Coupled Photodiode Imaging Array for Plasma Diagnostics Samuel Brockington, Andrew Case, F. Douglas Witherspoon HyperV Technologies has been developing an imaging diagnostic comprised of an array of fast, low-cost, long-record-length, fiber-optically-coupled photodiode channels to investigate plasma dynamics and other fast, bright events. By coupling an imaging fiber bundle to a bank of amplified photodiode channels, imagers and streak imagers can be constructed. By interfacing analog photodiode systems directly to commercial analog-to-digital converters and modern memory chips, a scalable solution for 100 to 1000 pixel systems with 14 bit resolution and record-lengths of 128k frames has been developed. HyperV is applying these techniques to construct a prototype 1000 Pixel framing camera with up to 100 Msamples/sec rate and 10 to 14 bit depth. Preliminary experimental results as well as future plans will be discussed. Work supported by USDOE Phase 2 SBIR Grant DE-SC0009492. [Preview Abstract] |
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JP12.00111: Two-photon Absorption Laser Induced Fluorescence in a Hot Deuterium Toroidal Plasma: Measurements of Absolute Density, Temperature, and Flow Speed of Deuterium Neutrals Drew Elliott, Derek Sutherland, Earl Scime, Aaron Hossack, Chris Everson, Kyle Morgan, Umair Siddiqui, Thomas Jarboe Two-photon Absorption Laser Induced Fluorescence (TALIF) has been demonstrated at WVU for measurements of krypton, hydrogen, deuterium, and xenon neutral particle distribution functions in helicon plasmas. We report now on deuterium measurements in a hot, ion temperatures up to 20 eV, spheromak. We have measured densities lower than 10$^{16}$ particles per meter cubed and neutral temperatures higher than 2 eV. The flow speeds suggest significant momentum coupling between the ions and neutral gas. The absolute density calibration is obtained by comparing a TALIF measurement of Kr gas of a known pressure with the TALIF signal from D and employing the known absorption cross section ratio for Kr and D along with other calibration factors. [Preview Abstract] |
(Author Not Attending)
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JP12.00112: Expansion of Collisional Radiative Model for Helium line ratio spectroscopy David Cinquegrani, Chris Cooper, Cary Forest, Jason Milhone, Jorge Munoz-Borges, Oliver Schmitz, Ezekial Unterberg Helium line ratio spectroscopy is a powerful technique of active plasma edge spectroscopy. It enables reconstruction of plasma edge parameters like electron density and temperature by use of suitable Collisional Radiative Models (CRM). An established approach is successful at moderate plasma densities ($\sim 10^{18}m^{-3}$ range) and temperature (30-300eV), taking recombination and charge exchange to be negligible. The goal of this work is to experimentally explore limitations of this approach to CRM. For basic validation the Madison Plasma Dynamo eXperiment (MPDX) will be used. MPDX offers a very uniform plasma and spherical symmetry at low temperature (5-20 eV) and low density ($10^{16}-10^{17}m^{-3})$. Initial data from MPDX shows a deviation in CRM results when compared to Langmuir probe data. This discrepancy points to the importance of recombination effects. The validated model is applied to first time measurement of electron density and temperature in front of an ICRH antenna at the TEXTOR tokamak. These measurements are important to understand RF coupling and PMI physics at the antenna limiters. Work supported in part by start up funds of the Department of Engineering Physics at the UW - Madison, USA and NSF CAREER award PHY-1455210. [Preview Abstract] |
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JP12.00113: Development progress of Correlation ECE and n-T cross-phase angle diagnostics for ASDEX-Upgrade Simon Freethy, Garrard Conway, Ivo Classen, Alex Creely, Anne White, Tim Happel, Branka Vanovac Relative turbulent temperature fluctuation amplitudes can be measured using Correlation ECE (CECE). This technique uses two narrow frequency-band radiometer channels, with an equivalent physical spacing within a turbulent radial correlation length. Correlation techniques select the common turbulent fluctuation while suppressing uncorrelated thermal noise. If such a diagnostic views the same part of the plasma as a reflectometer, then the coherence and cross-phase angle between temperature and density fluctuations can be measured. Two 2nd harmonic, X-mode, CECE radiometers have recently been installed on ASDEX Upgrade, one of which shares the quasi-optical steerable antenna of an existing Doppler reflectometer, i.e with a common line of sight of the plasma. We report on the progress of the installation and preliminary data from both systems. [Preview Abstract] |
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JP12.00114: 3D ELM fluctuation measurements with the new dual array ECE-Imaging diagnostic on ASDEX Upgrade Ivo Classen, Branka Vanovac, Calvin Domier, Neville Luhmann, Anton Bogomolov, Wolfgang Suttrop, Benjamin Tobias In a major upgrade, the (2D) electron cyclotron emission imaging diagnostic (ECE-Imaging) at ASDEX Upgrade (AUG) has been equipped with a second detector array, and has been successfully commissioned. The two detector arrays observe the plasma through the same vacuum window, both under a slight toroidal angle, to enable quasi-3D measurements of the electron temperature. The system measures a total of 288 channels, in two toroidally separated 2D arrays of approximately 50 cm vertically by 10 cm radially. The toroidal separation between the two poloidal observation planes is about 40 cm, such that the majority of the field lines is observed by both arrays simultaneously, thereby enabling a direct measurement of the 3D properties of plasma instabilities like ELM filaments. The toroidal separation of 40 cm is sufficient for the accurate measurement of both phase differences and transit times of (rotating) plasma structures, enabling a distinction between time varying 2D structures and true 3D structures (not possible with 2D diagnostics). The research will mainly focus on the investigation of the 3D structure of the temperature fluctuations related to edge localized modes (ELMs), in particular precursors and filaments. The first results on ELMs will be reported. [Preview Abstract] |
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JP12.00115: A simple frequency sweep linearization method for FM density profile reflectometry Adi Liu, Jianqiang Hu, Edward Doyle, Jin Zhang, Hong Li, Chu Zhou, Xiaohui Zhang, Mingyuan Wang, Tao Lan, Jinglin Xie, Wandong Liu, Changxuan Yu Frequency modulated, continuous wave (FMCW) reflectometry is widely used to measure the electron density profile on fusion devices. To ensure the output intermediate frequency signal is proportional to the propagation delay time, the frequency sweep should be linearized, especially for reflectometry with sweeping periods of only a few microseconds. We introduce a simple dynamic calibration technique to linearize the frequency sweep based on digital complex demodulation methods, without using a Fourier transform, which would induce a trade-off between frequency and time resolution. The technique is convenient as it can be done in the same conditions as for plasma measurements. The method is in use on the EAST profile reflectometer, and results will be presented.\\[4pt] Work was supported by the Natural Science Foundation of China (NSFC) under 11475173, National Magnetic Confinement Fusion Energy Development Program of China under 2013GB106002 and 2014GB109002, and US DOE Grants DE- SC0010424 and DE-SC0010469. [Preview Abstract] |
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JP12.00116: Upgrades to the profile and Doppler reflectometer systems on EAST Jian Qiang Hu, A. Di Liu, Edward J. Doyle, Guiding Wang, Hong Li, Chu Zhou, Xiao Hui Zhang, Ming Yuan Wang, Jin Zhang, Chang Xuan Yu The USTC reflectometer systems on the EAST Tokamak have been upgraded, including new Q- and V-band monostatic FMCW profile reflectometer systems with dynamic calibration, efficient transition lines with quasi-optical lenses and corrugated waveguides, dual polarization operation. The profile system is integrated with an 8-channel Doppler backscattering (DBS) system in a new flexible microwave front-end, and a second DBS system is at a separate toroidal location. The new systems cater for variable scenarios and allow for poloidal and toroidal turbulence correlations. We present the designs for these upgraded systems, system calibrations and measurements of the beam profile in laboratory, as well as the primary experimental results from EAST operation.\\[4pt] Work supported by the Natural Science Foundation of China 11475173, National Magnetic Confinement Fusion Energy Development Program of China 2013GB106002 and 2014GB109002, US DOE Grants DE- SC0010424 and DE-SC0010469, and China Scholarship Council [2014] 3026. [Preview Abstract] |
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JP12.00117: Faraday-Effect Polarimeter-Interferometer System for current density measurement on EAST Haiqing Liu, Yinxian Jie, Weixing Ding, David Lyn Brower, Zhiyong Zou, Jinping Qian, Weiming Li, Long Zeng, Shoubiao Zhang, Liqun Hu, Baonian Wan An eleven-channel far-infrared laser-based \textbf{PO}larimeter-\textbf{INT}erferometer (\textbf{POINT}) system utilizing the three-wave technique has been implemented for current density and electron density profile measurements in the EAST tokamak. Both polarimetric and interferometric measurement are obtained in a long pulse ($\sim$ 52s) discharge. The electron line-integrated density resolution of POINT is less than 5 $\times$ 10$^{16}$ m$^{-2}$ ($\sim$ 2$^{\circ}$), and the Faraday rotation angle rms phase noise is \textless 0.1$^{\circ}$. With the high temporal ($\sim$ 1 $\mu $sec) and phase resolution(\textless 0.1$^{\circ}$), density perturbations associated with the sawteeth cycle and tearing mode activities have been observed. It is evident that tearing modes are well correlated to dynamics of equilibrium current profile (or q-profile). Faraday rotation angle shows clear variation with low hybrid current drive while line-integrated density remains little changed, implying the current drive in the core. A Digital Phase Detector with 250 kHz bandwidth provides real-time Faraday rotation angle and density phase shift output, which will be integrated into current profile control system in a long pulse discharge in future. [Preview Abstract] |
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JP12.00118: Progress on Development of Low Pressure High Density Plasmas on the Helicon Plasma Experiment (HPX) Royce James, Phillip Azzari, Omar Duke-Tinson, John Frank, Jackson Karama, Jordan Hopson, Richard Paolino, Eva Sandri, Justin Sherman, Erin Wright, Jeremy Turk The small Helicon Plasma Experiment (HPX) at the Coast Guard Academy Plasma Lab (CGAPL), continues to progress toward utilizing the reputed high densities (10$^{13}$ cm$^{-3}$ and higher) at low pressure (.01 T) [1] of helicons, for eventual high temperature and density diagnostic development in future laboratory investigations. HPX is designed to create repeatedly stable plasmas ($\sim$ 20 - 30 ns) induced by an RF frequency in the 10 to 70 MHz range. HPX is constructing RF field corrected Langmuir probe raw data will be collected and used to measure the plasma's density, temperature, and potentially the structure and behavior during experiments. Our 2.5 J YAG laser Thomson Scattering system backed by a 32-channel Data Acquisition (DAQ) system is capable 12 bits of sampling precision at 2 MS/s for HPX plasma property investigations. Progress on the development of the RF coupling system, Helicon Mode development, magnetic coils, and observations from the Thomson Scattering, particle, and electromagnetic scattering diagnostics will be reported. [Preview Abstract] |
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JP12.00119: Design of a collective scattering system for electron gyroscale turbulence study in KSTAR Woochang Lee, Hyeon Park, Dongjae Lee, JuneEok Leem, Yongun Nam The design characteristics of a multi-channel collective (or coherent) scattering system for electron scale turbulence study in KSTAR, which is planned to be installed in 2016, are investigated. A few critical issues are discussed in depth such as effect of the Faraday rotation of the electric field polarization of probing and scattered, the probing wave frequency which is related to the optics for measurement of electron gyro scale turbulence, the wave polarization to minimize absorption of the probing power by electron cyclotron resonant layers, and the probing power. A proper and feasible optics with 300 GHz probing wave, which is based on these issues, provides a simultaneous measurement of electron density fluctuations at four discrete poloidal wave numbers up to 21 cm$^{-1}$. The upper limit corresponds to the normalized wave number $k_{\perp} \rho_e$ of 0.2 in KSTAR plasmas. To detect scattered wave power and extract phase information, a quadrature detection system consisting of four-channel antenna/detector array and electronics will be employed. [Preview Abstract] |
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JP12.00120: Measurement of RF electric field in high-$\beta $ plasma using a Pockels detector in magnetosphere plasma confinement device RT-1 Toshiki Mushiake, M. Nishiura, Z. Yoshida, Y. Yano, Y. Kawazura, H. Saitoh, M. Yamasaki, A. Kashyap, N. Takahashi, M. Nakatsuka, Atsushi Fukuyama The magnetosphere plasma confinement device RT-1 generates a dipole magnetic field that can confine high-$\beta $ plasma by using a levitated superconducting coil. So far it is reported that high temperature electrons (up to 50keV) exist and that the local electron $\beta_{e}$ value exceeds more than 100{\%}. However, the ion $\beta $ value $\beta_{i}$ remains low in the present high-$\beta $ state. To realize a high-$\beta_{i}$ state, we have started Ion Cyclotron Heating (ICH) experiments. For efficient ICH in a dipole topology, it is important to measure RF electric fields and characterize the propagation of RF waves in plasmas. On this viewpoint, we started direct measurement of local RF electric fields in RT-1 with a Pockels sensor system. A non-linear optical crystal in the Pockels sensor produces birefringence in an ambient electric field. The refractive index change of the birefringence is proportional to the applied electric field strength, which can be used to measure local electric fields. RF electric field distribution radiated from an ICH antenna was measured inside RT-1 in air, and was compared with numerical results calculated by TASK code. Results on the measurement of electric field distribution in high-$\beta $ plasma and evaluation of the absorbed RF power into ions will be reported. [Preview Abstract] |
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JP12.00121: Fully non-linear multi-species Fokker-Planck-Landau collisions for gyrokinetic particle-in-cell simulations of fusion plasma Robert Hager, E.S. Yoon, S. Ku, E.F. D'Azevedo, P. H. Worley, C.S. Chang We describe the implementation, and application of a time-dependent, fully nonlinear multi-species Fokker-Planck-Landau collision operator based on the single-species work of Yoon and Chang [Phys. Plasmas 21, 032503 (2014)] in the full-function gyrokinetic particle-in-cell codes XGC1 [Ku et al., Nucl. Fusion 49, 115021 (2009)] and XGCa. XGC simulations include the pedestal and scrape-off layer, where significant deviations of the particle distribution function from a Maxwellian can occur. Thus, in order to describe collisional effects on neoclassical and turbulence physics accurately, the use of a non-linear collision operator is a necessity. Our collision operator is based on a finite volume method using the velocity-space distribution functions sampled from the marker particles. Since the same fine configuration space mesh is used for collisions and the Poisson solver, the workload due to collisions can be comparable to or larger than the workload due to particle motion. We demonstrate that computing time spent on collisions can be kept affordable by applying advanced parallelization strategies while conserving mass, momentum, and energy to reasonable accuracy. We also show results of production scale XGCa simulations in the H-mode pedestal and compare to conventional theory. [Preview Abstract] |
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JP12.00122: Toroidal Particle-in-Cell Code with Gyro-kinetic Election and Fully-kinetic ion Jingbo Lin, Pengfei Liu, Wenlu Zhang, Zhihong Lin Current drive and auxiliary heating is critical for fusion plasmas. A kinetic simulation model has been developed using gyro-kinetic electron and fully-kinetic ion by removing fast gyro motion of electrons using the Lie-transform perturbation theory. A particle-in-cell kinetic code is developed based on this model in general magnetic flux coordinate systems, which is particularly suitable for simulations of magnetically confined fusion. Single particle motion and field solver are successfully verified respectively. Preliminary integral benchmark, for example the lower-hybrid wave (LHW) and ion Bernstein wave (IBW), shows a good agreement with theoretical results. This code could be used to investigate high frequency nonlinear phenomenon during lower-hybrid current drive (LHCD) and ion cyclotron radio frequency heating (ICRF). [Preview Abstract] |
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JP12.00123: Investigation of the effect of flow shear and the ITG on gyrokinetic MAST turbulence Louis Van Wyk, Edmund Highcock, Anthony Field, Alexander Schekochihin, Colin Roach We study the effect of flow shear $\gamma_{E}$ and ion temperature gradient $a/L_{Ti}$ on L-mode turbulence in MAST using gyrokinetic simulations. These parameters play a crucial role in regulating and driving turbulence and together with the ratio of the safety factor to the inverse aspect ratio, $q/\varepsilon $, define a ``zero-turbulence manifold'' (ZTM) that represents the critical values needed to sustain turbulence. Nonlinear simulations show that by varying $\gamma_{E}$ and $a/L_{Ti}$ within experimental errors the turbulence crosses the ZTM, implying that the experiment operates close to marginality. In this parameter regime flow shear is very effective at regulating the turbulence, which is found to be subcritical. Finally the structure of the turbulence was studied: statistical parameters such as radial, perpendicular and parallel correlation lengths and the correlation time were calculated and found to be in reasonable agreement with experimental results obtained using Beam Emission Spectroscopy. [Preview Abstract] |
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JP12.00124: A new Lagrangian particle scheme utilizing phase space grid for XGC1 edge gyrokinetic code Seung Hoe Ku, R. Hager, C.S. Chang, S. Parker A new Lagrangian numerical scheme has been developed that utilizes the phase space grid in addition to the usual marker particles. The new scheme splits the probability distribution function (PDF) of a kinetic equation into PDF of weighted particles and PDF of phase space grid. The former contains the fast varying part of the whole PDF and the later mostly contains the slowly varying part. The numerical scheme is implemented in the gyrokinetic particle code XGC1, which specializes in simulating the tokamak edge plasma. Since edge plasma across the magnetic separatrix and in contact with the wall is in non-thermal equilibrium with sources and sinks, the conventional delta-f technique is inapplicable. Deviation of the slowly varying PDF on velocity grid can be arbitrarily large. The weights of marker particles are determined by ``direct weight evolution'' instead of the differential form of weight evolution equations that conventional delta-f schemes use. Particle weight is slowly transferred to the phase space grid, suppressing the growth of particle weights. Comparison with the usual full-f and delta-f method confirms validity of the new scheme. The new scheme is compatible with massively parallel computing. [Preview Abstract] |
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JP12.00125: An Implicit $\delta f$ Sub-cycled and Orbit Averaged Lorentz Ion Model Benjamin Sturdevant, Scott Parker, Yang Chen, Benjamin Hause A second order implicit $\delta f$ Lorentz ion hybrid model with sub-cycling and orbit averaging has been developed to study low-frequency, quasi-neutral plasmas. This model may be useful for verifying gyrokinetic simulation models in applications where higher order terms may be important, for example, in the tokamak edge pedestal region, where the equilibrium gradient scale lengths are quite short. A significant challenge for simulations using Lorentz ions in the presence of a strong guide field is the small time step size requirements for fully resolving the ion gyromotion. In this work, we present a GPU accelerated sub-cycling and orbit averaging method which has been developed to make the Lorentz ion model more viable and has been successfully applied to a test bed model for ion Landau damped ion acoustic waves in a uniform magnetic field. Simulation results will be presented to demonstrate the accurate reproduction of finite-Larmor-radius effects using large macro time steps to advance the fields. Future plans to implement the method in the GEM gyrokinetic simulation code to study the toroidal ITG instability with Lorentz ions will also be presented. [Preview Abstract] |
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JP12.00126: A closed Vlasov-Maxwell simulation model for high-frequency nonlinear processes in plasma Pengfei Liu, Jingbo Lin, Wenlu Zhang, Ihor Holod, Liu Chen, Zhihong Lin A kinetic simulation model has been developed to investigate the plasma dynamics with a frequency lower than the electron cyclotron frequency. The electrons are described by gyro-kinetic or drift-kinetic equations that ignores the rapid electron cyclotron motions, and the ions are described by fully kinetic equations that captures the fast ion cyclotron motion. The gyrokinetic Vlasov equation for electrons is derived using the Lie-transform perturbation theory, and the electron charge density, current density and thermal pressure tensor is expressed in terms of the pull-back transformation from gyro-center distribution to particle distribution. A fully kinetic ion and gyro-kinetic electron Vlasov-Maxwell model in Tokamak geometry is then formulated with a realistic ion-to-electron mass ratio . Both full-f and $\delta$-f scheme have then been derived in magnetic coordinate systems. This model can be applied to global particle simulations of ion-cyclotron-radio-frequency (ICRF) heating and lower-hybrid current driven (LHCD), which are important issues for fusion plasmas. [Preview Abstract] |
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JP12.00127: Multiphysics/Multiscale Coupling of Microturbulence and MHD Equiliria W.W. Lee, E.A. Startsev, S.R. Hudson, W.X. Wang, S. Ethier We propose to investigate the multiphysics and multiscale coupling between a time-dependent gyrokinetic ``microscopic'' code for studying gyroradius-scale turbulence, associated with global ion-acoustic and shear-Alfven waves, and a ``macroscopic'' code for computing large-scale global equilibria based on the time-independent MHD equations, in order to identify a family of self-consistent global MHD equilibria that can minimize the electrostatic potentials responsible for turbulent transport by passing global parameters between the two codes. The codes involved are 1) the electromagnetic version [1] of the GTS code [2] for studying microturbulence, and 2) the SPEC code [3] for calculating three-dimensional MHD equilibria with or without chaotic fields. This concept is based on a newly found correlation between the gyrokinetic evolution and the MHD equilibrium when the electrostatic potential vanishes [4]. The proposed work involves the scales ranging from the electron skin depth to the machine size, and includes the physics of both gyrokinetics and MHD. \\[4pt] [1] E. A. Startsev et al., Sherwood Conference, New York (2015).\\[0pt] [2] W. X. Wang et al., Phys. Plasmas 13, 092525 (2006).\\[0pt] [3] S. R. Hudson et al., Phys. Plasmas 19, 112502 (2012).\\[0pt] [4] W. W. Lee, Sherwood Conference, New York (2015). [Preview Abstract] |
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JP12.00128: Bridging the fluid-kinetic gap with implicit particle methods Jorge Amaya, Fabio Bacchini, Maria Elena Innocenti, Wei Jiang, Vyacheslav Olshevsky, Giovanni Lapenta Plasmas have intrinsic kinetic scales determined by the response of electrons and ions. The kinetic scales determine the finest scales that are to be expected in a plasma. However, the intrinsic scales are tiny compared with the system scales of interest in most situations where a fluid approach is more efficient. When computers will be able to resolve all scales for the problems of interest now, we will no longer be interested in them and we will be attempting much bigger and more challenging problems. There is now and there always will be a grand canyon between the scales we can resolve with the supercomputers available and the system sizes we want to consider. Mathematics needs to be called into action to lay a bridge over this grand canyon. We consider two pontifex methods: the implicit method and the multi level-multi domain method. Our most recent developments will be described and their properties of stability, accuracy and ability to resolve the different characteristics will be discussed. [Preview Abstract] |
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JP12.00129: Gyrokinetic simulation of the collisional micro-tearing mode instability Edward Startsev, Wei-li Lee, Weixing Wang An application of recently developed perturbative particle simulation scheme for finite-$\beta$ plasmas in the presence of background inhomogeneities is presented. Originally [1], using similar scheme, we were able to simulate shear-Alfven waves, finite-$\beta$ modified drift waves and ion temperature gradient modes using a simple gyrokinetic particle code based on realistic fusion plasma parameters. Recently, we have successfully used the scheme for simulation of linear tearing and drift-tearing modes, in both collisionless semi-collisional regimes in slab geometry with sheared magnetic field. Here, we present further development of this scheme for the simulation of linear semi-collisional micro-tearing mode driven by electron temperature gradient [2] in high-aspect ratio cylindrical cross-section tokamak using the modified turbulence code GTS. \\[4pt] [1] E. A. Startsev and W. W. Lee,{\it Phys. Plasmas} {\bf 21}, 022505 (2014).\\[0pt] [2] J. F. Drake and Y. C. Lee, {\it Phys. Fluids} {\bf 20}, 1341 (1977). [Preview Abstract] |
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JP12.00130: MHD Simulations of Plasma Dynamics with Non-Axisymmetric Boundaries Chris Hansen, Jeffrey Levesque, Kyle Morgan, Thomas Jarboe The arbitrary geometry, 3D extended MHD code PSI-TET is applied to linear and non-linear simulations of MCF plasmas with non-axisymmetric boundaries. Progress and results from simulations on two experiments will be presented: 1) Detailed validation studies of the HIT-SI experiment with self-consistent modeling of plasma dynamics in the helicity injectors. Results will be compared to experimental data and NIMROD simulations that model the effect of the helicity injectors through boundary conditions on an axisymmetric domain. 2) Linear studies of HBT-EP with different wall configurations focusing on toroidal asymmetries in the adjustable conducting wall. HBT-EP studies the effect of active/passive stabilization with an adjustable ferritic wall. Results from linear verification and benchmark studies of ideal mode growth with and without toroidal asymmetries will be presented and compared to DCON predictions. Simulations of detailed experimental geometries are enabled by use of the PSI-TET code, which employs a high order finite element method on unstructured tetrahedral grids that are generated directly from CAD models. Further development of PSI-TET will also be presented including work to support resistive wall regions within extended MHD simulations. [Preview Abstract] |
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JP12.00131: Continued Development and Validation of Methods for Spheromak Simulation Thomas Benedett The HIT-SI experiment has demonstrated stable sustainment of spheromaks; determining how the underlying physics extrapolate to larger, higher-temperature regimes is of prime importance in determining the viability of the inductively-driven spheromak. It is thus prudent to develop and validate a computational model that can be used to study current results and provide an intermediate step between theory and future experiments. A zero-beta Hall-MHD model has shown good agreement with experimental data at 14.5 kHz injector operation. Experimental observations at higher frequency, where the best performance is achieved, indicate pressure effects are important and likely required to attain quantitative agreement with simulations. Efforts to extend the existing validation to high frequency ($\sim$ 36-68 kHz) using an extended MHD model implemented in the PSI-TET arbitrary-geometry 3D MHD code will be presented. Results from verification of the PSI-TET extended MHD model using the GEM magnetic reconnection challenge will also be presented along with investigation of injector configurations for future SIHI experiments using Taylor state equilibrium calculations. Work supported by DoE. [Preview Abstract] |
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JP12.00132: Development of the PARVMEC Code for Rapid Analysis of 3D MHD Equilibrium Sudip Seal, Steven Hirshman, Mark Cianciosa, Andreas Wingen, Ezekiel Unterberg, Robert Wilcox The VMEC three-dimensional (3D) MHD equilibrium has been used extensively for designing stellarator experiments and analyzing experimental data in such strongly 3D systems. Recent applications of VMEC include 2D systems such as tokamaks (in particular, the D3D experiment), where application of very small (delB/B $\sim$ 10$^{-3}$) 3D resonant magnetic field perturbations render the underlying assumption of axisymmetry invalid. In order to facilitate the rapid analysis of such equilibria (for example, for reconstruction purposes), we have undertaken the task of parallelizing the VMEC code (PARVMEC) to produce a scalable and temporally rapidly convergent equilibrium code for use on parallel distributed memory platforms. The parallelization task naturally splits into three distinct parts 1) radial surfaces in the fixed-boundary part of the calculation; 2) two 2D angular meshes needed to compute the Green's function integrals over the plasma boundary for the free-boundary part of the code; and 3) block tridiagonal matrix needed to compute the full (3D) pre-conditioner near the final equilibrium state. Preliminary results show that scalability is achieved for tasks 1 and 3, with task 2 still nearing completion. The impact of this work on the rapid reconstruction of D3D plasmas using PARVMEC in the V3FIT code will be discussed. [Preview Abstract] |
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JP12.00133: LOW TEMPERATURE, BREAKDOWN, THRUSTERS AND SHEATHS |
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JP12.00134: On the Geometrical Optics Approach in the Theory of Freely-Localized Microwave Gas Breakdown Michael Shapiro, Samuel Schaub, Jason Hummelt, Richard Temkin, Vladimir Semenov Large filamentary arrays of high pressure gas microwave breakdown have been experimentally studied at MIT using a 110 GHz, 1.5 MW pulsed gyrotron. The experiments have been modeled by other groups using numerical codes. The plasma density distribution in the filaments can be as well analytically calculated using the geometrical optics approach [1,2] neglecting plasma diffusion. The field outside the filament is a solution of an inverse electromagnetic problem. The solutions are found for the cylindrical and spherical filaments [1] and for the multi-layered planar filaments [2] with a finite plasma density at the boundaries. We present new results of this theory showing a variety of filaments with complex shapes. The solutions for plasma density distribution are found with a zero plasma density at the boundary of the filament. Therefore, to solve the inverse problem within the geometrical optics approximation, it can be assumed that there is no reflection from the filament. The results of this research are useful for modeling future MIT experiments. \\[4pt] [1] V. B. Gildenburg and S. V. Golubev, ZhETF, vol. 67, p. 89, 1974.\\[0pt] [2] V. E. Semenov, Sov. J. Plasma Phys., vol. 10, p. 328, 1984. [Preview Abstract] |
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JP12.00135: Rapid Formation of Distributed Plasma Discharges using X-Band Microwaves Xun Xiang, John Booske, John Scharer Observations of rapidly formed (\textless 50-300 ns) distributed plasma discharges using X-band microwaves are presented. A cylindrical stainless steel chamber enclosed with polycarbonate windows is used to observe microwave breakdown in Ar and Ne gas mixtures from 10 to 760 torr. The chamber is illuminated by the output of 25 kW, 0.8 $\mu$s pulse-width, 9.382 GHz magnetron through an X-band waveguide pressed against the polycarbonate window. Measured incident and reflected microwave power is used to detect the discharge and absorption and transmission characteristics as the pressure is varied. Measurements show 70{\%} reflected power once plasma is formed and a small amount of Argon in Neon shortens the breakdown time. An E-plane tapered waveguide is designed to enhance the electric field at the breakdown surface so that breakdown condition is improved. Additionally, an ICCD provides fast (\textless 50 ns) time-scale optical images of the plasma, revealing the plasma formation and decay processes. Optical emission spectroscopy branching ratio measurements provide plasma breakdown characteristics including electron temperatures and the electron energy distribution functions for different Ne/Ar gas mixture plasmas formed at 10-200 torr. Experiments show that inclusion of both red and blue argon lines significantly enhances the effective electron temperature and the distribution function solutions, compared with using only red lines. [Preview Abstract] |
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JP12.00136: 2-D Interferometric Measurements of Electron Density in an Air Breakdown Plasma Using a 124.5 GHz, 1 MW Gyrotron S.C. Schaub, J.S. Hummelt, W.C. Guss, M.A. Shapiro, R.J. Temkin A 1 MW, 124.5 GHz gyrotron was used to produce a linearly polarized, quasioptical beam in 2.2 $\mu $s pulses. The beam was focused to a 2.6 mm spot size, producing a peak electric field of 70 kV/cm, after transmission losses. This electric field is great enough to produce a breakdown plasma in air at pressures ranging from a few Torr up to atmospheric pressure. The resulting breakdown plasma spontaneously forms a two-dimensional array of filaments, oriented parallel to the polarization of the beam, that propagate toward the microwave source. A needlepoint initiator was placed at the focal point of the beam, creating highly reproducible plasma arrays. An intensified CCD, with a minimum exposure of 2 ns, was combined with a two-wavelength laser interferometer, operating at 532 and 635 nm, to make spatially and temporally resolved electron density measurements of the plasma array. [Preview Abstract] |
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JP12.00137: Numerical and analytic analysis of the effective secondary emission yield from velvet, foam, and other complex surfaces C. Swanson, I.D. Kaganovich Experiments measuring the secondary electron emission yield of complex surfaces [1] have suggested that the secondary electron emission may be suppressed by specially prepared complex surfaces. We model several surface types analytically and numerically and find large reduction in the secondary electron emission yield from the complex surface. We find a greatly reduced secondary electron emission yield for a velvet surface for nearly normal to the surface incident electrons. However foams can provide greatly reduced secondary electron emission yield for all incident electron angular distributions. We developed an algorithm for optimization of surfaces for suppressing secondary electron emission. \\[4pt] [1] R. Cimino et al., ``Search for New e-cloud Mitigator Materials for High Intensity Particle Accelerators,'' Proc. of IPAC2014, Dresden, Germany 2332-2334 (2014) [Preview Abstract] |
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JP12.00138: Non-LTE Steady-State Kinetics of He-Air Atmospheric Pressure Plasmas Tzvetelina Petrova, George Petrov, Eric Gillman, David Boris, Sandra Hern\'andez, Scott Walton A non-LTE, steady-state collisional-radiative kinetics model is developed to study discharges produced in mixtures of He, N$_{2}$ and O$_{2}$ (He-Air) at atmospheric pressures. The model is based on a self-consistent solution of coupled Boltzmann equation for the electron energy distribution function, electron energy balance equation, gas thermal balance equation, and a system of non-linear equations for species that govern plasma chemistry (electrons, ions, radicals, atoms and molecules in ground and excited states) [1]. The model and results can be applied to study a variety of atmospheric pressure plasmas generated in He-Air mixtures, such as plasma jets, dielectric barrier discharges, laser-induced plasmas, microwave plasmas, etc. In this talk, collisional rates and species densities are obtained as a function of He-to-air ratio and the results are benchmarked against available experimental data. \\[4pt] [1] Tz. B. Petrova, H. D. Ladouceur, and A. P. Baronavski, Phys. Rev. E \textbf{76} (2007) 006405. [Preview Abstract] |
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JP12.00139: Ion extraction and sheath physics in the Inductively Coupled Plasma Mass Spectrometer Ross Spencer The plasma in an Inductively Coupled Plasma Mass Spectrometer (ICP-MS) is weakly ionized and supersonic. Just before the ions in the instrument pass into the mass analyzer, the electrons are removed from the plasma by means of an ion lens which has a relatively large negative potential. This step in the process can be modeled by assuming collisionless ions and Boltzmann electrons coupled to the ion density. The calculation is carried out on an axisymmetric grid in cylindrical geometry using a banded-matrix direct solver for Poisson's equation and iterating on the ion trajectories and the electron density until a self-consistent arrangement of ion and electron densities is obtained. This is a sheath calculation, but without the need for a pre-sheath since the plasma has already been accelerated to high velocity before reaching the ion lens. The behavior of this interesting electrostatic sheath for various values of the extraction potential will be discussed. [Preview Abstract] |
(Author Not Attending)
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JP12.00140: Excitation of Ion Acoustic Waves in Plasmas with Electron Emission from Walls A.V. Khrabrov, H. Wang, I. D. Kaganovich, Y. Raitses, D. Sydorenko Various plasma propulsion devices exhibit strong electron emission from the walls either as a result of secondary processes or due to thermionic emission. To understand details of electron kinetics in plasmas with strong emission, we have performed kinetic simulations of such plasmas using EDIPIC code. We show that excitation of ion acoustic waves is ubiquitous phenomena in many different plasma configurations with strong electron emission from walls. Ion acoustic waves were observed to be generated near sheath if the secondary electron emission from the walls is strong. Ion acoustic waves were also observed to be generated in the plasma bulk due to presence of an intense electron beam propagating from the cathode. This intense electron beam can excite strong plasma waves, which in turn drive the ion acoustic waves. Research supported by the U.S. Air Force Office of Scientific Research. [Preview Abstract] |
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JP12.00141: Unifying Paschen Curve Conditions across Pressure and Gap Distance Amanda Loveless, Allen Garner, Agust Valfells The Paschen curve (PC) predicts the breakdown voltage of a gas by relating it to the product of pressure and gap distance (pd). Recent experiments deviate from the PC for microscale gaps at low pd. A scaling law incorporating field emission-driven breakdown and field enhancement to the macroscale Paschen law yields more accurate predictions for microscale gaps (A. Venkattraman and A. A. Alexeenko, Phys. Plasmas \textbf{19}, 123515 (2012).). While many studies consider low pd, deviations from the PC also arise at high pd, as demonstrated for gap distances between 0.0508 and 0.254 cm and pressures between 96.5 and 6900 kPa (W. J. Carey, A. J. Wiebe, R. D. Nord, and L. L. Altgilbers, in Proc. IEEE Pulsed Power Conf., 2011, pp. 741-744). High pd values are relevant for ongoing high voltage plasma experiments for food treatment and combustion. We attempt to elucidate the impact of large gap distances ($\sim$ 5 cm) and higher pressures ($\sim$ 200-300 kPa) on these deviations by connecting recent work at low pd to high pd by assessing scaling laws, analyzing field emission models, and using particle-in-cell codes. Implications on experimental design will be discussed, and the development of a universal Paschen curve will be explored. [Preview Abstract] |
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JP12.00142: Study of effective secondary electron emission in dc breakdown of argon with various metal electrodes Steven Adams, XuHai Huang, Kenneth Howe, Vladimir Demidov, Boyd Tolson An attractive aspect of Townsend's expression for the ionization coefficient, $\alpha \quad =$ A exp[-B/(E/p)], is that the exponential form allows a derivation of a neat analytical expression for the Paschen curve. Notwithstanding the elegance and fame of this expression, the theoretical Paschen curve does not always provide an accurate prediction for all E/p ranges and all gases. Deviations can be attributed to a variety of factors, including non-exponential behavior of $\alpha $ at higher E/p, variations of $\gamma $ with E/p and geometric effects. An experimental study of the effective secondary electron emission in Townsend breakdown has been conducted in Ar using a variety of electrodes. The threshold breakdown voltage was measured when the current became self-sustained, which corresponded to an effective secondary emission coefficient of $\gamma \quad =$ 1/[exp(($\alpha $/p)pd)-1]. This allowed a fundamental relationship to be derived between $\gamma $ and E/p from an experimental Paschen curve. In this work, argon gas was studied with copper, aluminum and platinum electrodes. The trends of the effective secondary electron emission are analyzed in different E/p ranges for various modes of secondary electron emission, including Ar ion impact, photon absorption, Ar metastable collisions and heavy-particle-ionization. [Preview Abstract] |
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JP12.00143: Measurements of neutral and ion velocity distribution functions in a Hall thruster Panagiotis Svarnas, Iavn Romadanov, Ahmed Diallo, Yevgeny Raitses Hall thruster is a plasma device for space propulsion. It utilizes a cross-field discharge to generate a partially ionized weakly collisional plasma with magnetized electrons and non-magnetized ions. The ions are accelerated by the electric field to produce the thrust. There is a relatively large number of studies devoted to characterization of accelerated ions, including measurements of ion velocity distribution function using laser-induced fluorescence diagnostic. Interactions of these accelerated ions with neutral atoms in the thruster and the thruster plume is a subject of on-going studies, which require combined monitoring of ion and neutral velocity distributions. Herein, laser-induced fluorescence technique has been employed to study neutral and single-charged ion velocity distribution functions in a 200 W cylindrical Hall thruster operating with xenon propellant. An optical system is installed in the vacuum chamber enabling spatially resolved axial velocity measurements. The fluorescence signals are well separated from the plasma background emission by modulating the laser beam and using lock-in detectors. Measured velocity distribution functions of neutral atoms and ions at different operating parameters of the thruster are reported and analyzed. [Preview Abstract] |
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JP12.00144: The Concept of the Pinch Helicon Magnetoplasma Thruster Celso Ribeiro A new concept of a high efficient electrodeless magnetoplasma-based electric thruster using a helicon plasma submitted to a pinch effect is proposed. This concept is intended to have high thrust for a short period, while reducing the plasma-wall contact. This proposal is highly suitable for rocket devices for faster satellites' orbital correction, a feasible way to lead manned missions into deep space, and for a more compact plasma sources for plasma materials for fusion studies. The pinch effect is created by a set of poloidal field coils placed around the insulating cylinder where the helical antenna is mounted. A pulsed current creates the pinch effect, which immediately insulates the plasma to the wall, thus reducing the perpendicular particle and energy losses. As a consequence, the axial flux, thus the thrust, should be increased because of particle balance. This may require an additional magnetic field produced by a single coil placed at the back of the helicon antenna (the gas entrance) to be acted during the pinch phase, leading to an unbalanced magnetic mirror-type configuration, therefore a net flux outwards. The combination with the double helicon structure may also maximize the efficiency. If everything is synchronized, there will be no gas or energy wasted, and the wall heat will be minimal, thus reducing the problems of material fatigue and failures, and making feasible the use of conventional materials. [Preview Abstract] |
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JP12.00145: Study of Ion Beam Forming Process in Electric Thruster Using 3D FEM Simulation Tao Huang, Xiaolin Jin, Quan Hu, Bin Li, Zhonghai Yang There are two algorithms to simulate the process of ion beam forming in electric thruster. The one is electrostatic steady state algorithm. Firstly, an assumptive surface, which is enough far from the accelerator grids, launches the ion beam. Then the current density is calculated by theory formula. Secondly these particles are advanced one by one according to the equations of the motions of ions until they are out of the computational region. Thirdly, the electrostatic potential is recalculated and updated by solving Poisson Equation. At the end, the convergence is tested to determine whether the calculation should continue. The entire process will be repeated until the convergence is reached. Another one is time-depended PIC algorithm. In a global time step, we assumed that some new particles would be produced in the simulation domain and its distribution of position and velocity were certain. All of the particles that are still in the system will be advanced every local time steps. Typically, we set the local time step low enough so that the particle needs to be advanced about five times to move the distance of the edge of the element in which the particle is located. [Preview Abstract] |
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JP12.00146: A Mini-helicon Thruster in Argon and Xenon Francis F. Chen Our small helicon discharge, 5 cm in diam and 5 cm long, using a commercial Nd permanent magnet, has been used previously\footnote{F.F. Chen, Phys. Plasmas \textbf{21}, 093511 (2014).} to eject an argon ion beam suitable for spacecraft propulsion. The specific impulse can be increased by biasing the conducting top plate of the discharge. Thrusters normally use xenon for propulsion because of its high mass and low ionization potential. We have now tested the mini-helicon thruster in xenon. The entire device is small enough to be brought to a poster session and shown there. [Preview Abstract] |
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JP12.00147: Modeling of Plasma Detachment from a Magnetic Nozzle Colin Glesner, Bhuvana Srinivasan The detachment of plasma from a magnetic nozzle is examined using numerical simulations based on the discontinuous galerkin method. Plasma detachment is of interest for its role in the development of plasma based space propulsion systems. The simulation parameters used, modeled after the computational and experimental work of Winglee et al,\footnote{R. Winglee, et al, Physics of Plasmas {\bf 14}, 063501 (2007).} result in $\beta \approx 0.01$, and $Re_{m} \approx 0.3$. In this low-$\beta$ regime perturbation of the initially imposed magnetic field is expected to be small. To more effectively study these perturbations, the ideal magnetohydrodynamic equations are modified by linearizing the magnetic field. The perturbative component is then evolved in the simulation rather than the total magnetic field, allowing for a clearer resolution of changes in the magnetic field produced by the plasma. Because of the intermediate range of magnetic Reynolds number present in this plasma configuration, the effect of introducing resistivity in the simulation is also examined. Further work will investigate the effect of varying the configuration of the magnetic field. [Preview Abstract] |
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JP12.00148: Experimental Validation of a Branched Solution Model for Magnetosonic Ionization Waves in Plasma Accelerators Thomas Underwood, Keith Loebner, Mark Cappelli Detailed measurements of the thermodynamic and electrodynamic plasma state variables within the plume of a pulsed plasma accelerator are presented. A quadruple Langmuir probe operating in current-saturation mode is used to obtain time resolved measurements of the plasma density, temperature, potential, and velocity along the central axis of the accelerator. This data is used in conjunction with a fast-framing, intensified CCD camera to develop and validate a model predicting the existence of two distinct types of ionization waves corresponding to the upper and lower solution branches of the Hugoniot curve. A deviation of less than 8\% is observed between the quasi-steady, one-dimensional theoretical model and the experimentally measured plume velocity. [Preview Abstract] |
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JP12.00149: Two- and three-dimensional particle-in-cell simulations of ExB discharges Johan Carlsson, Igor D. Kaganovich, Alexander V. Khrabrov, Yevgeny Raitses, Andrei Smolyakov The Large-Scale Plasma (LSP) Particle-In-Cell with Monte-Carlo Collisions (PIC-MCC) code has been used to simulate several crossed-field (ExB) discharges in two and three dimensions. Two-dimensional (2D) simulations of a cold-cathode electric discharge with power-electronics applications and a Penning discharge will be presented. Three-dimensional (3D) simulation results of a cylindrical Hall thruster with scaled plasma parameters will also be shown and compared to experiment~[Ellison2012]. To enable the 2D and 3D ExB discharge simulations, several improvements to the LSP code were made, including implementation of a new electrostatic field solver, external-circuit model and models for particle injection and secondary-electron emission. To ensure the correctness of the collision models used (and particularly important for the cold-cathode-discharge simulations), validation and code benchmarking was done with the LSP and EDIPIC codes in 1D for a glow discharge. Results and conclusions will be presented. L. Ellison, Y. Raitses and N. J. Fisch, ``Cross-field electron transport induced by a rotating spoke in a cylindrical Hall thruster,'' Physics of Plasmas \textbf{19}, 013503 (2012). Research supported by the U.S. Air Force Office of Scientific Research. [Preview Abstract] |
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JP12.00150: Optical emission spectroscopy of carbon arc for nanomaterial synthesis Vladislav Vekselman, Brentley Stratton, Yevgeny Raitses Arc plasma assisted synthesis of carbon nanostructures is one of the most efficient and simple production methods [1]. In spite of a long time use of this method in materials science research and industrial applications, the role of the plasma in nucleation and growth of nanostructures is not well understood. This is due to complexity of physico-chemical processes governing the plasma nanosynthesis. The objective of this work is to characterize the atmospheric pressure arc plasma used for synthesis of various carbon nanostructures. Optical emission spectroscopy was carried out to determine the distribution of temperature and density of carbon plasma in the synthesis zone as a function of arc discharge parameters. Accurate and detailed mapping of plasma parameters elucidate the general trend governing the formation of carbon nanostructures. \\[4pt] [1] C. Journet, W. K. Maser, P. Bernier, A. Loiseau, M. L. delaChapelle, S. Lefrant, P. Deniard, R. Lee, and J. E. Fischer, Large-scale production of single-walled carbon nanotubes by the electric-arc technique, Nature 388, 756 (1997). [Preview Abstract] |
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JP12.00151: Quantification of the error induced on Langmuir probe determined electron temperature and density due to an RF plasma potential Nischal Kafle, David Donovan, Elijah Martin An RF plasma potential can significantly effect the IV characteristic of a Langmuir probe if not properly compensated. A substantial research effort in the low temperature plasma community has been carried out to determine this effect and how to achieve the required compensation for accurate measurements. However, quantification of the error induced on the extracted electron temperature and density from an uncompensated Langmuir probe due to an RF plasma potential has not been explored. The research presented is the first attempt to quantify this error in terms of RF plasma potential magnitude, electron temperature, and electron density. The Langmuir probe IV characteristic was simulated using empirical formulas fitted to the Laframboise simulation results. The RF effected IV characteristic was simulated by adding a sinusoidal variation to the plasma potential and computing the time average numerically. The error induced on the electron temperature and density was determined by fitting the RF effected IV characteristic to the empirical formulas representing the standard Laframboise simulation results. Experimental results indicating the accuracy of this quantification will be presented. [Preview Abstract] |
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JP12.00152: Correlating Metastable-Atom Density, Reduced Electric Field, and Electron Energy Distribution in the Earlier Stages of a 1-Torr Argon Discharge M. Koepke, J.B. Franek, S.H. Nogami, V. Demidov, E.V. Barnat Temporal measurement of electron density, metastable-atom density, and reduced electric field are used to infer the dynamic behavior of the excitation rates describing electron-atom collision-induced excitation in the positive column of a pulsed Ar discharge plasma by invoking plausible assumptions about the shape of the electron energy distribution function (EEDF), specifically, inelastic electron-metastable collisions produce high-energy electrons and electron-electron collisions cause the EEDF to Maxwellianize.\footnote{\textit{PSST} 14, 722 (2005).} Direct observation of these excitation rates were used to predict the temporal behavior of metastable-atom density in the post-transient stage of a pulsed plasma discharge.\footnote{\textit{PSST} 23, 034009 (2015).} Ignoring the effect of electron-electron collisions allows for the examination, in this poster, of correlations between the aforementioned quantities in the transient stage of a discharge. We conclude that the observed and predicted line-emission ratio agree quantitatively in the transient phase of the discharge and agree qualitatively in the initiation phase of the discharge. Ignoring electron-electron collisions allows insight into hard-to-measure or expensive-to-measure plasma conditions and their time dependence. [Preview Abstract] |
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JP12.00153: Ion dynamics in a DC magnetron microdischarge measured with laser-induced fluorescence Christopher Young, Nicolas Gascon, Andrea Lucca Fabris, Tsuyohito Ito, Mark Cappelli We present evidence of coherent rotating azimuthal wave structures in a planar DC magnetron microdischarge operated with argon and xenon. The dominant stable mode structure varies with discharge voltage, and high frame rate camera imaging of plasma emission reveals propagating azimuthal waves in the negative $\vec{E} \times \vec{B}$ direction. This negative drift direction is attributed to a local field reversal arising from strong density gradients that drive excess ions towards the anode. Observed mode transitions are shown to be consistent with models of gradient drift-wave dispersion in such a field reversal when the fluid representation includes ambipolar diffusion parallel to the magnetic field direction. Time-averaged and time-resolved laser-induced fluorescence measurements interrogate xenon ion dynamics under the action of the field reversal. Time resolution is obtained by synchronizing with the coherent azimuthal wave frequency at fixed mode number. [Preview Abstract] |
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JP12.00154: Intense Underwater Laser Propagation and Ionization at Visible and Ultraviolet Wavelengths Ted Jones, Dmitri Kaganovich, Mike Helle, Tony Ting, John Palastro, Bahman Hafizi, Dan Gordon, Joe Penano, Yu-hsin Chen Intense underwater laser propagation, filamentation, and ionization are under investigation at NRL for applications including remote laser acoustic generation for low-frequency sonar. Time-resolved absorption spectroscopy of fs underwater laser ionization revealed hydrated electron density of 5.4 x 10$^{\mathrm{18}}$ cm$^{\mathrm{-3}}$ and lifetime of 350 ps. In addition, high-resolution fluorescence imaging of ns underwater laser propagation using two-photon absorbing dye, independently confirmed previous measurements of 100 micron diameter filament structures [Helle, et al., Appl. Phys. Lett. \textbf{103}, 121101]. A patented scheme for generating an elongated, meter-scale, high energy density underwater plasma [USP 9,088,123] is under study, in which such a filament structure could serve as a target for a second energetic ``heater'' laser pulse. Early experiments suggested improved ionization efficiency using the current configuration, with a 266 nm filament pulse, and a 532 nm heater pulse. 1- and 2-D simulations using a nonlinear laser propagation code are underway to predict beam envelope propagation, filamentation, and stimulated Raman and Brillouin scattering behavior. Results from recent experiments and simulations will be presented. [Preview Abstract] |
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JP12.00155: Laser-Bioplasma Interaction: The Blood Type Transmutation Induced by Multiple Ultrashort Wavelength Laser Beams V. Alexander Stefan The interaction of ultrashort wavelength multi laser beams with the flowing blood thin films leads to the transmutation of the blood types A, B, and AB into O type.\footnote{Henrik Clausen and the research group from the University of Copenhagen, \textit{Nature Biotechnology} \textbf{25}, 454 - 464 (2007); Published online on April 1, 2007.} This is a novel mechanism of importance for the transfusion medicine. Laser radiation is in resonance with the eigen-frequency modes of the antigen proteins\index{proteins} and forces the proteins to parametrically oscillate until they get kicked out from the surface. The stripping away of antigens is done by the scanning-multiple-lasers of a high repetition rate in the blue-purple frequency domain. The guiding-lasers are in the red-green frequency domain. The laser force, (parametric interaction with the antigen eigen-oscillation),\footnote{V. Stefan, B. I. Cohen, C. Joshi, \textit{Science}, 243, 4890, (Jan.27, 1989); V. Alexander Stefan, \textit{Neurophysics, Stem Cell Physics, and Genomic Physics}, (S-U-Press, La Jolla, CA, 2012); V. Alexander Stefan, APS-March-2013, {\#} H1.00208.} upon the antigen protein molecule must exceed its weight. The scanning laser beam is partially reflected as long as the antigen(s) is not eliminated. The process of the protein detachment can last a few minutes. [Preview Abstract] |
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JP12.00156: Low-temperature plasma needle effects on cultured metastatic breast cancer cells Sean Knecht, Sven Bilen, Michael Micci, Timothy Brubaker, Michael Wilson, Ian Cook, Nicholas Czesak, Garret Hipkins The Penn State Low-Temperature Plasma group is presently investigating the applications of low-temperature plasma for biomedical applications, including the effects on MDA-MB-231 metastatic breast cancer cells. A plasma needle system has been designed and constructed that consists of a 22-gauge stainless steel syringe needle, which acts as the high-voltage electrode, covered with PEEK tubing as the dielectric with a ring ground electrode on the outside. The system is driven by a low-frequency AC voltage amplifier, with typical operating conditions of 2--5 kV peak voltage at 5 kHz. Helium is used as the working fluid and produces a plasma jet with $\sim$ cm's visible extent. Cultured breast cancer cells were provided by our collaborator and exposed to the plasma needle for varying doses and detachment of cells was observed. The effects are attributed to reactive oxygen and nitrogen species generation and transport through the cell culture medium. Plasma needle characterization and the results of the breast cancer experiments will be presented. [Preview Abstract] |
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JP12.00157: Characterization of a DBD-Based Plasma Jet Using a Variable Pulse Width Nanosecond Pulser. Timothy Ziemba, Julian Picard, James Prager, Kenneth Miller, John Carscadden Most high voltage pulsers used to drive dielectric barrier discharges (DBDs), produce a single pulse shape (width and voltage), thus making it challenging to assess the effect of pulse shape on the production of different chemical species during a discharge. Eagle Harbor Technologies, Inc. (EHT) has developed a high voltage nanosecond pulser that enables independent control of the output voltage, pulse width, and pulse repetition frequency. This pulser has been specifically designed to drive dielectric barrier discharges (DBD). EHT has used this pulser to conduct a parametric investigation of a DBD-based jet utilizing spectroscopic diagnostics. A better understanding of this parameter dependency can allow for more targeted and effective application of plasma in medical, environmental, industrial, and other applications. Results comparing DBD voltage and current waveforms with plasma spectrographic measurements will be presented. [Preview Abstract] |
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JP12.00158: BiScO3-PbTiO3 Nanoceramics Prepared by Spark Plasma Sintering Xiaohui Wang Highly dense 0.37BiScO3-0.63PbTiO3 (BS-PT) ceramics with nano-sized grains were prepared by combination of Spark Plasma Sintering and two-step sintering method. The microstructures, phase and piezoelectric behaviors of BS-PT nanoceramics were investigated. TEM observations clearly showed that the average grain sizes of the ceramic samples were 23, 33 and 70 nm respectively. The contrasting morphologies of grain boundary region under different magnifications were caused by different contrast and imaging mechanisms. HRTEM image confirmed that the samples had dense and thin grain boundary regions. Perovskite phase was demonstrated for all the samples by XRD and SAED data. Local SPM measurements recorded well-formed butterfly and piezoelectric hysteresis loops for all the samples, suggesting that BS-PT nanoceramics retained a ferroelectric state and the polarizations were switchable with the average grain size as fine as 23 nm. There was a comparatively large fluctuation of local piezoelectric responses. A significant difference between local and macro piezoelectric coefficients was observed. The grain boundary regions with complex internal stress and charges/defects were the key factor to understand these unusual properties in ferroelectric nanoceramics. [Preview Abstract] |
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JP12.00159: Simulations of plasma sheaths using continuum kinetic models Bhuvana Srinivasan, Ammar Hakim Understanding plasma sheath physics is important for the performance of devices such as Hall thrusters due to the effect of energetic particles on electrode erosion. Plasma sheath physics is studied using kinetic and multi-fluid models with relevance to secondary electron emissions and plasma-surface interactions. Continuum kinetic models are developed to directly solve the Vlasov-Poisson equation using the discontinuous Galerkin method for each of the ion and electron species. A steady-state sheath is simulated by including a simple model for a neutral fluid. Multi-fluid simulations for the plasma sheath are also performed using the discontinuous Galerkin method to solve a complete set of fluid equations for each of the ion and electron species. The kinetic plasma sheath is compared to a multi-fluid plasma sheath. [Preview Abstract] |
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JP12.00160: Investigation of Ion-Ion Two-Stream Instabilities in the Presheath Ryan Hood, Fred Skiff, Scott Baalrud, Robert Merlino Ion velocity distribution functions (IVDF) have been measured for both argon and xenon in an experiment to investigate ion-ion two stream instabilities and their effect on ion flow velocities near sheaths. The device is a multidipole hot cathode discharge operated in a mixture of argon and xenon. IVDF data from Ar II and Xe II are collected simultaneously using optical tagging and laser induced fluorescence (LIF) in the presheath region of a negatively biased electrode. Fluctuations of the ion distribution functions will be observed to search for evidence of ion-ion two-stream instability. This instability is thought to be responsible for anomalous friction causing a merging of ion speeds near the sheath edge. Experiment updates and new results will be presented. [Preview Abstract] |
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JP12.00161: Fundamental Restructuring of the Collisional Presheath Near Electron-Emitting Surfaces Michael Campanell Recent work showed that intense electron emission can make the sheath potential become positive. This ``inverse sheath'' differs strongly from the ``space-charge limited sheath'' predicted in conventional models. Here we show that intense emission also makes the plasma interior restructure. The collisional presheath structure is dominated not by ion acceleration but by emission thermalization. Since the sheath potential is positive, emitted electrons are not accelerated away from the surface. They enter the presheath with low velocities and actually have a higher spatial density than the hotter plasma electrons by a ratio $\sim \sqrt{Tep/Temit}$. This leads to a surprising result that the quasineutral plasma density must increase from the bulk towards the presheath edge, opposite from the case of Bohm presheaths. The force balance throughout the plasma interior is altered by the electrons originating from the surface. Simulation and experimental evidence of ``inverted presheaths'' will be shown. The results could be pertinent to various plasma systems with intense emission. *M.D. Campanell, POP 22, 040702 (2015). [Preview Abstract] |
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JP12.00162: ABSTRACT WITHDRAWN |
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JP12.00163: Spectroscopic measurements for hydrogen dissociation degree in an helicon plasma C. Marini, P. Demolon, B. Duval, I. Furno, Ph. Guitienne, A. Howling, R. Jacquier, A. Karpushov, A. Simonin, K. Verhaegh Future fusion reactors, such as DEMO, will need a new generation of Neutral Beam (NB) systems to produce high power (up to 120 MW) and high energy (1 MeV) neutral beams. To achieve these requirements, the use of negative ion beams produced by a surface or volumetric plasma source is presently exploited by the fusion community. A new helicon plasma source based on a resonant birdcage network antenna is under development at the CRPP in collaboration with CEA-IRFM, and is installed on the linear Resonant Antenna Ion Device (RAID). One of the most important parameters for the negative ion production, other than electron temperature $T_e$ and density $n_e$, is the dissociation degree $\alpha$, that can be determined by using the passive spectroscopic method conceived by Lavrov et al. [2006 Plasma Sources Sci. Technol. 15 135]. The method exploits the dependence of the $H_\alpha$, $H_\beta$ and Fulcher (2,2)Q1 line intensity ratios on $T_e$, $\alpha$ and the gas temperature $T_{gas}$. A strong point of the method is that it employs only passive spectroscopic measurements that are non-perturbing and relatively easy to implement. The experimental setup is described and results are compared with actinometric estimations and 0D model predictions. [Preview Abstract] |
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JP12.00164: Probe Techniques for Plasma Potential Measurements in a Low-Temperature Magnetized Plasma Brian Kraus, Yevgeny Raitses We report results of measurements of plasma potential in a cross-field discharge of a cylindrical Penning configuration with magnetized electrons and non-magnetized ions. Measurements were conducted using three different probe types, including swept biased Langmuir probe, floating emissive probe and a magnetically-insulated probe [1]. Depending on the operating parameters of the Penning discharge, such as gas pressure (0.1-10 mTorr of Xenon, Argon) and magnetic field (10-100 Gauss), the plasma in this Penning discharge can be subject to the enhancement of electron cross-field transport compared to classical collisional transport [2]. We compare the three probe techniques and discuss the applicability of the magnetically-insulated probe for measurements in magnetized plasmas with such anomalous electron transport. \\[4pt] [1] V. I. Demidov, S. M. Finnegan, M. E. Koepke and E. W. Reynolds, Contrib. Plasma Phys. 44, 689 (2004).\\[0pt] [2] Y. Raitses, A. Smolyakov and I. Kaganovich, from 34th International Electric Propulsion Conference, Kobe, Japan, 2015. [Preview Abstract] |
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