K Hill

Publications (43)93.34 Total impact

• Article: Formation and stability of impurity "snakes" in tokamak plasmas.

  L Delgado-Aparicio, L Sugiyama, R Granetz, D A Gates, J E Rice, M L Reinke, M Bitter, E Fredrickson, C Gao, M Greenwald, K Hill, A Hubbard, J W Hughes, E Marmar, N Pablant, Y Podpaly, S Scott, R Wilson, S Wolfe, S Wukitch
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  ABSTRACT: New observations of the formation and dynamics of long-lived impurity-induced helical "snake" modes in tokamak plasmas have recently been carried out on Alcator C-Mod. The snakes form as an asymmetry in the impurity ion density that undergoes a seamless transition from a small helically displaced density to a large crescent-shaped helical structure inside q<1, with a regularly sawtoothing core. The observations show that the conditions for the formation and persistence of a snake cannot be explained by plasma pressure alone. Instead, many features arise naturally from nonlinear interactions in a 3D MHD model that separately evolves the plasma density and temperature.
  Physical Review Letters 02/2013; 110(6):065006. · 7.37 Impact Factor
• Article: X-ray imaging crystal spectroscopy for use in plasma transport research.

  M L Reinke, Y A Podpaly, M Bitter, I H Hutchinson, J E Rice, L Delgado-Aparicio, C Gao, M Greenwald, K Hill, N T Howard, A Hubbard, J W Hughes, N Pablant, A E White, S M Wolfe
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  ABSTRACT: This research describes advancements in the spectral analysis and error propagation techniques associated with x-ray imaging crystal spectroscopy (XICS) that have enabled this diagnostic to be used to accurately constrain particle, momentum, and heat transport studies in a tokamak for the first time. Doppler tomography techniques have been extended to include propagation of statistical uncertainty due to photon noise, the effect of non-uniform instrumental broadening as well as flux surface variations in impurity density. These methods have been deployed as a suite of modeling and analysis tools, written in interactive data language (IDL) and designed for general use on tokamaks. Its application to the Alcator C-Mod XICS is discussed, along with novel spectral and spatial calibration techniques. Example ion temperature and radial electric field profiles from recent I-mode plasmas are shown, and the impact of poloidally asymmetric impurity density and natural line broadening is discussed in the context of the planned ITER x-ray crystal spectrometer.
  The Review of scientific instruments 11/2012; 83(11):113504. · 1.52 Impact Factor
• Article: Molybdenum emission from impurity-induced m = 1 snake-modes on the Alcator C-Mod tokamak.

  L Delgado-Aparicio, M Bitter, R Granetz, M Reinke, P Beiersdorfer, D Gates, K Hill, N Pablant, Y Podpaly, J Rice, L Sugiyama
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  ABSTRACT: A suite of novel high-resolution spectroscopic imaging diagnostics has facilitated the identification and localization of molybdenum impurities as the main species during the formation and lifetime of m = 1 impurity-induced snake-modes on Alcator C-Mod. Such measurements made it possible to infer, for the first time, the perturbed radiated power density profiles from which the impurity density can be deduced.
  The Review of scientific instruments 10/2012; 83(10):10E517. · 1.52 Impact Factor
• Article: Overview of physics results from NSTX

  R. Raman, J-W. Ahn, J.P. Allain, R. Andre, R. Bastasz, D. Battaglia, P. Beiersdorfer, M. Bell, R. Bell, E. Belova, [......], K.L. Wong, J. Wright, Z. Xia, D. Youchison, G. Yu, H. Yuh, L. Zakharov, D. Zemlyanov, G. Zimmer, S.J. Zweben
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  ABSTRACT: In the last two experimental campaigns, the low aspect ratio NSTX has explored physics issues critical to both toroidal confinement physics and ITER. Experiments have made extensive use of lithium coatings for wall conditioning, correction of non-axisymmetric field errors and control of n = 1 resistive wall modes (RWMs) to produce high-performance neutral-beam heated discharges extending to 1.7 s in duration with non-inductive current fractions up to 0.7. The RWM control coils have been used to trigger repetitive ELMs with high reliability, and they have also contributed to an improved understanding of both neoclassical tearing mode and RWM stabilization physics, including the interplay between rotation and kinetic effects on stability. High harmonic fast wave (HHFW) heating has produced plasmas with central electron temperatures exceeding 6 keV. The HHFW heating was used to show that there was a 20–40% higher power threshold for the L–H transition for helium than for deuterium plasmas. A new diagnostic showed a depletion of the fast-ion density profile over a broad spatial region as a result of toroidicity-induced Alfvén eigenmodes (TAEs) and energetic-particle modes (EPMs) bursts. In addition, it was observed that other modes (e.g. global Alfvén eigenmodes) can trigger TAE and EPM bursts, suggesting that fast ions are redistributed by high-frequency AEs. The momentum pinch velocity determined by a perturbative technique decreased as the collisionality was reduced, although the pinch to diffusion ratio, Vpinch/χ, remained approximately constant. The mechanisms of deuterium retention by graphite and lithium-coated graphite plasma-facing components have been investigated. To reduce divertor heat flux, a novel divertor configuration, the 'snowflake' divertor, was tested in NSTX and many beneficial aspects were found. A reduction in the required central solenoid flux has been realized in NSTX when discharges initiated by coaxial helicity injection were ramped in current using induction. The resulting plasmas have characteristics needed to meet the objectives of the non-inductive start-up and ramp-up program of NSTX.
  Nuclear Fusion 08/2011; 51(9):094011. · 4.09 Impact Factor
• Source

  Available from: mit.edu

  Article: The Ar17 + Lyα2/Lyα1 ratio in Alcator C-Mod tokamak plasmas

  J E Rice, M L Reinke, J M A Ashbourn, A C Ince-Cushman, Y A Podpaly, M F Gu, M Bitter, K Hill, E Rachlew
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  ABSTRACT: High-quality spectra of hydrogen-like Ar17 + have been obtained from Alcator C-Mod tokamak plasmas using a spatially imaging high-resolution x-ray spectrometer system in an extensive study of the underlying high-n satellite lines. The ratio of Lyα2 (1S1/2–2P1/2) to Lyα1 (1S1/2–2P3/2) was found to be ~0.52 regardless of plasma parameters, which is somewhat greater than the ratio of the statistical weights of the upper n = 2 levels, 0.5. This difference is mainly due to the effects of collisional excitation of fine-structure sub-levels. For the observations presented here, electron densities were in an extended range from 3×1019 to 4×1020 m−3 with electron and ion temperatures between 1 and 4 keV. Experimental results are compared to calculations from COLRAD, a collisional-radiative modelling code, and good agreement is shown.
  Journal of Physics B Atomic Molecular and Optical Physics 07/2011; 44(16):165702. · 1.88 Impact Factor
• Article: OGLE-2005-BLG-153: Microlensing Discovery and Characterization of a Very Low Mass Binary

  K.-H. Hwang, A. Udalski, C. Han, Y.-H. Ryu, I. A. Bond, J.-P. Beaulieu, M. Dominik, K. Horne, A. Gould, B. S. Gaudi, [......], J. Wambsganss, A. Williams, K. Woller, M. Burgdorf, C. Snodgrass, M. Bode, D. L. Depoy, C.-U. Lee, B.-G. Park, R. W. Pogge
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  ABSTRACT: The mass function and statistics of binaries provide important diagnostics of the star formation process. Despite this importance, the mass function at low masses remains poorly known due to observational difficulties caused by the faintness of the objects. Here we report the microlensing discovery and characterization of a binary lens composed of very low mass stars just above the hydrogen-burning limit. From the combined measurements of the Einstein radius and microlens parallax, we measure the masses of the binary components of 0.10 ± 0.01 M ☉ and 0.09 ± 0.01 M ☉. This discovery demonstrates that microlensing will provide a method to measure the mass function of all Galactic populations of very low mass binaries that is independent of the biases caused by the luminosity of the population.
  The Astrophysical Journal 10/2010; 723(1):797. · 6.02 Impact Factor
• Article: The first experimental results from x-ray imaging crystal spectrometer for KSTAR.

  S G Lee, J G Bak, U W Nam, M K Moon, Y Shi, M Bitter, K Hill
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  ABSTRACT: The x-ray imaging crystal spectrometer (XICS) for the Korea Superconducting Tokamak Advanced Research has been first applied for the experimental campaign in 2009. The XICS was designed to provide measurements of the profiles of the ion and electron temperatures from the heliumlike argon (Ar XVII) spectra. The basic functions of the XICS are properly working although some satellites lines are not well matched with the expected theoretical values. The initial experimental results from the XICS are briefly described.
  The Review of scientific instruments 10/2010; 81(10):10E506. · 1.52 Impact Factor
• Article: Soft x-ray continuum radiation transmitted through metallic filters: an analytical approach to fast electron temperature measurements.

  L Delgado-Aparicio, K Tritz, T Kramer, D Stutman, M Finkenthal, K Hill, M Bitter
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  ABSTRACT: A new set of analytic formulas describes the transmission of soft x-ray continuum radiation through a metallic foil for its application to fast electron temperature measurements in fusion plasmas. This novel approach shows good agreement with numerical calculations over a wide range of plasma temperatures in contrast with the solutions obtained when using a transmission approximated by a single-Heaviside function [S. von Goeler et al., Rev. Sci. Instrum. 70, 599 (1999)]. The new analytic formulas can improve the interpretation of the experimental results and thus contribute in obtaining fast temperature measurements in between intermittent Thomson scattering data.
  The Review of scientific instruments 10/2010; 81(10):10E303. · 1.52 Impact Factor
• Source

  Available from: pppl.gov

  Article: Objectives and layout of a high-resolution x-ray imaging crystal spectrometer for the large helical device.

  M Bitter, K Hill, D Gates, D Monticello, H Neilson, A Reiman, A L Roquemore, S Morita, M Goto, H Yamada, J E Rice
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  ABSTRACT: A high-resolution x-ray imaging crystal spectrometer, whose concept was tested on NSTX and Alcator C-Mod, is being designed for the large helical device (LHD). This instrument will record spatially resolved spectra of helium-like Ar(16+) and will provide ion temperature profiles with spatial and temporal resolutions of <2 cm and ≥10 ms, respectively. The spectrometer layout and instrumental features are largely determined by the magnetic field structure of LHD. The stellarator equilibrium reconstruction codes, STELLOPT and PIES, will be used for the tomographic inversion of the spectral data.
  The Review of scientific instruments 10/2010; 81(10):10E328. · 1.52 Impact Factor
• Article: Frequency of Solar-like Systems and of Ice and Gas Giants Beyond the Snow Line from High-magnification Microlensing Events in 2005-2008

  A. Gould, Subo Dong, B. S. Gaudi, A. Udalski, I. A. Bond, J. Greenhill, R. A. Street, M. Dominik, T. Sumi, M. K. Szymański, [......], C. Liebig, L. Mancini, G. Masi, M. Mathiasen, S. Rahvar, D. Ricci, G. Scarpetta, J. Southworth, J. Surdej, C. C. Thöne
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  ABSTRACT: We present the first measurement of the planet frequency beyond the "snow line," for the planet-to-star mass-ratio interval –4.5 < log q < –2, corresponding to the range of ice giants to gas giants. We find at the mean mass ratio q = 5 × 10–4 with no discernible deviation from a flat (Öpik's law) distribution in log-projected separation s. The determination is based on a sample of six planets detected from intensive follow-up observations of high-magnification (A>200) microlensing events during 2005-2008. The sampled host stars have a typical mass M host ~ 0.5 M ☉, and detection is sensitive to planets over a range of planet-star-projected separations (s –1 max R E, s max R E), where R E ~ 3.5 AU(M host/M ☉)1/2 is the Einstein radius and s max ~ (q/10–4.3)1/3. This corresponds to deprojected separations roughly three times the "snow line." We show that the observations of these events have the properties of a "controlled experiment," which is what permits measurement of absolute planet frequency. High-magnification events are rare, but the survey-plus-follow-up high-magnification channel is very efficient: half of all high-mag events were successfully monitored and half of these yielded planet detections. The extremely high sensitivity of high-mag events leads to a policy of monitoring them as intensively as possible, independent of whether they show evidence of planets. This is what allows us to construct an unbiased sample. The planet frequency derived from microlensing is a factor 8 larger than the one derived from Doppler studies at factor ~25 smaller star-planet separations (i.e., periods 2-2000 days). However, this difference is basically consistent with the gradient derived from Doppler studies (when extrapolated well beyond the separations from which it is measured). This suggests a universal separation distribution across 2 dex in planet-star separation, 2 dex in mass ratio, and 0.3 dex in host mass. Finally, if all planetary systems were "analogs" of the solar system, our sample would have yielded 18.2 planets (11.4 "Jupiters," 6.4 "Saturns," 0.3 "Uranuses," 0.2 "Neptunes") including 6.1 systems with two or more planet detections. This compares to six planets including one two-planet system in the actual sample, implying a first estimate of 1/6 for the frequency of solar-like systems.
  The Astrophysical Journal 08/2010; 720(2):1073. · 6.02 Impact Factor
• Article: Addendum to papers from the NSTX Team, published in Review of Scientific Instruments.

  D A Gates, C Kessel, J Menard, G Taylor, J R Wilson, M G Bell, R E Bell, S Bernabei, J Bialek, T Biewer, [......], M Wade, R White, J Wilgen, M Williams, W Zhu, S J Zweben, R Akers, P Beiersdorfer, R Betti, T Bigelow
  The Review of scientific instruments 12/2009; 80(12):129901. · 1.52 Impact Factor
• Article: Lower hybrid heating and current drive on the Alcator C-Mod tokamak

  J.R. Wilson, R. Parker, M. Bitter, P.T. Bonoli, C. Fiore, R.W. Harvey, K. Hill, A.E. Hubbard, J.W. Hughes, A. Ince-Cushman, [......], C.K. Phillips, M. Porkolab, J. Rice, A.E. Schmidt, S. Scott, S. Shiraiwa, E. Valeo, G. Wallace, J.C. Wright, the Alcator C-Mod Team
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  ABSTRACT: On the Alcator C-Mod tokamak, lower hybrid current drive (LHCD) is being used to modify the current profile with the aim of obtaining advanced tokamak (AT) performance in plasmas with parameters similar to those that would be required on ITER. To date, power levels in excess of 1 MW at a frequency of 4.6 GHz have been coupled into a variety of plasmas. Experiments have established that LHCD on C-Mod behaves globally as predicted by theory. Bulk current drive efficiencies, n20IlhR/Plh ~ 0.25, inferred from magnetics and MSE are in line with theory. Quantitative comparisons between local measurements, MSE, ECE and hard x-ray bremsstrahlung, and theory/simulation using the GENRAY, TORIC-LH CQL3D and TSC-LSC codes have been performed. These comparisons have demonstrated the off-axis localization of the current drive, its magnitude and location dependence on the launched n∥ spectrum, and the use of LHCD during the current ramp to save volt-seconds and delay the peaking of the current profile. Broadening of the x-ray emission profile during ICRF heating indicates that the current drive location can be controlled by the electron temperature, as expected. In addition, an alteration in the plasma toroidal rotation profile during LHCD has been observed with a significant rotation in the counter-current direction. Notably, the rotation is accompanied by peaking of the density and temperature profiles on a current diffusion time scale inside of the half radius where the LH absorption is taking place.
  Nuclear Fusion 09/2009; 49(11):115015. · 4.09 Impact Factor
• Article: Climate impacts on the hydrology of high elevation catchments, Colorado Front Range

  M. Williams, K. Hill, N. Caine, J. Janke
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  ABSTRACT: Potential climate impacts on the hydrology of two seasonally snow covered catchments is evaluated using 24 years of data from Niwot Ridge Long Term Ecological Research Site, Colorado. At the larger (220 ha), higher elevation (3570 m) GL4 catchment annual discharge did not change significantly based on nonparametric trend testing. However, October streamflow volumes and groundwater storage did increase, despite drought conditions near the end of the record in 2000-2004. In contrast, at the smaller (8 ha), lower elevation (3400 m) MART catchment, annual discharge decreased significantly over the study period with the most substantial changes in July-September. The study period was separated into "wet", "normal", and "dry" years based on the 75th and 25th quartiles of annual precipitation. Results indicate that MART is particularly sensitive to changes in precipitation with dry years exhibiting decreased snowmelt peak flows, earlier snowmelt timing, decreased annual discharge, and reduced late-season flows. GL4 was less susceptible to changes in precipitation and surprisingly late-season flow volumes (Sept.-Oct.) were not significantly different between wet, normal, and dry conditions. Glacial melt from the Arikaree glacier may account for up to 43% of the increase in late-season flows based on ablation measurements. We downscaled a regional permafrost model based on topoclimatic variables to assess whether subsurface ice within permafrost and rock glaciers could account for the remaining deficiency. Results suggest that with only 1° C of warming over 1/3 of permafrost area would be lost. Over the study period mean annual minimum temperatures increased by 0.6° decade-1, with the some of most prominent increases occurring in July (1.5° C decade-1). Additionally, limited ground temperature measurements at an active rock glacier indicate a 1° C increase over the past decade. This suggests that the source of late-season streamflow at GL4 has shifted towards permafrost meltwater in recent warm, dry years. This study shows that seasonally snow covered catchments are particularly sensitive to changes in climate, but the hydrologic response may depend on landscape characteristics.
  03/2009; 11:6387.
• Source

  Available from: mit.edu

  Article: Observation of self-generated flows in tokamak plasmas with lower-hybrid-driven current.

  A Ince-Cushman, J E Rice, M Reinke, M Greenwald, G Wallace, R Parker, C Fiore, J W Hughes, P Bonoli, S Shiraiwa, A Hubbard, S Wolfe, I H Hutchinson, E Marmar, M Bitter, J Wilson, K Hill
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  ABSTRACT: In Alcator C-Mod discharges lower hybrid waves have been shown to induce a countercurrent change in toroidal rotation of up to 60 km/s in the central region of the plasma (r/a approximately <0.4). This modification of the toroidal rotation profile develops on a time scale comparable to the current redistribution time (approximately 100 ms) but longer than the energy and momentum confinement times (approximately 20 ms). A comparison of the co- and countercurrent injected waves indicates that current drive (as opposed to heating) is responsible for the rotation profile modifications. Furthermore, the changes in central rotation velocity induced by lower hybrid current drive (LHCD) are well correlated with changes in normalized internal inductance. The application of LHCD has been shown to generate sheared rotation profiles and a negative increment in the radial electric field profile consistent with a fast electron pinch.
  Physical Review Letters 02/2009; 102(3):035002. · 7.37 Impact Factor
• Source

  Available from: George Tynan

  Article: Overview of results from the National Spherical Torus Experiment (NSTX)

  D. A. Gates, J. Ahn, J. Allain, R. Andre, R. Bastasz, M. Bell, R. Bell, E. Belova, J. Berkery, R. Betti, [......], K. Wong, J. Wright, Z. Xia, X. Xu, D. Youchison, G. Yu, H. Yuh, L. Zakharov, D. Zemlyanov, S. Zweben
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  ABSTRACT: The mission of the National Spherical Torus Experiment (NSTX) is the demonstration of the physics basis required to extrapolate to the next steps for the spherical torus (ST), such as a plasma facing component test facility (NHTX) or an ST based component test facility (ST-CTF), and to support ITER. Key issues for the ST are transport, and steady state high b operation. To better understand electron transport, a new high-k scattering diagnostic was used extensively to investigate electron gyro-scale fluctuations with varying electron temperature gradient scale length. Results from n = 3 braking studies are consistent with the flow shear dependence of ion transport. New results from electron Bernstein wave emission measurements from plasmas with lithium wall coating applied indicate transmission efficiencies near 70% in H-mode as a result of reduced collisionality. Improved coupling of high harmonic fast-waves has been achieved by reducing the edge density relative to the critical density for surface wave coupling. In order to achieve high bootstrap current fraction, future ST designs envision running at very high elongation. Plasmas have been maintained on NSTX at very low internal inductance li [?] 0.4 with strong shaping (k [?] 2.7, d [?] 0.8) with bN approaching the with-wall b-limit for several energy confinement times. By operating at lower collisionality in this regime, NSTX has achieved record non-inductive current drive fraction fNI [?] 71%. Instabilities driven by super-Alfvenic ions will be an important issue for all burning plasmas, including ITER. Fast ions from NBI on NSTX are super-Alfvenic. Linear toroidal Alfven eigenmode thresholds and appreciable fast ion loss during multi-mode bursts are measured and these results are compared with theory. The impact of n > 1 error fields on stability is an important result for ITER. Resistive wall mode/resonant field amplification feedback combined with n = 3 error field control was used on NSTX to maintain plasma rotation with b above the no-wall limit. Other highlights are results of lithium coating experiments, momentum confinement studies, scrape-off layer width scaling, demonstration of divertor heat load mitigation in strongly shaped plasmas and coupling of coaxial helicity injection plasmas to ohmic heating ramp-up. These results advance the ST towards next step fusion energy devices such as NHTX and ST-CTF.
  Nuclear Fusion. 01/2009; 49(10):104016.
• Source

  Available from: arxiv.org

  Article: First Microlens Mass Measurement: PLANET Photometry of EROS BLG-2000-5

  Jin H. An, M. D. Albrow, J.-P. Beaulieu, J. A. R. Caldwell, D. L. DePoy, M. Dominik, B. S. Gaudi, A. Gould, J. Greenhill, K. Hill, [......], R. Martin, J. Menzies, R. W. Pogge, K. R. Pollard, P. D. Sackett, K. C. Sahu, P. Vermaak, R. Watson, A. Williams, and (The PLANET Collaboration
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  ABSTRACT: We analyze PLANET photometric observations of the caustic-crossing binary lens microlensing event, EROS BLG-2000-5, and find that modeling the observed light curve requires incorporation of the microlens parallax and the binary orbital motion. The projected Einstein radius (E = 3.61 ± 0.11 AU) is derived from the measurement of the microlens parallax, and we are also able to infer the angular Einstein radius (θE = 1.38 ± 0.12 mas) from the finite source effect on the light curve, combined with an estimate of the angular size of the source given by the source position in a color-magnitude diagram. The lens mass, M = 0.612 ± 0.057 M☉, is found by combining these two quantities. This is the first time that parallax effects are detected for a caustic-crossing event and also the first time that the lens mass degeneracy has been completely broken through photometric monitoring alone. The combination of E and θE also allows us to conclude that the lens lies in the near side of the disk, within 2.6 kpc of the Sun, while the radial velocity measurement indicates that the source is a Galactic bulge giant.
  The Astrophysical Journal 12/2008; 572(1):521. · 6.02 Impact Factor
• Article: Spectral resolution measurement of an x-ray imaging crystal spectrometer for KSTAR.

  S G Lee, J G Bak, U W Nam, M K Moon, J K Cheon, M Bitter, K Hill
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  ABSTRACT: A spectral resolution measurement of the x-ray imaging crystal spectrometer for the Korea Superconducting Tokamak Advanced Research machine utilizing a segmented position-sensitive detector and time-to-digital converter based delay-line readout electronics was performed by using an x-ray tube in a laboratory. The measured spectral resolution is about 12,600, which means the actual energy resolution is 0.32 eV for the x-ray tube's bremsstrahlung peak energy of 4 keV. The results from the spectral resolution measurement are described.
  The Review of scientific instruments 11/2008; 79(10):10E317. · 1.52 Impact Factor
• Article: A new DoD initiative: the Computational Research and Engineering Acquisition Tools and Environments (CREATE) program

  S Arevalo, C Atwood, P Bell, T D Blacker, S Dey, D Fisher, D A Fisher, P Genalis, J Gorski, A Harris, [......], R P Kendall, R L Meakin, S Morton, E T Moyer, D E Post, R Strawn, D v Veldhuizen, L G Votta, S Wynn, G Zelinski
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  ABSTRACT: In FY2008, the U.S. Department of Defense (DoD) initiated the Computational Research and Engineering Acquisition Tools and Environments (CREATE) program, a $360M program with a two-year planning phase and a ten-year execution phase. CREATE will develop and deploy three computational engineering tool sets for DoD acquisition programs to use to design aircraft, ships and radio-frequency antennas. The planning and execution of CREATE are based on the 'lessons learned' from case studies of large-scale computational science and engineering projects. The case studies stress the importance of a stable, close-knit development team; a focus on customer needs and requirements; verification and validation; flexible and agile planning, management, and development processes; risk management; realistic schedules and resource levels; balanced short- and long-term goals and deliverables; and stable, long-term support by the program sponsor. Since it began in FY2008, the CREATE program has built a team and project structure, developed requirements and begun validating them, identified candidate products, established initial connections with the acquisition programs, begun detailed project planning and development, and generated the initial collaboration infrastructure necessary for success by its multi-institutional, multidisciplinary teams.
  Journal of Physics Conference Series 08/2008; 125(1):012090.
• Source

  Available from: nstx.pppl.gov

  Article: A quantitative account of electron energy transport in a National Spherical Tokamak Experiment plasma

  K. L. Wong, S. Kaye, D. R. Mikkelsen, J. A. Krommes, K. Hill, R. Bell, B. LeBlanc
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  ABSTRACT: The first successful quantitative account of the electron thermal conductivity χe in a tokamak experiment due to imperfect magnetic surfaces is presented. The unstable spectrum of microtearing instabilities is calculated with the GS2 code for a well-behaved H-mode plasma in the National Spherical Tokamak Experiment [ M. Ono et al., Nucl. Fusion 40, 557 (2000) ], with 6 MW deuterium neutral beam heating at Ip = 0.75 MA, Bt = 0.5 T. The application of existing nonlinear theory shows that the unstable modes can produce overlapping magnetic islands leading to global stochastic magnetic fields. The calculated χe based on the present theory is in reasonable agreement with the values from transport analysis of the experimental data over the entire region (0.4<r/a<0.75) where the electron temperature gradient is strong enough to make microtearing the most unstable mode. There is no adjustable parameter in this comparison. This instability can be avoided by reversed magnetic shear or by heating the electrons to lower the electron-ion collision frequency.
  Physics of Plasmas 03/2008; 15(5):056108-056108-8. · 2.15 Impact Factor
• Source

  Available from: pppl.gov

  Article: Microtearing instabilities and electron transport in the NSTX spherical tokamak.

  K L Wong, S Kaye, D R Mikkelsen, J A Krommes, K Hill, R Bell, B Leblanc
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  ABSTRACT: We report a successful quantitative account of the experimentally determined electron thermal conductivity chi(e) in a beam-heated H mode plasma by the magnetic fluctuations from microtearing instabilities. The calculated chi(e) based on existing nonlinear theory agrees with the result from transport analysis of the experimental data. Without using any adjustable parameter, the good agreement spans the entire region where there is a steep electron temperature gradient to drive the instability.
  Physical Review Letters 10/2007; 99(13):135003. · 7.37 Impact Factor