S. L. Allen

Lawrence Livermore National Laboratory, Livermore, California, United States

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Publications (278)260.75 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Upgrades to core and divertor Thomson scattering (DTS) diagnostics at DIII-D have provided measurements of electron pressure profiles in the lower divertor from attached- to fully-detached divertor plasma conditions. Detailed, multistep sequences of discharges with increasing line-averaged density were run at several levels of Pinj. Strike point sweeping allowed 2D divertor characterization using DTS optimized to measure Te down to 0.5 eV. The ionization front at the onset of detachment is found to move upwards in a controlled manner consistent with the indication that scrape-off layer parallel power flux is converted from conducted to convective heat transport. Measurements of ne, Te and pe in the divertor versus Lparallel demonstrate a rapid transition from Te⩾15 eV to ⩽3 eV occurring both at the outer strike point and upstream of the X-point. These observations provide a strong benchmark for ongoing modelling of divertor detachment for existing and future tokamak devices.
    Journal of Nuclear Materials 02/2015; DOI:10.1016/j.jnucmat.2015.01.066 · 2.02 Impact Factor
  • Journal of Nuclear Materials 12/2014; DOI:10.1016/j.jnucmat.2014.12.052 · 2.02 Impact Factor
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    ABSTRACT: An imaging system with a wide-angle tangential view of the full poloidal cross-section of the tokamak in simultaneous infrared and visible light has been installed on DIII-D. The optical train includes three polished stainless steel mirrors in vacuum, which view the tokamak through an aperture in the first mirror, similar to the design concept proposed for ITER. A dichroic beam splitter outside the vacuum separates visible and infrared (IR) light. Spatial calibration is accomplished by warping a CAD-rendered image to align with landmarks in a data image. The IR camera provides scrape-off layer heat flux profile deposition features in diverted and inner-wall-limited plasmas, such as heat flux reduction in pumped radiative divertor shots. Demonstration of the system to date includes observation of fast-ion losses to the outer wall during neutral beam injection, and shows reduced peak wall heat loading with disruption mitigation by injection of a massive gas puff.
    Review of Scientific Instruments 11/2014; 85(11):11D855. DOI:10.1063/1.4892897 · 1.58 Impact Factor
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    ABSTRACT: A conceptual design for a divertor Thomson scattering (DTS) diagnostic has been developed for the NSTX-U device to operate in parallel with the existing multipoint Thomson scattering system. Higher projected peak heat flux in NSTX-U will necessitate application of advanced magnetics geometries and divertor detachment. Interpretation and modeling of these divertor scenarios will depend heavily on local measurement of electron temperature, Te, and density, ne, which DTS provides in a passive manner. The DTS design for NSTX-U adopts major elements from the successful DIII-D DTS system including 7-channel polychromators measuring Te to 0.5 eV. If implemented on NSTX-U, the divertor TS system would provide an invaluable diagnostic for the boundary program to characterize the edge plasma.
    Review of Scientific Instruments 11/2014; 85(11):11E825. DOI:10.1063/1.4894001 · 1.58 Impact Factor
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    ABSTRACT: New near infrared (NIR) spectroscopic measurements performed in the DIII-D tokamak divertor plasma suggest new viable diagnostic applications: divertor recycling and low-Z impurity flux measurements, a spectral survey for divertor Thomson scattering (DTS) diagnostic, and Te monitoring for divertor detachment control. A commercial 0.3 m spectrometer coupled to an imaging lens via optical fiber and a InGaAs 1024 pixel array detector enabled deuterium and impurity emission measurements in the range 800-2300 nm. The first full NIR survey identified D, He, B, Li, C, N, O, Ne lines and provided plasma Te, ne estimates from deuterium Paschen and Brackett series intensity and Stark line broadening analysis. The range 1.000-1.060 mm was surveyed in high-density and neon seeded divertor plasmas for spectral background emission studies for λ = 1.064 μm laser-based DTS development. The ratio of adjacent deuterium Paschen-α and Brackett Br9 lines in recombining divertor plasmas is studied for divertor Te monitoring aimed at divertor detachment real-time feedback control.
    Review of Scientific Instruments 10/2014; 85(11). DOI:10.1063/1.4891600 · 1.58 Impact Factor
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    ABSTRACT: In April 2010, two thermo-oxidation experiments ('O-bakes') were performed in the DIII-D tokamak. Internal surfaces of the tokamak, as well as a number of specimens inserted into the torus, were exposed to a mixture of 20% O2/80% He at a nominal pressure of 9.5 Torr (1.27 kPa) at a temperature of 350–360 °C for a duration of 2 h. Three primary conclusions have been drawn from these experiments: (1) laboratory measurements on the release of deuterium from tokamak codeposits by oxidation have been duplicated in a tokamak environment, (2) no internal tokamak components or systems were adversely affected by the oxidation and (3) the recovery of plasma performance following oxidation was similar to that following regular torus openings.
    Nuclear Fusion 05/2013; 53(7):073008. DOI:10.1088/0029-5515/53/7/073008 · 3.24 Impact Factor
  • T R Weber, S L Allen, J Howard
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    ABSTRACT: This work describes a coherence imaging spectrometer capable of making spatially resolved CIII flow measurements in the DIII-D lower divertor. The spectrometer exploits a periscope view of the plasma to produce line-of-sight averaged velocity measurements of CIII. From these chord averaged flow measurements, a 2D poloidal cross section of the CIII flow is tomographically reconstructed. Details of the diagnostic setup, acquired data, and data analysis will be presented, along with prospects for future applications.
    The Review of scientific instruments 10/2012; 83(10):10E102. DOI:10.1063/1.4728311 · 1.58 Impact Factor
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    ABSTRACT: Imaging interferometers have been used to measure the 2-D distribution of the Doppler shift of impurity emission in both the lower and upper DIII-D divertors. The interferometer design has been simplified to a single birefringent plate between two polarizers, and improved calibration techniques have been implemented, including temperature stabilization. Measurements of other impurity species such as CII have been added. An image-intensified camera in the upper divertor has enabled measurement of the flows in the crown of the plasma during lower single-null divertor operation. In general, flows are in opposite directions on the inner and outer scrape-off layers in the divertor, as expected from the magnetic geometry. Initial results from a wide view periscope of the whole plasma cross section will also be presented.
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    ABSTRACT: Temperature-controlled, 0.15 nm interference filters were installed on an edge-viewing system of the motional Stark effect (MSE) polarimeter on the DIII-D tokamak. The upgraded system provides a factor of two reduction in the bandpass compared to the previous design, and linear control of the bandpass, which is unaltered by wavelength tuning. With the new system, there is a reduced dependence of the inferred polarization angle on the filter wavelength calibration. Recent measurements from the calibrated edge-viewing system show increased agreement with other MSE arrays.
    The Review of scientific instruments 10/2012; 83(10):10E319. DOI:10.1063/1.4733342 · 1.58 Impact Factor
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    ABSTRACT: Direct analysis of internal magnetic field pitch angles measured using the motional Stark effect diagnostic shows m/n = 2/1 neoclassical tearing modes exhibit stronger poloidal magnetic flux-pumping than typical hybrids containing m/n = 3/2 modes. This flux-pumping causes the avoidance of sawteeth, and is present during partial electron cyclotron current drive suppression of the tearing mode. This finding could lead to hybrid discharges with higher normalized fusion performance at lower q95. The degree of edge localized mode-neoclassical tearing mode (ELM-NTM) coupling and the strength of flux-pumping increase with beta and the proximity of the modes to the ELMing pedestal. Flux-pumping appears independent of magnetic island width. Individual ELM-NTM coupling events show a rapid timescale drop in the island width followed by a resistive recovery that is successfully modeled using the modified Rutherford equation. The fast transient drop in island width increases with ELM size.
    Physics of Plasmas 02/2012; 19(2). DOI:10.1063/1.3684648 · 2.25 Impact Factor
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    ABSTRACT: An experiment was conducted in DIII-D to examine carbon deposition when a secondary separatrix is near the wall. The magnetic configuration for this experiment was a biased double-null, similar to that foreseen for ITER. 13C methane was injected toroidally symmetrically near the secondary separatrix into ELMy H-mode deuterium plasmas. The resulting deposition of 13C was determined by nuclear reaction analysis. These results show that very little of the injected 13C was deposited at the primary separatrix, whereas a large fraction of injected 13C was deposited close to the point of injection near the secondary separatrix. Six of the tiles were put back into DIII-D, where they were baked at 350-360 °C for 2 h at ~1 kPa in a 20% O2/80% He gas mixture. Subsequent ion beam analysis of these tiles showed that about 21% of the 13C and 54% of the deuterium were removed by the bake.
    12/2011; DOI:10.1088/0031-8949/2011/T145/014025
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    ABSTRACT: Recently, C^2+ flows have been measured at DIII-D using a recent, advanced technique in coherence imaging spectroscopy [1]. The diagnostic yields a two dimensional C^2+ flow measurement over of the entire lower divertor, with approximately ±1 cm resolution in space, and approximately ±2 km/s resolution in velocity. Preliminary analysis of data from a plasma shot during the 2010 campaign yields high flow (˜30 km/s) near the X-point and a flow reversal between the low and high field side of the scrape-off-layer. Results are in good agreement with predictions from the fluid plasma code, UEDGE. A discussion on the fundamental physics behind the flow as indicated by experimental and UEDGE results will be included. Data will likely prove useful in efforts to understand impurity transport processes critical in tokamak operations. 6pt [1] J. Howard, et al., Rev. Sci. Instrum. 81, 10E528 (2010).
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    ABSTRACT: Recent modifications to the motional Stark effect diagnostic on DIII D have improved detection of the Stark-split beam emission and reduced uncertainty in the inferred magnetic field line pitch angles. The core system's signal-to-noise ratio was increased and systematic errors reduced following the installation of redesigned monochrometers and by refining the in-vessel calibration procedure. Modeling of the neutral beam emission and MSE optics motivated the installation of narrowband optical filters for edge channels leading to reduced channel-to-channel offsets. The measurements are used to validate the modeling results, and constrain the edge magnetic field in equilibrium reconstructions of high confinement mode plasma scenarios. Modeling is used to assess the ability of the edge system to constrain the edge plasma current and the effect of mixed polarization states on the MSE pitch angles. Finally we investigate the specifications for future upgrades aimed at informing tearing mode stability studies.
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    ABSTRACT: We are designing a wide-angle tangential viewing system for DIII-D, with co-registered views in the visible and IR. We will examine toroidal and poloidal asymmetries of wall heating and particle flux during ELMs, magnetic perturbations, and disruptions; toroidal and poloidal mode structure of ELMs; poloidal distribution of particle flow velocities, and others. The system will simultaneously view the inner wall, outer wall, and upper and lower divertors, and will have an independent 3X optical zoom capability in visible and IR. Various parts of the image may be viewed at 3X magnification by translating the camera(s) vertically and laterally in the image plane. For IR we have a FLIR SC6000HS 3-5 μm camera, and for visible a Phantom V7.3. Both have high frame rate capability. Visible wavelength and neutral density filters may be selected, or interferometric flow measurement optics may be substituted for the filter system. This system was inspired by a design by CEA Cadarache for JET, and is similar to a system designed by LLNL for ITER upper ports.
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    ABSTRACT: Edge localized mode (ELM)-neoclassical tearing mode (NTM) coupling pumps poloidal flux from the core to the edge in hybrid discharges, contributing to flattening of the safety factor profile and avoidance of sawteeth. Direct motional Stark effect diagnostic analysis of internal magnetic field pitch angles show 2/1 NTMs exhibit stronger magnetic flux-pumping than typical hybrids, albeit at lower beta. This 2/1 flux-pumping is present during partial electron cyclotron current drive NTM suppression. This finding may lead to an alternative discharge with normalized fusion performance exceeding that required for Qfus=10 operation in ITER. The strength of flux-pumping increases with beta and proximity of the NTM to the ELMing pedestal. Individual ELM-NTM coupling events are successfully modeled using the modified Rutherford equation (MRE).
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    ABSTRACT: New scalings of the dependence of divertor heat flux peak and profile width, important parameters for the design of future large tokamaks, have been obtained from recent DIII-D experiments. We find the peak heat flux depends linearly on input power, decreases linearly with increasing density, and increases linearly with plasma current. The profile width has a weak dependence on input power, is independent of density up to the onset of detachment, and is inversely proportional to the plasma current. We compare these results with previously published scalings, and present mathematical expressions incorporating these results.
    Journal of Nuclear Materials 08/2011; 415(1). DOI:10.1016/j.jnucmat.2010.12.008 · 2.02 Impact Factor
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    ABSTRACT: Unbalanced double null ELMy H-mode configurations in DIII-D are used to simulate the situation in ITER high triangularity, burning plasma magnetic equilibria, where the second X-point lies close to the top of the vacuum vessel, creating a secondary divertor region at the upper blanket modules. The measured plasma conditions in the outer secondary divertor closely duplicated those projected for ITER. CH was injected into the secondary outer divertor to simulate sputtering there. The majority of the C found was in the secondary outer divertor. This material migration pattern is radically different than that observed for main wall CH injections into single null configurations where the deposition is primarily at the inner divertor. The implications for tritium codeposition resulting from sputtering at the secondary divertor in ITER are significant since release of tritium from Be co-deposits at the main wall bake temperature for ITER, 240 °C, is incomplete. The principal features of the measured C deposition pattern have been replicated by the OEDGE interpretive code.
    Journal of Nuclear Materials 08/2011; 415(1):513-. DOI:10.1016/j.jnucmat.2010.11.039 · 2.02 Impact Factor
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    ABSTRACT: Experiments are performed on the DIII-D tokamak to determine the retention rate in an all graphite first-wall tokamak. A time-dependent particle balance analysis shows a majority of the fuel retention occurs during the initial Ohmic and L-mode phase of discharges, with peak fuel retention rates typically ˜2 × 1021 D/s. The retention rate can be zero within the experimental uncertainties (<3 × 1020 D/s) during the later stationary phase of the discharge. In general, the retention inventory can decrease in the stationary phase by ˜20-30% from the initial start-up phase of the discharge. Particle inventories determined as a function of time in the discharge, using a "dynamic" particle balance analysis, agree with more accurate particle inventories directly measured after the discharge, termed "static" particle balance. Similarly, low stationary retention rates are found in discharges with heating from neutral-beams, which injects particles, and from electron cyclotron waves, which does not inject particles. Detailed analysis of the static and dynamic balance methods provide an estimate of the DIII-D global co-deposition rate of ⩽0.6-1.2×1020 D/s. Dynamic particle balance is also performed on discharges with resonant magnetic perturbation ELM suppression and shows no additional retention during the ELM-suppressed phase of the discharge.
    Journal of Nuclear Materials 08/2011; 415. DOI:10.1016/j.jnucmat.2011.02.028 · 2.02 Impact Factor
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    ABSTRACT: Various spatial heterodyne polarization interferometers for spectrally-resolved optical imaging of edge and core parameters in high temperature magnetized plasmas are described. Applications for such “coherence imaging” (CI) systems include imaging motional Stark effect and Zeeman effect polarimetry for determination of the magnetic field pitch angle, and passive and active (charge exchange recombination spectroscopy - CXRS) Doppler imaging of plasma temperature and flow. In this paper we describe spatial heterodyne coherence imaging systems and present first results of Doppler flow imaging in the DIII-D divertor.Instruments have been installed for imaging flows in the divertor and scrape-off-layer in the DIII-D tokamak and also for Doppler imaging on the H-1 heliac [1]. In the former case, single snapshot interferometric images of the plasma in CII 514nm, and CIII 465nm emission have been demodulated to obtain flow and ion temperature projections in both the scrape-off-layer and divertor. Flow field amplitudes in the divertor are found to be broad agreement with UEDGE modeling [2], and point the way towards experiments that address important divertor transport issues in future (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    Beiträge aus der Plasmaphysik 03/2011; 51(2‐3):194 - 200. DOI:10.1002/ctpp.201000062
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    ABSTRACT: Accurate measurement of internal magnetic field direction using motional Stark effect (MSE) polarimetry in the edge pedestal is desired for nearly all tokamak scenario work. A newly installed 500 kHz 32-channel digitizer on the MSE diagnostic of DIII-D allows full spectral information of the polarimeter signal to be recovered for the first time. Fourier analysis of this data has revealed magnetohydrodynamic (MHD) fluctuations in the plasma edge pedestal at ρ ≥ 0.92. By correlating edge localized mode fluctuations seen on lock-in amplifier outputs with MSE spectrograms, it has been shown that edge pedestal tearing mode fluctuations cause interference with MSE second harmonic instrument frequencies. This interference results in unrecoverable errors in the real-time polarization angle measurement that are more than an order of magnitude larger than typical polarimeter uncertainties. These errors can cause as much as a 38% difference in local q. By using a redundant measure of the linear polarization found at the fourth harmonic photo-elastic modulator (PEM) frequency, MHD interference can be avoided. However, because of poorer signal-to-noise the fourth harmonic signal computed polarization angle shows no improvement over the MHD polluted second harmonics. MHD interference could be avoided in future edge pedestal tokamak polarimeters by utilizing PEMs with higher fundamental frequencies and a greater separation between their frequencies.
    The Review of scientific instruments 03/2011; 82(3):033515. DOI:10.1063/1.3568827 · 1.58 Impact Factor

Publication Stats

1k Citations
260.75 Total Impact Points

Institutions

  • 1983–2014
    • Lawrence Livermore National Laboratory
      • Physics Division
      Livermore, California, United States
  • 2011
    • Johns Hopkins University
      • Department of Physics and Astronomy
      Baltimore, MD, United States
  • 2007
    • University of Texas at Austin
      • Institute for Fusion Studies
      Austin, Texas, United States
  • 2005–2007
    • University of Toronto
      • Institute for Aerospace Studies
      Toronto, Ontario, Canada
  • 1994–2003
    • Oak Ridge National Laboratory
      Oak Ridge, Florida, United States
  • 1997–2000
    • General Atomics
      San Diego, California, United States
  • 1987
    • Hebrew University of Jerusalem
      • Racah Institute of Physics
      Yerushalayim, Jerusalem, Israel