K. H. Burrell

General Atomics, San Diego, California, United States

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Publications (614)886.43 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The Phase Contrast Imaging (PCI) diagnostic on DIII-D provides a line-integrated measurement of density fluctuations covering wavenumbers 2 to 30 cm-1. An outer gap scan during QH-mode with stationary plasma parameters allowed the PCI to sample a large range in kr/kθ. A narrow peak in turbulence amplitude is seen near the LCFS. The ExB Doppler shift allows the location to be determined precisely, showing two distinct regions of turbulence at 0.5 and 0.2 cm inside the LCFS with kr>0 and kr<0 respectively, consistent with the expected effects of shear in the Er well. PCI measurements at 200 kHz show that kθ=0.8 cm-1 with poloidal correlation length Lθ=6 cm. Using a simple non-isotropic turbulence model, we find that kr=3 cm-1 and Lr=0.5 cm, with n/n˜25% in the pedestal for this high-kr turbulence. These fluctuations, which are outside the parameter range accessible to most turbulence diagnostics, are large enough in amplitude to play a role in setting the pedestal structure. These PCI observations are qualitatively similar to those made in ELM-free H-mode and between ELMs suggesting that similar large kr turbulence may be important.
    10/2012;
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    ABSTRACT: A range of experiments on DIII-D have investigated the role of externally applied torque and the associated toroidal rotation on confinement and stability in plasmas with high levels of normalized fusion performance. In standard H-mode plasmas, the confinement is initially enhanced by increasing rotation, but saturates at intermediate levels of rotation. However, the same effect is observed over a wider range of rotation in advanced inductive plasmas. Both ion and electron confinement improves with high rotation. Surprisingly, experiments in quiescent H-mode (QH-mode) plasmas have found the opposite trend, with improved confinement, performance and reduced turbulence levels at low rotation. The different behavior suggests that ExB shear, rather than rotation, is needed for improving confinement. In particular, for these QH-mode plasmas, it is found that the ExB shear near the edge is maintained or enhanced with torque from non-resonant magnetic fields, even at low rotation. In all scenarios, no major difference is observed in confinement whether the plasma is initiated with high rotation and slowed down, or formed with low rotation from the beginning.
    10/2012;
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    ABSTRACT: In DIII-D, localized electron cyclotron heating (ECH) is used to probe the critical gradient of, and onset of stiffness in, the electron temperature Te profile. While keeping the total injected ECH power constant, the deposition profile was varied to investigate the relationship between the Te gradient and the electron power balance heat flux. A critical temperature gradient was observed, above which both the heat diffusivity and the Te fluctuations increase sharply. To compare to modeling, efforts have been made to produce the most realistic equilibrium reconstructions by using kinetic pressure constraints and motional Stark effect measurements of the local magnetic pitch. With these reconstructions, the gyrokinetic stability codes GYRO and TGLF predict that there is a critical Te gradient, similar to the experimentally observed gradient, above which electron modes exist and whose growth rates dominate over the ion growth rates.
    10/2012;
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    ABSTRACT: Electron profile stiffness was studied in DIII-D L-mode discharges by systematically varying the heat flux in a narrow region with electron cyclotron heating and measuring the local change produced in {nabla}T{sub e}. Electron stiffness was found to slowly increase with toroidal rotation velocity. A critical inverse temperature gradient scale length 1/L{sub C} {approx} 3 m{sup -1} was identified at {rho}=0.6 and found to be independent of rotation. Both the heat pulse diffusivity and the power balance diffusivity, the latter determined by integrating the measured dependence of the heat pulse diffusivity on -{nabla}T{sub e}, were fit reasonably well by a model containing a critical inverse temperature gradient scale length and varying linearly with 1/L{sub T} above the threshold.
    Physics of Plasmas 08/2012; 19(8). · 2.38 Impact Factor
  • K. H. Burrell, J. M. Munoz Burgos
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    ABSTRACT: In plasmas equipped with neutral beam injection, excitation of atomic spectral lines via charge-exchange with neutral atoms is the basis of one of the standard plasma diagnostic techniques for ion density, temperature, and velocity. In order to properly interpret the spectroscopic results, one must consider the effects of the energy dependence of the charge-exchange cross-section as well as the motion of the ion after charge-exchange during the period when it is still in the excited state. This motion is affected by the electric and magnetic fields in the plasma. The present paper gives results for the velocity distribution function of the excited state ions and considers in detail the cross-section and ion motion effects on the post charge-exchange velocity. The expression for this velocity in terms of the charge-exchange cross-section and the pre charge-exchange velocity allows that latter velocity to be determined. The present paper is the first to consider the effect of the electric as well as the magnetic field and demonstrates that electric field and diamagnetic terms appear in the expression for the inferred velocity. The present formulation also leads to a novel technique for assessing the effect of the energy dependence of the charge-exchange cross-section on the inferred ion temperature.
    Physics of Plasmas 07/2012; 19(7). · 2.38 Impact Factor
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    ABSTRACT: Results from recent experiments demonstrate that quiescent H-mode (QH-mode) sustained by magnetic torque from non-axisymmetric magnetic fields is a promising operating mode for future burning plasmas. Using magnetic torque from n=3 fields to replace counter-I{sub p} torque from neutral beam injection (NBI), we have achieved long duration, counter-rotating QH-mode operation with NBI torque ranging from counter-I{sub p} to up to co-I{sub p} values of 1-1.3 Nm. This co-I{sub p} torque is 3 to 4 times the scaled torque that ITER will have. These experiments utilized an ITER-relevant lower single-null plasma shape and were done with ITER-relevant values of {nu}{sub ped}{sup *} and {beta}{sub N}{sup ped}. These discharges exhibited confinement quality H{sub 98y2}=1.3, in the range required for ITER. In preliminary experiments using n=3 fields only from a coil outside the toroidal coil, QH-mode plasmas with low q{sub 95}=3.4 have reached fusion gain values of G={beta}{sub N}H{sub 89}/q{sub 95}{sup 2}=0.4, which is the desired value for ITER. Shots with the same coil configuration also operated with net zero NBI torque. The limits on G and co-I{sub p} torque have not yet been established for this coil configuration. QH-mode work to has made significant contact with theory. The importance of edge rotational shear is consistent with peeling-ballooning mode theory. Qualitative and quantitative agreements with the predicted neoclassical toroidal viscosity torque is seen.
    Physics of Plasmas 05/2012; 19(5). · 2.38 Impact Factor
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    ABSTRACT: Bulk ion toroidal rotation plays a critical role in controlling microturbulence and MHD stability as well as yielding important insight into angular momentum transport and the investigation of intrinsic rotation. So far, our understanding of bulk plasma flow in hydrogenic plasmas has been inferred from impurity ion velocity measurements and neoclassical theoretical calculations. However, the validity of these inferences has not been tested rigorously through direct measurement of the main-ion rotation in deuterium plasmas, particularly in regions of the plasma with steep pressure gradients where very large differences can be expected between bulk ion and impurity rotation. New advances in the analysis of wavelength-resolved D{sub {alpha}} emission on the DIII-D tokamak [J. L. Luxon et al., Fusion Sci. Technol. 48, 807 (2002)] have enabled accurate measurements of the main-ion (deuteron) temperature and toroidal rotation. The D{sub {alpha}} emission spectrum is accurately fit using a model that incorporates thermal deuterium charge exchange, beam emission, and fast ion D{sub {alpha}} (FIDA) emission spectra. Simultaneous spectral measurements of counter current injected and co current injected neutral beams permit a direct determination of the deuterium toroidal velocity. Time-dependent collisional radiative modeling of the photoemission process is in quantitative agreement with measured spectral characteristics. L-mode discharges with low beam ion densities and broad thermal pressure profiles exhibit deuteron temperature and toroidal rotation velocities similar to carbon. However, intrinsic rotation H-mode conditions and plasmas with internal transport barriers exhibit differences between core deuteron and carbon rotation which are inconsistent with the sign and magnitude of the neoclassical predictions.
    Physics of Plasmas 05/2012; 19(5). · 2.38 Impact Factor
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    ABSTRACT: The EPED model predicts the H-mode pedestal height and width based upon two fundamental and calculable constraints: (1) onset of non-local peeling-ballooning modes at low to intermediate mode number, (2) onset of nearly local kinetic ballooning modes at high mode number. We present detailed tests of the EPED model in discharges with edge localized modes (ELMs), employing new high resolution measurements, and finding good quantitative agreement across a range of parameters. The EPED model is then applied for the first time to quiescent H-mode (QH), finding a similar level of agreement between predicted and observed pedestal height and width, and suggesting that the model can be used to predict the critical density for QH-mode operation. Finally, the model is applied toward understanding the suppression of ELMs with 3D resonant magnetic perturbations (RMP). Combining EPED with plasma response physics, a new working model for RMP ELM suppression is developed. We propose that ELMs are suppressed when a 'wall' associated with the RMP blocks the inward penetration of the edge transport barrier. A calculation of the required location of this 'wall' with EPED is consistent with observed profile changes during RMP ELM suppression and offers an explanation for the observed dependence on safety factor (q{sub 95}).
    Physics of Plasmas 05/2012; 19(5). · 2.38 Impact Factor
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    ABSTRACT: Direct evidence of zonal flow (ZF) predator-prey oscillations and the synergistic roles of ZF- and equilibrium E×B flow shear in triggering the low- to high-confinement (L- to H-mode) transition in the DIII-D tokamak is presented. Periodic turbulence suppression is first observed in a narrow layer at and just inside the separatrix when the shearing rate transiently exceeds the turbulence decorrelation rate. The final transition to H mode with sustained turbulence and transport reduction is controlled by equilibrium E×B shear due to the increasing ion pressure gradient.
    Physical Review Letters 04/2012; 108(15):155002. · 7.94 Impact Factor
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    ABSTRACT: The first systematic investigation of core electron thermal transport and the role of local ion temperature gradient/trapped electron mode/electron temperature gradient (ITG/TEM/ETG)-scale core turbulence is performed in high temperature, low collisionality H-mode plasmas in the DIII-D tokamak. Wavenumber spectra of L-mode and H-mode density turbulence are measured by Doppler backscattering. H-mode wavenumber spectra are directly contrasted for the first time with nonlinear gyrokinetic simulation results. Core ITG/TEM-scale turbulence is substantially reduced/suppressed by E × B shear promptly after the L–H transition, resulting in reduced electron thermal transport across the entire minor radius. For small kθρs, both experiment and nonlinear gyrokinetic simulations using the GYRO code show density fluctuation levels increasing with kθρs in H-mode (r/a = 0.6), in contrast to ITG/TEM-dominated L-mode plasmas. GYRO simulations also indicate that a significant portion of the remaining H-mode electron heat flux results directly from residual intermediate/short-scale TEM/ETG turbulence. Electron transport at substantially increased electron-to-ion temperature ratio (Te/Ti ≥ 1, r/a ≤ 0.35) has been investigated in ECH-assisted, quiescent H-mode plasmas. A synergistic increase in core electron and ion thermal diffusivity (normalized to the gyro-Bohm diffusivity) is found with applied ECH. From linear stability analysis, the TEM mode is expected to become the dominant linear instability with ECH due to increased electron-to-ion temperature ratio and a reduction in the ion temperature gradient. This is consistent with increased electron temperature fluctuations and core electron thermal diffusivity observed experimentally. The reduced ion temperature gradient likely results from a reduction in the ITG critical gradient due to increased Te/Ti and reduced E × B shear. These studies are performed at collisonality ( , r/a ≤ 0.6) and address transport in electron heat-dominated regimes, thought to be important in ITER due to α-particle heating.
    Nuclear Fusion 01/2012; 52(2):023003. · 2.73 Impact Factor
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    ABSTRACT: Edge localized mode (ELM) pacing using modulated n = 3 non-axisymmetric fields has been demonstrated on DIII-D over a wide range of conditions, including significant variations in temperature, βN, density and shape. At low collisionality, the pacing results in a clear reduction in the ELM size and peak heat flux to the divertor, up to a factor of 5–6 for short time windows, although only a factor of two for sustained periods with the present hardware capability. At higher collisionality, although similar increases in the ELM frequency have been demonstrated, no meaningful reduction in the heat flux is typically observed as a direct result of the pacing. However, it appears that the ELM size may be reduced indirectly via changes in the L–H power threshold as a result of density pumpout associated with the application of non-axisymmetric fields. At this stage, it remains unclear whether the failure to reduce the ELM size with modulated fields is a limitation associated with high collisionality, high density, or relative proximity to the L–H power threshold.
    Nuclear Fusion 01/2012; 52(3). · 2.73 Impact Factor
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    ABSTRACT: A critical gradient threshold has been observed for the first time in a systematic, controlled experiment for a locally measured turbulent quantity in the core of a confined high-temperature plasma. In an experiment in the DIII-D tokamak where LTe-1=|∇Te|/Te and toroidal rotation were varied, long wavelength (kθρs≲0.4) electron temperature fluctuations exhibit a threshold in LTe-1: below, they change little; above, they steadily increase. The increase in δTe/Te is concurrent with increased electron heat flux and transport stiffness. Observations were insensitive to rotation. Accumulated evidence strongly enforces the identification of the experimentally observed threshold with ∇Te-driven trapped electron mode turbulence.
    Physical Review Letters 01/2012; · 7.94 Impact Factor
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    ABSTRACT: High-power electron cyclotron (EC) waves are used to increase performance in several Advanced Tokamak (AT) regimes on DIII-D where there is a simultaneous need for high noninductive current and high beta. In the Quiescent High-confinement mode (QH-mode), a direct measurement of the electron cyclotron current drive (ECCD) profile is made using modulation techniques, and a trapped electron mode (TEM) dominated regime with core T{sub e}>T{sub i} is created. In the 'highq{sub min}' AT scenario, ECCD provides part of the off-axis noninductive current and helps to produce a tearing stable equilibrium. In the hybrid regime, strong central current drive from EC waves and other sources increases the noninductive current fraction to {approx_equal}100%. Surprisingly, the core safety factor remains above unity, meaning good alignment between the current drive profile and the desired plasma current profile is not necessary in this scenario.
    AIP Conference Proceedings. 12/2011; 1406(1).
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    Physics of Plasmas 11/2011; 18(11). · 2.38 Impact Factor
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    ABSTRACT: The TGLF Gyro-Landau-Fluid transport model has been successful in reproducing the observed density and temperature profiles in a wide variety of tokamak discharges from DIII-D, JET and TFTR. Recently, it was shown that the predicted fusion gain for ITER using TGLF is somewhat more pessimistic than previous GLF23 results due to finite aspect ratio effects that are only present in TGLF. A key ingredient in the TGLF predictions of ITER is profile stiffness. A consequence of the stiff core transport is that the fusion gain scales like βped and like 1/Paux at fixed βped. Since stiff core transport has an important role in our ITER predictions we seek to quantify the stiffness of TGLF. Stiffness (S) is defined as the ratio of the incremental energy diffusivity to the power balance energy diffusivity. To date, we find S ˜10 is typical in the plasma core and drops to less than 3 in the near edge region. The electron and ion stiffness is examined in recent DIII-D experiments and in previous L- and H-mode similarity discharges.
    11/2011;
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    ABSTRACT: Several heat flux scans have been performed in an L-mode discharge in DIII-D with the goal of investigating the stiffness and critical gradient in the electron channel at ρ=0.6, 0.4 and 0.3. The heat flux scans employed 6 gyrotrons operating for 3.5 s with a shot-by-shot variation in heat flux achieved by moving 1 gyrotron/shot from just outside to just inside the region of interest. The stiffness was studied as a function of 4 different toroidal rotation conditions, low rotation with ECH only, higher rotation with co-NBI and counter-NBI, and lower rotation with balanced-NBI. Preliminary results indicate that toroidal rotation does not appear to play a strong role in determining the stiffness of the electron profile. For the ECH only condition at ρ=0.6 very low values of temperature gradient were obtained, well below the critical gradient estimated by the gradient where the heat flux projects to zero.
    11/2011;
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    ABSTRACT: The Phase Contrast Imaging (PCI) diagnostic has been used on DIII-D to measure plasma turbulence from 2 to 30 cm-1 using three roughly vertical beam paths. Work here focuses on measurements of QH mode plasmas, with stationary plasma parameters and an outer gap scan that allowed the PCI to sample a range in poloidal angle and kr/kθ. The results show the largest edge turbulence has kθ,i> 0.4 and f > 200 kHz, consistent with the plasma velocity at the bottom of the Er well, and a radial coherence length much less than 1 cm. A sharp decrease in turbulence amplitude is seen between the midplane and θ= 20 deg away from the X-point with no similar drop between the midplane and θ= 20 deg toward the X-point. Another component to the turbulence is seen at roughly similar wavenumbers and f<100 kHz, consistent with the plasma velocity further inside the LCFS.
    11/2011;
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    ABSTRACT: The trigger mechanism of the L- to H-mode transition is not currently fully understood. Empirical scaling studies of the L-H transition power threshold have discovered global plasma parameter dependences, including a strong density dependence. The current work investigates the potential role of edge turbulence and flows in this density dependence by performing detailed measurements during a density scan experiment on DIII-D. Preliminary analysis indicates that the signatures of geodesic acoustic modes (GAMs) exist in both the perpendicular flow and electron temperature fluctuations (Te) prior to the L-H transition. Both Te/Te at the GAM frequency and Te/Te of broadband fluctuations are observed to decrease with increasing density. Measurements of density turbulence, ExB flow, together with linear stability analysis will also be reported.
    11/2011;
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    ABSTRACT: DIII-D experiments have shown that static 3D magnetic fields can be used to maintain the edge rotation shear required for ELM-stable operation in QH-mode even with zero-net torque from neutral beam injection (NBI). These results have been obtained in ITER-similar shape plasmas with ITER-level collisionality, normalized beta, and confinement quality. New experiments are planned to extend the previous results to conditions closer yet to those of ITER: 3D field application using coils external to the vessel, small co-Ip NBI torque, and low q95˜3. Results will be discussed.
    11/2011;
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    ABSTRACT: Core electron/ion thermal transport and its dependence on ITG/TEM/ETG-scale turbulence are examined in high temperature, strongly rotating QH-mode plasmas, at ITER-relevant collisionality (νe^* ˜0.05). To simulate central electron heating by α-particles, ECH has been used to achieve 0.6 <=Te/Ti<=1.1. ITG/TEM-scale density fluctuations remain virtually unchanged, while electron temperature fluctuations, and gyroBohm-normalized electron and ion diffusivities increase with Te/Ti. Linear stability calculations support a transition to a TEM-dominated regime due to increased Te/Ti and a reduced ion temperature gradient R/LTi with ECH. Initial GYRO nonlinear calculations will be shown. At reduced toroidal rotation, ITG-dominated QH-mode plasmas [Te(0)/Ti(0)˜0.6] exhibit 20% increased global energy confinement time and βN,
    11/2011;

Publication Stats

7k Citations
886.43 Total Impact Points

Institutions

  • 1988–2014
    • General Atomics
      San Diego, California, United States
  • 2003–2012
    • Princeton University
      • Princeton Plasma Physics Laboratory
      Princeton, NJ, United States
  • 1998–2012
    • University of California, Los Angeles
      • • Department of Electrical Engineering
      • • Department of Physics and Astronomy
      Los Angeles, CA, United States
  • 2011
    • University of Wisconsin, Madison
      • Department of Engineering Physics
      Madison, MS, United States
    • California College San Diego
      San Diego, California, United States
  • 1998–2011
    • Oak Ridge National Laboratory
      • Fusion Energy Division
      Oak Ridge, Florida, United States
  • 1988–2011
    • University of California, San Diego
      • Department of Mechanical and Aerospace Engineering (MAE)
      San Diego, CA, United States
  • 2004–2010
    • University of California, Irvine
      • Department of Physics and Astronomy
      Irvine, CA, United States
  • 2007
    • University of Texas at Austin
      • Fusion Research Center
      Austin, Texas, United States
    • CSU Mentor
      Long Beach, California, United States
  • 1995–2007
    • Massachusetts Institute of Technology
      • Plasma Science and Fusion Center (PSFC)
      Cambridge, MA, United States
  • 1995–2002
    • University of California, Berkeley
      Berkeley, California, United States
  • 2001
    • Lehigh University
      • Department of Physics
      Bethlehem, Pennsylvania, United States
  • 2000
    • Kurchatov Institute
      Moskva, Moscow, Russia
  • 1990–1998
    • Lawrence Livermore National Laboratory
      • Physics Division
      Livermore, California, United States