S. L. Allen

Lawrence Livermore National Laboratory, Livermore, California, United States

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Publications (295)308.41 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Externally applied non-axisymmetric magnetic fields are shown to have little effect on the impurity ion flow velocity and temperature as measured by the multichord divertor spectrometer in the DIII-D divertor for both attached and detached conditions. These experiments were performed in H-mode plasmas with the grad-B drift toward the target plates, with and without n = 3 resonant magnetic perturbations (RMPs). The flow velocity in the divertor is shown to change by as much as 30% when deuterium gas puffing is used to create detachment of the divertor plasma. No measurable changes in the C III flow were observed in response to the RMP fields for the conditions used in this work. Images of the C III emission are used along with divertor Thomson scattering to show that the local electron and C III temperatures are equilibrated for the conditions shown.
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    ABSTRACT: We survey the results of recent DIII-D experiments that tested the effectiveness of three innovative tokamak divertor concepts in reducing divertor heat flux while still maintaining acceptable energy confinement under neon/deuterium-based radiating divertor (RD) conditions: (1) magnetically unbalanced high performance double-null divertor (DND) plasmas, (2) high performance double-null “Snowflake” (SF-DN) plasmas, and (3) single-null H-mode plasmas having different isolation from their divertor targets. In general, all three concepts adapt well to RD conditions, achieving significant reduction in divertor heat flux (q⊥p) and maintaining high performance metrics, e.g., 50–70% reduction in peak divertor heat flux for DND and SF-DN plasmas that are characterized by βN ≅ 3.0 and H98(y,2) ≈ 1.35. It is also demonstrated that q⊥p could be reduced ≈50% by extending the parallel connection length (L||-XPT) in the scrape-off layer between the X-point and divertor targets over a variety of the RD and non-RD environments tested.
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    ABSTRACT: The snowflake divertor (SFD) control and detachment control to manage the heat flux at the divertor are successfully demonstrated at DIII-D. Results of the development and implementation of these two heat flux reduction control methods are presented. The SFD control algorithm calculates the position of the two null-points in real-time and controls shaping coil currents to achieve and stabilize various snowflake configurations. Detachment control stabilizes the detachment front fixed at specified distance between the strike point and the X-point throughout the shot.
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    ABSTRACT: Measurements show fast ion losses correlated with applied three-dimensional (3D) fields in a variety of plasmas ranging from L-mode to resonant magnetic perturbation (RMP) edge localized mode (ELM) suppressed H-mode discharges. In DIII-D L-mode discharges with a slowly rotating n = 2 magnetic perturbation, scintillator detector loss signals synchronized with the applied fields are observed to decay within one poloidal transit time after beam turnoff indicating they arise predominantly from prompt loss orbits. Full orbit following using M3D-C1 calculations of the perturbed fields and kinetic profiles reproduce many features of the measured losses and points to the importance of the applied 3D field phase with respect to the beam injection location in determining the overall impact on prompt beam ion loss. Modeling of these results includes a self-consistent calculation of the 3D perturbed beam ion birth profiles and scrape-off-layer ionization, a factor found to be essential to reproducing the experimental measurements. Extension of the simulations to full slowing down timescales, including fueling and the effects of drag and pitch angle scattering, show the applied n = 3 RMPs in ELM suppressed H-mode plasmas can induce a significant loss of energetic particles from the core. With the applied n = 3 fields, up to 8.4% of the injected beam power is predicted to be lost, compared to 2.7% with axisymmetric fields only. These fast ions, originating from minor radii ρ > 0.7, are predicted to be primarily passing particles lost to the divertor region, consistent with wide field-of-view infrared periscope measurements of wall heating in n = 3 RMP ELM suppressed plasmas. Edge fast ion Dα (FIDA) measurements also confirm a large change in edge fast ion profile due to the n = 3 fields, where the effect was isolated by using short 50 ms RMP-off periods during which ELM suppression was maintained yet the fast ion profile was allowed to recover. The role of resonances between fast ion drift motion and the applied 3D fields in the context of selectively targeting regions of fast ion phase space is also discussed.
    Nuclear Fusion 07/2015; 55(7):073028. DOI:10.1088/0029-5515/55/7/073028 · 3.06 Impact Factor
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    ABSTRACT: Periods of edge localized mode (ELM)-free H-mode with increased pedestal pressure and width were observed in the DIII-D tokamak when density fluctuations localized to the region near the separatrix were present. Injection of a powder of 45 µm diameter lithium particles increased the duration of the enhanced pedestal phases to up to 350 ms, and also increased the likelihood of a transition to the enhanced phase. Lithium injection at a level sufficient for triggering the extended enhanced phases resulted in significant lithium in the plasma core, but carbon and other higher Z impurities as well as radiated power levels were reduced. Recycling of the working deuterium gas appeared unaffected by this level of lithium injection. The ion scale, kθρs ~ 0.1–0.2, density fluctuations propagated in the electron drift direction with f ~ 80 kHz and occurred in bursts every ~1 ms. The fluctuation bursts correlated with plasma loss resulting in a flattening of the pressure profile in a region near the separatrix. This localized flattening allowed higher overall pedestal pressure at the peeling–ballooning stability limit and higher pressure than expected under the EPED model due to reduction of the pressure gradient below the 'ballooning critical profile'. Reduction of the ion pressure by lithium dilution may contribute to the long ELM-free periods.
    Nuclear Fusion 06/2015; 55(6):063018. DOI:10.1088/0029-5515/55/6/063018 · 3.06 Impact Factor
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    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; 463. DOI:10.1016/j.jnucmat.2015.01.066 · 1.87 Impact Factor
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    ABSTRACT: Recent DIII-D experiments assessed the snowflake divertor (SF) configuration in a radiative regime in H-mode discharges with seeding. The SF configuration was maintained for many energy confinement times (2–3 s) in H-mode discharges ( MA, MW, and down (favorable direction toward the divertor)), and found to be compatible with high performance operation (H98y2 ⩾ 1). The two studied SF configurations, the SF-plus and the SF-minus, have a small finite distance between the primary X-point and the secondary null located in the private flux region or the common flux region, respectively. In H-mode discharges with the SF configurations (cf. H-mode discharges with the standard divertor with similar conditions) the stored energy lost per the edge localized mode (ELM) was reduced, and significant divertor heat flux reduction between and during ELMs was observed over a range of collisionalities, from lower density conditions toward a higher density H-modes with the radiative SF divertor.
    Journal of Nuclear Materials 12/2014; 463. DOI:10.1016/j.jnucmat.2014.12.052 · 1.87 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 · 1.61 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 · 1.61 Impact Factor
  • V. A. Soukhanovskii · A. G. McLean · S. L. Allen ·
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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 · 1.61 Impact Factor
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    ABSTRACT: The triggering of rapid small edge localized modes (ELMs) by high frequency pellet injection has been proposed as a method to prevent large naturally occurring ELMs that can erode the ITER plasma facing components (PFCs). Deuterium pellet injection has been used to successfully demonstrate the on-demand triggering of edge localized modes (ELMs) at much higher rates and with much smaller intensity than natural ELMs. The proposed hypothesis for the triggering mechanism of ELMs by pellets is the local pressure perturbation resulting from reheating of the pellet cloud that can exceed the local high-n ballooning mode threshold where the pellet is injected. Nonlinear MHD simulations of the pellet ELM triggering show destabilization of high-n ballooning modes by such a local pressure perturbation.
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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 · 3.06 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 · 1.61 Impact Factor
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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 · 1.61 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: Thermo-oxidation is controlled exposure in an oxygen-containing atmosphere at elevated temperature and is being considered as a technique for the de-tritiation of carbon-based codeposits in ITER. In addition , unplanned oxidation may also occur during accidental air ingress. The impact of thermo-oxidation on ITER diagnostic mirrors causes concerns. A dedicated study was performed in DIII-D, where molybde-num and copper mirrors were installed in the main chamber, in the divertor, and at a location remote from the plasma and exposed for ;2 hours to a mixture containing 80% helium and 20% oxygen at a total pressure of 1.27 kPa. Mirrors in the main chamber and in the divertor were exposed at 3508C to 3608C whereas the temperature of mirrors in the remote area was ;1608C. Reflectivity of all mirrors was degraded after thermo-oxidation showing a decrease in the UV range from 60% to 10% for molybdenum mirrors and a 90% drop for copper mirrors at the wavelength 250 nm. The reflectiv-ity of mirrors exposed at lower temperature was less degraded. Surface analyses revealed formation of oxides on all mirrors. In ITER, shutters planned for mirror protection are ineffective against thermo-oxidation. Nevertheless, in-situ cleaning systems planned for ITER mirrors may efficiently remove oxide layers.
    Fusion Science and Technology 07/2012; 62(1):97. · 0.49 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 · 2.14 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; T145:4025-. DOI:10.1088/0031-8949/2011/T145/014025
  • T. R. Weber · S. L. Allen · D. N. Hill · W. H. Meyer · G. D. Porter · J. Howard ·
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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.

Publication Stats

2k Citations
308.41 Total Impact Points


  • 1983-2015
    • Lawrence Livermore National Laboratory
      • Physics Division
      Livermore, California, United States
  • 1980-2011
    • Johns Hopkins University
      • Department of Physics and Astronomy
      Baltimore, MD, United States
  • 2000
    • General Atomics
      San Diego, California, United States
  • 1994
    • Oak Ridge National Laboratory
      Oak Ridge, Florida, United States
  • 1987
    • Hebrew University of Jerusalem
      • Racah Institute of Physics
      Yerushalayim, Jerusalem, Israel